大别山地区沙坪沟斑岩型钼矿床蚀变及矿化特征研究

Characteristics of alteration and mineralization of Shapinggou porphyry molybdenum deposit,Dabie orogenic belt

沙坪沟钼矿床是大别山地区新近发现的、世界第二大的斑岩型钼矿床。本文在前期以及前人的工作基础上,通过详细的野外地质观察和系统的岩相学、矿相学以及探针测试分析工作,详细研究了沙坪沟钼矿床的蚀变与矿化特征。结果表明,沙坪沟钼矿床的围岩蚀变类型主要有钾硅酸盐化、青磐岩化和绢英岩化,绢英岩化又可细分为以石英为主和以绢云母为主。矿床中产出16种脉体类型,根据矿床的脉体类型划分了4个成矿阶段:(1)石英-钾长石阶段;(2)石英-硫化物阶段;(3)石英-绢云母阶段;(4)石英-萤石-石膏阶段。蚀变及矿化特征显示,沙坪沟钼矿床的钼矿化开始于石英-钾长石阶段晚期,结束于石英-绢云母阶段早期,石英-硫化物阶段是辉钼矿主要的形成阶段。沙坪沟钼矿床经历了多期次脉动式成矿流体的蚀变与矿化过程,在不同蚀变-矿化阶段中,成矿流体的物化条件和组分的变化是控制各阶段蚀变类型和脉体中矿物组合的主要因素。产于以伸展为主的板内环境的沙坪沟钼矿床与其他构造环境下的斑岩钼矿床在与成矿关系密切的岩石、蚀变分带等方面相似,但由于围岩性质的差异,矿体赋存位置及产状不同;而在矿物种类、矿化产出位置及产状、与矿化关系最为紧密的蚀变类型等方面存在差异。斑岩型钼矿床的构造背景可能控制了其岩浆的形成、演化以及含矿性,而岩浆岩最终定位的深度、围岩等条件也是控制蚀变和矿化特征的重要因素。

Shapinggou,the second largest porphyry molybdenum deposit in the world,is located in the Dabie orogenic belt. Prior to this study,only limited geological investigations of the alteration and mineralization had been limiting the features in the deposit were minor,restricting application of these guides to explore and to further study of the metallogenesis of the region. Previous work and our detailed field observations,as well as petrographical,mineralogical and electron micro-probe studies show that the ore-forming processes of the Shapinggou molybdenum deposit involved multiple stages of alteration and mineralization,including potassic,phyllic and propylitic alteration. The phyllic alteration can be further divided into two types depending on whether quartz or sericite is the predominant mineral. The ore-forming stages include quartz-K-feldspar,quartz-sulfide,quartz-sericite and quartz-fluorite-gypsum stages. The characteristics of alteration and mineralization at Shapinggou suggest that the molybdenite began to precipitate during the late quartz-K-feldspar stage reaching a peak during the quartz-sulfide stage. The early quartz-sericite stage also contributed some molybdenum to the ore system. The types of alteration and mineralization are controlled by the varying physical and chemical components of the ore-bearing fluids. The alteration and mineralization characteristics of the deposit are generally similar to the porphyry molybdenum deposits in different tectonic environments in other parts of the world. The ore-forming rocks,alteration types,and mineralization characteristics in Shapinggou deposit are broadly similar to deposits formed in lithospheric thinning,conversion from compression to extension,continental collision,continental rift,island arc and post-orogenic extension environments,but the shape and position of ore body at Shapinggou is different from those at other deposits. The tectonic setting may control the formation and evolution and ore-bearing potential of magma,but the depth of emplacement and the nature of the wall rocks control the alteration and mineralization characteristics of porphyry deposits.