The major, trace and precious metal geochemistry of some Permian layered intrusions, Central Queensland

© 1990 Shane John ReevesThe Bucknalla Complex, previously known as the Westwood Layered Intrusion, is a small, 10 km2, layered, tholeiitic, mafic-ultramafic intrusion located 50 km southwest of Rockhampton that was emplaced into an active continental margin environment in the Permian. The complex comprises clinopyroxenites, olivine clinopyroxenites, wehrlites, troctolites, hornblende gabbros, gabbros, anorthosites, leucogabbros and dolerites. It is a saucer-shaped lopolith (2200 m X 6 km at maximum stratigraphic intersection) which intruded Lower Permian spilitic pillow lavas, cherts and tuffs of the Rookwood Volcanics during the Lower Permian. It has subsequently been tilted vertically and in a northeast direction. It consists of over 15 laterally discontinuous igneous units ranging in thickness from 1-50 m. Plagioclase is a cumulus phase throughout the intrusion while orthopyroxene is absent until the very uppermost levels of the stratigraphy. The chromium composition of magnetite analysed by electron microprobe has been found to mimic whole-rock mg# and is a good measure of the degree of fractionation of the rocks. Electron microprobe analyses of samples from two traverses perpendicular to layering reveal cryptic variation in the primary phases (olivine: Fo69-83; plagioclase: An54-97; clinopyroxene mg#: Cpx67-87) which is not a simple function of stratigraphic height. Background PPGE (Pd & Pt), Au, S and Cu values for the intrusion are high while IPGE (Ir & Ru) are low. A total of 120 analyses has produced the following range of values: Pd, 2-70 ppb; Pt, 3-40 ppb; Au, 1-20 ppb; Ir, 0.01-0.07 ppb; Ru, 0.2-0.6 ppb; S, 150-400 ppm and Cu, 40-600 ppm. Platinum, Pd and Au display good correlations with Cu, particularly at more elevated levels, while Ir and Ru are better correlated with whole rock Ni and Cr. Palladium, Pt, Au, Cu & S are elevated in rocks which have intermediate whole-rock mg# (47-60). These trends suggest that the PGE are, to some extent, controlled by fractionation and that the high melting point PGEs (Ir, Ru) were precipitated with the early crystallising phases, such as olivine and clinopyroxene, whereas Pt, Pd and Au were removed from the magma by sulphides. Mantle normalized metal plots for both the mineralized and unmineralized rocks of the Bucknalla Complex display similar trends. Both plots display the anomalous low Ir content, PPGE enrichment and the clear control of sulphides on the distribution of the PGEs and Au. The ratio Pd/Ir is extremely high (1800-9300) indicating extreme fractionation of the PGEs. These trends may, in part, reflect PGE abundances inherited from the source (i.e. relatively low degrees of partial melting) but were exaggerated by the extraction of the IPGE during the early stages of fractional crystallization and by the precipitation of a PPGE-enriched sulphide component. The Complex is known to host minor Pd-Pt-Au-Cu mineralization, disseminated throughout the intrusion. The mineralization consists of chalcopyrite and bornite and their alteration products digenite and covellite, electrum (Au-Ag alloy), Pd-As, Pd-Sb, Pd-S, michenerite (PdBiTe2) and sperrylite (PtAs2). A common host rock is olivine gabbro and the silicate minerals are generally fresh. The mineralization is considered to be primary magmatic for a number of reasons, foremost of which are (i) the clear association of the PGMs with intercumulus (magmatic-textured) fresh or relict Cu-sulphides and (ii) a continuum in Pd/Pt, Cu/Pd, and Cu/Pt ratios from background to mineralized samples which strongly suggests that the processes responsible for the enhanced PGE content of the Bucknalla Complex were also responsible for mineralization. In as much as the former must have been produced by magmatic processes it is concluded that the higher grade PGE-Cu-S mineralization was also caused by primary magmatic processes. A model is proposed in which mineralization is sporadically generated by influxes of small batches of PGE-rich S-undersaturated magma into a magma chamber in which the resident magma has reached S-saturation due to fractional crystallization processes. Other intrusions in the region, namely; the Eulogie Park Complex, the Fred Creek intrusion and the Boogargan intrusion, are not considered prospective for stratiform PGE mineralization due to their low background PGE tenor, low Pd/S and Pd/Se ratios and high S contents