Trace-element composition of pyrite and its implications for hydrothermal process within the Mesoproterozoic metasedimentary sequences of the São Francisco Craton, northeastern Brazil

Abstract

The distribution of trace elements in pyrite has been documented for the first time in quartz veins hosted in the Mesoproterozoic metasedimentary sequence of the Tombador Formation, São Francisco Craton, northeast Brazil. In this study, Electron Microprobe Analyses (EPMA) were used to determine the trace-element compositions of pyrite in these hydrothermal quartz veins. Three pyrite types have been distinguished and interpreted from petrographical relationships and trace-element patterns. Pre-existing pyrite (Py1), derived from the host-rock quartzite, is Ni-poor with concentrations varying from 600 – 6,100 ppm. Elongated syn-tectonic pyrite (Py2) has similar trace-element composition to the Py1, with Ni amounts ranging between 830 and 7,870 ppm. In contrast, possibly post-tectonic, euhedral to subhedral hydrothermal pyrite (Py3), contains higher contents of Ni (7,970 – 26,120 ppm). Mafic and/or metasedimentary rocks from the Espinhaço Supergroup were probably the source of Ni for this fluid-flow event. Fluid generation is related to  the devolatilization of the base of the thickened crust, with migration of fluids by preexisting structures. Several shear zones and large-scale NNW-trending folds were developed during the inversion of the Espinhaço basin, as a result of the ca. 0.6 Ga Brasiliano orogenic event. Regional fluid movement through the crust at this time is supported by several mineralized veins and hydrothermal deposits in the São Francisco Craton and adjacent Neoproterozoic belts.