Origin of high-Cr podiform chromitites from Kabaena Island, Southeast Sulawesi, Indonesia: constraints from mineralogy and geochemistry

Abstract

ABSTRACT The East Sulawesi Ophiolite, one of the three largest ophiolites in the world, contains important podiform chromitites. However, the origin of these chromite deposits is not well constrained. Here, we present the detailed mineralogy and in situ chemistry of chromite and solid inclusions within chromite for podiform chromitites from the Kabaena Island, Southeast Sulawesi. Chromite grains have low TiO2 (0.17–0.27 wt%) and Al2O3 (13.4–15.1 wt%) contents and high Cr# [Cr/(Cr+Al), molar] of 0.70–0.74 and Mg# [Mg/(Mg+Fe2+), molar] of 0.69–0.75, which resemble those of other high-Cr podiform chromitites worldwide. The MORB-normalized patterns for some selected major-trace elements of chromite show slightly positive slopes from Al2O3 to Mn. Chromite-hosted silicate inclusions include olivine, clinopyroxene, orthopyroxene and amphibole and are characterized by higher Mg/Fe ratios than those of host peridotites. The geothermobarometry of chromite-hosted clinopyroxene-orthopyroxene pairs indicates that the estimated T-P conditions of the parental magma of the Kabaena chromitites are 950–1010°C and 7.0–8.4 Kbar. The extreme-Mg-rich olivine inclusions (Fo >96) with extremely high Ni (5300–8200 ppm) and Cr (1500–6700 ppm) contents are interpreted to have crystallized from high-Mg and Cr boninitic melts, and the Fo contents were then elevated by subsolidus re-equilibration (Fe–Mg and Fe–Ni exchange) with chromite. Sulfide inclusions contain base metal minerals containing millerite with rare pentlandite and chalcopyrite, and platinum-group minerals that include the laurite-erlichmanite series, Ir–Ni monosulfide, irarsite and cuproiridsite. This sulfide assemblage reveals that their parental magmas experienced the evolution of high T and low f(S2) to low T and high f(S2) from the early to late stage. Finally, combined with the palaeogeographic reconstruction in Sulawesi, we propose that the high-Cr chromitites in this region were formed by the reaction of depleted mantle and boninitic magma beneath a juvenile island arc, implying the initiation of a new subduction in the Miocene.