DISTRIBUTION AND GENETIC CONTRASTS OF THE ZN, PB, F MINERALIZATIONS OF THE PROTEROZOIC BAMBUI-TYPE, MG, BRAZIL

The Middle Proterozoic MVT/Bambui-type Zn-Pb-F deposits of the São Francisco Basin occur in different tectonic ambient affecting their paragenetic distribution and genetic evolution besides geochemical control. The reasons for the partition of mineralizations have puzzled economic geologists along time. Vazante and Morro Agudo deposits are located at the southwestern border of the basin while the deposits at the Januária, Itacarambi and Montalvania (JIM) at the central-eastern border show significantly different mineral associations and paragenesis (Robertson, 1963). Vazante mineralizations stratigraphically situated within greenish gray slates intercalated with pink stromatolitic dolomites are of willemite-type formed by silicification process succeeded dolomitization and sulfidization in short timespan, with subordinate sulfides, in Serra do Poço Verde Fm. Vazante Group (Dardenne, 1998). Vazante, interpreted as controlled by the Rio Escuro 12km-long fault system consists besides willemite of hematite, zincite, smithsonite and franklinite and subordinate sphalerite-galena. Hemimorphite-hydrozincite (calamine-type) forms in superficially derived collapsing breccia karst along foliation. Morro Agudo comprehends a stratabound, cement forming sphalerite-(Ag-poor)galena-pyrite-barite mineralization piled up by strong recumbent folding and thrusting controlled by north-south shear zone, consequently forming large ore reserves, with very minor willemite, hosted by stromatolitic/breccia dolomitic bioherms intercalated with reddish chert and siliceous dolomites of Morro do Calcário Formation. Central-eastern JIM-mineralizations, in contrast, occur as finely dispersed, fissure-vein and bedding replacements of ZnS-PbS- and CaF₂-types, separately within intraformational calcitic dolomites, along borders of gentle domes. Anglesite, cerussite, smithsonite, willemite, descloizite, vanadinite and Cu-minerals also exist. Geochemical constraint related to the mineralizations may be expressed in terms of chemical potential µ_SiO2-µ_H2O and µ_CO2-µ_H2O representing the influence of SiO₂, HF, H₂S and CO₂ in the aqueous fluids. Important phosphorite deposits lie within the dark gray slates intercalated with dolomite at Lower Vazante Gr.