The Proterozoic Vazante – Paracatu carbonate-hosted zinc district, Minas Gerais, Brazil: Siliciclastic rocks and their relationship with carbonate-hosted Zn-silicate and Zn-sulphide mineralization


The Proterozoic Upper Vazante Sequence (≈ 1300 – 1100 Ma) in Minas Gerais, Brazil is a mixed carbonate-siliciclastic succession that contains a number of hypogene Zn-silicate and Zn-sulphide deposits along a 250 km – long belt, forming the Vazante – Paracatu Zn District (Monteiro et al. 2006; Olivo et al. 2018). Host rocks in the mineral deposits include a variety of dolomitic rocks within a shallow marine succession (Dardenne 2000). The Zn deposits and their contained resources (data from Nexa Resources, personal communication; Olivo et al., 2018) include: 1. The Vazante – North Extension Zn silicate mines (combined resources of 37.6Mt @ 19.78% Zn, 0.48% Pb, 29.94g/t Ag) located in the southern part of the district, 2. The Morro Agudo Zn – Pb sulphide mine (resource of 20Mt @ 5% Zn, 2% Pb) located in the central-north part of the district, and 3. The Fagundes – Ambrosia Zn – Pb sulphide deposits located in the northern part of the district (No current published resource). It is thought that the majority of the carbonate-hosted Zn mineralization was formed during the Brasiliano Orogeny (850 - 650 Ma; Dn; Dardenne and Freitas-Silva 1999; Misi et al. 2014). However, little is known about the sources of metals.
The Serra do Garrote Formation, a thick (500+ m) meta-siliciclastic package dominated by phyllites, carbonaceous phyllites (≥1 wt.% TOC) and minor coarse rock types, underlies all the known carbonate-hosted Zn-deposits. The lithologies are made up of muscovite- and chlorite-bearing rocks that contain variable amounts of quartz, albite, titanite and organic matter. These rocks can be subdivided into three distinct subunits/protoliths (SG1, SG2, SG3) on the basis of immobile elements, specifically whole-rock Al/Ti ratios. Whole-rock geochemical analyses of these shows that these rocks are enriched in Zn (up to 0.46% Zn), and that SG1 (mean 550 ppm Zn) and SG2 (mean 360 ppm Zn) contain higher Zn compared to the SG3 subunit (mean 125 ppm Zn). 
Zn-enrichment is associated with various generations of hydrothermal sphalerite and pyrite that formed prior to the Brasiliano Orogeny. Whole-rock element correlation matrices provide a geochemical signature of Zn-enrichment, which is associated with Cd, Cu, Hg, In, V ± Sb, Se, Mo in the SG1 and SG2 subunits. This signature is similar to that documented in the carbonate-hosted Zn deposits (Olivo et al. 2018; Slezak et al. 2014; Monteiro et al. 2007). Hydrothermal sulphides were at least, partially remobilized during orogenesis, suggesting that the siliciclastic rocks of the Serra do Garrote Formation could have been the source of Zn and other ore-related elements in the carbonate-hosted Zn deposits. 


Neil Fernandes is a Ph.D. candidate at the Department of Geological Sciences and Geological Engineering, Queen’s University, Canada. He is currently in the final year of his studies working with Dr. Gema Olivo and Dr. Dan Layton-Matthews. The H.C. Morris Fellowship is sponsoring a self-designed experiential learning program (ELP) that is allowing him to explore his interest in sediment-hosted mineral deposits in Canada, U.S.A, Australia, Namibia, Ireland, Sweden, Peru and Brazil by funding visits to government geological surveys, mining companies and mines, and universities in all these countries. The objective of his ELP is to gain expertise in all facets of these mineral systems, from exploration to remediation. Prior to his Ph.D. studies, he worked in Chile for Barrick Gold exploration. He holds a M.Sc. from the University of Alberta, and a B.Sc. (Honours) from the University of Toronto.