新探明的玻利维亚银沙(Silver Sand)超大型银矿床地质特征和找矿评价

银沙银矿(Silver Sand silver deposit)是玻利维亚锡矿带内新探明的一个超大型中硫型浅成低温热液银矿床,已探明银金属资源量约1万t,平均银品位约120 g/t,并伴有少量铅、锌、铟、镓。银矿化与中新世中酸性侵入岩和次火山岩有关。矿体赋存于白垩系蚀变褪色石英砂岩中的密集构造裂隙带内。成矿作用可以划分为原生硫化物成矿期和表生氧化期,原生硫化物成矿期大致可分为3个矿化蚀变阶段,矿化蚀变阶段Ⅰ为大面积绢云母化褪色蚀变及玻利维亚型层状(manto)锡矿化、矿化蚀变阶段Ⅱ为细脉状和构造角砾间含银硫盐和硫化物,矿化蚀变阶段Ⅲ为方解石细脉。银沙银矿分布最为广泛的围岩蚀变类型为绢云母化,其次为黄铁矿化,以及少量硅化、碳酸盐化和重晶石化。矿化特征主要表现为在围岩白垩系石英砂岩构造裂隙内成密集含银硫盐和硫化物细脉及角砾岩脉。矿体埋藏浅,规模大,适合大规模露天开采。作为玻利维亚锡矿带的一个组成部分,该矿床形成于活动大陆弧后地区。根据成矿过程特点,文章初步提出银沙银矿的成矿模型,并根据两年以来找矿勘查工作,提出了进一步找矿方向,特别值得关注的是银沙银矿所在的成矿带呈近北北西走向,长约8 km,宽约2.5 km,区内遍布采矿遗迹,通过进一步系统勘探和开发,有望再探明一个世界级银矿区。

A newly explored world-class Silver Sand super large silver deposit, Bolivia: Geological characteristics and assessment

Silver Sand is an superlarge intermediate sulfidation epithermal silver deposit, characterized by veins and vein breccias of silver-containing sulfosalts and sulfides. It has siver resources of tenthousand ton with the ave-rage silver grade at 120 g/t, accompanied by a small amount of lead, zinc, indium, and gallium as by-product mi-nerals. The deposit is hosted in open tectonic fractures of decolored Cretaceous sandstones, associated with intermediate-felsic intrusions and subvolcanics of Miocene period. Mineralization includes primary sulfide mineralization stage and supergene oxidation stage. The primary sulfide mineralization consists of high temperature related early stage sericite discoloration and Bolivian-type layered (manto) tin mineralization followed by later stages of structure veinlets/breccia infilled by silver sulfide and sulfide mineralization as well as calcite veinlets. Sericitization is predominated wall rock alteration in Silver Sand deposit, followed by pyritization, silicification, carbonation and baritization. The mineralization is characterized by the formation of silver-rich sulfide veins/breccia in structural fissures of the Cretaceous quartz sandstone wall rock. Bulk silver mineralization is shallowly buried with good potential for open pit development. As a part of the Bolivian tin belt, the deposit was formed within the active continental back-arc background. Combined with characteristics of the mineralization itself, the authors initially propose the metallogenic model of the Silver Sand deposit. In addition, two years'' prospecting experience has given a positive view on the metallogenic belt where Silver Sand deposit is allocated. The district trend of Silver Sand is about 8 kilometers long in NNW direction and 2.5 kilometers wide in NEE direction extensively scattered with mining adits, wastes, and ruins of buildings for mining communities. The trend could be evolved into a new world-class mining district of silver with continuing modern exploration and development.