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

The Sossego iron oxide–copper–gold deposit in the Carajás 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.% 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 (~ 500 °C at 1.4 kbar). In the Sossego orebody, potassic alteration with orthoclase and Cl-rich biotite (up to 3.1 wt.%) 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 AlIV contents of the chlorite indicate crystallization at temperatures below 300 °C. Chalcopyrite occurs associated with pyrite (up to 2.3 wt.% Co and 0.2 wt.% Ni), native gold (up to 14.9 wt.% Ag), siegenite, millerite, vaesite, Pd-melonite, and hessite. Dilution and cooling of the hot metalliferous fluid (> 500 °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.