The hydrothermal evolution of the Alvo Açaí Cu (Au, Mo) skarn deposit, Carajás Province, Brazil.

Abstract

The Alvo Açaí Cu (Au, Mo) skarn deposit forms part of several poorly explored copper deposits in the western sector of the Carajás province in Brazil. Here, a 2.86 Ga metasyenogranitic basement and a 2.71 Ga quartzite of the Liberdade Group are crosscut by diabase and granitic pegmatite dikes in the deposit area. The sequence

of hydrothermal alteration at Alvo Açaí is marked by (1) early pervasive calcic-sodic alteration (hastingsite- albite) and (2) potassic iron alteration (biotite-almandine-grunerite) overprinted by (3) silicification (quartz), (4) prograde (grossular-hedenbergite), and (5) retrograde (actinolite-biotite-epidote) skarn stages. Late per- vasive chloritization along with epidote-calcite veinlets crosscut the previous alteration zones. The main stage of copper mineralization (I) is spatially and temporally related to the retrograde skarn alteration for which three chalcopyrite-bearing mineral assemblages are distinguished on the basis of textural relationships and mineral associations: (1) actinolite-chalcopyrite-pyrrhotite-pyrite-magnetite-molybdenite, (2) biotite-chalcopy- rite-pyrite-magnetite, and (3) epidote-pyrite-chalcopyrite-sphalerite. Minor chalcopyrite occurs along rare late- stage epidote-calcite veinlets (mineralization II). The evolution of a single hot H2O-NaCl-CaCl2–dominated magmatic fluid of moderate salinity (22.8–28.6 wt % NaCl + CaCl2 equiv) toward a cooler H2O-NaCl fluid, with likely variable amounts of FeCl2, MgCl2, and KCl of low to moderate salinity (0.1–33.2 wt % NaCl equiv). Fluid evolution as a result of progressive crystallization of the granitic pegmatite was likely the trigger for mineraliza- tion I as supported by calculated δ18OH2 O values from retrograde quartz. The chalcopyrite and pyrite δ34S values (–1.5, –1.1, and –0.7‰) point to a magmatic origin for the sulfur, which was most probably leached from sur- rounding igneous host rocks. The evolution of the Alvo Açaí deposit encompasses the development of the first copper skarn mineralization recognized in the Carajás province.