Silicate and base-metal sulfide inclusions in chromites from the Maqsad area (Oman ophiolite, Gulf of Oman): A model for entrapment

Abstract

Al-rich chromites from the Maqsad area (Oman ophiolite) have been studied to propose a genetic model for silicate and base-metal sulfide (BMS) inclusion entrapment. Samples were collected in both concordant and discordant nodular pods and in a peculiar stratiform ore; chromite ore contains olivine+clinopyroxene+plagioclase as interstitial silicates. Regardless of the ore type, the silicate inclusions are, in decreasing order of abundance, composed of Ti- and Na-rich phlogopite, Ti-pargasite, enstatite, Mg-diopside, olivine, albite and secondary hydrothermal products. Chromite-hosted BMS are dominated by pentlandite, heazlewoodite, millerite and are nearly devoid of Fe- and Cu-sulfides. The samples which come from concordant ores are very poor in mineral inclusions, a consequence of the strain-induced grain boundary migration and recrystallization which they suffered during plastic deformation. In contrast, the nodular and stratiform samples which escaped post-magmatic deformation and densification are inclusion rich. The inclusions display negative crystal shape following the chromite symmetry. If present in nodular samples, they may be arranged as coronas which mimic the shape of the host chromite grains. All these textural features are considered to be evidence for mineral inclusion entrapment during magmatic precipitation of chromite. By reference to a previous dynamic model of chromite ore genesis, we consider that the nodules of chromite crystallized from basaltic magma were flowing inside a narrow cavity in which a non-turbulent convective system was established. We deduce from the presence of several inclusion coronas or -patches in the largest nodules and their aabsence in the smallest,that the inclusions were trapped preferentially at the point in which the primitive magma flow meets the convective current. The peculiar mineralogy of mineral inclusions dominated by hydroxyl-sodic phases indicates a complex interaction at the magmatic stage between the chromite-precipitating magma and an unrelated aqueous fluid phase. This fluid, which was sodium- and sulfur-bearing, travelled through the top of the mantle sequence and the basis of cumulate rocks of the ophiolite. It could have been injected at certain stages of its evolution in the same conduits as the chromite-precipitating magma, locally giving rise to volatile-enriched differentiated melts. The latter have been sealed together with the liquidus phases of magma (olivine, clinopyroxene, orthopyroxene and plagioclase) as inclusions during chromite growth and/or dissolution. Olivine and pyroxenes have partly reacted with the trapped volatile-rich melt, producing phlogopite and pargasite several hundred degrees below trapping temperatures. BMS have formed through sulfidization reactions with enclosed silicates and/or host chromite.