The Pb-Zn (Ba) Nonsulfide Mineralizations at Bou Caïd (Ouarsenis, Algeria): Mineralogy, Isotope Geochemistry, and Genetic Inferences
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Abstract:
The ore deposits of Bou Caïd (Ouarsenis, Algeria) occur in Jurassic and Cretaceous sedimentary rocks. The barite and Pb-Zn (Fe, Cu, and F) ore deposits of Bou Caïd belong to vein- and karst-type. The mineralization is represented in the whole area by a mixture of barite (currently still exploited) and nonsulfides consisting of hemimorphite, smithsonite, cerussite, hydrozincite, and Fe-oxy-hydroxides, with remnants of galena and sphalerite in variable proportions. Mineralogical and geochemical analyses were carried out on the Bou Caïd nonsulfides. Several samples representing nonsulfide mineralization (Grand Pic and at Srâa Abdelkader) were subjected to a multidisciplinary analytical approach, using optical microscopy (OM), powder X-ray diffraction (PXRD), Scanning Electron Microscopy with Energy Dispersive Spectrometry (SEM-EDS). Nonsulfide mineralization consists of a mixture of hemimorphite, hydrozincite, smithsonite, cerussite, and Fe-oxy-hydroxides, often with zebra-like textures. In the proposed paragenetic scheme, covellite and chalcocite are followed by cerussite, jarosite, smithsonite, and hydrozincite. Then, hemimorphite crystallizes, accompanied by mimetite, traces of malachite and clay minerals (also Zn-bearing), precipitate. Fe-(Mn)-oxy-hydroxides can form during various phases of the supergene stage. Small amounts of late barite can be related to partial remobilization and occur as reprecipitation products. Stable isotope analyses were performed on the calcites and metal carbonates of the supergene ores. Carbon and oxygen isotope values of smithsonite and hydrozincite were comparable to published supergene Zn carbonate data. The isotope values of the Bou Caïd calcites fell both into the hydrothermal carbonate and in the supergene fields.Keywords:
Supergene (geology)
Covellite
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
Sulfide Minerals
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Chalcocite
Hypogene
Covellite
Supergene (geology)
Bornite
Copper sulfide
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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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