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Geochemical mapping of porphyry deposits and associated alteration through transported overburden Rich, Shane Daniel

Abstract

Discoveries of porphyry copper deposits (PCDs) in British Columbia have declined over the past 20 years. Undiscovered PCDs are predicted to occur in the Intermontane Belt, an assemblage of volcanic arc terranes considered important for hosting large porphyry deposits. The Deerhorn Cu-Au porphyry, located within this belt, is covered with up to 60 m of undisturbed moraine. Detailed regolith mapping based on a new classification scheme for BC, combined with aqua regia and deionised water assays, sequential leaching, physicochemical measurements and hydrocarbon analysis, was undertaken to determine the response in the till to buried mineralisation. Geochemical relationships dependent on the regolith type were identified and are particularly evident in organic rich areas. To minimise background noise, data analysis was constrained to the dominent regolith unit: the undisturbed moraine (DrM), resulting in enhanced anomaly identification. Regolith mapping in advance of a soil survey can therefore allow targeted efficient sampling. Normalisation to organic carbon (Corg) to counteract the influence of organic matter on trace element variability resulted in anomalous response for elements As, Cs, Cu, Mo, Tl, U and W by conventional aqua regia on a 180-micron fraction B-upper-horizon soil. The response for six of these elements is peripheral to mineralisation in the surface environment. Multi-element evaluation is superior for mineralisation detection than individual element evaluation. Light hydrocarbon results indicate a surface response with a distinct depletion over mineralisation. Sequential leaching on selected anomalous Cu samples indicates that an aqua regia extraction was the optimal first choice for anomalous Cu detection. Copper-Fe-oxide-bearing grains interpreted as oxidised sulphides were recovered from an anomalous Cu-bearing sample and confirm a contribution of glacial clastic transport. Visual screening for clastic fragments allows a reprioritisation of anomalous response. Copper isotope results do not support Cu-ion migration from the buried mineralisation, but instead support a clastic contribution. Anomaly formation is interpreted as a combination of clastic transport, ionic migration of H⁺, vegetation uptake and surface redistribution.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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