Mineral chemistry of ore and hydrothermal alteration at the Sossego iron oxide-copper-gold deposit, Carajas Mineral Province, Brazil

摘要

The Sossego iron oxide-copper-gold deposit in the Carajas Mineral Province comprises two major orebodies, Sequeirinho and Sossego. Sodic alteration (albite-hematite) and sodic-calcic alteration zones represented by albite, ferro-edenite/hastingsite (up to 3.8 wt. percent Cl), actinolite/magnesiohornblende, magnetite, titanite, epidote, and calcite are predominant at Sequeirinho. Magnetite bodies with envelopes of apatite-rich actinolitite were formed with the sodic-calcic event at high temperatures (approx 500 deg C at 1.4 kbar). In the Sossego orebody, potassic alteration with orthoclase and Cl-rich biotite (up to 3.1 wt. percent) and chloritization are the main alteration types. Mineralized breccias in both orebodies have coarse-grained zoned actinolite/ferro-actinolite, Cl-apatite, and magnetite within the matrix. Sulfides occur in equilibrium with a paragenetically late calcite-quartz-chlorite-epidote-allanite assemblage. The Al~(IV) contents of the chlorite indicate crystallization at temperatures below 300 deg C. Chalcopyrite occurs associated with pyrite (up to 2.3 wt. percent Co and 0.2 wt percent Ni), native gold (up to 14.9 wt. percent Ag), siegenite, millerite, vaesite, Pd-melonite, and hessite. Dilution and cooling of the hot metalliferous fluid (>500 degg C) by mixing with meteoric fluids may have been the main mechanisms responsible for the deposition of metals transported as metal chloride complexes in both orebodies of the Sossego deposit.