Features of the Cuobuzha podiform chromitite along western Yarlung Zangbo suture zone in Tibet:Implications for its tectonic setting

Most of chromitite ores produced in China are from the Yarlung Zangbo ophiolite belt (YZSZ), which is divided into three parts:the eastern part (Qu Shui to Mo Tuo), the middle part (Ang Ren to Ren Bu) and the western part (Saga to the Sino-Indian border). Furthermore, the western part contains north sub-belt and south sub-belt. For a long time, most of studies have focused on the eastern part and middle part, with very insufficient studies conducted on the western part, especially on the north subbelt, which inhibits mineral exploration along the YZSZ. Cuobuzha ophiolite is located in the north sub-belt, which occurs along a NW-SE trending belt and is mainly composed of harzburgite with diabase dykes. The field investigation discovered several chromite orebodies 0.5-1 m long and 0.2-0.5 m thick within the ophiolite. All of the ores are massive. The analytical results by electron-probe micro-analyzer (EPMA) show that the Cuobuzha chromitite is of typical high-Cr variety with Cr numberCr^#=100×Cr/(Cr + Al) ranging from 75 to 78, and Mg numberMg^#=100×Mg/(Mg + Fe²⁺) ranging from 66 to 69. Calculated results show that the ratio of FeO/MgO ranges from 0.51 to 0.65, and that the values of Al₂O₃ and TiO₂ are 11.27%-12.1% and 0.19%-0.4%, respectively, revealing that the Cuobuzha chromitite has boninitic affinities. However, the occurrence of clinopyroxene exsolution indicates that it should have experienced a deep process.