Mineralogical and chemical characterization of supergene copper-bearing minerals: Examples from Chile and Burkina Faso

We document the occurrence of inclusions of a (Ag,I)-rich mineral in supergene chalcocite from the Mantos de la Luna argentiferous stratabound Cu depos it in the Coastal Range of northern Chile. In this deposit, located 30 km south of Tocopilla, Cu mineralization occurs preferentially in the lower levels of amygdaloidal and porphyritic horizons. Mineral paragenesis is simple and composed exclusively of Ag-bearing supergene chalcocite (digenite), atacamite, and chrysocolla. EMPA observations reveal the presence of discrete, micron-sized (1-10 μm) inclusions of a Ag iodide mineral in supergene chalcocite. The inclusions were identified as iodargyrite by means of EDS and WDS elemental mapping. The Ag concentrations in the inclusions vary from 1.0-67.6 wt% and they are contaminated by Cu and S from chalcocite. The small size and the beam-sensitivity of the Ag-I inclusions precluded the precise descript ion of its chemical formula. However, the Ag and I elementa l maps strongly correlate with the inclusions, whereas the WD S maps of Cu and S correlate well with the chalcocite sulfide host. The occurrence of iodargyrite inclusions in supergene chalcocite suggests the involvement of iodine-rich waters during supergene enrichment at the Mantos de la Luna Cu deposit. Considering the fact that the occurrence of iodargyrite is restricted to extremely arid environments [1], our observations strongly suggest the prevalence of hyperarid conditions during the latest stag es of supergene enrichment of the Mantos de la Luna argentiferous Cu deposit in northern Chile. This suggests that supergene enrichment processes of Cu deposits in the hyperarid Atacama Desert are dynamic in nature and do not exclusively require the presence of meteoric water. Further studies are needed not only to address the isotopic signature (and age) of iodine-rich waters involved in supergene enrichment of these deposits (e.g. deep formation waters), but also to constrain the origin of iodine in the extensive nitrate deposits occurr ing in the eastern flank of the Coastal Range.

Chalcocite

Supergene (geology)

Hypogene

Bornite

Porphyritic

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Supergene mineralisation of the Boyongan porphyry copper-gold deposit, Surigao del Norte, Philippines

AM Ignacio

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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Citations (3)

Bacteria as Mediators of Copper Sulfide Enrichment During Weathering

Science (1996)

Richard H. Sillitoe Robert L. Folk Nicolás Saric

Supergene chalcocite enrichment during weathering is an economically vital natural process that may lead to severalfold increases in the copper content of sulfide deposits. A scanning electron microscope study of chalcocite (Cu2S) from major enriched copper deposits in northern Chile revealed myriad bacterioform bodies in original growth positions near replacement interfaces with remnant hypogene sulfide grains. These minute (0.03 to 0.2 micrometers) chalcocite bodies are interpreted as fossilized and metallized nannobacteria that promoted the fixation of mobilized copper ions. Bacterial activity may thus be a fundamental factor in supergene enrichment of copper deposits.

Chalcocite

Hypogene

Supergene (geology)

Copper sulfide

Pseudomorph

Bornite

Covellite

10.1126/science.272.5265.1153

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Citations (84)

Pleistocene recycling of copper at a porphyry system, Atacama Desert, Chile: Cu isotope evidence

Mineralium Deposita (2010)