| Zoige uranium ore fields are located in the western part of the uranium metallogenic belt in southern Qinling Mountains of Zoige, Sichuan, the main part is the White Longjiang anticlinorium composed of Paleozoic strata, Zoige uranium ore fields is an important C-Si pelite type uranium deposit area in China. Deposit series are mainly produced in a metamorphic silicon-limestone system of the Yang-changgou group, Ta-er group and La-long group in the Lower Silurian, it is strictly controlled by lithology, tectonic and geochemical environment, which is the largest number, the largest scale, the largest industrial significance and economic significance deposit in China at present, and the development of C-Si pelite type uranium ore field is obvious, thus many geological workers have attention on it.The siliceous rock is one of the important ore-bearing rocks in Zoige C-Si pelite type uranium ore field. The siliceous rocks in this area are mainly divided into micritic mud silicolite, metacrystal silicolite, silicefied silicolite and calcified silicolite according to the structure and causes. The article mainly find out the origin of siliceous rocks in major element geochemistry, trace element geochemistry, REE geochemistry and isotope geochemistry. Using factor analysis method as the foundation, the article analyzed the correlation of the ore hosting rocks, hydrothermal minerals(quartz, calcite) and trace element of ores, it explored the relationship between siliceous rocks and uranium mineralization. Through this research, the following results have been achieved recognition:1.The major element geochemistry characteristics of siliceous rocksThe average content of SiO2 in siliceous rocks is 95.72%, explaining that there is almost free of terrigenous sediment. The value of Al/(Al+Fe+Mn) is 0.21, the value of Fe/Ti is 55.69, the value of(Fe + Mn) /Ti is 61.78, which are all reflecting the hot water characteristics of siliceous rocks in Zogie uranium ore field. The Al-Fe-Mn diagram and the Al2O3-SiO2 diagram of standard model both showed that the hot water sedimentary characteristics. The value of MnO/TiO2 is 5.226, reflecting that the siliceous rocks could be formed in the deep sea basin environment. The average value of MgO/Al2O3 is 0.940, indicating that the sedimentary environment was the high salinity sedimentary environment which have few marginal material.2.The trace element geochemistry characteristics of siliceous rocksThe trace elements of siliceous rocks in Zoige have the characteristics of high value of U and low value of Zr, suggesting that it may be hot genesis and the environment of far from terrigenous. The value of Th / U and Sr / Ba are both far less than l, which showed that significant causes of hot water, while the low value of Rb / Sr, reflecting that the mafic-rich material source added when time of siliceous sedimentary, indicating that there is was a strong expansion lysis.3.The REE geochemistry characteristics of siliceous rocksThe shale-normalized REE distribution patterns of North America of siliceous rocks in Zoige was a leftist, most samples have the characteristics of negative Ce anomalies and weak positive Eu anomalies, the average of δCe is 0.80, the average of δEu is 1.2, which indicated the hot water sedimentary characteristics of the siliceous rocks.4.The Si isotope geochemistry characteristics of siliceous rocksThe ?30Si of district gray and black silicon rocks is-0.4‰~+0.2‰(the average is 0.017‰), which appeared the depositional environment was far from the mainland. The ?30Si of quartz veins is-0.1‰~+0.5‰(the average is 0.25‰), reflecting that the hot water sedimentary characteristics. The ?30Si of silica fume transition rocks is-0.2‰, indicating that the process of silicification with underground water participated. The ?30Si of mineralized siliceous rocks is-0.1‰~+0.5‰, which explained the chemical sedimentary siliceous rocks in the original deep hot water have transformed again by the mineralization of ore bearing hydrothermal solution.5.The relationship between ore rocks and uranium mineralizationThe trace element geochemistry characteristics of rocks in ore-bearing rock series of Zoige uranium ore fields indicate that the enriched trace element types of siliceous rocks、mineralization siliceous rocks、siliceous limestone and siliceous ore are similar, there are two different distributin patterns of the primitive mantle standardization of trace element spider diagram, and slate and dacite porphyry have the similar distributin characteristics, the distributin patterns of siliceous rocks、mineralized siliceous rocks、siliceous limestone、siliceous ore、mineralized siliceous limestone、limestone、calcite vein and quartz vein are generally consensus, reflecting that uranium ore-forming have intimate connection with siliceous rocks、siliceous limestone、calcite vein and quartz vein.There are six factors of ore-bearing rocks which are representative. The element of F1 factor are Cr-Ga-Zr-Nb-Hf-Ta-W-Pb-Th, which represents the deep source dacite porphyry and slate. The element of F2 factor are Li-Be-Sc-Ga-Rb-Zr-Nb-Cs-Hf-Th, which represents the trace element combination of terrigenous microclastic rocks. The element of F3 factor are Co-Ni-Zn-YMo-U, which represents the ore mineralization calcite veins. The element of F4 factor are V-Cu-Mo-U, which represents the siliceous rock and some ore calcite veins. The element of F5 factor are Bi-Pb-Ba, representing sedimentary siliceous rocks. The element of F6 factor is Sr, representing Sedimentary limestone and silicified limestone. Wherein, F5 and F6 factors are closely related to each other, which reflects the marine chemical sedimentary origin; F2 、F3 and F4 factors are closely related to each other, which reflects the mineralizationprocess; F1 factor is independent. It is seen that the mineralization is closely related to siliceous rocks、 siliceous limestone、calcite vein and quartz vein.6.The mineralization significance of siliceous rocksThe average value of uranium of siliceous rocks in Zoige is 10.955×10-6, the average value of thorium is 10.955×10-6, and the value of Th/U is only 0.059, which is belonging to the depositon construction with rich uranium. The extremely low value of Th/U suggests the uranium in rocks is active. The high content of uranium and low content of Th/U of siliceous rocks are very favorable for uranium mineralization, the siliceous rock series formed by the hot water have laid a good foundation for the later mineralization of uranium. The calcium silicon rocks(siliceous limestone) have not only big void, but also it is tending to produce brittle deformation and fracture zone between cambium, that the ore-bearing hydrothermal can transport and precipitation preconcentration easy in it. Therefore, the siliceous rocks are not only the sources bed of ore-forming, the calcium silicon rocks(siliceous limestone) formed by carbonation siliceous rocks also provide a very favorable space for mineraliztion.The conclusion aboved are important for the study of the causes of siliceous rocks in Zoige, it is also meaningful for the work of uranium prospecting exploration. |
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