Composition and provenance of dark sand and gravel in the northern Kumtagh Desert

In the autumn of 2020 and 2021, our research group conducted two comprehensive desert scientific expeditions in the northern region of the Kumtagh Desert, obtaining 73 sample points of sand dune deposits. After laboratory systematic tests, 81 samples were analyzed for particle size, 39 groups(434 samples) of color statistics were conducted, 59 geographical location points(223 samples) were micro-identified for detrital minerals, and 6875 thin section lithic fragments were identified. The northern part of Kumtagh Desert exhibits a high content of fine gravel and coarse sand, with a significant portion being black, dark gray, and brown in color. The proportion of dark particles in the sediment is closely correlated with grain size, whereby larger grains tend to contain a higher concentration of dark particles. The thin sections of lithic fragment and detrital minerals identification reveal that the material composition of this dark sand and gravel is intricate, consisting of both dark heavy minerals and dark light components, as well as more abundance of dark lithics. The rock composition exhibits a wide diversity, encompassing over 90 different species. The predominant rocks include rhyolite, felsite, volcanic glass, trachyte, and andesite(with a content exceeding 5%). Additionally, the composition comprises latite, granite, and tuff(with a content ranging from 3%to 5%). Furthermore, there are 14 types of rocks and detrital minerals(with a content surpassing 1%), such as quartzite, basalt, feldspar, and chalcedony. The presence of dark detrital minerals is primarily observed in fine sand to coarse silt, predominantly consisting of opaque ferrous metal minerals(limonite, magnetite, hematite, and ilmenite) as well as translucent dark nonmetallic minerals such as hornblende and epidote. In most samples, the proportion of dark heavy minerals is relatively low, accounting for less than 1%of the total sediment. The black and gray debris primarily originates from the inherent color of the rock itself, while the high concentration of debris is associated with the nature and origin of the parent rock in the primary source area, Altyn Tagh. On the other hand, brown debris is closely linked to both rock and mineral composition as well as particle surface adhesion(iron and manganese oxides and clay minerals). Prolonged drought conditions coupled with intense solar radiation lead to iron and manganese oxidation along with a thickening desert varnish layer. Consequently, particles containing iron exhibit a deepened surface coloration.