Gold geochemistry in a semiarid weathering environment : a case study of the Fazenda Brasileiro deposit, Bahia, Brazil

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1987

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Abstract

Oxidation of sulfide- and carbonate-rich vein gold deposits in a semiarid environment can be represented as a three-stage process, each creating supergene conditions conducive to the dissolution and reprecipitation of gold-silver alloys. These processes have been documented at the Fazenda Brasileiro gold deposit within the Rio Itapicuru Greenstone Belt, an Archean volcanosedimentary sequence in northeastern Brazil, about 170 km from the Atlantic coast. The deposit consists of a mineralized zone 10 km long, 100 to 200 m wide, with proven reserves of 100 metric tons of gold. Average ore grade is 7 g/ton (7 ppm), but gold can be economically recovered from oxidized surficial material containing less than 1 ppm Au. Present monthly production is 60 kg of gold bullion. The initial stage of weathering involves oxidation of sulfides under moderately oxidizing conditions in a neutral to alkaline environment. Under these conditions carbonates are dissolved and leached and sulfates and arsenates are precipitated. Sulfide oxidation releases a variety of sulfur species into solution, including SO₄=, H₂S, HS⁻, S=, and HSO₄⁻, as well as metastable species such as S₂O₃= and SO₃=. Under these conditions gold-silver alloys are soluble and enter solution as MeS₂O₃ complexes, which can be readily destabilized at low pH, by variation of oxidation potential, and by dilution. The silver and gold complexes formed under these conditions are unstable, and the reprecipitated alloy is enriched in silver. The second stage takes place within the oxidized zone above the water table. The previously formed sulfates and arsenates dissolve in O₂- and Cl-rich meteoric water. Oxidation of remnant sulfides, dissolution of sulfates and arsenates and the oxidation of Fe⁺² to Fe⁺³ creates the acid oxidizing conditions favorable to dissolution and transport of gold and silver as chloride complexes. AgCl⁰ (or AgCl₂⁻), the silver complex formed under these conditions, is more stable than AuCl₄⁻, the gold complex. Consequently, upon reprecipitation of gold by decreasing oxidation potential, by dilution and lowering of pH of the solutions, or by reaction of the solutions with albite, adsorption on quartz and/or iron oxides, silver remains in solution. Thus, the fineness of this type of secondary gold increases. The final stage involves dehydration of hydrous iron oxides formed during stage 2. During long periods of drought the hydrous iron oxides lose their bound water. Some sulfates persist from stage 2 in association with the hydrous iron phases, and there is an enrichment of manganese in the iron hydroxides. As ferrihydrite dehydrates to goethite and/or hematite, the hydrous iron minerals develop desiccation cracks. The H₂SO₄-bearing acid water generated migrates through the cracks and carries dissolved metal species, including gold as the AuCl₄⁻ complex. Further dehydration may destabilize the metals from solution causing their precipitation in these cracks. Also, input of fresh rain water dilutes the acid dehydration water, contributing to metal precipitation. Gold formed by this process is also silver depleted

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