Abstract

Traditional, near surface geochemical techniques have been effective in mineral discovery but to explore in deeper transported cover (>10 m) and find new resources, as demanded by a growing global population, new and improved exploration methods are required. As mineral exploration transitions from exploring in residual outcropping terrains into deeper covered terrains, geochemical signatures are diluted and the ability to successfully discern subtle geochemical signatures is essential. At the ‘geochemically-blind’ North Miitel Ni deposit in Western Australia, we compared passive soil-gas hydrocarbons and weakly extracted elements from soil (10 – 20 cm depth) with both proving successful. The primary mineralization is >200 m below the surface, but a weak zone of secondary Ni enrichment occurs in the saprolite at 15 – 20 m depth. This enriched zone is covered by 10 – 15 m of transported cover. Minimum hypergeometric probability (MHP) statistics were used to evaluate the near surface geochemical signatures of c. 100 samples from three traverses over the mineralization. Soil samples from 10–20 cm were subjected to distilled water, 0.1 M cold hydroxylamine hydrochloride and aqua regia extractions. The water-extractable concentrations of Ni, Co, Mo, Sb, and Sn were successful in identifying mineralization (MHP <1%, type II error). The hydrocarbons, 2-methylbutane, pentane and 1-pentene were also successful (MHP <1%) at identifying the zone of mineralization using the Amplified Geochemical Imaging (AGI) passive soil-gas collectors. Of the techniques used, the water extraction performed the best using MHP classified accuracy to identify mineralization. The passive soil-gas data were the second most effective, and superior to the stronger partial extractions using hydroxylamine and aqua regia that did not identify the mineralized zone (MHP>>1%). The water extraction and the passive soil-gas showed a greater degree of variability than the stronger extractions. The results indicate that depth of sampling, interaction with organic carbon and potential mechanisms of metal migration greatly influence the geochemical anomaly near the surface. These mechanisms are evident as hydromorphic dispersion at depth, with potential capillarity and gaseous migration up through the profile above the water table. Integrating an understanding of metal migration mechanisms, genesis and evolution of target and pathfinder compounds (particularly hydrocarbons) related to deposit types will improve future exploration. Extending into much deeper cover (>20 m), the viability of passive soil-gas methods may become more relevant and warrant further study for mineral exploration.