This paper reviews the geochemistry of weathered nickeliferous
sulphidic and lateritic profiles, the supergene behaviour of nickel
and ore-associated elements, and their implications for
exploration in an attempt to distinguish between barren and
mineralised nickeliferous profiles. Nickel is an abundant accessory element in ultramafic rocks
with background concentrations in some units up to 0.25 weight
per cent. Ore-grades in fresh rock are mostly associated with
massive and disseminated sulphide zones within magnesium-rich
layered ultramafic sequences. In lateritic weathered terrain,
nickel is also concentrated in the regolith profile. This process
not only upgrades nickel in the gossanous weathered product of
sulphidic deposits, but for sub-economic sulphide zones and
some ultramafic rocks, is sufficient to result in a supergene
resource. Ore-grade accumulations of nickel in regolith after
sulphide-poor ultramafic rocks are referred to as lateritic deposits.
Nickel sulphide zones and their ferruginous weathering products
are collectively referred to as sulphidic deposits and are the major
source of nickel in Western Australia. Mineral exploration in the Yilgam Block is hampered by deep
lateritic weathered profiles (eg Butt and Sheppy, 1975; Smith,
1977; 1982). During the nickel boom of 1967 - 1972, most
discoveries resulted from the location of outcropping
nickeliferous gossan and ironstone. Exposed ultramafic terrain
with nickel sulphide potential were explored intensively, mostly
by gossan and soil sampling. However, up to 85 per cent of the
area of the Yilgarn Block is buried by transported regolith. In
these covered areas, barren and mineralised rock responses at the
surface are similar and soil sampling produced unreliable results
due to many false anomalies. A statistical, multi-element
approach to regional geochemical exploration has evolved.
Potential trace element indicators investigated, include Co, Cr,
Cu, Ir, Mn, Pb, Pd, Pt, Sc, Se, and Zn. In recent years, trace
element ratios have also been tested with varying degrees of
Success.