Element ratios in nickel sulphide exploration: vectoring towards ore environments

The use of element ratios in geochemical exploration can help to distinguis h mineralised from barren geological complexes, predict deposit types, and assist in estimating and evaluating dispersion of ore indicator elements. E xploration at Kambalda Nickel Operations, Western Australia, has focused on developing a repetitive robust geochemical method that can vector towards Ni-Cu-PGE sulphide deposits. One method has been developed so as to utilise element ratios in a manner which are compatible with the genetic geologica l exploration model for komatiite-associated massive Ni sulphide deposits. A particularly successful element ratio is expressed as [(Ni/Cr) x (Cu/Zn)] . The [(Ni/Cr) x (Cu/Zn)] ratio is effective in vectoring towards channelli zed komatiitic environments. This paper presents key examples that demonstr ate the power of the [(Ni/Cr) x (Cu/Zn)] ratio in fresh komatiite rock, its intensely weathered equivalent, and in surface materials. Analytical techn iques employing weaker acid attacks (HCl) and finer size fractions from sur face materials enhance ratio contrasts. Exploration implications are discus sed. Komatiites associated with Ni sulphides are low in Cr compared to baro n komatiites. Low Cr values persist along the flow from Ni sulphide mineral isation in channellized komatiites. Mineralised basal komatiites flows are enriched in Ni compared to barren flows. Thus, utilising a Ni :Cr ratio, po tentially mineralised flows (high Ni, low Cr) can be distinguished from bar ren flows (low Ni, high Cr) with Ni:Cr ratio values typically >1. This Ni:C r signature can be traced down-plunge, on the basal komatiite, for >14 km a nd through the komatiitic stratigraphy. In addition, this signature is, in part, preserved through the weathered komatiite and in surface material. It has successfully identified new Ni sulphide occurrences and highlighted si gnificant areas of exploration interest. Sulphide-bearing sedimentary horiz ons of variable thickness occur at the base of, and within, the komatiitic successions. These meta-sedimentary rocks typically have three times the Zn concentration (1500 ppm) to that of Cu (500 ppm). However, because nickel sulphides have a greater concentration of Cu than Zn, the Cu:Zn ratio can a ssist in discriminating Ni sulphide-bearing zones from those with no Ni sul phides. Combining the Ni:Cr and Cu:Zn ratios has the effect of reducing the influence of particular lithologies (e.g. sulphidic meta-sedimentary rocks ) and aids in the delineation of Ni sulphide mineralisation. Multiplying th e ratios enhances the contrast of element ratios related to mineralisation and subdues the effect of lithologies. Utilisation of the [(Ni/Cr) x (Cu/Zn )] ratio increases contrast between background and mineralisation, and thus the ability to vector towards basal komatiite channellized environments th rough the distinctive behaviour of Ni, Cr, Cu and Zn. (C) 1999 Elsevier Sci ence B.V. All rights reserved.