滇西铜厂沟斑岩-矽卡岩型矿床钼富集机制：岩浆-热液演化过程榍石示踪

Porphyry-related deposits are the most important sources of molybdenum, while the geochemical behavior and enrichment mechanisms of Mo during magmatic-hydrothermal evolution, including magmatic fractionation, magma-fluid exsolution and phase separation, are difficult to trace. As the common accessory minerals in calc-alkaline magmatic rocks, titanites have experienced the entire magmatic-hydrothermal process. They are enriched in REEs, HFSEs and molybdenum, tin and tungsten etc., and have been proven to be geochronometer, geo-thermometer and porphyry fertility indicator, which could be applied to investigate the enrichment mechanisms of metals in magmatic-hydrothermal deposit. The Tongchanggou deposit is located in the southernmost section of the Late Cretaceous Yidun porphyry-skarn Mo-Cu metallogenic belt, with the largest molybdenum resource and the highest molybdenum grade. On the basis of detailed field investigation and microscopic observations, different generations of titanites from porphyries and skarn are identified in the Tongchanggou deposit. The in situ microstructure, major and trace elements, U-Pb and (U-Th)/He dating and Nd, Mo isotopic compositions of three-type titanites are analyzed, in order to 1) investigate the crystallization conditions (P-T-fO2) and evolutional processes of titanites, 2) trace the magmatic-hydrothermal transitional processes of the Tongchanggou deposit, 3) constrain the sources and enrichment processes of molybdenum, 4) lay a foundation for further building a fine genetic model of porphyry system, and 5) provide theoretical support for revealing regional metallogenic regularity to guide ore-prospecting and exploration.

斑岩矿床是钼最重要的来源，钼在岩浆-热液体系中的富集机制一直是该类矿床的研究难点。榍石常见于钙碱性岩浆岩中，经历岩浆-热液完整演化过程，富集稀土元素、高场强元素和钼等金属元素，是灵敏的定年剂、温度-压力-氧逸度计和成矿潜力指示矿物，已成为揭示岩浆-热液矿床金属富集机制的理想研究对象。铜厂沟是义敦地体晚白垩世斑岩-矽卡岩型钼铜矿带上钼储量最大、品位最高的矿床，发育岩浆榍石、岩浆-热液期的热液榍石和矽卡岩期的热液榍石。项目拟针对铜厂沟矿床不同世代榍石，开展LA-ICP-MS U-Pb和(U-Th)/He定年，厘定岩浆-热液活动时限，约束岩浆结晶冷却时间和热液出溶-聚集效率；利用榍石原位成分变化揭示岩浆-热液演化过程P-T-fO2变化及其对钼在流体/熔体分配的影响；根据Mo、F含量和Nd、Mo同位素比值变化示踪钼来源及富集机制；将有益于构建精细的斑岩成矿系统模型，为揭示区域成矿规律提供理论支撑。

金属的富集取决于初始岩浆化学还是岩浆-热液演化过程，是数十年来矿床学界一直争论的问题。就钼的富集而言，最新研究表明钼在源区的预富集不如岩浆房的大小以及岩浆流体出溶和聚集的效率等其他因素重要。榍石普遍存在于长英质斑岩和矽卡岩中，是灵敏的定年剂、温度-压力-氧逸度计和成矿潜力指示矿物，且富含Mo，有望成为揭示钼富集机制的理想矿物。本项目通过详细的镜下观察，在铜厂沟新鲜/矿化花岗闪长斑岩体和矽卡岩矿体中识别出三个世代的四类榍石，分别为新鲜岩体中发育典型韵律环带的岩浆榍石（Ttn-1）、钾化岩体中发育熔蚀结构、部分或全部被钛铁矿交代的热液榍石（Ttn-2-1）和与金红石、钛铁矿、磷灰石、氟碳钙铈矿、重晶石和方解石共生的细粒热液榍石（Ttn-2-2）以及矽卡岩中与方解石、石榴子石和黄铜矿共生的晶形较完好、颗粒较小的热液榍石（Ttn-3），并针对不同世代的榍石开展了系统的电子探针、LA-ICP-MS U-Pb测年和微量元素分析。结果表明岩浆榍石的U-Pb年龄为83.7±1.0 Ma，锆石U-Pb年龄为84.6±0.7 Ma，结合先前发表的辉钼矿Re-Os和锆石裂变径迹年龄，反演了铜厂沟花岗闪长斑岩的冷却历史，揭示出铜厂沟成矿岩体经历了漫长的岩浆-热液演化过程。新鲜和蚀变斑岩（Ttn-1、Ttn-2-1和Ttn-2-2）的岩浆和岩浆-热液榍石具有一致的地球化学特征（TiO2和∑REE含量相对较高，F含量较低），属于岩浆成因的榍石。Ttn-3则具有相对较高的（Al2O3+FeO）、F和高场强元素（HFSEs、Zr、Hf、W、Sc和Cr）丰度、较高的Al/Fe和Nb/Ta比值、较低的TiO2和∑REE含量，与矽卡岩系统中的热液榍石相似。岩浆榍石、锆石、磷灰石和全岩均无Eu负异常，表明其母岩浆是富水且氧化的，这有利于Mo替代榍石中的Ti并在富Ti矿物相富集。从Ttn-1到Ttn-3，钼含量发生骤降，由67ppm下降至3ppm，这可能与Ttn-3的形成晚于辉钼矿沉淀有关。通过全球典型富钼和无钼矽卡岩系统岩浆和热液榍石中钼含量的系统对比，本研究认为初始熔体中的中高钼含量、高氧逸度和高含水量以及出溶流体高效的MoS2沉淀共同控制了钼的富集和大型钼矿床的形成。