FLUIDOS ASSOCIADOS À MINERALIZAÇÃO DA MINA DE OURO SCHRAMM, COMPLEXO GRANULÍTICO LUIS ALVES (SC)

Abstract

The gold mineralization at the Schramm mine, in Santa Catarina State, southern Brazil, is associated with quartz and carbonate veins within a ductile-brittle, NNE-striking, subvertical shear zone, of Neoproterozoic age. The shear zone crosscuts Archean basic granulites and gneisses of the Luiz Alves complex, next to the contact with the volcano-sedimentary sequence of the Itajai Group. Within the vein vicinities gneisses have been altered to quartz and muscovite and granulites to chlorite and albite; carbonate alteration is pervasive in all host rocks. Both carbonate (siderite, ankerite and dolomite) and quartz veins in the mineralized zone contain As, Ni, Co, Cu, Zn, Pb and Fe sulfides, but the high grade zones (above 500 ppm Au) are mostly within the former. On the basis of their modes of occurrence and microthermometric behavior, four types of fluid inclusions were identified in carbonates (C1, C2, C3, and C4) and three in quartz (Q1, Q2, and Q3). Primary H 2 O-CO 2 inclusions with salinity values in the range of 0,2 to 14 wt% NaCl eq. and total homogenization temperatures between 280°C and 310°C are represented by the inclusion types C1, C2 and Q1. The inclusion types Q2 and C3 are primary aqueous, with salinities between 4 and 11 wt% NaCl eq. and total homogenization temperatures within the interval of 205°C and 270°C. The inclusion types Q3 and C4 are of secondary origin and contain an aqueous solution with salinities varying from 3 e 13 wt% NaCl eq. and total homogenization between 140oC e 190oC. These inclusions were trapped during the quartz and carbonate recrystallization, as a result of the reactivation of the shear zone and formation of the Itajai Basin. The primary inclusion types C1, C2, C3 and Q1 and Q2 in carbonate and quartz, respectively, are considered as representatives of the gold-mineralizing fluid, which are considered originally a low sulfur, CH 4 – and N 2 - free H 2 O-CO 2 solution, rich in CO 2 , Fe, Mg, Ca, Na, K, Cl, Au, Ag, Ni , As and SiO 2 , with Zn and Co in subordinate amounts. Massive carbonate veins were the first to precipitate from this fluid during the temperature rise of the hydrothermal system, together with the gold and sulfides, due to the reaction of this fluid with iron rocks intercepted by the shear zone. This reaction may have led to the desestabilization of the metal-bearing sulfur complexes, reduction of the gold and precipitation of the sulfides. Sulfide-rich, gold-poor quartz veins crystallized afterwards, in the waning stages of temperature drop due to the mixing of the H 2 O-CO 2 fluid with meteoric waters.