ABSTRACT During the 1970s and 1980s, the Applied Geochemistry Research Group, Imperial College of Science and Technology, London, investigated gas geochemistry for mineral exploration in regions of exotic overburden. Mercury vapour anomalies were determined over structurally controlled mineralization in arid terrain but the necessity of adequate trace levels of mercury in the mineralization at depth to provide detectable signals in the vapour phase near surface was recognized as a limitation. Anomalous soil air disequilibria of oxygen and carbon dioxide were found above the suboutcrop of several types of oxidizing sulphide mineralization: oxygen in soil air is depleted due to oxygen consumption by sulphide oxidation reactions; these produce sulphur dioxide which, as sulphuric acid, reacts with any carbonates associated with the mineralization to generate, and increase the soil air content of, carbon dioxide. Carbonyl sulphide, one of the sparingly soluble metastable gases produced in trace quantities by sulphide oxidation, was found adsorbed on soil particles over concealed sulphide mineralization; conspicuously strong anomalies were found in glacial clay overlying fault-hosted mineralization, consistent with a numerical model of gas migration in complex media. Although, in the subsequent two decades, models and methods for geochemical exploration in regions of exotic overburden have increasingly focused on metal ion transport from depth to surface, a steady flow of reports of research into, and successful applications of, mercury, oxygen, carbon dioxide and metastable trace gases generated by sulphide oxidation continues.

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