The role of mineralogy and geochemistry in forming anomalies on interfaces and in areas of deep basin cover: implications for exploration

Abstract

This paper examines how geochemical dispersion may be used in areas of deep transported cover to locate buried mineralization. Two regions with contrasting weathering history and transported cover were investigated. These are the Mt Isa region of northern Queensland and the Yilgarn Craton of Western Australia. At both locations, transported cover is from 20–70 m thick. A range of techniques and instruments, including bulk geochemistry, SEM, electron microprobe and selective extraction analyses were used to understand the vertical distribution and nature of ore-related elements. At the Eloise Cu-Au Deposit in the Mt Isa region, Mesozoic cover lies over prospective Proterozoic basement which is largely unweathered. At Eloise, the degree of weathering, below the unconformity and transported cover, is minimal so weathering-related dispersion in basement and cover is also minimal. There is no dispersion of indicator elements into the upper parts of the unweathered Mesozoic sediments. Mechanical dispersion dominates as is indicated by the SEM and petrographic study and is restricted to basal sediments (last few metres). Unweathered sulphides occur in the basement and in the fragments of clastic basement material and fossil organic matter in the silty matrix of the Mesozoic sediments. The palaeotopography governs the direction of dispersion and needs to be thoroughly understood before meaningful interpretation. The Lancefield South Au Deposit has a cover of highly weathered Permian to Quaternary sediments on Archaean basement of the Yilgarn Craton. Stacked weathering profiles in each sedimentary unit allowed transfer of Au, As, Cu and Zn across the weathering profiles (paleosols) by hydrogeochemical and vegetation interactions as they evolved. High paleo-watertables within the Permian sediments have allowed upward migration of metals reaching close to the surface of the Permian sediments. Bioturbation and transpiration pumped the anomalous metals into the overlying Tertiary and Quaternary sediments from the Permian. Although regolith anomalies are weakened, especially in the Tertiary and Quaternary cover by post-depositional weathering processes over a long time, they still clearly remain. Thus, a highly weathered sedimentary cover is likely to trap indicator elements more effectively than an unweathered sedimentary cover, such as at Eloise. In addition to basal sediments or interfaces, ferruginous horizons in weathered sediments high in the sequence can also be anomalous in ore-related metals.