Shale-hosted Ni–(Cu–PGE) mineralisation: a global overview

Abstract

Shale-hosted Ni–Cu–platinum group element (PGE) sulphide occurrences represent significant resources of not only Ni, Cu and the PGEs but also, in some cases, other elements such as Mo, Zn, Mn, V and U. The largest known deposit of this type that is currently being mined is the Talvivaara Zn–Ni–Cu–Mn–U shale deposit in Finland. The total resource of this deposit is ∼1·550 Bt of ore, at 0·22%Ni, 0·13%Cu, 0·49%Zn, 200 ppm Co and 17 ppm U, all of which can be extracted using a bioheapleaching mineral processing approach. Other significant examples of this deposit type are the Mo–Ni–Zn–Au–PGE deposits of the Niutitang Formation in China and the Ni–PGE–Zn–Mo–Re–Ag mineralisation of the Nick deposit in the Selwyn Basin, Yukon, Canada. However, although they provide insights into the processes responsible for mineralisation in shale-hosted Ni deposits, the Chinese and Selwyn Basin deposits are of too low a tonnage to make them high priority exploration targets. Other potential resources discussed here include the Okcheon shales in Korea, the European Alum Shale and the Chattanooga Shale of the Appalachian Basin. Possible modern analogues for the shale-hosted Ni–Cu–PGE deposits at Talvivaara, Nick, and in China include the Rainbow and Logatchev vent fields of the Mid-Atlantic Ridge, where hydrothermal fluids interact with ultramafic rocks, stripping them of metals before venting them at the seawater–ocean floor boundary, and sapropel formation in the Mediterranean, where organic carbon-rich sediments are interlayered with organic carbon-poor sediments. These organic carbon-rich layers are associated with significantly elevated base metal and sulphur concentrations when compared to the carbon-poor layers, indicating preferential deposition of base metals during sapropel sedimentation. Two genetic models for shale-hosted Ni–Cu–PGE mineralisation are currently favoured: synsedimentary mineralisation, with metals being scavenged from the overlying water column, and exhalative hydrothermal mineralisation, with metals being deposited into sedimentary basins from hydrothermal vents. This paper focuses on shale-hosted Ni mineralisation and the potential for these shales as an economic source of unconventional Ni, Cu and the PGEs.