Gold distribution and the relationship to pyrite trace element geochemistry at the Ernest Henry deposit, Queensland

Abstract

The Ernest Henry deposit is situated within the Eastern Fold Belt of the Mount Isa Inlier, NW Queensland, and is the largest iron oxide copper-gold (IOCG) deposit in the Proterozoic Cloncurry district. The hydrothermal deposit is hosted in brecciated intermediate metavolcanic and metasedimentary rocks with a biotite-calcite-chalcopyrite-gold-magnetite-pyrite-quartz mineral assemblage.

This study investigates the mineralogical, textural and geochemical association between gold and pyrite with samples collected from three drill holes (EH768, EH859 and EH864) at ~700m vertical depth within the ore body.

Majority of the gold (~98%) at Ernest Henry is in the form of free gold, which is commonly observed in pyrite microfractures, associated with chalcopyrite infill. Free gold has been interpreted to have entered the system with the main economic Cu-Au mineralisation stage. We propose that the semi-conducting potential of pre-existing pyrite surfaces have acted as a catalyst for the precipitation of free gold.

Implications from this study may assist in the improvement of gold recovery at Ernest Henry and provide a better understanding of the timing of the ‘C’ and ‘G’ in ‘IOCG’ deposits in the local Cloncurry District.