Genesis And An Evaluation For Granite-Pegmatite Type Columbotantalite Ore-Forming Potential In The Northern Margin Of The Qaidam Basin, Qinghai, P.R. China

The northern marginal metallogenic belt of Qaidam Basin, where is rich in ore-formingelements, such as non-ferrous metals, Cr-Ni and rare metals, possesses good material base offorming endogenetic metal ores. The pegmatites in Shengge area are the main explorationtargets for rare metal-gem deposits. In the thesis, by doing research on field geologicalfeatures, petrography, mineralogy and geochemistry of the niobium-tantalum-rich graniticpegmatites, we will study the genesis and mineralizing process of pegmatite-typecolumbotantalite. At the same time, we would discuss the formation mechanism of magmaliquid segregation, occurrence of niobium and tantalum and metallogenic evolution. We havetentatively set up a metallogenic model of the pegmatite-type columbotantalite. Throughstudying the pegmatitic metallogenesis, we have analyzed the features of metallogenic seriesand evaluated the niobium-tantalum-forming potential. The results obtained are as follows:1) The pegmatitic veins in the study area, which are small in scale and large in quantity,are distributed alternately in groups and belts. Being controlled by the regional secondaryfaults and fractures, most of the pegmatites are veined or lentoid. The pegmatites are mainlymicrocline-type, microcline-albite-type, albite-type and muscovite-type. The pegmatitic veinshave high ratio of mineralized ones to the total, uneven mineralization, high grade andconsiderable resource.2) The pegmatites develop metasomatic phenomenon widely, which is characteristic ofalbitization and muscovitization. It shows positive correlation between metasomatism andniobium-tantalum mineralization. Most of the niobium and tantalum occurs in independentmineral of columbite and columbite-tantalite, while a small quantity of them occurs in spheneand hematite by isomorphism. The tectonic superimposed stress, which occurred in and afterdiagenetic stage, resulted in obvious brittle-ductile deformation in the rocks.3) Do systemic research about the geochemical characteristics of pegmatite. Thepegmatite is alkali-rich and peraluminous. The content of REE is low and exhibit a M-typetetrad effect. The contents of Li, Rb, Nb, Ta and REE change as the alkali metasomatism(K-Na-(Li, K)) development regularly. At the same time, there is mineralization of Nb-(Nb,Ta)-Nb. 4) The pegmatitic veins are the result of magma liquid segregation. After high degree offractionation, pegmatite magma, which was rich in volatile components and rare elements,moved along structure fractures and formed abundant pegmatite veins. Rare elements, such asNb and Ta, assembled, migrated, concentrated and precipitated during this process.5) Pegmatitic veins of the same types but different geochemical characteristics have beenfound in the research area. They are all muscovite-albite pegmatite veins, but the onesoccurred in the southeast margin of the area have higher content of rare and radioactiveelements, such as the REE, Nb and Th. This feature suggests that there should be more thanone (at least two) stages of pegmatite mineralization.6) The studied object is similar to the metallogenic series of I-type granite inWuyi-Yunkai area, and can be contrast to the metallogenic setting and genetic model oftypical pegmatite-type rare metal deposits in China. With advantageous tectonic magmatichydrothermal mineralization of Nb and Ta, the area shows a great metallogenic potential.