| 其他摘要 | The ore-forming process of REE deposits is closely related to the evolution of the crust, tectonic movement and magmatism. The REE mineralization suddenly occurred and largely distributed on the Earth in Middle Proterozoic metallogenetic epoch in the evolution of the Earth. The Yinachang Fe-Cu-REE deposit is one of typical deposits and mainly occurs in Yinmin Formation of Kunyang Group of Middle Proterozoic epoch. This paper mainly reports the results of investigation on the REE geochemistry of various rocks and minerals, the sources of ore -forming materials and the age of mineralization. The Preliminary mechanism of sudden REE enrichment and mineralization in Middle Proterozoic has been interpreted. In this paper, the main conclusion have been put forword as following: Yinachang Fe-Cu-REE deposit occurred in Wuding -Lufeng volcanic fault deporssion during the initial stage of Kunyang rift. The Kunyang Group is distributed in the marrow and long belt constrained by the Luzhijiang lithospheric fault and the Xiaojiang-Yimeng fault. The wall rock of ore-bodies is the siliceous dolostone and alkaline volcanic rock (trachy andesite) in the Yinachang deposit. Based on the shape of ore-bodies and the structure and texture of ores, the deposit was identificed as synsedimentary with wall rocks. There occurred REE minerals, such as bastnaesite , monazite and allanite., associated with apatite and fluorite with REE contents to a certain extent. Both The hanging wall and the footwall rock of ore bodies are garnet biotite schist, protoliths are alkaline volcanic rocks (tracyh andesite). According to characteristics of trace elements assemblages and the setting of tectonics, these alkaline volcanic rocks were derived from the lower degree melting of metasomatic and enriched mantle (EMI) in the initial stage of Middle Proterozoic epoch, they strongly enriched in Large Ion lithophile elements (LILE), such as Ba, Cs, Rb, K and LREE, depleted Zr, Hf ,Ti, Sr and HREE. Comparative studies of REE geochemical characteristics of various geologic bodies in the deposit indicate that the various ores and alkaline volcanic rocks contain abundant REE, specially LREE (￡REE=645 * 10-6~4443 * 10-6; (La/Yb)N=17.3~81.1), the chondrite-normalized patterns of both ores and alkaline volcanic rocks show a strong LREE enrichment and positive Eu anomaly, in contrasting with the dolostones which show slight LREE enrichment and moderately negative Eu anomaly. REE patterns of ores are similar to hydrothermal sediment core in the East Pacific Rise, whereas REE patterns of dolostones are similar to those of PA AS. 3. The composition of trace elements of ores are similar to those of hydrothermal sediment core in the East Pacific Rise, obviously distinguished from the composition of carbonatite-type REE deposits. In diagram (Al-Fe-Mn, Fe/Ti-Al/(A1+Fe+Mn), U-Th, and Y-P2O5). the element composition of ores exhibits hydrothermal sedimentary signification. The ratio of Y/Ho in ores approximates to the ratio of black chimney in TAG seafloor. The ore-forming fluids show high temperature and redox characteristics. In conjunction with the geological setting of the deposit, the primary ore-forming fluids might certain higher REE and higher volatile elements derived from the mantle degassing or the alkaline volcanic magmas. The genesis of Yinachang Fe-Cu-RE deposit might belong to volcanic exhalation-hydrothermal sedimentary. 4 REE geochemistry of fluorite, siderite, quartz, magnetite, calcite in ores mainly controlled by ore-forming fluids, in contrasting to those of the metamorphic minerals which controlled by crystal structure. The ore-forming fluids of initial stage show larger LREE/HREE fractionation than that of later stage. A narrow range of 5 14S valus for early stage chalcopyrite (-0.3%0 to 2.8%o) suggested that S originated from the mantle and the 8 13C and 5 18O valus of siderite range from -11.17%o to -15.37%o and from 8%0~9.1%o, respectively, it indicates that the C and O of ore-forming fluids were derived from the magma or organogenous sediment. The main REE complex in ore-forming fluids may be (RE(CO3)3F)δ (RE(CO3)3F2)5- RE(F,C1)- finally they deposited to form REE minerals such as bastnaesite when the temperature and press are droped. 5 Sm-Nd isochron ages of minerals (δ-points) and fluorite (δ-points) are 1621 + llOMa and 1538±43Ma, respectively, which are similar to zircon U-Pb age (1676Ma) of alkaline volcanic rock (trachy andesite) and the zircon U-Pb age (1685Ma) of quartz syenite porphyry in the Yinachang ore district, and approximating to Yinmin Formation strata age (1765Ma). The mineralization age is much the same as alkaline volcanic eruption events in early stage of Middle Proterozoic in Kunyang rift. The signatures of Nd(t) for: -2.87- -3.6 and for flourite f Nd(t): -3.93-5.49 indicated they have been derived from the enriched mantle. 6% There is strong similarity in REE mineralizaton among the Yinachang Fe-Cu-REE deposit, the Bayan Obo REE-Fe-Nb superlarge deposit and the Olympic Datn O. Au-U-Ag-REE superlarge deposit. The tectonic setting of mineralization are all in the marginal rift of the continents, the mineralizing events mainly took place in the Middle-Proterozoic epoch, and the REE are mainly generated the anorogenic magmatism which recorded the extensional tectonic activity that took place in the final stage of the formation of a 1.5Ga supercontinent and in the initial stages of its breakup, or derived from metacomatic enriched mantle. |
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