U-Pb geochronology & geochemistry of apatite within the Marshall Shear Zone of the Ernest Henry Deposit, NW Queensland

Abstract

Ernest Henry is the largest known Iron Oxide Copper Gold (IOCG) deposit within the Eastern Fold Belt of the Proterozoic Mount Isa Inlier. The deposit is hosted in meta-andesitic volcanics, the temporal equivalent of the Mount Fort Constantine volcanics. The orebody formed in a hydrothermally altered and brecciated system with the four shear zones: Hangingwall Shear Zone (HWSZ), Footwall Shear Zone, Inter-lens and the Marshall Shear Zone (MSZ). The Marshall Shear Zone has been overlooked due to its relative distance from the orebody. However, it plays an important role in improving our understanding of the timing of alteration, formation and mineralisation within the deposit. This study uses geochronology and geochemical techniques to investigate apatite grains within the MSZ and E1 deposits.

Apatite from the MSZ provided U-Pb ages between 1508 ± 45 Ma and 1463 ± 38 Ma, synchronous with the late stage of Cu-Au mineralisation and regional D3 deformation event of the Isan Orogeny. The E1-North apatite dated at 1550 ± 16 Ma, synchronous with early K (-Ba)-feldspar, apatite, biotite and magnetite of the stage 2 alteration at E1 and premineralisation at Ernest Henry. However, investigations of the paragenesis and fluid chemistry of apatite within the MSZ suggest that the apatite could have undergone fluid-induced resetting and could be older than the age found in this study.

Trace element compositions of the MSZ and E1 apatite suggest the fluids that formed the apatite were likely of magmatic and hydrothermal origin, rich in S + REE, but sourced from different magmatic bodies, similar to the Williams and Naraku Batholiths. The metasomatising fluid affecting the MSZ and E1-North apatite populations are likely a combination of the magmatic-hydrothermal fluids mentioned above, and a highly saline upper crustal brine, or an S-bearing fluid from the Corella Formation.