滇西马厂箐铜钼多金属矿床成矿流体来源及演化过程——来自石榴子石原位微量元素及H-O同位素组成的限定

Origin and evolution of ore-forming fluids of the Machangqing Cu-Mo polymetallic deposit in western Yunnan:Constraints from the in-situtrace elements and H-O isotopic composition of garnet

滇西马厂箐铜钼多金属矿床位于三江特提斯成矿域,是一个与喜马拉雅期富碱斑岩侵入有关的多金属矿床。前人研究表明,马厂箐铜钼多金属矿床形成于斑岩-矽卡岩成矿系统,但由于缺乏系统矿物学研究,目前对矽卡岩矿化过程和成矿效应仍不清楚,限制了对该矿床成矿过程的全面认识。因此,本文以马厂箐矿床矽卡岩型矿化中的石榴子石为研究对象,利用H-O同位素、电子探针(EPMA)以及LA-ICP-MS原位微区技术开展了同位素及成分分析,限定成矿流体来源以及反演成矿流体演化过程。石榴子石可分为早期自形石榴子石(Grt I)和晚期他形石榴子石(Grt II)。Grt I与辉石共生,并伴有黄铁矿、黄铜矿等金属矿物组合,属于钙铝-钙铁榴石固溶体(And_(49.37~99.58)Gro_(0~49.79));Grt II更富Fe,属钙铁榴石系列(And_(67.50~99.85)Gro_(0~31.84))。两期石榴子石均富含Th、U、Nd,亏损Ba、Sr、Hf、Nb,富轻稀土。基于石榴子石矿物化学特征认为,Grt I可能是在弱酸性、氧化、低水岩比(W/R)条件下形成的,而Grt II可能是在酸性、氧化和高W/R条件下形成的。另外,两期石榴子石δDV-SMOW值为-122.6‰~-98.6‰,δ18O流体值为5.95‰~8.25‰,暗示成矿流体主要来自岩浆水。综合研究认为,在矽卡岩化过程中,成矿流体的氧逸度、温度整体呈现降低趋势,流体pH逐渐由酸性向弱酸性—中性演化,这一转变可能是触发马厂箐矿床矽卡岩型矿化铜钼硫化物沉淀的主要机制。

The Machangqing Cu-Mo deposit,located in the Sanjiang Tethyan metallogenic belt,is a polymetallic deposit related to the intrusion of alkaline porphyry in the Himalayan period.Previous studies have shown that the Machangqing Cu-Mo polymetallic deposit was formed in a porphyry-skarn metallogenic system.However,due to the lack of systematic mineralogical research,a mineralization process and metallogenic effects of skarn are still unclear,limiting a comprehensive understanding of the mineralization process of this deposit.Herein,this paper considers garnet in skarn mineralization in Machangqing deposit as the research object,and uses H-O isotope,electron probe(EPMA)and LA-ICP-MS in-situ microregion technology to carry out isotope and composition analysis to define the source of ore-forming fluids and invert the evolution process of ore-forming fluids.The garnets can be divided into early euhedral garnets(Grt I)and late anhedral type(Grt II).The Grt I,coexisting with the pyroxene,pyrite and chalcopyrite,belongs to the grossular-andradite garnet solid solution(And_(49.37~99.58)Gro_(0~49.79)).The Grt II has elevated Fe compared to Grt I,and mainly consists of andradite(And_(67.5~99.85)Gro_(0~31.84)).Both the types of garnets are enriched in Th,U,LREEs and Nd,depleted in Ba,Sr,Hf and Nb.Based on the chemical compositions of garnet,the Grt I may have formed under mildly acidic,oxidized and low W/R conditions,whereas the Grt II may have formed under acidic,oxidation,and high W/R conditions.In addition,theδD_(V-SMOW)values andδ~(18)O_(fluid)values of these garnets range from-122.6‰to-98.6‰and 5.95‰to 8.25‰,respectively,implying that the hydrothermal ore-forming fluid originated from magma-derived fluid.A comprehensive consideration of data indicated that the oxygen fugacity and temperature of the ore-forming fluid presented a decreasing trend during the skarn mineralization process,and the pH of the fluid gradually changes from acid to weak acid neutral.This process may have been the main mechanism to trigger the precipitation of skarn type mineralized copper molybdenum sulfide in the Machangqing deposit.