Use of geochemistry as a guide to platinum group element potential of mafic-ultramafic rocks: examples from the west Pilbara Block and Halls Creek Mobile Zone, Western Australia

Abstract

Integrated chemical and isotope studies of late Archaean mafic-ultramafic layered complexes and associated extrusive volcanic rocks in the west Pilbara Block, Western Australia, suggest that they were derived from komatiite magmas through processes of crustal assimilation followed by fractional crystallization. On the basis of Nd isotope data the contaminants are believed to be felsic basement rocks of mainly early Archaean age. Parent magmas of these layered complexes are siliceous high magnesian basalts (SHMB) and can be regionally correlated with two types of komatiites. The Cooya Pooya Dolerite and the Negri Volcanics could be related to group 1 komatiite, characterized by near chondritic Al 2O 3/TiO 2 (∼21), Ti/Sc (∼74) and flat heavy rare earth element (REE) patterns, whereas the Munni Munni, Mt. Sholl, Dingo, Maitland and Andover complexes could be related to the group 2 komatiites characterized by depletion in Al (Al 2O 3/TiO 2∼10), heavy REE and Sc. Komatiites are products of very large degrees (e.g. 30–50%) partial melting of the convective mantle. They commonly have Pd∼8−10 ppb, Au ∼ ppb, Pt 10−14 ppb ppb and are strongly undersaturated in sulfur (∼ 500 ppb?). Archaean SHMB commonly have higher Pd(∼ 15 ppb), but are still S-undersaturated (< 1000 ppm) unless contaminated by S-rich crustal rocks. Komatiites and SHMB commonly have Ti/Pd (∼2−3 × 10 5), and Pd/Cu ( ∼−3 × −4 ) similar to estimates of the primitive mantle. The west Pilbara SHMB have PGE contents (Pd ∼10−15 ppb) similar to the estimated parent magma of the Bushveld complex, and other late Archaean SHMB. Some late Archaean major layered complexes and early Proterozoic dykes in the Yilgarn Block, Western Australia arc inferred to have similar values. These magmas are believed to have potential for PGE mineralization through sulfide saturation processes in magma chambers and feeder dykes. Early Proterozoic mafic-ultramafic intrusives in the Halls Creek Mobile Zone, have parent magmas with a chemical affinity to tholeiites generated in continental rift environments. Among them, parent magmas of the Woodward Dolerite, Lamboo Complex, and Panton Sill have high inferred abundances of Pd (up to 15 ppb), Pt and Au. Abundances of some incompatible elements (Ti, Zr, Y and P) in these rocks are lower than in mid-ocean ridge basalts suggesting magma generation by large degrees (>/25%) of mantle melting. Their primitive parent magmas, when first scaparated from the mantle, are inferred to have been S-undersaturated and to have had Pd, Pt and Au contents similar to the Archaean SHMB. With the exception of the Woodward Dolerite, they were generally saturated with sulfur prior to emplacement resulting in occurrences of minor basal chromitite layers with PGE-enriched sulfides generated during injection of new pulses of Saturated magma into the chamber (e.g. Panton Sill). Such rocks are therefore not believed to be favourable hosts for Merensky Reef style PGE mineralization. Feeder dykes of these intrusives and the Woodward Dolerite may have potential for mineralization similar to the NiCuCuPGE sulfide deposits in feeder dykes of Permian-Triassic continental flood basalts in west Siberia. The key factor for PGE potential in mafic-ultramafic magmas is S-undersaturation which can be achieved either by high temperature (> 1400°C) and ⩾10% partial melting under high pressure (⩾ 25kb) and/or large degrees (⩾25%) melting of MORB type mantle source under low pressures. Their mantle sources appear to have fairly constant Pd (∼4 ppd), Pt (∼6 ppb), Au (∼1 ppb) and S (∼250 ppb) from Archaean to the present. S-undersaturation may also be achieved by smaller degrees of meltung of residual, refractory mantle poor in Ca, Al, Ti, Fe and S ⪡ 250 ppm) (with Pd, Pt ⪡ 4 ppb, Au ⩽ 1 ppb) of the continental lithosphere or wedge above a subduction zone. Rocks of the first association include komatiites, continental flood basalts and picrites, which are believed by some to be related to mantle plume activity. These PGE rich magmas could be further enhanced by melting of the refractory lithospheric mantle, whereas the second type of rocks include boninites, low-Ti tholeiites, some lamprophytes and shoshonites.