南澜沧江带勐满热泉型金矿床成矿作用:热液蚀变与元素地球化学制约

Metallogenesis of the Mengman hot-spring type gold deposit,southern Lancangjiang zone: Constraints by hydrothermal alteration and trace element composition

勐满金矿床是西南三江特提斯造山带迄今为止报导的为数不多的热泉型金矿床之一,也是南澜沧江带唯一发现的该成因类型的金矿床。然而目前对勐满金矿床热液蚀变特征的分析不足和地球化学数据的缺乏,一直制约着对其成矿过程的深入理解。勐满金矿床原生矿体产于早古生代澜沧群曼来组片岩和侏罗纪花开左组碎屑岩不整合面附近,断裂构造导流、控矿作用显著。矿区两类围岩均发生强烈蚀变,但蚀变类型简单,仅为硅化和高岭土化,与金成矿密切相关。热液高岭土化的大量发育,反映围岩中的长石等含铝矿物与呈酸性的流体发生作用。全岩微量元素组成对比研究表明,近矿围岩蚀变过程中未发生明显的微量元素迁移。镜下观察到Au与黄铜矿等金属硫化物共生,元素相关性分析显示Au与Ag、Cu、Pb、As、S、Sb等元素有正相关趋势,表明它们由统一热液系统携带并发生卸载。在弱酸性成矿流体中,Au主要以金硫络合物的形式进行迁移。当含Au流体运移至地层不整合面附近时,与围岩反应并发生强烈高岭土化,导致流体中的SiO_2和高岭石含量急剧增加,逐渐在矿区导流断裂中沉淀下来。断裂变窄甚至封闭,流体内压持续升高,最终发生爆破,成矿流体强烈减压沸腾,引发金硫络合物失稳,Au发生卸载并沉淀。该过程反复多次发生,形成了矿区含金硅质角砾岩及蚀变岩型矿石。

Mengman gold deposit is one of the few hot-spring type gold deposits in Sanjiang Tethys orogen,as well as the only one in the southern Lancangjiang zone. However,it restricts the further comprehension of the metallogenic process that there is not enough analysis of hydrothermal alteration and the absence of geochemical data. The primary ore bodies of Mengman gold deposit exist around the unconformity between the Early-Paleozoic Manlai schist of Lancang Group and Jurassic Huakaizuo clastic rocks,the fault structure of which plays an important role in fluid-directing and ore-controlling. Both the two types of wall-rocks have experienced intensive hydrothermal alteration,while the alteration type is not complicated,only silication and kaolinization,and they are closely related to gold mineralization. The hydrothermal kaolinization reflects that it is the acid fluid that reacted with feldspars of wall-rocks. Comparison among whole-rock trace element composition indicates no obvious trace element migration,and the positive correlation trend between Au and Ag,Cu,Pb,As,S,Sb means these elements are in the same hydrothermal system,combined with gold existing with metal sulfides such as chalcopyrite. Meanwhile,Au mainly exists as Au（ HS）2-in the faintly acid ore-forming fluid. When the fluid arrived in the unconformity,it reacted with the wall-rocks,with strong hydrothermal kaolinization,contributing to the rapid increase of quartz and kaolinite in the fluid. Moreover,the increase caused precipitation of quartz and kaolinite,resulting in the fault becoming narrow and the pressure gradually arising,and eventually causing the explosion,which directly leads to the boiling of the ore-forming fluid and the precipitation of the gold. It repeatedly happened,therefore the gold-bearing siliceous breccia and altered rock type ores are finally formed.