Texture and chemistry of pyrite from the Waihi epithermal system, Hauraki Goldfield, New Zealand

Abstract

The Waihi epithermal system is a low sulfidation epithermal system that hosts one of the largest epithermal Au-Ag deposits in the Southern Hemisphere. Various sulfide minerals are present within the system, including pyrite (FeS2), one of the most abundant sulfide minerals on Earth. Pyrite is an archive for hydrothermal fluid chemistry due to its crystal structure enabling incorporation of other elements such as Co, Ni, Se, As, Te. The presence or absence of these elements reflects the changes in the hydrothermal fluid chemistry throughout the growth of the grain. The textural variations in the grains indicates changes in growth conditions for each grain at the time of formation. The chemistry of each different zone, coupled with stages of growth, indicates the chemical influences on the pyrite in the Waihi system at that time.

The use of petrographic, EPMA and LA-ICPMS analyses demonstrates distinctive textural zones within pyrite grains, varying in chemistry and appearance. Some zones contain elevated concentrations of Au and Ag, and varying concentrations of Cu, Pb or Zn. The textural observations indicate pyrite growth was interrupted by periods of dissolution before the grain continued growing. Most grains begin growth with high trace element concentrations. The lowest concentrations of trace elements occur in the outer growth phase. Inclusions are rich in Pb, Ca, Se, Zn, Au and Ni, and cause the ‘dirty’ texture observed in grains. The distinctive textural features are attributed to the varying concentrations in Au, As, Cu, Zn, Ag and Te.

This research showcases the dynamics of hydrothermal fluid flow at Waihi, revealing that the gold-forming hydrothermal system at Waihi are more complex than initially thought. There is both evidence for periods of low gold pyrite growth, and periods of high pyrite growth.