Southwest China, especially Guizhou Province, has geological conditions for the formation of black rock series deposits. At present, large and super-large phosphate deposits have been discovered in the Sinian Doushantuo Formation and the Cambrian Niutitang Formation, including Kaiyang phosphate, Wengfu phosphate and Zhijin Xinhua rare earth phosphate (Zhang Lunwei et al., 2007). There are three large-scale deposits (Zunyi Song Lin Nickel-Molybdenum Mine, Zhijinyuan Nickel-Molybdenum Mine and Zunyi Huangjiawan Nickel-Molybdenum Mine), 1 medium-sized deposit (Jinsha Rock Hole Nickel-Molybdenum Mine) and four small-scale deposits (Jinsha Ganhe Molybdenum Mine, Xingyi Bangzha Molybdenum Mine, Kaiyang Baimadong Molybdenum Mine and Tongren Dam Molybdenum Mine) (Fan Zhenglie, 2006).

During the Sinian-Cambrian period, the Yangtze landmass composed of continental crust was dominated by stable carbonate deposits, and the southeast of Hunan and Guangxi, the southwest of Guizhou and the southeast of Yunnan were transitional crusts. The passive marginal deposits of the mainland deepened to the southeast, consisting of marl, siliceous rocks and carbonaceous mudstone, and the southeast of Hunan was a deep sea area. The Yunan movement at the end of Cambrian formed folds, uplifts and NE-trending thrust belts in Yunkai area, which started the collage of Cathaysian and Yangtze landmasses and ended the tectonic stage dominated by extensional activities in the early Caledonian. This stage is an important period for the formation of black rock series. During the Sinian Doushantuo period in the upper Yangtze region, nearshore clastic rocks-carbonate rocks and phosphorite-shale, carbonate rocks and phosphorite (Ye Lianjun et al., 1998) were gradually deposited from west to east, and no large phosphorite deposits were found in the clastic rock assemblage. Doushantuo Formation is a set of transgressive deposits integrated or pseudo-integrated on the moraine of Nantuo Formation. In the paleomagnetic study of Doushantuo Formation in South China, Zhiming Sun et al. (2004) thought that South China was located in the low latitude area near the equator. Li et al. (1996) found that when the Rodinia supercontinent disintegrated, South China was on the fault zone between the central ancient land and Australia, but it was likely to be separated from the Lauren ancient land and Australia in Sinian (Shi Chunhua, 2005). The sedimentary distribution and environmental changes of the Sinian Doushantuo phosphorite are controlled by the ancient Yangtze plate and tectonic factors. During the geological history from 800 million years to11million years, strong orogeny occurred, which made the study area form a relatively stable platform basement (Liu Hongyun et al., 199 1). After Doushantuo period, it was deposited as carbonate rock of Dengying Formation.

The black rock series is mainly composed of black shale, siliceous rock and phosphorite, and its relationship with the underlying strata is contact with the Dengying Formation in Taiwan Province. In the slope-basin area, it is in integrated contact with Hexapo Formation or Laobao Formation. The sequence from bottom to top is silicalite-phosphorite-black shale. The thickness varies greatly, generally from several meters to 100 meters or more.

At the beginning of the Early Cambrian, the pan-continent disintegrated, which led to the strong extension of the Yangtze plate and Cathaysian plate (Liu Baojun et al., 1993), and the tectonic subsidence intensified. At this time, due to transgression, the sea level rose rapidly and the water body deepened rapidly to the land. Combined with tectonic subsidence, the accommodation space increases, the sedimentation rate decreases, and the wave base surface of seawater and the interface of photosynthesis also increase, resulting in the Yangtze platform being flooded in a large area and becoming a flooded platform environment. In this environment, the lower water body is below the wave bottom, and the photosynthesis is very weak, thus forming an anoxic reducing environment. Hot water injection leads to the enrichment of polymetallic elements in black rock series. During this period, volcanic ash ejected by volcanic activity not only caused changes in the physical and chemical conditions of seawater, but also caused changes in climate, which caused the melting of continental glaciers, promoted the rapid rise of sea level and accelerated the formation of anoxic environment. Minerals related to this set of black rock series are mainly divided into two categories: ① black rock series: sedimentary-reformed phosphorus, vanadium, molybdenum and uranium; ② Carbonate rock: sedimentary reformed lead-zinc deposit.

The uranium content in black rock series is generally high because it contains carbonaceous, fine pyrite and other reducing substances. In the crust, the abundance of uranium in black shale is much greater than that in other sedimentary rocks. The uranium content in black rock series existing in southwest China is generally high, which provides an important uranium source for later uranium mineralization. The content of uranium and other metal elements in black rock series in southwest China is relatively high. For example, the U of the black rock series of Qianzhusi Formation in Deze, Yunnan Province is 665,438+0 times that of the continental crust (Xia Qinglin et al., 2008). During the study of black rock series in Guizhou, the project team sampled and analyzed Niutitang Formation, Jiumenchong Formation, Shillengshui Formation, Liuchapo Formation, Dengying Formation and Doushantuo Formation respectively. The results are shown in Table 5. 1 and Table 5.2. Table 5. 1 shows that the U content in black rock series is obviously higher than the crustal abundance value (2.07μg/g) (thomas lee, 1992), which is 6 ~ 20 times of the crustal abundance value, while the U content in normal dolomite of Dengying Formation is obviously lower than the crustal abundance value, far lower than that of black rock series. Table 5.2 gives the statistics of U content in black rock series. It can be seen that the uranium content in black rock series, especially (mineralized) phosphorite, is high, even reaching the level of exploitation and utilization. This part of uranium is widely distributed in southwest China, and the total amount is considerable, which is a potential uranium resource. Through the study of black rock series, Li Chaoyang and others also think that the black shale of Niutitang Formation in Lower Cambrian is a source bed rich in U, Ag, Se, PGE, V, Mo, Hg and other ore-forming elements (Li Chaoyang and others, 2003). It can be seen that the black rock series is the source bed of uranium mineralization and other elements, which is of great significance to uranium prospecting in the black rock series. Therefore, in this sense, the Nordic continental cracking event provides a place and opportunity for uranium mineralization in southwest China, which is the pre-enrichment period of uranium.

Table 5. 1 U content and its enrichment coefficient in some black rock series in Guizhou

Note: The samples were analyzed by the Analysis and Testing Center of Beijing Institute of Geology, Nuclear Industry. * According to thomas lee and others, 1992.

Table 5.2 Statistical Table of U Content in Black Rock Series from Central Guizhou to Southeast Guizhou