南秦岭杨家坝多金属矿区中的碳酸岩岩相学及成矿地球化学

Petrography and geochemistry of mineralization for carbonatite in Yangjiaba polymetal field of South Qinling

秦岭造山带是我国重要的成矿区带之一。研究发现,南秦岭杨家坝多金属矿区中元古界碧口群火山沉积岩系中原以为所夹的"白云岩"在产状上具侵入接触关系,并且从岩相学、元素和同位素地球化学分析论证,确认其为源自地幔的碳酸岩;岩石总体表现为明显富集轻稀土、大离子亲石元素,尤以Sr、Ba相对富集,而过渡元素,尤以Ti、Cr、Ni相对亏损,高场强元素则表现为矿化蚀变较之弱蚀变和无蚀变相对富集,这与岩相学研究伴随硅化和硫化物蚀变而发育多金属矿化,以及同位素系列研究表现强烈相似于EMⅡ型富集地幔背景,并具碳酸岩与碳酸盐岩的过渡特征形成呼应,暗示矿区碳酸岩及相关矿化的形成,可能与秦岭造山带从中元古代到中新生代发生同生成矿,或构造体制转换并伴随后造山期强烈陆内造山作用导致的壳幔叠加改造密切相关,是重大深部地质事件的标志。碳酸岩的发现和确认,为论证本区深部地质地球化学动力学事件和过程,以及壳幔混染对成矿的贡献提供了新的岩石学证据。

Qinlin orogene is one of the important metallogenetic belt for China. Our research detectes that previous 'dolomite' displayed the occurrence of intrusive contact in volcanite series for Mid-Proterozoic Bikou Group, and the rock is confirmed as carbonatite which derived from mantle based on the studies of petrography, geochemistry of elements and isotopes. The carbonatite had an ensemble displays in the aspects of enriching LREE and LILE (large-ion lithophile elements), especially Sr and Ba, and depleting transition elements, especially Ti, Cr and Ni. Else as HFSE (high field strength elements) had more enrichment in metallizing alteration carbonatite than in weak or no alteration rocks. The understand is compatible with petrographic features, which polymetallization was accompanied by silication and sulfide alteration, and isotopic study results that setting was similar as EM II enriched mantle and the rock had intergraded from carbonatite to carbonate rock. The research conclusions suggested that the metallization related to the formation of carbonatite were probably correlate intimately to crust-mantle superimposed alter, which was resulted in syngentic mineralization or intensive land orogenesis while the tectonic framework was transformed from Mid-Proterozoic to Meso-Cenozoic of Qinling orogene. The carbonatite was a sign of important deep geologic event. Therefore, discovery and confirmation of carbonatite provide new petrological evidence for studying deep geo-geochemistry dynamic event and process, and contribution that crust-mantle contamination superimposed alter promoted metallization.