Dingqing ophiolite chromite in the eastern segment of Bangong Co-Nujiang suture zone, Tibet:Occurrence characteristics and classifications

The Dingqing ophiolite is located in the eastern segment of the Bangong Co-Nujiang suture zone which is juxtaposed with the Yarlung Zangbo suture zone, and is an important area for finding chromite deposits in China. The ophiolite body is distributed in the southeast direction, with a total area of nearly 600 km². It is mainly composed of mantle peridotite, pyroxenite, gabbro, diabase, basalt, plagiogranite, siliceous rock and argillaceous rock. According to the spatial distribution, the Dingqing ophiolite is divided into east and west ophiolite bodies. On the basis of previous work, 83 chromite ore spots were discovered through geological mapping, measured sections, trench recording and drilling, including 27 in the east ophiolite body and 56 in the west ophiolite body. According to the production characteristics of the chromite spots and types of wall rock, Dingqing chromite can be divided into four types:Type Ⅰ:chromite orebodies are vein-like, the wall rock is stripe-like or lenticular dunite and massive harzburgite; Type Ⅱ:chromite orebodies are lenticular, podiform or irregularly agglomerated, the wall rock is thin-shelled dunite and porphyry or massive harzburgite; Type Ⅲ:chromite is diffusely distributed in dunite, and the wall rock is stripe-shaped dunite and massive or porphyry harzburgite; Type Ⅳ:chromite orebodies are produced in stripes, and the wall rock is stripe-like or lenticular dunite and directional harzburgite. According to the structural characteristics of chromitites, they are mainly divided into four types:massive, vein, disseminated and disseminated stripe. Massive and veined chromitites are the main types of chromitites, a small amount is disseminated and disseminated stripe, with a small amount of nodular or bean-like structure developed in local dunite. In this study, 13 representative chromite ore spots were selected for detailed studies of petrology, mineralogy, and mineral chemistry. According to the mineral chemistry characteristics of chrome spinel in the chromitites, the chromite bodies can be divided into four types, i.e., high chromium (Cr^#78-85), medium-high chromium (Cr^#60-74), medium chromium (Cr^#30-51) and low chromium (Cr^#9-14)(Cr^#=100×Cr/(Cr+Al)). The east ophiolite body contains medium-high chromium and medium chromium type chromitite, with the lack of high chromium type chromitite; the west ophiolite body contains high chromium and medium chromium type chromitite, and lacks medium-high chromium type chromitite. At the same time, there is a very low Cr# chromitite in the east rock mass and the west rock mass, which is tentatively assigned to "low chromium type chromitite". These different types of chromitites have a certain correspondence with the field output, and may also restrict their geneses. This content will be expanded in subsequent articles. A comparison with Luobusa podiform chromitite and peridotite in mineral assemblages and mineral chemistry shows that they have many similar characteristics, even with the probable existence of a good prospecting space.