Alteration signatures and footprints of the Ernest Henry deposit and camp: Spectral mineralogy and geochemistry

Courteney Dhnaram1, Vladimir Lisitsin1, Suraj Gopalakrishnan1, Daniel Killen1

1Geological Survey of Queensland, Brisbane, Australia

The Ernest Henry Cu-Au deposit is the best-known – and the most economically significant to date – example of Iron-oxide copper-gold (IOCG) deposits in the Cloncurry district of the eastern Mount Isa Province in Queensland, Australia. Geological Survey of Queensland (GSQ) has undertaken systematic sampling and mineralogical and geochemical analyses of drill core samples to characterise geochemical and mineralogical zonation of hydrothermal alteration associated with the Ernest Henry deposit, at a scale of hundreds to thousands of metres.

Short-wave infrared (SWIR) and thermal infrared data was collected using the GSQ’s HyLogger-3 machine across 19 drill holes (9 continuous drill holes and 10 sub-sampled drill cores), with >200 samples collected across these drill holes analysed for multi-element geochemistry (four-acid digestion – ICP-MS / OES; lithium metaborate fusion; fire assay – Au,Pt, Pd; Leco furnace – C, S; KOH fusion – ion chromatography – F; and Aqua regia – ICP-MS, Hg, Se, Te; 48-67 elements). 

Multi-element geochemistry from samples within the orebody identified a Cu-Ag-S-Te-As-Bi-Mo-W-Co-Se-Re (Pb-In-U-Sb-Sn) signature of the main phase of copper mineralisation. This signature is mostly seen within and near the orebody (up to 100m) and shows a strong association with potassic alteration.

Chlorite composition within both SWIR and TIR spectra varied from Mg-rich chlorite to Fe-rich chlorite with increasing distance to Ernest Henry orebody.  This trend was seen both along the NE and NW trending drill holes up to a distance of ~1km. Both SWIR and TIR spectra were used to discriminate early regional Phlogopite from pre-ore Biotite within the Dark Mica mineral group, with Biotite alteration extending to ~2km NE of the deposit and only 500m to the NW.

Potassic alteration from both geochemistry and mineralogy was identified ~1 km outwards of the deposit (within the proximal zone).  K-feldspar group compositional variations were identified using TIR, with potassic alteration consisting of microcline within the proximal to ore zone and orthoclase associated with copper mineralisation.

Multiple phases of the same mineral was identified in the hyperspectral data across the study area, specifically minerals previously thought to be only associated with early regional alteration (actinolite, albite, epidote etc).  Understanding which phases of these minerals are associated with pre-ore and ore mineralisation for similar Cu-Au systems will hopefully aid exploration in the future.


Biography

Courteney Dhnaram is a Senior Geoscientist within the Mineral Geoscience group at the Geological Survey of Queensland. Cpurteney has experience working across the state working on regional-camp scale projects, recently focusing on copper-gold mineral systems within North-West Queensland.

About the GSA

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