METAMORPHISM OF HYDROTHERMALLY ALTERED ROCKS IN A VOLCANOGENIC MASSIVE SULFIDE DEPOSIT: THE PALMEIRÓPOLIS, BRAZIL, EXAMPLE

Abstract

The Palmeiropolis Cu-Zn (Pb) volcanogenic massive sulfide deposit, Brazil, consists of three ore bodies enclosed by hydrothermal alteration zones. The ore bodies and the alteration zones were metamorphosed under amphibolite facies conditions. The Palmeiropolis alteration zones are characterized by a great diversity of bulk rock composition that originated a wide variety of low variance mineral assemblages. These assemblages are composed of orthoamphiboles (anthophyllite and gedrite), hornblende, biotite, garnet, staurolite, sillimanite, gahnite and, rarer, cordierite. Based on analyses of mineral chemistry and mineral assemblages, temperatures are estimated to have been 550-625 degrees C and pressures 2-5.5 kbar. The temperature of metamorphism that prevailed at the Palmeiropolis deposit is comparable to other amphibolite facies massive sulfide deposits, such as Geco and Linda, Canada; Falun, Sweden; and Bleikvassli, Norway. The mineralogy of the alteration zones is similar in all these deposits even though they were metamorphosed at different pressure conditions, reflected by the crystallization of one of Al (sub 2) SiO (sub 5) phase (andalusite, sillimanite or kyanite).