STUDY ON ISOTOPIC GEOCHEMISTRY OF ORIGIN OF THE 3701 URANIUM DEPOSIT

Abstract: The ore deposit occured in carbonate rocks located in an exocontact belt (O- 120m) of the L granite batholith (Fig. 1) .The deposit was hosted by argillaceous limestone of the Yingtang Formation, Middle Devonian.The L granite body with an area of 238km²has K-Ar age of 202-318Ma. U-Pb ages of pitchblende from the deposit are 65.0±0.6Ma, 59.2 ± 0.6Ma, 30.7 ± 0.6Ma.The time-gap between the age of granite and the time of ore deposition is large, which indicates there is not a direct relationship between the mineralization and intrusive activity of the L granite body. Sulfur isotopic compositions of 30 pyrites from the ores range from δ³⁴S values of+0.8 to -39.81 per mil which are approximately similar to those of the host rocks (ranging from -8.05 to -20.49 per mil for pyrites (Table 2) . The 4 analyzed calcites from ores have δ¹³C values between -0.23 and -1.53 per mil (Table 4) . The 4 analyzed host rocks have δ¹³C values between +1.004 and -0.965 per mil.These data are very similar. U-Pb isotopic compositions of 3 K-feldspars from the granite, 3 galenas associated with pitchblende and 7 granitic rocks have been determined (Table 3, 5) .The calculative results suggest: (1) .All leads belong to an anomalous type low in radiogcnctic lcad. These data possess a good linear relation in ²⁰⁷Pb/ ²⁰⁴Pb-₂₀₆Pb /²⁰⁴Pb plots (Fig.4), implying a genetic connection between the granite and the ore. (2) .Average loss of uranium in granitc has been 59.51% since the granite is formed, while average addition of uranium in granite near limestone has been 49.56%.It is an important uranium source for the mineralization. The δ¹⁸O values ( SMOW, 6 samples) of ore-forming fluid at 120°-281℃ in equilibrum with microcrystallinc quartzs and calcitcs range from -3.14 to+13.92 per mil (Table 6), which are higher than those of ancient meteoric water in the area.It may be due to the oxygen isotopic exchange between the meteoric water (δ¹³O_H2o = -9~-10pcr mil) and limestone (δ¹⁸O_H2o ≠ +12.68~+26.15 per mil) . There arc reasons to believe that in the ore constituents S, C were mainly derived from the host rocks (argillaceous limestone), and U, Pb mainly from the granite.The ore-forming fluid was dominated by meteoric water. It can be imagined that the surface water in the period of continental weathering and oxidation (Cretaceous) infiltrated downwards, leaching out large amounts of uranium from the granitic rocks to form infiltration solution. A part of uranium in solution was adsorpted by argillaceous limestones. The solution was heated at large depth and then moved upwards, during which it dissolved constantly uranium from the host rocks to form the deposit. Therefore, the 3701 uranium deposit is a stratabound one of the multi-source and later multi-superimposition of hydrothermal transformation.