原位分析两类硬石膏成分示踪西藏驱龙巨型斑岩铜矿硫的来源与成矿演化

Porphyry Cu deposits related to the India-Asia continental collision mainly cluster in the Gangdese Metallogenic belt, Tibet. Their formation is related to the magmatic generation caused by the continent-continent collision. The previous studies principally focus on the petrogenesis of Cu porphyries, the characteristics of magma source, the genetic link between the formation of porphyry deposits and Tibetan collisional processes and etc. However, the ultimate metal and sulfur origin of porphyry deposits, the generation condition of sulfur-rich and high-oxidized magma and magmatic-hydrothermal processes of porphyry Cu deposits have been conducted very weekly. These significant questions hamper the further understanding to the fertile magma origin and ore-forming mechanism of the collision-related porphyry deposits. Thus, in order to answer to the questions above, this proposal takes the unique magmatic anhydrite and wide-occurred hydrothermal anhydrite in Qulong giant porphyry Cu deposit as the research object. We plan to perform the in situ analysis of the trace element, sulfur isotope and strontium isotope on two types of anhydrite and coexisted sulfides by laser ablation multi-collector ICP-MS (LA-MC-ICPMS) and Nano Secondary Ion Mass Spectrometry (NanoSIMS), combined with routine microthermometric study of fluid inclusions in different hydrothermal stages and electron probe microanalysis on the composition of the rock-forming minerals and alteration minerals in anhydrite-bearing rocks. Through this research, the sulfur source of Qulong magmatic system will be constrained and the genesis of magmatic anhydrite in Qulong porphyry Cu deposit will be explained. The sulfur-rich and high-oxidized magmatic-hydrothermal systematic evolution and the mechanism of sulfide precipitation will be illustrated. Furthermore, the key controlling factors, which the anhydrite-bearing magma and related hydrothermal system help to the formation of super-large porphyry Cu deposits in the Tibetan continental orogen, will be ascertained. This study will complete and advance the theory of metallogenesis of collision-related porphyry deposits.

与印亚大陆碰撞过程有关的斑岩铜矿主要产出在西藏冈底斯成矿带内。其形成与陆-陆碰撞引起的深部岩浆过程相关。前期研究集中在带内斑岩铜矿的岩浆岩性质、岩浆源区特征和构造背景等方面，缺乏对成矿物质来源、高硫-高氧化岩浆形成条件及岩浆-热液成矿过程等科学问题的研究，制约对碰撞环境斑岩铜矿成岩-成矿过程的理解。本申请拟以西藏驱龙巨型斑岩铜矿为研究对象，运用激光剥蚀多接收等离子体质谱和纳米离子探针原位分析技术，进行岩浆硬石膏、热液硬石膏及共生硫化物的微量元素与S-Sr同位素组成研究，结合对不同成矿阶段流体的显微测温和电子探针分析造岩矿物-蚀变矿物组成，查明驱龙铜矿成矿岩浆中硫的来源和岩浆硬石膏成因，阐明高硫-高氧化条件下驱龙铜矿岩浆-流体演化过程和矿质沉淀机制，揭示碰撞环境中富硫岩浆-热液体系对巨型斑岩铜矿形成的控制因素，为进一步发展大陆碰撞环境斑岩矿床成矿理论做出贡献。

本研究拟以中国最大的铜矿——中新世西藏驱龙巨型斑岩铜矿为研究对象，运用激光剥蚀多接收等离子体质谱进行岩浆硬石膏和热液硬石膏的微量元素分析，并结合对多期侵入体内锆石-磷灰石成分研究和多种温度计估算，查明驱龙铜矿岩浆-热液硬石膏微量元素-同位素特征和熔体中硫元素含量变化，解释成矿岩浆中岩浆硬石膏成因和不同成矿阶段热液硬石膏微量元素变化。同时对驱龙巨型斑岩铜钼矿的中新世侵入体开展锆石和磷灰石的(U-Th)/He研究。在研究侵入岩的野外关系、热液脉系与侵入体的穿切关系和评估前人的年代学研究的基础上，全面系统揭示驱龙斑岩铜矿的岩浆-热液-成矿-冷却-剥露过程，探讨其剥露过程、保存条件与青藏高原构造隆升的关系，为冈底斯成矿带中新世斑岩矿床的剥露过程提供新的制约。研究结果显示，岩浆硬石膏与寄主岩石（花岗闪长斑岩）具有相似的稀土元素配分特征，结合硬石膏在熔体中溶解度模型和锆石EuN/EuN*与Hf组成，查明驱龙铜矿成矿岩浆中岩浆硬石膏可能来源于岩浆的混合或补给过程，即在含岩浆硬石膏的花岗闪长斑岩形成过程中，岩浆房发生基性岩浆（可能是超钾质岩浆）补给，之后在斑岩结晶时捕获在岩浆房中CaO升高的斜长石斑晶和硬石膏斑晶／巨晶。不同成矿阶段热液硬石膏Eu异常变化揭示成矿过程中流体温度和氧逸度的变化趋势，即从早期钾硅酸盐-硫化物阶段到硫化物大规模沉淀的石英-绢云母-硫化物阶段，成矿流体的氧逸度可能不变或升高，发生降温过程。而到晚期硅化-粘土化-硫化物阶段，成矿流体的氧逸度可能发生降低的趋势。驱龙斑岩铜钼矿床中成矿斑岩经历单向快速冷却的过程（>200°C /m.y.），剥露时间为13.6-12.4 Ma。结合矿区上覆火山岩地层厚度、流体包裹体测温确定的成矿深度及岩体的角闪石压力计，综合建立该巨型斑岩铜矿的岩浆-热液-剥露过程模型，并估算剥露速率为0.16-0.24 mm/y。该速率与前人研究冈底斯岩基的剥露速率相近。中新世斑岩铜矿的剥露时代几近于冈底斯岩基南部的剥露峰期(~20-15 Ma)。