Jul 31, 1990 - Geological, mineralogical and geochemical aspects of Archean Banded Iron-Formation-hosted gold deposits: Some examples from Southern ...
Mineral. Deposita 25 [Suppl] S 125-S 135 (1990)
MINERALIUM DEPOSITA 9 Springer-Verlag 1990
Geological, mineralogical and geochemical aspects of Archean Banded Iron-Formation-hosted gold deposits: Some examples from Southern Africa T. Oberthiir 1, R. Saager 2 and H.-P. Tomschi 3
1 Bundesanstalt ffir Geowissenschaftenund Rohstoffe, Stilleweg2, D-3000 Hannover 51, Federal Republic of Germany 2 Institut fiir Kristallographie und Petrographie, Swiss Federal Institute of Technology, ETH Zentrum, CH-8092 Ziirich, Switzerland 3 Shashe Mines (Pty) Ltd., P.O. Box 919, Francistown, Botswana Received: December 1989/Accepted: July 31, 1990 Abstract. This paper reports on different styles of gold mineralization observed in Archean gold deposits hosted by Algoma-type Banded Iron-Formations (BIF) in southern Africa. Genetic aspects of various occurrences are discussed in the context of mineralogical as well as geochemical data of BIFs from the greenstone terranes of the Zimbabwe and Kaapvaal cratons. The study revealed that, in spite of their different provenance and age (3.5 to 2.6 Ga), the BIFs are geochemically similar, whereas observed mineralogical differences reflect various degrees of metamorphic overprint. Generally, the BIFs belong to mixed oxide-carbonate-(+ sulfide)-facies. REE distribution patterns of the investigated Archean BIF samples exhibit positive Eu-anomalies, which suggest a strongly reducing nature of the solutions which also provided the distinctive element contents now present in the chemical sediments. Irrespective of their formation, gold enrichment in BIF only occurs if the S- and/or As-contents of the BIFs exceed specific threshold values, i.e. gold mineralization is always associated with increased contents of the iron-sulfides pyrite, arsenopyrite and pyrrhotite. The studies indicate that BIF-hosted gold occurrences are not products of a single universal metallogenic process, but may be explained by several different genetic processes such as primary syn-sedimentary formation, diagenetic changes, metamorphic remobilization, and epigenetic hydrothermal emplacement.
Banded Iron-Formations (BIFs) are chemical sedimentary rocks consisting of alternating thin layers of chert (quartz) and iron minerals (iron-oxides, -carbonates, -silicates and/or -sulfides) (James 1954). BIFs are of Archean to Proterozoic age and, on a worldwide scale, constitute prominent lithological sequences in shield terranes. BIFs contain the major portion of the world's iron ores and, in addition, host a number of gold deposits. Most students of BIFs agree on a synsedimentary origin of the iron, silica and carbonates in these rocks and regard them as chemical sediments (Trendall and Morris
1983). However, the metallogenic aspects of gold mineralization hosted in BIFs (syn- or epigenetic) are currently much disputed (Phillips et al. 1984; Foster et al. 1986; Saager et al. 1987; Foster and Gilligan 1987; Master et al. 1989; Saager and Oberthfir 1989; Lhotka and Nesbitt 1989; and others). This paper discusses the geology, mineralogy and geochemistry of some BIF-hosted gold deposits in Zimbabwe and considers mineralogical and geochemical aspects of Archean BIFs from southern which were investigated. The purpose of the paper is to report on various features of the mineralization and to discuss different genetic aspects. General geology
The samples studied were collected from the Archean greenstone belts of the Kaapvaal and the adjacent Zimbabwe cratons (Fig. 1). The two cratons are separated by the high-grade granulite facies metamorphic rock successions of the Limpopo Mobile Belt. Reviews on greenstone belt development in southern Africa were recently given by Anhaeusser (1986) and Foster et al. (1986). The investigated samples were obtained from different stratigraphic positions within the various greenstone belts, with ages ranging from about 3.5 to 2.6 Ga. Associated rocks are mainly mafic or ultramafic volcanics and, less commonly, sedimentary rocks. All the rocks sampled underwent metamorphism of at least lower greenschist facies, but distinctive mineral assemblages (amphiboles, garnet) indicate that locally a higher degree of metamorphic overprint was attained (Table 1).
Gold mineralization in Banded Iron-Formations
The following type localities contain BIF-hosted gold occurrences that represent contrasting styles of mineralization. Additional aspects of the different styles of mineralization have been discussed previously (Fripp 1976, Fos-
S 126
BROOMSTOCK MINE _~7-" ZIMB,ABWE CRATON
Distribution of ] ore zones on 4-level
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Fig. 1. Generalized geological map of southern Africa showing greenstone belt terranes and sample localities
Fig. 2. Distribution of ore zones on 4-level (100 m level), Broomstock mine (modified after Nutt et al. 1988)
ter et al. 1986; Saager et al. 1987; Ille 1987; Nutt et al. 1988; Saager and Oberthtir 1989).
pods which display an erratic distribution (Fig. 2). Gold ores are characterized by the occurrence of visible arsenopyrite/pyrite mineralization which is generally confined to a delicate set of flat-lying, irregular veins and joints in the jaspilite. These structures are usually filled with ankerite and quartz, and commonly contain fragments of jaspilite. Needles of arsenopyrite and cubes of pyrite are found in the jaspilite adjacent to the veins and joints. A conspicuously higher concentration of sulfides is present near the veins and joints. Away from these and extending for up to about 10 cm into the jaspilite, the mineralization becomes increasingly disseminated. Significantly, the bright reddish jaspilite is bleached to a light grey to yellowish rock in areas showing sulfide mineralization (Fig. 3). This observation suggests that the emplacement of sulfides coincided with a concurrent consumption of hematite. The gold ores are refractory, and although a large number of polished sections were studied, no microscopic gold was observed, even in samples reaching gold grades of 30 g/t.
Broomstock mine, Kwekwe greenstone belt, Zimbabwe The Kwekwe greenstone belt in central Zimbabwe (locality No. 3 in Fig. 1) is composed of three major stratigraphic units of Bulawayan age, termed the Felsic, Maliyami and Mafic Formations (Harrison 1970). This greenstone belt contains numerous gold-quartz veins as well as gold-bearing sulfidic and antimonian veins which occur in different types of host rocks including granitic gneiss, metabasalt, serpentinite and jaspilitic iron formation. Talc-schist and metabasalts of the Mafic Formation host the gold-bearing jaspilitic BIF at Broomstock Mine (Nutt et al. 1988). Two isolated lenses of BIF are separated by barren metabasalt. The BIFs are up to 10 m wide and extend for about 100 m along strike and at least 125 m down dip. The BIF at Broomstock consists of alternating hematitic and cherty layers (Fig. 8a), between 0.05 and 1.5 mm wide. Locally, they alternate with layers of Mgsiderite and some magnetite and in places, massive, reddish jaspilite bodies are also present. Along the margins of the jaspilite bodies breccias are widespread. Gold mineralization is associated with brecciation and fracturing of the BIF and is confined to irregular
Golden Kopje mine, Chinhoyi greenstone belt, Zimbabwe The Golden Kopje mine is situated at the south-eastern extremity of the Chinhoyi greenstone belt (locality No. 1 in Fig. 1). The lithological sequence at the mine is of Bulawayan age (Stagman 1961; Foster et al. 1986) and consists of talc schists in the footwall of the ore-bearing,
S 127 steeply NW-dipping BIF. Surface exposures in the footwall of the talc schist show the presence of an intrusive granodioritic batholith which, however, is not exposed in underground workings. The gold-bearing BIF is between 25 and 50 m thick and is exposed for several hundred metres along strike and down to the 160 m level in the mine. Chlorite schists locally containing lenses of argillites and chert are present in the hanging wall of the BIF horizon (Fig. 4). The entire sequence has been metamorphosed at lower greenschist facies conditions and is discordantly overlain by conglomerates and grits of Shamvaian age. The BIF at Golden Kopje mine consists of alternating, 1 to 10 mm thick layers of chert and magnetite which are accompanied by accessory ankerite and some sulfides. Several ore zones are confined to the BIF horizon.
//// / / / ,,V / ,
5 cm
They are between 1 and 3 m wide and extend up to some tens of metres along strike. The ore zones are characterized by visible sulfide mineralization consisting of pyrite and pyrrhotite in varying proportions carrying microscopic gold. On the mine scale, some of the ore zones transect the entire BIF, whereas others form subparallel or, locally, even podiform lenses (Fig. 4). The sulfides occur disseminated or concentrated in stringers, single layers or sets of layers, and occasionally form massive mineralization. Commonly the sulfide layers are oriented parallel to the bedding. However, in some exposures they transect the bedding of the BIE Gold occurs as inclusions or heals micro-fractures in pyrite and pyrrhotite. Mean gold grain-sizes are in the order of 10 -20 I.tm, with an observed maximum diameter of about 75 Ixm. A continuous, concordant, 0.3-0.5 m wide band of massive pyrite is intercalated in the stratigraphically upper portion of the BIE However, gold values of about 0.3 g/t in this type of pyrite are uneconomic.
Bar 20 and Vubachikwe mine, Gwanda greenstone belt, Zimbabwe ................. i : " ; ' " . .
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bedding of jaspilite mineralization (FeS 2 + FeAsS)
Fig. 3. Detailed sketch of typical mineraliation style as found at Broomstock mine. A - bleached, light grey jaspilite; B - ankeritic vein with fragments of jaspilite; C - antimonian mineralization (mainly tetrahedrite); D ankerite veinlet; E - jaspilite breccia cemented by ankerite; F - massive pyrite
The Bar 20 and Vubachikwe mines are situated in the northwestern portion of the Gwanda greenstone belt in southern Zimbabwe (locality 7 in Fig. 1; Fig. 5). This belt consists of a thick pile of weakly metamorphosed basaltic to andesitic volcanic and intercalated sedimentary rocks of Bulawayan age (Tyndale-Biscoe 1940; Wilson et al. 1978). Stopes at the Bar 20 mine expose two distinctly different orebodies. The No. 1 orebody represents a BIF lying concordantly within the enclosing meta-volcanic chlorite-schists (Figs. 5 and 6). The No. 2 orebody represents epigenetic vein-type gold mineralization and was discussed elsewhere (Saager et al. 1987). The BIF (No. 1 orebody) displays fine banding of white chert layers which alternate with greenishblack layers of Mg-siderite and fine-grained pyrrhotite. The orebody ranges from 2 to 5 m in thickness, and its lateral extension both along strike and downdip is in
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= 50 m
Fig. 4. Generalized geological plan showing setting and distribution of ore zones at Golden Kopje mine (125 m level)
S 128
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GEOLOGY of the GWANDA GREENSTONE BELT
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Fig. 6. Detailed geological sections of the Vubachikwe and the Bar 20 mine (modified after Saager et al. 1987)
excess of 100 m. Individual layers are between a few millimeters and one centimetre thick and can be traced for metres underground. Locally the sulfide minerals occur concentrated in the lower portions of the carbonate-rich layers. This feature was interpreted as a result of gravity settling of amorphous sulfidic precipitates during sedimentation (Saager et al. 1987). In response to tectonic overprinting of the rocks, the competent BIF was fractured obliquely to the bedding. These discontinuous and small-scale fractures displaced individual BIF layers by some millimeter to centimeters only, and provided structures along which the sulfides were remobilized and quartz as well as carbonates recrystallized. The ore is refractory, and in spite of its appreciable gold grade of 5 - 1 0 g/t, no microscopic gold was observed. The hanging-wall of the No. I orebody consists of a graphitic schist up to 3 m wide. It contains abundant
Fig. 5. Generalized geological map of the western part of the Gwanda greenstone belt showing the regional distribution of BIFs and locations of active gold mines (modified after Tyndale-Biscoe 1940)
sphalerite, pyrrhotite and chalcopyrite as well as traces of Pb-Sb-sulfides. At the Vubachikwe deposit gold ores are present in three stratigraphic positions (Fig. 6). The three orebodies are confined to subparallel BIF horizons, which are traceable for about 3 km along strike and at least 1 km down-dip. The BIF horizons are separated by interlayered amphibolite of tholeiitic composition and are 1 to 25 m thick. They consist of light-coloured layers of chert and carbonate (ankerite, calcite) and darker layers of fine-grained chlorite, grunerite-cummingtonite and coarse sulfides (arsenopyrite, pyrrhotite, pyrite). Locally, rare almandine, gahnite and biotite are present in the ores. Individual layers usually are up to 1 cm thick. In contrast to the layered ore of Bar 20 mine, layering in the Vubachikwe ores is much more irregular, and locally massive remobilized sulfides form stratabound ore-shoots. According to Fripp (1974, 1976), these ore-shoots resulted from boudinageing of the BIF horizons and suffered the same structural deformations as the surrounding country rocks. Gold mineralization is associated with sulfides, especially arsenopyrite, and is confined to the BIF horizons. Microscopic gold is common in the Vubachikwe ores (Fig. 8 b), foiming particles of free gold up to 60 lam in diameter, or the precious metal overgrows or heals microfractures predominantly in arsenopyrite. Fine grinding of the ore to - 325 mesh leads to a gold recovery by cyanide leaching of about 80%. It should be noted that recent drilling on 27-level, Vubachikwe mine, revealed the presence of graphitic schists close to the Long John orebody (U. Lotz, personal communication 1990).
Mineralogical observations Detailed studies of the various BIF-samples, including transmitted and reflected light microscopy as well as mi-
S 129 Table 1. Mineralogical variation of the investigated BIF-samples
Locality Sample ( ) locality in Fig. 1 Barberton, S.A. Mount Morgan Devonian 78/4-Fig Tree
(11)
Murchison, S.A. M 37
(10)
Pietersburg, S.A. 77/2, 50, 60 Pm 61, 67, 68
(9)
Zimbabwe Broomstock Redcliff Buchwa Giant Golden Kopje Belvedere Camperdown Old Wanderer Bar 20 Vubachikwe Lennox
(3) (3) (8) (2) (1) (6) (4) (4) (7) (7) (5)
Other occurrences Witwatersrand, S.A. Thabazimbi, S.A. Middleback R., Aus. Sao Bento, Brazil
Oxides
Carbonates
Silicates
Sulfides
Gold
(mt) mt, hm
sid, ank sid sid, ank
(musc) (cl)
py, (a) py, a (py)
* -
mt
sid
-
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-
mt, hm mt, hm (goe)
.
-
-
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sid, ank sid . cc ank sid sid, ank sid, ank ank, cc cc ?
py, a py, a
-
po py, po py, a py, (a) py po, (a) a, py, po po
* * * * *
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-
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*
.
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.
.
. amph cl amph, cl (cl, musc) (el, musc) cl amph, cl amph, gt
.
. .
. .
. .
-
Abbreviations:
Oxides: m t - magnetite; h m - hematite; g o e - goethite Carbonates: e c - calcite; s i d - siderite; a n k - ankerite Silicates: e l - chlorite; m u s e - muscovite; g t - garnet; a m p h Sulfides: p y - pyrite; a arsenopyrite; p o - pyrrhotite Gold: - not detected; * - present microscopically
-
amphibole (tremolite, actinolite, grunerite)
Carbonates
CaCO3
M g - s i d e r i t e , a n k e r i t e a n d , less often, calcite are c o m m o n c o n s t i t u e n t s in m o s t o f the samples. T h e c a r b o n a t e s f o r m small, discrete crystals w h i c h o c c u r d i s s e m i n a t e d in aggregates o f chert o r f o r m thin layers o r r e m o b i l i z a t i o n s filling j o i n t s a n d fractures. Sulfide-rich l a y e r in B I F s a r e c o m m o n l y c h a r a c t e r ized b y the presence o f a p p r e c i a b l e a m o u n t s o f c a r b o n ates. M i c r o p r o b e a n a l y s e s (Fig. 7 a n d Table 1) d e m o n strate t h a t M g - s i d e r i t e m a y be the o n l y c a r b o n a t e p r e s e n t o r t h a t a l t e r n a t i v e l y , at o t h e r localities, it co-exists w i t h ankerite. S a m p l e s f r o m the G o l d e n K o p j e m i n e c o n t a i n ankerite, a n d t h o s e f r o m the G i a n t m i n e calcite only. A t V u b a c h i k w e , a n k e r i t e is p r e s e n t in the B I F w h e r e a s b o t h a n k e r i t e a n d calcite o c c u r in late d i s c o r d a n t veins. MgC03
....
J,
,
(Fo,Mn)C03
Fig. 7. Compositional ranges (in mol %) of carbonate minerals from southern African BIFs c r o p r o b e analyses, r e v e a l e d v a r i a t i o n s in a b u n d a n c e o f the m a i n m i n e r a l p h a s e s as s h o w n in Table 1. A d d i t i o n a l d a t a on the m i n e r a l c h e m i s t r y a n d the r o c k g e o c h e m i s t r y o f the B I F s will be p r e s e n t e d elsewhere ( O b e r t h i i r et al. in prep.).
Silicates and quartz X e n o m o r p h i c q u a r t z , in the f o r m o f m i c r o c r y s t a l l i n e c h e r t (grain size 5 - 5 0 ~tm) is u b i q u i t o u s in m o s t o f the samples. L o c a l s t r u c t u r a l d e f o r m a t i o n a n d / o r m e t a m o r phic o v e r p r i n t has led to distinct e n l a r g e m e n t s o f the g r a i n sizes o f q u a r t z . A s a result, m e a n d i a m e t e r s r a n g i n g f r o m 100 to 240 p m were m e a s u r e d ( A b o l f a t h 1988) in
S 130 amphibole-bearing BIFs which were metamorphosed at upper greenschist facies conditions (Table 1). Traces of white mica and chlorite were observed in only some of the BIF samples, whereas fibrous, colourless to green amphiboles of grunerite/cummingtonite and the tremolite-actinolite series are present in all samples originating from the Pietersburg greenstone belt and in some samples from Zimbabwe (Table 1). These observations indicate a higher metamorphic overprint in these localities. The main chemical reaction leading to amphibole formation in BIF is of the type (Winkler 1976; Klein 1983): 7 siderite + 8 quartz + grunerite + 7 CO 2
HzO
5 dolomite + 8 quartz + HzO --* tremolite + 3 calcite + 7 CO2
(1) (2)
According to Klein (1983), these reactions are active during medium-grade metamorphic overprint of ironformation assemblages, at temperatures ranging from 450 ~ to 600 ~ Garnet (almandine), actinolite and biotite were observed at the Lennox and locally at the Vubachikwe mines.
Oxides Magnetite is the most c o m m o n and abundant iron oxide, constituting between 5 and 30% of the samples. Individual magnetite grains, between 30 and 200 jam in diameter, display idiomorphic to subidiomorphic shapes. The magnetite grains are optically and chemically homogeneous and devoid of inclusions. Hematite may be the product of martitization of earlier magnetite, especially in oxidized samples from surface outcrops. However, fine needles up to 20 lam long and scales of specularite occur disseminated in chert in some samples and are probably of early synsedimentarydiagenetic origin. Goethite is a supergene constituent and is present only in weathered surface samples.
Sulfides Pyrite forms either fine-grained disseminations or wellcrystallized grains up to several millimeters in diameter. The mineral is concentrated in layers parallel to the bedding and it is also present as massive aggregates or as fillings in cross-cutting veins. Zonation marked by different porosity, and rows of oriented inclusions, is visible in some of the coarser pyrite grains, although microprobe analyses failed to reveal any chemical zoning in differing Ni, Co or As-contents. Arsenopyrite is always idiomorphic (Fig. 8b). Its lath-shaped crystals reach up to 300 lain in length, and mutual intergrowths of arsenopyrite and pyrite are common. Optical zonation is prominent in many arsenopyrite grains, and chemical zonation in arsenopyrite from various occurrences was established by microprobe analyses.
Fig. 8. a Well-banded jaspilite. Mainly chert (light areas) and finegrained hematite (black) as well as coarse-grained idiomorphic pyrite and arsenopyrite. Broomstock mine. Transmitted light; horizontal length of photomicrograph is 3 mm. b Gold (white) enclosed by pyrrhotite (medium grey) together with gangue (black). Idiomorphic arsenopyrite (light grey, left). Vubachikwe mine. Reflected light, oil immersion; horizontal length of photomicrograph is 0.5 mm
The general trend observed is an increase of the As/Sratios from core to rim of individual grains at constant Fe-contents. Ni, Co and Sb were detected as additional elements in arsenopyrite only in a few exceptional cases. Kretschmar and Scott (1976) suggested the use of the As-contents of arsenopyrites as a geothermometer, provided co-existing sulfides allow an estimate of the sulfuractivity during mineral formation. Using our data of arsenopyrite core compositons, (Fig. 9), formation temperatures from about < 300 ~ to 500 ~ are deduced for the various occurrences. These temperature estimates are in general agreement with the observed medium and upper greenschist facies metamorphic grades of the country rocks. However, Sharpe et al. (1985) pointed out that arsenopyrite geothermometry is inconsistent with low-temperature deposits, to which many of our studied occurrences belong. In addition, most of our data points fall into temperature ranges which lie close to the lower temperature limit (about 300 c~C) of experimental calibration of the arsenopyrite geothermometer. A further restriction for the use of the arsenopyrite for geothermometric mea-
S 131 28
30
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33
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35
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Mount M o r g a n
n=6
o
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o Devonian
n=17
o
.9
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n=8
Camperdown
n=5
Lennox
posed of magnetite, siderite and/or ankerite, is most common. At higher metamorphic rank, the mineral assemblages of the oxide-carbonate facies are superseded by those defining the oxide-silicate facies. According to the metamorphic reactions (1) and (2) described above, carbonates are consumed and amphiboles are formed. Varying amounts of sulfides, in addition to oxides and carbonates, lead to a mixed oxide-carbonate-sulfide facies. However, care must be exercised in interpreting the origin of the sulfides or their precursor minerals, because they might be either products of synsedimentary, diagenetic or epigenetic processes.
n=3
Geochemical investigations Sao
Bento
n=7
B a r 20 n=26
O n=62
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O
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Vubachikwe i
:
i
: cores
i
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Fig. 9. Compositional ranges of arsenopyrites from various localities, in terms of at % As surements arises from the observed chemical zonation in arsenopyrite. It indicates changes of crystallization conditions during growth, and chemical disequilibrium within the particular grains. The use of chemical data of arsenopyrite for geothermometric measurements and as possible genetic indicators should therefore be treated with some reserve. Xenomorphic pyrrhotite is somewhat more abundant at localitites which underwent a higher metamorphic overprint. In some cases, as at the Giant and Lennox mines in Zimbabwe, pyrrhotite is the only iron-sulfide present. Chalcopyrite, galena and shalerite are trace constituents. They occur a xenomorphic, irregular grains usually intergrown with pyrite, pyrrhotite or arsenopyrite. Gold
Microscopic gold was observed, closely associated with sulfides, in samples from six different localities (Table 1). It forms small (5-30 lam) rounded inclusions in the sulfides, or heals micro-fractures in cataclastic pyrite or arsenopyrite. Occasionally it occurs as free grains attaining a grain size of up to 75 ~tm in diameter (Fig. 8 b). Gold particles from the Vubachikwe, Golden Kopje, Camperdown and Devonian mines contain silver ranging from 4 to 8 wt.-% Ag. These descriptions and the data in Table 1 demonstrate that most of these BIFs are of mixed facies types (James 1954). The oxide-carbonate facies, usually corn-
Geochemical investigations using XRF, AAS and NAA were performed on more than 100 samples collected at 24 localities. Some remarkable findings based on 34 samples are presented below. The REE distributions of the investigated Archean BIFs show flat patterns with distinct positive Eu-anomalies (Fig. 10). Depleted absolute REE contents and positive Eu-anomalies are typical for Archean ironrich sediments (Fryer 1977, 1983; Graf 1978). In order to produce a positive Eu-anomaly in a chemical sediment, Europium must be mobilized as Eu (II). According to Graf (1978) the positive Eu-anomalies in BIFs indicate an input of strongly reducing hydrothermal solutions into the waters from which the BIFs precipitated. However, recent work by Danielson (1989) showed that Eu (III) is dominant in an environment in which iron-hydroxides are precipitated. Therefore, the positive Eu-anomalies observed in the present study do not necessarily reflect the redox-conditions of the depository, but could probably be inherited from the source of REE mobilization, i.e. oceanic basalts which undergo sea-floor alteration at elevated temperatures. Eh-values capable of Eu-reduction are thought to prevail under such conditions, and Danielson (1989) further pointed out that the positive Eu-anomaly found in Archean, but not in Proterozoic iron-formations, may reflect a higher heat flow in the oceanic crust during Archean as compared to Proterozoic time. The REE signatures of these southern African BIFs, therefore, suggest that sea-floor leaching processes played an important role in providing their REE and possibly other elemental contents. It is noteworthy that the REE distribution displayed by the Contorted Bed of the lower Witwatersrand Supergroup resembles a pattern typical for Proterozoic BIFs, in spite of the recently established Archean age of the Witwatersrand Supergroup (ca. 2.72.9 Ga; Barton et al. 1989). Gold shows a statistically significant positive correlation with arsenic and a less pronounced one with sulfur. Inspection of the raw data using probability graphs (Sinclair 1978; one example is shown in Fig. 11) indicates that the distributions of sulfur, arsenic and gold are essentially polymodal, but could to some extent be approximated by bimodal distributions. Accordingly, the populations of each of the three elements were subdivided into two sub-
S 132 2.
2 Zimbabwe
Barberton
I
-*
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0 9 9
BAR 20, o r e GOLDEN KOPJE ( n - 3 )
9
M O U N T MORGAN~ oflP
* 9 DEVONIANI o r e 11r FIG TREE
VUBACHIKWE, ores
O OAYLIGHTr o r e
-1
-1
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Nd
Sm Eu
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u
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Yb Lu
Nd
Sm Eu
L
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L
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2.
REE distributions
Pietersburg (n- 5)
Kwekwe jaspilites ,1~ SROOMSTOCK.
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.
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i
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Zimbabwe
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;
i
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SmEu
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. OLD WANDERER O
,
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Chondrite-normalized REE distribution patterns of BIFs from Zimbabwe (left column) and South Africa (right column). I~EE distribution patterns obtained for Proterozoic BIFs (Thabazimbi, Transvaal, South Africa; Contorted Bed, Witwatersrand, South Africa; Middleback Ranges, Australia) are shown for comparison (bottom right column)
Fig. 10.
U
-
-
~ . . ~ . . ................ . ~ .
._m O
9
LENNOX. o r s
MIDOLEBACK RANGES, AUSTRALIA
9
THASAZIMEII, T r a n s v a a l
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/It LENNOX I
Lace
Table
Nd
Sin Eu
Tb
I
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-1
=
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Total (n = 34)
Population "Background . . . . Excess" (n = 17) (n = 17)
S (%) As (ppm) Au (ppb)
3.34 3900 4860
0.04 70 12
Tb
I
I
Yb Lu
populations, termed "background" and "excess" population, possessing the arithmetic mean values shown in Table 2.
2. Arithmetic mean values
Element
SmEu
7.41 8 750 10 325
Inspection of the elemental distribution patterns, furthermore, showed that the gold-rich samples possess elevated contents of arsenic and/or sulfur (Fig. 11). The mineralogical finding that gold is found only in BIFs which carry the sulfides pyrite, pyrrhotite or arsenopyrite is thus further substantiated by these statistical analyses.
S 133 probability (%) 10 i
i
30
,
i
5O
70
i i I i I
90 *
i
i
50000
xx '"...
10000
Au (ppb) ~
' L t t i
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,, I t
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i
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i
Fig. 11. Probabilitydistribution of gold in the BIF sample set, and partitioning of the data into two sub-populations (termed "background" and "excess" population) according to the method described by Sinclair (1978). Differentsymbolsare used for samples belonging to the respective "background" and "excess" populations of the distributions of sulfur and/or arsenic: star excess population of S and As; triangle - e x c e s s populationof As;fullcircle - excesspopulationof S; open circle - backgroundpopulationof As and S
Discussion and conclusions Gold mineralization
Different styles of gold mineralization in BIFs were described above, and some mineralogical as well as geochemical data relevant to the presence and distribution of the noble metal in these rocks were presented. This data indicates that the jaspilite-hosted, stratabound gold mineralization at Broomstock mine is related to tectonic fracturing of the competent BIF and the subsequent introduction of hydrothermal fluids carrying gold, arsenic and sulfur. Some of the observed relationships (Fig. 3) could also result from diagenetic reactions in yet unconsolidated sea-floor sediments. Fluid-wallrock reactions, as suggested for example by the dissolution and consumption of hematite (Fig. 3), led to a replacement-type mineralization with arsenopyrite and pyrite carrying submicroscopic gold. The ultimate source of the hydrothermal fluids and their contained elements may be related to metamorphic dewatering and leaching of greenstone assemblages at greater depth.
At Golden Kopje mine, gold is closely associated with stratabound pyrite and pyrrhotite mineralization. Most of the gold-rich mineralization appears to be related to epigenetic structures formed in response to metamorphic overprinting and concomitant tectonization. These structures were used as channelways for hydrothermal solutions which introduced gold and sulfides. Fluid-wallrock reactions are obscure in the magnetite-bearing BIF at Golden Kopje mine. Possible syngenetic, concordant sulfide mineralization with subeconomic gold contents is present in subordinate quantity. Gold mineralization at Golden Kopje mine may be related to either the introduction of gold from sources external to the iron formation, or to the metamorphic remobilization and upgrading of subeconomic, synsedimentary sulfide/gold mineralization that was previously contained in the BIF horizon. The BIF-hosted gold deposits in the Gwanda greenstone belt (Bar 20 and Vubachikwe) have many features in common, including the stratiform nature of the ores, their extreme lateral continuity and their mineralogical and geochemical characteristics. Arsenopyrite and pyrrhotite are the major sulfides present. Gold attains sizes of up to 60 ~tm in the Vubachikwe ores, but the noble metal is submicroscopic in the ores of Bar 20 mine. The No. 1 orebody at Bar 20 mines occurs in chlorite schists, whereas the Vubachikwe ores are hosted by amphibolites. According to Saager et al. (1987) these different host rocks reflect an increase in metamorphic grade in a northwesterly direction from Bar 20 towards Vubachikwe mine. This metamorphic trend is also underlined by the mineralogical assemblages of the auriferous BIF, which exhibit a paragenetic change from quartz-Mg-siderite/chlorite at the Bar 20 mine to quartz/ ankerite/calcite/grunerite (+ almandine, _ biotite) at the Vubachikwe mine. The trend is furthermore accompanied by an increase of the grain sizes of quartz, arsenopyrite and gold which unquestionably points to ore emplacement prior to the peak of regional metamorphism. It is proposed that further mineralogical studies should be undertaken to confirm the suggested metamorphic trend in the Gwanda greenstone belt. In addition to their genetic significance, the results are thought to provide important information on, first, the possible interdependence of gold mineralization and metamorphism, and, second, the amenability of the auriferous ores to metallurgical treatment. The No. 1 orebody at Bar 20 mine is considered to represent a relatively undisturbed, low-metamorphic counterpart of the Vubachikwe orebodies. These latter, in contrast, suffered higher metamorphic overprinting and pronounced boundinageing, tectonization and remobilization of the ore components. A synsedimentary genetic process by subaqueous hot springs or exhalations for the BIF-hosted gold ores in the Gwanda greenstone belt was proposed by Fripp (1976) and Saager et al. (1987). Major aspects in support of a syngenetic origin of the mineralization include the confinement of gold and sulfides to the BIF horizons, the absence of any widespread wallrock alteration, the absence of sulfide/gold mineralization in the iron-rich host rocks, the considerable lateral persistence of individual, narrow BIF layers, their rhyth-
S 134 mical banding, their volcanogenic-exhalative geochemical characteristics (especially their REE patterns), and the associated graphitic schists carrying elevated base metal contents at Bar 20 mine.
combination of superimposed different genetic proccess such as synsedimentary deposition, diagenetic reactions, metamorphic remobilization and epigenetic hydrotherreal mineralization.
Mineralogical and geochemical aspects
Summary
Despite major differences amongst these Archean BIFs from southern Africa, such as their ages and metamorphic histories, their mineral assemblages and many geochemical characteristics, they display far-reaching similarities. Mineralogical investigations revealed that most of the BIFs belong to mixed oxide-carbonate-facies of iron formations, and that the silicate-facies BIFs probably formed from mixed oxide-carbonate-facies iron-formations due to elevated metamorphic conditions. Therefore, most of the samples originally represent the mixed oxide-carbonate facies. Magnetite and sidertite/ankerite are the principal carriers of iron in these rocks. Sulfides present in the BIF include pyrite, pyrrhotite and arsenopyrite. In general, pyrite is idiomorphic and coarse-grained, whereas xenomorphic pyrrhotite aggregates display mosaic textures with common triple junctions. Idiomorphic arsenopyrite is often chemically zoned which is indicative of disequilibrium (re-)crystallization. Grain sizes, grain shapes and compositional variations of the sulfides are regarded as evidence for their metamorphic reconstitution.
1. All Archean BIFs from southern Africa investigated in this study represent mixed oxide-carbonate-facies of iron-formations. 2. These Archean BIFs possess similar geochemical characteristics which point to common processes during their formation. 3. Strongly reducing hydrothermal fluids were involved in the mobilization from sea-floor rocks of the elements now present in BIFs. 4. Gold mineralization in BIFs is always stratabound but only sometimes stratiform. 5. Gold mineralization in BIFs is associated with higher abundances of arsenic and/or sulfur, i.e. the presence of the sulfides arsenopyrite, pyrite and pyrrhotite. 6. Mineralogical and geochemical data are equivocal in interpreting the genesis of gold mineralization in BIFs. 7. BIF-hosted gold mineralization cannot be regarded as the product of a single mineralizing event. Instead, different particular or a combination of superimposed processes including synsedimentary deposition, diagenetic reactions, metamorphic remobilizations, and epigenetic hydrothermal emplacement must be taken into account in metallogenetic concepts for gold mineralization in BIE
Metallogenic considerations Submarine volcanic activity was widespread during the formation of the southern African greenstone belts, as documented by thick piles of massive and pillowed lavas, tufts and volcaniclastic sedimentary strata present in the Archean rock successions. It is suggested that gold and associated elements were extracted from sea-floor rocks by strongly reducing hydrothermal fluids. The dissolved element suite, including gold, sulfur and arsenic, was precipitated at the loci of BIF sedimentation. Locally, gold concentrations reached levels constituting ore and, therefore, at least some of the gold mineralization is synsedimentary in origin. Diagenetic processes - reworking of already deposited sediments by evolved fluids may have played an additional role in concentrating gold and sulfides in the ferruginous sediments. Epigenetic gold mineralization formed after consolidation of the BIF through the action of auriferous hydrothermal fluids, which followed dilatant fracture systems in the competent rocks. Deposition of gold and associated sulfides apparently was mainly controlled by chemical reactions between the dissolved species of the fluids and the iron-rich rocks. The mineralogical and geochemical data of this study are equivocal in regard to the genesis of gold mineralization in BIF. However, field observations combined with laboratory data indicate that the various types of mineralizations cannot be regarded as products of a single universal process, but may be explained by a
Acknowledgements. The authors are indebted to a large number of individuals, institutions and companies who aided them through their great hospitality, able advice and technical support during fieldwork in Zimbabwe. Cooperation with Prof. K.A. Viewing and staff, Institute of Mining Research, University of Zimbabwe, made the project possible. The authors also wish to acknowledge the support provided by numerous geologists and explorationists through valuable discussions or assistance in the field. They include P. Bourhill, D. Chigonda, P. Chaka, D. McDonald, T. Langerman, R. Pingstone, T. Nutt, D. Muirhead, C. Castelin, M. Ralph, A. Ncube, M. and D. Thompson and A. Marsh. D. Opper, Mineralogical Institute, Universitfit K61n, performed most of the XRF analyses. The Institute of Nuclear Chemistry, Universit/it K61n, provided access to NAA facilities. M. Frey, Windeck, deserves special thanks for editorial suggestions and discussions. Ms. G. G6decke, Hannover, kindly and ably typed the manuscript. Ms. G. Wedlich-Gartelmann, Hannover, is thanked for drafting some of the figures. Constructive comments by two reviewers significantly improved the manuscript. Financial support was obtained through grants Sa 210/12 - 1, 2, 3 of the Deutsche Forschungsgemeinschaft (DFG), and by the German Society for Technical Cooperation (GTZ).
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