滇西北拉巴燕山晚期花岗岩岩石成因及其成矿指示——黑云母和角闪石矿物化学证据

在滇西北香格里拉拉巴地区,近年通过钻探新发现了燕山晚期花岗岩体及伴生的超大型钼(-铜)-多金属矿床。调查发现,岩浆成因黑云母和角闪石记录了其形成时的岩浆温度、压力、氧逸度以及物质来源等岩石成因信息,这些物理化学条件制约了成矿元素在熔体相与流体相之间的分配,成为约束岩浆过程、岩石成因及成矿机制的重要因素。本文对拉巴矿区花岗岩中黑云母和角闪石进行了详细的矿相学和成分分析,据此厘定了岩石形成的物理化学条件,探讨其岩石成因及成矿效应。结果显示,花岗岩中黑云母的Fe2+/(Fe2++Mg)值较为均一,具有无钙或贫钙的特点,Ti阳离子数为0. 31～0. 52,属于岩浆成因;角闪石的Si阳离子数为6. 68～7. 20,Ti阳离子数为0. 09～0. 13,属于岩浆成因;计算获得岩浆结晶温度为705～903℃,结晶压力为59～449 MPa,侵位深度为2. 2～17. 0 km。黑云母和角闪石的矿物化学特征指示,寄主花岗岩体为I型花岗岩,具有幔源物质参与特点,形成于较高的氧逸度环境中;黑云母的卤族元素(F、Cl)含量为0. 17%～0. 58%,指示岩浆出溶流体为富含F、Cl的流体,利于Mo、Cu等元素的富集成矿,暗示本区具有很大的成矿潜力。

Petrogenesis of the late Yanshanian Laba granite in northwestern Yunnan Province and its metallogenic implications: Evidence from mineral chemistry of biotites and amphiboles

The late Yanshanian Laba porphyritic granite,associated with a superlarge Mo(-Cu)-polymetallic deposit,was recently identified by drilling exploration in Shangrila area of northwestern Yunnan Province. The magmatic biotites and amphiboles have recorded the detailed petrogenesis information including the temperature,pressure,oxygen fugacity,and the source origin of the parent magma. The authors investigated the mineralogical composition of the biotites and amphiboles in the porphyritic granite from the Laba Mo(-Cu) deposit,and constrained the physico-chemical conditions of the ore-bearing magmatic rocks as well as the metallogenic potential. Electron microprobe analyses(EMPA) show that the biotites have uniform Fe^2 +/(Fe^2 ++ Mg) values,without or with minor CaO,and the Ti cation number is 0. 31 ~ 0. 52,indicating that they are of magmatic origin. The Si,Ti cation numbers of the amphiboles are 6. 68 ~ 7. 20 and 0. 09 ~ 0. 13 respectively, indicating that they are also of magmatic origin. The authors have reached the conclusion that the formation temperature, pressure and depth of the porphyritic granite are 705 ~ 903℃, 59 ~ 449 MPa and 2. 2 ~ 17. 0 km respectively. The features of mineral geochemistry of the biotites and amphiboles suggest that the porphyritic granite belongs to the I-type granite with the addition of mantle-derived materials, and was formed in an environment with high oxygen fugacity. The values of F and Cl in the biotites are 0. 17% ~ 0. 58%, showing that the fluids exsolved from the granitic magmas were rich in F and Cl,which was beneficial for enrichment and mineralization of Mo and Cu. These results shed light on the correlation between the rock-forming and ore-forming processes for the superlarge Laba Mo(-Cu)-polymetallic deposit. Meanwhile, they also indicate that the Laba orefield has a giant potential for future exploration.