VMS型矿床成矿流体来源及其演化过程：来自四川呷村矿床的锂同位素证据

As one of the world’s most important deposit type, VMS deposits represent a significant source of the world's Cu, Zn, Pb, Au, and Ag ores, and thus their genesis has caught much attention of the geoscience community. It is generally accepted that ore-forming fluids of such deposits were derived from the mixing of convective seawater with magmatic hydrothermal fluids, but specific proportions of the two types of fluids in ore-forming hydrothermal solution and their roles in the formation of VMS deposits still remain the subject of much debate. Additionally, detailed mixing process and subsequent evolution path of ore-forming fluids during formation of such deposits are still poorly understood. Traditional isotope tracer techniques had been intensively applied to determine the source and evolution process of ore-forming fluids in VMS deposits, but these methods are doomed to be ineffective. In the study, we plan to use lithium isotope tracer, a recently-emerged technique that can differ magmatic hydrothermal fluids from seawater easily due to their distinctly different lithium isotopic composition, to investigate the source and evolution process of ore-forming fluids in the Gacun deposits, Sichuan. During the study, samples from stringer-stockwork zones in the footwall, massive ore zones in the hanging wall, and alteration zones surrounding main orebodies will be systematically collected for lithium isotopic measurement. The aims of this study are to: (1) identify the source of ore-forming fluids in VMS deposits; (2) track mixing and subsequent evolution processes of different fluid end-members; and (3) reinforce genetic model for VMS deposits.

海底喷流热水流体的来源一直是一个长期争论的问题，其成矿流体系统究竟是以海水为主还是以岩浆出溶流体为主？这两者分别对海底热水流体成矿系统产生多大的贡献？在成矿过程中两者如何混合又如何运移？传统技术方法难以给出确切的证据。基于以上考虑，本项目以四川呷村—赋存于晚三叠世海相钙碱性酸性火山岩系的典型块状硫化物(VMS)矿床为研究对象，从三维空间系统采集从下盘脉状-网脉状矿体热液石英脉、上盘块状矿体中的脉石石英，以及围绕矿体的热液蚀变矿物，开展系统的锂同位素研究，利用在同位素分馏过程中锂同位素表现出的独特个性，再加上海水中和岩浆水两个源区中的锂同位素比值差异较大的特性，并结合氧同位素分析，鉴别成矿流体系统中不同流体端员及其来源, 追踪和揭示端员流体混合过程, 详细再塑块状硫化物矿床的成矿过程。

火山成因块状硫化物(VMS)矿床，也称为以火山岩为容矿岩石的块状硫化物(VHMS)矿床. 由于其成因不仅涉及到成矿学的一般理论问题，而且涉及到岩石圈/水圈/生物圈的相互作用，因此，一直是国际成矿学界的研究热点。本项目选择我国最典型的VMS矿床——四川呷村大型铅锌铜多金属矿床，系统地对呷村矿床含矿岩系上长英质火山岩与流体反应过程中元素的迁移规律、氧同位素和锂同位素组成的变化进行研究，厘定了在水-岩反应过程中成矿元素的迁入迁出情况，确定了成矿物质来源呷村矿床的成矿流体为海水和岩浆水的混合流体。而在海水比例确定的条件下，我们确定了呷村矿床各个位置的W/R值，得知0、2、5号勘探线中的MSO矿带区域为主要的流体汇集区域。W/R值的确定与成矿元素Cu、Pb、Zn的变化很好地限定了热液流体运移的空间结构与规模尺度，并再现了呷村矿床形成时的水-岩反应过程。重点针对成矿流体来源问题， 通过大量石英-流体包裹体Li同位素分析和均一温度测定，深入探讨了矿物-流体Li同位素分馏，初步建立了石英-流体Li同位素分馏经验公式：△δ7LiQuartz-fluid=-8.9382×（1000/T）+22.22（线性相关系数R2=0.98），为利用热液矿床广泛发育的热液石英示踪成矿流体来源奠定了重要基础；并首次将锂同位素用于矿床成矿流体研究中，获得了呷村矿床一批热液石英和流体包裹体的Li同位素分析数据，综合分析呷村矿床下覆脉状-网脉状矿带和上部块状矿体的锂-氧同位素组成及其空间变化规律，定量估算了成矿流体中岩浆水-海水混合比例，提供了岩浆水大量参与成矿的Li同位素新证据，再塑了两端元流体混合过程及其空间变化，完善了呷村矿床热水成矿模型。