大厂锡—多金属矿床热液喷气沉积成因的证据——容矿岩石的微量元素及稀土元素地球化学

EVIDENCE FOR HYDROTHERMAL EXHALATIVE SEDIMENTARY ORIGIN OF THE DACHANG TIN-POLYMETALLIC DEPOSITS——GEOCHEMISTRY OF RARE EARTH ELEMENTS AND TRACE ELEMENTS OF THE HOST ROCKS

对大厂锡矿床容矿岩石稀土元素地球化学研究表明,本矿区主要容矿岩石——硅质岩、富长石岩及电气石岩稀土元素总量低、具弱的Ce亏损、明显的Eu负异常。这些特点与某些有代表性矿床中的热液喷气沉积岩——条带状燧石岩及电气石岩十分相似,证明了它们属于喷气沉积成因。相反,与主要容矿岩石互成条带的部分绢云母岩、含绢云母的长石岩等则稀土总量高,轻重稀土分馏明显,与北美页岩相似,具有陆源沉积或混合成因的特征。运用聚类分析的方法,这些岩石的微量元素地球化学分类与上述结果完全一致,为其成因进一步提供了佐证。

Results of a REE study of the Devonian host rocks of the Dachang tinpolymetallic deposits indicate that the main host rocks including siliceous rocks, K-feldspar-rich rocks and tourmalinites are generally lower in total REE content, and Show negative cerium anomalies and marked europium depletions. The ratios of LREE's vs. HREE's are lower than those of contemporaneous sediments derived from terrigenous material, All of these characteristics are similar to those of bedded iron-bearing formations of middlc-upper Proterozoic and younger age, and also to exhalites including bedded chert and tourmalinate in some massive sulfide deposits. This suggests that the main host rocks of the Dachang deposits are of exhalative sedimentary origin. On the contrary, minor thin layers of sericitic and sericite-bearing feldspar-rich rock which are interbedded with the major siliceous host rock are generally higher in total REE content, and show an enrichment of LREE's over HREE's. These, therefore, have similar REE characteristics to those of the 40(reference) North American shales. This indicates that the above two kinds of host rocks in Dachang were formed by a combination of terrigenous and exhalative chemical sedimentary processes. Also, the contents of eight trace elements (Ti, Zr, Nb, Y, Sc, Ga, Ce, P) of the host rocks in the Dachang ore deposits were determined. Simply comparing the variation in trace element contents of the host rocks in the Dachang ore deposits with that of bedded chert and tourmalinite in some massive sulfide deposits as well as with that of sandstone, limestone and shale, the authors fail to reveal any significant regularities among them. However, according to cluster analysis with the Euclidean distance of the trace element composition based on eleven parameters including contents of the above eight elements and ratios of Zr/TiO_2, Nb/Y, Ga/Sc, the geochemical classification of the host rocks in terms of the trace elements in the Dachang ore deposits shows markedly genetic significance: siliceous rocks, K-feldspar-rich rocks and tourmalinites could be classified as the group of exhalite-chert, and sericitic rocks and sericite-bearing feldspar-rich rocks could be assigned to the group of shales. Obviously, the geochemical characteristics of the trace elements, just like the REE characteristics, can provide new evidence for the role of exhalative processes in the formation of the Devonian host rocks in the Dachang tin-polymetallic ore field.