The nature of gold-bearing fluids in Atud gold deposit, Central Eastern Desert, Egypt

Electron microprobe analyses of gold and associated ore minerals as well as stable isotope analyses of sulphide and carbonate minerals were performed in order to determine the metal and fluid sources and temperature of the mineralizing systems to better understand the genesis of the Atud gold deposit hosted in the metagabbro–diorite complex of Gabal Atud (Central Eastern Desert, Egypt). The gold can be classified as electrum (63.6–74.3 wt.% Au and 24.6–26.6 wt.% Ag) and is associated with arsenopyrite and As-bearing pyrite in the main mineralization (gold-sulphides) phase within the main mineralized quartz veins and altered host rocks. Based on the arsenopyrite geothermometer, As-contents (29.3–32.7 atom%) in arsenopyrite point to deposition in the Log ƒS2 and T ranges of −10.5 to −5.5 and 305–450°C, respectively, during the main mineralizing phase. Based on the δ34S isotopic compositions of the sulphides, they are originated from magmatic fluids in which the sulphur is either sourced directly from magma or remobilized from the magmatic rocks (gabbroic rocks). On the other hand, calcite formed from fluids having mainly magmatic mixed with variable metamorphic signatures based on its δ13C and δ18O values. This work concluded that the gold-bearing ores at Atud deposit have magmatic sources leaching from the country intrusive rocks during water/rock interactions then remobilized during a metamorphic event. Therefore, the Atud gold deposit is classified as an intrusion-related gold deposit, in which the gabbro–diorite host intrusion acted as the source of metals which were mobilized and deposited as a result of the effects of NW–SE shearing.

为明确成矿系统的金属来源、流体来源及成矿温度，以深入理解产于埃及中东部沙漠地区加拜勒阿图德（Gabal Atud）的变辉长岩-闪长岩（metagabbro–diorite）杂岩中的阿图德金矿床（Atud gold deposit）成因，本次研究对金及伴生矿石矿物开展了电子探针（electron microprobe）分析，并对硫化物与碳酸盐矿物开展了稳定同位素分析。该矿床中的金可被归为银金矿（electrum，金含量63.6~74.3 wt.%，银含量24.6~26.6 wt.%），在主矿化（金-硫化物）阶段，金与毒砂（arsenopyrite）及含砷黄铁矿（As-bearing pyrite）伴生，赋存于主矿化石英脉及蚀变围岩中。基于毒砂地质温压计（arsenopyrite geothermometer），毒砂中的砷含量（29.3~32.7 原子%）指示主矿化阶段的硫化物活度对数（Log ƒS₂）区间为-10.5~-5.5，成矿温度区间为305~450℃。根据硫化物的δ³⁴S同位素组成，其硫源来自岩浆流体，硫既可以直接源自岩浆，也可从岩浆岩（辉长岩类）中活化迁移而来。另一方面，结合方解石的δ¹³C与δ¹⁸O同位素值，其形成流体以岩浆流体为主，混入了不同程度的变质流体特征。本研究得出结论：阿图德金矿床的含金矿石其金属源为岩浆源，在水-岩相互作用过程中从围岩侵入岩中淋滤萃取，随后在变质事件中发生活化迁移。因此，阿图德金矿床可被归为与侵入岩相关的金矿床（intrusion-related gold deposit），其中作为容矿岩的辉长岩-闪长岩侵入体为金属源，金属在北西-南东向（NW–SE）剪切作用下发生活化迁移并沉淀就位。