Uranium mineralization of Paleoproterozoic and Triassic in the Huayangchuan area of Xiaoqinling belt, China: Implications from U-Pb geochronology of zircon and monazite from uranium-bearing pegmatite

The Huayangchan uranium deposit in the west of Xiaoqinling intracontinental orogenic belt, southern margin of North China Craton, is a polymetallic deposit, which is dominated by uranium, niobium and associated lead, rare earth elements. This deposit was discovered in the 1950s, and has been known as carbonatite type uranium deposit containing betafite for a long time. A lot of previous research work has also focused on carbonatite type uranium deposit. In recent years, new geological work has found a large number of uranium-bearing pegmatites (or fine crystalline rocks) and granite veins in the Huayangchuan area. This paper studies the petrography, mineralogy, zircon and monazite U-Pb chronology of uranium-bearing pegmatites located in the west of Huayangchuan uranium mining area and Huangjiagou area. It is found that zircons in uranium-bearing pegmatites are mainly of magmatic origin, whose U-Pb ages are concentrated in 1842 +/- 11Ma similar to 1829 +/- 12Ma. The co-exist of zircon and betafite minerals in the rocks indicates that mineralization in diagenetic stage also occurred, which reflects the uranium mineralization effect of granite magma activity during basement cratonization in the southern margin of North China Craton in Paleoproterozoic (about 2. 0 similar to 1. 8Ga). There are also many hydrothermal monazites in uranium-bearing pegmatites. The U-Pb isotopic analysis of these monazites obtained two groups of age data of 268.7 +/- 5. 9Ma similar to 259 +/- 4. 6Ma and 232.5 +/- 4. 7Ma similar to 228.2 +/- 9. 2Ma respectively, reflecting two stages of tectonic thermal events happened in Huayangchuan area from Late Paleozoic to Early Mesozoic. Among them, the Late Paleozoic age data (268. 7 +/- 5. 9Ma similar to 259 +/- 4. 6Ma) show that a long-range tectonic thermal effect of the deep plate subduction and collision mechanism from the Yangtze Plate to the southern edge of the North China Plate (and South Qinling micro block) happened in Huayangchuan area; while the Early Paleozoic age data (232.5 +/- 4. 7Ma similar to 228. 2 +/- 9. 2Ma) are very close to the metallogenic age data of the carbonatite type uranium and molybdenum deposits in Huayangchuan area, which provides new evidence for the existence of Early Mesozoic uranium molybdenum polymetallic mineralization in Huayangchuan area and even the East Qinling Mountains.