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Surficial geochemical tools for Cu-Mo porphyry exploration in till-covered terrain Chouinard, Rachel L.M.

Abstract

As the discovery of porphyry Cu deposits from outcropping mineralised or altered bedrock decreases, mineral exploration is required to adapt new strategies and techniques for exploring deeper. This is especially important in areas with disconnected transported cover, such as the Quaternary glacial sediments that cover much of Canada. Surficial geochemical exploration strategies to detect the presence of known bedrock mineralisation through transported cover were evaluated at two buried porphyry Cu-Mo targets, Highmont South and J.A., at Highland Valley Copper. The Highmont South target subcrops below 2‒10 metres of till, whereas the J.A. target is concealed by sequences of pre-Quaternary to Holocene sediments up to 347 metres in thickness. Detailed surficial mapping at both targets followed by upper B horizon soil sampling, physicochemical measurements, profile sampling, biogeochemical sampling, and installation of soil hydrocarbon collectors was undertaken to characterise the mineralogical and chemical changes that develop in the surficial environment after glacial dispersal and soil development. Anomalous Cu, Mo, Ag, Bi, Sb, As, and W in upper B horizon soils overlying mineralisation at Highmont South is attributed to local glacial transport of weathered porphyry material. Anomalous Cu, Ag, Sb, and W in upper B horizon soils above the West Highmont fault, which crosscuts mineralisation, is attributed to metal-bearing groundwater migrating along the fault. Vegetation uptake and cycling is responsible for elevated concentrations of Mo in lodgepole pine needles sampled from trees growing above and proximal to mineralisation, as well as contributing to elevated soil Mo concentrations in the corresponding area. A response of long-chain normal alkane hydrocarbons is identified in the soil overlying mineralisation and the West Highmont fault. There is no evidence for a surficial geochemical response to deeply buried mineralised bedrock at J.A. Surficial geochemistry is controlled by differences in several surficial material types, both glacial and post-glacial, as well as hydromorphic processes and potential anthropogenic influences. A potential surficial signal generated by vertical ion migration from underlying bedrock could be confounded by a number of factors including: the thickness of cover, low porosity and permeability of the pre-Quaternary sequences, the high variability of surficial material types, and/or anthropogenic influences.

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