Sistemática de isótopos de oxigênio e carbono aplicada ao estudo da evolução metalogenética do depósito de Zn-Pb de Morro Agudo (MG)

Abstract

The Vazante-Paracatu region represents the mineral district responsible for all the Brazilian zinc production. In this region, mineral deposits are characterized by predominantly sulphide-rich ores (sphalerite, galena, pyrite, marcasite), as in Morro Agudo, and silicatic ores with willemite at Vazante. In the Morro Agudo deposit, the host rocks (dolomitic breccias and dolarenites) have evidence of early dolomitization, besides marine, meteoric and burial diagenesis. Three mineralization episodes have been defined and are associated with processes of (1) replacement of host rock by fine-grained sphalerite, (2) open-space filling in vugs and veins by sphalerite, galena, pyrite, marcasite, quartz, hydrothermal dolomite and bitumen, and (3) formation of late veins and veinlets with sphalerite within brittle structures. The isotopic signatures of the diagenetic dolomite indicate processes of interaction with mineralizing fluids, while the isotopic composition of the hydrothermal dolomite, associated with sulphides, suggests contribution of reduced carbon for the hydrothermal system originated from the stratigraphically upper argilic-dolomitic unit. This could imply in downward flow of cold and reduced fluids in addition to ascendant migration of hot and metalliferous brines during mineralization. The calculated oxygen isotopic compositions for hydrothermal fluids suggest formational origin for the ascendant and descendant fluids similar to those associated to the genesis of Mississippi Valley Type Pb-Zn deposits.