Deformed Chromitite Layers in the Coobina Intrusion, Pilbara Craton, Western Australia

The Archean Coobina intrusion contains a number of tectonically dislocated and folded chromitite seams within a strongly deformed ultramafic-mafic layered intrusion. The intrusion consists of predominantly serpentinized olivine cumulates, overlain by gabbroic rocks. The entire sequence, along with the overlying greenstone belt sediments and volcanic rocks, shows evidence for multiple episodes of ductile deformation and a later phase of shearing associated with the Capricorn orogen. This history has resulted in isoclinal folding and boudinage of the chromite layers and disruption of original lateral continuity. This deformation appears to have been accommodated by micrometer-scale microshears within the massive chromitites rather than by penetrative fabric development. The Coobina chromitites have compositions at the more Cr-rich end of the range observed in layered intrusions worldwide. Chrome contents and Cr/Al ratios are indicative of a parent magma of komatiitic affinity. Mg/Fe ratios vary unsystematically and have probably been modified by varying degrees of subsolidus exchange of Mg and Fe with surrounding olivine-rich cumulates. Chromitites are consistently Pt and Pd poor; have typical 100-ppb levels of Ru and Ir, and show no systematic relationship to stratigraphy or major element chromite chemistry. The lack of Pt and Pd enrichment is attributed to formation from uniformly S-undersaturated magmas, consistent with evidence for high temperatures of formation. Crystal sizes in the Coobina chromites show linear trends on crystal-size distribution plots. Such trends are considered to be characteristic of homogeneous populations of growing crystals. There is no evidence of grain-size sorting that might be expected from crystal settling processes, and a mechanism of in situ nucleation and growth is inferred. Chromite seams are interpreted as the result of magma-mixing processes during magma chamber recharge, but with miming taking place entirely between olivine-saturated magmas, accounting for the lack of pyroxene-bearing cumulates in the sequence, and the higher than usual Cr/Al ratios.