Geochemistry Characteristics Of Migmatite Rocks And The Relationship With Uranium Mineralization In The 505 Area, Panzhihua District, Southwestern China

Kangdian geological axis is in the western margin of the Yangtze Block. Its complex geological structure, strata and mineral resource have always been the focus and hot spots in gelogical study in the area. There are various types of uranium mineralization and uranium occurences. Among them, the uranium mineranization in metamorphic rocks and migmatites is very important one. Based on comprehensive analysis of petrology and geochemistry in 505 Area, the paper discussed the main ore-bearing rock, major and trace element geochemistry, tectonic setting and relationship with uranium mineralization, and mainly achieve the following understandings:(1) Amphibolic rocks (schist, granulite) and plagioclase gneiss are mainly migmatized in the 505 area. The migmatization developed a wide range of metasomatic texture, including metasomatic myrmekitic texture, perthitic texture and corrosion texture, and so; leucosomes of new quartz, quartz-plagioclase and pure plagioclase; and melanosome of biotite. Some of them have granitic texture. In space, migmatized rocks mainly occur in the Proterozoic strata which are between Datian quartz diorite and Heime granite of Jinningian, where Agmatite, dictyonite and venite were developed, some homogeneous migmatites (diatexite) were also occured.(2) The migmatite(homogeneous) in the 505 area is petrology-chemically peraluminous, calc-alkaline rocks, and is belonging granodiorite or tonalitic rocks in the classiffication,with chemical composition of SiO₂,65%⁶9%,Na₂O>K₂O. Constant element of the migmatite does not show clear trends of magmatic evolution. In REE, it is generally enriching LREE and depleting HREE, with positive anomalies and negative Eu anomalies, or no Eu anomalies. Depented on La / Yb andΣREE , two kinds of the migmatite can be distinguished: the muscovite-bearing homogeneous migmatite(La/Yb>20,ΣREE≦100); The biotite-bearing homogeneous migmatite, including metamorphic rocks (La/Yb<10,ΣREE=50-300).The homogeneous migmatite is slightly higher trace element of Rb, lower Nb, Y compering with the metamorphic recks. (3) In the 505 area, the characteristics of leucosomes are mainly enriching sodium, enriching plagioclase(sodium-oligoclase) and quartz, and appearing K-feldspar.Compared with chemical composition of migmatite and metamorphic rocks, some oxides in the migmatite have a significant increase, such as Na₂O, SiO₂, Al₂O₃; have a slight increase of K₂O, and other activities components; some oxides such as MgO,CaO,Fe₂O₃ and other non-active components , are significantly reduced; another oxides or elements have no essential difference. The causes can be explained by anatexis. Contrast with convention anatexis, there are some minor differences, such as REE geochemistry showing differentiation; total alkaline in migmatites (or neosome) increased and so on. All of these phenomenons emphysised that it isn't a complete equal chemical process. Not all types of migmatization are equal chemical process. And a magmatic differentiation in the migmatization process may be happened, or added some non-situ anatectic materials. In the process of migmatite appears sodium account and K-feldspar account, is the result of the late differentiation, is different from alkaline fluid metasomatism.(4) All of uranium anomalies and mineralization which have been found in this area are alonging the E-W fault zone of F₁, F₂ in the west side of regional fault zone of F3. The ore-controlling faults developed in the migmatite band or near the contact zone. In the fault zones specifically developed retrograde alteration such as zoisite- epidote-sericite-muscovite alteration, and uranium mineralization. Also hydrothermal alteration, such as pyrite-silicification-carbonate-chlorite alteration is developed in the fault zones showing the characteristics of hydrothermal activity. In the surface, the fault zone is composed by multiple paralled shear structures. In the section it may be echelon-type shear zone. Hydrothermal mineralization is the main period of uranium mineralization, and is confirmed the existence of industrial uranium bodies by drilling in the 505 area.(5) The combination of low temperature hydrothermal alteration in the migmatite shows that hydrothermal fluids may come from atmospheric water precipitation and hot water circulation. In the study area, Extensive uranium-bearing carbonate alteration developed in the deep and mostly uranium-bearing silica alteration developed in the surface or shallow, indicate the possibility of deep fluid. However, there is recently non evidences of obvious large-scale account of alkali and other metierals from the deep, the uranium-forming fluid whether from the deep or from atmospheric water precipitation and hot water circulation needs a further study.(6) The ore-bearing rocks have strong negative Eu anomalies, LREE and HREE flat, a clear V-type of REE pattern.Their change of La/Yb andΣREE is similar to migmatite and metamorphic rocks, indicating that uranium mineralization related to the role from the migmatite hydrothermal. The uranium content of amphibolic rocks is low, 20×10-6, uranium background values of migmatite increased significantly, up to 30×10-6, in the fault zone in migmatite uranium content increased, up to 39×10^-6; U-Th in the mineralization lot is separated, uranium may be from the host rocks. Migmatization and faults are the main ore-controlling factors. According to all of above metioned, a possible model of uranium mineralization in the 505 Area was proposed.