岩浆热液型银矿床、银矿省及形成的控制因素

岩浆热液型银矿床主要指与岩浆热液作用相关的独立银矿床和共生银矿床(Ag平均品位一般大于100g/t),它是银最重要的来源。本文对全球80多个典型的大型-超大型岩浆热液型银矿床进行了梳理和总结,将其主要分为浅成低温热液型(低硫型、中硫型和高硫型)、矽卡岩型、斑岩型和五元素型四种类型,其中浅成低温热液型占主导,斑岩型和矽卡型数量较少。全球大型-超大型的岩浆热液型银矿床主要分布在东太平洋俯冲带和中亚造山带东段,这些银矿床均位于陆壳基底之上。按照发育地区不同可分为六大银成矿省,即中国兴蒙银成矿省、美国西部盆岭银成矿省、墨西哥西北银成矿省、秘鲁中部银多金属成矿省、玻利维亚银锡成矿省和俄罗斯远东银锡成矿省。成矿时代主要集中在中、新生代。这些银成矿省与大规模酸性-中酸性岩浆活动密切相关,包括发育大量酸性熔结凝灰岩的长英质大火成岩省,或者富锡流纹岩、黄玉流纹岩和石英斑岩等高演化岩浆岩。这些大规模岩浆热液银成矿作用通常与区域大地构造背景转换相关,比如从挤压到伸展或者伸展到挤压。相对富银的含水大陆下地壳源区、大规模高分异的岩浆作用、银对熔体中共存硫化物和磁铁矿相对弱的相容性、高盐度的流体、成矿流体集中运移的通道和高效的沉淀机制是银大规模成矿的有利控制因素。银矿床的研究工作相对于铜、金矿床远远落后,银成矿省和酸性大火成岩省的内在联系、控制斑岩钼-银/锡-银两种银成矿系统的机制、岩浆演化对银成矿的控制、银矿潜力区的勘查找矿等关键问题仍亟待解决。

Magmatic-hydrothermal silver deposits, argentiferous provinces and the main controlling factors of formation

Magmatic-hydrothermal silver deposits (MHSD) mainly include silver-only and silver-base metal deposits (generally average Ag grade >100g/t) associated with magmatic-hydrothermal activities. MHSD are the major source of silver. In this contribution, more than 80 typical large and super-large MHSD all over the world are sorted and summarized, which are mainly classified into four types: epithermal (low-, intermediate-, and high-sulfidation), porphyry, skarn and five-element deposits, among which the epithermal type is most important whereas porphyry and skarn types are subordinate. The large and super-large MHSD are mainly located in the East Pacific subduction zone and the eastern Central Asian orogenic belt, all of which are located on the continental basement, and the age is mainly in the Mesozoic and Cenozoic. The six silver metallogenic provinces are the China Xing-Meng argentiferous province, the United States Western Basin Ridge argentiferous province, Northwest Mexico argentiferous province, the Central Peru argentiferous-polymetallic province, the Bolivia silver-tin argentiferous province, and Russia Far East silver-tin argentiferous province. The argentiferous provinces are genetically related to large-scale acidic to intermediate-acidic magmatism, such as the silicic large igneous provinces with massive rhyolite ignimbrite, or the highly evolved magmatic rocks characterized by tin-rich rhyolite, topaz rhyolite, and quartz porphyry. These large-scale magmatic-hydrothermal activities are formed during tectonic transitions, such as from compression to extension or extension to compression. Relatively silver-rich hydrous continental lower crustal source, large-scale highly evolved magma, relatively weak preferentially partition of silver into sulfides and magnetite, high salinity fluids, focused fluid flow and efficient precipitation are important controlling factors for large-scale silver mineralization. The studies for silver are far behind that of copper and gold, and there are still many questions about magmatic-hydrothermal silver deposits, for instance, the linkage between argentiferous provinces and silicic large igneous provinces, the formation of silver mineralization related to molybdenum and tin system, the control of silver mineralization by magmatic evolution, and the exploration for silver prospects.