江南造山带基底变质沉积岩中锂元素分布和富集机制及对锂成矿的制约

内容提要:江南造山带是我国重要的稀有金属成矿带,已发现多处与锂相关的花岗伟晶岩型稀有金属矿床。锂等稀有金属元素在源区基底岩石中的富集是花岗伟晶岩型锂稀有金属矿床成矿的物质基础,但是江南造山带基底岩石中锂的分布及其富集机制仍不清楚。本文详细调查研究了江南造山带东段新元古代冷家溪群、双桥山群、溪口岩群和板溪群变质沉积岩和星子杂岩。这些基底变质岩的岩石类型包括变质砂岩、泥质板岩、千枚岩和云母片岩以及少量片麻岩。地球化学分析结果显示,冷家溪群、双桥山群和溪口岩群变质沉积岩具有相似的成分,变质砂岩和云母片岩-片麻岩整体上比泥质板岩和千枚岩具有较高的Si O₂含量,较低的Ti O₂、Al₂O₃、K₂O、Mg O和TFe₂O₃含量。泥质板岩和千枚岩含有更高的稀有金属元素含量,其中锂含量达到61.8×10^-6,而变质砂岩的锂丰度为44.9×10^-6。板溪群具有最低的稀有金属元素含量(Li=30.8×10

Distribution and enrichment mechanism of lithium in meta sedimentary rocks in the Jiangnan orogen and implications for lithium mineralization

The Jiangnan orogen is one of the important metallogenic belts of rare metals in China. Several granite- pegmatite type lithium rare metal deposits have been found in this belt. The enrichment of lithium and other rare metal elements in the basement rocks is the material basis for the mineralization of the granite- pegmatite type lithium rare metal deposit. However, the distribution and enrichment mechanism of lithium in the basement rocks of the Jiangnan orogen are still unclear. In this paper, the Neoproterozoic Lengjiaxi Group, the Shuangqiaoshan Group, the Xikou Group and the Banxi Group meta- sedimentary rocks and the Xingzi complex in the eastern section of Jiangnan orogen are investigated in detail. These basement rocks include metasandstone, argillite, phyllite, mica schist and less gneiss. Analytical results show that meta- sedimentary rocks of the Lengjiaxi, Shuangqiaoshan and Xikou groups have similar compositions, and metasandstones, mica schists and gneisses have higher SiO2 and lower TiO2, Al2O3, K2O, MgO and TFe2O3 than argillites and phyllites. The argillites and phyllites generally contain higher rare metal element contents, with mean lithium content up to 61. 8×10-6, whereas the metasandstones have lower lithium abundances with a mean of 44. 9×10-6. The Banxi Group has lower rare metal element content (Li=30. 8×10-6). Lithium content of meta- sedimentary rocks of the Lengjiaxi, Shuangqiaoshan and Xikou groups exhibit a good positive correlation with the chemical index of alteration (CIA), MgO, TFe2O3, and with the clay minerals in the rocks, especially chlorite. Therefore, lithium concentrations of sedimentary rocks are obviously controlled by chemical weathering, that is, the mechanism of lithium enrichment is the adsorption of clay minerals, especially montmorillonite and chlorite. The majority of lithium concentrates are in biotite, muscovite and staurolite in middle- and high- grade metamorphic rocks. The modeling of partial melting of the meta- sedimentary rocks of the Lengjiaxi Group shows that the decomposition and melting of mica minerals are the most important cause of the increase of lithium content in the melt during partial melting. The lithium concentration of the melt reaches the peak when the mica is completely melted. Continued melting will dilute lithium content in the melt. Lithium concentration in the melt are affected greatly by lithium content in the source and the partial melting degree, which depends on major element compositions of the source rocks and P- T conditions. The modelling results also show that increasing pressure is conducive to lithium enrichment in the melt. Under the conditions of 1. 0 GPa and 0. 75% water content, the melting degree reaches 10. 7%, and lithium content of the melt is highest, up to 9. 3 times of lithium content of the source. Therefore, partial melting under medium pressure facies series and water- deficiency may be the most favorable for lithium enrichment in melt.