Siderophile and chalcophile metal variations in basalts: Implications for the sulfide saturation history and Ni–Cu–PGE mineralization potential of the Tarim continental flood basalt province, Xinjiang Province, China

Abstract

It is well known that a genetic link exists between the formation of ultramafic-hosted Ni–Cu–PGE sulfide deposits and the eruption of associated chalcophile- and siderophile-depleted continental flood basalts. However, until recently little research has been undertaken on the Permian Tarim Basin continental flood basalt province in Xinjiang province, China. Here, we present new trace, siderophile and chalcophile element data from continental flood basalts that are well exposed and preserved in the Keping area of Xinjiang province and we discuss their sulfide saturation history and Ni–Cu mineralization potential. Ultramafic–mafic units dominated by basaltic rocks outcrop over an area of about 250,000km2 in the Tarim Basin; these Lower Permian basaltic rocks are hosted by the Kupukuziman and Kaipaizileike Formations, a mixture of basalts, trachybasalts, amygdaloidal basalts and basaltic pyroclastics.Major, trace and rare earth element (REE) data indicate that basalts within both formations are cogenetic, whilst low concentrations of Ni, Cu and platinum-group elements (PGE) indicate that they are chalcophile element depleted. The basalts have very low (Cu/Zr)PM and extremely low (Pd/Zr)PM and (Pd/Cu)PM ratios; both these ratios and the concentrations of Ni, Cu and the PGEs are invariant with changes in MgO concentrations. Furthermore, the magmas represented by the Keping basalts have very low calculated sulfur concentration at sulfide saturation (SCSS) capacities, and only needed to assimilate small amounts of sulfur-bearing crustal material to breach the SCSS and form immiscible sulfide liquids. This assimilation of crustal material appears to have occurred at depth; this is evidenced by the clear link between crustal contamination (as expressed by (Th/Nb)PM ratios) and the removal and depletion of chalcophile elements in the magmas that formed the basalts in the Keping area (as expressed by (Pd/Cu)PM and (Pd/Zr)PM ratios). The geochemistry of the basalts is consistent with an enriched mantle source; all of the basalts assimilated significant amounts of crustal material, became sulfur saturated and formed immiscible magmatic sulfides at depth; these magmatic Ni–Cu–PGE sulfides scavenged chalcophile elements from the magma and remained at depth whilst the chalcophile-depleted basalts were erupted.The identification of the segregation of magmatic sulfides from ultramafic–mafic magmas within the Tarim Basin indicates that the area should be considered highly prospective for mineral exploration for magmatic Ni–Cu–PGE deposits. Magmatic Ni–Cu–PGE sulfide enriched units may well exist in magma conduits or at the base of shallow magma chambers within the basin, and any ultramafic sills within the basin that are cogenetic and coeval with the Lower Permian Keping area basalts, for example the layered ultramafic–mafic bodies in the Bachu area, should be considered as prospective targets.