江西足洞和关西花岗岩的稀土元素地球化学及矿化特征

足洞和关西岩体分别为花岗岩风化壳离子吸附型重、轻稀土矿床的原岩。足洞岩体的∑REE¹⁾为264ppm,LREE/HREE²⁾值为0.81-024,平均的钇对∑REE占有率为35.8-54.5%。这主要是由于岩浆结晶演化及晚期有交代钠长石化、白云母化和萤石-氟碳钙钇矿化的结果。这些蚀变产生了钇族稀土氟碳酸盐、硅酸盐和砷酸盐等内生矿化作用。

REE GEOCHEMISTRY AND MINERALIZATION CHARACTERISTICS OF THE ZUDONG AND GUANXI GRANITES, JIANGXI PROVINCE

The Zudong rock body is a HREE-rich granite intrusion with an isotopic age of 124 Ma and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7185, and the Guanxi rock body is a LREE-rich granite intrusion with an isotopic age of 170 Ma and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7056. These two intrusions were emplaced in the southern Jiangxi post-Caledonian uplift area of the South China Caledonian fold belt, and are in intrusivecontact with the Lower Jurassic volcanics. They were both controlled by fault zones trending nearly E-W and NNE (see Fig. 1).In the Zudong intrusive, as a result of magmatic evolution and metasomatism, most of the early-formed biotite K-feldspar granite (ZBG) was transformed into muscovite-K-feldspar——alkaline feldspar granite (ZMG). These two types of rocks show a transitional contact in outcrop. The evolution from ZBG (∑REE 266 ppm) into ZMG (∑REE 262 ppm) shows the following REE geochemical characteristics: the LREE/HREE ratios changed from 0.81 to 0.24, and the δEu values from 0.17 to 0.03; the REE underwent a clear fractionation, i.e. the (La/Yb)_N ratios changed from 1.58 to 0.36, and the Y/∑REE ratios increased from 35.8 to 54.5% (cif. Table 4 and Fig. 3). These changes were closely related to magmatic evolution and late-stage autometasomatism: albitization, muscovitization and fluorite-doveritization, and reflect the features of enrichment of yttrium in ZMG. The studies show that the entire magmatic system presents a trend of HREE enrichment from the early magmatic stage to the postmagmatic hy-drothermal stage. However, in the early stage, REE were accumulated with only 8% forming xenotime, monazite and zircon, and ca. 22% dispersing into rock-forming minerals (see Table 5). In the alkaline stage of K and Na activity, most of REE especially HREE were increasingly active, and formed fluorine complexes with fluorine (on average 0.14%) in residual melt-solution. As a consequence of fluoritization, these complexes reacted with CO₂, Ca, As, Si etc. in late magmatic hydrothermal solutions to form yttrium-rich REE endogenic mineralizations such as doverite, bastnaesite-(Y), gadolinite and chernovite. Under appropriate physico-chemical conditions of wearthering process, these REE minerals were decomposed, thus providing a material source for enrichment of HREE and Y in the wearthering crust of the intrusion. By contrast, in the Guanxi intrusion, besides relative enrichment of LREE in the original magma, the autometasomatic albitization, muscovitization and fluoritization in the late stage of magmatic crystallization were much weaker than in the Zhdong rock body. At the same time, granitophyre derived from the Guanxi granitic magma is also characterized by LREE concentration (LREE/HREE ratio =3.8). As mentioned above, the Zudong and Guanxi granites belong to the middle-late and early Yanshanian products respectively. The main reasons for the two intrusions belonging to HREE- and LREE-enriched bodies respectively are the difference in the HREE and LREE contents between their original magmas and the different characteristics of their evolution and autometasomatism.