Formation of the giant Aynak copper deposit, Afghanistan: evidence from mineralogy, lithogeochemistry and sulphur isotopes
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Abstract:
Aynak is the largest known copper deposit in Afghanistan, with indicated resources of 240 Mt grading 2.3% Cu placing it in the 'giant' category. Host rocks are Neoproterozoic metasediments comprising dolomitic marble, carbonaceous quartz schist and quartz-biotite-dolomite schist containing garnet, scapolite and apatite. Chalcopyrite and bornite dominate the hypogene ore with lesser pyrite, pyrrhotite, cobaltite and chalcocite, and rare sphalerite, molybdenite, uraninite and barite. Sulphides occur as bedding-parallel laminae, disseminations, metamorphic segregations and crosscutting veins. Sulphide δ34S ratios range –14.5 to +17.3‰ in bedded and disseminated sulphides (n = 34). This broad range favours biogenic reduction of seawater sulphate as a major source of sulphur, although thermochemical reduction processes are not precluded. The narrower δ34S range of –6 to +12.2‰ in vein and segregation sulphides (n = 21) suggests localized redistribution and partial homogenization during metamorphism. Geochemical associations suggest that Al, P, Ca, Ti and Fe were primary sedimentary constituents whereas Cu, Mg, S, Se, As, Co and Bi were introduced subsequently. We infer that Aynak originated as a shale- and carbonate-hosted stratabound replacement deposit, resembling orebodies of the Central African Copperbelt, although underlying red-beds are absent at Aynak and mafic volcanics were the probable copper source. These giant deposits formed worldwide in the Cryogenian probably due to marine enrichment in copper, magnesium and sulphate coincident with profuse basaltic volcanism and ocean oxidation.Keywords:
δ34S
Covellite
Bornite
Molybdenite
Hypogene
Chalcocite
The Journal of the Japanese Association of Mineralogists Petrologists and Economic Geologists (1961)
The authors have arrived at the following results upon studying the idaite-bearing ores from ten different localities in Japan. 1) Idaite have been found in several deposits in Japan. The specimens from Yakuwa Mine, Yamagata Prefecture, have been unequivocally identified by X-ray powder method. The number of sites yielding idaite in Japan is expected to increase with further study. 2) Idaite occurs in the secondary enrichment zone of copper deposit in the form of secondary decomposition product of chalcopyrite and bornite, except in two sites, where it is present as final crystallization product in copper deposits formed under low temperature and pressure. 3) Hypogene idaite is contained in small quantity in bornite, together with digenite and chalcopyrite. Supergene idaite forms lattices, lamellae and veinlets buried in chalcopyrite and bornite. In some cases, it forms films between chalcopyrite and supergene chalcocite, or replaces pyrite in company with supergene chalcocite, digenite and covellite. 4) The process of secondary enrichment and oxidation of chalcopyrite and bornite may be summarized as follows; chalcopyrite→supergene bornite hypogene bornite idaite chalcocite digenite→covellite cuprite tenorite native copper
Bornite
Chalcocite
Covellite
Supergene (geology)
Hypogene
Marcasite
Copper sulfide
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Chalcocite
Covellite
Hypogene
Bornite
Supergene (geology)
Malachite
Sulfide Minerals
Pyrolusite
Laterite
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Boyongan is a blind copper-gold porphyry deposit that was discovered by Anglo
American Exploration (Philippines), Inc. in August 2000. It is located in Surigao del Norte,
Philippines. Current inferred mineral resources for Boyongan are estimated at 219 million
tonnes of combined oxide and sulfide material with an average grade of 0.51% copper and
0.74 grams of gold per tonne. Most of the high-grade mineral resource is within the oxide
(supergene zone).
Deep oxidation at Boyongan has developed a thick supergene enrichment profile (up to
600 meters) which has a complex supergene mineralogy, consisting of chalcocite, digenite,
pseudo-covellite, native copper, cuprite, malachite, pseudo-malachite, azurite, chrysocolla,
pseudo-chrysocolla, and pseudo-neotocite. Fine gold (<100μm) has been observed in
goethite, chalcocite, chrysocolla, and malachite. Supergene mineralisation is associated
with iron oxides (goethite with minor hematite) and clays (kaolinite, halloysite, illite and
montmorillonite). Oxidation and the development of supergene minerals has been
controlled mainly by fracturing and the availability of hypogene sulfides. The low pyrite
content of hypogene mineralisation at Boyongan allowed supergene mineralisation to
develop in-situ from near-neutral pH groundwaters.
The initial stages of supergene mineralisation involved the replacement of hypogene
sulfides such as chalcopyrite and bornite by chalcocite, digenite and pseudo-covellite. In
some places, chalcocite replaced pyrite. Goethite formed during the weathering of pyrite,
chalcopyrite, bornite and chalcocite. Copper that was released into solution precipitated as
native copper, which has replaced chalcocite locally. Native copper was then oxidised to
form cuprite, and also acicular and euhedral crystals of chalcotricite. Some cuprite may
have precipitated directly from solution, and also where chalcocite reacted with oxygenated
groundwaters. The final stages of supergene copper mineralisation at Boyongan produced copper carbonate (malachite, pseudo-malachite, azurite and pseudo-neotocite) and a copper
silicate overprint (chrysocolla and pseudo-chrysocolla) onto earlier-formed copper oxides
and sulfides.
Copper generally has a more dispersed or erratic distribution than gold. Gold is restricted
spatially to the early mineral intrusions. Copper grades in the cuprite-dominated zone in the
west generally decrease with depth toward zones of patchy native copper. The copper
carbonate (malachite-azurite)-dominated blanket above the cuprite zone contains both high
grade copper and gold (>1% and >2 g/t, respectively). Chalcocite zones that have partially
replaced hypogene copper sulfides have higher grades (>0.5% Cu and >1g/t Au) compared
to zones of chalcocite replacing pyrite (<0.5% Cu and <0.5g/t Au). Chrysocolla and/or
pseudo-chrysocolla is confined to zones that contain high copper and gold grades (>0.5%
and >1 g/t, respectively).
Isotopic compositions of malachite and azurite from Boyongan are consistent with
deposition from ambient temperature (15°C to 20°C) meteoric water. These low
temperatures are consistent with Boyongan being a low-sulfide porphyry system. Higher
pyrite contents would probably have led to greater degrees of sulfide oxidation as well as
higher groundwater temperatures. δ13C values of malachite are consistent with an organic
carbon (soil?) source suggesting that malachite may have formed when Boyongan was
uplifted and exposed. δ13C values of azurite are much higher, and could be derived from
seawater, or by remobilisation of an inorganic carbon from carbonate wallrocks, or by
sulfide oxidation by supergene-related bacteria above the water table.
Chalcocite
Bornite
Supergene (geology)
Covellite
Hypogene
Pyrolusite
Alunite
Cuprite
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The Kyaukse Sabe Taung copper deposit is located in the western margin of the Shan Plateau, central Myanmar. It has estimated reserves of 1.6 Mt of ore grading 3.15% Cu, with additional Ag and Au enrichment. This deposit is typically hosted by Middle Ordovician limestone of the Wunbye Formation and the mineralization is controlled by the regional structures. The ore-mineral assemblage consists of pyrite, chalcopyrite, tetrahedrite, chalcocite, covellite, bornite, native copper, malachite, azurite, cuprite, tenorite, silver amalgams and Bi-sulphosalts, members of hammarite-aikinite series. Gangue minerals are quartz, dolomite, ankerite and calcite. Silver mineralization is represented by acanthite and by silver amalgams with a high content of Hg, ranging from 2.15 to 26.68 wt.%. Silver minerals are a result of supergene alteration of Ag- and Hg-bearing tetrahedrite and could increase economic value of the deposit. Silver association is located within the lowermost part of the supergene zone in the deposit, and tracking the redox barrier position can help to identify parts of the orebody with silver enrichment. The d34S values of chalcopyrite are +5.7 to +10.7‰ and those of tetrahedrite are +1.6 to +1.7‰.
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Bornite
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Tetrahedrite
Ankerite
Malachite
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Cuprite
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The giant Tizert copper deposit is considered as the largest copper resource in the western Anti-Atlas (Morocco). The site is characterized by Cu mineralization carried by malachite, chalcocite, covellite, bornite and chalcopyrite; azurite is not observed. The host rocks are mainly limestones (Formation of Tamjout Dolomite) and sandstones/siltstones (Basal Series) of the Ediacaran/Cambrian transition. The supergene enrichment is most likely related to episodes of uplift/doming (last event since 30 Ma), which triggered the exhumation of primary/hypogene mineralization (chalcopyrite, pyrite, galena, chalcocite I and bornite I), generating their oxidation and the precipitation of secondary/supergene sulfides, carbonates and Fe-oxyhydroxides. The Tizert supergene deposit mainly consists of (i) a residual patchwork of laterite rich in Fe-oxyhydroxides; (ii) a saprolite rich in malachite, or “green oxide zone” where primary structures such as stratification are preserved; (iii) a cementation zone containing secondary sulfides (covellite, chalcocite II and bornite II). The abundance of Cu carbonates results from the rapid neutralization of acidic meteoric fluids, due to oxidation of primary sulfides, by carbonate host rocks. Chlorite is also involved in the neutralization processes in the sandstones/siltstones of the Basal Series, in which supergene clays, such as kaolinite and smectites, subsequently precipitated. At Tizert, as can be highlighted in other supergene Cu-deposits around the world, azurite is absent due to low pCO2 and relatively high pH conditions. In addition to copper, Ag enrichment is also observed in weathered rocks; Fe-oxyhydroxides contain high Zn, As, and Pb contents. However, these secondary enrichments are quite low compared to Cu in the whole Tizert site, which is therefore, considered as relatively homogeneous.
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This study was planned with the aim of identifying the nature and circumstances of the high-graded central core and increasing trend of copper content through depth of 1000 m in Miduk PCD. Mechanisms of high-grading, refer to hypogene enrichment (HE), in PCDs poorly understood. Two main hypotheses for hypogene enrichment formation assumed addition of extra copper to the system, alternatively hypogene leaching and enrichment. In order to obtain alteration-mineralization-geochemical pattern both horizontally and vertically, all macroscopic data extracted from relogging of 6800 m' drill core along an east-west profile, compiled with microscopic observations from studying of 550 thin-polished sections and copper grades of 3400 samples analyzed by XRF and ICP-OES. Our findings proved hypogene enrichment events at deposit. HE evidences in macroscopic and microscopic scales identified almost as various replacement textures between Fe-Cu sulphides and also vein-reopening by later Cu-mineralization and new generation of disseminated or vein type mineralization. In addition, appearance of dark halo, as consuming intermediate chalcocite phase, around pyrite and chalcopyrite which gradually evolves as bornite, also extruding extra iron as fibrous hematite at the outer edge of bornite product replaced chalcopyrite, partially replacement of bornite and chalcopyrite to hypogene chalcocite and covellite-digenite in deep potassic are other HE evidences in the case study. Here, we draw on microscopic observations and SEM-BSE-EDS results, secondary hypogene genesis for some of bornite and chalcopyrite as a hypogene enrichment evidence. Observations from relogging show that potassic alteration has a relatively good preservation in the center of the deposit from depth to surface, but affected by intense overprinting of subsequent alterations towards margins. Evident function of ore-leaching at margins, also elevated copper grades in central parts of the deposit strongly suggest leaching-fixation mechanism. Where buffer potential of the rock is preserved copper fixation and where it totally eliminated almost complete leaching of copper happened. Consequently, we introduce leaching-fixation as index processes in hypogene enrichment at the case study. We suggest that identifying the nature of hypogene enrichment processes and its characterizations not only improve understanding about PCD's hydrothermal evolution, but also achieve exploration indicators, furthermore, industrial benefits in the production line.
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Bornite
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