The Perseverance Ultramafic Complex, Western Australia: The Product of a Komatiite Lava River

Abstract

The Perseverance ultramafic complex is a body of olivine-rich komatiitic rocks spatially associated with the Agnew nickel deposit, in the Agnew-Wiluna greenstone belt of the Archaean Yilgarn Block in Western Australia. The complex consists of a central lenticular body, up to 700 m thick, of olivine adcumulates, flanked by laterally extensive sheet-like bodies of olivine orthocumulates and spinifextextured komatiite flows. Rocks progressively further away from the central lens have chemical compositions reflecting higher original proportions of komatiite liquid to cumulus olivine. Parent liquids had MgO contents between 25 and 32% MgO, approximately chondritic Al/Ti ratios and HREE patterns, and moderate depletion in LREE. Olivines within the adcumulate lens show a progressive increase in forsterite content from Fo93 at the bottom to Fo94..5 at the top. Calculated original olivine compositions in ihe flanking rocks are similar to those at the base of the central lens. Original olivine nickel contents show a symmetrical variation from maximum values of 3500 ppm at the top of the central lens, through minimum values of 1000 ppm at the base and margins of the central lens to intermediate values in the distal rocks. The complex as a whole shows evidence for nickel depletion relative to other komatiite suites. These observations are explained in terms of prolonged eruption and flow of komatiitic lava down a major flow channel or lava river. Adcumulates crystallized on the floor and sides of the central channel, which was formed at an early stage by thermal erosion of floor rocks. Episodic overflow of the central channel produced distal 'flood plain' rocks consisting of olivine orthocumulates and layered flows. Lavas became more magnesian and nickel-rich with time, giving rise to the observed spatial variation in primary olivine composition. Nickel depletion of the earliest lavas is attributed to pre-eruption segregation of large volumes of immiscible Fe-Ni-sulfide, which were concentrated to form the underlying Agnew nickel deposit.