The komatiite-hosted Perseverance Ni-sulphide deposit, Agnew-Wiluna greenstone belt, Western Australia; new insights into the Perseverance komatiite channel and footwall lithostratigraphy

Abstract

The paper presents new geological and geochemical data on the Perseverance komatiite complex (PKC) that hosts the Perseverance Ni-sulphide deposit and the volcanic and sedimentary rocks forming the footwall of the PKC. Using the 3D reconstruction of the PKC and its’ footwall volcano-sedimentary succession the heterogeneity of the komatiite channel has been characterised on the basis of spatial distribution of the geochemical petrogenetic indicators.The study shows that the komatiite facies proximal to the mineralised bodies and the main pathway are expressed in the 3Dmodels as the segments with anomalously high MgO concentrations coupled with low contents of Al2O3, TiO2and CaO. The geochemical indicators of komatiite fertility, Ni/Cr and Ni/Ti ratios, commonly are in the range of 1 to 3 for Ni/Cr and 2 to 5 for Ni/Ti within the mainpathway, The Ni/Crexceeds 5and Ni/Ti20 in the mineralised parts of the channel. In the peripheral zone komatiites thesevalues commonly drop to less than 1, corresponding to barren komatiites. Similarly large variations are exhibited by the ratios of mantle normalized Pt and Pd to Al and Ti over short distances from 1 to 25. Closely spaced high and low values of these ratios hinder their use for discriminating fertile from barren komatiites. However, within the komatiite main lava pathway they can be used as geochemical pathfinders for magmatic sulphides.Reconstruction of the structurally complex komatiite sequences can be facilitated by using lithogeochemical correlation of the host volcano-sedimentary successions and particularly tracing geochemically distinctive marker units. For these purposes a special reference in this study was made to the geochemical systematics of rocks that make up footwall succession of the PKC complex. It was shown that amphibolites of the host sequence can be geochemically subdivided into different types, including mafic volcaniclastic rocks (mafic-1) and intrusive mafic rocks, apparently metagabbro sills (mafic-2). The mafic volcaniclastic rocks are strongly enriched in LREE, possibly due to presence of sedimentary epiclastic material or reflecting magma contamination by the crustal material during ascent through the crust. Additionally, they have notably high-Th contents, allowing them to be classified as having high-Th geochemical affinities. The second group (mafic-2), interpreted as coherent in origin, are characterised by flat REE patterns and lower overall abundances of the rare earths and Zr corresponding to a tholeiitic composition. Contents of Th and TiO2of these rocks are clustered in the two narrow intervals of compositions, corresponding to low-Th and intermediate-Th basalts, which form different amphibolite bodies. Mafic volcanics intercalate with the felsic volcanics, and quartz-feldspar-biotite-amphibole (QFBA) schists. It is shown that QFBA schist composition corresponds to metamorphosed clastic sedimentary and volcaniclastic rocks of mixed composition, which contain both clastic materials derived from an Archaean provenance and volcaniclastics material derived from mafic and felsic tuffs. The different geochemical affinities of the mafic rocks distributed in the footwall succession of the PKC apparently suggests different magma sources and, possibly, can also reflect the position of the rocks within the mantle plume structure. In conclusion, abundances, and ratios of the trace elements in composition of the mafic rocks matches that of the different parts of the Agnew Wiluna greenstone belt and can be used for overall stratigraphic correlations.