西藏驱龙巨型斑岩Cu-Mo矿床的富S、高氧化性含矿岩浆——来自岩浆成因硬石膏的证据

S-rich,Highly-Oxidized Ore-bearing Magma in the Qulong Giant Porphyry-Type Cu-Mo Deposit in Southern Tibet——Evidence from Magmatogenic Anhydrite

西藏冈底斯成矿带上的驱龙巨型斑岩Cu-Mo矿床以发育大量的硬石膏为特征。详细的岩相学研究发现:驱龙矿床不仅发育热液脉状的硬石膏,含矿斑岩中还产出岩浆成因硬石膏,这在国内系首次报道。观察表明,在岩浆演化早期的花岗闪长岩、黑云母二长花岗岩中,岩浆硬石膏以矿物包体的形式产于斜长石、石英中;在主成矿期的二长花岗斑岩、花岗闪长斑岩中,岩浆硬石膏以矿物包体产于斜长石斑晶中,局部以斑晶形式产出,并伴有富S的磷灰石(SO3含量为0.11%～0.44%)、磁铁矿发育。后期热液活动也形成了大量的硬石膏±石英+硫化物脉,是矿区主要的矿化类型之一。电子探针分析结果显示,后期的热液硬石膏与岩浆硬石膏相比,在微量成分上明显富集Sr(分别为0.24%和0.03%),可能是由于在岩浆-热液演化过程中,Sr的不相容性或者/以及粘土化蚀变造成的。岩浆硬石膏以及与之共生的富S磷灰石的出现,明确指示驱龙矿床成矿岩浆具有富S、高氧逸度的特征;同时也为研究S在斑岩型矿床岩浆演化过程中的状态、行为,提供了很好的研究对象。

The Qulong giant porphyry Cu-Mo deposit in the Gangdese metallogenic belt,southern Tibet,is characterized with a great amount of well-developed anhydrite. Detailed petrographic study discovered firstly that the Qulong deposit contains not only hydrothermal vein-like anhydrite but magmatic anhydrite. Our observation showed that magmatic anhydrite at granodiorite and biotite monzogranite stock formed at early stage of magmatic evolution occurs as inclusions in plagioclase and quartz. Accompanied with the occurrence of S-rich apatite（0.11%～0.44% of SO2） and magnetite,magmatic anhydrite occurs as mineral inclusions in plagioclase phenocrysts hosted in monzogranite porphyry and granodiorite porphyry at the main mineralization period,with minor as phenocrysts. Late hydrothermal activity resulted in formation of a large amount of anhydrite＋quartz＋sulfide veins,constituting one of the dominating mineralization forms in this deposit. Electron microprobe analyses showed that the magmatic anhydrite is enriched in Sr（0.24%） compared with late-phase hydrothermal anhydrite（Sr 0.03%） ,and this probably resulted from incompatible behavior of Sr or/and from advanced argillic alteration during the hydrothermal evolution of magma. The coexistence of magmatic anhydrite and sulfur-rich apatite indicated that the magmas that formed the Qulong giant porphyry Cu-Mo deposit is characterized with high S and high oxygen fugacity,providing an ideal case for studying the state and behavior of sulphur in the magma evolution of porphyry-type deposits.