Trace element zoning in pyrite determined by PIXE elemental mapping: evidence for varying ore-fluid composition and electrochemical precipitation of gold at the Spitskop deposit, Saldania Belt, South Africa.

The geochemistry of pyrite associated with mesothermal ‘invisible’ gold mineralization in the Neoproterozoic Saldania Belt was studied with micro-PIXE. The petrologically observed weak zoning of pyrite was clearly visible on elemental maps generated by this analytical method. It also allowed the positive identification of two generations of pyrite. Early, irregular pre- to syn-tectonic grains are As-poor and display strongly zoned later, possibly post-tectonic, overgrowths rich in As, Ni, Sb and Pb. This late phase also produced a generation of individual, distinctly zoned euhedral grains of similar composition. Gold is extremely fine-grained and hosted by the zoned pyrite grains as submicroscopic specks located on the boundary between As-poor and As-rich bands. This association is indicative of the electrochemical precipitation of gold. Zoning of the elements Ni, Sb and Pb in the late-stage pyrite is attributed to selective precipitation in response to changes in composition and physico-chemical conditions of the mineralizing fluids that have been derived from devolatilization of the enclosing metamorphosed volcano-sedimentary pile. Copyright © 2004 John Wiley & Sons, Ltd.