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New technologies for predicting the occurrence of high-grade gold ore shoots


The overall aim of this project was "To develop a predictive capability for detecting and evaluating high-grade gold ore shoots in vein-type gold deposits using new, cost effective analytical and software technologies". A key strategy was to make better use of diamond drill core material to provide geostatistical estimates of the distribution of high-grade ore not intersected during drilling, which is 'value added' information. Using gold deposits in Portugal, Ireland, Greece and the Czech Republic, it has been demonstrated how process data (i.e. data relating to the formation of the ores), if acquired during the early stages of exploration, can be used very effectively to direct and focus investigations. Data include petrography, mineralogy, microstructures, fluid inclusions and quartz geochemistry. Emphasis is given to the use of 3D visualisation techniques for interpreting this information. This approach allows SMEs to gain the earliest possible understanding of the spatial aspects of mineralisation, thereby providing an informed judgment as to the optimum location and orientation of exploration drill holes. Case studies are given showing: - Use of geochemical ratios as proxies for gold. - Maximisation of limited assay data. - Fluid pathways as scale independent exploration vectors. - Rapid logging of hydrothermal alteration using infrared spectrometry. - Predictive indicator kriging and 3D modelling of geochemistry and alteration mineralogy. To underpin the acquisition of process data, several new instruments have been developed or redesigned including: - Automated system for the microthermometric analysis of fluid inclusions. - Micro-coring drill rig for on-site sampling of drill cores (minimum disturbance to reference cores). - New procedures for the rapid, multi-element analysis of vein quartz and sulphides by laser ablation ICP-mass spectrometry. Vein quartz geochemistry affords a new exploration tool that complements conventional geochemical methods and can be used to discriminate between mineralised and unmineralised quartz veins in reconnaissance programmes. Building upon research already carried out by the RTD performers, a new generic model has been developed for the formation of high grade gold ores that incorporates processes not accounted for by current ore deposit models. It highlights the important role of late stage hydrothermal overprinting and provides SMEs with the scientific rationale for exploration at a local scale. The metallogenic model is widely applicable to gold deposits throughout extensive areas of western and eastern Europe, especially in the Variscan and Caledonide terranes of Iberia, France, central Europe, Ireland, UK and Scandinavia.