Review of lithogeochemical exploration tools for komatiite-hosted Ni-Cu-(PGE) deposits

© 2016.
High grade type 1 (stratiform contact) Komatiite-hosted Ni-Cu-(PGE) deposits are composed of extremely valuable but small and often deep-seated ore bodies. This paper describes, reviews and compares the current exploration techniques for these challenging exploration targets, with new scientific advances and recently developed exploration tools including latest studies on the nature and size of secondary hydrothermal haloes surrounding primary magmatic deposits. The focus is on understanding at which scales these various tools should be used and compare their accuracy. The scale, geological context and density of the available data will affect the effectiveness of different geochemical exploration tools, therefore the uncertainties on our understanding of these criteria will have a significant impact on the outcome of exploration protocols. At the deposit scale, many effective traditional geochemical indicators (e.g., Ni/Cr, PGE variations) as well as many geological indicators, potentially extending 50 to 450 m from ore, will be rendered inadequate by structural deformation of the system. In such cases, hydrothermal haloes will still be effective up to km scale and should be favoured, especially when combined with EM surveys, as demonstrated by case studies in deposits in Western Australia such as Spotted Quoll in the Forrestania greenstone belt, Maggie Hays and Emilie Ann in the Lake Johnson greenstone belt, and Harmony in the Agnew-Wiluna greenstone belt. When stepping up to prospect scale, indicators of favourable tectonic settings, crustal contamination and magmatic processes will be more useful, in particular the use of ratios of Ni, Ti, Zr and Cr that are immobile during alteration and moderate degrees of weathering and can be detected using portable XRF. Such techniques will in some cases allow the generation of regional targets. In any case, and whatever the scale, the best exploration outcomes rely on a smart combination of the various techniques described here, along with a hint of luck.