Hydrothermal Pd-Au Deposits Hosted by Proterozoic, Lake Superior-Type Iron-Formation in the Itabira District, Brazil.
The hydrothermal Au-Pd deposits are hosted by the jacutinga in the Lake Superior-type iron-formation of the Itabira Group, Minas Supergroup. Jacutinga is a highly sheared, metamorphosed, hydrothermally altered iron-formation composed of hematite, magnetite, quartz, tourmaline, talc and phlogopite (Cauê) or muscovite (Conceição). In the Cauê mine, the highest concentrations of Pd-Au occur in quartz and hematite veins parallel to the S1 foliation (Corpo Y, CorpoX, Aba Leste and Central orebodies) or parallel to the S2 foliation (Aba Norte orebody, which contains the lowest amount of Pd). The main orebody (Corpo Y) is on average 0.2 m thick and 70m long and extends for 250 m downdip in the direction of the Le elongation lineation. In the Conceição mine, the orebody consists of a single quartz vein parallel to S1 foliation. It extends 100 m down-dip in the direction of the Le elongation lineation, and 400 m along strike. The hydrothermal ore minerals are palladian gold (up to 20 wt% Pd), palladium antimonides-arsenides (arsenopalladinite and mertierite II) and a palladium selenide (palladeisite). Gold bullions average 91.21% Au, 6.27% Pd, 0.96% Ag and 0.85% Cu in the Cauê mine and 96.48% Au, 2.83% Pd, 0.63% Ag and 0.24% Cu in the Conceição mine.
The similarities between the Au-Pd orebodies at the Conceição and Cauê deposits suggest that they were formed by similar processes and their characteristics favor an epigenetic model for gold and palladium concentration in the iron-formation. The main mineralizing event is interpreted to be contemporaneous with the D1 deformation and peak metamorphism. The mineral assemblage and oxygen isotopic data suggest that the hydrothermal fluids responsible for transport of precious metals were hot (600° C), acidic (i.e., muscovite-stable), oxidizing (buffer by hematite-magnetite pair) and very poor in S, as denoted by the almost complete absence of sulfides and sulfates in the Pd-Au ore. Under these conditions, Au and Pd are transported preferentially as chloride complexes. The reaction of the fluid with the dolomitic carbonate (the probably protolith of the jacutinga) would have dissolved carbonate and K-feldspar to form muscovite or phlogopite and tourmaline, resulting in an increase in solution pH and a decrease of Cl concentration and subsequent deposition of gold and palladium minerals.
Since the Cretaceous, these Archean and Proterozoic rocks have been weathered, and the hydrothermal silicates and oxides have been replaced by kaolinite and Fe(Mn)-hydroxides; gold and palladium have been remobilized and reprecipitated as pure metals or Pd-oxides.
Gold, supergene, Sao Francisco Craton, iron formation, jacutinga, epigenetic, Itabira District, palladium, Brazil, hydrothermal