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A revised classification scheme of pyrite in the Witwatersrand Basin and application to placer gold deposits
Journal Pre-proof A revised classification scheme of pyrite in the Witwatersrand Basin and application to placer gold deposits
G. Costa, A. Hofmann, A. Agangi PII:
S0012-8252(19)30505-7
DOI:
https://doi.org/10.1016/j.earscirev.2019.103064
Reference:
EARTH 103064
To appear in:
Earth-Science Reviews
Received date:
26 July 2019
Revised date:
1 December 2019
Accepted date:
10 December 2019
Please cite this article as: G. Costa, A. Hofmann and A. Agangi, A revised classification scheme of pyrite in the Witwatersrand Basin and application to placer gold deposits, Earth-Science Reviews(2019), https://doi.org/10.1016/j.earscirev.2019.103064
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Journal Pre-proof
A revised classification scheme of pyrite in the Witwatersrand Basin and application to placer gold deposits G. Costa1, A. Hofmann 1,* [email protected], A. Agangi1,2 1 Department of Geology, University of Johannesburg, Auckland Park 2006, South Africa 2 Department of Earth Resource Science, Akita University, Akita 010-8502, Japan * Corresponding author. Abstract Pyrite is the most abundant ore mineral in gold-bearing quartz pebble conglomerates (QPCs) of the Witwatersrand Basin and similar Archaean sedimentary units. Much of the pyrite in Archaean conglomerates is detrital in origin, implying that it survived
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weathering, transport, and reworking under anoxic conditions. Detrital pyrite is
post-depositional
processes. As
pyrite
is
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generally found together with authigenic pyrite formed in situ as a result of syn- to frequently associated with gold
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mineralization, many authors have developed different pyrite classification schemes in
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order to better understand the nature and origin of the mineralization, although the different classifications cannot always be easily compared as they largely based on
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inferred pyrite genesis. We propose a revised, entirely descriptive classification scheme for detrital and authigenic pyrite in Archaean QPCs. The scheme was created
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in order to provide a consistent framework for the petrographic description of pyrite in
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clastic sedimentary rocks. Detrital pyrite is subdivided into massive, inclusion-bearing, and coarsely crystalline types. Authigenic pyrite includes euhedral, overgrowth, infill,
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aggregate, and pseudomorphic types. We evaluate the methods used in pyrite classification, propose formation pathways for detrital and authigenic pyrites, and apply the classification scheme to some well-known occurrences of detrital pyritebearing conglomerates.
Keywords: Archaean, detrital pyrite, gold mineralization, Witwatersrand Basin
1. Introduction Pyrite (cubic FeS2) is known to undergo oxidation when exposed to the modern atmosphere and hydrosphere (Bierens de Haan, 1991). Its preservation as detrital grains in sedimentary rocks older than c. 2.4 Ga implies an atmospheric oxygen level < 0.01 atm at that time and is one of the main arguments in favour of an oxygen-
Journal Pre-proof deficient atmosphere in the Archaean (Grandstaff, 1980; Holland, 1984; Krupp et al., 1994; Johnson et al., 2014). Only rarely is detrital pyrite found in sedimentary successions younger than that time, which has become known as the Great Oxidation Event (GOE) (Holland, 1984). Preservation of detrital pyrite in oxic environments requires specific conditions, such as rapid burial in water-saturated, reduced sediments, as for example recorded by Cretaceous to Recent sedimentary strata in southern New Zealand (Youngson et al., 2006). Archaean quartz pebble conglomerates (QPCs) with abundant detrital pyrite are
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commonly of economic interest. The Witwatersrand (or Wits) Basin in South Africa
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(Fig. 1) is the best example of a sedimentary succession known for its gold and uranium mineralization associated with detrital pyrite. The Mesoarchaean sedimentary rocks of
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the Witwatersrand Supergroup host the richest and the single most important gold province in the world, with ∼ 53 kt of gold mined in total (Frimmel, 2019). The term
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Witwatersrand Basin is used here in a broad sense to encompass all Archaean volcano-
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sedimentary units that accumulated in the central part of the Kaapvaal Craton between 3.1 and 2.6 Ga ago. These include the Dominion Group and the
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Witwatersrand, Ventersdorp and basal Transvaal supergroups. Different types of both detrital and authigenic pyrite occur in QPCs and have been
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described and investigated from many different angles. As different terms are used by
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different authors and as existing schemes mix descriptive and genetic terminology, a revision of existing pyrite classification schemes is thus overdue. In addition, much recent work on Archaean pyrite has shed new light on its elemental and isotopic composition and origin. In view of these previous results, presented below is a new classification scheme that synthesizes and homogenizes older classifications using examples of the Wits Basin. The classification is strictly descriptive and widely applicable to Archaean QPCs.
2. The Witwatersrand Basin 2.1. Pre-Witwatersrand basement and the Dominion Group The Archaean Kaapvaal Craton in southern Africa developed over a protracted time span between 3.7 and 2.7 Ga (Eglington and Armstrong, 2004). The central-eastern part of the craton (Fig. 1) stabilized c. 3.1 Ga ago, after which it acted as basement for
Journal Pre-proof the development of intracontinental basins. A period of extension at 3.07 Ga gave rise to deposition of the volcano-sedimentary Dominion Group, which lies unconformably on granitoid-greenstone basement and comprises a predominantly volcanic bimodal sequence up to three kilometres in thickness (Marsh, 2006). A basal siliciclastic unit (Rhenosterspruit Formation), including QPCs known as the Lower and Upper Dominion Reefs, hosts marginally economic gold and uranium mineralization. The Rhenosterhoek Formation contains in excess of 1100 m of mafic to intermediate, amygdaloidal lava with minor felsic volcanic rocks. The uppermost unit, the Syferfontein Formation,
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conformably overlies the Rhenosterhoek Formation and is dominated by feldspar-
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quartz-phyric felsic volcanic rocks, basaltic to andesitic lava, tuff and breccia locally up to 3000 m thick (Jackson, 1992). A sample of the Syferfontein Formation has been
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2.2. The Witwatersrand Supergroup
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dated at 3074 ± 6 Ma (ion probe U–Pb zircon age; Armstrong et al., 1991).
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The Witwatersrand Supergroup is the classic example of a palaeoplacer gold deposit hosted in QPCs and associated sandstones. It is a predominantly sedimentary
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sequence subdivided into the West Rand and Central Rand groups (Fig. 1). The West Rand Group is a shallow-marine succession of sandstone and shale overlying the
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Palaeoarchaean granitoid-greenstone basement and the Dominion Group (Guy et al.,
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2010), with the youngest detrital zircon ages of 2985 ± 14 and 2902 ± 13 Ma obtained from near the base and top, respectively (Kositcin and Krapez, 2004). The Central Rand Group consists predominantly of fluvial to marginal marine sandstones and conglomerates. A minimum age of deposition for the Central Rand Group was provided by xenotime dated at ∼ 2780 Ma and interpreted to be diagenetic in origin (Kositcin et al., 2003). Gold and uranium-bearing QPCs of the Wits Basin, also known as reefs, host abundant detrital and authigenic (i.e. formed in situ) pyrite, specifically in the Central Rand Group. Besides detrital pyrite, the reefs are enriched in a large array of detrital heavy minerals, such as apatite, arsenopyrite, cassiterite, chromite, cobaltite, columbite, corundum, diamond, garnet, molybdenite, monazite, uraninite, zircon and many others as cited by Feather and Koen (1975), Phillips and Law (2000), Law and Phillips (2005) and Agangi et al. (2013), indicating prolonged times of sedimentary reworking
Journal Pre-proof and mineral concentrations under oxygen-deficient conditions. Following deposition, hydrothermal and metamorphic fluids modified, during several events, the primary mineralogy of the reefs, giving rise to remobilization of some of the primary ore constituents (Law and Phillips, 2005).
2.3. Ventersdorp Supergroup The volcano-sedimentary Ventersdorp Supergroup overlies the Witwatersrand Supergroup (Fig. 1). Its basal unit is marked by the richly gold-endowed Ventersdorp
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Contact Reef (VCR). The VCR consists of conglomerates interbedded with sandstones
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with a maximum thickness of 7 m and overlies the Central Rand Group unconformably (Krapez, 1985; Agangi et al., 2015). The VCR is conformably overlain by a succession of
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flood basalts with local intercalations of felsic volcanic and sedimentary rocks. U–Pb zircon ages indicate a first phase of continental flood basalt eruption as early as 2.78
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Ga (Cornell et al., 2017; Humbert et al., 2019), which overlaps with the youngest
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detrital zircon age of 2780 ± 5 Ma for the VCR (Robb and Meyer, 1995). In spite of belonging to a stratigraphic unit very different from the Witwatersrand
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Supergroup, the VCR displays a similar style of mineralization, with both detrital and authigenic pyrite being a common mineral (Utter, 1978; Smits, 1994; Agangi et al.,
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2013). Gold mineralization in the VCR is mainly found in areas where it unconformably
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overlies reefs of the Central Rand Group (McCarthy, 1994), this being the reason to consider the VCR as part of the Wits Basin at least in terms of its mineral endowment. The same reasoning applies to the base of the Transvaal Supergroup, characterized by the Black Reef.
2.4. Transvaal Supergroup The Transvaal Supergroup (Fig. 1) is characterized by the basal siliciclastic Black Reef Formation, chemical sedimentary rocks (carbonate, chert, banded iron formation (BIF)) of the Chuniespoort Group, and predominantly siliciclastic rocks of the Pretoria Group (Els et al., 1995; Eriksson et al., 2006). A SHRIMP U–Pb zircon age of 2588 ± 7 Ma was obtained from a tuff from the basal unit of the Chuniespoort Group (Martin et al., 1998). In the Wits Basin, the Black Reef Formation lies unconformably on the Ventersdorp or Witwatersrand Supergroup (Barton and Hallbauer, 1996; Fuchs et al.,
Journal Pre-proof 2016). It consists of a fining-upward sequence with basal sandstone and pyrite-rich QPC, known as the Black Reef, overlain by carbonaceous shale (Fuchs et al., 2016). The Black Reef resembles other Witwatersrand QPCs in terms of economic concentration of gold, mineralogy, and its depositional environment (Barton and Hallbauer, 1996; Fuchs et al., 2016). A supposed source of gold in the Black Reef resides in the erosion of underlying Witwatersrand Reefs as suggested by Papenfus (1964) and Pouroulis and Austin (1989). However, the genetic relationship between the two is still unclear (Fuchs et al., 2016). Despite erratic Au distribution throughout
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the Black Reef, it has been mined in parts of the Wits Basin around Johannesburg
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(Henry and Master, 2008).
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2.5. Gold mineralization in the Witwatersrand basin
Different hypotheses have been proposed for the genesis of the gold mineralization of
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the Wits Basin, which has been a controversy for many decades. The placer model
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proposes that gold and related heavy minerals (e.g. pyrite, uraninite) were introduced into the basin as clastic particles by sedimentary processes; the hydrothermal or
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epigenetic model proposes that gold precipitated from hydrothermal aqueous solutions well after deposition; and the modified placer model proposes that gold and
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related minerals were of detrital origin, but were variably remobilized after burial,
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causing changes in the composition and textural position within the ore (Pretorius, 1976; Phillips and Law, 2000; Law and Phillips, 2005; Frimmel, 2005). The latter model has been the most widely accepted in recent years. A fourth model, the syngenetic model, argues for gold to have precipitated in the reefs from solution, potentially aided by microbial mediation (e.g. Reimer, 1984; Mossman et al., 2008; Horscroft et al., 2011). This model has gained some momentum in recent years (Agangi et al., 2015; Frimmel and Hennigh, 2015; Heinrich, 2015). For example, Heinrich (2015) proposed dissolution of gold in the hinterland by acid rain, transport as sulfur complexes in runoff water, followed by chemical reduction of the dissolved gold onto organic material in shallow lakes. An additional aspect of the gold ore involves the presence of carbonaceous matter in the reefs, as bitumen seams and nodules. Although a matter of debate for decades,
Journal Pre-proof bitumen was largely derived from oil that was generated at different times in the evolution of the Wits Basin (England et al., 2002a).
3. A new pyrite classification scheme The new classification scheme is multi-layered and allows classification of pyrite grains in different textural classes of increasing detail. The two main classes of pyrite occurring in Wits Basin reefs compose the first layer: 1) detrital (or allogenic) pyrite (Table 1), and 2) authigenic pyrite that formed in situ after deposition (Table 2).
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Some authors also differentiated between syn-genetic (or syn-sedimentary),
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diagenetic, and epigenetic pyrite (e.g. Hallbauer, 1986; Guy et al., 2014). The syngenetic type refers to reworked intrabasinal sedimentary material and is thus detrital
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in nature, even though the degree of reworking and transport may have been small. Diagenetic pyrite is regarded to have formed in situ during early diagenesis of QPCs,
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whereas epigenetic is a term that refers to pyrite that formed after consolidation of
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the sediment (Maynard, 1983). As the timing of in situ pyrite growth can frequently not unambiguously be determined, diagenetic and epigenetic pyrites are herein
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included in the authigenic type.
Detrital pyrite can generally be differentiated from authigenic pyrite by evidence of
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abrasion. While this evidence may be found in the truncation of internal structures
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(e.g. zonation, growth banding, micro-veining) present in rounded grains in pyrite (e.g., MacLean and Fleet, 1989; England et al., 2002b), the more direct approach is to simply regard rounding as evidence of abrasion (Da Costa et al., 2017). Rounded pyrite grains representing sulfidized detrital rock or mineral fragments (herein referred to as authigenic pseudomorphic pyrite) do occur (Hirdes and Saager, 1983; Myers et al., 1993), although sulfidization is frequently not complete, thus allowing distinction between truly detrital and pseudomorphic pyrite. In addition, detrital pyrite may have overgrowths of authigenic pyrite, which may only be revealed by etching or microbeam analytical techniques (Fleet, 1998; Agangi et al., 2013; Da Costa et al., 2017).
4. Methods for the study of pyrite in Archaean QPCs
Journal Pre-proof The main variables to differentiate detrital from authigenic pyrite and to evaluate the provenance of detrital pyrite include the morphology and texture of pyrite grains, the nature of primary mineral inclusions, the trace element composition, and the isotopic composition (Da Costa et al., 2017). Pyrite morphology and texture has usually been investigated in situ by reflected light microscopy of polished thin sections and block mounts with or without oil immersion (Ramdohr, 1958; Saager, 1970, 1976; England et al., 2002b; Guy et al., 2014). Hydrofluoric acid liberation (Neuerburg, 1975) has been widely used to release pyrite grains from the rock matrix in order to investigate their
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morphology and their single grain chemical and isotopic composition (Utter, 1978;
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Hallbauer and Gehlen, 1983; Barton and Hallbauer, 1996; Hofmann et al., 2009). It is the rounded morphology and truncation of internal texture that most clearly
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differentiates detrital from authigenic grains. In addition, pit marks and scours on liberated rounded pyrite grains also support their detrital origin (Hallbauer, 1986;
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Utter, 1978).
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Optical microscopy has been complemented by scanning electron microscopy, either by imaging the surface features of liberated grains or investigating intergrowths with
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other minerals (Utter, 1978; Hallbauer and von Gehlen, 1983; Hallbauer, 1986; England et al., 2002b; Guy et al., 2012; Agangi et al., 2015). More recently, the distribution of
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major and trace elements was determined in situ using X-ray compositional mapping
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with electron probe micro-analyzer (EPMA), proton-induced X-ray emission (PIXE), or element mapping using laser-ablation inductively coupled plasma mass-spectrometry (LA-ICPMS; Large et al., 2013; Agangi et al., 2013; Fuchs et al., 2016). These techniques allow for the evaluation of chemical growth zoning, recording the evolution of mineralizing fluids, but also secondary events of brecciation, veining, resorption, and recrystallization. The combination of orientation contrast imaging and electron backscatter diffraction (EBSD) further allow to establish the deformation history of pyrite grains (Reddy and Hough, 2013). Trace elements in pyrite include a large range of siderophile and chalcophile elements, the concentrations and ratios of which can provide information on the setting of pyrite formation (Koglin et al., 2010; Large et al., 2013; Agangi et al., 2013; Guy et al., 2014). Trace element analysis of single pyrite grains liberated from the rock matrix can be problematic for provenance analysis, because of the common occurrence of inclusions
Journal Pre-proof and evidence of remobilization of elements susceptible to hydrothermal alteration, such as Au, Ni and As (Meyer et al., 1990; England et al., 2002b; Hofmann et al., 2009). Multiple S and Fe isotope analysis of detrital pyrite grains provide information on their provenance, specifically if pyrite was derived from a magmatic or sedimentary source (Hofmann et al., 2009). Sulfur isotopes can fractionate by mass-dependent, both biotic and abiotic processes. Thermochemical sulfate reduction coupled to the oxidation of organic matter or ferruginous iron-bearing minerals in hydrothermal systems can result in large fractionation of δ34S between sulfate and sulfide (Ohmoto and
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Goldhaber, 1997). At lower temperatures during early diagenesis, microorganisms can
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impart fractionations during their sulfur metabolism, particularly during the process of sulfate reduction (Canfield et al., 2000). Mass-independent fractionation (MIF) of S
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isotopes is caused by SO 2 photolysis under ultraviolet radiation in the anoxic Archaean atmosphere (Farquhar et al., 2001). Multiple S isotope analysis thus allows to
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distinguish between S that passed through the Archaean atmosphere (Δ 33S ≠ 0 ‰) and
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magmatic S incorporated into hydrothermal fluids (Δ 33S = 0 ‰). The degree of MIF-S was muted during parts of the Mesoarchaean (Farquhar et al.,
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2007), and mixing between S-pools with opposite Δ33S values can cancel out MIF signals, thus making multiple S isotope analysis not always entirely reliable to
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differentiate magmatic from sedimentary sources in the Wits Basin. Additional
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constraints can thus be obtained from the analysis of Fe isotopes. Fe isotope fractionation occurs during redox changes, fluid-mineral interactions, and biological processes (see Dauphas and Rouxel, 2006, Anbar and Rouxel, 2007, and Johnson et al., 2008 for reviews), and can frequently be observed in sedimentary pyrite (Hofmann et al., 2009). Lead isotope analysis following dissolution of liberated pyrite or in situ can provide constraints on pyrite formation age, based on a model Pb growth curve, and on the Pb source, which may provide information of pyrite provenance or post-depositional processes (Barton and Hallbauer, 1996; Poujol, et al., 1999; Meffre et al., 2008). Similarly, the Re-Os isotope system can be used as a geochronometer and as a process tracer for gold and sulfide minerals (Kirk et al., 2001; Schaefer et al., 2010; Mathur et al., 2013).
Journal Pre-proof 5. Detrital pyrite A large literature base exists on the description of detrital, i.e. rounded, pyrite and many different types have been observed (Liebenberg, 1955; Ramdohr, 1958; Saager, 1970; Feather and Koen, 1975; Utter, 1978; Hallbauer, 1986; Barton and Hallbauer, 1996; England et al., 2002b; Guy et al., 2014). The term buckshot pyrite has been widely used for rounded pyrite of granule size in the Wits Basin (Viljoen, 1967; Hallbauer, 1986), a term that should be abandoned as it relates to a time when a rifle was part of the prospecting geologist’s field kit.
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Since pyrite and gold are frequently closely associated in the reefs (Feather and Koen,
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1975), the careful evaluation of different types of detrital pyrite has the potential to provide constraints on the provenance of the ore components of the reefs. For this
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reason, many authors have developed different pyrite classification schemes from the study of various gold-bearing QPCs of the Wits Basin in order to better understand the
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nature and origin of the reefs and the environmental conditions of their formation.
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The morphology and texture of pyrite grains, the presence or absence of inclusions, and their spatial arrangement, trace element composition, and S and Fe isotope
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composition are the main variables that have been used to group pyrite into different varieties (e.g. Ramdohr, 1958; Barton and Hallbauer, 1996; England et al., 2002b;
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Hofmann et al., 2009; Guy et al., 2012, 2014; Agangi et al., 2015; Da Costa et al., 2017).
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We attempted a review of the most commonly cited detrital pyrite classification schemes, by comparing the description of the different textures, shapes, sizes, and geochemical information. This synthesis resulted in a new detrital pyrite classification scheme as proposed in this article and summarized in Table 1. Among the different detrital pyrites, two main groups of pyrite can be distinguished based on texture: massive and inclusion-bearing pyrite. The boundary between the two types is herein set at ∼ 10 vol.% of mineral inclusions. This is a value that seems to best differentiate massive from inclusion-bearing pyrite, keeping in mind that massive pyrites do sometimes contain minor amounts of inclusions. However, this value should be regarded as a broad guideline only. Both massive and inclusion-bearing types are commonly present together in many reefs, and may even form different portions of the same compound grain (e.g. Hofmann et al., 2009). Inclusions are mainly constituted of a variety of primary minerals, i.e. those that are part of the pyrite source
Journal Pre-proof and provide provenance information, and secondary ones, which formed after sedimentation (Saager, 1970; Utter, 1978; Hallbauer, 1986; Agangi et al., 2013). Thus, although not always possible, the identification of the origin of inclusions is important in evaluating their significance. Mineral inclusions are silicate phases (mica -group minerals, quartz), oxides (ilmenite, rutile and uraninite), phosphates (apatite, monazite), sulfide phases (chalcopyrite, galena, pentlandite, pyrrhotite, molybdenite and sphalerite) and fine-grained (silt-clay-sized) sediment, now represented by a finegrained mesh of phyllosilicates. Note that in some previous studies the TiO 2 phase was
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identified as rutile, for instance using Raman spectroscopy (Agangi et al., 2015),
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although in some other cases structural information is lacking, so that TiO 2 polymorphs (rutile, anatase and brookite) cannot be distinguished, and the term rutile is used in a
inclusion-bearing
pyrites; grains
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generic sense for any mineral made of TiO 2. Carbonaceous matter may be abundant in containing
particularly high proportions of
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carbonaceous matter have been referred to as sooty pyrite (e.g. Large et al., 2013).
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The unifying criterion for the identification of detrital pyrite, irrespective of it being massive or with inclusions, is evidence of rounding. Truncation of textures at the grain
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margin may provide additional evidence of abrasion and, as in the case of abraded concentrically laminated grains (Agangi et al., 2015), may provide information on the
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former size of unabraded pyrite grains. The size of detrital grains, being controlled by
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processes in the sedimentary environment, varies from reef to reef, rather than between different pyrite types. The size is related to the original size of pyrite in source rocks, distance of transport from the source, and degree of reworking. Some reefs contain exceptionally large grains of pebble-size pyrite, sometimes exceeding 10 mm in diameter, such as the Black Reef in the East Rand, while other reefs, such as the Vaal and Basal reefs, have grains from fine- to coarse-grained sand size. Coetzee (1965) reported median values of ∼ 0.15–0.2 mm for massive pyrite from various reefs of the Central Rand Group, while those from the VCR and Black Reef were observed to be on average larger (∼ 0.2-0.3 mm). Minter (1976) reported an average pyrite size of ∼ 0.23 mm from the Vaal Reef. In addition, a decrease in the grain size of pyrite was noted in several reefs along inferred palaeoslopes, with larger “nodular” pyrite becoming removed a few kilometres away from inferred basin entry points (Minter, 1978).
Journal Pre-proof The delicate textures of some inclusion-bearing pyrite types suggest relatively short transport distance and, therefore, give indications on the conditions near the site of final deposition, whereas massive pyrite may have been transported for longer distances, and thus may provide information about the hinterland. As has been shown for the Southland placers of New Zealand, pyrite can be transported for tens of kilometres in fluvial systems, even under modern overall oxic conditions (Craw et al., 2003).
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5.1. Type DM: Massive detrital pyrite
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Massive pyrite has frequently been referred to as “compact” pyrite in the literature (Table 1), possibly because of the scarcity or lack of inclusions and a more “closely
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packed” nature of pyrite sub-grains, although most massive pyrite consists of abraded single crystals (e.g. Fleet, 1998). This type of pyrite has been observed in all reefs of the
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Wits Basin. Massive pyrite varies in size from < 0.2 to 2 mm (Saager, 1970; Utter, 1978)
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and grains frequently have equant shapes (Fig. 2 A). Some massive pyrite exhibits nearly cubic outlines with abraded edges (Figs. 2 B), indicating reworking of euhedral
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pyrite crystals. Common inclusions present in pyrite type DM (0 to ∼ 10 % in abundance) are sulfides (chalcopyrite, sphalerite, galena) and silicates (quartz,
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muscovite, chlorite and biotite), with a size < 20 µm and typically ∼ 5 µm (Hallbauer,
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1986; Barton and Hallbauer, 1996; England et al., 2002b; Agangi et al., 2013). In comparison to inclusion-rich pyrite, massive pyrite typically exhibits lower concentrations in a suite of minor and trace elements (Supplementary Table 1) that includes As, Ni, Sb, Tl, Pb, Mn, Au and Ag (Agangi et al., 2013, 2015; Large et al., 2013). Internal zonation marked by variations in As, Ni and other trace metals is common and reflects crystal growth textures or post-depositional element remobilization (MacLean and Fleet, 1989; Agangi et al., 2013). Gold occurs as purported primary inclusions (due to high Ag content), as inclusions of secondary gold along cracks, and as small amounts of “invisible gold” or sub-microscopic inclusions with Au concentration in massive pyrite typically < 1 ppm (Saager, 1970, 1976; Agangi et al., 2013; Large et al., 2013).
Journal Pre-proof 5.2. Type I: Inclusion-bearing pyrite The term “porous” has widely been used in the literature for inclusion-bearing pyrite (Saager, 1970; Hallbauer and von Gehlen, 1983; England et al., 2002b), as under reflected light the inclusions are generally non-reflective and thus superficially have the appearance of voids. We however urge to refrain from using this term, as mineral inclusions,
or
their
metamorphic/pseudomorphic
precursors,
were
likely
trapped/incorporated during the growth of pyrite and, therefore, they were never pores at any time.
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Inclusion-bearing pyrite comprises a group of pyrite types characterized by mineral
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inclusions (>10 vol.%) arranged in different textures. The presence of inclusions is largely related to the nature of this pyrite type, as it consists of aggregates of small (5
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to 50 µm, England et al., 2002b), euhedral to subhedral pyrite crystals, with inclusions frequently representing the interstices of the crystals. A skeletal growth of pyrite
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crystals, starting from delicate octahedral axial crosses has been described by Saager
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(1970) and referred to as loosely-knit texture. Arrangement, density, and morphology of pyrite crystals within a single grain vary and this results in different arrangements of
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the inclusions. The inclusions may be randomly oriented or arranged in planar-parallel or concentric laminae. As such, this pyrite type is subdivided into several sub-types
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depending on the arrangement of the inclusions within a grain: random inclusion-
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bearing pyrite (type DIR), planar-laminated pyrite (type DIP), and concentrically laminated pyrite (type DIC). A fourth sub-type, characterized by randomly oriented sub-spherical inclusions, is herein referred to as microspherical pyrite (type DIM). In contrast to authigenic (or coarsely crystalline) pyrite, inclusions are not developed in crystallographic directions and zonal arrangements (e.g. Saager, 1970). While inclusion-bearing pyrite is typically found as sand-sized grains, it can occur as pebbles up to several centimetres across.
5.3. Type DIR: Detrital pyrite with randomly distributed inclusions Pyrite type DIR consists of grains < 2 mm in diameter, with some up to 5 mm (Saager, 1970; Utter, 1978). The numerous inclusions are randomly distributed in the grain, with a size range from a few µm up to tens of µm (Figs. 3 A to D). It is commonly observed in all reefs of the Wits Basin, but much less commonly in the Dominion
Journal Pre-proof Group (Saager, 1970; Hallbauer, 1986; England et al., 2002b; Rantzsch et al., 2011; Guy et al., 2014). Plastic deformation features, such as radial cracks, indentations and hard minerals pressed onto the surface of DIR-type pyrite have been reported (Utter, 1978). In situ analyses by electron microprobe and laser ablation ICP-MS (Supplementary Table 1) have revealed that concentrations of trace elements such as As, Ni, Mn, Sb, Tl, Pb and Au in pyrite type DIR from the VCR and Carbon Leader Reef are relatively high in comparison with massive pyrite from the same reefs (e.g. As ≤ 1 wt % and Ni ≤ 0.8 wt %; Agangi et al., 2013). Concentrations of Au, Sb and in some cases Te are typically
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positively correlated (Large et al., 2013). Whole-grain analyses from several reefs also
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found the highest Au concentrations (up to 1400 ppm Au) in inclusion-bearing pyrite in comparison to massive pyrite (Utter, 1978). Saager and Mihálik (1967) reported
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mosaic-like, intimately intergrown isotropic and anisotropic pyrite in DIR grains, with
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the latter type being less hard, less reflective, and enriched in As, Co and Ni.
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5.3.1 Composite pyrite
A distinct variety of inclusion-bearing detrital pyrite is referred to as composite. This
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type consists of granules to small pebbles of pyrite-cemented, heavy mineral-rich sandstone, containing detrital grains of quartz, pyrite, chromite, rutile and zircon, with
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a typical size from < 0.1 to 0.3 mm. Composite grains have been observed in the Steyn,
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Elsburg, and Black reefs (Schidlowski, 1967; England et al., 2002b; this study Figure 4). The degree of packing of the detrital grains is generally low, indicating early diagenetic cementation of sandy sediment. Some of the pyrite present in composite grains, instead of being cement, might represent detrital Fe (-Ti)-oxide grains replaced by pyrite (Ramdohr, 1958).
5.3.2 Carbonaceous pyrite “Sooty” pyrite, herein referred to as carbonaceous pyrite, is another variety of inclusion-bearing detrital pyrite. It is characterized by a fine intergrowth of pyrite, mica-group minerals (e.g. sericite), and abundant carbonaceous matter, and has a diameter range from < 0.25 to ∼2 mm. It has been described from the Carbon Leader Reef and Black Reef (Large et al., 2013; Fuchs et al., 2016; this study; Fig. 5).
Journal Pre-proof 5.3.3 Aggregate pyrite England et al. (2002b; their Figure 3A) described a DIR-type pyrite (“porous aggregated pyrite”) to represent early diagenetic nodules that formed in situ. An authigenic origin was suggested on the basis of the absence of a sharp, abraded bounding surface. However, in situ corrosion of the margin of detrital grains (England et al., 2002b) or overgrowth of authigenic pyrite may have resulted in the more diffuse grain margins. We thus regard this pyrite type (Fig. 6) to be largely detrital.
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5.4. Type DIP: Planar-laminated detrital pyrite
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Terms such as layered, banded or laminated have previously been used to describe this pyrite type (Utter, 1978; Hallbauer, 1986; England et al., 2002b). It generally has a
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tabular shape under the microscope with long axes up to 4 mm. It is constituted by laminae that differ in the abundance of inclusions (massive vs inclusion-rich) or in the
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size of pyrite crystals forming the laminae. Laminae are typically thinner than 1 mm
Pr
and mostly continuous over the scale of the grain (Utter, 1978; Hallbauer, 1986; this study; Fig. 7). This pyrite type has been described in various reefs (e.g. Steyn Reef,
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Basal Reef, middle and upper Elsburg reefs, and VCR; Hallbauer, 1986; England et al., 2002b, Guy et al., 2014). It shows a wide range of trace element concentrations within
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different reefs in the Wits Basin (Koglin et al., 2010; Supplementary Table 1).
5.5. Type DIC: Concentrically (rarely colloform) laminated detrital pyrite This type has previously been referred to as oolitic or colloform pyrite (Table 1). Concentrically laminated pyrite occurs as spherical grains or fragments thereof (Fig. 8), with a diameter of ∼ 0.6 to 1 mm, and exceptionally up to 7 mm (Hallbauer, 1986; Guy et al., 2014; Agangi et al., 2015; this study). It consists of concentric sets of µm-scale laminae alternating with thicker laminae (tens of µm) and non-laminated domains (Agangi et al., 2015; this study). Some grains have a core of radiating bladed crystals surrounded by a concentrically laminated rim (England et al., 2002b). In contrast to concentrically laminated grains, colloform pyrite shows only domains of curved laminations that may or may not be concentric, and sometimes several domains may be stacked in an asymmetric mode (e.g. Fig. 5C of England et al., 2002b).
Journal Pre-proof Inclusions of quartz, chlorite, muscovite, rutile, apatite, monazite and chalcopyrite have been reported in concentrically laminated pyrite from the VCR (Agangi et al., 2013). The occurrence of secondary native gold inclus ions in the interstices between laminae was observed by Agangi et al. (2015). In contrast, Utter (1978) described small gold inclusions with a maximum size of 5 µm homogeneously dispersed in the interstices between individual pyrite particles that form pyrite grains; these Au inclusions were interpreted as primary inclusions. The pyrite type DIC has been observed in several reefs (e.g. Basal, Vaal, Kimberley, Upper Elsburg), but it may be
f
most abundant in the VCR (Hirdes and Saager, 1983; Guy et al., 2014; Agangi et al.,
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2015; this study).
In situ analyses of DIC-type pyrite from the VCR, B-Reef and Elsburg Reef
pr
(Supplementary Table 1) indicate that this type of pyrite is enriched in a suite of elements that includes Ni, As, Au, Ag, Pb, Sb, Tl and Mn in comparison to massive
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detrital pyrite (Agangi et al., 2013; Koglin et al., 2010).
5.6. Type DIM: Microspherical detrital pyrite
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This type is characterized by µm-scale spherical to subspherical structures in detrital pyrite previously referred to as “mineralized bacteria” and described in the Basal, Vaal,
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Elsburg and Black reefs (Ramdohr, 1958; Schidlowski, 1965; Saager, 1970, 1976). They
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occur in detrital pyrite grains ∼ 0.2 mm to 6.0 mm in size. The microspheres range in diameter from ∼ 0.01 mm up to 0.045 mm and occur as isolated, or occasionally compound, spheres, characterized by a single or double rim of micro-inclusions mainly of quartz (Schidlowski, 1965; Saager, 1970) (Fig. 9). The microstructures were interpreted as pyritized bacterial colonies, or remains of microbial life forms, based on morphological observations (Schidlowski, 1965; Saager, 1970, 1976). In order to refrai n from genetic terminology, especially since a microbial origin for the microspheres remains unproven, this pyrite type is herein referred to as microspherical pyrite. No published data so far exists on the trace element and isotopic composition of this pyrite type.
Journal Pre-proof 5.7. Type DC: Coarsely crystalline detrital pyrite Coarsely crystalline pyrite consists of massive or inclusion-bearing grains, mostly from 0.2 to 1.5 mm (but up to 5 mm) in diameter. The grains consist of pyrite crystals typically larger than 0.05 mm and with a variety of textures, including dendritic, branching (leaf-like), cone-in-cone, chevron, or radiating textures (Fig. 10; Ramdohr, 1958; Barton and Hallbauer, 1996; England et al., 2002b; Guy et al., 2014). These differ from the relatively fine-grained euhedral to subhedral crystals that form many inclusion-bearing pyrite types. We use the crystal texture as a qualifier (e.g. radial
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crystalline pyrite) in the classification. Type DC pyrite occurs in the Basal, Steyn,
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Virginia, Upper Elsburg, Vaal and Black reefs (Hallbauer, 1986; England et al., 2002b) and probably in many others. Only limited compositional data of this type of pyrite are
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available in the literature. In the Black Reef, this type is characterized by enrichment in
6. Detrital pyrite provenance
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1996) (Supplementary Table 1).
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Co, Ni, As, Bi, Cu and Ag in comparison with massive pyrite (Barton and Hallbauer,
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Detrital pyrite can be derived from erosion of any pyrite-bearing lithology, as long as conditions during weathering, transport, deposition, diagenesis, and metamorphism
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will allow pyrite to be preserved in sedimentary rocks. The wide range of textures and
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compositions of detrital pyrite indicates an origin from a variety of source rocks in the basin hinterland and/or from intrabasinal reworking of early diagenetic or hydrothermal sulfides. A number of potential sources for detrital pyrite have been proposed, such as: 1) sedimentary sources, e.g. diagenetic pyrite in fine-grained sedimentary rocks (Dimroth et al., 1979; Guy et al., 2010; Hofmann et al., 2009); 2) syn-genetic sedimentary sources (reworked early diagenetic pyrite; Hallbauer, 1986, Guy et al., 2014, Agangi et al., 2015); and 3) hydrothermal sources. The latter includes exhalative and syn-genetic sulfide deposits (Hutchinson and Viljoen, 1988), hydrothermally altered granites (Klemd and Hallbauer, 1987; Robb and Meyer, 1995), vein mineralization, and/or iron formations of greenstone belts (Saager et al., 1982; Utter, 1978). However, some of the latter sources may not necessarily have had pyrite in the abundances required to have contributed to the large amounts of detrital pyrite found in many reefs. Magmatic sources for detrital pyrite are generally not considered,
Journal Pre-proof as pyrrhotite is the dominant sulfide in igneous rocks (Hall, 1986). Metamorphic rocks may equally yield pyrite upon erosion. However, pyrite does start to recrystallize at greenschist facies grade and may convert to pyrrhotite during metamorphism at lower amphibolite facies grade associated with host rock dehydration reactions (Craig and Vokes, 1993; Tomkins, 2010). Erosion of volcanogenic massive sulfide (VMS) deposits is also considered unlikely, due to the general absence of detrital base metal sulfides (e.g. Guy et al., 2014). Some studies of pyrite geochemistry focused on the Co/Ni ratio as a potential
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provenance indicator. A Co/Ni ratio lower than one has been proposed to be
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associated with a syn-sedimentary or diagenetic origin, whereas a Co/Ni ratio greater than one may be indicative of a hydrothermal source (Loftus-Hills and Solomon, 1967;
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Koglin et al., 2010). However, different studies have found no significant differences in Co/Ni between different pyrite types (Utter, 1978) or that Co/Ni of authigenic pyrite
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plots prominently across the Co/Ni = 1 line (Agangi et al., 2013), implying that this
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discriminant criterion has no broad applicability in Witwatersrand reefs (also see Meyer et al., 1990; Koglin et al., 2010). Guy et al. (2010) noted that diagenetic pyrite
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that formed in ferruginous shale of the Wits Basin had a much higher Co/Ni ratio (> 10) than pyrite in Fe-poor shales (< 1), indicating: (1) a strong control exerted by the host
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sediment in which the pyrite formed, and (2) a much larger variability of Co/Ni ratios of
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sedimentary pyrite than previously assumed. Sulfur isotope analysis of detrital Witwatersrand pyrite was undertaken by England et al. (2002b), Hofmann et al. (2009), Guy et al. (2014) and Agangi et al. (2015). A compilation of these analyses is presented in Figure 11. Sulfur isotopes can provide fingerprints of potential pyrite sources. Sulfides of magmatic origin should plot close to the origin in a δ34S vs Δ33S diagram. Sulfides present in VMS deposits that formed by seawater circulation through hydrothermal systems and seawater sulfate reduction commonly carry near to zero or relatively small negative Δ 33S values and a small range in δ34S (Farquhar and Wing, 2005; Jamieson et al., 2006; Ueno et al., 2008). In contrast, disseminated sulfides in Archaean black shales have mostly positive Δ 33S values inherited from reduction of photochemically-produced S species such as elemental S (Farquhar and Wing, 2003, 2005; Ono et al., 2009), while pyrite nodules in Archaean
Journal Pre-proof organic matter-rich shales have negative Δ33S values due to seawater sulfate reduction in sediments during diagenesis. The large heterogeneity of δ34S and Δ33S of detrital pyrite from single samples indicated contrasting formation processes and thus provenance (Hofmann et al., 2009; Guy et al., 2014; Agangi et al., 2015). Figure 11 shows that none of the samples plot along the Archaean Reference Array (ARA), which is typically observed in marine deposits (Ono et al., 2003). In contrast, limited available data on S isotopes from nonmarine deposits indicate muted Δ33S values that typically do not follow the ARA (e.g. Maynard et al., 2013). Some samples (e.g. Black Reef, irrespective of pyrite type) show
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a positive correlation in δ34S and Δ33S space, suggesting specific atmospheric photochemical fractionation processes during the time of deposition. However, the
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bulk of the data lacks a clear MIF-S signal and has slightly positive δ34S values. The reason for this different MIF-S signal is not understood, but may relate to microbial
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reworking of an atmospheric signal, mixing with non-photolytic S species, variations in
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atmospheric photochemistry during the Archaean, or a combination thereof (Guy et al., 2014). In the specific case of inclusion-rich pyrite types, which are interpreted to
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have been locally derived, the muted MIF-S may be due to inefficient separation of sulfate and elemental S pools (which carry Δ33S of opposite sign) in a non-marine
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diagenetic pyrite.
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environment, so that the two pools can combine during formation of sedimentary-
A large range of Fe isotope values has been reported from detrital Witwatersrand pyrite. Bulk pyrite analysis yielded δ56Fe values between −1.1 and 1.2 ‰ (Hofmann et al., 2009), while in situ ion probe spot analysis yielded an even larger range (−1.5 to +5.04‰.; Agangi et al., 2015). In the latter study, negative δ56Fe values were found in concentrically laminated grains and the highest positive values were found in random inclusion pyrite. While igneous rocks and siliciclastic sedimentary rocks with low carbon and sulfur contents have limited Fe isotope variations, diagenetic pyrites in Archaean black shales show highly variable and mostly negative values ranging from 3.5 to +0.5 ‰ (Rouxel et al., 2005; Yamaguchi et al., 2005; Archer and Vance, 2006; Marin-Carbonne et al., 2014). In contrast, oxidized forms of iron, such as found in Feoxides in Archaean BIFs, are frequently characterized by positive δ56Fe values up to 1.6 ‰ (Dauphas et al., 2004; Rouxel et al., 2005; Whitehouse and Fedo, 2007; Johnson et
Journal Pre-proof al., 2008). The large range of Fe isotope values reported from Witwatersrand pyrite suggests its derivation from a supracrustal source, and is entirely consistent with reworking of diagenetic sulfides, also incorporating oxidized forms of iron. Köppel and Saager (1974) compared Pb isotopes of detrital pyrite concentrates with sulfides from Archaean greenstone belt-hosted gold deposits of South Africa. A similar isotopic composition was regarded to indicate a greenstone belt provenance for the detrital pyrite. However, studies using bulk samples, unless carefully hand-picked into different pyrite types, need to be treated with care, as any concentrate of detrital
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pyrite will include different types. Mobility of Pb during deformation and
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metamorphism also needs to be considered (Fougerouse et al., 2019). Pb-Pb dating of single detrital pyrite grains from the < 2.7 Ga old Black Reef by Barton and Hallbauer
pr
(1996) indicated a derivation from ca. 3.2 to 2.8 Ga old source region. These pre-
provide information on the provenance.
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sedimentation ages confirm a detrital origin for this pyrite, although they do not
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Four pyrite particles from a whole rock sample of Vaal Reef were subjected to Re-Os analysis (Kirk et al., 2001) and defined an isochron with an age of 2.99 ± 0.11 Ga. While
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the type of pyrite was not specified by Kirk et al. (2001), a detrital rather than authigenic origin was inferred, as the age either predates or overlaps the age of
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deposition of the Central Rand Group in which the Vaal Reef is situated.
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Mathur et al. (2013) analyzed Re and Os contents and isotopic ratios of Au-rich sulfides from the Carbon Leader Reef, the VCR, and the Black Reef. Sulfides included unspecified types of mainly pyrite and likely included mixtures of detrital and authigenic pyrite. Isochron ages for individual reefs were found to be younger than Witwatersrand sediment deposition, suggesting hydrothermal remobilization of sulfides. These results may, once again, emphasize the importance of a detailed study of pyrite (and other ore mineral) types prior to chemical and isotopic analysis of compositionally complex samples. Contrasting results are to be expected between bulk and in situ analyses, potentially unnecessarily fueling the debate on the origin of the Witwatersrand gold deposit.
Journal Pre-proof 6.1. Massive detrital pyrite Massive detrital pyrite should be the most resistant to mechanical abrasion. The poor cleavage of pyrite together with a hardness of 6.5 (Tauson et al., 2015) may have allowed extensive transport and reworking. However, many massive grains have nearly cubic outlines with abraded edges, indicating relatively limited reworking of euhedral pyrite crystals. Such grains may have been derived from hydrothermal sources or from metamorphosed, and thus recrystallized, sedimentary rocks (Saager, 1970; Utter, 1978; Hallbauer and von Gehlen, 1983; Barton and Hallbauer, 1996; Guy et al., 2014).
f
They could even have been derived from early diagenetic pyrite filling porosity in
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sediments. Trace element zoning in massive pyrite (e.g. MacLean and Fleet, 1989; Agangi et al., 2013) may indicate growth from hydrothermal fluids. MacLean and Fleet
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(1989) suggested pyrite with pronounced zoning in As to have been derived from greenstone-hosted gold deposits. Rare inclusions of spessartine and corundum in
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massive pyrite have been interpreted by England et al. (2002b) to reflect derivation
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from pegmatites and VMS deposits, respectively. Massive pyrite grains from the Central Rand Group show internal plastic deformation features that indicate pyrite
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deformation prior to their incorporation into the conglomerates (Reddy and Hough, 2013), consistent with an extra-basinal source. Large et al. (2013) obtained Pb–Pb ages
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of approximately 2.95–2.75 Ga for massive detrital pyrite from the Carbon Leader
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Reef, confirming the detrital origin. A hydrothermal source indicated by trace element analysis, high Co/Ni ratios and low Au concentrations was proposed by Koglin et al. (2010). Massive detrital pyrite of the Dominion Reef and VCR has narrow δ34S (ca. −5 to +4‰) and no significant MIF signal (-0.2 < Δ33S < 0.2‰), which is compatible with a crustal as opposed to atmospheric, source of S (Hofmann et al., 2009; Agangi et al., 2015). In contrast, significant MIF signal with negative Δ33S values in the Kimberley and Black reefs is compatible with a sedimentary source (Hofmann et al., 2009; Fig. 11). However, a sedimentary source is also inferred for massive detrital pyrite from the VCR having δ34S and Δ33S close to 0‰, but negative Fe isotope values. In summary, the provenance of massive pyrite is complex and likely included multiple sources as well as very different sources for different reefs.
Journal Pre-proof 6.2. Detrital pyrite with randomly oriented inclusions Detrital pyrite rich in inclusions is common among all the reefs in the Wits Basin. The composition/texture of the inclusions as well as trace element and isotopic composition suggests a sedimentary origin. Because of the preservation of delicate textures and the occasional evidence for transport in a soft state, inclusion-rich pyrite is interpreted to have formed locally, and to record environmental conditions near the site of final deposition. The presence of quartz and phyllosilicate inclusions as well a s rare rounded oxide and phosphate grains is consistent with the growth in a fine-
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grained clastic sediment host. Carbonaceous pyrite formed in a sedimentary host rich
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in organic carbon, such as organic-rich muds or microbial mats. Composite pyrite grains represent reworked sand intraclasts that were subjected to early diagenetic
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pyrite cementation prior to erosion. While sedimentary gangue mineral inclusions are likely primary, it is rarely evident if gold and base metal sulfide inclusions are indeed
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primary or were introduced during secondary hydrothermal-metamorphic processes.
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In general, this can be evaluated based on the distribution of inclusions, which will be controlled by cracks or grain margins for secondary minerals. In addition, it has been
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shown that secondary Au inclusions along cracks have higher Au/Ag (∼ 10) in
al., 2015).
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comparison with invisible gold which has Au/Ag of ca. 0.1 (Large et al., 2013; Agangi et
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The abundance of trace elements such as Co, Ni, Mo, As, Cu, and V associated with low Au/Ag ratio indicate that DIR pyrite was sourced from carbonaceous mudstone (Utter, 1978; Large et al., 2009) or formed in a suboxic to anoxic environment with organic matter available (Agangi et al., 2013 and references therein). These trace elements might indicate environmental conditions in the provenance area that may have given rise to their mobilization due to variations in redox conditions, pH or bacterial activity. Bulk gold analysis of concentrates of different pyrite types from the Klerksdorp Goldfield indicated that samples dominated by inclusion-rich pyrites also had the highest bulk gold contents (Utter, 1978). Sulfur isotope analyses of pyrite type DIR from different reefs show a large spread in δ34S and Δ33S space (Fig. 11), indicative of incorporation of both elemental and oxidized S species and reworking of the isotopic signal by microbial sulfate reduction.
Journal Pre-proof 6.3. Planar-laminated detrital pyrite The
lamination
in
this
type
of
pyrite
is
either
derived
from
pyrite
deposition/precipitation as laminae or diagenetic pyrite growth in laminated sediments (Tucker, 1980; Hallbauer, 1986, England et al., 2002b). A sedimentary source is thus indicated, compositionally similar to finely laminated sediments consisting of admixtures of pyrite, mud, and organic matter, as for example observed by Dimroth et al. (1979) at Randfontein Mine.
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6.4 Concentrically (rarely colloform) laminated detrital pyrite
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Sulfidation of coated grains in oolitic ironstone was proposed by Ramdohr (1958) for the origin of concentrically laminated pyrite, whereas England et al. (2002b) a
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proposed sulfidation of carbonate or evaporite ooids. The latter authors inferred that the ooids were likely replaced in the source area before transport and deposition into
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the basin rather than in situ, as indicated by a core-rim with different isotope values of
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δ34S (England et al., 2002b). Colloform laminated pyrite was interpreted by England et al. (2002b) to potentially indicate derivation from VMS deposits.
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The reworking of ooid beds or VMS deposits would yield multi-grain or multicomponent clasts, but such are not observed. The non-massive texture, formerly
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plastic nature, and the low resistance to long distance transport of DIC grains suggest
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that they formed near the site of deposition, in a low energy subaqueous environment, such as tidal mud flats or floodplains, where accretion of pyrite mud and other finegrained detritus took place (Hallbauer, 1986; Agangi et al., 2015). Growth of these grains took place lamina by lamina during early diagenesis , either while the grains were moving on the surface of a muddy substrate or under shallow sediment cover, with a possibility of successive reworking followed by reburial (Agangi et al., 2015). Reworking gave rise to fragmentation of the pyrite grains. Truncation of concentric layering in diagenetic marcasite has been observed in Tertiary palaeoplacers, but only in proximity to quartz clasts (Falconer et al., 2006), a feature uncommon in Witwatersrand QPCs. The mineral inclusions found in DIC pyrite are rather similar to those found in DIR pyrite and both types are enriched in a similar suite of trace elements, suggesting a similar environment of deposition. Variations in δ34S values between laminae of single
Journal Pre-proof grains suggest changes in water sulfate concentrations likely caused by microbial sulfate reduction in fluvial channels, ephemeral ponds or interchannel flats with elevated concentrations of sulfate (Guy et al., 2014; Agangi et al., 2015). Concentrically laminated pyrite always has negative Δ33S values (Fig. 11), which indicates that S was derived from photolytic (atmospheric) sulfate (Guy et al., 2014; Agangi et al., 2015). This contrasts with positive Δ33S values found in many coexisting DIR-type pyrites (Fig. 11). Both these types are considered to be indicative of the local environment of
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deposition, but clearly derived their S from separate pools.
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6.5 Microspherical detrital pyrite
The origin of pyrite type DIM has been the subject of controversy. Ramdohr (1958)
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interpreted these grains as deriving from pyritized granular ironstone. Schidlowski (1965) and Saager (1970) interpreted the microspherical pyrite as pyritized bacterial
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colonies or, more generally, fossil remains of primitive life forms. However, a biogenic
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origin was based only on morphological features and other origins cannot be ruled out (Schidlowski, 1965). Guy et al. (2014) suggested that some of the microspheres
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represent former pyrite framboids. Despite the various interpretations, a sedimentary
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origin of microspherical pyrite is indicated.
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6.6. Coarsely crystalline detrital pyrite The radial crystal texture of some grains has been regarded to be identical to those in diagenetic marcasite nodules (Barton and Hallbauer, 1996; England et al., 2002b), although marcasite is no longer preserved, possibly due to its conversion to pyrite at elevated temperatures. Identical textures have also been found in VMS deposits (Barrie et al., 2009). However, pyrite with elongate, commonly skeletal pyrite crystals are fragile, thus they formed more or less in situ, close to the site of final deposition, after only limited transport and reworking, in line with a sedimentary rather than VMS provenance. Grains with skeletal crystals imply unidirectional growth in open-pores (England et al., 2002b), probably indicating syn-depositional pyrite growth in the conglomerates themselves,
followed by sedimentary reworking. Alternatively, they formed
displacively in sediment with high plasticity and fluid content (Sellés-Martínez, 1996).
Journal Pre-proof Pyrite nodules with very similar features have been described from the West Rand Group (Guy et al., 2010) and Phanerozoic black shale successions (Carstens, 1985).
7. Classification of authigenic pyrite The characteristic feature of authigenic pyrite is its euhedral to subhedral shape. It is most commonly massive, although inclusion-bearing authigenic pyrite does occur. Most formed in situ after QPC sedimentation, during diagenetic, hydrothermal and/or metamorphic processes. Some authigenic pyrite formed during sedimentation and can
f
thus be found as abraded detrital grains. Authigenic pyrite occurs as individual crystals
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or crystal clusters of euhedral cubic to pyritohedral shape or as an overgrowth on older pyrite generations. Massive authigenic pyrite also occurs as veins cutting through
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detrital pyrite or other minerals, a clear feature of a post-depositional origin. A review of previous work on authigenic pyrite was done by comparing the description of the
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different textures, shapes, sizes, and geochemical information (Ramdohr, 1958;
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Saager, 1970; Utter, 1978; Hallbauer, 1986; England et al., 2002b; Guy et al., 2014). A new authigenic pyrite classification scheme was derived from this synthesis (Table 2),
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whereby authigenic pyrite is classified into 4 subtypes: authigenic euhedral/subhedral (type E), authigenic overgrowth (AO), authigenic infill (AI), and authigenic aggregates
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(AA). Pseudomorphic pyrite (type AP) also falls into this group. Authigenic pyrite is
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characterized by extremely variable contents of trace element, such as As, Ni and Co, even in single grains, due to strong chemical zonation (Agangi et al., 2013). These chemical zones can be correlated between grains in the same sample, thus further confirming in situ growth. Authigenic pyrite is mostly Au-poor in comparison with inclusion-rich pyrite, but contains some Au particles, either trapped during growth or along cracks. These particles demonstrate post-sedimentary Au mobilization.
7.1. Type AE: Euhedral / subhedral authigenic pyrite Euhedral / subhedral authigenic pyrite is characterized by cubic, pentagonal, octahedral or dodecahedral crystals or crystal outlines. It occurs as single crystals or clustered into aggregates, with a wide diameter range, but typically ∼ 10 µm to 3 mm (Fig. 12) (Saager, 1970, 1976; Hallbauer and von Gehlen, 1983; England et al., 2002b, this study). Inclusions are uncommon and, if present, account for less than 10 vol. % in
Journal Pre-proof most cases. They are present as random inclusions or in crystallographically oriented zonal arrangements and comprise chalcopyrite, chlorite, quartz, sphalerite, galena, pyrrhotite, a U-Ti mineral phase and native Au (-Ag) (Saager, 1970; Agangi et al., 2013). Some of these inclusions, such as chlorite, sulfides and gold, are demonstrably secondary in origin where they occur along cracks. Many AE grains combine a core of detrital massive pyrite (DM) and authigenic overgrowths (AO), which is however not always obvious using ore microscopy alone. The presence of multiple growth zones may explain why analyses of this pyrite type yielded trace element concentrations and
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isotopic compositions that vary widely (Supplementary Table 2).
7.2. Type AO: Authigenic pyrite overgrowth
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Authigenic pyrite frequently forms overgrowths on rounded detrital grains (commonly DM and, less frequently, DIR types), thus giving rise to pyrite grains with a core-rim
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structure (Fig. 13). The proportion of AO in detrital pyrite can reach up to 30% (England
Pr
et al., 2002b). Overgrowth can consist of a single idiomorphic crystal or as idiomorphic to hypidiomorphic pyrite aggregates (Utter, 1978; Guy et al., 2014).
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Authigenic pyrite overgrowths are mostly massive, but inclusions can sometimes be abundant. In frequent cases, especially when pyrite overgrowths occur on massive
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detrital grains, the detrital core remains invisible under the optical microscope, but it
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can be made visible by etching of the thin section with nitric acid (Saager and Mihálik, 1967). Element mapping by SEM or EPMA also allows the identification of authigenic overgrowth (Frimmel et al., 2005; Agangi et al., 2013). Previous studies have identified high concentrations of As, Ni, Co, and Au in AO pyrite as compared to detrital pyrite cores (England et al., 2002b; Koglin et al., 2010; Large et al., 2013; Supplementary Table 2).
7.3. Type AI: Authigenic pyrite infill This type of pyrite fills fractures and potentially primary porosity. Pyrite veinlets (Fig. 14) pre-dating QPC deposition cross-cut detrital pyrite grains, but are truncated by their margins. Post-depositional veinlets filling fractures a few μm wide cross -cut detrital pyrite and other detrital grains (Ramdohr, 1958; Saager, 1970; Utter, 1978; Hofmann et al., 2009; Guy et al., 2012, 2014). Some post-depositional veinlets are
Journal Pre-proof contiguous with authigenic pyrite overgrowths (England et al., 2002b). Pore-filling authigenic pyrite in detrital grains has been described by England et al. (2002b). Pyrite infill is trace element poor (see Supplementary Table 2), but is associated with inclusions of sphalerite, galena, chalcopyrite and electrum, which gives anomalous concentrations of some trace elements (Agangi et al., 2013).
7.4. Type AA: Authigenic pyrite aggregates This pyrite type was earlier described as aggregate pyrite (Fig. 6) and regarded to be
f
largely detrital (subtype of DIR). Nevertheless, early diagenetic (authigenic) formation
pr
result in its detrital origin to become obscured.
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of this pyrite may be possible in some cases. Minor reworking without abrasion will
7.5. Type P: Pseudomorphic pyrite
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Pseudomorphic pyrite represents primarily non-sulfidic detrital grains that have been
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replaced by pyrite. These pyritized grains contain relict structures and compositions that allow them to be identified as pyrite pseudomorphs after individual minerals or
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rock fragments (Ramdohr, 1958; Saager, 1970; England et al., 2002b). Sand-sized pyrite with rutile laths are generally interpreted to represent pseudomorphs after ilmenite or
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titaniferous magnetite that exsolved to rutile and Fe-oxides, the latter having been
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replaced by pyrite (Ramdohr, 1958; England et al., 2002). A detrital pyrite with dendritic texture of rutile from the Vaal Reef is shown in Figure 15. Pebbles of chert, shale and BIF in many reefs show partially replacement by pyrite due to selective sulfidation (Myers et al., 1993; Phillips and Dong, 1994; Phillips and Law, 2000; Fig. 16). In summary, pyritization is rarely complete, thus allowing differentiation from truly detrital pyrite grains.
8. Origin of authigenic pyrite Authigenic pyrite is of in situ origin and formed after deposition, with different types having formed at different times under different physico-chemical conditions during diagenetic and epigenetic (hydrothermal and metamorphic) processes (Hallbauer and von Gehlen, 1983). While diagenetic processes included pyrite formation by microbial sulfate reduction (MSR) that must have operated shortly after deposition, there were
Journal Pre-proof several episodes of hydrothermal and metamorphic overprints in the history of the Wits Basin. These include regional metamorphism, thermal metamorphism during the emplacement of the Bushveld Igneous Complex, impact cratering and uplift of the Vredefort Dome, but also the intrusion of mafic sills and dykes at different times in Earth history (Frimmel et al., 2005; Phillips and Law, 2000). Although formation and recrystallization of authigenic pyrite may be related to any of these events, it is difficult to obtain direct age information on single stages of pyrite growth. Coarsely crystalline authigenic pyrite is commonly observed in localities which have been subjected to
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higher temperatures, such as near dykes (Hallbauer and von Gehlen, 1983). As such,
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early diagenetic pyrite overgrowths are regarded to be smaller than those related to growth during metamorphic/hydrothermal fluid flow (Guy et al., 2014). Large et al.
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(2013) obtained Pb–Pb data of overgrowth pyrite from the Carbon Leader Reef of ca. 2100–2020 Ma, an age range that overlaps with the tectono-thermal events of the
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Vredefort impact and Bushveld intrusion.
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Authigenic pyrite types, including reworked and thus diagenetic types, have a relatively narrow range of δ34S -0.5 ‰ to + 4 ‰ (England et al., 2002b; Guy et al., 2014) and
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mostly no MIF-S signature similar to the crustal S reservoir. Non-zero Δ33S values in bulk samples (Hofmann et al., 2009) may reflect the presence of a detrital core or
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sulfidic sediments.
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remobilization of atmospheric S, sourced locally from detrital pyrite or from other
Authigenic pyrite infills equally formed at different times depending on whether they fill primary porosity or post-depositional fracture porosity. It has been debated whether the pseudomorphic replacement of iron-bearing rocks and mineral grains by pyrite giving rise to type P occurred before, during, or after deposition (Ramdohr, 1958; Reimer and Mossman, 1990; England et al., 2002b). Given the sheer abundance of syn-sedimentary and authigenic pyrite in the Witwatersrand reefs, most pseudomorphic replacement likely took place after deposition.
9. Pyrite occurrence in other palaeoplacer deposits Beside the QPCs of the Witwatersrand Supergroup and immediately overlying stratigraphic units, a number of other pyrite-bearing QPCs exist that span in age from the Archaean to the Phanerozoic and are enriched in gold, albeit not always
Journal Pre-proof economical. Most of them have very similar pyrite varieties, and are thus amenable to the application of the new classification scheme. This similarity in pyrite morphology and composition also point to similar formation processes. In the following, a short description of gold- and/or uranium-bearing QPC palaeoplacer occurrences enriched in pyrite from different periods in geological time is given. The description of pyrite will follow the terminology proposed in this study. A schematic representation of the different pyrite types is provided in Figure 17 to aid in comparison.
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9.1. Moodies Group, Barberton Greenstone Belt, South Africa
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Detrital pyrite has been observed in conglomerates and sandstones of the Moodies Group of the Barberton Greenstone Belt (Saager et al., 1982; Nabhan et al., 2016). The
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Moodies Group consists predominantly of alluvial conglomerate and braided fluvial, tidal, and shallow-marine sandstones that were deposited ∼ 3.23 Ga ago (Heubeck et
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al., 2013). Detrital pyrite in conglomerate is massive and occurs together with a
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number of other detrital accessory minerals, including rare carbon and uraninite grains. It is poorly rounded and sorted, suggestive of short transport distances and
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poor sediment reworking (Saager et al., 1982). Nabhan et al. (2016) observed rounded grains of massive and inclusion-bearing pyrite with authigenic overgrowths. Detrital
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cores have δ34S values of ~ 0 ± 5‰, while the rims have values as low as -24.5‰, which
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has been related to biogenic fractionation of sulfur, and thus formation of the overgrowths during diagenesis (Nabhan et al., 2016). No economic concentrations of placer gold have been reported from the Moodies conglomerates. This may relate to the fact that gold mineralization in the Barberton Greenstone Belt postdates deposition of the Moodies Group by more than 100 Ma (Agangi et al., 2019).
9.2. Jacobina Group, São Francisco Craton, Brazil The Jacobina Basin is located in the east-central portion of Bahia state, in the São Francisco Craton. The Jacobina Group includes the Serra do Córrego Formation, which is characterized by interbedded coarse-grained sandstone, grit, and QPC that host AuU-pyrite mineralization (Teles et al., 2015). Metamorphic grade is greenschist facies, but sedimentary structures are locally well preserved. A braided stream to fluvio-
Journal Pre-proof deltaic depositional environment has been proposed. U–Pb ages obtained from detrital zircon of the Jacobina Group are as young as 3.2 Ga, and the abundance of detrital pyrite suggests deposition prior to the GOE (Teles et al., 2015). Pyrite occurs in the gold-bearing conglomerates together with quartz, muscovite, and other phyllosilicates, but also accessory zircon, chromite, thorite–uraninite, monazite, ilmenite, magnetite, tourmaline, and rutile (Milesi et al., 2002). Pyrite occurs as sandsized detrital and authigenic grains. Detrital grains are mainly massive, but inclusionbearing types (DIR, DIP) also occur. Authigenic pyrite includes AE crystals and AI pyrite
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veins (Milesi et al., 2002; Teles et al., 2015). Gold is spatially associated with pyrite and
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occurs as small flakes, as inclusions in pyrite, and intergrown with AE. A modified placer origin is inferred for the gold mineralization in the Jacobina Basin (Milesi et al.,
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2002; Teles et al., 2015).
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9.3. Dominion Group
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The Rhenosterspruit Formation hosts marginally economic gold and uranium mineralization in QPCs of the Lower and Upper Dominion reefs. The Lower Reef is a
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matrix-supported pebble to cobble conglomerate that has been sporadically mined for its gold content. The Upper Reef is a small-pebble conglomerate laterally more
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uniform than the Lower Reef, is enriched in a suite of heavy minerals with abundant
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garnet and has principally been mined for its uranium enrichment (Malan, 1959; Hiemstra, 1968; Rantzsch et al., 2011). Pyrite is the most abundant heavy mineral in the Dominion reefs. The most common detrital pyrite is massive with minor inclusion-bearing pyrite (Malan, 1959; Hiemstra, 1968; Rantzsch et al., 2011; this study). Relatively unfractionated values of δ 34S and Δ33S (0.0 ± 0.02‰) and the small range of δ56Fe values similar to igneous rocks for massive pyrite is consistent with high-temperature hydrothermal or crustal derivation, such as sulfide-quartz veins hosted in granitoid-greenstone terrains (Hofmann et al., 2009). This interpretation is supported by (1) the texture of detrital pyrite, consisting mainly of massive sand-sized grains, and (2) the setting of the Dominion Group resting directly on granitoid basement (Hofmann et al., 2009).
Journal Pre-proof 9.4. Pongola Supergroup, Swaziland and South Africa Detrital pyrite is locally present in conglomerates of the Pongola Supergroup, which has been correlated with the West Rand Group by Beukes and Cairncross (1991). The Pongola Supergroup is divided into the basal Nsuze Group and the upper Mozaan Group and has been subjected to low-grade regional metamorphism (Luskin et al., 2019). The Pongola Supergroup nonconformably overlies Archaean granitoidgreenstone basement with the oldest volcanic unit in the Nsuze Group dated at 2980 ± 10 Ma (Mukasa et al., 2013).
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The Nsuze Group is a volcano-sedimentary sequence of basaltic volcanic and
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volcaniclastic rocks intercalated with sandstone, shale and, locally, dolomite. The Mozaan Group consists of an alternating assemblage of sandstone, shale, banded iron-
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formation and rare conglomerates. The depositional environment was to a large extent an epicontinental shallow sea (Wilson et al., 2013; Luskin et al., 2019). Saager et
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al. (1986) described pyrite-bearing QPCs from one locality in the Nsuze Group and
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from several localities in the Mozaan Group. The heavy mineral detrital fraction of the conglomerates includes, in decreasing order of abundance, pyrite, rutile/leucoxene,
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chromite, zircon, monazite, and arsenopyrite, amongst others. Gold occurs as reconstituted grains and as inclusion in massive pyrite; erratic gold concentrations
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from 1 to 4 g/t supported a number of small-scale mining operations (Saager et al.,
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1986). Generally, the conglomerates are discontinuous, polymictic (vein quartz, quartzite and chert) and matrix-supported with sub-angular to subrounded clast, suggestive of predominantly fluvial deposition and subordinate marine reworking. The most extensively studied QPC is the Mozaan Contact Reef, situated at the base of the Mozaan Group above an angular unconformity (Hicks and Hofmann, 2012). The detrital pyrite types present in the reef are mainly massive, random inclusion-bearing and rare radial crystalline types. Authigenic euhedral pyrites are common (Saager et al., 1986; Hofmann et al., 2009). All pyrites from the Mozaan Contact Reef analysed by Hofmann et al. (2009) have small but consistently positive Δ 33S values and negative δ56Fe values in support of a sedimentary origin. Crystalline pyrite has highly negative δ34S values, suggesting microbial disproportionation of elemental sulfur.
Journal Pre-proof 9.5. Bababudan Group, Dharwar Craton, India The Bababudan Group in the province of Karnataka, South India, is a volcanosedimentary sequence deposited between 2.9 and 2.7 Ga with a basal quartz pebble conglomerate that hosts detrital pyrite, uraninite and gold overlying a granitoidgreenstone basement (Srinivasan and Ojakangas, 1986; Sunder Raju et al., 2014). Clasts are less than 10 cm in diameter, include vein quartz, metachert and quartzite, and occur in a quartz sand matrix with phyllosilicates and heavy minerals such as zircon and ilmenite (Srinivasan and Ojakangas, 1986). Pyrite occurs as rounded detrital
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grains of both massive and random inclusion-bearing types as well as euhedral
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authigenic types (Srinivasan and Ojakangas, 1986; Arora, 1991; Udayakumar and Philip, 1997). No economic concentration of Au and U has yet been discovered, and the
9.6. Huronian Supergroup, Canada Palaeoproterozoic
Huronian Supergroup in Canada is a predominantly
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The
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source of the pyrite remains to be investigated.
sedimentary succession. In its lower part, pyrite-bearing fluvial QPCs, which are locally
formations (Frimmel, 2014).
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auriferous and/or uraniferous, are present in the c. 2.4 Ga Matinenda and Miss issagi
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Pyrite present in the Matinenda Formation is mainly detrital (Roscoe, 1969) with a
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variety of massive and inclusion-bearing pyrite types with a diameter from ∼ 0.8 to 2.9 mm (Roscoe, 1969; Theis, 1979). Minor authigenic pyrite occurs as euhedral to anhedral crystals ∼ 1 mm in diameter as well as authigenic overgrowths on detrital grains (Roscoe, 1969; Theis, 1979). In the Mississagi Formation, authigenic pyrite is more abundant than massive and inclusion-bearing detrital pyrite (Mossman and Harron, 1983; Koglin et al., 2010). In their study of QPCs in the basal 30 m of the Mississagi Formation, Ulrich et al. (2011) distinguished three types of pyrite. The first type is of unequivocal detrital origin and includes grains of inclusion-bearing (types DIR, DIP, DIC) as well as (radial and dendritic) crystalline (type DC) pyrite 5 to 10 mm in diameter; this pyrite is enriched in Au, Ag, Te, Bi, Mn, Cu, Zn, Pb, Sb and Hg. The second type consists of subhedral to euhedral, massive to inclusion-bearing pyrite 0.5−2 mm in diameter of either authigenic (AE) or recrystallized detrital (type DM or DIR) origin; it is enriched in As and
Journal Pre-proof Co and depleted in Au. The third type is regarded hydrothermal in origin and forms irregular masses interstitial to clastic grains and filling veins (type AI) that cross-cut matrix and clasts; this type is variably enriched in Ni, Cu, Ag, Co and Zn and depleted in Au. All pyrite types have overlapping, generally positive δ 34S values (0.97−9.26‰), whereas Δ33S values in detrital grains tend to be negative and those in authigenic pyrites unfractionated or positive (Ulrich et al., 2011).
9.7. Waimumu and Otago areas, New Zealand
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Gold-bearing conglomerate placer deposits remarkably similar to those of the
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Witwatersrand Basin occur in the south of New Zealand in the Waimumu and Otago districts and include detrital and authigenic sulfide minerals and gold (Youngson et al.,
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2006; Falconer et al., 2006). The Waimumu QPCs occur in non-marine, fluvial and
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colluvial sedimentary units of late Oligocene to Pliocene age (Falconer et al., 2006). Sulfides in the matrix of QPC occur as rounded clasts of sand- up to cobble-sized
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aggregates of framboidal marcasite with a radial texture. Marcasite overgrowths are common on quartz clasts, while marcasite is also found cementing QPC at the cm
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scale, incorporating host quartz grains (Falconer et al., 2006). Sulfide textures thus resemble those found in inclusion-bearing and radial crystalline pyrite types from the
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Wits Basin. Authigenic euhedral massive pyrite forms single cubes, less commonly
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intergrown cubes, and rarely, pyritohedra with an average size of ∼ 2 mm (Falconer et al., 2006). The marcasite is characterized by fine compositional layering with variations in Ni, Co, and As contents and has an average Co:Ni ratio of 0.6. A wide range of δ34S values between -45 to +18‰ is consistent with a diagenetic origin for the Fe-sulfides involving microbial sulfate reduction (Falconer et al., 2006).
10. Conclusion This new compilation of detrital and authigenic pyrite types, their textures and morphologies, as well as their geochemical composition helped to clarify the source of some of the elements hosted by pyrite as well as the setting in which this mineral formed. The origin of pyrite in the Wits Basin, as well as in many other placer deposits around the world, has long been a disputed issue. There is now however overwhelming evidence that most of the detrital pyrite is sedimentary in origin, while
Journal Pre-proof much of the authigenic pyrite is syn-sedimentary to diagenetic in origin. Such proposals have been made in the past (e.g. Dimroth et al., 1979), but are now more firmly based on more abundant trace element and isotope data of pyrite. With the exception of massive pyrite, all other detrital pyrite types discussed in this study can be associated with a sedimentary origin. This largely involved synsedimentary or diagenetic growth of pyrite framboids, nodules, concretions, microspheres and cement, later subjected to erosion, reworking and deposition in QPCs forming the various inclusion-bearing pyrites, coarsely crystalline pyrites, as well
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as composite grains. Based on S isotope evidence, much of the sedimentary-diagenetic
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pyrite formation can be directly attributed to microbial sulfate reduction and elemental sulfur disproportionation. Concentrically-laminated pyrites were derived
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solely from sulfate reduction. Precipitation of pyrite in euxinic conditions could also have played a role, forming planar-laminated pyrite muds. Some massive pyrites
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frequently show S isotopic composition similar to some of the more obvious diagenetic
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pyrite types in the same reefs. Together with a fractionated Fe isotopic composition, some of it too was largely of sedimentary derivation.
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The formation of abundant diagenetic pyrite requires sulfate and reactive iron (Berner, 1984). The latter appears not to have been in short supply, assuming largely oxygen-
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deficient conditions as well as some evidence for sulfidization of Fe-Ti-oxides. Sulfate
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could have been delivered via sulfate aerosols following volcanic eruptions or localised oxidative weathering of pyrite in the hinterland. That the time of Wits Basin sedimentation is also the time of the formation of the first oxygen oasis related to enhanced photosynthetic activity is now well established (Planavsky et al., 2014; Eickmann et al., 2018; Ossa et al., 2019). Massive pyrite may have derived from many different sources, including greenstone belts hundreds of kilometres away. What is important to point out is the generally low Au content of massive pyrite, in contrast to much higher gold contents of some sedimentary pyrite. The textures of some sediment-derived pyrite grains (e.g. type DIC) indicate that they are locally-sourced, and the primary (mostly invisible) gold in these grains was taken up from the surrounding environment during their formation, likely from solution, aided by sulfate reducers (Lengke and Southam, 2007). This gold may have been derived from the dissolution of detrital gold, which was concentrated
Journal Pre-proof in the placer deposits, under acidic conditions that may have been a response to sulfide oxidation related to the onset of oxidative weathering around that time (Agangi et al., 2015; Frimmel and Hennigh, 2015). It is furthermore noteworthy that very similar pyrite textures can be found in different gold placers of very different ages around the world, indicating very similar pyrite-forming processes.
Acknowledgments This research was supported by the DST-NRF Centre of Excellence for Integrated
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Mineral and Energy Resource Analysis (CIMERA) via a Doctoral Fellowship to Giuliana
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Costa. We thank Evan Cook (Gold1), Simone Hartmann (AngloGold Ashanti), and Sunet Dercksen (Sibanye Gold) for access to drill core and arranging underground visits.
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Thanks are extended to Sebastian Fuchs, George Henry, and Bradley Guy for discussion
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and samples.
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Figure 1. Geological map of the Witwatersrand Basin and simplified stratigraphic column. BR, Black Reef; VCR, Ventersdorp Contact Reef; CLR, Carbon Leader Reef; DR, Dominion Reef; VD, Vredefort Dome. Modified from Agangi et al. (2015).
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Figure 2. Photomicrographs (reflected polarized light) of massive detrital pyrite (type DM) from the Black Reef, Moderfontein Mine. (A) Well rounded massive pyrite. (B) Massive cubic pyrite with abraded edges.
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Figure 3. Back-scattered electron images of two examples (A and C) of random inclusionbearing detrital pyrite (type DIR), Black Reef, Moderfontein Mine. (B and D) Details of inclusions of muscovite (Ms), chlorite (Chl), titanite, and quartz (Qtz).
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Figure 4. (A) Photomicrograph (reflected polarized light) of a composite pyrite grain (subtype of DIR pyrite) from the Black Reef, Moderfontein Mine. (B) Detail of detrital grains of chromite (Chr), zircon (Zr), rutile (Rt), and a grain replaced by muscovite (Ms). (C) Detail of detrital rutile with inclusions of brannerite (Brt) and muscovite (Ms).
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Figure 5. (A and B) Back-scattered electron image of carbonaceous pyrite (subtype of DIR pyrite) from the Black Reef, Modderfontein Mine. (C) Raman maps of carbonaceous matter (CM), pyrite and muscovite. Mapped area is shown in B.
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Figure 6. Back-scattered electron image of aggregate pyrite (subtype of DIR pyrite). (A) Aggregate pyrite grain with chlorite (Chl) inclusions and authigenic pyrite overgrowth obscuring the inferred detrital nature of the grain; Carbon Leader Reef, Tau Tona Mine. (B) Aggregate pyrite grain from the VCR, TauTona Mine. (C) Detail of (B) showing euhedral, micron-sized grains of pyrite; the pyrite grains are embedded in quartz (Qtz) and muscovite (Ms).
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Figure 7. (A) Photomicrograph (reflected polarized light) of planar laminated pyrite (type DIP), Upper Elsburg Reef, Cooke Mine. (B) Back-scattered electron image of a portion of the grain shown in A. Inclusions consist of chalcopyrite (Ccp) and galena (Gn). Quartz (Qtz) fills a microcrack.
Figure 8. Photomicrograph (reflected polarized light) of concentrically laminated pyrite (type DIC) from the Ventersdorp Contact Reef, Tau Tona Mine. (A) Typical concentrically laminated pyrite. (B) Fragment of concentrically laminated pyrite.
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Figure 9. Back-scattered electron images of microspherical pyrite (type DIM), Basal Reef, Free State Goldfield. (A) Detrital grain of microspherical pyrite with some authigenic pyrite overgrowth. (B) Detail of microspherical structures with spherical domain of quartz (Qtz) inclusions. An inclusion of rutile (Rt) is also present.
Figure 10. Images of diverse types of coarsely crystalline detrital pyrite (type DC). (A) Photomicrograph (reflected polarized light) of radial crystalline pyrite. (B) Back-scattered electron image of pyrite with chevron texture. (C) Back-scattered electron image of crystalline pyrite showing growth zoning as defined by variations in the abundance of inclusions. Secondary rutile (Rt) occupies fractures. (D) Photomicrograph (reflected polarized light) of crystalline pyrite showing growth zoning. (A) is from the Mozaan Contact Reef, while the other samples are from the Black Reef, Modderfontein Mine.
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Figure 11. Sulfur isotope chart of detrital pyrite from various reefs of the Witwatersrand basin. Compiled from Hofmann et al. (2009), Guy et al. (2014) and Agangi et al. ( 2015). Pyrite types: massive (M), coarsely crystalline (C), random inclusion-bearing (IR), planar-laminated (IP), concentrically laminated (IC).
Figure 12. Photomicrographs (reflected polarized light) of authigenic euhedral/subhedral pyrite (type AE). (A) Single euhedral pyrite grains from the Carbon Leader Reef. (B) Cluster of euhedral pyrite grains from the Basal Reef, Free State Goldfields. The centre of the cluster appears to enclose a detrital grain of sparsely inclusion-bearing massive pyrite.
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Figure 13. Photomicrographs (reflected polarized light) of authigenic overgrowth pyrite (type AO). (A) Pyrite overgrowing a cluster of several massive detrital pyrite grains, Carbon Leader Reef. (B) Pyrite overgrowth on an inclusion-bearing pyrite, Basal Reef, Free State Goldfield.
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Figure 14. Photomicrographs (reflected polarized light) of authigenic infill pyrite (indicated by arrows). (A) Pyrite filling fracture in detrital quartz grain, Vaal Reef, Kopanang Mine. (B) Veinlets of massive pyrite in a detrital inclusion-bearing pyrite grain. The veins are truncated at the grain margin and therefore formed prior to pyrite reworking, Black Reef, Modderfontein Mine.
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Figure 15. Back-scattered electron image and element maps of pyrite pseudomorph after ilmenite or titaniferous magnetite from the Vaal Reef, Kopanang Mine.
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Figure 16. Pebble of banded iron formation affected by pyritization along the margin and in discrete laminae. Photo of drill core from the Kimberley Formation. Field of view is 3 cm.
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Figure 17. Schematic representations of detrital and authigenic pyrite types (inspired by Guy et al., 2014). Detrital pyrite is identified by evidence for abrasion (i.e. rounding). It is subdivided into massive, inclusion-bearing, and coarsely crystalline types. Authigenic pyrite includes euhedral, overgrowth, infill, aggregate, and pseudomorphic types. It may have formed as a result of early diagenesis, burial diaganesis, and/or epigenetic processes.
Journal Pre-proof Declaration of interests The authors declare that they have no known competing financial interests or personal Ramdohr (1958)
Saager (1970)
Utter (1978)
Hallbauer (1986)
England et al., (2002b)
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al
Pr
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pr
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This study
relationships that could have appeared to influence the work reported in this paper.
Table 1. Detrital pyrite classification
Guy et al., (2014 )
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Skeletal and radial concretions
Porous rounded pyrite; concretionar y pyrite
Allogenic detrital pyrite
Compact pyrite
DET1
Synsedimenta ry pyrite (mudball pyrite)
Porous pyrite (rounded aggregated pyrite; concretionar y pyrite)
SYN2, DIA2, DIA-5
Synsedimenta ry pyrite (rounded, layered pyrite)
Porous pyrite (banded pyrite)
DIA-5
Collofor m pyrite
Synsedimenta ry pyrite (oolitic pyrite)
Porous pyrite (ooliticcolloform pyrite)
SYN1, DIA-5
-
Synsedimenta ry pyrite (framboidal pyrite)
-
DIA1, DIA12
Radial, concretionary pyrite
Porous pyrite (concretiona ry pyrite; dendritic pyrite; miscellaneo us forms)
DIA5, DIA-6
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Concentric concretions
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Type DIC: Concentricall y laminated
-
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Type DIP: Planarlaminated
Type DIM: Microspheric al
Type DC: Coarsely crystalline detrital pyrite (radial, dendritic, chevron; massive or with inclusions)
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pr
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Compact rounded pyrite
Pyrite pseudomorp hs
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Type DI: inclusion-bearing detrital pyrite (∼ > 10% inclusions)
Type DIR: Random inclusions (various subtypes)
Pebble pyrite
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Type DM: Massive detrital pyrite (∼ < 10% inclusions)
Type 2: Allogeni c rounded compact pyrite (al. c. p) Type 3: Allogeni c rounded porous pyrites (al. p. p) – well rounded variety; inclusio ns in pyrite type 3 Type 3: Allogeni c rounded porous pyrites (al. p. p) – banded variety
Mineralized bacteria
Concretiona ry pyrite; skeletal concretions; compact, radial concretions
Pyrite nodules containing microspheric al structures (“mineralize d bacteria”)
Concretionar y pyrite
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Type AO: Authigenic overgrowth pyrite
Compact pyrite encrustatio ns; porous or skeletal pyrite growths
Compact and porous pyrite encrustation s
Fissurecoatings
Xenomorphi c pyrite (veinlets and fracture fillings)
Type AA: Authigenic aggregate pyrite
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Pyrite pseudomor phs
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Rounded pseudomorp hic pyrite
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Type AP: Pseudomorphic pyrite
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Type 1: Authigenic idiomorphic to hypidiomorp hic pyrite (au. i. p)
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Type AI: Authigenic infill pyrite
Table 2. Authigenic pyrite classification
Guy et al. (201 4)
Authigenic pyrite (euhedral to subhedral pyrite)
DIA2; DIA6; EPI-1
Authigenic pyrite (pyrite overgrowths )
DIA11; EPI-2
Authigenic pyrite (porefill pyrite; vein pyrite)
EPI-3
-
Porous pyrite (in situ aggregated pyrite)
-
Synsedimenta ry pyrite (coarsegrained aggregates of quartz and pyrite)
Authigenic pyrite (pyrite pseudomorp hs)
DIA12; EPI-4
Authigenic postsediment ary pyrite
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Idiomorphic pyrite
Compact and porous idiomorphic to hypidiomorp hic pyrite
England et al. (2002b)
Hallbauer (1986)
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Type AE: Authigenic euhedral/subhe dral pyrite
Utter (1978)
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Saager (1970)
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Ramdohr (1958)
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This study
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G. Costa, A. Hofmann, A. Agangi PII:
S0012-8252(19)30505-7
DOI:
https://doi.org/10.1016/j.earscirev.2019.103064
Reference:
EARTH 103064
To appear in:
Earth-Science Reviews
Received date:
26 July 2019
Revised date:
1 December 2019
Accepted date:
10 December 2019
Please cite this article as: G. Costa, A. Hofmann and A. Agangi, A revised classification scheme of pyrite in the Witwatersrand Basin and application to placer gold deposits, Earth-Science Reviews(2019), https://doi.org/10.1016/j.earscirev.2019.103064
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A revised classification scheme of pyrite in the Witwatersrand Basin and application to placer gold deposits G. Costa1, A. Hofmann 1,* [email protected], A. Agangi1,2 1 Department of Geology, University of Johannesburg, Auckland Park 2006, South Africa 2 Department of Earth Resource Science, Akita University, Akita 010-8502, Japan * Corresponding author. Abstract Pyrite is the most abundant ore mineral in gold-bearing quartz pebble conglomerates (QPCs) of the Witwatersrand Basin and similar Archaean sedimentary units. Much of the pyrite in Archaean conglomerates is detrital in origin, implying that it survived
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weathering, transport, and reworking under anoxic conditions. Detrital pyrite is
post-depositional
processes. As
pyrite
is
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generally found together with authigenic pyrite formed in situ as a result of syn- to frequently associated with gold
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mineralization, many authors have developed different pyrite classification schemes in
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order to better understand the nature and origin of the mineralization, although the different classifications cannot always be easily compared as they largely based on
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inferred pyrite genesis. We propose a revised, entirely descriptive classification scheme for detrital and authigenic pyrite in Archaean QPCs. The scheme was created
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in order to provide a consistent framework for the petrographic description of pyrite in
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clastic sedimentary rocks. Detrital pyrite is subdivided into massive, inclusion-bearing, and coarsely crystalline types. Authigenic pyrite includes euhedral, overgrowth, infill,
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aggregate, and pseudomorphic types. We evaluate the methods used in pyrite classification, propose formation pathways for detrital and authigenic pyrites, and apply the classification scheme to some well-known occurrences of detrital pyritebearing conglomerates.
Keywords: Archaean, detrital pyrite, gold mineralization, Witwatersrand Basin
1. Introduction Pyrite (cubic FeS2) is known to undergo oxidation when exposed to the modern atmosphere and hydrosphere (Bierens de Haan, 1991). Its preservation as detrital grains in sedimentary rocks older than c. 2.4 Ga implies an atmospheric oxygen level < 0.01 atm at that time and is one of the main arguments in favour of an oxygen-
Journal Pre-proof deficient atmosphere in the Archaean (Grandstaff, 1980; Holland, 1984; Krupp et al., 1994; Johnson et al., 2014). Only rarely is detrital pyrite found in sedimentary successions younger than that time, which has become known as the Great Oxidation Event (GOE) (Holland, 1984). Preservation of detrital pyrite in oxic environments requires specific conditions, such as rapid burial in water-saturated, reduced sediments, as for example recorded by Cretaceous to Recent sedimentary strata in southern New Zealand (Youngson et al., 2006). Archaean quartz pebble conglomerates (QPCs) with abundant detrital pyrite are
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commonly of economic interest. The Witwatersrand (or Wits) Basin in South Africa
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(Fig. 1) is the best example of a sedimentary succession known for its gold and uranium mineralization associated with detrital pyrite. The Mesoarchaean sedimentary rocks of
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the Witwatersrand Supergroup host the richest and the single most important gold province in the world, with ∼ 53 kt of gold mined in total (Frimmel, 2019). The term
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Witwatersrand Basin is used here in a broad sense to encompass all Archaean volcano-
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sedimentary units that accumulated in the central part of the Kaapvaal Craton between 3.1 and 2.6 Ga ago. These include the Dominion Group and the
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Witwatersrand, Ventersdorp and basal Transvaal supergroups. Different types of both detrital and authigenic pyrite occur in QPCs and have been
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described and investigated from many different angles. As different terms are used by
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different authors and as existing schemes mix descriptive and genetic terminology, a revision of existing pyrite classification schemes is thus overdue. In addition, much recent work on Archaean pyrite has shed new light on its elemental and isotopic composition and origin. In view of these previous results, presented below is a new classification scheme that synthesizes and homogenizes older classifications using examples of the Wits Basin. The classification is strictly descriptive and widely applicable to Archaean QPCs.
2. The Witwatersrand Basin 2.1. Pre-Witwatersrand basement and the Dominion Group The Archaean Kaapvaal Craton in southern Africa developed over a protracted time span between 3.7 and 2.7 Ga (Eglington and Armstrong, 2004). The central-eastern part of the craton (Fig. 1) stabilized c. 3.1 Ga ago, after which it acted as basement for
Journal Pre-proof the development of intracontinental basins. A period of extension at 3.07 Ga gave rise to deposition of the volcano-sedimentary Dominion Group, which lies unconformably on granitoid-greenstone basement and comprises a predominantly volcanic bimodal sequence up to three kilometres in thickness (Marsh, 2006). A basal siliciclastic unit (Rhenosterspruit Formation), including QPCs known as the Lower and Upper Dominion Reefs, hosts marginally economic gold and uranium mineralization. The Rhenosterhoek Formation contains in excess of 1100 m of mafic to intermediate, amygdaloidal lava with minor felsic volcanic rocks. The uppermost unit, the Syferfontein Formation,
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conformably overlies the Rhenosterhoek Formation and is dominated by feldspar-
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quartz-phyric felsic volcanic rocks, basaltic to andesitic lava, tuff and breccia locally up to 3000 m thick (Jackson, 1992). A sample of the Syferfontein Formation has been
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2.2. The Witwatersrand Supergroup
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dated at 3074 ± 6 Ma (ion probe U–Pb zircon age; Armstrong et al., 1991).
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The Witwatersrand Supergroup is the classic example of a palaeoplacer gold deposit hosted in QPCs and associated sandstones. It is a predominantly sedimentary
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sequence subdivided into the West Rand and Central Rand groups (Fig. 1). The West Rand Group is a shallow-marine succession of sandstone and shale overlying the
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Palaeoarchaean granitoid-greenstone basement and the Dominion Group (Guy et al.,
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2010), with the youngest detrital zircon ages of 2985 ± 14 and 2902 ± 13 Ma obtained from near the base and top, respectively (Kositcin and Krapez, 2004). The Central Rand Group consists predominantly of fluvial to marginal marine sandstones and conglomerates. A minimum age of deposition for the Central Rand Group was provided by xenotime dated at ∼ 2780 Ma and interpreted to be diagenetic in origin (Kositcin et al., 2003). Gold and uranium-bearing QPCs of the Wits Basin, also known as reefs, host abundant detrital and authigenic (i.e. formed in situ) pyrite, specifically in the Central Rand Group. Besides detrital pyrite, the reefs are enriched in a large array of detrital heavy minerals, such as apatite, arsenopyrite, cassiterite, chromite, cobaltite, columbite, corundum, diamond, garnet, molybdenite, monazite, uraninite, zircon and many others as cited by Feather and Koen (1975), Phillips and Law (2000), Law and Phillips (2005) and Agangi et al. (2013), indicating prolonged times of sedimentary reworking
Journal Pre-proof and mineral concentrations under oxygen-deficient conditions. Following deposition, hydrothermal and metamorphic fluids modified, during several events, the primary mineralogy of the reefs, giving rise to remobilization of some of the primary ore constituents (Law and Phillips, 2005).
2.3. Ventersdorp Supergroup The volcano-sedimentary Ventersdorp Supergroup overlies the Witwatersrand Supergroup (Fig. 1). Its basal unit is marked by the richly gold-endowed Ventersdorp
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Contact Reef (VCR). The VCR consists of conglomerates interbedded with sandstones
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with a maximum thickness of 7 m and overlies the Central Rand Group unconformably (Krapez, 1985; Agangi et al., 2015). The VCR is conformably overlain by a succession of
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flood basalts with local intercalations of felsic volcanic and sedimentary rocks. U–Pb zircon ages indicate a first phase of continental flood basalt eruption as early as 2.78
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Ga (Cornell et al., 2017; Humbert et al., 2019), which overlaps with the youngest
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detrital zircon age of 2780 ± 5 Ma for the VCR (Robb and Meyer, 1995). In spite of belonging to a stratigraphic unit very different from the Witwatersrand
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Supergroup, the VCR displays a similar style of mineralization, with both detrital and authigenic pyrite being a common mineral (Utter, 1978; Smits, 1994; Agangi et al.,
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2013). Gold mineralization in the VCR is mainly found in areas where it unconformably
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overlies reefs of the Central Rand Group (McCarthy, 1994), this being the reason to consider the VCR as part of the Wits Basin at least in terms of its mineral endowment. The same reasoning applies to the base of the Transvaal Supergroup, characterized by the Black Reef.
2.4. Transvaal Supergroup The Transvaal Supergroup (Fig. 1) is characterized by the basal siliciclastic Black Reef Formation, chemical sedimentary rocks (carbonate, chert, banded iron formation (BIF)) of the Chuniespoort Group, and predominantly siliciclastic rocks of the Pretoria Group (Els et al., 1995; Eriksson et al., 2006). A SHRIMP U–Pb zircon age of 2588 ± 7 Ma was obtained from a tuff from the basal unit of the Chuniespoort Group (Martin et al., 1998). In the Wits Basin, the Black Reef Formation lies unconformably on the Ventersdorp or Witwatersrand Supergroup (Barton and Hallbauer, 1996; Fuchs et al.,
Journal Pre-proof 2016). It consists of a fining-upward sequence with basal sandstone and pyrite-rich QPC, known as the Black Reef, overlain by carbonaceous shale (Fuchs et al., 2016). The Black Reef resembles other Witwatersrand QPCs in terms of economic concentration of gold, mineralogy, and its depositional environment (Barton and Hallbauer, 1996; Fuchs et al., 2016). A supposed source of gold in the Black Reef resides in the erosion of underlying Witwatersrand Reefs as suggested by Papenfus (1964) and Pouroulis and Austin (1989). However, the genetic relationship between the two is still unclear (Fuchs et al., 2016). Despite erratic Au distribution throughout
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the Black Reef, it has been mined in parts of the Wits Basin around Johannesburg
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(Henry and Master, 2008).
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2.5. Gold mineralization in the Witwatersrand basin
Different hypotheses have been proposed for the genesis of the gold mineralization of
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the Wits Basin, which has been a controversy for many decades. The placer model
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proposes that gold and related heavy minerals (e.g. pyrite, uraninite) were introduced into the basin as clastic particles by sedimentary processes; the hydrothermal or
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epigenetic model proposes that gold precipitated from hydrothermal aqueous solutions well after deposition; and the modified placer model proposes that gold and
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related minerals were of detrital origin, but were variably remobilized after burial,
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causing changes in the composition and textural position within the ore (Pretorius, 1976; Phillips and Law, 2000; Law and Phillips, 2005; Frimmel, 2005). The latter model has been the most widely accepted in recent years. A fourth model, the syngenetic model, argues for gold to have precipitated in the reefs from solution, potentially aided by microbial mediation (e.g. Reimer, 1984; Mossman et al., 2008; Horscroft et al., 2011). This model has gained some momentum in recent years (Agangi et al., 2015; Frimmel and Hennigh, 2015; Heinrich, 2015). For example, Heinrich (2015) proposed dissolution of gold in the hinterland by acid rain, transport as sulfur complexes in runoff water, followed by chemical reduction of the dissolved gold onto organic material in shallow lakes. An additional aspect of the gold ore involves the presence of carbonaceous matter in the reefs, as bitumen seams and nodules. Although a matter of debate for decades,
Journal Pre-proof bitumen was largely derived from oil that was generated at different times in the evolution of the Wits Basin (England et al., 2002a).
3. A new pyrite classification scheme The new classification scheme is multi-layered and allows classification of pyrite grains in different textural classes of increasing detail. The two main classes of pyrite occurring in Wits Basin reefs compose the first layer: 1) detrital (or allogenic) pyrite (Table 1), and 2) authigenic pyrite that formed in situ after deposition (Table 2).
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Some authors also differentiated between syn-genetic (or syn-sedimentary),
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diagenetic, and epigenetic pyrite (e.g. Hallbauer, 1986; Guy et al., 2014). The syngenetic type refers to reworked intrabasinal sedimentary material and is thus detrital
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in nature, even though the degree of reworking and transport may have been small. Diagenetic pyrite is regarded to have formed in situ during early diagenesis of QPCs,
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whereas epigenetic is a term that refers to pyrite that formed after consolidation of
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the sediment (Maynard, 1983). As the timing of in situ pyrite growth can frequently not unambiguously be determined, diagenetic and epigenetic pyrites are herein
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included in the authigenic type.
Detrital pyrite can generally be differentiated from authigenic pyrite by evidence of
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abrasion. While this evidence may be found in the truncation of internal structures
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(e.g. zonation, growth banding, micro-veining) present in rounded grains in pyrite (e.g., MacLean and Fleet, 1989; England et al., 2002b), the more direct approach is to simply regard rounding as evidence of abrasion (Da Costa et al., 2017). Rounded pyrite grains representing sulfidized detrital rock or mineral fragments (herein referred to as authigenic pseudomorphic pyrite) do occur (Hirdes and Saager, 1983; Myers et al., 1993), although sulfidization is frequently not complete, thus allowing distinction between truly detrital and pseudomorphic pyrite. In addition, detrital pyrite may have overgrowths of authigenic pyrite, which may only be revealed by etching or microbeam analytical techniques (Fleet, 1998; Agangi et al., 2013; Da Costa et al., 2017).
4. Methods for the study of pyrite in Archaean QPCs
Journal Pre-proof The main variables to differentiate detrital from authigenic pyrite and to evaluate the provenance of detrital pyrite include the morphology and texture of pyrite grains, the nature of primary mineral inclusions, the trace element composition, and the isotopic composition (Da Costa et al., 2017). Pyrite morphology and texture has usually been investigated in situ by reflected light microscopy of polished thin sections and block mounts with or without oil immersion (Ramdohr, 1958; Saager, 1970, 1976; England et al., 2002b; Guy et al., 2014). Hydrofluoric acid liberation (Neuerburg, 1975) has been widely used to release pyrite grains from the rock matrix in order to investigate their
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morphology and their single grain chemical and isotopic composition (Utter, 1978;
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Hallbauer and Gehlen, 1983; Barton and Hallbauer, 1996; Hofmann et al., 2009). It is the rounded morphology and truncation of internal texture that most clearly
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differentiates detrital from authigenic grains. In addition, pit marks and scours on liberated rounded pyrite grains also support their detrital origin (Hallbauer, 1986;
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Utter, 1978).
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Optical microscopy has been complemented by scanning electron microscopy, either by imaging the surface features of liberated grains or investigating intergrowths with
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other minerals (Utter, 1978; Hallbauer and von Gehlen, 1983; Hallbauer, 1986; England et al., 2002b; Guy et al., 2012; Agangi et al., 2015). More recently, the distribution of
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major and trace elements was determined in situ using X-ray compositional mapping
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with electron probe micro-analyzer (EPMA), proton-induced X-ray emission (PIXE), or element mapping using laser-ablation inductively coupled plasma mass-spectrometry (LA-ICPMS; Large et al., 2013; Agangi et al., 2013; Fuchs et al., 2016). These techniques allow for the evaluation of chemical growth zoning, recording the evolution of mineralizing fluids, but also secondary events of brecciation, veining, resorption, and recrystallization. The combination of orientation contrast imaging and electron backscatter diffraction (EBSD) further allow to establish the deformation history of pyrite grains (Reddy and Hough, 2013). Trace elements in pyrite include a large range of siderophile and chalcophile elements, the concentrations and ratios of which can provide information on the setting of pyrite formation (Koglin et al., 2010; Large et al., 2013; Agangi et al., 2013; Guy et al., 2014). Trace element analysis of single pyrite grains liberated from the rock matrix can be problematic for provenance analysis, because of the common occurrence of inclusions
Journal Pre-proof and evidence of remobilization of elements susceptible to hydrothermal alteration, such as Au, Ni and As (Meyer et al., 1990; England et al., 2002b; Hofmann et al., 2009). Multiple S and Fe isotope analysis of detrital pyrite grains provide information on their provenance, specifically if pyrite was derived from a magmatic or sedimentary source (Hofmann et al., 2009). Sulfur isotopes can fractionate by mass-dependent, both biotic and abiotic processes. Thermochemical sulfate reduction coupled to the oxidation of organic matter or ferruginous iron-bearing minerals in hydrothermal systems can result in large fractionation of δ34S between sulfate and sulfide (Ohmoto and
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Goldhaber, 1997). At lower temperatures during early diagenesis, microorganisms can
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impart fractionations during their sulfur metabolism, particularly during the process of sulfate reduction (Canfield et al., 2000). Mass-independent fractionation (MIF) of S
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isotopes is caused by SO 2 photolysis under ultraviolet radiation in the anoxic Archaean atmosphere (Farquhar et al., 2001). Multiple S isotope analysis thus allows to
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distinguish between S that passed through the Archaean atmosphere (Δ 33S ≠ 0 ‰) and
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magmatic S incorporated into hydrothermal fluids (Δ 33S = 0 ‰). The degree of MIF-S was muted during parts of the Mesoarchaean (Farquhar et al.,
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2007), and mixing between S-pools with opposite Δ33S values can cancel out MIF signals, thus making multiple S isotope analysis not always entirely reliable to
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differentiate magmatic from sedimentary sources in the Wits Basin. Additional
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constraints can thus be obtained from the analysis of Fe isotopes. Fe isotope fractionation occurs during redox changes, fluid-mineral interactions, and biological processes (see Dauphas and Rouxel, 2006, Anbar and Rouxel, 2007, and Johnson et al., 2008 for reviews), and can frequently be observed in sedimentary pyrite (Hofmann et al., 2009). Lead isotope analysis following dissolution of liberated pyrite or in situ can provide constraints on pyrite formation age, based on a model Pb growth curve, and on the Pb source, which may provide information of pyrite provenance or post-depositional processes (Barton and Hallbauer, 1996; Poujol, et al., 1999; Meffre et al., 2008). Similarly, the Re-Os isotope system can be used as a geochronometer and as a process tracer for gold and sulfide minerals (Kirk et al., 2001; Schaefer et al., 2010; Mathur et al., 2013).
Journal Pre-proof 5. Detrital pyrite A large literature base exists on the description of detrital, i.e. rounded, pyrite and many different types have been observed (Liebenberg, 1955; Ramdohr, 1958; Saager, 1970; Feather and Koen, 1975; Utter, 1978; Hallbauer, 1986; Barton and Hallbauer, 1996; England et al., 2002b; Guy et al., 2014). The term buckshot pyrite has been widely used for rounded pyrite of granule size in the Wits Basin (Viljoen, 1967; Hallbauer, 1986), a term that should be abandoned as it relates to a time when a rifle was part of the prospecting geologist’s field kit.
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Since pyrite and gold are frequently closely associated in the reefs (Feather and Koen,
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1975), the careful evaluation of different types of detrital pyrite has the potential to provide constraints on the provenance of the ore components of the reefs. For this
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reason, many authors have developed different pyrite classification schemes from the study of various gold-bearing QPCs of the Wits Basin in order to better understand the
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nature and origin of the reefs and the environmental conditions of their formation.
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The morphology and texture of pyrite grains, the presence or absence of inclusions, and their spatial arrangement, trace element composition, and S and Fe isotope
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composition are the main variables that have been used to group pyrite into different varieties (e.g. Ramdohr, 1958; Barton and Hallbauer, 1996; England et al., 2002b;
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Hofmann et al., 2009; Guy et al., 2012, 2014; Agangi et al., 2015; Da Costa et al., 2017).
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We attempted a review of the most commonly cited detrital pyrite classification schemes, by comparing the description of the different textures, shapes, sizes, and geochemical information. This synthesis resulted in a new detrital pyrite classification scheme as proposed in this article and summarized in Table 1. Among the different detrital pyrites, two main groups of pyrite can be distinguished based on texture: massive and inclusion-bearing pyrite. The boundary between the two types is herein set at ∼ 10 vol.% of mineral inclusions. This is a value that seems to best differentiate massive from inclusion-bearing pyrite, keeping in mind that massive pyrites do sometimes contain minor amounts of inclusions. However, this value should be regarded as a broad guideline only. Both massive and inclusion-bearing types are commonly present together in many reefs, and may even form different portions of the same compound grain (e.g. Hofmann et al., 2009). Inclusions are mainly constituted of a variety of primary minerals, i.e. those that are part of the pyrite source
Journal Pre-proof and provide provenance information, and secondary ones, which formed after sedimentation (Saager, 1970; Utter, 1978; Hallbauer, 1986; Agangi et al., 2013). Thus, although not always possible, the identification of the origin of inclusions is important in evaluating their significance. Mineral inclusions are silicate phases (mica -group minerals, quartz), oxides (ilmenite, rutile and uraninite), phosphates (apatite, monazite), sulfide phases (chalcopyrite, galena, pentlandite, pyrrhotite, molybdenite and sphalerite) and fine-grained (silt-clay-sized) sediment, now represented by a finegrained mesh of phyllosilicates. Note that in some previous studies the TiO 2 phase was
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identified as rutile, for instance using Raman spectroscopy (Agangi et al., 2015),
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although in some other cases structural information is lacking, so that TiO 2 polymorphs (rutile, anatase and brookite) cannot be distinguished, and the term rutile is used in a
inclusion-bearing
pyrites; grains
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generic sense for any mineral made of TiO 2. Carbonaceous matter may be abundant in containing
particularly high proportions of
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carbonaceous matter have been referred to as sooty pyrite (e.g. Large et al., 2013).
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The unifying criterion for the identification of detrital pyrite, irrespective of it being massive or with inclusions, is evidence of rounding. Truncation of textures at the grain
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margin may provide additional evidence of abrasion and, as in the case of abraded concentrically laminated grains (Agangi et al., 2015), may provide information on the
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former size of unabraded pyrite grains. The size of detrital grains, being controlled by
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processes in the sedimentary environment, varies from reef to reef, rather than between different pyrite types. The size is related to the original size of pyrite in source rocks, distance of transport from the source, and degree of reworking. Some reefs contain exceptionally large grains of pebble-size pyrite, sometimes exceeding 10 mm in diameter, such as the Black Reef in the East Rand, while other reefs, such as the Vaal and Basal reefs, have grains from fine- to coarse-grained sand size. Coetzee (1965) reported median values of ∼ 0.15–0.2 mm for massive pyrite from various reefs of the Central Rand Group, while those from the VCR and Black Reef were observed to be on average larger (∼ 0.2-0.3 mm). Minter (1976) reported an average pyrite size of ∼ 0.23 mm from the Vaal Reef. In addition, a decrease in the grain size of pyrite was noted in several reefs along inferred palaeoslopes, with larger “nodular” pyrite becoming removed a few kilometres away from inferred basin entry points (Minter, 1978).
Journal Pre-proof The delicate textures of some inclusion-bearing pyrite types suggest relatively short transport distance and, therefore, give indications on the conditions near the site of final deposition, whereas massive pyrite may have been transported for longer distances, and thus may provide information about the hinterland. As has been shown for the Southland placers of New Zealand, pyrite can be transported for tens of kilometres in fluvial systems, even under modern overall oxic conditions (Craw et al., 2003).
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5.1. Type DM: Massive detrital pyrite
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Massive pyrite has frequently been referred to as “compact” pyrite in the literature (Table 1), possibly because of the scarcity or lack of inclusions and a more “closely
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packed” nature of pyrite sub-grains, although most massive pyrite consists of abraded single crystals (e.g. Fleet, 1998). This type of pyrite has been observed in all reefs of the
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Wits Basin. Massive pyrite varies in size from < 0.2 to 2 mm (Saager, 1970; Utter, 1978)
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and grains frequently have equant shapes (Fig. 2 A). Some massive pyrite exhibits nearly cubic outlines with abraded edges (Figs. 2 B), indicating reworking of euhedral
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pyrite crystals. Common inclusions present in pyrite type DM (0 to ∼ 10 % in abundance) are sulfides (chalcopyrite, sphalerite, galena) and silicates (quartz,
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muscovite, chlorite and biotite), with a size < 20 µm and typically ∼ 5 µm (Hallbauer,
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1986; Barton and Hallbauer, 1996; England et al., 2002b; Agangi et al., 2013). In comparison to inclusion-rich pyrite, massive pyrite typically exhibits lower concentrations in a suite of minor and trace elements (Supplementary Table 1) that includes As, Ni, Sb, Tl, Pb, Mn, Au and Ag (Agangi et al., 2013, 2015; Large et al., 2013). Internal zonation marked by variations in As, Ni and other trace metals is common and reflects crystal growth textures or post-depositional element remobilization (MacLean and Fleet, 1989; Agangi et al., 2013). Gold occurs as purported primary inclusions (due to high Ag content), as inclusions of secondary gold along cracks, and as small amounts of “invisible gold” or sub-microscopic inclusions with Au concentration in massive pyrite typically < 1 ppm (Saager, 1970, 1976; Agangi et al., 2013; Large et al., 2013).
Journal Pre-proof 5.2. Type I: Inclusion-bearing pyrite The term “porous” has widely been used in the literature for inclusion-bearing pyrite (Saager, 1970; Hallbauer and von Gehlen, 1983; England et al., 2002b), as under reflected light the inclusions are generally non-reflective and thus superficially have the appearance of voids. We however urge to refrain from using this term, as mineral inclusions,
or
their
metamorphic/pseudomorphic
precursors,
were
likely
trapped/incorporated during the growth of pyrite and, therefore, they were never pores at any time.
f
Inclusion-bearing pyrite comprises a group of pyrite types characterized by mineral
oo
inclusions (>10 vol.%) arranged in different textures. The presence of inclusions is largely related to the nature of this pyrite type, as it consists of aggregates of small (5
pr
to 50 µm, England et al., 2002b), euhedral to subhedral pyrite crystals, with inclusions frequently representing the interstices of the crystals. A skeletal growth of pyrite
e-
crystals, starting from delicate octahedral axial crosses has been described by Saager
Pr
(1970) and referred to as loosely-knit texture. Arrangement, density, and morphology of pyrite crystals within a single grain vary and this results in different arrangements of
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the inclusions. The inclusions may be randomly oriented or arranged in planar-parallel or concentric laminae. As such, this pyrite type is subdivided into several sub-types
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depending on the arrangement of the inclusions within a grain: random inclusion-
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bearing pyrite (type DIR), planar-laminated pyrite (type DIP), and concentrically laminated pyrite (type DIC). A fourth sub-type, characterized by randomly oriented sub-spherical inclusions, is herein referred to as microspherical pyrite (type DIM). In contrast to authigenic (or coarsely crystalline) pyrite, inclusions are not developed in crystallographic directions and zonal arrangements (e.g. Saager, 1970). While inclusion-bearing pyrite is typically found as sand-sized grains, it can occur as pebbles up to several centimetres across.
5.3. Type DIR: Detrital pyrite with randomly distributed inclusions Pyrite type DIR consists of grains < 2 mm in diameter, with some up to 5 mm (Saager, 1970; Utter, 1978). The numerous inclusions are randomly distributed in the grain, with a size range from a few µm up to tens of µm (Figs. 3 A to D). It is commonly observed in all reefs of the Wits Basin, but much less commonly in the Dominion
Journal Pre-proof Group (Saager, 1970; Hallbauer, 1986; England et al., 2002b; Rantzsch et al., 2011; Guy et al., 2014). Plastic deformation features, such as radial cracks, indentations and hard minerals pressed onto the surface of DIR-type pyrite have been reported (Utter, 1978). In situ analyses by electron microprobe and laser ablation ICP-MS (Supplementary Table 1) have revealed that concentrations of trace elements such as As, Ni, Mn, Sb, Tl, Pb and Au in pyrite type DIR from the VCR and Carbon Leader Reef are relatively high in comparison with massive pyrite from the same reefs (e.g. As ≤ 1 wt % and Ni ≤ 0.8 wt %; Agangi et al., 2013). Concentrations of Au, Sb and in some cases Te are typically
f
positively correlated (Large et al., 2013). Whole-grain analyses from several reefs also
oo
found the highest Au concentrations (up to 1400 ppm Au) in inclusion-bearing pyrite in comparison to massive pyrite (Utter, 1978). Saager and Mihálik (1967) reported
pr
mosaic-like, intimately intergrown isotropic and anisotropic pyrite in DIR grains, with
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the latter type being less hard, less reflective, and enriched in As, Co and Ni.
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5.3.1 Composite pyrite
A distinct variety of inclusion-bearing detrital pyrite is referred to as composite. This
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type consists of granules to small pebbles of pyrite-cemented, heavy mineral-rich sandstone, containing detrital grains of quartz, pyrite, chromite, rutile and zircon, with
rn
a typical size from < 0.1 to 0.3 mm. Composite grains have been observed in the Steyn,
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Elsburg, and Black reefs (Schidlowski, 1967; England et al., 2002b; this study Figure 4). The degree of packing of the detrital grains is generally low, indicating early diagenetic cementation of sandy sediment. Some of the pyrite present in composite grains, instead of being cement, might represent detrital Fe (-Ti)-oxide grains replaced by pyrite (Ramdohr, 1958).
5.3.2 Carbonaceous pyrite “Sooty” pyrite, herein referred to as carbonaceous pyrite, is another variety of inclusion-bearing detrital pyrite. It is characterized by a fine intergrowth of pyrite, mica-group minerals (e.g. sericite), and abundant carbonaceous matter, and has a diameter range from < 0.25 to ∼2 mm. It has been described from the Carbon Leader Reef and Black Reef (Large et al., 2013; Fuchs et al., 2016; this study; Fig. 5).
Journal Pre-proof 5.3.3 Aggregate pyrite England et al. (2002b; their Figure 3A) described a DIR-type pyrite (“porous aggregated pyrite”) to represent early diagenetic nodules that formed in situ. An authigenic origin was suggested on the basis of the absence of a sharp, abraded bounding surface. However, in situ corrosion of the margin of detrital grains (England et al., 2002b) or overgrowth of authigenic pyrite may have resulted in the more diffuse grain margins. We thus regard this pyrite type (Fig. 6) to be largely detrital.
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5.4. Type DIP: Planar-laminated detrital pyrite
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Terms such as layered, banded or laminated have previously been used to describe this pyrite type (Utter, 1978; Hallbauer, 1986; England et al., 2002b). It generally has a
pr
tabular shape under the microscope with long axes up to 4 mm. It is constituted by laminae that differ in the abundance of inclusions (massive vs inclusion-rich) or in the
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size of pyrite crystals forming the laminae. Laminae are typically thinner than 1 mm
Pr
and mostly continuous over the scale of the grain (Utter, 1978; Hallbauer, 1986; this study; Fig. 7). This pyrite type has been described in various reefs (e.g. Steyn Reef,
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Basal Reef, middle and upper Elsburg reefs, and VCR; Hallbauer, 1986; England et al., 2002b, Guy et al., 2014). It shows a wide range of trace element concentrations within
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different reefs in the Wits Basin (Koglin et al., 2010; Supplementary Table 1).
5.5. Type DIC: Concentrically (rarely colloform) laminated detrital pyrite This type has previously been referred to as oolitic or colloform pyrite (Table 1). Concentrically laminated pyrite occurs as spherical grains or fragments thereof (Fig. 8), with a diameter of ∼ 0.6 to 1 mm, and exceptionally up to 7 mm (Hallbauer, 1986; Guy et al., 2014; Agangi et al., 2015; this study). It consists of concentric sets of µm-scale laminae alternating with thicker laminae (tens of µm) and non-laminated domains (Agangi et al., 2015; this study). Some grains have a core of radiating bladed crystals surrounded by a concentrically laminated rim (England et al., 2002b). In contrast to concentrically laminated grains, colloform pyrite shows only domains of curved laminations that may or may not be concentric, and sometimes several domains may be stacked in an asymmetric mode (e.g. Fig. 5C of England et al., 2002b).
Journal Pre-proof Inclusions of quartz, chlorite, muscovite, rutile, apatite, monazite and chalcopyrite have been reported in concentrically laminated pyrite from the VCR (Agangi et al., 2013). The occurrence of secondary native gold inclus ions in the interstices between laminae was observed by Agangi et al. (2015). In contrast, Utter (1978) described small gold inclusions with a maximum size of 5 µm homogeneously dispersed in the interstices between individual pyrite particles that form pyrite grains; these Au inclusions were interpreted as primary inclusions. The pyrite type DIC has been observed in several reefs (e.g. Basal, Vaal, Kimberley, Upper Elsburg), but it may be
f
most abundant in the VCR (Hirdes and Saager, 1983; Guy et al., 2014; Agangi et al.,
oo
2015; this study).
In situ analyses of DIC-type pyrite from the VCR, B-Reef and Elsburg Reef
pr
(Supplementary Table 1) indicate that this type of pyrite is enriched in a suite of elements that includes Ni, As, Au, Ag, Pb, Sb, Tl and Mn in comparison to massive
Pr
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detrital pyrite (Agangi et al., 2013; Koglin et al., 2010).
5.6. Type DIM: Microspherical detrital pyrite
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This type is characterized by µm-scale spherical to subspherical structures in detrital pyrite previously referred to as “mineralized bacteria” and described in the Basal, Vaal,
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Elsburg and Black reefs (Ramdohr, 1958; Schidlowski, 1965; Saager, 1970, 1976). They
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occur in detrital pyrite grains ∼ 0.2 mm to 6.0 mm in size. The microspheres range in diameter from ∼ 0.01 mm up to 0.045 mm and occur as isolated, or occasionally compound, spheres, characterized by a single or double rim of micro-inclusions mainly of quartz (Schidlowski, 1965; Saager, 1970) (Fig. 9). The microstructures were interpreted as pyritized bacterial colonies, or remains of microbial life forms, based on morphological observations (Schidlowski, 1965; Saager, 1970, 1976). In order to refrai n from genetic terminology, especially since a microbial origin for the microspheres remains unproven, this pyrite type is herein referred to as microspherical pyrite. No published data so far exists on the trace element and isotopic composition of this pyrite type.
Journal Pre-proof 5.7. Type DC: Coarsely crystalline detrital pyrite Coarsely crystalline pyrite consists of massive or inclusion-bearing grains, mostly from 0.2 to 1.5 mm (but up to 5 mm) in diameter. The grains consist of pyrite crystals typically larger than 0.05 mm and with a variety of textures, including dendritic, branching (leaf-like), cone-in-cone, chevron, or radiating textures (Fig. 10; Ramdohr, 1958; Barton and Hallbauer, 1996; England et al., 2002b; Guy et al., 2014). These differ from the relatively fine-grained euhedral to subhedral crystals that form many inclusion-bearing pyrite types. We use the crystal texture as a qualifier (e.g. radial
f
crystalline pyrite) in the classification. Type DC pyrite occurs in the Basal, Steyn,
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Virginia, Upper Elsburg, Vaal and Black reefs (Hallbauer, 1986; England et al., 2002b) and probably in many others. Only limited compositional data of this type of pyrite are
pr
available in the literature. In the Black Reef, this type is characterized by enrichment in
6. Detrital pyrite provenance
Pr
1996) (Supplementary Table 1).
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Co, Ni, As, Bi, Cu and Ag in comparison with massive pyrite (Barton and Hallbauer,
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Detrital pyrite can be derived from erosion of any pyrite-bearing lithology, as long as conditions during weathering, transport, deposition, diagenesis, and metamorphism
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will allow pyrite to be preserved in sedimentary rocks. The wide range of textures and
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compositions of detrital pyrite indicates an origin from a variety of source rocks in the basin hinterland and/or from intrabasinal reworking of early diagenetic or hydrothermal sulfides. A number of potential sources for detrital pyrite have been proposed, such as: 1) sedimentary sources, e.g. diagenetic pyrite in fine-grained sedimentary rocks (Dimroth et al., 1979; Guy et al., 2010; Hofmann et al., 2009); 2) syn-genetic sedimentary sources (reworked early diagenetic pyrite; Hallbauer, 1986, Guy et al., 2014, Agangi et al., 2015); and 3) hydrothermal sources. The latter includes exhalative and syn-genetic sulfide deposits (Hutchinson and Viljoen, 1988), hydrothermally altered granites (Klemd and Hallbauer, 1987; Robb and Meyer, 1995), vein mineralization, and/or iron formations of greenstone belts (Saager et al., 1982; Utter, 1978). However, some of the latter sources may not necessarily have had pyrite in the abundances required to have contributed to the large amounts of detrital pyrite found in many reefs. Magmatic sources for detrital pyrite are generally not considered,
Journal Pre-proof as pyrrhotite is the dominant sulfide in igneous rocks (Hall, 1986). Metamorphic rocks may equally yield pyrite upon erosion. However, pyrite does start to recrystallize at greenschist facies grade and may convert to pyrrhotite during metamorphism at lower amphibolite facies grade associated with host rock dehydration reactions (Craig and Vokes, 1993; Tomkins, 2010). Erosion of volcanogenic massive sulfide (VMS) deposits is also considered unlikely, due to the general absence of detrital base metal sulfides (e.g. Guy et al., 2014). Some studies of pyrite geochemistry focused on the Co/Ni ratio as a potential
f
provenance indicator. A Co/Ni ratio lower than one has been proposed to be
oo
associated with a syn-sedimentary or diagenetic origin, whereas a Co/Ni ratio greater than one may be indicative of a hydrothermal source (Loftus-Hills and Solomon, 1967;
pr
Koglin et al., 2010). However, different studies have found no significant differences in Co/Ni between different pyrite types (Utter, 1978) or that Co/Ni of authigenic pyrite
e-
plots prominently across the Co/Ni = 1 line (Agangi et al., 2013), implying that this
Pr
discriminant criterion has no broad applicability in Witwatersrand reefs (also see Meyer et al., 1990; Koglin et al., 2010). Guy et al. (2010) noted that diagenetic pyrite
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that formed in ferruginous shale of the Wits Basin had a much higher Co/Ni ratio (> 10) than pyrite in Fe-poor shales (< 1), indicating: (1) a strong control exerted by the host
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sediment in which the pyrite formed, and (2) a much larger variability of Co/Ni ratios of
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sedimentary pyrite than previously assumed. Sulfur isotope analysis of detrital Witwatersrand pyrite was undertaken by England et al. (2002b), Hofmann et al. (2009), Guy et al. (2014) and Agangi et al. (2015). A compilation of these analyses is presented in Figure 11. Sulfur isotopes can provide fingerprints of potential pyrite sources. Sulfides of magmatic origin should plot close to the origin in a δ34S vs Δ33S diagram. Sulfides present in VMS deposits that formed by seawater circulation through hydrothermal systems and seawater sulfate reduction commonly carry near to zero or relatively small negative Δ 33S values and a small range in δ34S (Farquhar and Wing, 2005; Jamieson et al., 2006; Ueno et al., 2008). In contrast, disseminated sulfides in Archaean black shales have mostly positive Δ 33S values inherited from reduction of photochemically-produced S species such as elemental S (Farquhar and Wing, 2003, 2005; Ono et al., 2009), while pyrite nodules in Archaean
Journal Pre-proof organic matter-rich shales have negative Δ33S values due to seawater sulfate reduction in sediments during diagenesis. The large heterogeneity of δ34S and Δ33S of detrital pyrite from single samples indicated contrasting formation processes and thus provenance (Hofmann et al., 2009; Guy et al., 2014; Agangi et al., 2015). Figure 11 shows that none of the samples plot along the Archaean Reference Array (ARA), which is typically observed in marine deposits (Ono et al., 2003). In contrast, limited available data on S isotopes from nonmarine deposits indicate muted Δ33S values that typically do not follow the ARA (e.g. Maynard et al., 2013). Some samples (e.g. Black Reef, irrespective of pyrite type) show
oo
f
a positive correlation in δ34S and Δ33S space, suggesting specific atmospheric photochemical fractionation processes during the time of deposition. However, the
pr
bulk of the data lacks a clear MIF-S signal and has slightly positive δ34S values. The reason for this different MIF-S signal is not understood, but may relate to microbial
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reworking of an atmospheric signal, mixing with non-photolytic S species, variations in
Pr
atmospheric photochemistry during the Archaean, or a combination thereof (Guy et al., 2014). In the specific case of inclusion-rich pyrite types, which are interpreted to
al
have been locally derived, the muted MIF-S may be due to inefficient separation of sulfate and elemental S pools (which carry Δ33S of opposite sign) in a non-marine
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diagenetic pyrite.
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environment, so that the two pools can combine during formation of sedimentary-
A large range of Fe isotope values has been reported from detrital Witwatersrand pyrite. Bulk pyrite analysis yielded δ56Fe values between −1.1 and 1.2 ‰ (Hofmann et al., 2009), while in situ ion probe spot analysis yielded an even larger range (−1.5 to +5.04‰.; Agangi et al., 2015). In the latter study, negative δ56Fe values were found in concentrically laminated grains and the highest positive values were found in random inclusion pyrite. While igneous rocks and siliciclastic sedimentary rocks with low carbon and sulfur contents have limited Fe isotope variations, diagenetic pyrites in Archaean black shales show highly variable and mostly negative values ranging from 3.5 to +0.5 ‰ (Rouxel et al., 2005; Yamaguchi et al., 2005; Archer and Vance, 2006; Marin-Carbonne et al., 2014). In contrast, oxidized forms of iron, such as found in Feoxides in Archaean BIFs, are frequently characterized by positive δ56Fe values up to 1.6 ‰ (Dauphas et al., 2004; Rouxel et al., 2005; Whitehouse and Fedo, 2007; Johnson et
Journal Pre-proof al., 2008). The large range of Fe isotope values reported from Witwatersrand pyrite suggests its derivation from a supracrustal source, and is entirely consistent with reworking of diagenetic sulfides, also incorporating oxidized forms of iron. Köppel and Saager (1974) compared Pb isotopes of detrital pyrite concentrates with sulfides from Archaean greenstone belt-hosted gold deposits of South Africa. A similar isotopic composition was regarded to indicate a greenstone belt provenance for the detrital pyrite. However, studies using bulk samples, unless carefully hand-picked into different pyrite types, need to be treated with care, as any concentrate of detrital
f
pyrite will include different types. Mobility of Pb during deformation and
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metamorphism also needs to be considered (Fougerouse et al., 2019). Pb-Pb dating of single detrital pyrite grains from the < 2.7 Ga old Black Reef by Barton and Hallbauer
pr
(1996) indicated a derivation from ca. 3.2 to 2.8 Ga old source region. These pre-
provide information on the provenance.
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sedimentation ages confirm a detrital origin for this pyrite, although they do not
Pr
Four pyrite particles from a whole rock sample of Vaal Reef were subjected to Re-Os analysis (Kirk et al., 2001) and defined an isochron with an age of 2.99 ± 0.11 Ga. While
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the type of pyrite was not specified by Kirk et al. (2001), a detrital rather than authigenic origin was inferred, as the age either predates or overlaps the age of
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deposition of the Central Rand Group in which the Vaal Reef is situated.
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Mathur et al. (2013) analyzed Re and Os contents and isotopic ratios of Au-rich sulfides from the Carbon Leader Reef, the VCR, and the Black Reef. Sulfides included unspecified types of mainly pyrite and likely included mixtures of detrital and authigenic pyrite. Isochron ages for individual reefs were found to be younger than Witwatersrand sediment deposition, suggesting hydrothermal remobilization of sulfides. These results may, once again, emphasize the importance of a detailed study of pyrite (and other ore mineral) types prior to chemical and isotopic analysis of compositionally complex samples. Contrasting results are to be expected between bulk and in situ analyses, potentially unnecessarily fueling the debate on the origin of the Witwatersrand gold deposit.
Journal Pre-proof 6.1. Massive detrital pyrite Massive detrital pyrite should be the most resistant to mechanical abrasion. The poor cleavage of pyrite together with a hardness of 6.5 (Tauson et al., 2015) may have allowed extensive transport and reworking. However, many massive grains have nearly cubic outlines with abraded edges, indicating relatively limited reworking of euhedral pyrite crystals. Such grains may have been derived from hydrothermal sources or from metamorphosed, and thus recrystallized, sedimentary rocks (Saager, 1970; Utter, 1978; Hallbauer and von Gehlen, 1983; Barton and Hallbauer, 1996; Guy et al., 2014).
f
They could even have been derived from early diagenetic pyrite filling porosity in
oo
sediments. Trace element zoning in massive pyrite (e.g. MacLean and Fleet, 1989; Agangi et al., 2013) may indicate growth from hydrothermal fluids. MacLean and Fleet
pr
(1989) suggested pyrite with pronounced zoning in As to have been derived from greenstone-hosted gold deposits. Rare inclusions of spessartine and corundum in
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massive pyrite have been interpreted by England et al. (2002b) to reflect derivation
Pr
from pegmatites and VMS deposits, respectively. Massive pyrite grains from the Central Rand Group show internal plastic deformation features that indicate pyrite
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deformation prior to their incorporation into the conglomerates (Reddy and Hough, 2013), consistent with an extra-basinal source. Large et al. (2013) obtained Pb–Pb ages
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of approximately 2.95–2.75 Ga for massive detrital pyrite from the Carbon Leader
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Reef, confirming the detrital origin. A hydrothermal source indicated by trace element analysis, high Co/Ni ratios and low Au concentrations was proposed by Koglin et al. (2010). Massive detrital pyrite of the Dominion Reef and VCR has narrow δ34S (ca. −5 to +4‰) and no significant MIF signal (-0.2 < Δ33S < 0.2‰), which is compatible with a crustal as opposed to atmospheric, source of S (Hofmann et al., 2009; Agangi et al., 2015). In contrast, significant MIF signal with negative Δ33S values in the Kimberley and Black reefs is compatible with a sedimentary source (Hofmann et al., 2009; Fig. 11). However, a sedimentary source is also inferred for massive detrital pyrite from the VCR having δ34S and Δ33S close to 0‰, but negative Fe isotope values. In summary, the provenance of massive pyrite is complex and likely included multiple sources as well as very different sources for different reefs.
Journal Pre-proof 6.2. Detrital pyrite with randomly oriented inclusions Detrital pyrite rich in inclusions is common among all the reefs in the Wits Basin. The composition/texture of the inclusions as well as trace element and isotopic composition suggests a sedimentary origin. Because of the preservation of delicate textures and the occasional evidence for transport in a soft state, inclusion-rich pyrite is interpreted to have formed locally, and to record environmental conditions near the site of final deposition. The presence of quartz and phyllosilicate inclusions as well a s rare rounded oxide and phosphate grains is consistent with the growth in a fine-
f
grained clastic sediment host. Carbonaceous pyrite formed in a sedimentary host rich
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in organic carbon, such as organic-rich muds or microbial mats. Composite pyrite grains represent reworked sand intraclasts that were subjected to early diagenetic
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pyrite cementation prior to erosion. While sedimentary gangue mineral inclusions are likely primary, it is rarely evident if gold and base metal sulfide inclusions are indeed
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primary or were introduced during secondary hydrothermal-metamorphic processes.
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In general, this can be evaluated based on the distribution of inclusions, which will be controlled by cracks or grain margins for secondary minerals. In addition, it has been
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shown that secondary Au inclusions along cracks have higher Au/Ag (∼ 10) in
al., 2015).
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comparison with invisible gold which has Au/Ag of ca. 0.1 (Large et al., 2013; Agangi et
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The abundance of trace elements such as Co, Ni, Mo, As, Cu, and V associated with low Au/Ag ratio indicate that DIR pyrite was sourced from carbonaceous mudstone (Utter, 1978; Large et al., 2009) or formed in a suboxic to anoxic environment with organic matter available (Agangi et al., 2013 and references therein). These trace elements might indicate environmental conditions in the provenance area that may have given rise to their mobilization due to variations in redox conditions, pH or bacterial activity. Bulk gold analysis of concentrates of different pyrite types from the Klerksdorp Goldfield indicated that samples dominated by inclusion-rich pyrites also had the highest bulk gold contents (Utter, 1978). Sulfur isotope analyses of pyrite type DIR from different reefs show a large spread in δ34S and Δ33S space (Fig. 11), indicative of incorporation of both elemental and oxidized S species and reworking of the isotopic signal by microbial sulfate reduction.
Journal Pre-proof 6.3. Planar-laminated detrital pyrite The
lamination
in
this
type
of
pyrite
is
either
derived
from
pyrite
deposition/precipitation as laminae or diagenetic pyrite growth in laminated sediments (Tucker, 1980; Hallbauer, 1986, England et al., 2002b). A sedimentary source is thus indicated, compositionally similar to finely laminated sediments consisting of admixtures of pyrite, mud, and organic matter, as for example observed by Dimroth et al. (1979) at Randfontein Mine.
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6.4 Concentrically (rarely colloform) laminated detrital pyrite
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Sulfidation of coated grains in oolitic ironstone was proposed by Ramdohr (1958) for the origin of concentrically laminated pyrite, whereas England et al. (2002b) a
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proposed sulfidation of carbonate or evaporite ooids. The latter authors inferred that the ooids were likely replaced in the source area before transport and deposition into
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the basin rather than in situ, as indicated by a core-rim with different isotope values of
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δ34S (England et al., 2002b). Colloform laminated pyrite was interpreted by England et al. (2002b) to potentially indicate derivation from VMS deposits.
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The reworking of ooid beds or VMS deposits would yield multi-grain or multicomponent clasts, but such are not observed. The non-massive texture, formerly
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plastic nature, and the low resistance to long distance transport of DIC grains suggest
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that they formed near the site of deposition, in a low energy subaqueous environment, such as tidal mud flats or floodplains, where accretion of pyrite mud and other finegrained detritus took place (Hallbauer, 1986; Agangi et al., 2015). Growth of these grains took place lamina by lamina during early diagenesis , either while the grains were moving on the surface of a muddy substrate or under shallow sediment cover, with a possibility of successive reworking followed by reburial (Agangi et al., 2015). Reworking gave rise to fragmentation of the pyrite grains. Truncation of concentric layering in diagenetic marcasite has been observed in Tertiary palaeoplacers, but only in proximity to quartz clasts (Falconer et al., 2006), a feature uncommon in Witwatersrand QPCs. The mineral inclusions found in DIC pyrite are rather similar to those found in DIR pyrite and both types are enriched in a similar suite of trace elements, suggesting a similar environment of deposition. Variations in δ34S values between laminae of single
Journal Pre-proof grains suggest changes in water sulfate concentrations likely caused by microbial sulfate reduction in fluvial channels, ephemeral ponds or interchannel flats with elevated concentrations of sulfate (Guy et al., 2014; Agangi et al., 2015). Concentrically laminated pyrite always has negative Δ33S values (Fig. 11), which indicates that S was derived from photolytic (atmospheric) sulfate (Guy et al., 2014; Agangi et al., 2015). This contrasts with positive Δ33S values found in many coexisting DIR-type pyrites (Fig. 11). Both these types are considered to be indicative of the local environment of
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deposition, but clearly derived their S from separate pools.
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6.5 Microspherical detrital pyrite
The origin of pyrite type DIM has been the subject of controversy. Ramdohr (1958)
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interpreted these grains as deriving from pyritized granular ironstone. Schidlowski (1965) and Saager (1970) interpreted the microspherical pyrite as pyritized bacterial
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colonies or, more generally, fossil remains of primitive life forms. However, a biogenic
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origin was based only on morphological features and other origins cannot be ruled out (Schidlowski, 1965). Guy et al. (2014) suggested that some of the microspheres
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represent former pyrite framboids. Despite the various interpretations, a sedimentary
rn
origin of microspherical pyrite is indicated.
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6.6. Coarsely crystalline detrital pyrite The radial crystal texture of some grains has been regarded to be identical to those in diagenetic marcasite nodules (Barton and Hallbauer, 1996; England et al., 2002b), although marcasite is no longer preserved, possibly due to its conversion to pyrite at elevated temperatures. Identical textures have also been found in VMS deposits (Barrie et al., 2009). However, pyrite with elongate, commonly skeletal pyrite crystals are fragile, thus they formed more or less in situ, close to the site of final deposition, after only limited transport and reworking, in line with a sedimentary rather than VMS provenance. Grains with skeletal crystals imply unidirectional growth in open-pores (England et al., 2002b), probably indicating syn-depositional pyrite growth in the conglomerates themselves,
followed by sedimentary reworking. Alternatively, they formed
displacively in sediment with high plasticity and fluid content (Sellés-Martínez, 1996).
Journal Pre-proof Pyrite nodules with very similar features have been described from the West Rand Group (Guy et al., 2010) and Phanerozoic black shale successions (Carstens, 1985).
7. Classification of authigenic pyrite The characteristic feature of authigenic pyrite is its euhedral to subhedral shape. It is most commonly massive, although inclusion-bearing authigenic pyrite does occur. Most formed in situ after QPC sedimentation, during diagenetic, hydrothermal and/or metamorphic processes. Some authigenic pyrite formed during sedimentation and can
f
thus be found as abraded detrital grains. Authigenic pyrite occurs as individual crystals
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or crystal clusters of euhedral cubic to pyritohedral shape or as an overgrowth on older pyrite generations. Massive authigenic pyrite also occurs as veins cutting through
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detrital pyrite or other minerals, a clear feature of a post-depositional origin. A review of previous work on authigenic pyrite was done by comparing the description of the
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different textures, shapes, sizes, and geochemical information (Ramdohr, 1958;
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Saager, 1970; Utter, 1978; Hallbauer, 1986; England et al., 2002b; Guy et al., 2014). A new authigenic pyrite classification scheme was derived from this synthesis (Table 2),
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whereby authigenic pyrite is classified into 4 subtypes: authigenic euhedral/subhedral (type E), authigenic overgrowth (AO), authigenic infill (AI), and authigenic aggregates
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(AA). Pseudomorphic pyrite (type AP) also falls into this group. Authigenic pyrite is
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characterized by extremely variable contents of trace element, such as As, Ni and Co, even in single grains, due to strong chemical zonation (Agangi et al., 2013). These chemical zones can be correlated between grains in the same sample, thus further confirming in situ growth. Authigenic pyrite is mostly Au-poor in comparison with inclusion-rich pyrite, but contains some Au particles, either trapped during growth or along cracks. These particles demonstrate post-sedimentary Au mobilization.
7.1. Type AE: Euhedral / subhedral authigenic pyrite Euhedral / subhedral authigenic pyrite is characterized by cubic, pentagonal, octahedral or dodecahedral crystals or crystal outlines. It occurs as single crystals or clustered into aggregates, with a wide diameter range, but typically ∼ 10 µm to 3 mm (Fig. 12) (Saager, 1970, 1976; Hallbauer and von Gehlen, 1983; England et al., 2002b, this study). Inclusions are uncommon and, if present, account for less than 10 vol. % in
Journal Pre-proof most cases. They are present as random inclusions or in crystallographically oriented zonal arrangements and comprise chalcopyrite, chlorite, quartz, sphalerite, galena, pyrrhotite, a U-Ti mineral phase and native Au (-Ag) (Saager, 1970; Agangi et al., 2013). Some of these inclusions, such as chlorite, sulfides and gold, are demonstrably secondary in origin where they occur along cracks. Many AE grains combine a core of detrital massive pyrite (DM) and authigenic overgrowths (AO), which is however not always obvious using ore microscopy alone. The presence of multiple growth zones may explain why analyses of this pyrite type yielded trace element concentrations and
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isotopic compositions that vary widely (Supplementary Table 2).
7.2. Type AO: Authigenic pyrite overgrowth
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Authigenic pyrite frequently forms overgrowths on rounded detrital grains (commonly DM and, less frequently, DIR types), thus giving rise to pyrite grains with a core-rim
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structure (Fig. 13). The proportion of AO in detrital pyrite can reach up to 30% (England
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et al., 2002b). Overgrowth can consist of a single idiomorphic crystal or as idiomorphic to hypidiomorphic pyrite aggregates (Utter, 1978; Guy et al., 2014).
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Authigenic pyrite overgrowths are mostly massive, but inclusions can sometimes be abundant. In frequent cases, especially when pyrite overgrowths occur on massive
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detrital grains, the detrital core remains invisible under the optical microscope, but it
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can be made visible by etching of the thin section with nitric acid (Saager and Mihálik, 1967). Element mapping by SEM or EPMA also allows the identification of authigenic overgrowth (Frimmel et al., 2005; Agangi et al., 2013). Previous studies have identified high concentrations of As, Ni, Co, and Au in AO pyrite as compared to detrital pyrite cores (England et al., 2002b; Koglin et al., 2010; Large et al., 2013; Supplementary Table 2).
7.3. Type AI: Authigenic pyrite infill This type of pyrite fills fractures and potentially primary porosity. Pyrite veinlets (Fig. 14) pre-dating QPC deposition cross-cut detrital pyrite grains, but are truncated by their margins. Post-depositional veinlets filling fractures a few μm wide cross -cut detrital pyrite and other detrital grains (Ramdohr, 1958; Saager, 1970; Utter, 1978; Hofmann et al., 2009; Guy et al., 2012, 2014). Some post-depositional veinlets are
Journal Pre-proof contiguous with authigenic pyrite overgrowths (England et al., 2002b). Pore-filling authigenic pyrite in detrital grains has been described by England et al. (2002b). Pyrite infill is trace element poor (see Supplementary Table 2), but is associated with inclusions of sphalerite, galena, chalcopyrite and electrum, which gives anomalous concentrations of some trace elements (Agangi et al., 2013).
7.4. Type AA: Authigenic pyrite aggregates This pyrite type was earlier described as aggregate pyrite (Fig. 6) and regarded to be
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largely detrital (subtype of DIR). Nevertheless, early diagenetic (authigenic) formation
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result in its detrital origin to become obscured.
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of this pyrite may be possible in some cases. Minor reworking without abrasion will
7.5. Type P: Pseudomorphic pyrite
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Pseudomorphic pyrite represents primarily non-sulfidic detrital grains that have been
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replaced by pyrite. These pyritized grains contain relict structures and compositions that allow them to be identified as pyrite pseudomorphs after individual minerals or
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rock fragments (Ramdohr, 1958; Saager, 1970; England et al., 2002b). Sand-sized pyrite with rutile laths are generally interpreted to represent pseudomorphs after ilmenite or
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titaniferous magnetite that exsolved to rutile and Fe-oxides, the latter having been
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replaced by pyrite (Ramdohr, 1958; England et al., 2002). A detrital pyrite with dendritic texture of rutile from the Vaal Reef is shown in Figure 15. Pebbles of chert, shale and BIF in many reefs show partially replacement by pyrite due to selective sulfidation (Myers et al., 1993; Phillips and Dong, 1994; Phillips and Law, 2000; Fig. 16). In summary, pyritization is rarely complete, thus allowing differentiation from truly detrital pyrite grains.
8. Origin of authigenic pyrite Authigenic pyrite is of in situ origin and formed after deposition, with different types having formed at different times under different physico-chemical conditions during diagenetic and epigenetic (hydrothermal and metamorphic) processes (Hallbauer and von Gehlen, 1983). While diagenetic processes included pyrite formation by microbial sulfate reduction (MSR) that must have operated shortly after deposition, there were
Journal Pre-proof several episodes of hydrothermal and metamorphic overprints in the history of the Wits Basin. These include regional metamorphism, thermal metamorphism during the emplacement of the Bushveld Igneous Complex, impact cratering and uplift of the Vredefort Dome, but also the intrusion of mafic sills and dykes at different times in Earth history (Frimmel et al., 2005; Phillips and Law, 2000). Although formation and recrystallization of authigenic pyrite may be related to any of these events, it is difficult to obtain direct age information on single stages of pyrite growth. Coarsely crystalline authigenic pyrite is commonly observed in localities which have been subjected to
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higher temperatures, such as near dykes (Hallbauer and von Gehlen, 1983). As such,
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early diagenetic pyrite overgrowths are regarded to be smaller than those related to growth during metamorphic/hydrothermal fluid flow (Guy et al., 2014). Large et al.
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(2013) obtained Pb–Pb data of overgrowth pyrite from the Carbon Leader Reef of ca. 2100–2020 Ma, an age range that overlaps with the tectono-thermal events of the
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Vredefort impact and Bushveld intrusion.
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Authigenic pyrite types, including reworked and thus diagenetic types, have a relatively narrow range of δ34S -0.5 ‰ to + 4 ‰ (England et al., 2002b; Guy et al., 2014) and
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mostly no MIF-S signature similar to the crustal S reservoir. Non-zero Δ33S values in bulk samples (Hofmann et al., 2009) may reflect the presence of a detrital core or
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sulfidic sediments.
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remobilization of atmospheric S, sourced locally from detrital pyrite or from other
Authigenic pyrite infills equally formed at different times depending on whether they fill primary porosity or post-depositional fracture porosity. It has been debated whether the pseudomorphic replacement of iron-bearing rocks and mineral grains by pyrite giving rise to type P occurred before, during, or after deposition (Ramdohr, 1958; Reimer and Mossman, 1990; England et al., 2002b). Given the sheer abundance of syn-sedimentary and authigenic pyrite in the Witwatersrand reefs, most pseudomorphic replacement likely took place after deposition.
9. Pyrite occurrence in other palaeoplacer deposits Beside the QPCs of the Witwatersrand Supergroup and immediately overlying stratigraphic units, a number of other pyrite-bearing QPCs exist that span in age from the Archaean to the Phanerozoic and are enriched in gold, albeit not always
Journal Pre-proof economical. Most of them have very similar pyrite varieties, and are thus amenable to the application of the new classification scheme. This similarity in pyrite morphology and composition also point to similar formation processes. In the following, a short description of gold- and/or uranium-bearing QPC palaeoplacer occurrences enriched in pyrite from different periods in geological time is given. The description of pyrite will follow the terminology proposed in this study. A schematic representation of the different pyrite types is provided in Figure 17 to aid in comparison.
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9.1. Moodies Group, Barberton Greenstone Belt, South Africa
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Detrital pyrite has been observed in conglomerates and sandstones of the Moodies Group of the Barberton Greenstone Belt (Saager et al., 1982; Nabhan et al., 2016). The
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Moodies Group consists predominantly of alluvial conglomerate and braided fluvial, tidal, and shallow-marine sandstones that were deposited ∼ 3.23 Ga ago (Heubeck et
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al., 2013). Detrital pyrite in conglomerate is massive and occurs together with a
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number of other detrital accessory minerals, including rare carbon and uraninite grains. It is poorly rounded and sorted, suggestive of short transport distances and
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poor sediment reworking (Saager et al., 1982). Nabhan et al. (2016) observed rounded grains of massive and inclusion-bearing pyrite with authigenic overgrowths. Detrital
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cores have δ34S values of ~ 0 ± 5‰, while the rims have values as low as -24.5‰, which
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has been related to biogenic fractionation of sulfur, and thus formation of the overgrowths during diagenesis (Nabhan et al., 2016). No economic concentrations of placer gold have been reported from the Moodies conglomerates. This may relate to the fact that gold mineralization in the Barberton Greenstone Belt postdates deposition of the Moodies Group by more than 100 Ma (Agangi et al., 2019).
9.2. Jacobina Group, São Francisco Craton, Brazil The Jacobina Basin is located in the east-central portion of Bahia state, in the São Francisco Craton. The Jacobina Group includes the Serra do Córrego Formation, which is characterized by interbedded coarse-grained sandstone, grit, and QPC that host AuU-pyrite mineralization (Teles et al., 2015). Metamorphic grade is greenschist facies, but sedimentary structures are locally well preserved. A braided stream to fluvio-
Journal Pre-proof deltaic depositional environment has been proposed. U–Pb ages obtained from detrital zircon of the Jacobina Group are as young as 3.2 Ga, and the abundance of detrital pyrite suggests deposition prior to the GOE (Teles et al., 2015). Pyrite occurs in the gold-bearing conglomerates together with quartz, muscovite, and other phyllosilicates, but also accessory zircon, chromite, thorite–uraninite, monazite, ilmenite, magnetite, tourmaline, and rutile (Milesi et al., 2002). Pyrite occurs as sandsized detrital and authigenic grains. Detrital grains are mainly massive, but inclusionbearing types (DIR, DIP) also occur. Authigenic pyrite includes AE crystals and AI pyrite
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veins (Milesi et al., 2002; Teles et al., 2015). Gold is spatially associated with pyrite and
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occurs as small flakes, as inclusions in pyrite, and intergrown with AE. A modified placer origin is inferred for the gold mineralization in the Jacobina Basin (Milesi et al.,
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2002; Teles et al., 2015).
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9.3. Dominion Group
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The Rhenosterspruit Formation hosts marginally economic gold and uranium mineralization in QPCs of the Lower and Upper Dominion reefs. The Lower Reef is a
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matrix-supported pebble to cobble conglomerate that has been sporadically mined for its gold content. The Upper Reef is a small-pebble conglomerate laterally more
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uniform than the Lower Reef, is enriched in a suite of heavy minerals with abundant
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garnet and has principally been mined for its uranium enrichment (Malan, 1959; Hiemstra, 1968; Rantzsch et al., 2011). Pyrite is the most abundant heavy mineral in the Dominion reefs. The most common detrital pyrite is massive with minor inclusion-bearing pyrite (Malan, 1959; Hiemstra, 1968; Rantzsch et al., 2011; this study). Relatively unfractionated values of δ 34S and Δ33S (0.0 ± 0.02‰) and the small range of δ56Fe values similar to igneous rocks for massive pyrite is consistent with high-temperature hydrothermal or crustal derivation, such as sulfide-quartz veins hosted in granitoid-greenstone terrains (Hofmann et al., 2009). This interpretation is supported by (1) the texture of detrital pyrite, consisting mainly of massive sand-sized grains, and (2) the setting of the Dominion Group resting directly on granitoid basement (Hofmann et al., 2009).
Journal Pre-proof 9.4. Pongola Supergroup, Swaziland and South Africa Detrital pyrite is locally present in conglomerates of the Pongola Supergroup, which has been correlated with the West Rand Group by Beukes and Cairncross (1991). The Pongola Supergroup is divided into the basal Nsuze Group and the upper Mozaan Group and has been subjected to low-grade regional metamorphism (Luskin et al., 2019). The Pongola Supergroup nonconformably overlies Archaean granitoidgreenstone basement with the oldest volcanic unit in the Nsuze Group dated at 2980 ± 10 Ma (Mukasa et al., 2013).
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The Nsuze Group is a volcano-sedimentary sequence of basaltic volcanic and
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volcaniclastic rocks intercalated with sandstone, shale and, locally, dolomite. The Mozaan Group consists of an alternating assemblage of sandstone, shale, banded iron-
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formation and rare conglomerates. The depositional environment was to a large extent an epicontinental shallow sea (Wilson et al., 2013; Luskin et al., 2019). Saager et
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al. (1986) described pyrite-bearing QPCs from one locality in the Nsuze Group and
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from several localities in the Mozaan Group. The heavy mineral detrital fraction of the conglomerates includes, in decreasing order of abundance, pyrite, rutile/leucoxene,
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chromite, zircon, monazite, and arsenopyrite, amongst others. Gold occurs as reconstituted grains and as inclusion in massive pyrite; erratic gold concentrations
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from 1 to 4 g/t supported a number of small-scale mining operations (Saager et al.,
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1986). Generally, the conglomerates are discontinuous, polymictic (vein quartz, quartzite and chert) and matrix-supported with sub-angular to subrounded clast, suggestive of predominantly fluvial deposition and subordinate marine reworking. The most extensively studied QPC is the Mozaan Contact Reef, situated at the base of the Mozaan Group above an angular unconformity (Hicks and Hofmann, 2012). The detrital pyrite types present in the reef are mainly massive, random inclusion-bearing and rare radial crystalline types. Authigenic euhedral pyrites are common (Saager et al., 1986; Hofmann et al., 2009). All pyrites from the Mozaan Contact Reef analysed by Hofmann et al. (2009) have small but consistently positive Δ 33S values and negative δ56Fe values in support of a sedimentary origin. Crystalline pyrite has highly negative δ34S values, suggesting microbial disproportionation of elemental sulfur.
Journal Pre-proof 9.5. Bababudan Group, Dharwar Craton, India The Bababudan Group in the province of Karnataka, South India, is a volcanosedimentary sequence deposited between 2.9 and 2.7 Ga with a basal quartz pebble conglomerate that hosts detrital pyrite, uraninite and gold overlying a granitoidgreenstone basement (Srinivasan and Ojakangas, 1986; Sunder Raju et al., 2014). Clasts are less than 10 cm in diameter, include vein quartz, metachert and quartzite, and occur in a quartz sand matrix with phyllosilicates and heavy minerals such as zircon and ilmenite (Srinivasan and Ojakangas, 1986). Pyrite occurs as rounded detrital
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grains of both massive and random inclusion-bearing types as well as euhedral
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authigenic types (Srinivasan and Ojakangas, 1986; Arora, 1991; Udayakumar and Philip, 1997). No economic concentration of Au and U has yet been discovered, and the
9.6. Huronian Supergroup, Canada Palaeoproterozoic
Huronian Supergroup in Canada is a predominantly
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The
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source of the pyrite remains to be investigated.
sedimentary succession. In its lower part, pyrite-bearing fluvial QPCs, which are locally
formations (Frimmel, 2014).
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auriferous and/or uraniferous, are present in the c. 2.4 Ga Matinenda and Miss issagi
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Pyrite present in the Matinenda Formation is mainly detrital (Roscoe, 1969) with a
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variety of massive and inclusion-bearing pyrite types with a diameter from ∼ 0.8 to 2.9 mm (Roscoe, 1969; Theis, 1979). Minor authigenic pyrite occurs as euhedral to anhedral crystals ∼ 1 mm in diameter as well as authigenic overgrowths on detrital grains (Roscoe, 1969; Theis, 1979). In the Mississagi Formation, authigenic pyrite is more abundant than massive and inclusion-bearing detrital pyrite (Mossman and Harron, 1983; Koglin et al., 2010). In their study of QPCs in the basal 30 m of the Mississagi Formation, Ulrich et al. (2011) distinguished three types of pyrite. The first type is of unequivocal detrital origin and includes grains of inclusion-bearing (types DIR, DIP, DIC) as well as (radial and dendritic) crystalline (type DC) pyrite 5 to 10 mm in diameter; this pyrite is enriched in Au, Ag, Te, Bi, Mn, Cu, Zn, Pb, Sb and Hg. The second type consists of subhedral to euhedral, massive to inclusion-bearing pyrite 0.5−2 mm in diameter of either authigenic (AE) or recrystallized detrital (type DM or DIR) origin; it is enriched in As and
Journal Pre-proof Co and depleted in Au. The third type is regarded hydrothermal in origin and forms irregular masses interstitial to clastic grains and filling veins (type AI) that cross-cut matrix and clasts; this type is variably enriched in Ni, Cu, Ag, Co and Zn and depleted in Au. All pyrite types have overlapping, generally positive δ 34S values (0.97−9.26‰), whereas Δ33S values in detrital grains tend to be negative and those in authigenic pyrites unfractionated or positive (Ulrich et al., 2011).
9.7. Waimumu and Otago areas, New Zealand
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Gold-bearing conglomerate placer deposits remarkably similar to those of the
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Witwatersrand Basin occur in the south of New Zealand in the Waimumu and Otago districts and include detrital and authigenic sulfide minerals and gold (Youngson et al.,
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2006; Falconer et al., 2006). The Waimumu QPCs occur in non-marine, fluvial and
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colluvial sedimentary units of late Oligocene to Pliocene age (Falconer et al., 2006). Sulfides in the matrix of QPC occur as rounded clasts of sand- up to cobble-sized
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aggregates of framboidal marcasite with a radial texture. Marcasite overgrowths are common on quartz clasts, while marcasite is also found cementing QPC at the cm
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scale, incorporating host quartz grains (Falconer et al., 2006). Sulfide textures thus resemble those found in inclusion-bearing and radial crystalline pyrite types from the
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Wits Basin. Authigenic euhedral massive pyrite forms single cubes, less commonly
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intergrown cubes, and rarely, pyritohedra with an average size of ∼ 2 mm (Falconer et al., 2006). The marcasite is characterized by fine compositional layering with variations in Ni, Co, and As contents and has an average Co:Ni ratio of 0.6. A wide range of δ34S values between -45 to +18‰ is consistent with a diagenetic origin for the Fe-sulfides involving microbial sulfate reduction (Falconer et al., 2006).
10. Conclusion This new compilation of detrital and authigenic pyrite types, their textures and morphologies, as well as their geochemical composition helped to clarify the source of some of the elements hosted by pyrite as well as the setting in which this mineral formed. The origin of pyrite in the Wits Basin, as well as in many other placer deposits around the world, has long been a disputed issue. There is now however overwhelming evidence that most of the detrital pyrite is sedimentary in origin, while
Journal Pre-proof much of the authigenic pyrite is syn-sedimentary to diagenetic in origin. Such proposals have been made in the past (e.g. Dimroth et al., 1979), but are now more firmly based on more abundant trace element and isotope data of pyrite. With the exception of massive pyrite, all other detrital pyrite types discussed in this study can be associated with a sedimentary origin. This largely involved synsedimentary or diagenetic growth of pyrite framboids, nodules, concretions, microspheres and cement, later subjected to erosion, reworking and deposition in QPCs forming the various inclusion-bearing pyrites, coarsely crystalline pyrites, as well
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as composite grains. Based on S isotope evidence, much of the sedimentary-diagenetic
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pyrite formation can be directly attributed to microbial sulfate reduction and elemental sulfur disproportionation. Concentrically-laminated pyrites were derived
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solely from sulfate reduction. Precipitation of pyrite in euxinic conditions could also have played a role, forming planar-laminated pyrite muds. Some massive pyrites
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frequently show S isotopic composition similar to some of the more obvious diagenetic
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pyrite types in the same reefs. Together with a fractionated Fe isotopic composition, some of it too was largely of sedimentary derivation.
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The formation of abundant diagenetic pyrite requires sulfate and reactive iron (Berner, 1984). The latter appears not to have been in short supply, assuming largely oxygen-
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deficient conditions as well as some evidence for sulfidization of Fe-Ti-oxides. Sulfate
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could have been delivered via sulfate aerosols following volcanic eruptions or localised oxidative weathering of pyrite in the hinterland. That the time of Wits Basin sedimentation is also the time of the formation of the first oxygen oasis related to enhanced photosynthetic activity is now well established (Planavsky et al., 2014; Eickmann et al., 2018; Ossa et al., 2019). Massive pyrite may have derived from many different sources, including greenstone belts hundreds of kilometres away. What is important to point out is the generally low Au content of massive pyrite, in contrast to much higher gold contents of some sedimentary pyrite. The textures of some sediment-derived pyrite grains (e.g. type DIC) indicate that they are locally-sourced, and the primary (mostly invisible) gold in these grains was taken up from the surrounding environment during their formation, likely from solution, aided by sulfate reducers (Lengke and Southam, 2007). This gold may have been derived from the dissolution of detrital gold, which was concentrated
Journal Pre-proof in the placer deposits, under acidic conditions that may have been a response to sulfide oxidation related to the onset of oxidative weathering around that time (Agangi et al., 2015; Frimmel and Hennigh, 2015). It is furthermore noteworthy that very similar pyrite textures can be found in different gold placers of very different ages around the world, indicating very similar pyrite-forming processes.
Acknowledgments This research was supported by the DST-NRF Centre of Excellence for Integrated
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Mineral and Energy Resource Analysis (CIMERA) via a Doctoral Fellowship to Giuliana
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Costa. We thank Evan Cook (Gold1), Simone Hartmann (AngloGold Ashanti), and Sunet Dercksen (Sibanye Gold) for access to drill core and arranging underground visits.
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Thanks are extended to Sebastian Fuchs, George Henry, and Bradley Guy for discussion
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and samples.
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Figure 1. Geological map of the Witwatersrand Basin and simplified stratigraphic column. BR, Black Reef; VCR, Ventersdorp Contact Reef; CLR, Carbon Leader Reef; DR, Dominion Reef; VD, Vredefort Dome. Modified from Agangi et al. (2015).
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Figure 2. Photomicrographs (reflected polarized light) of massive detrital pyrite (type DM) from the Black Reef, Moderfontein Mine. (A) Well rounded massive pyrite. (B) Massive cubic pyrite with abraded edges.
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Figure 3. Back-scattered electron images of two examples (A and C) of random inclusionbearing detrital pyrite (type DIR), Black Reef, Moderfontein Mine. (B and D) Details of inclusions of muscovite (Ms), chlorite (Chl), titanite, and quartz (Qtz).
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Figure 4. (A) Photomicrograph (reflected polarized light) of a composite pyrite grain (subtype of DIR pyrite) from the Black Reef, Moderfontein Mine. (B) Detail of detrital grains of chromite (Chr), zircon (Zr), rutile (Rt), and a grain replaced by muscovite (Ms). (C) Detail of detrital rutile with inclusions of brannerite (Brt) and muscovite (Ms).
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Figure 5. (A and B) Back-scattered electron image of carbonaceous pyrite (subtype of DIR pyrite) from the Black Reef, Modderfontein Mine. (C) Raman maps of carbonaceous matter (CM), pyrite and muscovite. Mapped area is shown in B.
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Figure 6. Back-scattered electron image of aggregate pyrite (subtype of DIR pyrite). (A) Aggregate pyrite grain with chlorite (Chl) inclusions and authigenic pyrite overgrowth obscuring the inferred detrital nature of the grain; Carbon Leader Reef, Tau Tona Mine. (B) Aggregate pyrite grain from the VCR, TauTona Mine. (C) Detail of (B) showing euhedral, micron-sized grains of pyrite; the pyrite grains are embedded in quartz (Qtz) and muscovite (Ms).
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Figure 7. (A) Photomicrograph (reflected polarized light) of planar laminated pyrite (type DIP), Upper Elsburg Reef, Cooke Mine. (B) Back-scattered electron image of a portion of the grain shown in A. Inclusions consist of chalcopyrite (Ccp) and galena (Gn). Quartz (Qtz) fills a microcrack.
Figure 8. Photomicrograph (reflected polarized light) of concentrically laminated pyrite (type DIC) from the Ventersdorp Contact Reef, Tau Tona Mine. (A) Typical concentrically laminated pyrite. (B) Fragment of concentrically laminated pyrite.
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Figure 9. Back-scattered electron images of microspherical pyrite (type DIM), Basal Reef, Free State Goldfield. (A) Detrital grain of microspherical pyrite with some authigenic pyrite overgrowth. (B) Detail of microspherical structures with spherical domain of quartz (Qtz) inclusions. An inclusion of rutile (Rt) is also present.
Figure 10. Images of diverse types of coarsely crystalline detrital pyrite (type DC). (A) Photomicrograph (reflected polarized light) of radial crystalline pyrite. (B) Back-scattered electron image of pyrite with chevron texture. (C) Back-scattered electron image of crystalline pyrite showing growth zoning as defined by variations in the abundance of inclusions. Secondary rutile (Rt) occupies fractures. (D) Photomicrograph (reflected polarized light) of crystalline pyrite showing growth zoning. (A) is from the Mozaan Contact Reef, while the other samples are from the Black Reef, Modderfontein Mine.
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Figure 11. Sulfur isotope chart of detrital pyrite from various reefs of the Witwatersrand basin. Compiled from Hofmann et al. (2009), Guy et al. (2014) and Agangi et al. ( 2015). Pyrite types: massive (M), coarsely crystalline (C), random inclusion-bearing (IR), planar-laminated (IP), concentrically laminated (IC).
Figure 12. Photomicrographs (reflected polarized light) of authigenic euhedral/subhedral pyrite (type AE). (A) Single euhedral pyrite grains from the Carbon Leader Reef. (B) Cluster of euhedral pyrite grains from the Basal Reef, Free State Goldfields. The centre of the cluster appears to enclose a detrital grain of sparsely inclusion-bearing massive pyrite.
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Figure 13. Photomicrographs (reflected polarized light) of authigenic overgrowth pyrite (type AO). (A) Pyrite overgrowing a cluster of several massive detrital pyrite grains, Carbon Leader Reef. (B) Pyrite overgrowth on an inclusion-bearing pyrite, Basal Reef, Free State Goldfield.
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Figure 14. Photomicrographs (reflected polarized light) of authigenic infill pyrite (indicated by arrows). (A) Pyrite filling fracture in detrital quartz grain, Vaal Reef, Kopanang Mine. (B) Veinlets of massive pyrite in a detrital inclusion-bearing pyrite grain. The veins are truncated at the grain margin and therefore formed prior to pyrite reworking, Black Reef, Modderfontein Mine.
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Figure 15. Back-scattered electron image and element maps of pyrite pseudomorph after ilmenite or titaniferous magnetite from the Vaal Reef, Kopanang Mine.
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Figure 16. Pebble of banded iron formation affected by pyritization along the margin and in discrete laminae. Photo of drill core from the Kimberley Formation. Field of view is 3 cm.
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Figure 17. Schematic representations of detrital and authigenic pyrite types (inspired by Guy et al., 2014). Detrital pyrite is identified by evidence for abrasion (i.e. rounding). It is subdivided into massive, inclusion-bearing, and coarsely crystalline types. Authigenic pyrite includes euhedral, overgrowth, infill, aggregate, and pseudomorphic types. It may have formed as a result of early diagenesis, burial diaganesis, and/or epigenetic processes.
Journal Pre-proof Declaration of interests The authors declare that they have no known competing financial interests or personal Ramdohr (1958)
Saager (1970)
Utter (1978)
Hallbauer (1986)
England et al., (2002b)
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This study
relationships that could have appeared to influence the work reported in this paper.
Table 1. Detrital pyrite classification
Guy et al., (2014 )
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Skeletal and radial concretions
Porous rounded pyrite; concretionar y pyrite
Allogenic detrital pyrite
Compact pyrite
DET1
Synsedimenta ry pyrite (mudball pyrite)
Porous pyrite (rounded aggregated pyrite; concretionar y pyrite)
SYN2, DIA2, DIA-5
Synsedimenta ry pyrite (rounded, layered pyrite)
Porous pyrite (banded pyrite)
DIA-5
Collofor m pyrite
Synsedimenta ry pyrite (oolitic pyrite)
Porous pyrite (ooliticcolloform pyrite)
SYN1, DIA-5
-
Synsedimenta ry pyrite (framboidal pyrite)
-
DIA1, DIA12
Radial, concretionary pyrite
Porous pyrite (concretiona ry pyrite; dendritic pyrite; miscellaneo us forms)
DIA5, DIA-6
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Concentric concretions
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Type DIC: Concentricall y laminated
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Type DIP: Planarlaminated
Type DIM: Microspheric al
Type DC: Coarsely crystalline detrital pyrite (radial, dendritic, chevron; massive or with inclusions)
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Compact rounded pyrite
Pyrite pseudomorp hs
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Type DI: inclusion-bearing detrital pyrite (∼ > 10% inclusions)
Type DIR: Random inclusions (various subtypes)
Pebble pyrite
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Type DM: Massive detrital pyrite (∼ < 10% inclusions)
Type 2: Allogeni c rounded compact pyrite (al. c. p) Type 3: Allogeni c rounded porous pyrites (al. p. p) – well rounded variety; inclusio ns in pyrite type 3 Type 3: Allogeni c rounded porous pyrites (al. p. p) – banded variety
Mineralized bacteria
Concretiona ry pyrite; skeletal concretions; compact, radial concretions
Pyrite nodules containing microspheric al structures (“mineralize d bacteria”)
Concretionar y pyrite
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Type AO: Authigenic overgrowth pyrite
Compact pyrite encrustatio ns; porous or skeletal pyrite growths
Compact and porous pyrite encrustation s
Fissurecoatings
Xenomorphi c pyrite (veinlets and fracture fillings)
Type AA: Authigenic aggregate pyrite
-
Pyrite pseudomor phs
-
Rounded pseudomorp hic pyrite
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Type AP: Pseudomorphic pyrite
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Type 1: Authigenic idiomorphic to hypidiomorp hic pyrite (au. i. p)
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Type AI: Authigenic infill pyrite
Table 2. Authigenic pyrite classification
Guy et al. (201 4)
Authigenic pyrite (euhedral to subhedral pyrite)
DIA2; DIA6; EPI-1
Authigenic pyrite (pyrite overgrowths )
DIA11; EPI-2
Authigenic pyrite (porefill pyrite; vein pyrite)
EPI-3
-
Porous pyrite (in situ aggregated pyrite)
-
Synsedimenta ry pyrite (coarsegrained aggregates of quartz and pyrite)
Authigenic pyrite (pyrite pseudomorp hs)
DIA12; EPI-4
Authigenic postsediment ary pyrite
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Idiomorphic pyrite
Compact and porous idiomorphic to hypidiomorp hic pyrite
England et al. (2002b)
Hallbauer (1986)
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Type AE: Authigenic euhedral/subhe dral pyrite
Utter (1978)
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