伊朗Angouran Zn-Pb-Ag矿床特征与成因

Characteristics and Genesis of the Angouran Zn-Pb-Ag Deposit in Iran

Angouran Zn-Pb-Ag矿床是伊朗目前正在开采的最大的铅锌矿床。它位于Zagros造山带内的Sanandaj-Sirjan变质带的西北部,赋存于新元古界—寒武系大理岩和片岩建造内。矿体上部呈筒状出现在大理岩中,向下逐渐过渡为似层状、层状,出现在大理岩与云母片岩的接触带上。由上至下,出现碳酸盐型矿体、硫化物-碳酸盐混合矿体、硫化物矿体,由角砾状和块状矿石组成。硫化物主要为闪锌矿,分早期富Fe和晚期贫Fe两种,次为方铅矿。碳酸盐矿物主要是菱锌矿和方解石,前者包括早期内生成因的和晚期为次生成因的两种类型。硫化物阶段成矿期流体具有低温、高盐度的特征,为Na+-Ca2+-Cl--SO2+4卤水体系,来自蒸发的盆地卤水。C-O、S和Pb同位素特征反映出成矿流体与大理岩围岩发生了强烈的相互作用,沉淀的碳酸盐矿物一定程度继承了围岩的C-O同位素特征,硫化物中的硫主要来源于硫酸盐的热化学还原作用,金属为壳源成因。Angouran矿床缺乏岩体、后生矿化、(主要)碳酸盐岩赋矿、角砾岩容矿、成矿流体为盆地卤水来源等特征与密西西比河谷型(MVT)矿床相似,而与火山成因块状硫化物(VMS)、沉积喷流型(SEDEX)、侵入体有关的热液矿床的特征差别明显,反映该矿床的硫化物成矿可能是一个MVT矿床。硫化物成矿后叠加了与火山作用有关的内生菱锌矿矿化,最后在表生作用下形成了次生的菱锌矿矿化。

The Angouran Zn-Pb-Ag deposit, the biggest Zn-Pb mine in Iran, is situated in the northwestern Sanandaj-Sirjan metamorphic zone of the Zagros orogen, and hosted by the Neoproterozoic-Cambrian marble-micaschist sequence. The orebodies occur in a pipe-shaped body within the marble, which are downwards changed into stratiform or stratoid orebodies in the contact between the marble and the underlying micaschist. Massive or brecciated carbonate ore, mixed sulfide and carbonate ore, and sulfide ore sequentially occur in the shallow to deep parts relative to the current surface. The sulfides consist of sphalerite and subordinate galena, including the early stage of Fe-rich sphalerite and the late stage of Fe- poor sphalerite. The carbonate ore comprises hypogene and supergene smithsonite and calcite. Fluid inclusion data demonstrate that the ore-forming fluid in the sulfide stage is characterized by a Na＋-Ca2＋- CI--SO2＋4 brine system with low temperature and high salinity, which may originate from highly evaporative brine. The characteristics of C-O, S and Pb isotope indicate that the ore-forming fluid have strongly interacted with the host marble; Sulfur in the sulfides is a result of thermal-chemical sulfate reduction; Metals is derived from crust. In summary, the Angouran deposit is characteristic of absence of igneous intrusions, epigenetic mineralization, brecciated carbonate-hosted ore, and the ore-forming fluid being basinal brine origin. This indicates that the deposit is similar to classical Mississippi valley-type （MVT） deposit but clearly distinct from VMS, SEDEX, and intrusion-related hydrothermal deposits. The hypogene smithsonite at Angouran is probably related to Neogene-Quaternary volcanic activity, which overprinted the sulfide mineralization. The supergene smithsonite is the result of the oxidation of the sulfide ores under a surface environment.