Formation of the giant Aynak copper deposit, Afghanistan: evidence from mineralogy, lithogeochemistry and sulfur isotopes

This project is based on PhD research undertaken by Hamidullah Waizy at the University of Brighton, 2014-2018. The files are linked to an article published on 4th October 2020 in 'International Geology Review', doi: 10.1080/00206814.2020.1824129 Abstract: Aynak is the largest known copper deposit in Afghanistan, with indicated resources of 240 Mt grading 2.3% Cu placing it in the ‘giant’ category. Host rocks are Neoproterozoic metasediments comprising dolomitic marble, carbonaceous quartz schist and quartz-biotite-dolomite schist containing garnet, scapolite and apatite. Chalcopyrite and bornite dominate the hypogene ore with lesser pyrite, pyrrhotite, cobaltite and chalcocite, and rare sphalerite, molybdenite, uraninite and barite. Sulfides occur as bedding-parallel laminae, disseminations, metamorphic segregations and crosscutting veins. Sulfide δ34S ratios range –14.5 to +17.3‰ in bedded and disseminated sulfides (n=34). This broad range favours biogenic reduction of seawater sulfate as a major source of sulfur, although thermochemical reduction processes are not precluded. The narrower δ34S range of –6 to +12.2‰ in vein and segregation sulfides (n=21) suggests localised redistribution and partial homogenization during metamorphism. Geochemical associations suggest that Al, P, Ca, Ti and Fe were primary sedimentary constituents whereas Cu, Mg, S, Se, As, Co and Bi were introduced subsequently. We infer that Aynak originated as a shale- and carbonate-hosted stratabound replacement deposit, resembling orebodies of the Central African Copperbelt, although underlying red-beds are absent at Aynak and mafic volcanics were the probable copper source. These giant deposits formed worldwide in the Cryogenian probably due to marine enrichment in copper, magnesium and sulfate coincident with profuse basaltic volcanism and ocean oxidation.

本项目基于哈米杜拉·瓦伊齐（Hamidullah Waizy）于2014—2018年在布莱顿大学完成的博士研究。相关文件关联一篇2020年10月4日发表于《国际地质评论（International Geology Review）》的论文，DOI：10.1080/00206814.2020.1824129。
摘要：艾纳克（Aynak）是阿富汗已知规模最大的铜矿床，探明资源量达2.4亿吨，铜品位2.3%，属于“巨型”矿床范畴。其赋矿围岩为新元古代（Neoproterozoic）变沉积岩，包括白云质大理岩、碳质石英片岩以及含石榴子石（garnet）、方柱石（scapolite）和磷灰石（apatite）的石英-黑云母-白云石片岩。原生矿石以黄铜矿（chalcopyrite）和斑铜矿（bornite）为主，伴生少量黄铁矿（pyrite）、磁黄铁矿（pyrrhotite）、钴毒砂（cobaltite）和辉铜矿（chalcocite），罕见闪锌矿（sphalerite）、辉钼矿（molybdenite）、晶质铀矿（uraninite）和重晶石（barite）。硫化物以顺层纹层、浸染状、变质分异体及穿切脉体的形式产出。层状与浸染状硫化物的硫同位素δ³⁴S比值范围为-14.5‰至+17.3‰（n=34），这一宽泛区间表明海水硫酸盐的生物还原作用是硫的主要来源，但未排除热化学还原过程的参与。脉状与分异型硫化物的δ³⁴S区间更窄，为-6‰至+12.2‰（n=21），指示变质作用过程中发生了局部分馏与部分均一化。地球化学组合特征显示，铝、磷、钙、钛和铁为原始沉积组分，而铜、镁、硫、硒、砷、钴和铋则为后期引入的组分。研究推断，艾纳克矿床最初为页岩和碳酸盐岩容矿的层控交代型矿床，类似中非铜矿带（Central African Copperbelt）的矿体，尽管艾纳克地区缺失下伏红层，且镁铁质火山岩可能为铜的源岩。这类巨型矿床在全球范围内的成冰纪（Cryogenian）时期形成，其成因可能与海水在富铜、富镁和富硫酸盐的条件下富集，伴随强烈的玄武质火山活动与海洋氧化作用相关。