Stratigraphic and textural variations in the chromite composition of the ophiolitic Sakhakot-Qila Complex, Pakistan

The deposits and their host rocks compare well with podiform ores hosted by alpine-type peridotites of ophiolites. Unlike most alpine-type complexes, predominant stratigraphic variations in the chemical composition of chromite exist. The chromite deposits fall in two major zones, the first consisting of three subzones. Regular compositional correlations between the accessory chromites of the host rocks and the associated segregated chromites are drawn, indicating relatively later crystallization of the former. Relict cumulate chromitite textures cover the podiform range and are examined in the light of compositional data. The inequigranular textural varieties probably developed by crystallization of successive crops from magmatic fractionation, with their coarser grained, massive chromite components always being slightly higher in Mg/(Mg + Fe (super +2) ) than the associated finer grained chromite components. The Mg-Fe (super +2) exchange between chromite and silicates seems to start during the cumulus stage and continues during the postcumulus stage, but is reduced to a very small intensity during the subsolidus stage until it ends at about 700 degrees C.--Modified journal abstract.