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Chapter 11.1 Mineral Deposits Associated with Stromatolites
11. MINERALIZATION ASSOCIATED WITH STROMATOLITES
Chapter 11.1
MINERAL DEPOSITS ASSOCIATED WITH STROMATOLITES Felix Mendekohn
INTRODUCTION
Many mineral deposits are closely associated with stromatolites which occur either as bioherms (reefs), or biostromes that formed as widespread stromatolitic beds or algal mats. It is likely that there are many more than the ones dealt with here, but these are sufficiently numerous and widespread to suggest that they give a fair sampling of the class. The information available on different deposits shows a wide range in quality and quantity, probably reflecting the knowledge of and interest in the stromatolites themselves, the degree of preservation, and the effects of metamorphism. The term reef has been rather loosely used in many discussions of ore deposits to indicate any mound-, ridge-, or reef-like organic structure; since it is not always possible to be certain of their precise meaning, “reef” is used as the authors use it, and should be understood to include bioherms as well as reefs in the restricted sense of protruding wave-resistant structures. In the descriptions that follow, deposits are grouped according to the valuable element(s) they contain; a summary is presented in Table I. COPPER
The association between stromatolites and ore deposits is well demonstrated and documented in the copper province of Zambia/Zaire. On the Zambian Copperbelt the two main groups of ore deposits are those in argillite (ore shale) and those in quartzite. Bioherms present in many of the ore-shale deposits were not recognized as stromatolitic rocks for a long time because the moderate grade of metamorphism, local shearing along the stromatolite zone, and supergene alteration t o talcose cherty dolomite resulted in the partial destruction of the internal structure. However, the intersection of a number of well-preserved bodies at depth leaves little doubt that all or most are stromatolitic. Stromatolitic bioherms, forming a nearshore dolomitic limestone facies of the argillaceous Ore Formation (Garlick,
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Fig. 1. Portion of the Roan Antelope orebody, in the vicinity of the Irwin shaft barren gap, shown unrolled and restored as nearly as possible to its attitude and relations before folding, but with the vertical scale much exaggerated. The barren dolomite, considered to be part of a stromatolite bioherm, occupies the whole of the Ore Formation and rests on a hill of basement rocks; beyond, the Ore Formation rests on footwall beds. The barren dolomite thins to a feather edge and is then replaced by argillite; beyond, the widespread carbonate at the base of the Ore Formation, containing copper/iron sulphides and forming a second orebody where marked “mineralized”, is an impure dolomitic limestone representing an original algal mat. The copper-rich zone in the argillite spreads outward from the bioherm and away to the right like a plume, but decreases to normal grade in this direction.
1964), occur in many instances over ridges or hills of basement that formed promontories or islands, projecting to or slightly above the base of the Ore Formation during deposition. The bioherms themselves are barren of copper and related minerals except in shaly inclusions, but the adjacent ore shale is richly mineralized, the metal content decreasing outwards and being reflected by a mineral zoning of bomite-chalcopyrite-pyrite (Fig. 1;Garlick, 1964). A number of stromatolitic bioherms are now known for 50 km along the eastem margin of the ore shale deposits, from Roan Antelope through Nkana and on to Chambishi (Annels, 1974; Clemmey, 1974);associated layers of impure dolomite, notably at the base of the Ore Formation (Fig. l), are considered to represent original algal mats with included detritus. The
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Fig. 2. Stromatolite bioherms at Mufulira, in argillite immediately beneath the quartzite of the ‘B’ orebody. It can be seen how stromatolite growth started in the lower shale, but was inhibited and almost stopped by a thin layer of sand (white); in the succeeding dark shale growth again became vigorous, t o form the bioherm, which is about double the width shown. The ‘B’ orebody quartzite can be seen at the top, starting at the handle of the hammer; the quartzite immediately above the bioherm is mottled for a few feet by poorly developed algal growths.
zone of bioherms and sandy sediments marks the eastern shoreline, offshore is a zone of shale about 2 km wide that contains the copper orebodies, and beyond is a 3-8 km zone of pyritic carbonaceous shale; beyond this again are (deeper-water) carbonates (Garlick, 1961, p. 157), or terrestrial sediments (Annels, 1974). These dimensions and rock types indicate that deposition took place in an interdeltaic coastal marine lagoonal environment (Hamblin, 1973). Van Eden (1974)concludes that broadly similar conditions of deposition, with offshore shoals, existed during the deposition of the sands that now form the Mufulira C orebody, one of the major quartzite deposits. The quartzite orebodies are in general devoid of stromatolites, but the first stromatolites recognized on the Copperbelt were in a bioherm in argillaceous beds immediately below the Mufulira B orebody fringe (Malan, 1964;Figs. 2, 3);Paltridge (1968)found that an extensive algal biostrome, containing bodies of less well-developed stromatolites, spread away from the original occurrence. Malan (1964) reports a low copper content in the
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Fig. 3. Close-up of stromatolites of the Mufulira bioherm, in shale, showing shape and structure. The dark lines in the stromatolites are micaceous material similar t o the adjacent argillite and represent sediment that collected on the flat tops of the stromatolites during growth; little or none is present on the steep flanks.
bioherm, proportional to the amount of argdlaceous material included in or between the stromatolites; Paltridge reports a similar low copper content in the carbonate part of the biostrome, but as much as 7% in fine-grained algalbedded sandy sediment, which in places forms the foundation for stromatolites. In the continuation of the copper-bearing Katanga formations in the Shaba (Katanga) province of Zdire, the Sene des Mines is considered to be the stratigraphic equivalent of the ore-bearing formations of Zambia (Franqois, 1974). The ore deposits within the Sene des Mines consist of two mineralized formations, the upper a dolomitic shale like that of the Zambian argillite deposits, and the lower a finely laminated siliceous dolomite (Oosterbosch, 1962). The intervening regular bed of dolomitic limestone in places consists wholly or partly of stromatolites, identified as Collenia undosa (Oosterbosch, 1962,p. go),forming an algal biostrome; in places bioherms protrude into the overlying dolomitic shale (Fig. 4).As in Zambia the stromatolite formation is barren of copper, except for occasional inclusions of cupriferous shale. The stromatolites here are cylindrical, and on weathering are partly replaced by silica to form hollow cylinders and the rock is known as “roches siliceuses cellulaires” (Fig. 5 ) .
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Fig. 4. A bioherm in the Musonoi open pit, Shaba, Zaire, projecting about 4 m into the dolomitic shale o f the upper orebody, which is in part draped over the bioherm. The underlying stromatolitic carbonate formation (roches siliceuses cellulaires, RSC) from which the bioherm grew is hidden under the broken rock on the right.
At Matsitama in Botswana, in an isolated basin of metamorphosed formations that might be correlated with the Katanga, a deposit of potential economic significance occurs in quartz-carbonate rock, dolomite, and shale. A quartz-carbonate body that is probably a bioherm is more richly mineralized with copper than the laterally adjacent shale and biostromal beds. A t Mount Isa, Queensland, the Urquhart shales of the Middle Proterozoic Mount Isa Group (1,600 m.y.) contain important lead-zinc and copper deposits; the Urquhart shales consist of dolomitic shales and siltstones, dolomite, and tuffaceous shale. Galena, sphalerite, and pyrite occur in distinct concordant layers throughout the Urquhart shales, and where sufficiently concentrated, form the fourteen known orebodies, which are arranged en echelon, plunge south, and have in places suffered folding and faulting (Bennet, 1965). There is a broad but distinct mineral zoning, from pyrite in the north into sphalerite-pyrite-pyrrhotite, then rich galena-sphalerite orebodies that terminate against silica-dolomite bodies. The silica-dolomite contains pyrite, pyrrhotite, and chalcopyrite, and in places close to the lead-zinc bodies but separate from them, are the copper orebodies. The coarse ‘disordered-looking’ silica-d olomite bodies have been interpreted as
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Fig. 5 . Siliceous tubes of the RSC stromatolitic carbonate that led to the naming of the formation the “roches siliceuses cellulaires”. The outer, presumably purer carbonate portions of the columnar stromatolites (see Fig. 3) were preferentially replaced by silica, and the interior portion was weathered away; remnants of the stromatolite layering can be seen inside the tubes.
being algal reefs and reef-breccias, the finely bedded shale being off-reef organic tuffaceous sediment (Garlick, 1964;Stanton, 1972). In the Middle Proterozoic Belt Supergroup in Montana, a stratiform copper deposit has been found in feldspathic quartzite and siltstone of the Revett Formation, and subeconomic or unproven copper- silver concentrations are known in various places through the higher Belt formations (Harrison, 1974). In the Helena Formation of the middle Belt carbonate sequence, disseminated copper minerals are associated with algal stromatolite mounds and other structures of probably algal affinity (Fig. 6). The Infracambrian formations of the western Anti-Atlas of Morocco, consisting of sandstones, schists, and limestones, contain a number of small copper deposits at various stratigraphic horizons, many on the flanks of basement highs (Chazan and Fauvelet, 1962). The Tamjout series, consisting of recrystallized reef dolomites containing Collenia, overlain by a layer containing lenses of white quartz, is widely but weakly mineralized with lead, zinc, vanadium, and pyrite, and has several copper deposits.
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Fig. 6. Stromatolitic mounds in the Helena dolomite of the Belt series, Montana, U.S.A. In the cliff face behind the mounds are other features that are probably algal, and disseminated copper minerals occur in similar rocks nearby.
In the Bijawar formations of Madya Pradesh, in an old copper-mining district, Collenia-bearing dolomite is overlain by a more siliceous layer containing cyclindrical Conophyton (Balasundaram and Mahadevan, 1972). Chalcopyrite and galena occur disseminated and as veinlets in the formations containing the stromatolites, in places within the stromatolite zones. LEAD-ZINC
Lead-zinc (barite- fluorite-copper-cobalt) deposits are widespread in the Mississipi Valley of the United States, the great majority in carbonate rock ranging from limestone to dolomite (Beales and Onasich, 1970), and the class of deposit represented here is known as the Mississipi Valley type. The deposits are spread through almost the complete range of Phanerozoic rocks, but over 80% of the ore lies in formations of Cambrian-Ordovician and Mississipian -Pennsylvanian age. Stromatolite reefs are associated with a number of the deposits that are present in Late Cambrian and Early Ordovician formations in southeast Missouri (Wagner, 1961;Myers, 1966;Snyder
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PLAN
0
S ECT 10N
-
30 60 r rn
VERTICAL EXAGGERATION l o x
/
RIDGE
Fig. 7. Stromatolite reef over topographic high in the southeast Missouri lead belt; lead minerals occur mainly on the upper and lateral margins of the reef, and also in the overlying formations (after Wagner, 1961).
and Gerdemann, 1968) and to a lesser extent in Tennessee (Kendall, 1960), but in the upper Mississippi district stromatolites are absent from the ore formations (Heyl, 1968). The southeast Missouri lead belt is situated on the flank of a basement high and here Late Cambrian reefs, now interpreted as algal banks (Larsen, 1973), of digitate or columnar Collenia or Gymnosolen, lie over ridges of either basement rocks or Early Cambrian sediments (Fig. 7), accompanied by widespread discontinuous biostromes formed from stromatolite mounds. The sulphide minerals are closely associated with the stromatolitic formations, the major concentrations commonly being along the tops or edges of the elongate reefal structures. Associated black pyritic shales interrupted in places by patch reefs represent the muddy deposits accumulated in lagoons within fringing reefs (Myers, 1966). In east Tennessee, zincsulphide concentrations associated with reefs in the Late Cambrian Longview
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Formation form a significant proportion of the ore in some mines in this area. These reefs are 3- 5 m across and up to 2 m high and are built up of fairly even laminae, without columnar structures, but are considered to be stromatolitic (Kendall, 1960);sphalerite occurs mainly as distinct caps, up to 30 cm thick, on the reefs, with some along other margins but little internally. On Baffin Island, Northwest Territories, Canada, the Middle Proterozoic Society Cliffs Formation is predominantly an algal carbonate that is brecciated and contains specks and massive bands of sphalerite, galena, and pyrite (Geldsetzer, 1973). The carbonate consists mainly of finely to irregularly laminated stromatolitic dolomite with occasional biohermal and nodular structures, interpreted by Geldsetzer as being of subtidal to intertidal origin. In Bahia, Brazil, lead-zinc orebodies in the Bambui are associated with Collenia (Cassedanne, 1966,1969). PHOSPHATE
In the Aravalli metasediments of Udaipur, India, phosphorite is.closely associated with stromatolitic dolomite overlying orthoquartzite (Banejee, 1971b), the stromatolites concerned being Collenia symmetrica, Minjaria, and Baicalia. In detail, the phosphate occurs as: concentrations in stromatolite columns, laminar algal (stromatolitic) phosphorite, reworked fragments of stromatolites forming silicified conglomeratic or brecciated phosphorite, massive-bedded phosphorite with sandy and clayey laminae, and disseminated pellets and nodules in dolomite. Phosphorites are widely distributed, the P,O, content ranges from 10%to 37%, and the major ore mineral is apatite. Bannerjee also refers to the occurrence of Collenia and oncolites in other Late Precambrian phosplioritic formations in India (Gangolihat Dolomite), Russia, and China. IRON
Several of the major deposits of iron formation of Early Proterozoic age, around 2,000-2,200 m.y. old, are associated with stromatolites and carbonate rocks, and there is evidence that there was algal and bacterial activity in a substantial number of them (Cloud, 1973;La Berge, 1973). The Biwabik Iron Formation in the Mesabi district of the Lake Superior iron province contains two zones of cherty columnar stromatolites (Bayley and James, 1973), and underlying dolomite also contains stromatolites. On the Ontario side of Lake Superior, the equivalent Gunflint Iron Formation consists of interbedded stromatolitic chert, bedded chert, taconite, jasper, limestone, and black shale (Hohann, 1969b; Douglas, 1970, p. 118); Moorehouse and Beales (1962)describe jaspery and carbonate stromatolites
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(Collenia and Gymnosolen ?), and also “reefs” (bioherms), one with a core of stromatolitic chert and jasper but a surface consisting mainly of calcarenite and taconite. In black shale of the overlying Rove Formation at Kakabeka Falls powerhouse a cherty reef-like structure has a capping of pyrite about 5 cm thick, and oncolites occur nearby in the shale. At Steep Rock, Ontario, iron ore in the Archaean to Early Proterozoic formations (Jolliffe, 1966; Cloud, 197313) overlies dolomite containing columnar stromatolites. In South Africa, iron formation of the Transvaal sequence overlies a thick dolomite that contains abundant stromatolites (Truswell and Eriksson, 1972), and stromatolitic limestone occurs immediately beneath the iron formation in both the Cape and Transvaal Provinces. In the Cape region, pyritic stromatolitic limestone and pyritic carbonaceous shale underlies the ferruginous chert and pyritic carbonaceous shale of the transition zone (Beukes, 1973).In the eastern Transvaal the transition zone between the two consists of interbedded stromatolitic limestone, pyritic dolomite and mudstone, and iron formation; the limestone contains laminar, columnar, and domal stromatolitic structures (Button, 1973c), and Button (1976) also emphasizes the relation between carbonate and iron formation. MANGANESE
A t Kisenge in the western part of Shaba province, Zaire, a stratiform manganese deposit in sericite schists of the Lukoshi Complex is cut by a pegmatite dated at 1,850 m.y. (Doyen, 1973). Below the oxide zone, the deposit consists of alternating beds of manganese carbonate, “gondite”, and graphite schist; the carbonate consists of almost pure rhodochrosite, and the gondite of spessartite, a manganese-bearing garnet. The carbonate ore, with about 40% manganese, contains spessartite and small amounts of nickel and cobalt sulphides. Within the carbonate ore are several layers of stromatolites that are stated by Doyen to resemble Collenia. Doyen concludes that the manganese was of sedimentary origin and that subsequent metamorphism led to the development of garnet in the more impure carbonate layers. In the Transvaal Black Reef Formation in Botswana small stromatolites consisting of and embedded in manganese oxide, are associated with a layer of manganese nodules having a general similarity to those being formed today on the ocean floors; it is not known whether the manganese oxide replaces carbonate or is an original constituent (Litherland and Malan,
1973).
Monty (1973)found that matiganese nodules from the Blake Plateau and the south Atlantic have a very fine wavy and regular lamination, characteristic of stromatolitic structures, and consisting of fine dark brown and lightcoloured laminae; in the dark laminae, 6-8pm thick, are manganese-bearing bacterial rods and tiny filaments. The nodules result from the rhythmic
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growth of filamentous bacteria concentrating manganese and iron minerals within their sheaths, and it is possible that regional variations in composition might be dependent on the biochemical activity of the bacteria. Monty considers that they can be regarded as deep-water oceanic stromatolites. GOLD AND URANIUM
The Early Proterozoic (2,300-2,600m.y.) Witwatersrand formations of South Africa consist of an upper arenaceous division and a lower more argillaceous division. Most of the gold-uranium deposits occur in the lower part of the upper division, in or adjacent to conglomerate layers at the bases of cycles of sedimentation (Pretorius, 1974). In the western and southwestern parts of the goldfield, the bulk of the gold and uranium is present in carbon seams at the top of certain of the cycles of sedimentation; Pretorius (quoted in Payne, 1974) estimates that in recent years, 40% of the gold produced came from carbon seams, and the proportion is probably increasing. These carbon seams are considered to represent algal mats (Pretorius, 1974) or, according to Hallbauer and Van Warmelo (1974),mats of a material resembling modem lichen and probably consisting of algal and fungal organisms. Hallbauer and Van Warmelo present evidence to show that gold and uranium were assimilated by these plants during growth, and also physically trapped as detrital grains between carbon columns. In the Rum Jungle area of Northern Territory, Australia, uranium and associated copper, lead, and gold deposits occur in Early Proterozoic (1,800-2,200m.y.) formations. The deposits are closely associated with silicified limestone breccias, partly reefal in origin, or occur in sediments of off-reef facies, siltstone and carbonaceous shale with intercalated limestone lenses (Condon and Walpole, quoted in Garlick, 1964). Crohn (in Walpole et al., 1968) reports both agreement and disagreement with the interpretation of a reefal environment by subsequent workers. Based on his work at Nabarlek, Cooper (1973)suggests that regional uranium mineralization took place in Late Proterozoic times (710-815m.y.). FLUORITE
In the western Transvaal, South Africa, fluorite is present in significant amounts at the top of the Dolomite of the Transvaal Supergroup, in what was a minor lead-zinc district in the past. The presence of abundant stromatolites in the Dolomite has already been referred to, and a good deal of the fluorite in this area is associated with stromatolites, economic concentrations being largely confined to stromatolite bioherms.
TABLE I Summary of data on mineral deposits and associated stromatolites Formation
Age
Lo cat ion
Form of stromatolites
Elements
Nodules Bonneterre
Recent L. Cambrian
Ocean SE Missouri, U.S.A.
Mn, Fe (Cu, Co, Ni) Pb, Zn
Longview Tamjout Unknown Unknown Aravalli
L. Cambrian Infracambrian Middle Riphean Sinian Precambrian (?)
Tennessee, U.S.A. Morocco Russia China Rajasthan, India
Bacterial Collenia (Gymnosolen)
Gangolihat Bambui Katanga Katanga
Middle Riphean Middle Riphean L. Proterozoic : 900 m.y. L.Proterozoic: 900 m.y.
Pithoragarh, India Bahia, Brazil Copperbelt, Zambia Shaba, Zaire
Society Cliffs Belt Mount Isa Matsitama Gunflint
L. Proterozoic: 1,000 m.y. M. Proterozoic: 1,200 m.y. M. Proterozoic: 1,700 m.y. L.Proterozoic (?) E. Proterozoic: 2,000 m.y.
N.W.T., Canada Montana, U.S.A. Queensland, Australia Botswana Ontario, Canada
Biwabik Steep Rock ?
E. Proterozoic: 2,000 m.y. E. Proterozoic/Archaean E. Proterozoic: 1,800-2,200m.y.
Transvaal Transvaal
E. Proterozoic: 2,250m.y. E. Proterozoic: 2,250m.y.
Lukoshi Bijawar
E. Proterozoic: Precambrian
Minnesota, U.S.A. Ontario, Canada Northern Territory, Australia Cape Province, S. Africa Transvaal, South Africa; Botswana Shaba, Zaire Madya Pradesh, India
Witwatersrand
E. Proterozoic: 2,500m.y.
> 1,850m.y.
South Africa
?
Collenia Collenia oncolites Baicala Minjaria Collenia sy m me trica ?
Collenia Collenia Collenia undosa
Zn cu P P P
P Pb (Zn) c u (CO, U) c u , co (U)
? ? ? ?
Collenia Gymnosolen (?) ? ? ?
Fe Fe U (Cu, Pb, P)
?
Fe Fe, Mn
?
Collenia Collenia Conophy ton stratiform
Mn (Co, Ni) Cu, Pb Au, U
9
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ANALOGOUS DEPOSITS ASSOCIATED WITH OTHER BIOHERMS (Table I)
A substantial number of mineral deposits in Phanerozoic host rocks are associated with bioherms built by corals, rudistids, orbitilinids, stromatoporoids, and other animals, or with bodies loosely referred to as reefs, biohems, etc., and with unknown (or partial algal) affiliation. Because of their general similarity to many of the deposits in stromatolitic formations already described, some of these are listed or briefly described below. Lead-zinc deposits in or closely associated with reefs of Devonian or Carboniferous age are those of Pine Point, Canada (Campbell, 1967);Meggen, West Germany; and Tynagh and Silvermines, Ireland (Pereira, 1967).In reefs associated with lead- zinc deposits in the Canning Basin, Australia, Monseur and Pel (1973)list the reef-building organisms as stromatoporoids (see also Ch. 10.4 herein). Maucher and Schneider (1967)show the Triassic Alpine type lead-zinc deposits to be restricted to a few distinct beds of the thick limestone-dolomite sequence; the Ladinian deposits are bound to reef complexes, occurring mainly in a few layers in the back-reef facies, with a few iron (zinc) bodies in the coral reef itself. In northwest Africa, Monseur and Pel (1973)list the following Jurassic stratiform lead-zinc deposits that occur in reefs, many also extending into the back-reef facies: Bou Dahar (Pb), Bou Arhous (Pb), Tagount (Pb), Touissit (Zn, Pb) in Morocco; El Abed (Zn, Pb), Dominique Luciani (Zn, Pb), and Deglen (Zn, Pb) in Algeria. In addition there are a number of nonstratified deposits in reef complexes, and several deposits containing manganese and/or iron, such as Tiharatine, Youdi, and Bou Arfa in Morocco. Lower Cretaceous deposits include Quenza and Bou Khadra (Fe), and Mesloula (Pb) in Algeria, and Reocin (Zn, Fe, Pb) in Spain, as well as a number of non-stratified deposits. The Ruby Creek deposit in Alaska is a stratiform copper deposit in interbedded dolomite, limestone, and phyllite that form part of a Middle Devonian reef complex, containing stromatoporoids, corals, brachiopods, and gastropods (Runnels, 1969). In the Jurassic Todilto Limestone, Grants, New Mexico (Perry, 1963),uranium is present within and especially on the edges of elongate to arcuate reefs. Perry considers that the reefs are of algal origin, though they are coarsely crystalline and no internal structure is preserved. Fluorite pipes occur in reef complex limestone of the Carboniferous limestones of Derbyshire (Ford, 1969). DISCUSSION
The mineral deposits associated with stromatolites fall into the broad class of stratiform deposits in sedimentary rocks, since they are with few exceptions either stratiform in habit, or are portions of stratiform deposits. They
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occur in biohermal or biostromal stromatolitic rocks, or in closely associated rocks of both the fore-reef and back-reef environments, so that the hosts are dominantly carbonates, dolomite and limestone, or argillites , but also range to arenaceous and even coarser sediments in places; interbedded volcanic rocks are common, but intrusive magmatic rocks are conspicuously absent. These deposits occur in rocks of Proterozoic or Early Phanerozoic age, encompassing a 2-b.y. period (2.6-0.5 b.y) during which stromatolites reached their greatest development (e.g. Glaessner, 1968). In rocks of postEarly Ordovician age, broadly similar deposits are associated with bioherms built by corals, stromatoporoids, rudistids, etc., rather than the blue-green algae that were the main builders of the stromatolites. It is not clear to what extent algae and bacteria were concerned with the building of the Phaneroozoic bioherms; perhaps they were important but were not preserved, or their role was originally unimportant because they were destroyed by the action of grazing and burrowing animals, as described by Garret (1970b). There have been several references to the temporal partition of elements in the broad class of stratiform mineral deposits in sedimentary (volcanic) rock sequences, such as those by King (1965), Mendelsohn (1967), and Pereira (1967). For example, King describes a broad depositional pattern in which iron concentrations appear first, then copper, followed by lead-zinc and phosphate. Turning to the biohermal environment, Monseur and Pel (1973) find that copper is frequent in Precambrian and Infracambrian reefs, whereas zinc and to a lesser degree lead prevail in the Paleozoic and Mesozoic reefs, and that the associated rock also changes gradually from dolomite to limestone over this period. A wide range of valuable elements is concentrated in the deposits associated with stromatolites, and though the data are not sufficient to provide a complete picture, there is a suggestion that the different elements tend to be confined to host rocks laid down in particular periods. The major Superior-type iron deposits of the world were laid down during the Early Proterozoic (Goldich, 1973), and it is therefore not surprising that the iron deposits associated with stromatolites are confined to host rocks of this age. The Witwatersrand gold-uranium hosts are also Early Proterozoic, slightly older than the iron deposits, as are other gold-uranium-pyrite conglomerates. The first major copper and lead-zinc deposits associated with stromatolites appear at 1,700 m.y., though there seems to be a concentration of cupriferous deposits between about 1,200 and 700 m.y.; lead and zinc hosts are confined mainly to the period from 1,000 to 500 m.y., though they again become important in Late Paleozoic and Mesozoic biohermal deposits. Phosphorus is concentrated in midRiphean rocks, laid down between 1,350 and 950 m.y. Manganese occurs in Early Proterozoic hosts, then again in the modem oceanic nodules. The genesis of stratiform deposits in sedimentary rocks has been the subject of considerable debate for many years, and there has been a substantial swing away from earlier concepts of introduction of elements from late
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intrusive bodies, to those concerned with syngenetic and diagenetic processes. The genesis of mineral deposits associated with stromatolites must be considered as a part of this broad picture, though they have special aspects and problems, some of which are considered below. Banerjee (1971b) concludes that the Udaipur phosphorite was directly related to the action of stromatolitic algae, which led to the precipitation of phosphate and the trapping of phosphatic sediment in a shallow basin. Trudinger and Mendelsohn (Ch. 11.2) discuss the precipitation of phosphorus in shelf areas by chemical activity, or its accumulation by phytoplankton, though there is no direct evidence of phosphorus concentration related to stromatolite formation. There is little doubt that the gold and uranium in the Witwatersrand formations were deposited and concentrated during the deposition of the host rocks (Pretorius, 1974). The organisms that form the “algal” mats, whether they were algae or a lichen-like form, seem to have played an active part in collecting a substantial part of the gold and uranium, both physically and organically. In the case of the uranium (gold-lead-copper) deposits of northern Australia, the evidence is conflicting (Walpole et al., 1968), and it does not seem likely that stromatolite growth contributed directly to ore deposition, though the role of the bioherms as a host during later processes could have been important. It is generally accepted that the Early Proterozoic iron-formations are chemical sediments, though there are many differences regarding the mechanisms and conditions of derivation, transport, deposition, and diagenesis of the iron, silica, and associated elements (James and Sims, 1973). In many basins the iron formation is closely associated with stromatolitic carbonate rocks, although they may not be exactly contemporaneous, and with bacteria and blue-green algae. The ability of microorganisms to precipitate iron is discussed by Trudinger and Mendelsohn (Ch. 11.2). Cloud (1973b) suggests that, in Early Proterozoic time, the interaction of oxygen-producing photosynthetic blue-green algae and ferrous iron led to the removal of excess free oxygen, the conversion of ferrous iron to ferric iron, and the precipitation of iron as ferric oxides or hydroxides. Cloud suggests that at this time the pH was generally too low for the formation of carbonates, though Button (1975) suggests that in the Transvaal, offshore bars of clastic limestone led to the formation of barred basins within which evaporation led to iron deposition. It seems that, while microorganisms were probably associated with iron deposition, and algal mats might occur, stromatolitic carbonate rocks are unlikely to be intimately associated with iron formations, though they are an integral part of the ore-forming cycle and some forms could tolerate a substantial amount of iron in the environment. Manganese is known to accumulate in stratiform deposits during sedimentation (Park and McDiarmid, 1964), and the work of Doyen (1973) and Litherland and Malan (1973) suggests the possibility that stromatolites were
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at least able to grow under conditions where manganese was precipitating. Evidence suggesting the direct precipitation of manganese in modern oceanic nodules by stromatolite-forming microorganisms is given by Monty (1973), and Trudinger and Mendelsohn (Ch. 11.2) show the close relation that can exist between manganese deposition and the action of microorganisms. It seems likely that some manganese deposits formed during, and perhaps related to, the growth of stromatolites. Copper, lead, zinc, and iron sulphides, and associated minerals such as other sulphides, barite, and fluorite, provide some of the more spectacular and important examples of mineral deposits associated with stromatolites, as well as with other biogenic rocks. For the Mississipi Valley lead-zinc and related deposits, and many of the Phanerozoic deposits related to biogenic rocks that have been listed, a widely accepted idea is that the valuable elements were introduced by circulating or connate brines that were heated and moved up-dip from the depths of the depositional basins during diagenesis. The porous reef rocks provided channel ways for the solutions, and contained organic material and sulphur that created a reducing environment and led to precipitation of these elements as sulphides (Campbell, 1966;Beales and Onasich, 1970,etc.). The evidence is persuasive, and there can be little doubt that this process did operate in a number of these deposits. For the Baffin Island lead-zinc deposit, Geldsetzer (1973) attributes syngenetic dolomitization and sulphide mineralization to seawater-derived brine that circulated through karst-brecciated algal carbonates. In the Zambia-Zaire copper-cobalt province, Garlick (1965,1972) has summarized many of the arguments for syngenesis, while Bartholomd (1974) makes a convincing case for the diagenetic introduction of copper and cobalt at Kamoto; Annels (1974) proposes a similar process for the Zambian Copperbelt, and Van Eden (1974) suggests a combination of the two at Mufulira. Clemmy (1974)suggests a combination of tidal-flat, marine, and lagoonal condtions of sedimentation for the Ore Formation at Nkana; the copper content is related to shoreline, facies, and tidal directions, and mineralization took place during sedimentation. The bioherms in this province, which could be expected to have had the greatest permeability during diagenesis, are barren of sulphide minerals, whereas formations that represent algal mats and other stromatolitic biostromes are mineralized, the copper content commonly being related to the argillaceous content. These features suggest that mineralization was contemporaneous with sedimentation for the ore-shale deposits, and that the metal was introduced into the host rock together with the clay minerals; the stromatolitic formations and the organisms that formed them played an important role in the sedimentation of the host rock. Diagenetic processes were active in creating or enriching some deposits in this province. The partition of metals is evident in many of 'these deposits, and this is well illustrated at Mount Isa, where copper in biohermal rocks is closely
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associated with the stratigraphically equivalent bedded lead- zinc deposits. Bennett (1965) considers that all sulphides there were formed by the precipitation of metals during sedimentation, the sulphur being biogenic, and that the copper bodies formed by local re-mobilization during folding. Trudinger and Mendelsohn (Ch. 11.2) show that the deposition of metal sulphide minerals could take place during sedimentation as a result of biogenic sulphate reduction, or of evaporitic concentration and the subsequent formation of organic complexes. Rickard (1974)considers that biogenic synsedimentary copper sulphide ores are possible, provided an adequate metal source exists; potential environments for stratiform copper-sulphide ore formation are sediments of active oceanic ridges, partly connate subsurface water systems, and near-shore organic-rich sediments near a mineralized hinterland. CONCLUSIONS
The process of mineralization of stratiform deposits associated with stromatolites is an integral part of the whole development of sedimentary basins during sedimentation and diagenesis, and it is perhaps significant that many of the deposits discussed are in intracratonic basins. This general statement can be extended to include deposits associated with other biogenic formations, and even to a large part of the class of stratiform mineral deposits in sedimentary (volcanic) rocks. The favoured sedimentary environment is the barred basin-lagoon- tidal flat-evaporite-sabkha group of ocean-edge environments, of which stromatolitic bioherms and biostromes and algal mats often form an integral part. The valuable elements could be supplied from the hinterland by erosion, from the ocean itself, or from volcanic emanations in the vicinity; depending on the rate and nature of the supply and the conditions of sedimentation, these elements would be widely dispersed in the sediments or concentrated locally. Where mineral deposits are formed, the contribution of the microorganisms could be indirect, helping to create a favourable physical and chemical environment, or more direct in trapping enriched material, helping to precipitate the valuable elements organically, or supplying other necessary elements such as sulphur. Altematively the organisms could merely survive in an environment where deposition of these elements is taking place, without playing any part in the process. Soon after sedimentation and through the diagenetic period, mineralization would take place through the agency of connate brines that leach the valuable elements from their wide dispersion or from already-formed deposits, or perhaps through brines bringing them from a source external to the sediments; the brines would move, generally up-dip, till precipitation takes place in a favourable environment. In this process, the biogenic
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formations could serve by providing channel ways for the solutions, organic material to create a reducing environment, and sulphur in the form of sulphate or biogenically reduced sulphide to precipitate metal sulphides. Later processes could be superimposed on earlier ones, leading to further enrichment or leaching of existing deposits, and also providing conflicting evidence of ore genesis. Late diagenetic and metamorphic processes could be expected to modify deposits by recrystallization or even remobilization and redeposition, so that original textures, characteristics, and processes become obscured; this could be expected to be significant in many stromatoliterelated deposits, which are among the earliest known stratiform deposits. The reason for the temporal partition of elements in these deposits is not entirely clear, but it is in part related to the changing supply of particular elements at or near the surface of the Earth. An example of this is iron, which became abundantly available when the evolving conditions at the surface reached a particular stage in Early Proterozoic times, to form a group of major deposits that has not been duplicated since. Lead does not appear in any stratiform deposit before about 1,700 m.y., and indeed in few known major deposits formed before this time. Copper and zinc are common in Archaean volcanogenic deposits, and copper occurs in Early Proterozoic stratiform deposits, but both appear, with lead, for the first time in stromatolite-related deposits in the Middle Proterozoic at around 1,700 m.y. In addition to the changing supply of the elements to the environment in which stromatolites formed, perhaps there was an evolution of the microorganisms or changes in their ability to survive in or react with concentrations of these elements.
Chapter 11.1
MINERAL DEPOSITS ASSOCIATED WITH STROMATOLITES Felix Mendekohn
INTRODUCTION
Many mineral deposits are closely associated with stromatolites which occur either as bioherms (reefs), or biostromes that formed as widespread stromatolitic beds or algal mats. It is likely that there are many more than the ones dealt with here, but these are sufficiently numerous and widespread to suggest that they give a fair sampling of the class. The information available on different deposits shows a wide range in quality and quantity, probably reflecting the knowledge of and interest in the stromatolites themselves, the degree of preservation, and the effects of metamorphism. The term reef has been rather loosely used in many discussions of ore deposits to indicate any mound-, ridge-, or reef-like organic structure; since it is not always possible to be certain of their precise meaning, “reef” is used as the authors use it, and should be understood to include bioherms as well as reefs in the restricted sense of protruding wave-resistant structures. In the descriptions that follow, deposits are grouped according to the valuable element(s) they contain; a summary is presented in Table I. COPPER
The association between stromatolites and ore deposits is well demonstrated and documented in the copper province of Zambia/Zaire. On the Zambian Copperbelt the two main groups of ore deposits are those in argillite (ore shale) and those in quartzite. Bioherms present in many of the ore-shale deposits were not recognized as stromatolitic rocks for a long time because the moderate grade of metamorphism, local shearing along the stromatolite zone, and supergene alteration t o talcose cherty dolomite resulted in the partial destruction of the internal structure. However, the intersection of a number of well-preserved bodies at depth leaves little doubt that all or most are stromatolitic. Stromatolitic bioherms, forming a nearshore dolomitic limestone facies of the argillaceous Ore Formation (Garlick,
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Fig. 1. Portion of the Roan Antelope orebody, in the vicinity of the Irwin shaft barren gap, shown unrolled and restored as nearly as possible to its attitude and relations before folding, but with the vertical scale much exaggerated. The barren dolomite, considered to be part of a stromatolite bioherm, occupies the whole of the Ore Formation and rests on a hill of basement rocks; beyond, the Ore Formation rests on footwall beds. The barren dolomite thins to a feather edge and is then replaced by argillite; beyond, the widespread carbonate at the base of the Ore Formation, containing copper/iron sulphides and forming a second orebody where marked “mineralized”, is an impure dolomitic limestone representing an original algal mat. The copper-rich zone in the argillite spreads outward from the bioherm and away to the right like a plume, but decreases to normal grade in this direction.
1964), occur in many instances over ridges or hills of basement that formed promontories or islands, projecting to or slightly above the base of the Ore Formation during deposition. The bioherms themselves are barren of copper and related minerals except in shaly inclusions, but the adjacent ore shale is richly mineralized, the metal content decreasing outwards and being reflected by a mineral zoning of bomite-chalcopyrite-pyrite (Fig. 1;Garlick, 1964). A number of stromatolitic bioherms are now known for 50 km along the eastem margin of the ore shale deposits, from Roan Antelope through Nkana and on to Chambishi (Annels, 1974; Clemmey, 1974);associated layers of impure dolomite, notably at the base of the Ore Formation (Fig. l), are considered to represent original algal mats with included detritus. The
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Fig. 2. Stromatolite bioherms at Mufulira, in argillite immediately beneath the quartzite of the ‘B’ orebody. It can be seen how stromatolite growth started in the lower shale, but was inhibited and almost stopped by a thin layer of sand (white); in the succeeding dark shale growth again became vigorous, t o form the bioherm, which is about double the width shown. The ‘B’ orebody quartzite can be seen at the top, starting at the handle of the hammer; the quartzite immediately above the bioherm is mottled for a few feet by poorly developed algal growths.
zone of bioherms and sandy sediments marks the eastern shoreline, offshore is a zone of shale about 2 km wide that contains the copper orebodies, and beyond is a 3-8 km zone of pyritic carbonaceous shale; beyond this again are (deeper-water) carbonates (Garlick, 1961, p. 157), or terrestrial sediments (Annels, 1974). These dimensions and rock types indicate that deposition took place in an interdeltaic coastal marine lagoonal environment (Hamblin, 1973). Van Eden (1974)concludes that broadly similar conditions of deposition, with offshore shoals, existed during the deposition of the sands that now form the Mufulira C orebody, one of the major quartzite deposits. The quartzite orebodies are in general devoid of stromatolites, but the first stromatolites recognized on the Copperbelt were in a bioherm in argillaceous beds immediately below the Mufulira B orebody fringe (Malan, 1964;Figs. 2, 3);Paltridge (1968)found that an extensive algal biostrome, containing bodies of less well-developed stromatolites, spread away from the original occurrence. Malan (1964) reports a low copper content in the
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Fig. 3. Close-up of stromatolites of the Mufulira bioherm, in shale, showing shape and structure. The dark lines in the stromatolites are micaceous material similar t o the adjacent argillite and represent sediment that collected on the flat tops of the stromatolites during growth; little or none is present on the steep flanks.
bioherm, proportional to the amount of argdlaceous material included in or between the stromatolites; Paltridge reports a similar low copper content in the carbonate part of the biostrome, but as much as 7% in fine-grained algalbedded sandy sediment, which in places forms the foundation for stromatolites. In the continuation of the copper-bearing Katanga formations in the Shaba (Katanga) province of Zdire, the Sene des Mines is considered to be the stratigraphic equivalent of the ore-bearing formations of Zambia (Franqois, 1974). The ore deposits within the Sene des Mines consist of two mineralized formations, the upper a dolomitic shale like that of the Zambian argillite deposits, and the lower a finely laminated siliceous dolomite (Oosterbosch, 1962). The intervening regular bed of dolomitic limestone in places consists wholly or partly of stromatolites, identified as Collenia undosa (Oosterbosch, 1962,p. go),forming an algal biostrome; in places bioherms protrude into the overlying dolomitic shale (Fig. 4).As in Zambia the stromatolite formation is barren of copper, except for occasional inclusions of cupriferous shale. The stromatolites here are cylindrical, and on weathering are partly replaced by silica to form hollow cylinders and the rock is known as “roches siliceuses cellulaires” (Fig. 5 ) .
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Fig. 4. A bioherm in the Musonoi open pit, Shaba, Zaire, projecting about 4 m into the dolomitic shale o f the upper orebody, which is in part draped over the bioherm. The underlying stromatolitic carbonate formation (roches siliceuses cellulaires, RSC) from which the bioherm grew is hidden under the broken rock on the right.
At Matsitama in Botswana, in an isolated basin of metamorphosed formations that might be correlated with the Katanga, a deposit of potential economic significance occurs in quartz-carbonate rock, dolomite, and shale. A quartz-carbonate body that is probably a bioherm is more richly mineralized with copper than the laterally adjacent shale and biostromal beds. A t Mount Isa, Queensland, the Urquhart shales of the Middle Proterozoic Mount Isa Group (1,600 m.y.) contain important lead-zinc and copper deposits; the Urquhart shales consist of dolomitic shales and siltstones, dolomite, and tuffaceous shale. Galena, sphalerite, and pyrite occur in distinct concordant layers throughout the Urquhart shales, and where sufficiently concentrated, form the fourteen known orebodies, which are arranged en echelon, plunge south, and have in places suffered folding and faulting (Bennet, 1965). There is a broad but distinct mineral zoning, from pyrite in the north into sphalerite-pyrite-pyrrhotite, then rich galena-sphalerite orebodies that terminate against silica-dolomite bodies. The silica-dolomite contains pyrite, pyrrhotite, and chalcopyrite, and in places close to the lead-zinc bodies but separate from them, are the copper orebodies. The coarse ‘disordered-looking’ silica-d olomite bodies have been interpreted as
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Fig. 5 . Siliceous tubes of the RSC stromatolitic carbonate that led to the naming of the formation the “roches siliceuses cellulaires”. The outer, presumably purer carbonate portions of the columnar stromatolites (see Fig. 3) were preferentially replaced by silica, and the interior portion was weathered away; remnants of the stromatolite layering can be seen inside the tubes.
being algal reefs and reef-breccias, the finely bedded shale being off-reef organic tuffaceous sediment (Garlick, 1964;Stanton, 1972). In the Middle Proterozoic Belt Supergroup in Montana, a stratiform copper deposit has been found in feldspathic quartzite and siltstone of the Revett Formation, and subeconomic or unproven copper- silver concentrations are known in various places through the higher Belt formations (Harrison, 1974). In the Helena Formation of the middle Belt carbonate sequence, disseminated copper minerals are associated with algal stromatolite mounds and other structures of probably algal affinity (Fig. 6). The Infracambrian formations of the western Anti-Atlas of Morocco, consisting of sandstones, schists, and limestones, contain a number of small copper deposits at various stratigraphic horizons, many on the flanks of basement highs (Chazan and Fauvelet, 1962). The Tamjout series, consisting of recrystallized reef dolomites containing Collenia, overlain by a layer containing lenses of white quartz, is widely but weakly mineralized with lead, zinc, vanadium, and pyrite, and has several copper deposits.
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651
Fig. 6. Stromatolitic mounds in the Helena dolomite of the Belt series, Montana, U.S.A. In the cliff face behind the mounds are other features that are probably algal, and disseminated copper minerals occur in similar rocks nearby.
In the Bijawar formations of Madya Pradesh, in an old copper-mining district, Collenia-bearing dolomite is overlain by a more siliceous layer containing cyclindrical Conophyton (Balasundaram and Mahadevan, 1972). Chalcopyrite and galena occur disseminated and as veinlets in the formations containing the stromatolites, in places within the stromatolite zones. LEAD-ZINC
Lead-zinc (barite- fluorite-copper-cobalt) deposits are widespread in the Mississipi Valley of the United States, the great majority in carbonate rock ranging from limestone to dolomite (Beales and Onasich, 1970), and the class of deposit represented here is known as the Mississipi Valley type. The deposits are spread through almost the complete range of Phanerozoic rocks, but over 80% of the ore lies in formations of Cambrian-Ordovician and Mississipian -Pennsylvanian age. Stromatolite reefs are associated with a number of the deposits that are present in Late Cambrian and Early Ordovician formations in southeast Missouri (Wagner, 1961;Myers, 1966;Snyder
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652
PLAN
0
S ECT 10N
-
30 60 r rn
VERTICAL EXAGGERATION l o x
/
RIDGE
Fig. 7. Stromatolite reef over topographic high in the southeast Missouri lead belt; lead minerals occur mainly on the upper and lateral margins of the reef, and also in the overlying formations (after Wagner, 1961).
and Gerdemann, 1968) and to a lesser extent in Tennessee (Kendall, 1960), but in the upper Mississippi district stromatolites are absent from the ore formations (Heyl, 1968). The southeast Missouri lead belt is situated on the flank of a basement high and here Late Cambrian reefs, now interpreted as algal banks (Larsen, 1973), of digitate or columnar Collenia or Gymnosolen, lie over ridges of either basement rocks or Early Cambrian sediments (Fig. 7), accompanied by widespread discontinuous biostromes formed from stromatolite mounds. The sulphide minerals are closely associated with the stromatolitic formations, the major concentrations commonly being along the tops or edges of the elongate reefal structures. Associated black pyritic shales interrupted in places by patch reefs represent the muddy deposits accumulated in lagoons within fringing reefs (Myers, 1966). In east Tennessee, zincsulphide concentrations associated with reefs in the Late Cambrian Longview
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653
Formation form a significant proportion of the ore in some mines in this area. These reefs are 3- 5 m across and up to 2 m high and are built up of fairly even laminae, without columnar structures, but are considered to be stromatolitic (Kendall, 1960);sphalerite occurs mainly as distinct caps, up to 30 cm thick, on the reefs, with some along other margins but little internally. On Baffin Island, Northwest Territories, Canada, the Middle Proterozoic Society Cliffs Formation is predominantly an algal carbonate that is brecciated and contains specks and massive bands of sphalerite, galena, and pyrite (Geldsetzer, 1973). The carbonate consists mainly of finely to irregularly laminated stromatolitic dolomite with occasional biohermal and nodular structures, interpreted by Geldsetzer as being of subtidal to intertidal origin. In Bahia, Brazil, lead-zinc orebodies in the Bambui are associated with Collenia (Cassedanne, 1966,1969). PHOSPHATE
In the Aravalli metasediments of Udaipur, India, phosphorite is.closely associated with stromatolitic dolomite overlying orthoquartzite (Banejee, 1971b), the stromatolites concerned being Collenia symmetrica, Minjaria, and Baicalia. In detail, the phosphate occurs as: concentrations in stromatolite columns, laminar algal (stromatolitic) phosphorite, reworked fragments of stromatolites forming silicified conglomeratic or brecciated phosphorite, massive-bedded phosphorite with sandy and clayey laminae, and disseminated pellets and nodules in dolomite. Phosphorites are widely distributed, the P,O, content ranges from 10%to 37%, and the major ore mineral is apatite. Bannerjee also refers to the occurrence of Collenia and oncolites in other Late Precambrian phosplioritic formations in India (Gangolihat Dolomite), Russia, and China. IRON
Several of the major deposits of iron formation of Early Proterozoic age, around 2,000-2,200 m.y. old, are associated with stromatolites and carbonate rocks, and there is evidence that there was algal and bacterial activity in a substantial number of them (Cloud, 1973;La Berge, 1973). The Biwabik Iron Formation in the Mesabi district of the Lake Superior iron province contains two zones of cherty columnar stromatolites (Bayley and James, 1973), and underlying dolomite also contains stromatolites. On the Ontario side of Lake Superior, the equivalent Gunflint Iron Formation consists of interbedded stromatolitic chert, bedded chert, taconite, jasper, limestone, and black shale (Hohann, 1969b; Douglas, 1970, p. 118); Moorehouse and Beales (1962)describe jaspery and carbonate stromatolites
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(Collenia and Gymnosolen ?), and also “reefs” (bioherms), one with a core of stromatolitic chert and jasper but a surface consisting mainly of calcarenite and taconite. In black shale of the overlying Rove Formation at Kakabeka Falls powerhouse a cherty reef-like structure has a capping of pyrite about 5 cm thick, and oncolites occur nearby in the shale. At Steep Rock, Ontario, iron ore in the Archaean to Early Proterozoic formations (Jolliffe, 1966; Cloud, 197313) overlies dolomite containing columnar stromatolites. In South Africa, iron formation of the Transvaal sequence overlies a thick dolomite that contains abundant stromatolites (Truswell and Eriksson, 1972), and stromatolitic limestone occurs immediately beneath the iron formation in both the Cape and Transvaal Provinces. In the Cape region, pyritic stromatolitic limestone and pyritic carbonaceous shale underlies the ferruginous chert and pyritic carbonaceous shale of the transition zone (Beukes, 1973).In the eastern Transvaal the transition zone between the two consists of interbedded stromatolitic limestone, pyritic dolomite and mudstone, and iron formation; the limestone contains laminar, columnar, and domal stromatolitic structures (Button, 1973c), and Button (1976) also emphasizes the relation between carbonate and iron formation. MANGANESE
A t Kisenge in the western part of Shaba province, Zaire, a stratiform manganese deposit in sericite schists of the Lukoshi Complex is cut by a pegmatite dated at 1,850 m.y. (Doyen, 1973). Below the oxide zone, the deposit consists of alternating beds of manganese carbonate, “gondite”, and graphite schist; the carbonate consists of almost pure rhodochrosite, and the gondite of spessartite, a manganese-bearing garnet. The carbonate ore, with about 40% manganese, contains spessartite and small amounts of nickel and cobalt sulphides. Within the carbonate ore are several layers of stromatolites that are stated by Doyen to resemble Collenia. Doyen concludes that the manganese was of sedimentary origin and that subsequent metamorphism led to the development of garnet in the more impure carbonate layers. In the Transvaal Black Reef Formation in Botswana small stromatolites consisting of and embedded in manganese oxide, are associated with a layer of manganese nodules having a general similarity to those being formed today on the ocean floors; it is not known whether the manganese oxide replaces carbonate or is an original constituent (Litherland and Malan,
1973).
Monty (1973)found that matiganese nodules from the Blake Plateau and the south Atlantic have a very fine wavy and regular lamination, characteristic of stromatolitic structures, and consisting of fine dark brown and lightcoloured laminae; in the dark laminae, 6-8pm thick, are manganese-bearing bacterial rods and tiny filaments. The nodules result from the rhythmic
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growth of filamentous bacteria concentrating manganese and iron minerals within their sheaths, and it is possible that regional variations in composition might be dependent on the biochemical activity of the bacteria. Monty considers that they can be regarded as deep-water oceanic stromatolites. GOLD AND URANIUM
The Early Proterozoic (2,300-2,600m.y.) Witwatersrand formations of South Africa consist of an upper arenaceous division and a lower more argillaceous division. Most of the gold-uranium deposits occur in the lower part of the upper division, in or adjacent to conglomerate layers at the bases of cycles of sedimentation (Pretorius, 1974). In the western and southwestern parts of the goldfield, the bulk of the gold and uranium is present in carbon seams at the top of certain of the cycles of sedimentation; Pretorius (quoted in Payne, 1974) estimates that in recent years, 40% of the gold produced came from carbon seams, and the proportion is probably increasing. These carbon seams are considered to represent algal mats (Pretorius, 1974) or, according to Hallbauer and Van Warmelo (1974),mats of a material resembling modem lichen and probably consisting of algal and fungal organisms. Hallbauer and Van Warmelo present evidence to show that gold and uranium were assimilated by these plants during growth, and also physically trapped as detrital grains between carbon columns. In the Rum Jungle area of Northern Territory, Australia, uranium and associated copper, lead, and gold deposits occur in Early Proterozoic (1,800-2,200m.y.) formations. The deposits are closely associated with silicified limestone breccias, partly reefal in origin, or occur in sediments of off-reef facies, siltstone and carbonaceous shale with intercalated limestone lenses (Condon and Walpole, quoted in Garlick, 1964). Crohn (in Walpole et al., 1968) reports both agreement and disagreement with the interpretation of a reefal environment by subsequent workers. Based on his work at Nabarlek, Cooper (1973)suggests that regional uranium mineralization took place in Late Proterozoic times (710-815m.y.). FLUORITE
In the western Transvaal, South Africa, fluorite is present in significant amounts at the top of the Dolomite of the Transvaal Supergroup, in what was a minor lead-zinc district in the past. The presence of abundant stromatolites in the Dolomite has already been referred to, and a good deal of the fluorite in this area is associated with stromatolites, economic concentrations being largely confined to stromatolite bioherms.
TABLE I Summary of data on mineral deposits and associated stromatolites Formation
Age
Lo cat ion
Form of stromatolites
Elements
Nodules Bonneterre
Recent L. Cambrian
Ocean SE Missouri, U.S.A.
Mn, Fe (Cu, Co, Ni) Pb, Zn
Longview Tamjout Unknown Unknown Aravalli
L. Cambrian Infracambrian Middle Riphean Sinian Precambrian (?)
Tennessee, U.S.A. Morocco Russia China Rajasthan, India
Bacterial Collenia (Gymnosolen)
Gangolihat Bambui Katanga Katanga
Middle Riphean Middle Riphean L. Proterozoic : 900 m.y. L.Proterozoic: 900 m.y.
Pithoragarh, India Bahia, Brazil Copperbelt, Zambia Shaba, Zaire
Society Cliffs Belt Mount Isa Matsitama Gunflint
L. Proterozoic: 1,000 m.y. M. Proterozoic: 1,200 m.y. M. Proterozoic: 1,700 m.y. L.Proterozoic (?) E. Proterozoic: 2,000 m.y.
N.W.T., Canada Montana, U.S.A. Queensland, Australia Botswana Ontario, Canada
Biwabik Steep Rock ?
E. Proterozoic: 2,000 m.y. E. Proterozoic/Archaean E. Proterozoic: 1,800-2,200m.y.
Transvaal Transvaal
E. Proterozoic: 2,250m.y. E. Proterozoic: 2,250m.y.
Lukoshi Bijawar
E. Proterozoic: Precambrian
Minnesota, U.S.A. Ontario, Canada Northern Territory, Australia Cape Province, S. Africa Transvaal, South Africa; Botswana Shaba, Zaire Madya Pradesh, India
Witwatersrand
E. Proterozoic: 2,500m.y.
> 1,850m.y.
South Africa
?
Collenia Collenia oncolites Baicala Minjaria Collenia sy m me trica ?
Collenia Collenia Collenia undosa
Zn cu P P P
P Pb (Zn) c u (CO, U) c u , co (U)
? ? ? ?
Collenia Gymnosolen (?) ? ? ?
Fe Fe U (Cu, Pb, P)
?
Fe Fe, Mn
?
Collenia Collenia Conophy ton stratiform
Mn (Co, Ni) Cu, Pb Au, U
9
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ANALOGOUS DEPOSITS ASSOCIATED WITH OTHER BIOHERMS (Table I)
A substantial number of mineral deposits in Phanerozoic host rocks are associated with bioherms built by corals, rudistids, orbitilinids, stromatoporoids, and other animals, or with bodies loosely referred to as reefs, biohems, etc., and with unknown (or partial algal) affiliation. Because of their general similarity to many of the deposits in stromatolitic formations already described, some of these are listed or briefly described below. Lead-zinc deposits in or closely associated with reefs of Devonian or Carboniferous age are those of Pine Point, Canada (Campbell, 1967);Meggen, West Germany; and Tynagh and Silvermines, Ireland (Pereira, 1967).In reefs associated with lead- zinc deposits in the Canning Basin, Australia, Monseur and Pel (1973)list the reef-building organisms as stromatoporoids (see also Ch. 10.4 herein). Maucher and Schneider (1967)show the Triassic Alpine type lead-zinc deposits to be restricted to a few distinct beds of the thick limestone-dolomite sequence; the Ladinian deposits are bound to reef complexes, occurring mainly in a few layers in the back-reef facies, with a few iron (zinc) bodies in the coral reef itself. In northwest Africa, Monseur and Pel (1973)list the following Jurassic stratiform lead-zinc deposits that occur in reefs, many also extending into the back-reef facies: Bou Dahar (Pb), Bou Arhous (Pb), Tagount (Pb), Touissit (Zn, Pb) in Morocco; El Abed (Zn, Pb), Dominique Luciani (Zn, Pb), and Deglen (Zn, Pb) in Algeria. In addition there are a number of nonstratified deposits in reef complexes, and several deposits containing manganese and/or iron, such as Tiharatine, Youdi, and Bou Arfa in Morocco. Lower Cretaceous deposits include Quenza and Bou Khadra (Fe), and Mesloula (Pb) in Algeria, and Reocin (Zn, Fe, Pb) in Spain, as well as a number of non-stratified deposits. The Ruby Creek deposit in Alaska is a stratiform copper deposit in interbedded dolomite, limestone, and phyllite that form part of a Middle Devonian reef complex, containing stromatoporoids, corals, brachiopods, and gastropods (Runnels, 1969). In the Jurassic Todilto Limestone, Grants, New Mexico (Perry, 1963),uranium is present within and especially on the edges of elongate to arcuate reefs. Perry considers that the reefs are of algal origin, though they are coarsely crystalline and no internal structure is preserved. Fluorite pipes occur in reef complex limestone of the Carboniferous limestones of Derbyshire (Ford, 1969). DISCUSSION
The mineral deposits associated with stromatolites fall into the broad class of stratiform deposits in sedimentary rocks, since they are with few exceptions either stratiform in habit, or are portions of stratiform deposits. They
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occur in biohermal or biostromal stromatolitic rocks, or in closely associated rocks of both the fore-reef and back-reef environments, so that the hosts are dominantly carbonates, dolomite and limestone, or argillites , but also range to arenaceous and even coarser sediments in places; interbedded volcanic rocks are common, but intrusive magmatic rocks are conspicuously absent. These deposits occur in rocks of Proterozoic or Early Phanerozoic age, encompassing a 2-b.y. period (2.6-0.5 b.y) during which stromatolites reached their greatest development (e.g. Glaessner, 1968). In rocks of postEarly Ordovician age, broadly similar deposits are associated with bioherms built by corals, stromatoporoids, rudistids, etc., rather than the blue-green algae that were the main builders of the stromatolites. It is not clear to what extent algae and bacteria were concerned with the building of the Phaneroozoic bioherms; perhaps they were important but were not preserved, or their role was originally unimportant because they were destroyed by the action of grazing and burrowing animals, as described by Garret (1970b). There have been several references to the temporal partition of elements in the broad class of stratiform mineral deposits in sedimentary (volcanic) rock sequences, such as those by King (1965), Mendelsohn (1967), and Pereira (1967). For example, King describes a broad depositional pattern in which iron concentrations appear first, then copper, followed by lead-zinc and phosphate. Turning to the biohermal environment, Monseur and Pel (1973) find that copper is frequent in Precambrian and Infracambrian reefs, whereas zinc and to a lesser degree lead prevail in the Paleozoic and Mesozoic reefs, and that the associated rock also changes gradually from dolomite to limestone over this period. A wide range of valuable elements is concentrated in the deposits associated with stromatolites, and though the data are not sufficient to provide a complete picture, there is a suggestion that the different elements tend to be confined to host rocks laid down in particular periods. The major Superior-type iron deposits of the world were laid down during the Early Proterozoic (Goldich, 1973), and it is therefore not surprising that the iron deposits associated with stromatolites are confined to host rocks of this age. The Witwatersrand gold-uranium hosts are also Early Proterozoic, slightly older than the iron deposits, as are other gold-uranium-pyrite conglomerates. The first major copper and lead-zinc deposits associated with stromatolites appear at 1,700 m.y., though there seems to be a concentration of cupriferous deposits between about 1,200 and 700 m.y.; lead and zinc hosts are confined mainly to the period from 1,000 to 500 m.y., though they again become important in Late Paleozoic and Mesozoic biohermal deposits. Phosphorus is concentrated in midRiphean rocks, laid down between 1,350 and 950 m.y. Manganese occurs in Early Proterozoic hosts, then again in the modem oceanic nodules. The genesis of stratiform deposits in sedimentary rocks has been the subject of considerable debate for many years, and there has been a substantial swing away from earlier concepts of introduction of elements from late
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intrusive bodies, to those concerned with syngenetic and diagenetic processes. The genesis of mineral deposits associated with stromatolites must be considered as a part of this broad picture, though they have special aspects and problems, some of which are considered below. Banerjee (1971b) concludes that the Udaipur phosphorite was directly related to the action of stromatolitic algae, which led to the precipitation of phosphate and the trapping of phosphatic sediment in a shallow basin. Trudinger and Mendelsohn (Ch. 11.2) discuss the precipitation of phosphorus in shelf areas by chemical activity, or its accumulation by phytoplankton, though there is no direct evidence of phosphorus concentration related to stromatolite formation. There is little doubt that the gold and uranium in the Witwatersrand formations were deposited and concentrated during the deposition of the host rocks (Pretorius, 1974). The organisms that form the “algal” mats, whether they were algae or a lichen-like form, seem to have played an active part in collecting a substantial part of the gold and uranium, both physically and organically. In the case of the uranium (gold-lead-copper) deposits of northern Australia, the evidence is conflicting (Walpole et al., 1968), and it does not seem likely that stromatolite growth contributed directly to ore deposition, though the role of the bioherms as a host during later processes could have been important. It is generally accepted that the Early Proterozoic iron-formations are chemical sediments, though there are many differences regarding the mechanisms and conditions of derivation, transport, deposition, and diagenesis of the iron, silica, and associated elements (James and Sims, 1973). In many basins the iron formation is closely associated with stromatolitic carbonate rocks, although they may not be exactly contemporaneous, and with bacteria and blue-green algae. The ability of microorganisms to precipitate iron is discussed by Trudinger and Mendelsohn (Ch. 11.2). Cloud (1973b) suggests that, in Early Proterozoic time, the interaction of oxygen-producing photosynthetic blue-green algae and ferrous iron led to the removal of excess free oxygen, the conversion of ferrous iron to ferric iron, and the precipitation of iron as ferric oxides or hydroxides. Cloud suggests that at this time the pH was generally too low for the formation of carbonates, though Button (1975) suggests that in the Transvaal, offshore bars of clastic limestone led to the formation of barred basins within which evaporation led to iron deposition. It seems that, while microorganisms were probably associated with iron deposition, and algal mats might occur, stromatolitic carbonate rocks are unlikely to be intimately associated with iron formations, though they are an integral part of the ore-forming cycle and some forms could tolerate a substantial amount of iron in the environment. Manganese is known to accumulate in stratiform deposits during sedimentation (Park and McDiarmid, 1964), and the work of Doyen (1973) and Litherland and Malan (1973) suggests the possibility that stromatolites were
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at least able to grow under conditions where manganese was precipitating. Evidence suggesting the direct precipitation of manganese in modern oceanic nodules by stromatolite-forming microorganisms is given by Monty (1973), and Trudinger and Mendelsohn (Ch. 11.2) show the close relation that can exist between manganese deposition and the action of microorganisms. It seems likely that some manganese deposits formed during, and perhaps related to, the growth of stromatolites. Copper, lead, zinc, and iron sulphides, and associated minerals such as other sulphides, barite, and fluorite, provide some of the more spectacular and important examples of mineral deposits associated with stromatolites, as well as with other biogenic rocks. For the Mississipi Valley lead-zinc and related deposits, and many of the Phanerozoic deposits related to biogenic rocks that have been listed, a widely accepted idea is that the valuable elements were introduced by circulating or connate brines that were heated and moved up-dip from the depths of the depositional basins during diagenesis. The porous reef rocks provided channel ways for the solutions, and contained organic material and sulphur that created a reducing environment and led to precipitation of these elements as sulphides (Campbell, 1966;Beales and Onasich, 1970,etc.). The evidence is persuasive, and there can be little doubt that this process did operate in a number of these deposits. For the Baffin Island lead-zinc deposit, Geldsetzer (1973) attributes syngenetic dolomitization and sulphide mineralization to seawater-derived brine that circulated through karst-brecciated algal carbonates. In the Zambia-Zaire copper-cobalt province, Garlick (1965,1972) has summarized many of the arguments for syngenesis, while Bartholomd (1974) makes a convincing case for the diagenetic introduction of copper and cobalt at Kamoto; Annels (1974) proposes a similar process for the Zambian Copperbelt, and Van Eden (1974) suggests a combination of the two at Mufulira. Clemmy (1974)suggests a combination of tidal-flat, marine, and lagoonal condtions of sedimentation for the Ore Formation at Nkana; the copper content is related to shoreline, facies, and tidal directions, and mineralization took place during sedimentation. The bioherms in this province, which could be expected to have had the greatest permeability during diagenesis, are barren of sulphide minerals, whereas formations that represent algal mats and other stromatolitic biostromes are mineralized, the copper content commonly being related to the argillaceous content. These features suggest that mineralization was contemporaneous with sedimentation for the ore-shale deposits, and that the metal was introduced into the host rock together with the clay minerals; the stromatolitic formations and the organisms that formed them played an important role in the sedimentation of the host rock. Diagenetic processes were active in creating or enriching some deposits in this province. The partition of metals is evident in many of 'these deposits, and this is well illustrated at Mount Isa, where copper in biohermal rocks is closely
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associated with the stratigraphically equivalent bedded lead- zinc deposits. Bennett (1965) considers that all sulphides there were formed by the precipitation of metals during sedimentation, the sulphur being biogenic, and that the copper bodies formed by local re-mobilization during folding. Trudinger and Mendelsohn (Ch. 11.2) show that the deposition of metal sulphide minerals could take place during sedimentation as a result of biogenic sulphate reduction, or of evaporitic concentration and the subsequent formation of organic complexes. Rickard (1974)considers that biogenic synsedimentary copper sulphide ores are possible, provided an adequate metal source exists; potential environments for stratiform copper-sulphide ore formation are sediments of active oceanic ridges, partly connate subsurface water systems, and near-shore organic-rich sediments near a mineralized hinterland. CONCLUSIONS
The process of mineralization of stratiform deposits associated with stromatolites is an integral part of the whole development of sedimentary basins during sedimentation and diagenesis, and it is perhaps significant that many of the deposits discussed are in intracratonic basins. This general statement can be extended to include deposits associated with other biogenic formations, and even to a large part of the class of stratiform mineral deposits in sedimentary (volcanic) rocks. The favoured sedimentary environment is the barred basin-lagoon- tidal flat-evaporite-sabkha group of ocean-edge environments, of which stromatolitic bioherms and biostromes and algal mats often form an integral part. The valuable elements could be supplied from the hinterland by erosion, from the ocean itself, or from volcanic emanations in the vicinity; depending on the rate and nature of the supply and the conditions of sedimentation, these elements would be widely dispersed in the sediments or concentrated locally. Where mineral deposits are formed, the contribution of the microorganisms could be indirect, helping to create a favourable physical and chemical environment, or more direct in trapping enriched material, helping to precipitate the valuable elements organically, or supplying other necessary elements such as sulphur. Altematively the organisms could merely survive in an environment where deposition of these elements is taking place, without playing any part in the process. Soon after sedimentation and through the diagenetic period, mineralization would take place through the agency of connate brines that leach the valuable elements from their wide dispersion or from already-formed deposits, or perhaps through brines bringing them from a source external to the sediments; the brines would move, generally up-dip, till precipitation takes place in a favourable environment. In this process, the biogenic
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formations could serve by providing channel ways for the solutions, organic material to create a reducing environment, and sulphur in the form of sulphate or biogenically reduced sulphide to precipitate metal sulphides. Later processes could be superimposed on earlier ones, leading to further enrichment or leaching of existing deposits, and also providing conflicting evidence of ore genesis. Late diagenetic and metamorphic processes could be expected to modify deposits by recrystallization or even remobilization and redeposition, so that original textures, characteristics, and processes become obscured; this could be expected to be significant in many stromatoliterelated deposits, which are among the earliest known stratiform deposits. The reason for the temporal partition of elements in these deposits is not entirely clear, but it is in part related to the changing supply of particular elements at or near the surface of the Earth. An example of this is iron, which became abundantly available when the evolving conditions at the surface reached a particular stage in Early Proterozoic times, to form a group of major deposits that has not been duplicated since. Lead does not appear in any stratiform deposit before about 1,700 m.y., and indeed in few known major deposits formed before this time. Copper and zinc are common in Archaean volcanogenic deposits, and copper occurs in Early Proterozoic stratiform deposits, but both appear, with lead, for the first time in stromatolite-related deposits in the Middle Proterozoic at around 1,700 m.y. In addition to the changing supply of the elements to the environment in which stromatolites formed, perhaps there was an evolution of the microorganisms or changes in their ability to survive in or react with concentrations of these elements.