Expanding the Footprint of Orogenic Gold –Trace Elements in Sulphides

The Macraes orogenic gold deposit in Central Otago, New Zealand was formed within the Otago Schist in the Early Cretaceous during long-lived terrane accretion and crustal thickening because of subduction along the Gondwana margin. This gold deposit is primarily composed of refractory, sulphide-hosted gold with a minor component of quartz vein-hosted free gold. The mineralising fluids are thought to have originated through metamorphic dehydration of rocks at depth at the greenschist to amphibolite facies transition and this fluid scavenged gold from the metamorphic pile. The gold-bearing fluid was subsequently focused, and gold and gold-bearing sulphides were deposited along various shears of the Hyde-Macraes Shear Zone (HMSZ). The conventional method of gold exploration across Otago and elsewhere in the world is via the use of fire assay and/or portable x-ray fluorescence (pXRF) to search for anomalous pathfinder elements like arsenic which are detectable in soil and rock at much higher concentrations than gold. This method of exploration is highly effective at identifying and evaluating large regional-scale, mineralised structures such as the HMSZ. This study examines the sulphides of the Macraes deposit directly to determine the concentration of trace elements (such as gold and arsenic) and establish their spatial dependency, zonation and inter-relationships. This project makes use of optical petrography, itrax, energy dispersive spectroscopy (SEM-EDS) and laser ablation-inductively coupled plasma-mass spectroscopy (LA-ICP-MS) to determine how the concentrations of trace elements behave with respect to each other and distance from a known ore zone in drill core. With these data, we seek to allow exploration within a discontinuously mineralised structure to vector towards hydrothermal sulphides rich in gold rather than metamorphic sulphides and by extension, characterise distal and proximal hydrothermal sulphides in mesoscopic structures such as graphitic shears. The itrax proved itself to have potential as an assaying tool since even at rapid count times and a relatively coarse sampling interval, arsenic counts were found to have a distribution that mimicked the results of traditional assay. With a longer count time and finer sampling interval, gold count distributions were useful for identifying potential grains of free gold in the core material that would easily have been missed by traditional core logging and assay. When compared to elements that may interfere with the gold count signal, such as tungsten and lead, it was found that gold could be somewhat reliably distinguished, this, when combined with the presence of free gold grains in the vicinity of high gold peaks means the itrax has the potential to reliably identify gold distributions in core. Sulphide-gold (within the sulphide structure) has a strong positive correlation with arsenic in pyrite and weaker correlations with copper, silver, bismuth and tellurium, sulphide-gold was found to not have an obvious correlation with lead, nickel or zinc. Micronugget gold (discrete particulate gold within sulphides) appears to be more abundant in arsenic-poor regions of sulphides, suggesting that substitution of arsenic forsulphur in pyrite may promote incorporation of sulphide-gold and prevent micro-nuggets from forming due to changes in the surface charge conditions of the sulphides. When the concentrations of sulphide-gold, arsenic, copper, and zinc are plotted against depth, there appears to be a systematic increase in sulphide gold concentration in regions of whole rock enrichment and the ore zone can be defined with greater detail using the methodologies set out in this this work than with traditional whole-rock methodologies. In addition to enhanced ore zone definition, this methodology could aid in exploration since regions with rare but auriferous sulphides can be detected and pursued. There also appeared to be a reduction in base metal concentrations in the ore zone suggesting potential in locating regions of base metal dearthity in sulphide-focused exploration