Pyrite geochemistry and textures in the epithermal Au-Ag mineralisation at Waihi

Abstract

The Waihi epithermal system is a low sulphidation epithermal system that hosts one of the largest high grade ore deposits in the Southern Hemisphere. The use of petrographic and EPMA analyses demonstrate there are distinctive textural zones within pyrite grains that vary in chemistry and appearance. Some of these zones contain elevated concentrations of Au and Te, and varying concentrations of Fe, S, Cu, Pb or Zn. The textural observations indicate that in some samples, pyrite growth was interrupted by periods of dissolution. Grains begin growth with overall low trace element concentrations, with the highest concentrations of trace elements occurring in the middle phase of the grain’s growth. Inclusions are rich in Pb, Ca, Se, Zn and Ni, and are attributed to the ‘dirty’ texture of some samples. Inclusion-rich areas are also high in Au. The nature of the presence of Te suggests either a single source of magmatic fluid, released in pulses, or periodical dilution of the magmatic fluid by meteoric waters.