Study On Mineralization Of Hongshanzi Uranium - Molybdenum Deposit In Keshiketeng Banner, Inner

Guyuan-Hongshanzi is a very important uranium-polymetal metallogenic belt in north margin of north china block, china. Hongshanzi U-Mo deposit is a tapical volcanic uranium deposit in this belt. Based on the further systematic studies of deposit geology, magmatism, uranium mineralization ore-controlling Factors and metallogenic fluid features, we analyzed the sources of mineralizing fluids and the mechanism of uranium migration and concentration and established the metallogenic model of hongshanzi deposit by using hot spot metallogenic theory.⑴ The results of petrological geochemistry indicate that all the magmatic rocks belong to calc-alkaline series and have REE fractionation and negative Eu and positive Ce anomalies. The diabase show high Nb, Ta, Ti and low Rb, U, Th contents, the other magmatic rocks show high Rb, Th, U and low Nb, Ta, Ti contents. Studies on SHRIMP U-Pb zircon geochronology showed that the age of andesitic tuff breccia is 277.19±4.7Ma, the age of granodiorite is 268±3Ma, the age of trachyte is 159±4Ma, the age of rhyolite porphyry is 160.6±2.9Ma, the age of syenite porphyry is 160.2±1.9Ma, the age of granite is 132.9±1.4Ma and the age of diabase is 134.2±2.7Ma. On the basis of previous researches and results of Sr-Nd isotope and petrological geochemistry, the author found syenite porphyry, granite porphyry, trachyte and rhyolite porphyry originated from lower crust source region and may have mantle components and biotite granite and diabase originated from the depleted mantle source region, the components of magmatic rock have a tendency from enriched mantle EM1 to depleted mantle.⑵ Wall rock alterations mainly included albitization, fluoritization, hematitization, chloritization, potassium alteration and silicification. Uranium mineralization had close relationship with albitization-dominated alkaline hydrothermal alteration. Thorium mineralization closely associated with acidic fluoritization. Molybdenum mineralization closely related to silicification. Wall rock alteration had obvious horizontal zoning characteristics. Centered with Tiantaiyong-Heshengyuan fault were well-developed silicification hematitization and chloritization. Albitization overlay on the hematitization. Alkaline mineralized alteration and the newly found acidic mineralized alteration constituted the acid-base alteration zonation in space. Alkaline mineralized alteration developed in the northeast, while acid mineralized alteration developed in the southwest.⑶ The uranium ore bodies mainly outcropped in volcanic basins. The morphology of ore body was controlled by the edge faults F1 and F2 of the volcanic graben basin and volcanic neck. Tiantaiyong-Heshengyuan fault was the primary ore-controlling structure, basin edge faults F1, F2 and volcanic neck were secondary ore-controlling structures. Uranium mineralization mainly located in the broken trachyte of the hanging wall of the basin edge fault, and also developed in the inner contact zone between rhyolitic porphyry and volcanic neck. Uranium was mainly in the form of uraninite. Molybdenum mineralization mainly occurred in rhyolitic porphyry and granite, and molybdenum developed in the form of molybdenite. Molybdenum mineralization layered on the uranium mineralization. Thorium deposited in the fluoritization quartz vein. Thorium existed in the form of thorianite and thorite.⑷ On the basis of previous studies, we have noticed that the mineralization of U/Th and Mo has occurred obviously later than the trachyte and rhyolite porphyry generated during volcanic activity, with the evidence of the characteristics of U/Th and Mo mineralization. The time of mineralization was limited to the early Cretaceous Epoch. The mineralization of U/Th happened a little bit earlier than the mineralization of Mo. In time, the mineralization was relatively independent of magmatic rocks generated during volcanic activities. And in space it showed the characteristics of mineralization stacking above the magmatic rocks.⑸ The features of mineralization, wall-rock alteration and fluid inclusions in the study area indicate that the ore-forming fluids of U were reductive, rich in F, Na, U, Th and from deep source with high temperature. U existed and migrated in the reducing fluids in the form of UCl3+, UF3+ and inorganic hybrid complexes. When the ore-forming fluids from deep source got into the favorable position for mineralization, the conditions of temperature, pressure, Eh, pH and oxygen fugacity would change abruptly, which could cause the vapor components to escape, and UCl3+, UF3+ and U-bearing inorganic hybrid ligand to decompose rapidly in the fluids. In that case, U4+ would reach saturation in solution and coagulate in the form of colloidal sol, and the residual liquid would escape, as well as the occurrence of multi-elements mineral precipitation and metasomatic alteration.