A 3-STAGE GENETIC MODEL FOR THE IGARAPE BAHIA LATERITIC GOLD DEPOSIT, CARAJAS, BRAZIL

A significant gold deposit in laterite is under development in the Carajas region of Brazil. The lateritic profile of the Igarape Bahia deposit, Carajas, can be divided into six horizons: basal unweathered rocks followed by saprolite, a pallid zone, a mottled zone, a ferruginous zone, and topsoil. Native gold and electrum are found within iron oxide-hydroxide nodules in the ferruginous zone where gold orebodies have been delineated. Most of the gold grains are less than 10 mum in size. There are three morphological types: (1) rounded grains in voids, associated with colloform goethite and needlelike goethite; (2) irregularly shaped grains filling shrinkage cracks in amorphous goethite; and (3) subidiomorphic crystals associated with amorphous goethite or needlelike goethite. The morphological features and occurrences of the gold suggest that it has been mobilized and reprecipitated during lateritization. In the ferruginous zone, gold grains from the upper part contain more silver than those from the lower part, i.e., the fineness of gold increases downward in the profile. Quartz veinlets and sulfides in bedrock suggest that the primary rocks were enriched in gold. In the first weathering stage thiosulfate ions from weathering of sulfides are hypothesized to transport both gold and silver into the saprolite and mottled zones where secondary electrum and silver-rich gold were formed. In the second stage, gold and silver were remobilized and incorporated into the ferruginous zone as the weathering proceeded and the erosion surface was lowered. In the third stage, after uplift and incision of the landscape, chloride leaching separated silver from gold grains to purify the electrum. The survival of electrum in the upper part of the ferruginous zone can be attributed to lower chloride concentrations in near-surface ground water and to enrichment of organic matter which can lower the redox potential and increase the stability of native silver. Some silver was then adsorbed by manganese oxide minerals in the lower part of the ferruginous zone.