西南三江老厂Pb-Zn块状硫化物矿床成矿作用与金属富集机制

The Laochang deposit is located in the Changning–Menglian Paleotethyan suture, Sanjiang orogenic belt, SW China. The Pb–Zn massive sulfide ores with enormous tonnage in Pb, Zn, and other metals show clear association with the Carboniferous volcanogenic submarine exhalation. In comparison with the typical VMS deposits, the Laochang is characterized by the Oceanic Island Basalt (OIB)-type volcanic rocks as orebody host, the Pb>Zn>Cu mineralization, and the absence of sulfate and Fe-oxides. This kind of VMS ore deposit, enriched in base metal and hosted in the OIB volcanic rocks, is rarely discovered. Our previous study tentatively proposed that the enormous base metal enrichment of the Laochang deposit and its host volcanic rocks was related to the melting of recycled enriched material in the deep mantle. The Laochang deposit is thus considered as a key medium to research the mantle heterogeneity and its significance in base metal metallogenesis. The ambiguities, including the nature of the recycled material, the VMS metallogenic process, and the control on the metallogenic reducing environment, need to be further addressed. In this proposal, we plan to assess the Li and O isotopes of host quartz and primary fluid inclusions from various types of ores in the submarine hydrothermal system. This together with the study on the types and thermodynamic parameters of the primary fluid inclusions could reveal the VMS metallogenic process and elucidate the controls on the reducing environment. It further plan to analyze the whole-rock Li–Os and zircon Hf isotopic systematics of the OIB-type volcanic rocks based on their zircon U–Pb age obtained by the applicant. This new approach could detect the type of the recycled enriched materials involved in deep mantle and its genetic link with the Pb–Zn accumulation. Via introducing the state-of-the-art isotopic systems, the proposal thus intended to deepen our understanding on the genesis of VMS deposit and bring forward new information for the mantle heterogeneity.

老厂矿床位于西南三江昌宁－孟连缝合带内，其VMS型成矿明显，规模巨大，且具以下特色：石炭纪OIB型火山岩容矿、Pb>Zn>Cu金属组合、原生矿石缺乏氧化物。与OIB有关的贱金属VMS型矿床全球罕见，申请者前期研究认为老厂矿床Pb异常富集系OIB地幔源区中再循环富集物质回返的结果。该矿床为研究地幔不均一性与贱金属成矿提供了重要窗口，然而再循环物质的属性仍然不清；VMS型成矿作用过程和还原成矿环境成因也尚待查明。本项目拟对老厂VMS型矿体各类矿石中的石英进行Li–O同位素组成分析，结合原生流体包裹体类型和热力学参数，查清成矿流体中海水和岩浆水配比及其空间变化，阐释成矿作用过程和成矿环境控制因素；在前期OIB型火山岩锆石U–Pb定年基础上，重点研究其全岩Li–Os和锆石Hf同位素组成特征，揭示地幔源区物质组成及其对金属富集的制约。本研究有望丰富VMS型矿床成矿模式，为理解地幔不均一性提供新信息。

西南三江造山带昌宁－孟连古特提斯大洋扩张过程中产生了系列OIB型火山岩及与其相关的Pb–Zn–Cu块状硫化物矿床，以老厂规模最大，为理解古特提斯洋地幔物质组成和贱金属富集成矿机制提供了良好窗口。项目对昌宁－孟连缝合带糯良、老厂和芒信火山岩垂向物质结构进行分析，室内开展SHRIMP锆石U–Pb年龄、全岩元素组成和Sr–Nd同位素组成研究，揭示其系~320 Ma的 OIB火山活动产物。部分样品同位素组成较典型地幔范围明显富集由陆壳物质混染所致，结合火山岩普遍夹粉砂岩和页岩等陆源碎屑物质，认为火山机构产于大陆架近海岸带环境。对老厂矿床进行矿石学和矿相学、流体包裹体热力学和成分以及S–Pb同位素组成研究，结果表明成矿流体系岩浆释气流体和海水的混合，块状硫化物堆积于洋底还原海水环境，成矿过程中生物活动不明显，含矿热液与海水接触的温度骤降是导致硫化物沉淀的重要因素。提出造成老厂块状硫化物矿床同时富集亲地壳元素Pb、Zn与亲地幔元素Cu的特殊金属组合的关键原因在于OIB火山活动发生于大陆边缘，火山热机驱动流体循环导致火山岩的Cu、S和陆壳基底或沉积物的Pb、Zn发生活化、迁移与最终集聚，从而认为老厂矿床系VMS–SEDEX过渡成因。