NUMERICAL MODELING OF HYDROTHERMAL MINERALIZATION IN THE VAZANTE ZINC DEPOSIT, BRAZIL
The Vazante deposit (29 Mt, 20% Zn) in Minas Gerais, central Brazil represents the world's largest known example of hypogene non-sulfide zinc (NSZ) mineralization, and is the principal deposit in the Vazante-Unaí trend, the largest Zn-Pb district in South America. Hypogene NSZ mineralization like that at Vazante is a mode of zinc enrichment in the Earth's crust that until recently had not been widely recognized, and the processes from which it originates are only beginning to be characterized and understood. Hypogene NSZ deposits have similarities to other sediment-hosted base metal deposits such as Mississippi Valley-type deposits in that they also are hosted predominantly by carbonates and appear to have been precipitated from heated brines. They differ in other important respects, however, particularly in terms of mineralogy, which in NSZ deposits is dominated by willemite instead of sphalerite.
In the present study, numerical modeling is being used to gain insights into the origin of NSZ mineralization at Vazante. A leading hypothesis for the formation of hypogene NSZ mineralization is mixing between cooler, oxidizing meteoric fluids and hotter, metal-rich basinal fluids. In the scenario tested thus far, mineralization is considered largely to have post-dated Neoproterozoic Brasiliano deformation such that fluid flow was strongly influenced by structures of this age. Results indicate that hydraulic head gradients on the order of 10-3 initiated during orogenesis would have been sufficient to allow deep basinal fluids to migrate up steeply dipping strata where they would have displaced and mixed with shallower resident meteoric fluids. The results also document the role of permeability contrasts in focusing fluid flow and mineralization in features such as the Vazante shear zone.