The geochemistry of gossans associated with Sarcheshmeh porphyry copper deposit, Rafsanjan, Kerman, Iran: Implications for exploration and the environment
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Supergene (geology)
Chalcocite
Hypogene
Jarosite
Marcasite
Bornite
Acid Mine Drainage
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
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Covellite
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Marcasite
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Chalcocite
Bornite
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Covellite
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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.
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The Cerattepe mine, one of the volcanogenic massive sulfide deposits in northeastern Turkey, is hosted within the late Cretaceous volcanic, intrusive and sedimentary rocks. Deposit's main ore body contains high-grade massive copper sulfides and a gold-silver and barium rich oxide zone, characterized by dense alteration stages, is situated on top of it. Replacement, cataclastic, breccia, dissemination, dendritic, concentric growth, colloform, and framboidal textures were identified. Pyrite, sphalerite, marcasite, chalcopyrite, bornite, galena, tennantite-tetrahedrite, gold, covellite, digenite, chalcocite, cuprite and cubanite constitute the mineral paragenesis where quartz, calcite and barite account for the gangue minerals. Limonite, hematite, lepidocrocite, malachite, azurite and jarosite developed in the oxidation zone.
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The Manus back arc basin is well known for hosting submarine hydrothermal deposits of Vienna Wood, Pacmanus, Onsen site and Solwara 1-12. The Vienna Wood located in the Central Manus basin is a typical Cu-Zn type of mineralization hosted by mid-oceanic ridge basalt and consists predominantly of sphalerite, wurtzite, chalcopyrite and gangue minerals of anhydrite, gypsum and silica. In contrast, the Pacmanus hydrothermal deposit in the eastern part of the Manus basin is hosted by rocks ranging from basalt to dacite and rhyodacite. The mineralization is of the polymetallic Zn-Cu-Pb-Au type consists of ore minerals of sphalerite, chalcopyrite, bornite, wurtzite, pyrite, marcasite, enargite, tennantite, galena, Pb-As-Sulphosalt, gold, covellite, digenite, chalcocite and gangue minerals of barite, amorphous silica, anhydrite and gypsum. The chemical composition of sphalerite, galena, gold and tennantite-tetrahedrite indicate high concentrations of Lead, Arsenic, Silver, Iron, Copper and Antimony compared to their stoichiometric composition. The Onsen site in the Eastern Manus Basin is the first deep sea acid sulfate type of mineralisation and consist of enargite, covellite, chalcopyrite, pyrite, marcasite and gangue minerals of pyrophyllite, alunite, quartz, cristoballite, amorphous silica and native sulfur. The PNG Government has granted exploration and mining licenses to Nautilus Mineral for commercial exploitation of these deposits and Solwara 1 deposit in the Eastern Manus Basin. Nautilus minerals has done extensive exploration and reports indicated and inferred a mineral resource of 1030 kt and 1540 kt respectively for their Solwara 1 project east of Pacmanus site at a 2.6% Cu equivalent cut off grade. Whether this mineral resource is sufficient to deliver PNG first deep sea mining or not will depend on the success of the trial mining method that is currently being developed. Studies done on nearby Pacmanus and Onsen submarine hydrothermal deposits indicate that the deposits contain a much higher proposition of deleterious elements such As, Pb and Sb. Nautilus did not disclose the full mineralogy and assay results of the Solwara 1 and 2 projects to the public domain apart from Cu, Au, Ag and Zn. Nautilus did not suggest how it will address toxic and heavy metals that are associated with the ore concentrate in its Environmental Impact Studies.
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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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