Low-T formation of high-Cr spinel with apparently primary chemical characteristics within podiform chromitite from Rayat, northeastern Iraq

摘要

Chemical modification of chromian spinel at low-T alteration was examined in detail for a podiform chromitite from a Tethyan ophiolitic mélange belt at Rayat, northeastern Iraq. The chromitite is highly brecciated and the matrix has been completely altered, producing chlorite and carbonate (dolomite and calcite). High-Cr, low-Fe3+ spinel has formed along the margins and cracks of chromian spinel grains throughout the alteration, associated with unaltered primary spinel and magnetite without ferritchromite. In associated harzburgites, only ferritchromite is found instead of the high-Cr, low-Fe3+ spinel. The high-Cr, low-Fe3+ secondary spinel apparently has chemical properties of mantle origin, plotted at the extension of ordinary mantle spinels on compositional spaces. The character is due to subtraction of Al as chlorite with the addition of an amount of magnetite component from the silicate matrix, which is small in volume relative to peridotite and composed of highly magnesian olivine (up to Fo₉₇). We should treat high-Cr chromian spinels with caution in highly altered mantle-derived rocks, especially chromitite and other rocks with highly magnesian olivine, as well as in detrital particles for provenance study.