Gold nugget morphology and geochemical environments of nugget formation, southern New Zealand

Ore Geology Reviews 79 (2016) 301–315

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Gold nugget morphology and geochemical environments of nugget formation, southern New Zealand Dave Craw ⁎, Kat Lilly Geology Department, University of Otago, PO Box 56, Dunedin 9054, New Zealand

a r t i c l e

i n f o

Article history: Received 24 February 2016 Received in revised form 31 May 2016 Accepted 5 June 2016 Available online 11 June 2016 Keywords: Pyrite Gold Supergene Crystal Placer Groundwater

a b s t r a c t Gold nuggets (centimetre scale) have formed in a supergene alteration zone on hydrothermal gold deposits, and occur intergrown with quartz and iron oxyhydroxide pseudomorphs after sulphide minerals, and along fractures in quartz and host rocks. The supergene alteration was driven by groundwater-driven water-rock interaction near to a regional unconformity beneath fluvial sediments, and involved clay alteration and oxidation that extended up to 50 m below the unconformity. Oxidation of pyrite and arsenopyrite produced temporary thiosulphate ligands that mobilised microparticulate gold encapsulated in the sulphide minerals. The nuggets have some crystalline form, and internally they consist of anhedral grains, elongated gold plates, and intimate intergrowths of gold and iron oxyhydroxide. Nugget surfaces have further micron scale overgrowths of microparticulate gold, gold plates, and gold crystals. Nuggets were eroded and recycled into nearby proximal Miocene quartz pebble conglomerates, where they concentrated in placers near the basal unconformity. Later recycling transferred gold into Pleistocene fluvial channels. Gold dissolution and redeposition as plates and crystals occurred on the exterior surfaces of placer gold particles, with little change in mass. All groundwater maintained high pH throughout the geological history because there was sufficient calcite in the basement rocks to neutralise any acid generated by pyrite oxidation. Hence, gold mobility in sediments was driven by thiosulphate complexes as for the in situ nuggets, albeit with lower dissolved sulphur concentrations. Despite aridification of the climate in the late Cenozoic, with resulting localised high dissolved chloride concentrations, chloride complexation did not contribute to gold mobility. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Low temperature gold dissolution and reprecipitation has been well documented with laboratory experiments and field observations around the world (Boyle, 1979; Webster and Mann, 1984; Webster, 1986; Vasconcelos and Kyle, 1991; Bowell et al., 1993; Hough et al., 2008; Usher et al., 2009; Yesares et al., 2014, 2015). These processes typically occur within the weathering environment, and can result in localised concentration of gold, leading to so-called supergene enrichment zones on pre-existing hydrothermal deposits (Mann, 1984; Stoffregen, 1986; Bowell, 1992; Gao et al., 1995; Hong and Tie, 2005; Yesares et al., 2014, 2015). The gold remobilisation can be driven by inorganic chemistry of the environment, mediated by biological processes, or a combination of these mechanisms (Mann, 1984; Webster, 1986; Vlassopoulos and Wood, 1990; Reith et al., 2007, 2012; Usher et al., 2009). Despite the abundant evidence for gold mobility in the surficial environment, the formation of gold nuggets in this environment is less ⁎ Corresponding author. E-mail address: [email protected] (D. Craw).

http://dx.doi.org/10.1016/j.oregeorev.2016.06.001 0169-1368/© 2016 Elsevier B.V. All rights reserved.

studied and more controversial (Boyle, 1979; Bowell, 1992; Hough et al., 2007, 2009; Reith et al., 2010; Kamenov et al., 2013). This controversy arises in part because there is good evidence for formation of coarse gold nuggets in some hydrothermal deposits prior to weathering (Boyle, 1979; Hough et al., 2007, 2009; Cabral et al., 2008; Kamenov et al., 2013). Nevertheless, while there is widespread observational support for at least some gold addition to such nuggets within the surficial environment, the relative proportions of hydrothermal and supergene gold in nuggets remains a point of discussion (Boyle, 1979; Hough et al., 2007, 2009; Reith et al., 2010, 2012; Kamenov et al., 2013). Similarly, the relative proportions of hydrothermal and supergene gold that constitute detrital flakes in fluvial placers is controversial (Youngson and Craw, 1995; Chapman et al., 2000, 2011; Reith et al., 2010, 2012), although these more distal gold occurrences are beyond the scope of the present study. In this study, we provide observational evidence for formation of gold nuggets solely in the near-surface environment within the supergene zone of hydrothermal gold deposits. We document the external and internal textures of these in situ nuggets, and provide some inferences on the geochemical environment in which they formed. We provide textural observations on nuggets that have been eroded into

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nearby proximal placer deposits, to document the changes that have occurred to the nuggets during the initial stages of sedimentary transport and post-depositional alteration. We then place all these processes into the context of evolving geochemical conditions that prevailed during nugget formation and sedimentary recycling as the climate changed from moist maritime warm temperate conditions to an arid inland rain shadow with localised high evaporative salinity. From these observations and inferences, we define the physical and chemical processes that have been most important for gold nugget formation through the Cenozoic tectonic and climatic evolution of the supergene environment. 2. General setting The Mesozoic Otago Schist belt of southern New Zealand hosts numerous orogenic and associated placer gold deposits (Fig. 1; Williams, 1974; Mortensen et al., 2010; Craw, 2013). The orogenic deposits formed during Cretaceous uplift and exhumation of the schist belt, and the placer deposits resulted from recycling and re-concentration of eroded gold from Cretaceous to Recent (Mortensen et al., 2010; Youngson and Craw, 1995; Youngson et al., 2006; Craw, 2010, 2013). The placer recycling process has been driven by progressive uplift and erosion as the region underwent tectonic rejuvenation through the Cenozoic (Table 1; Youngson and Craw, 1995; Craw, 2010). The region has produced N 8 Moz (~250 t) of placer gold and N4 Moz (~130 t) of orogenic gold (Williams, 1974; Moore and Doyle, 2015). Nearly all of the orogenic gold production has been from the modern Macraes mine (Fig. 1; opened 1990), which has a total gold resource of N 10 Moz (~300 t; Moore and Doyle, 2015). Apart from a short period of complete, or near-complete, marine inundation in the Oligocene, the goldfield has been exposed to non-

marine weathering and erosion with localised deposition of gold-bearing fluvial sediments since at least the middle Cretaceous (Landis et al., 2008; Craw, 2010). The inland portion of the goldfield of interest in this study (Fig. 1) has only minor preservation of pre-Pliocene sediments, and the oldest preserved placer deposits occur in remnants of Eocene quartz pebble conglomerates (Youngson et al., 2006; Craw, 2010, 2013). These sediments were variably recycled into Miocene quartz pebble conglomerates during Miocene uplift from the marine inundation as a major new plate boundary, the Alpine Fault, was initiated (Figs. 1, 2A; Table 1; Youngson et al., 2006; Craw, 2010, 2013). In particular, uplift of the Old Man Range (Fig. 1) during the Miocene (Upton et al., 2014) shed Eocene sediments and gold, combined with fresh debris from the rising range that included gold eroded from exposed orogenic vein systems (Figs. 1, 2A). Much of the goldfield was covered by a middle Miocene shallow lake that was partially dammed by the rising Old Man Range (Fig. 2A; Douglas, 1986; Craw, 2013; Upton et al., 2014). Most of the topography of the inland parts of the goldfield has resulted from Pleistocene folding of the basement and overlying sediments, exposing antiformal ridges of schist separated by sedimentary basins (Fig. 1; Bennett et al., 2005; Craw, 2013; Craw et al., 2013). Related, but subordinate, faults have locally offset basement and sediments on range margins. The Pleistocene deformation has enhanced the Old Man Range topography (Fig. 1) with antiformal folding of the main range and reverse faulting on the lower slopes. Miocene auriferous quartz pebble conglomerates have been eroded from the tops of ranges, and remnants are exposed along the margins of the basins where they have been mined historically (Fig. 1; Williams, 1974; Bennett et al., 2005). The weakly dissected schist ranges are dominated by the folded but low-relief surface of an exhumed unconformity on which the auriferous sediments were originally deposited (Craw, 1994; Bennett et al., 2005). Groundwater alteration of nonmarine sediments and underlying

Fig. 1. Hillshade image of the Otago Schist goldfield in southern New Zealand (see inset for location).

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Table 1 Summary of environmental changes during evolution of gold nuggets and associated placer deposits in the Otago Schist goldfield, from Mildenhall and Pocknall (1989); Lee et al. (2001); Pole (2003); Craw et al., 2013, and contained references. Age

Setting

Climate

Environment

Present

Rain shadow; inland basins & plateaus

Pleistocene

Rain shadow

Uplift of fold ranges Shrublands, grasslands, minor & fluvial basins forest; anthropogenic modification Uplift of fold ranges Shrublands, grasslands, minor & fluvial basins forest all fluctuate

Pliocene

Initiation of rain shadow to E of mountains

Arid; salt pans; rainfall = 300–400 mm/year Episodically periglacial, arid; loess plumes; salt pans? Cool temperate to arid; loess plumes

Vegetation

Uplifting ridges; alluvial fans & plains Rising relief; fluvial, lacustrine

Gold veins

Placer gold

Erosion of nuggets

Recycling

Enhancement & erosion of nuggets

Recycling; Au addition

Shrublands, grasslands, forest, retreating

Erosion of nuggets

Recycling; Au addition?

Forest

Enhancement of nuggets at unconformity Enhancement & erosion of nuggets

Minor recycling

Middle-Late Localised uplift & Miocene subsidence

Temperate

Early Miocene

Expanding landmass at initiation of Alpine Fault

Warm temperate, maritime

Minor relief, fluvial

Forest

Oligocene

Marine incursion

Warm temperate

Coastal and/or inundated

Sedimentary burial

Eocene

Ocean island

Warm temperate, maritime

Emergent land rare/absent Lowlands

Proximal & distal recycling; Au addition Sedimentary burial

Forest

Nugget formation at unconformity

Proximal, distal, recycled

basement occurred throughout the Cenozoic history of the goldfield (Craw, 1994; Chamberlain et al., 1999; Craw et al., 2013). The Cenozoic climate history of the goldfield was initially dominated by moist temperate maritime conditions that supported abundant forest in low relief landscapes (Table 1). Rise of mountains along the Alpine Fault plate boundary, especially in the Pliocene, caused development of a prominent rain shadow over the goldfield, culminating in an arid climate periodically punctuated by periglacial conditions (Table 1). Rainfall is currently sufficiently low to facilitate formation of surface salt deposits derived by evaporation of marine aerosols in rainwater (Fig. 2B; Table 1; Craw et al., 2013; Druzbicka et al., 2015). Salt pans are particularly common on exposed areas of clay-altered basement, which form an impermeable substrate for evaporative accumulation (Fig. 2B; Druzbicka et al., 2015). A range of evaporative minerals, dominated by halite, has formed on exposed surfaces of these clay-rich alteration zones (Fig. 2B). Associated less altered basement rocks and overlying quartz pebble conglomerates also have evaporative minerals, dominated by gypsum (from oxidation of pyrite) and calcite (Fig. 2B). 3. Methods Sampling for this study was carried out at abandoned historic orogenic and placer mine sites. Gold nuggets were extracted using handheld electronic metal-detecting equipment by hobby miners over many years, and specimens from their collections were lent for this study. Additional samples were obtained by us by traditional gold panning techniques. Gold nuggets were obtained from in situ vein systems at Blackstone Hill and Ophir (Figs. 1, 2D). Observations and samples were obtained from proximal placers in Miocene quartz pebble conglomerates that formed close to, or across, orogenic vein systems at Gilberts Gully and Butchers Dam (Fig. 1, 2A–C; Stephens et al., 2015). Part of the Gilberts Gully site has recently been designated a scientific reserve for preservation of salt pans, and our observations on salt environments were obtained from this reserve (Fig. 2B; Chapman Road; Druzbicka et al., 2015). Similarly, an area adjacent to the Butchers Dam gold placer has recently been reserved and also includes salt pans. Mineralogy and geochemical data of an in situ substratal alteration zone were obtained from drill core through basal Cenozoic sediments into the underlying altered rocks, as described by Stein et al. (2011). This drill core was extracted from the distal equivalents, in Southland (Fig. 1), of the Miocene sediments in the goldfield of this study. The basement for the drillhole is metagreywacke, rather than schist, but has similar mineralogy and chemical composition to the schist basement in the goldfield. Rock major element data on the drill core were

obtained by X-ray fluorescence using a Philips 2400 instrument at University of Otago, with international standards for calibration (Stein et al., 2011). Volatile contents, mainly water, was determined by loss on ignition by heating at 1100 °C for 1 h. Geochemical data on surface and groundwater compositions were compiled from a range of sources outlined in Craw and Nelson (2000); Craw (2000); Litchfield et al. (2002); Craw et al. (2013), and Druzbicka et al. (2015). Orogenic gold exploration data were compiled from numerous unpublished company reports, as described by Craw et al. (2015). This compilation includes fire assay results on variably mineralised rocks, with a lower cut-off of 0.1 mg/kg Au selected for clarity of presentation, and 0.5 mg/kg for some of the Macraes mine data set because of the very large number of lower values. A Zeiss scanning electron microscope (SEM) with energy dispersion analytical attachment (University of Otago Centre for Electron Microscopy) was used for examination of surface and internal textures of gold particles. Operating voltage was varied between 5 and 30 kV in order to observe gold textures at different depths through iron oxyhydroxide coatings. Internal textures of some nuggets were examined on broken surfaces, and in polished sections after etching with 50% aqua regia for ~5 min, using both light and electron microscopy. 4. Substratal alteration Eocene and Miocene quartz pebble conglomerates were sufficiently permeable to channel extensive groundwater flow, and extensive water-rock interaction reactions resulted. Silica mobility has facilitated some cementation of conglomerates with quartz, to form hard silcrete layers that persist as boulders and cobbles throughout the subsequent recycling processes. In addition, some authigenic pyrite cement has formed in many parts of the quartz pebble conglomerates. More extensively, the immediately underlying basement has been transformed to clays in a zone up to 20 m thick over the whole goldfield area (Craw, 1994; Chamberlain et al., 1999). Most of this clay-rich substratal alteration zone has since been stripped from the rising antiformal ranges, but is preserved beneath Miocene sediments in intervening basins and in Southland (Figs. 1, 2A,B). The groundwater alteration processes caused extensive leaching of some elements from the basement rocks (Fig. 3). Decomposition of albite, muscovite, and chlorite released alkalis and silica, and metamorphic calcite was dissolved and removed from the rocks (Figs. 2B, 3). The residual rocks are distinctly more hydrated, as they are dominated by kaolinite and smectite (Figs. 2B, 3). Beneath the most clay-rich alteration zone, rocks have been affected alteration reactions to a lesser

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Fig. 2. Stratigraphic and lithological setting for supergene gold nuggets and placers. A. Schematic section (not to scale) showing sites of early Cenozoic nugget formation uplifted and eroded in Miocene to Recent (left), with placers formed in Miocene and Pleistocene fluvial deposits (centre and right). B. Section through the clay-altered unconformity beneath Miocene sediments that formed proximal to in situ nugget-bearing vein systems. Mineral suites are indicated for fresh rocks (left) and present exposed outcrops with arid climate evaporation (right). C. Subrounded gold nugget from a proximal Miocene placer, Gilberts Gully. D. Angular nugget from a supergene zone on an orogenic vein, Ophir. E. Photomicrograph (incident light) of relict gold particle in an oxidised orogenic vein, Old Man Range. F. Photomicrograph (incident light) of gold particle encapsulated in fresh pyrite, Macraes mine.

extent, but some alteration effects extend to N50 m below the unconformity. In particular, variable oxidation of iron minerals has occurred in this zone over large areas (Craw, 1994). 5. Supergene gold Formation of supergene zones on Otago gold deposits resulted in three principal changes to the primary (hydrothermal) gold textures: (a) liberation of encapsulated gold as sulphide minerals were oxidised (Fig. 2E,F); (b) enrichment of gold grades in a narrow zone near the base of the oxidised zone (Figs. 2B, 4A,B, 5A,C); and (c) increase in

gold particle size to form nuggets (Figs. 2D, 4B–E, 5B). These three processes were all inter-related with the formation of the alteration zone associated with the regional unconformity. Consequently, the historic mines that were developed in orogenic gold deposits all began with relatively enriched gold grades, in the oxidised rocks exposed at the unconformity surface (Figs. 1, 4A,B, 5A,C; Craw et al., 2015). The gold content of the ore at many of the historic mines is not known with precision, but typically exceeded 30 mg/kg (~ 1 oz/t; Fig. 4A; Williamson, 1939; Williams, 1974). These sorts of elevated grades are widespread in oxidised samples taken from the surface workings during more modern exploration programmes (Figs. 4A, 5A). Visible

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Weight % 0

1

2

Weight %

3

4

5

0

0

10

20

30

40

50

60

0 K2O

2

4

6

CaO

8

Clay-altered rock K2O

10

12

14

Transition to fresh rock

CaO Na2O

Depth below unconformity (metres)

Depth below unconformity (metres)

Clay-altered rock 2 Al2O3

4

6

LOI

SiO2

Clayaltered rock

8

10

12

14

Transition to fresh rock

Fig. 3. X-ray fluorescence analyses, with associated loss on ignition data, of rocks down a drillhole through the basement alteration zone beneath Miocene sediments (data from Stein et al., 2011).

gold is common in these oxidised rocks, and was probably the norm during historic mining. Historic gold extraction typically involved mercury amalgamation of this visible gold after coarse crushing. Gold nuggets obtained for this study were extracted either directly from the oxidised rocks in the mineralised vein systems or from lithic colluvium immediately adjacent to the vein systems (Fig. 4C–E), and these are

probably representative of at least some of the gold that the historic miners were targeting. Historic mining ceased once the unoxidised rocks were reached at the base of the unconformity-related alteration zone (Park, 1908). Below this level, the gold contents of the rocks decreased markedly, and the gold became difficult to extract because it is finely particulate

Fig. 4. Nugget formation in the supergene zone beneath the regional basement unconformity. Localities are indicated in Fig. 1. A. Compilation of historic and modern gold assay data for historic sites at Barewood, Nenthorn, and Oturehua, and modern Macraes mine. B. Reconstructed supergene alteration profile. C. Gold nugget intergrown with prismatic quartz and minor iron oxyhydroxide, from Blackstone Hill. D. Gold nugget from colluvium near a supergene-altered vein system, Ophir. E. Gold nuggets from colluvium at Ophir (centre; as in D), compared to nuggets derived from nearby Miocene placers (left and right).

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Fig. 5. Contrasting supergene and hydrothermal gold in two Otago orogenic deposits. A. Bar graph of gold grades in 1 kg samples of oxidised and unoxidised rock in Oturehua vein swarm, compared to historic production from the supergene zone. B. Image of nugget with intergrown gold crystals from the supergene zone of veins at Blackstone Hill, along strike from Oturehua. C. Bar graph of a representative selection of gold grades of 1 kg samples of sulphidic ore from Macraes mine, compared to historic production from the supergene zone. Note the logarithmic scale, necessary to display the large number of low grade sulphidic samples.

(commonly micron scale; Fig. 2E,F) and encapsulated in sulphides (pyrite and arsenopyrite) that form 5–10% of the mineralised rocks. Hence, few historic mines extended deeper than ~50 m (Park, 1908; Williams, 1974; Craw et al., 2015). This decrease in gold contents with depth has been confirmed by drilling beneath several of the historic workings during modern exploration programmes (Fig. 4A). Likewise, the change from relatively coarse grained visible gold near the surface to microparticulate encapsulated gold at depth, in unoxidised rocks, has been confirmed by exploration drilling. These changes in gold textures with depth are summarised in Fig. 2C–E. One instructive example of the above-described localisation of high gold grades in the supergene zone of historic mines is displayed by the vein system exposed at Oturehua and its along-strike extension on Blackstone Hill (Figs. 1; 5A; Craw et al., 2015). Historic mining of the oxidised rocks at Oturehua produced ore with bulk Au contents N40 mg/kg (Fig. 5A), and mining ceased when sulphidic rocks were encountered below the oxidised zone. The miners left behind oxidised rocks, with some visible gold (mm scale), that have Au grades up to 60 mg/kg (Fig. 5A). In contrast, modern exploration drillhole data shows that the sulphidic rocks at depth have distinctly lower Au grades

and finer gold particles encapsulated in sulphides (Fig. 5A). The Blackstone Hill extension of this vein system was not mined as extensively because of remoteness, and more of the coarse gold remains in the oxidised rocks, including some centimetre scale nuggets (Fig. 5B; Craw et al., 2015). Unoxidised sulphidic rocks at the Blackstone Hill localities have low Au grades and the gold is fine and encapsulated in the sulphide minerals. The only modern mine in an orogenic hydrothermal system, at Macraes (Fig. 1) also demonstrates the change in grade and particle size of gold with depth through the oxidised unconformity zone (Figs. 4A; 5C). Historic mining focussed on relatively high grade oxidised rocks (bulk 5–10 mg/kg; Fig. 4C; Williamson, 1939), and gold was extracted using mercury amalgamation until sulphidic rocks were reached. Mining was abandoned below the base of the oxidation zone because the gold grades were distinctly lower than in the oxidised rocks, and the encapsulated gold was not amenable to extraction with mercury amalgamation. At the modern mine, small volumes of this relatively high grade oxidised rock containing free gold particles are stockpiled and processed separately by direct cyanidation. However, the modern mine mainly processes very low grade sulphidic ore (Figs. 2E, 5C; Au b 1.5 mg/kg), and even the highest grade sulphidic samples from the mine have Au typically b4 mg/kg with few higher grade samples (Fig. 5C). Visible gold (up to 100 μm) is rare, and no attempts are made to save this component in the processing system that involves extraction of micron-scale particles encapsulated in sulphides by pressure oxidation and cyanidation (Craw, 2003). After N 4 Moz of production from as deep as 500 m below the oxidation zone, no discarded coarse particulate gold has been encountered during monitoring of the mine tailings. The above observations demonstrate that supergene gold liberation from sulphides, and subsequent enrichment into a narrower zone, occurred in the oxidation zone beneath the regional unconformity (Figs. 2A,B; 5A,B). This supergene mobilisation of gold locally produced coarse, nuggety, gold in the oxidised zone. The supergene gold mostly occurs in close association with iron oxyhydroxide, and is commonly accompanied by scorodite and clay minerals (Fig. 2B). Some visible gold persists in close association with relict sulphides, where oxidation of the sulphides has been incomplete near to the oxidation front (Craw et al., 2015). Massive blocks of vein quartz within the oxidation zone or at its base contain partially oxidised sulphide grains, and irregular gold particles occur within the sulphide pseudomorphs (Fig. 2E) and in immediately adjacent partially oxidised rock. Some remobilised supergene gold has overgrown sulphide pseudomorphs, locally with remnants of primary sulphides. Supergene gold has significant Ag contents (up to 8 wt%), similar to the primary gold that typically contains 5– 10 wt% Ag (Craw et al., 2015). Copper levels are below microprobe detection limits in both hydrothermal and supergene gold. Mercury levels are highly variable in both hydrothermal and supergene gold (MacKenzie and Craw, 2005), although historic use of mercury amalgamation confounds detailed evaluation of Hg at many sites. 6. Nuggets in situ Gold nuggets occur in situ in the supergene zone of hydrothermal quartz veins (Fig. 4B–E), within the quartz itself, in irregular masses of iron oxyhydroxide that have replaced sulphide minerals, and in adjacent fractures within the quartz. Some nuggets occur in fractures in the immediate host rocks, up to 1 m from the veins. In addition, nuggets that have been liberated from the quartz veins during erosion are disseminated through adjacent colluvium, especially near to the base of the colluvium close to the bedrock surface. Downslope creep has transported some nuggets in colluvium, resulting in minor rounding of the nuggets (Fig. 4E). The gold nuggets have complex and convoluted shapes, commonly with delicate protrusions, and some are intimately intergrown with prismatic vein quartz crystals (Figs. 2C,D; 4C,D). Within these complex

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Fig. 6. A-D. SEM backscatter images (at indicated voltages) of the surface of an in situ gold nugget from Blackstone Hill (as in Fig. 5B). Dark patches are iron oxyhydroxide, which contains microparticulate gold, as further revealed at higher voltages. E. Polished section through part of the same nugget, etched with aqua regia to show internal grain structure.

gold masses, smaller crystal outlines are common (Fig. 6A), many with several octahedra or dodecahedra locally intergrown. The nugget surfaces are partially coated with iron oxyhydroxide, especially in crevices (Fig. 6A–D). The iron oxyhydroxide coatings commonly contain microparticulate gold (Fig. 6B–D). In detail, nugget surfaces are covered in delicate gold plates and additional microparticulate gold (Fig. 6C,D), and these plates commonly merge to form elongate forms that are interfingered, and also protrude into and over cavities. The internal structure of nuggets consists of coarse irregular anhedral grains, with

the exterior plates forming finer grained margins (Fig. 6E). The nugget in Fig. 6 has a uniform Ag content of 2.7 wt% across both coarse grained core and finer plated margins. Gold-rich iron oxyhydroxide in the supergene zone is pervaded by fine delicate and irregular gold seams and veinlets (Fig. 7A–C). This fine gold locally coalesces to form coarser patches, and networks of these patches constitute nuggets which are 25–50% gold on the millimetre scale (Fig. 7A). Some gold patches have grown into elongated plates with linear ribs (Figs. 7D, 8A) that are similar in scale and

Fig. 7. A-D. SEM backscatter images (at indicated voltages) of interior broken surfaces of an in situ gold nugget with abundant intergrown iron oxyhydroxide (black) from Blackstone Hill. Fine-scale veinlets of gold in iron oxyhydroxide in B are revealed at higher voltage in C. D shows internal interfingered elongate gold plates.

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Fig. 8. SEM backscatter images (at 15 kV) of interior broken surfaces of an in situ gold nugget with abundant intergrown iron oxyhydroxide (black) from Blackstone Hill. A, Gold plates are irregularly stacked on each other (left and right), with a zone of more regular stacking of plates (centre). B–D. Layered and stepped crystal forms, shown at three different scales.

geometry to the exterior plates on other nuggets. Plates are generally irregularly stacked on each other, but locally are more ordered to develop crystal forms (Fig. 8A–D). Some of the internal crystal forms consist of stacked sub-crystals, which are locally pseudo-hexagonal in shape (Fig. 8B–D). All the gold in Figs. 7 and 8 has 3 wt% Ag. 7. Physically recycled gold Placer gold has been recycled into Miocene quartz pebble conglomerates over most of the Otago Schist goldfield (Fig. 1; Youngson and Craw, 1995; Youngson et al., 2006). Most of this gold is highly flattened and flaky, reflecting long distance transport and associated reworking (Youngson and Craw, 1995). The hosting quartz pebble conglomerates contain clasts of rounded quartz recycled from older (Eocene) deposits, and angular to subrounded first-cycle quartz pebbles. The latter clasts were mostly derived from metamorphic veins in the basement schist that was decomposed by the sub-stratal alteration. Likewise, abundant kaolinite in the matrix of the Miocene conglomerates, and in associated fine grained sediments, has been derived from the pre-existing (Eocene) clay-rich alteration zone (Fig. 2A). Eocene pollen has been recycled into some Miocene deposits (Mildenhall and Pocknall, 1989). In contrast to the widespread flaky recycled gold, some gold nuggets and smaller angular particles occur in Miocene placers where the hosting conglomerates have been formed on or close to hydrothermal vein systems (Fig. 2A,B). These more proximal Miocene placers are most prominent near the base of the Old Man Range, which was undergoing uplift at the time of placer formation (Figs. 1; 2A). Cobbles in basal conglomerate layers at these sites include subrounded hydrothermal quartz, some of which contain visible gold particles, attesting to the proximity of vein source(s) within the eroded basement. The gold-bearing conglomerates rest on schist basement that has since been further clay-altered and variably oxidised (Fig. 2A,B). Outcrops of the goldbearing unconformity are almost invariably coated with evaporative salts to some extent (Fig. 2B; Druzbicka et al., 2015). The gold particles in the Miocene proximal placers are relatively large, commonly irregularly-shaped, and angular, albeit with some rounded surfaces (Figs. 4E, 9A–E, 10A,B). Remnants of crystal shapes

are still preserved on the exterior of some nuggets, especially in surface re-entrants and cavities that have been protected from abrasion (Fig. 9A–D). Cavities and crevices have been filled with quartz sand during or after deposition (Fig. 9B,D), and some surfaces have been coated by post-depositional iron oxyhydroxide (Fig. 10B). Internally, the original grain structure is largely preserved, with straight grain boundaries that have suffered negligible deformation (Fig. 9E). In detail, recycled particle surfaces are smooth, undulating, and locally pitted (Figs. 9A–D; 10C,D). Anhedral grain boundaries and other discontinuities in the gold have been preferentially etched by dissolution of the particle surfaces, to expose the internal grain structures (Fig. 9C,D). Thin gold overgrowths occur on the smoothed outsides of the particles (Fig. 9E), similar to overgrowths on in situ nuggets (Fig. 6E) and these may have been inherited from that stage of formation. In addition, some more delicate and finer grained, locally crystalline, overgrowths protrude from surfaces and infill dissolution pits (Fig. 10D–F). Small (micron scale) semi-spherical overgrowing grains have coalesced in places to form incipient plates, some of which are elongated with linear ribs (Fig. 10C–F). Crystalline gold forms include hexagonal plates and dodecahedra (Fig. 10E,F). This gold has low Ag contents (0–3 wt%), although only qualitative analysis is possible because of the small particle size and the presence of Ag-bearing gold in the background. On-going uplift has caused recycling of coarse gold from Miocene placers into nearby (b1 km) Pleistocene channels (Fig. 2A; Stephens et al., 2015). The Pleistocene channel sediments are dominated by angular and subrounded immature schist clasts, with only minor remnants of quartz pebbles from the Miocene deposits. At least some of the Pleistocene channel debris is colluvium and/or debris flow deposits. However, some water-washing has apparently occurred, as gold particles are commonly more rounded and flattened than in the nearby Miocene deposits (Fig. 11A). Some surfaces of the particles show the same evidence for etching, dissolution, and pitting as in the Miocene deposits. However, the abraded and etched surfaces have commonly been coated with micron-scale gold plates (Fig. 11B,C). These plates are mainly made up of amorphous and vermiform secondary gold which has locally coalesced into elongate plates (Fig. 11B). Some of the plates consist of

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Fig. 9. Surface and internal textures of a nugget in colluvium eroded from a proximal Miocene placer at Ophir. A–D. Stereomicroscopic views of the surface. Remnant gold crystal outlines, rounded during transport, are visible in A (centre), B (in cavity) and some are enlarged in C and D. White debris in crevices is remnant quartz sand from the Miocene sediments. E. Etched (aqua regia) polished section through the nugget, sliced at right of A, showing internal grain structure (boundaries indicated with white arrows) and thin overgrowths (indicated with black arrows).

Fig. 10. SEM backscatter images (at 15 kV) of exterior surfaces of nuggets (as in A, B) from a proximal Miocene placer deposit, Butchers Dam (Fig. 1). C. Close view of nugget surface, showing naturally etched surface with enhanced dissolution that displays internal grain boundaries and discontinuities (arrowed, lower right). D. Close view of naturally dissolved internal grain boundaries, with a strongly etched grain triple junction (arrowed). Etched surface has been subsequently coated by gold plates, some of which are elongated (centre and left). E. Microparticulate gold, gold plates, and a hexagonal crystal coat the etched particle surface. F. Close view of added microparticulate gold and gold crystals, locally coalescing to form elongated gold plates (upper right).

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Fig. 11. SEM backscatter images (at 15 kV) of exterior surface of gold particles from a Pleistocene channel near to Butchers Dam (Fig. 1). A. Equant but abraded detrital particle. B. Vermiform secondary gold coalesces to form plates, some of which are elongate, on a variably etched particle surface. C. Plates have straight edges reflecting microcrystalline gold makeup. D, E. Stepped crystalline forms, with some intergrown and overlain elongate gold plates (e.g., bottom of D).

microcrystalline gold that results in straight crystal boundaries and intergrowths in the plates (Fig. 11C). More rarely, stacked crystalline plates are intergrown with, or overgrown by, elongate plates (Fig. 11D,E). These stacked crystals strongly resemble those seen in in situ nuggets (Fig. 8B–D). 8. Water compositions 8.1. Modern water analogues The textural observations presented above show that gold has been dissolved and reprecipitated in the supergene zone to form nuggets, and again after recycling into placers. Uplift and erosion since the nuggets were formed, and then recycled, has changed the hydrogeological settings in which this gold mobility occurred. However, it is still possible to obtain some insights into the geochemical environment of gold mobilisation by examining modern water compositions in the same rock types and general settings. The in situ nugget formation, in particular, occurred while groundwater was extensively chemically interacting with schist basement, in environments where sulphide minerals were being oxidised. To investigate this, we use groundwater data from a range of modern environments relevant to these processes to deduce some general chemical conditions under which nugget formation occurred. There has also been substantial change in climate since initial supergene gold mobilisation, and aridification has led to localised elevated salinity of near-surface waters. Hence, we also use data from a range of climatic settings to evaluate potential effects of climate-driven salination on gold mobilisation and reprecipitation. To address the potential salinity effects, we use groundwater data from schist basement and schist-dominated gravels aquifers in coastal and inland sites, and saline water compositions measured and reconstructed at inland sites (Litchfield et al., 2002; Craw et al., 2013;

Druzbicka et al., 2015). To examine the chemical effects of water-rock interaction involving sulphide-bearing schist basement, we use an extensive groundwater database obtained around the modern Macraes mine (Fig. 1; Craw, 2000; Craw and Nelson, 2000). This database includes groundwater sites that are in schist basement peripheral to the mine operation and were selected to represent typical background water compositions. A set of data is included for shallow groundwater moving through unmineralised schist debris, with minor pyrite, that was excavated from above the ore zone. These data sets are compared to waters that have interacted with sulphide-bearing ore zones and associated variably mineralised schist, which were exposed in open pit mining operations. These latter waters in particular provide an analogue for waters responsible for supergene gold mobilisation processes through geological time. 8.2. Pyrite and calcite dissolution All schist basement rocks contain pyrite (b 1%) and calcite (2–5%) as metamorphic minerals (Fig. 2B). In addition, both these minerals have been remobilised by groundwater below the water table, and occur on joint surfaces. These two minerals are generally the most reactive in the groundwater system and tend to dominate the resultant water compositions (Craw, 2000; Craw et al., 2013). While oxidation of pyrite can cause localised acidification, this is rapidly neutralised by calcite dissolution (Craw, 2000). The resultant pH of most groundwaters is nearneutral to weakly alkaline (Fig. 12A,B). Pyrite dissolution during oxidation ultimately contributes dissolved sulphate to most near-surface waters (Fig. 12A,B). Oxidation of pyrite and arsenopyrite in mineralised rocks results in strongly elevated dissolved sulphate concentrations, although the pH remains neutral to alkaline (Fig. 12B). Dissolution of calcite results in loss of Ca from the rocks (Fig. 3), and elevated dissolved Ca2 + and alkalinity in waters (Fig. 12C,D). Most

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groundwaters have compositions along a trend with molar Ca2+ to alkalinity ratio of 1:2 (Fig. 11C). This ratio reflects dissolution of calcite facilitated by atmospheric carbon dioxide (carbonic acid; Stumm and Morgan, 1995): CaCO3 þ CO2 þ H2 O ¼ Ca2þ þ 2HCO3 −

ð1Þ

In contrast, calcite dissolution that accompanies pyrite oxidation in mineralised rocks results in a data trend that leads to a molar Ca2+ to alkalinity ratio of 1:1 (Fig. 11D). This reflects calcite dissolution driven by the sulphuric acid generated by the pyrite oxidation, with associated hydrogen ions (Stumm and Morgan, 1995; Craw, 2000): CaCO3 þ Hþ ¼ Ca2þ þ HCO3 −

ð2Þ

Both reactions neutralise all carbonic and sulphuric acids to maintain the high pH. 8.3. Salination influence Marine aerosols in rainwater also contribute dissolved sulphate, albeit at modest levels in most coastal and inland groundwaters (Fig. 12A). Evaporative saline sites have relatively high dissolved sulphate, and also typically maintain near-neutral to alkaline pH (Fig. 12A). The molar Ca2+ to alkalinity ratio of waters associated with highly saline sites is distinctly different from those involving calcite dissolution from the schist basement, reflecting seawater values with only minor basement schist interaction (Fig. 12C). The seawater influence is most apparent with Na+ and Cl− relationships, especially for coastal groundwaters and saline sites (Fig. 13A,B). Similarly, Mg2+ and K+ relationships for many groundwaters reflect the contributions of marine aerosols (Fig. 13C,D). Despite the common seawater influence to water compositions, chemical interactions between groundwaters and schist cause deviations in dissolved elemental relationships from seawater trends. Inland

groundwaters have Na+ contents greater than expected for marine aerosol input, and this reflects hydrolysis of albite in the host rocks (Figs. 2B, 3; Craw and Nelson, 2000; Craw et al., 2013). Mobilisation of K from alteration of muscovite (Figs. 2B, 3) is less apparent but contributes to deviations from the seawater trends (Fig. 13C,D). Deviations from seawater trends for sodium is most apparent in the waters that have interacted with mineralised rocks and oxidised the sulphides, and this interaction has been sufficient to completely obscure and overprint the presence of marine aerosols in nearby waters (Fig. 13B). Similarly, many of the waters that interacted with mineralised rocks have relatively high Mg2+ compared to nearby marine aerosol-affected waters (Fig. 13D). Dissolution of chlorite from the schist host is the most likely reason for this deviation (Craw, 2000). The large basement-hosted groundwater reservoir affected the chemistry of waters in overlying Miocene quartz pebble conglomerates, especially with respect to the neutral to alkaline pH (pH 7–8; Barker et al., 2004). Sulphate from marine aerosols in rainwater was reduced to HS−, particularly around organic debris and lignite beds, and this was combined with Fe2 + from alteration of chlorite in the basement to form authigenic pyrite cement (Fig. 2B; Craw, 1994; Youngson, 1995). Uplift and dissection of these sediments causes reversal of this process, with oxidation of pyrite contributing additional sulphate to the groundwater system while maintaining high pH as in the schist basement. Evaporative concentration of groundwater emanations from these rocks forms sulphate mineral encrustations (Fig. 2B; Youngson, 1995). 9. Discussion 9.1. Distinguishing hydrothermal from supergene gold The strongest indicator of supergene gold at a regional scale is the occurrence of a gold enrichment zone within, and near the base of, a zone of extensive groundwater-driven alteration and oxidation (Figs. 2, 4; Yesares et al., 2014, 2015; Craw et al., 2015). This enrichment in Otago was approximately ten-fold compared to the original

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hydrothermal deposits, implying mobilisation of most of the gold from up to 100 m of overlying altered rock (Figs. 2–4). Associated with this supergene enrichment, gold particle size in Otago has typically increased, locally forming centimetre scale nuggets. Coarse gold particles (N0.1 mm) are very rare in the underlying unaltered hydrothermal deposits in Otago and nuggets have never been found in the unaltered hydrothermal deposits. Similar substratal supergene enrichment of gold has occurred in Spain, albeit without formation of nuggets (Yesares et al., 2014, 2015). In contrast, extensive (locally N200 m deep) unconformity-related oxidation of gold-bearing sulphides in the Yukon goldfield of Canada has not caused significant supergene gold enrichment, although some localised, micron scale, liberation of gold from sulphides has occurred (e.g., MacKenzie et al., 2015). It may be this lack of confounding supergene gold mobilisation in the Yukon goldfield that has enabled linkage of detrital gold with hydrothermal sources (Chapman et al., 2010, 2011). Similarly, removal of any supergene alteration zone by regional glaciation, as in northern Europe, may permit more ready identification of hydrothermal features in gold deposits and detrital particles derived from them (e.g., Chapman et al., 2000). At the scale of individual gold particles, there is considerable overlap between hydrothermal and supergene gold textures. Detrital gold particles with hydrothermal origins can contain inclusions of sulphide minerals associated with initial emplacement (Chapman et al., 2000, 2010, 2011), and supergene gold deposited near to the redox front in Otago has locally overgrown relict sulphide minerals, although typically with abundant iron oxyhydroxide. Distinctive metals such as Ag, Cu and Hg have been useful for linking detrital gold to hydrothermal sources elsewhere (Chapman et al., 2000, 2010, 2011). However, Ag is the most consistent minor metal in Otago gold and this Ag was mobilised with the Au during supergene enrichment. Internal grain textures of supergene nuggets in Otago (e.g., Figs. 6E, 9E) superficially resemble hydrothermal textures (Hough et al., 2007, 2009), although the supergene textures are primarily a result of impinging crystals during growth (Figs. 5B, 9A–D)

rather than hypogene annealing as in some hydrothermal gold. Our observations (above) show that these supergene crystalline textures survive some transport in proximal fluvial systems (e.g., Fig. 11). 9.2. Water compositions and gold solubility High pH is a robust feature of all the waters outlined above, and this is maintained by the large calcite reservoir in the basement schist (Fig. 12C,D). Apart from highly localised saline sites, sulphate is the principal anion in the waters, derived from marine aerosols and from oxidation of pyrite (Fig. 12A,B). These chemical compositions imply that sulphur complexation is the most likely mechanism for gold remobilisation in the near-surface environments described above (Fig. 14A; Webster, 1986). Even where chloride concentrations do become elevated, at saline sites, the pH remains sufficiently high to preclude significant development of gold-chloride complexes, which require acid conditions (Fig. 14A; Mann, 1984; Usher et al., 2009). Oxidation of pyrite to produce sulphate ions commonly involves temporary formation of intermediate metastable thiosulphate ions (S2O2− 3 ; Goldhaber, 1983; Rimstidt and Vaughan, 2003; Kleinjan et al., 2005; Melashvili et al., 2015). The process involves several steps that reflect progressive increases in the oxidation states of the sulphur (Rimstidt and Vaughan, 2003; Melashvili et al., 2015), including: FeS2 þ 1:5O2 þ H2 O ¼ 2FeIII OOH þ 4S 4S þ 4OH− ¼ 2HS− þ S2 O3 2 2HS− þ 2O2 ¼ S2 O3 2

−

−

þ H2 O

þ H2 O

ð3Þ ð4Þ ð5Þ

Under acid conditions, the thiosulphate ions immediately decompose during interaction with dissolved Fe3 + derived from the iron oxyhydroxide (Rimstidt and Vaughan, 2003). However, thiosulphate

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migration of dissolved gold as bisulphide complexes may have occurred below the water table, where sulphide minerals were more stable. These bisulphide complexes are readily destabilised by minor changes in either pH or redox state, and this may have led to deposition of gold with sulphide minerals at the base of the supergene zone. Likewise, any downward-migrating Au-thiosulphate complexes would also have been destabilised when the ligands decomposed via reversal of Eqs. (4) and (5) below the water table, and more gold may have precipitated with sulphides from these reactions (Fig. 14A,B). All of the above processes occur via inorganic reactions that have been verified under laboratory conditions and rates (Webster, 1986; Rimstidt and Vaughan, 2003; Melashvili et al., 2015). In addition, it is possible that these or similar reactions may have been mediated, or rate-enhanced, by microbiological processes (Chen et al., 2014; Dockrey et al., 2014; Corkhill and Vaughan, 2009). Most such microbiological activity occurs under acidic conditions (Chen et al., 2014), but Dockrey et al. (2014) suggest that acidic microenvironments on oxidising sulphide mineral surfaces can host appropriate bacteria in an overall circumneutral pH environment, such as the nugget-forming sites described herein.

9.3. Nugget recycling and climate evolution

Fig. 14. Summary of nugget formation processes and associated gold mobility. A. Generalised gold solubility diagram (compiled from Webster, 1986; Usher et al., 2009; Heinrich, 2015) showing the observed pH range and the most likely ligands responsible for gold dissolution and reprecipitation (see text). B. Sketch cross section to show the principal physical and chemical processes that have operated to form and recycle gold nuggets.

ions are more long-lived under alkaline conditions, although they oxi2− dise progressively to S3O2− and ultimately SO2− (Rimstidt and 6 , SO3 4 Vaughan, 2003; Melashvili et al., 2015). Formation of thiosulphate ions during oxidation of pyrite under alkaline conditions is important for gold solubility as thiosulphate complexes (Fig. 14A; Webster, 1986). Gold-thiosulphate complexation is being widely investigated for industrial gold extraction, as an alternative to cyanide (Grosse et al., 2003; Senanyake, 2007; Melashvili et al., 2015). Given the robust neutral to alkaline pH predicted for the sites of nugget formation in this study, and the associated abundant pyrite oxidation in the supergene zone, we suggest that gold-thiosulphate complexes were the principal mode of gold mobility and concentration in those settings (Fig. 14A,B). Hence, nugget formation was apparently driven by initial dissolution of microparticulate gold from within oxidising pyrite grains, followed by reprecipitation of the mobilised gold as the thiosulphate ions were further oxidised (Fig. 14A,B; Webster, 1986; Craw et al., 2015). There was probably a contribution of thiosulphate ions from oxidation of arsenopyrite (Corkhill and Vaughan, 2009), which also contains microparticulate gold in the Otago Schist goldfield. Gold is also soluble as bisulphide complexes, Au(HS)− 2 (Webster, 1986; Heinrich, 2015), and these may have formed as part of the pyrite oxidation process, following on from Eq. (4) (Fig. 14A,B). Like the thiosulphate ligands, the bisulphide ligands are only temporarily present in the pyrite-oxidising environment (Eq. (5)). However, bisulphide ions are more abundant immediately below the water table, where weakly reducing conditions prevailed (Heinrich, 2015). Hence,

There has been dissolution and reprecipitation of gold within the Miocene sediments after nugget recycling (Figs. 9, 10), and similar processes have occurred after Pleistocene recycling (Fig. 11). Surfaces have been etched, locally deeply, and there has been deposition of microparticulate gold, gold plates, and gold crystals (Figs. 10, 11). These processes have operated mainly at the micron scale on the surfaces of the recycled nuggets (Figs. 10, 11). Consequently, there appears to have been little or no change in masses of particle, although the shapes have been modified by physical transport and post-transport chemical processes (Figs. 2E, 9–11). The erosion and recycling processes removed the nuggets from their original sulphur-rich environment and redeposited them in sediments with relatively minor pyrite contents (Figs. 2A,B; 14B). This is the equivalent to transferring the nuggets from the most concentrated waters to the background waters in Figs. 12B,D and 13B,D. Further, the climate aridification that accompanied the development of the rain shadow (Table 1) facilitated localised increases in groundwater salinity (Figs. 12A, 13A). Nevertheless, the pH remained high throughout these processes and it is unlikely that gold solubility was facilitated by chloride complexes even in the most saline sites, and sulphur complexes probably continued as the principal agent of gold mobility despite the changes to climate and the environments of the gold (Fig. 14A). High groundwater salinity in arid environments has been invoked as a driver for separation of Au and Ag during supergene gold mobilisation, leading to redeposition of low-Ag gold (Mann, 1984; Webster and Mann, 1984; Usher et al., 2009; Yesares et al., 2015). In contrast, moist tropical environments facilitate parallel Au and Ag mobility as thiosulphate complexes, with associated reprecipitation of Ag-bearing supergene gold (Webster and Mann, 1984; Webster, 1986), although some segregation of Au and Ag can occur (Bowell, 1992; Bowell et al., 1993). In the Otago Schist goldfield, nuggets contain similar Ag contents to the primary hydrothermal gold, supporting the inferences above that thiosulphate complexation was the dominant mobilisation mechanism (Fig. 14A,B). These nuggets initially formed while the area had temperate moist maritime climate, before the onset of aridification (Table 1). Microparticulate gold and gold plates on the outside of the in situ nuggets (Fig. 6) may reflect later additions, perhaps at the same times as similar additions to the recycled nuggets (Figs. 10, 11) that occurred during the changes in climate in the late Cenozoic. These additions to the nuggets can contain significant Ag (up to ~3 wt%) and the gold mobility that caused them presumably had the same mechanisms as the rest of the nuggets.

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10. Conclusions Supergene gold nuggets (cm scale) have grown within hydrothermal gold-bearing vein systems in the Otago Schist goldfield, principally in the oxidised zone in close association with iron oxyhydroxides. Some coarse grained gold has also formed with primary pyrite and arsenopyrite near the base of the oxidised supergene zone. The supergene zone initially formed during intense groundwater-driven water-rock interaction near (b50 m) to the basement unconformity beneath Eocene fluvial quartz pebble conglomerates, and similar nugget formation processes may have continued through the late Cenozoic. The primary hydrothermal gold is microparticulate and encapsulated in pyrite and arsenopyrite, and this gold was liberated, mobilised, and concentrated by supergene oxidation. Historic mining targeted this high grade supergene nuggety gold, and these mines were abandoned when low grade refractory sulphidic rock was encountered. In contrast, the modern Macraes mine is developed mainly in the deeper sulphidic rocks, and extracts microparticulate refractory gold but not nuggety gold. Nuggets that are still in situ in oxidised vein systems are commonly coarsely crystalline, although internal structures also include coarse anhedral grains. Crystalline forms include pseudohexagonal plates, octahedra and dodecahedra. In addition, elongate gold plates occur within nuggets and on their exterior surfaces, along with microparticulate gold intergrown with iron oxyhydroxide. Nugget gold typically contains ~ 3 wt% Ag. Nuggets occur within quartz veins where they are intergrown with prismatic quartz, iron oxyhydroxide pseudomorphs after sulphides, and along fractures in quartz veins and adjacent host rocks. Some nuggets were eroded and recycled in the Miocene into nearby fluvial quartz pebble conglomerates, where they form components of placer deposits on the underlying unconformity. Further groundwater-driven alteration has affected the Miocene sediments and underlying basement, and this has facilitated further gold dissolution and reprecipitation, although this has occurred mainly at the micron scale. Likewise, further similar alteration has affected gold that was recycled into nearby Pleistocene fluvial channels. There has been physical deformation of the recycled gold particles, but only minor changes in mass from chemical processes. The host schist basement contains sufficient metamorphic calcite to maintain groundwater pH to near-neutral or alkaline levels despite any localised acidification from oxidation of pyrite. Hence, all waters in the modern goldfield have high pH and moderate to high sulphate concentrations, and these water compositional characteristics have persisted throughout the nonmarine history of the goldfield despite any marine influences associated with the maritime setting. Gold mobility in the supergene zone was facilitated by thiosulphate complexation that occurred temporarily during oxidation of sulphides. Additional gold mobility may have occurred with bisulphide complexation. However, pH was too high for chloride complexation of gold at any time in the goldfield's geological history. Even when aridification of the climate occurred in the late Cenozoic, and evaporative salt concentrations developed with localised halite deposition, the redox chemistry of sulphur complexation remained the dominant control on gold mobility and reprecipitation. Acknowledgements This research was supported financially by Marsden Fund (Royal Society of New Zealand; UOO1313), NZ Ministry for Business Innovation and Employment, and the University of Otago. The SEM images were produced at the University of Otago Centre for Electron Microscopy. Generous access to the Oceana Gold Ltd. groundwater database helped our understanding of water chemical evolution in basement rocks. Numerous discussions with Noel Becker, Jim Mortensen, Donna Falconer, Peter Grieve, Mark Hesson, and John Youngson were helpful. The enthusiastic gold-finding activities of Peter Grieve, Mark Hesson, Bill Hinchey

and Shaun McLellan were invaluable in providing material for study. Sam Stephens provided excellent background geology, and collected and photographed Fig. 2E. A sceptical but positive and helpful review by Rob Chapman substantially improved the presentation. References Barker, S.L.L., Kim, J.P., Craw, D., Frew, R.D., Hunter, K.A., 2004. Processes affecting the chemical composition of Blue Lake, an alluvial gold-mine pit lake in New Zealand. Mar. Freshw. Res. 55, 201–211. Bennett, E.R., Youngson, J.H., Jackson, J.A., Norris, R.J., Raisbeck, G.M., Yiou, F., Fielding, E., 2005. Growth of South Rough Ridge, Central Otago, New Zealand: using in situ cosmogenic isotopes and geomorphology to study an active blind reverse fault. J. Geophys. Res. 110, B020404. http://dx.doi.org/10.1029/2004JB003184. Bowell, R.J., 1992. Supergene gold mineralogy at Ashanti, Ghana: implications for the supergene behaviour of gold. Mineral. Mag. 56, 545–560. Bowell, R.J., Foster, R.P., Gize, A.P., 1993. 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