滇西马厂箐钼铜金矿床中赋矿斑状花岗岩定年及其地质意义

Dating of Ore-Bearing Porphyritic Granite from the Machangqing Mo-Cu-Au Deposit in Western Yunnan Province and Its Geologic Significance

马厂箐钼铜金多金属矿床是滇西地区金沙江—哀牢山构造带内与新生代富碱斑岩有关的典型矿床之一。根据赋矿斑状花岗岩中锆石的阴极发光图像、LA-ICP-MS微量元素分析和U-Pb定年以及辉钼矿Re-Os法测年研究等,结合前人研究成果,应用透岩浆流体成矿理论,进一步讨论了马厂箐矿床的成因机制。研究表明:该矿床的多金属成矿与赋矿斑状花岗岩的成岩基本同时,其成岩成矿过程统一受制于与该区大规模活动的新生代富碱岩浆和深部地壳重熔的花岗质岩浆同步运移的成矿流体作用。这种成矿流体是包含于岩浆并与其互不混溶的含矿地幔流体。但在上侵运移过程中,伴随岩浆的成岩作用,流体与岩浆发生不同程度分离,表现为:产于富碱斑状花岗岩体内,形成正岩浆成矿的斑岩型钼矿;产于岩体与围岩接触带,形成接触交代成矿的矽卡岩型铜(钼)矿;产于地层围岩中则形成构造破碎蚀变岩型金矿。在这一成岩成矿过程中,地幔流体可以运载和沿途活化成矿物质至适宜容矿部位集中,并随其对地壳岩石的交代蚀变以及深度和环境变化引起的物理化学条件变化,其流体属性由熔浆流体→超临界流体→热液流体转化,促使壳幔物质相互作用而叠加成矿,导致在不同部位形成不同矿种和不同类型的系列成矿效应。

The Machangqing Mo- Cu- Au polymetal deposit is a typical deposit which is associated with the Cenozoic alkaline porphyry along the Jinshajiang - Ailaoshan fault. Based on the results of CL images, LA- ICP - MS trace elements analysis, U - Pb dating of zircon from ore-bearing porphyritic granite and Re - Os dating of molybdenite, referring to theory of the transmagmatic fluid and former reseach, genetic mechanism of the Machangqing deposit are discussed in detail here. It is suggested that the Machangqing polymetal deposit and the ore-bearing porphyritic granite were formed at the same time. The process of diagenesis and mineralization are controlled by mineralizing fluid effect caused by large scale event of Cenozoic alkali-rich magma and granitic magma from deep crust remelting. The mineralizing fluid is the ore-bearing mantle fluid contained in magma unmixingly. During migration and ascent of the mineralizing fluid, the mineralizing fluid and magma separated in varying degrees, following crystallization diagenesis of magma. It shows three kinds of typical mineralization, porphyry copper deposit of orthomagmatic mineralization in the inner alkali-rich porphyritic granite, skarn copper （molybdenum） deposit of contact metasomatic mineralization at the contact zones between porphyry and surrounding rocks and the tectonoclastic altered rock type gold deposit in strata. In these processes, the property of mantle fluid was changed from magma---~supercritical fluid---,~hydrothermal fluid following the changes in depth and environment ,and the corresponding changes of physical and chemical conditions, and the moving fluid carried, activated and enriched ore materials to suitable places; it was the action of fluid that accelerated crust-mantle materials overlapping mineralization, even resulted different commodities and varied types of metallogenetic influence in different localizations.