Zincian, manganiferous chrome spinel from the Swat valley ophiolite, NW Pakistan

Abstract

Metamorphosed basic volcanic rocks of the Swat valley ophiolite locally contain trace to accessory amounts of disseminated chrome spinel. The grains of chrome spinel are invariably altered along margins and fractures. They, therefore, consist of low reflectance (grey) cores/ patches that are bordered and traversed by highly reflectant (bright) zones. Compositionally, the cores are high-Cr, almost Mg-free chromites, containing unusually high amounts of Zn and Mn, and may thus be described as a solid solution of three end-member spinel compositions, i.e. chromite, gahnite and galarite. The bright zones of alteration lack any notable amount of Zn or Mn and range from Cr-mangnetite to magnetite. Compositional and textural relationships suggest that the markedly zincian and manganoan character of the spinel grains is the result of a hydrothermal activity that took place before the magnetite-forming alteration process. The Zn-, Mil-bearing hydrothermal fluids either reacted with the parental lava prior to or during its solidification or, alternatively, invaded the host rocks after their formation leading to the addition of Zn and Mn to, and concomitant removal of Mg from, the already crystallized magmatic chromite through dissolution-recrystallization.