Peridotites and Chromitites from the Dingqing Ophiolite in the Eastern Segment of Bangong–Nujiang Suture Zone, Tibet: Occurrence Characteristics and Classifications

The Dingqing ophiolite is located in the eastern segment of the Bangong-Nujiang suture zone. This suture zone is W–E trending parallel with the Yarlung–Zangbo suture zone and is an strategic area for exploring chromite deposits in China. The Dingqign ophiolite is distributed in near SE-NW direction. According to the spatial distribution, the Dingqing ophiolite is sudivided into two massifs, including the East and the West massifs. The Dingqing ophiolite covers an area of nearly 600 km2. This ophiolite is composed of peridotite, pyroxenite, gabbro, diabase, basalt, plagiogranite and chert(Fig. 1). The peridotite is the main lithology of the Dingqing ophiolite. The peridotite covers about 90% of the total area of the Dingqing ophiolite. The Dingqing ophiolite is dominated by harzburgite with a small amounts of dunite. The Dingqing harzburgite displays different textures, such as massive, Taxitic, oriented and spherulitic textures(Fig. 2d–i). These four types of harzburgite occur in both the East and West massifs, especially in the Laraka area of the eastern part of the East massif. Dunites have different occurrences in the field outcrops, such as lenticular or stripshaped, thin-shell and agglomerate varieties(Fig. 2a–c). On the basis of detailed field work, we have discovered 83 chromitite bodies, including 27 in the East massif and 56 in the West massif. According to the occurrence scale and quantity of the chromitite bodies, we have identified four prospecting areas, namely Laraka, Latanguo, Langda and Nazona. Chromitites in the Dingqing ophiolite show different textures, including massive, disseminated, veined and disseminated-banded textures(Fig. 3). On the basis of the Cr#(=Cr/(Cr+Al)×100) of chromite, we have classified the Dingqing chromitite into high-Cr, medium high chromium type, medium chromium type and low chromium type chromitite(Figs. 4, 5). Among them, low chromium type chromitite Cr# is extremely low, ranging from 9.23 to 14.01, with an average of 11.89;TiO2 content is 0.00% to 0.04%, and the average value is 0.01%, which may be a new output type of chromitite. These different types of chromitites have different associations/assemblages of mineral inclusions. The inclusions in high chromium type chromitite are mainly clinopyroxene and a small amount of olivine;medium high chromium chromitite are mainly amphibole, a small amount of clinopyroxene and phlogopite;while low-chromium chromite rarely develops mineral inclusions, and micron-sized clinopyroxene inclusions are common in olivines which are gangue mineral in it. These different types of chromite ore bodies have a certain correspondence with the field output, and may also restrict their genesis. This part will be further developed in the follow-up work.