南秦岭竹溪县天宝乡粗面岩地球化学特征与铌成矿

Geochemical characteristics of trachyte and Nb mineralization process in Tianbao Township,Zhuxi County,Southern Qinling

南秦岭造山带北大巴山东段大面积出露早志留世粗面岩,夹于早志留世梅子垭组(S_1m)下部层位含炭粉砂质板岩之中,整体呈北西-南东向展布。这套粗面质火山岩与铌矿关系密切。竹溪县天宝铌矿床产出于火山喷发沉积韵律的中下部层位,赋矿岩石以粗面质熔结凝灰岩为主。粗面质熔岩地球化学数据显示其富碱质,稀土元素总量高且轻稀土元素明显富集,高场强元素Nb、Ta、Zr、Hf等富集,Sr、P、Ti等亏损,具有类似于OIB的地球化学性质,来源于幔源玄武质岩浆的演化,是形成于大陆裂谷环境的低压型粗面岩。矿体粗面岩类(熔结凝灰岩)与非矿体粗面岩(粗面质熔岩)相比,具有略低的SiO_2含量,高的Al_2O_3、K_2O、CaO含量,低的P_2O_5含量,更为富集的高场强元素(Nb、Ta、Zr、Hf)和轻稀土元素(La、Ce),更强的负Eu异常、正Ce异常,略强的P、Ti负异常以及更高的挥发分(F、CO_2)含量。天宝铌矿含铌矿物为易解石,产出于早期矿物的边缘、裂隙或晚期细脉中,成矿阶段处于岩浆作用的晚期。铌的富集与挥发分(F、CO_2等)的运移、保存条件密切相关。该铌矿为火山岩型稀有金属矿床,兼具岩浆型和岩浆热液型矿床特征,与庙垭和杀熊洞铌-稀土矿床属同一成矿系列的不同类型矿床。

The large area of the eastern section of the North Dabei Mountain lies in North Qinling orogenic belt with extensive exposed trachyte, which was formed in early Silurian, sandwiched in lower level carbonaceous silty slates of the Early Silurian Meiziya Group （S1m） and spread from northwest to southeast. This trachyte is closely related to niobium mineralization. The Tianbao Nb deposit occurs in the middle to lower part of the volcanic sedimentary rhythm. The most important host rock is ignimbrite. The trachytes are alkali-rich, with high ∑REE and enriched LREE. High field strength elements such as Nb, Ta, Zr and Hf are enriched and Sr, P, and Ti are depleted, showing the characteristics of OIB. These characteristics of the trachytes indicate that trachyte magma originated from the evolution of mantle-derived basaltic magma. They are low pressure type trachytes formed in an intraplate rift tectonic environment. Compared with the barren trachyte, the ore-forming trachyte has lower SiO2, higher Al2O3, K2O, CaO, lower P2O5, higher high field strength elements（Nb, Ta, Zr, Hf）and light rare earth elements（La,Ce）, stronger negative Eu anomalies, positive Ce anomalies, slightly strongly depleted in P and Ti, and higher volatiles（F, CO2）. Niobium-containing mineral is aeschynite, which was generated along the early mineral edge, cracks or late veins. The metallogenic stage was in the late stage of magmatsim. The enrichment process of element Nb was closely related to the migration and preservation of the volatiles （F, CO2 etc.）. This Nb deposit is a volcanic rare metal deposit, with the characteristics of magmatic type and magmatic hydrothermal deposits. The deposit type is different from that of Miaoya and Shaxiongdong rare earth deposits; nevertheless, they belong to the same mineralization series.