Styles of hydrothermal alteration and gold mineralizations associated with the Nova Lima Group, Quadrilátero Ferrífero: Part I, description of selected gold deposits

Abstract

Three main mineralization styles are recognized in a number of mesothermal, lode-gold deposits associated with rocks of the Nova Lima Group, Rio das Velhas Archean Greenstone Belt, Quadrilatero Ferrifero, Minas Gerais, Brazil. They comprise 1) structurally controlled, sulfide-replacement zones in metamorphosed oxide- and carbonate-facies BIF; 2) shear-related, disseminated sulfides in nydrothermal alteration zones within metamorphosed mafic volcanic- or sedimentary rocks; and 3) Auriferous quartz-carbonate-sulfide veins and veinlet systems. These styles reflect variations in fluid to rock interaction, and thereby a distinctive ore mineral compoSition. In addition, distinctive mineral associations characterize hydrothermal alteration zonal patterns around orebodies. Det alled investigations in Cuiaba, Juca Vieira, Raposos, Santana and Sao Bento mines indicate that, as is the case with other similar deposits worldwide, the processes of ore depoSition are the same processes that give rise to the alteration assemblages. Hydrothermal alteration overprints metamorphic assemblages, and is overall dominated by zones of chlorite, carbonate and white mica development in proximity to ore. Sulfidation and silicification abound in the latter two, minerallogically simple, inner alteration zones. These characteristics suggest variations in the H2O to CO2-ratio of the sulfur-bearing, aqueous-carbonic ore fluid and interaction at varying fluid to rock ratios with the progression of hydrothermal alteration.