Weathering, element distribution and geochemical dispersion at Mt Keith, Western Australia: implication for nickel sulphide exploration

Abstract

The komatiite–dunite-hosted Ni sulphide deposit at MKD5 (Mt Keith) has a complex regolith consisting of a residual profile (>75 m depth), partly collapsed towards the top, overlain by 40 m of exotic sediments. The weathered komatiites are composed predominantly of degraded serpentine, neo-formed hydrated Mg-silicates, carbonates, silica and Fe oxides. Magnesium concentrations decrease upwards through the lower regolith, whereas those of Fe and Si increase, reflecting degradation of Mg-bearing minerals. A sharp decline of Mg within the profile marks the Mg discontinuity , separating Mg-saprolite from the overlying Fe saprolite. The Fe saprolite, dominated by Fe oxides and silica, has pseudo-sedimentary structures indicating partial collapse of the profile. At the unconformity with the sediments, eluviation, mixing and churning have produced zones with chemical and mineralogical characteristics of the underlying, overlying and adjacent lithologies. Nickel, Co, Mn, Cr and PGE are dominantly hosted by secondary Fe oxides in the regolith, but above the Mg-discontinuity, Ni and Co are locally enriched by co-precipitation with Mn oxides. The oxidation potential, controlled by ferrolysis, ranges from −0.4 to +0.2 volts, and the pH, buffered by bicarbonate ion, irrespective of the presence of sulphides, decreases from 9.5 to 6.0 up through the profile. Copper is not significantly mobile under such conditions and precipitates as inert native metal, commonly spatially associated with Fe oxides. Separation of Ni and Cu occurs in the upper saprolite and pedolith, because metallic Cu (±Pd) is retained and residually concentrated in collapsed Fe saprolite, whereas Ni, adsorbed to goethite, is redissolved and reprecipitated below the Mg-discontinuity. High Ni concentrations in the regolith cannot alone be used to distinguish between Ni sulphide mineralized (NiS) and barren komatiitic protoliths. The strong correlation between chalcophile elements evident in mineralized protolith is lost with the oxidation of sulphides, and Ni derived from sulphides is indistinguishable from that derived from silicates. Ultramafic-derived regolith materials having Ni/Cu ratios of 7–19 and >19 may indicate the presence of massive and disseminated sulphides, respectively. The ratio at MKD5 is 60. These relationships may be distorted by preferential leaching, or by the influx of either element from wall-rocks. High concentrations of PGE, especially Pt, Pd and Ru, may also indicate mineralized protolith. However, the very low secondary mobility of PGE and Cu greatly restricts target size. Larger targets are given by Ni/Cr and Cu/Zn ratios which, either singly or multiplied together, can be used to identify mineralized komatiitic sequences and provide vectors towards ore. Their value is diminished if the ore and host sequence are decoupled during emplacement.