A possible new chromium mineralization： the formation of chromium spinel in Gangshang ultramafic rocks of Sulu UHP belt

Abstract: Gangshang ultramafic rocks are mainly composed of garnet peridotites and dunite, which contain olivine, chromium spinel, diopside, endiopside, or/and garnet, orthopyroxene, amphibole and phlogopite. Chromium spinels in Gangshang ultramafic rocks are compositionally variable, with Cr^#(molar100Cr/(Cr+Al)) varying from 51 to 89, and are associated with four stages of ultramafic rocks. With the increase of Cr^# in chrome spinels Mg^#（molar100Mg/(Mg+Fe²⁺)）of chromium spinels decreases,whereas oxygen fugacity increases, as reflected by the composition of Gangshang chromium spinels. In the process of retrometamorphism of amphibole-facies and greenschist-facies, Cr, Mg and Al of chromium spinels decrease, whereas Fe relatively increase. In addition, Gangshang ultramafic rocks experienced shearing reformation and CO metasomatism, which was helpful to the enrichment of chromium and the formation of chromium spinel. This mechanism is also useful to understanding the development of chromite deposits in some metamorphic areas and the enrichment of some chromite ore bodies.