Petrology and platinum-group-element geochemistry of Archaean layered mafic-ultramafic intrusions, west Pilbara Block, Western Australia

In a group of Archaean, layered, mafic-ultramafic
intrusions in the west Pilbara Block, Western Australia, the Munni Munni Complex (2925 Ma)
contains a substantial resource of PGEs (20-30 Mt @ 2.9 ppm Pt + Pd + Au) and other intrusions
contain subeconomic resources of Ni and Cu. Contrasting PGE distribution patterns within the group
indicate that the timing and mechanism of S-saturation were critical for the formation of PGE-enriched
sulphide units. Generally, overlying mafic units are thicker than the rhythmically layered ultramafic
components. Olivine and clinopyroxene were usually the first minerals to crystallise and cumulus
orthopyroxene is restricted to the upper parts of the ultramafic zones. Chromium partitioned into early
crystallising clinopyroxene, thus downgrading the potential for PGE-chromite associations. The parent
magmas were probably siliceous, high-Mg basalts of Aldepleted komatiitic affinity. In this study,
the variations of PGEs and incompatible trace elements were documented up the entire stratigraphic
successions. In the Munni Munni Complex (the intrusion studied in most detail) the PGEsulphide
mineralisation is directly below the ultramafic/gabbroic contact and is thought to have resulted
from crystal fractionation and mixing of S-undersaturated and S-saturated magmas; as in the Great
Dyke of Zimbabwe, it does not occur at or near the base of clearly defined cyclic units. The other
(smaller,mainly sheet-like) intrusions studied typically contain basal segregations of PGE-poor
Ni-Cu sulphides; the magmas were generally S-saturated early in their evolution and there was no
PGE enrichment at higher levels. In exploration, variations in the ratios of precious metals and
the trends of the incompatible lithophile and chalcophile elements should be used in association
with field and mineralogical evidence to indicate potential PGE-enrichment processes, such as
S-saturation, crustal contamination, crystal fractionation, magma mixing, and/or hydrothermal
remobilisation.