Contribution To Shaanxi Province Uranium Orefield Ore-controlling Factors And Metallogenic Model Research

Lantian uranium ore field is located in the northwestern segment of Muhuguan granite intrusion, borded by Teritary fault-depression basin on the west. Much effort is made to discuss uranium mineralization mechanism of the Lantian uranium ore field, involving in ore-controlling structures, geology and geochemistry characteristics of wall rocks and mafic dikes. A possible genetic mineralization model of the Lantian uranium ore filed is proposed. All of the following statemens can be concluded.1. Lantian uranium ore field is controlled by3large-scale groups of faults. That is NE compressive faults, formed by NW and NWW principal compressive stress, nearly NS shearing faults and NW tensile faults. Manganitude and strength of ore-forming faults effected uranium mineralization and wall rocks alteration much. Meanwhile, Uranium mineralization is also controlled by NE-NNE dominant faults and their derivatives secondary fractures, extending about2～10m normally. In addition, uranium mineralization accompany with cataclastic-alterated granites. Outwards, fine grain granitic fractures, granitic fractures and fractured granites have be indentified in turn. Clayzation, hematitization and fluoritation can be found made out easily. Vertically, mineralization structures extend in succession, that can be exampled by10floor of mining section.2. Wall rocks from fresh granites to weak alterated granites, medium alterated granites, and then to ore-bearing granites, their geochemistry characteristics change obviously. In particular, Si, Ca, Mg, Fe2+. Na, Mn, CO2reduce gradually or rapidly. Whereas, K, Fe3+, S increase gradually on the contrary. In addition, E REE reduce also. Fraction degree of their HREE and LREE can be disingusihed easily. And following uranium mineralization intensively, this distinction reduces by degrees. In ore-bearing graites, K, Sr, and Ti are depleted. and Ba, Nb are enriched.In the mineralization process part of Si of the normal rock have replaced to the ore-forming fluid, it appears as quartz veins. Part of K of ore-forming fluid have replaced to the normal rock, it appears as the red alteration. With decompression boiling, the CO2content decreased. Fe2+decreasing meanwhile Fe3+increasing, in the mineralization process (UO2)2+ and Fe2+reaction, formation of pitchblende and hematite.3. There is two types of mafic dikes in Lantian uranium field. One is dark green lamprophyre, minette (according to the IUGS recommended lamprophyre classification scheme). According to the Rittmann standard, belong to inferior alkali rock series, In the SiO2-Na2O+K2O diagram, the sample falls in Rock (1990) delineation of the ultramafic lamprophyres nearby. MgO is negatively correlated to SiO2, TiO2, K2O, and positively correlated to Al2O3, MnO, CaO, P2O5, that exists in the process of crystallization fractionation or volatile migration. TheEE content is higher, in the Chondrites REE distribution patterns diagram, display the performance of the LREE enrichment and right allocation mode, and LREE enrichment Strongly, LREE and HREE fractionation strongly. The other one is light grey diorite porphyries, according to the Rittmann standard, belong to inferior alkali rock series. In the SiO2-K2O diagram, sample fell in the shoshonite series. Compared with the minette, show lower Ti, Fe, Mg content, higher Si, Al, K content. TheEE content is higher also, in the Chondrites REE distribution patterns diagram, display the performance of the LREE enrichment and right allocation mode, and LREE enrichment Strongly, LREE and HREE fractionation strongly. The pattern of the minette and the diorite-porphyrite is consistent in the Chondrites REE distribution patterns diagram, but the SREE and SHREE content of the minette is two times of the diorite-porphyrite. Compared with the Muhuguang granites mass, the SREE content of the above two mafic dikes is higher, but the pattern in the Chondrites REE distribution patterns diagram is consistent. Above-mentioned geochemical characteristics imply that the source rock compositions of the two mafic dikes may be consistent, but the genesis of minette may be derived directly partial melting of the source rock, the genesis of the diorite-porphyrite May be assimilated the upper crust material in the process of emplacement.4. Lantian uranium field mineralization mechanism has been preliminary analysised. The formation of theyγ52-2, The preenrichment of U caused by the multiple magmatic activity. Alkali-alternation causing by mantle fluid activate the uranium of the accessory minerals, and the mantle fluid provide abundant ΣCO2. Uranium which is moved as UO2(CO3)34+deposit in right physic-chemical circumstance. The phase of overprinting and reworked mineralization of oxidation and leachina.