江西密坑山火山岩型铍矿的发现及其意义

火山岩型铍矿是金属铍重要的矿床类型, 它每年提供了全球80%以上的铍资源量, 因此, 寻找和发现该类型矿床对于提高我国铍资源的安全供给能力至关重要。本文作者经过前期野外地质调查、分析测试和野外地质剖面测量等工作, 在江西密坑山陆相火山盆地发现上湾、松岽和荣荆坝等三处火山岩型铍矿。按照赋矿围岩的不同, 可以将它们分为碎斑熔岩中黄玉硫化物石英脉型和凝灰岩中破碎带蚀变岩型两种成矿类型。碎斑熔岩中黄玉硫化物石英脉型主要呈大脉、细脉和网脉状或团块状产出, 矿石主要由黄玉、石英、白云母、黑云母、闪锌矿、方铅矿、锡石、黑钨矿、辉钼矿、硅铍石和羟硅铍石等矿物组成; 已发现碎斑熔岩黄玉硫化物石英脉约20余条, Be含量一般在106×10^-6~850×10^-6之间。凝灰岩中破碎带蚀变岩型主要沿凝灰岩中的破碎带展布, 矿石主要由石英、萤石、绿泥石、闪锌矿、方铅矿、锡石、绿柱石、硅铍石和羟硅铍石等矿物组成; 已发现凝灰岩破碎带蚀变岩型矿化体4条, Be含量一般在40×10^-6~2790×10^-6之间, 且随着蚀变强度的增强而升高。与铍矿化有关的热液蚀变主要为黄玉化、白云母化、萤石化、绿泥石化和碳酸盐化。本文获得碎斑熔岩锆石SHRIMP U-Pb年龄为136.4±1.8Ma; 获得碎斑熔岩和凝灰岩铍矿化体中与铍矿物共生的锡石U-Pb年龄分别为137.4±1.9Ma和134.7±4.8Ma。碎斑熔岩和凝灰岩中铍矿化体形成时间基本相同, 且接近于或稍晚于碎斑熔岩和晶屑凝灰岩的锆石U-Pb年龄, 显示铍矿化与该地区的火山岩浆活动密切相关。该地区火山岩型铍矿的发现, 不仅为在区域火山盆地中寻找该类型的矿床提供了依据, 也为在发育斑岩型、次火山岩型锡矿地区寻找铍矿提供借鉴。

The discovery and geological significance of the volcanic-hosted beryllium occurrences at Mikengshan district,Jiangxi Province,South China

Volcanic-hosted beryllium deposit is an important type of beryllium resource, it provides over 80% of global beryllium resources annually. Therefore, the exploration and discovery of this type is crucial to improve the safe supply capacity of beryllium resources in China. On the basis of preliminary field geological investigation, analysis and testing, and geological profile measurement, three volcanic-hosted beryllium occurrences, such as Shangwan, Songdong and Rongjingba, were discovered in the terrestrial volcanic rock area of Mikengshan, Jiangxi Province, South China. According to the host rock, it can be divided into two types of mineralization: (1) topaz-bearing sulfide quartz vein in pyroclastic lava, and (2) altered rock type in the fractured zone in the tuff. Topaz-bearing sulfide quartz vein occur as large veins, veinlets, stockwork, and/or massive. It is mainly composed of topaz, quartz, muscovite, biotite, sphalerite, galena, cassiterite, wolframite, molybdenite, phenakite and bertrandite. At present, more than 20 topaz-bearing sulfide quartz vein have been discovered, with Be content range from 106×10-6 to 850×10-6. The altered rock type is mainly distributed along the fracture zone in the tuff, and the ore is mainly composed of quartz, fluorite, chlorite, sphalerite, galena, cassiterite, beryl, phenakite and bertrandite. Four altered fracture zones have been found in the tuff, which with Be content range from 40×10-6 to 2790×10-6. The Be concentration in altered fracture zone increased with the degree of chloritization alteration. The hydrothermal alterations related to beryllium mineralization are mainly composed of topazization, muscovitization, fluoritization, chloritization, and carbonatization. This paper obtains a zircon SHRIMP U-Pb age of 136.4±1.8Ma for porphyroclastic lava, two cassiterite U-Pb ages of 137.4±1.9Ma from topaz-bearing sulfide quartz vein in porphyroclastic lava and 134.7±4.8Ma from altered rock in the fractured zone in the tuff type, respectively. These two events formed almost simultaneously within the error range, which suggests the beryllium mineralization is close to or slightly later than the crystalline tuff deposition and porphyroclastic lava erupted in the region, and is related to volcanic magmatic activity. The discovery of volcanic-hosted beryllium occurrences not only provides a clue to search for this type of deposit in volcanic-sedimentary basins in the region, but also provides reference for the exploration for volcanic-hosted beryllium deposits in areas where porphyry and sub-volcanic tin deposits are developed.