Hydrothermal remobilisation of base metals and platinum group elements around komatiite-hosted nickel-sulphide deposits: applications to exploration methods

[Truncated] The purpose of this project was to enlarge the detectable footprint of magmatic nickelsulphide deposits, with an emphasis on Archaean Western Australian komatiite-hosted systems. Komatiite-hosted nickel sulphide deposits are extremely challenging exploration targets, primarily due to the limited extent of their magmatic footprint. The circulation of hydrothermal fluids through these deposits and within the surrounding rocks has the potential to create a significantly larger geochemical halo which would serve as a powerful exploration tool. The aim of this project is to determine whether hydrothermal fluids can remobilise and re-precipitate base metals and platinum group elements from magmatic sulphide deposits into the country rocks, and what is the nature, the geometry and the extent of the geochemical halo thus created. Four komatiite-hosted nickel-sulphide deposits, differing in geological setting, hydrothermal alteration style and metamorphic processes, were selected as case studies. This empirical approach of the problem aimed at defining the possible nature and geometry of a geochemical halo around these diverse komatiite-hosted nickel-sulphide deposits. It was combined with an empirical study of the mobility of base metals and PGE within an ore body during alteration, using the intrusion-hosted Kevitsa deposit as a natural laboratory. Results of the various case studies highlighted the role of arsenic as a carrier within hydrothermal fluids. In two of the case studies, the Miitel and Sarah’s Find deposits, a Ni-Co- As-Pd-Pt geochemical halo was observed extending respectively 250 and 1,780 meters away from the massive sulphides within the country rocks, along preferred fluid pathways. The size of this geochemical halo is larger than any observed magmatic footprint around komatiite-hosted nickel-sulphide deposits. The remobilised Ni, Co, Pd and Pt are precipitated as arsenides and/or sulfarsenides, concentrated within veinlets along lithological contacts and/or within shear zones intersecting the ore body. No geochemical halo was observed for the two other case studies, the Perseverance deposit and the Otter-Juan and Durkin (Kambalda) mineralised system, where there is no evidence of arsenic metasomatism.