Characteristics of the Gyaca chromite deposit in the eastern segment of the Yarlung−Zangbo suture zone in Xizang, China and their geological significances

The mantle peridotites of the Gyaca ophiolite in the eastern segment of the Yarlung Zangbo Suture Zone (YZSZ) are composed of harzburgites and dunites. Several typical podiform chromite deposits (occurrences) are developed within the ophiolite massif. The chromite orebodies occurred as vein-like or lenticular bodies hosted in harzburgites, and are generally enveloped by thin crust of dunite. In order to clarify the tectonic affinity and chromite metallogenic potential of the Gyaca ophiolite, in this paper, we conducted systematic petrological, mineralogical and geochemical investigations of the mantle peridotites and their hosted chromites. The results show that the Gyaca chromite deposit is a typical high-Cr chromite deposit (with Cr# values of chromites ranging from 71.8 to 77.7), and its parental ore-forming magma has boninitic melt affinity. The chromian spinels in chromite ores are characterized with synchronous depletion of Zn, Co and Mn but enrichment of Ni, and contain amphibole inclusions, confirming that the parental melt had enriched water. The mantle peridotites of the Gyaca ophiolite possess a type of affinity of the single supra-subduction zone (SSZ). The chromite was product of the intereaction between the mantle peridotite and water-rich and sulfur-undersaturated boninitic melt. It recorded the metallogenic evolution process under the SSZ tectonic setting. This study clarified the metallogenic mechanism and tectonic setting of the Gyaca chromite deposit, and provided an important reference for the exploration of chromite resources in small ophiolite massifs within the YZSZ.