Variation in composition of chromites from chromite-arsenide deposits in the peridotites of Malaga, Spain

Abstract

The composition of chromites in Ni-arsenide-bearing chromitites and of orthopyroxenes in orthopyroxenites from several Cr-Ni-deposits in the Malaga peridotites, Spain, shows little variation in each deposit but strong variation between the deposits. The variations in chromite composition from different deposits are caused by peculiar features, namely, sympathetic increase of Cr 2 O 3 /R 2 O 3 and MgO/RO ratios, unusually high V and Zn decreasing with an increase in Cr 2 O 3 /R 2 O 3 and MgO/RO, and irregular variation in Ni and Co. The chromites and associated orthopyroxenes in the different deposits have recrystallized under similar metamorphic conditions. The variation in mineral chemistry between the deposits is presumably controlled by differences in bulk compositions and is related to the compositions of the magmas from which the chromitites and orthopyroxenites crystallized. Small variations in chromite composition within each of the deposits are characterized by normal trends with Cr 2 O 3 /R 2 O 3 increasing with a decrease in MgO/RO. This latter trend is due to partial reequilibration of the chromites to later conditions of lower temperature during serpentinization of the peridotites. The data presented are considered not inconsistent with the hypothesis suggested in an earlier paper (Oen, 1973), that the arsenide-bearing chromitites have crystallized from Cr-rich oxide-arsenide liquids, which originated by liquid immiscibility in pyroxene-crystallizing residual magmas of special composition.