CHLORITIZATION OF THE HYDROTHERMALLY ALTERED BEDROCK AT THE IGARAPE-BAHIA GOLD DEPOSIT, CARAJAS, BRAZIL

The Ingarape Bahia gold deposit has developed from weathering of a nea r-vertical hydrothermal Cu (Au) mineralization zone. The unweathered b edrock composed of chlorite schists is mainly metamorphosed basalts, p yroclastic and elastic sedimentary rocks and iron formation. Contents and Fe/(Fe + Mg) ratios of chlorites increase from distal country rock towards the mineralization zone, which can be attributed to different water/rock ratios and locations in a hydrothermal system. In the hydr othermal system high salinity fluids convected through basin-floor roc ks, stripping metals from the recharge zones with precipitation in dis charge zones. The chlorite with lower Fe/(Fe + Mg) ratios indicates al teration by relatively unreacted Mg-rich fluids, occurring within rech arge zones. By contrast, the chlorite with higher Fe/(Fe + Mg) ratios in the mineralization zone formed from solutions rich in Fe, Mn, Au, C u, H2S and SiO2 within a discharge zone. The iron formation could also be formed within the discharge zone or on the basin floor from the Fe -rich fluids. The distal country rock with less chlorite content is a hydrothermal product at low water/rock ratios whereas the proximal cou ntry rock and the host rock with more chlorite content formed at high water/rock ratio conditions. The AI(IV) contents of chlorites indicate that the formation temperatures of these rocks range from 204 to 266 degrees C, with temperatures slightly increasing from distal country r ock towards the mineralization zone.