Carbonate alteration of ophiolitic rocks in the Arabian–Nubian Shield of Egypt: sources and compositions of the carbonating fluid and implications for the formation of Au deposits

Ultramafic portions of ophiolitic fragments in the Arabian–Nubian Shield (ANS) show pervasive carbonate alteration forming various degrees of carbonated serpentinites and listvenitic rocks. Notwithstanding the extent of the alteration, little is known about the processes that caused it, the source of the CO₂ or the conditions of alteration. This study investigates the mineralogy, stable (O, C) and radiogenic (Sr) isotope composition, and geochemistry of suites of variably carbonate altered ultramafics from the Meatiq area of the Central Eastern Desert (CED) of Egypt. The samples investigated include least-altered lizardite (Lz) serpentinites, antigorite (Atg) serpentinites and listvenitic rocks with associated carbonate and quartz veins. The C, O and Sr isotopes of the vein samples cluster between −8.1‰ and −6.8‰ for δ¹³C, +6.4‰ and +10.5‰ for δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr of 0.7028–0.70344, and plot within the depleted mantle compositional field. The serpentinites isotopic compositions plot on a mixing trend between the depleted-mantle and sedimentary carbonate fields. The carbonate veins contain abundant carbonic (CO₂±CH₄±N₂) and aqueous-carbonic (H₂O-NaCl-CO₂±CH₄±N₂) low salinity fluid, with trapping conditions of 270–300°C and 0.7–1.1 kbar. The serpentinites are enriched in Au, As, S and other fluid-mobile elements relative to primitive and depleted mantle. The extensively carbonated Atg-serpentinites contain significantly lower concentrations of these elements than the Lz-serpentinites suggesting that they were depleted during carbonate alteration. Fluid inclusion and stable isotope compositions of Au deposits in the CED are similar to those from the carbonate veins investigated in the study and we suggest that carbonation of ANS ophiolitic rocks due to influx of mantle-derived CO₂-bearing fluids caused break down of Au-bearing minerals such as pentlandite, releasing Au and S to the hydrothermal fluids that later formed the Au-deposits. This is the first time that gold has been observed to be remobilized from rocks during the lizardite–antigorite transition.

阿拉伯-努比亚地盾（Arabian-Nubian Shield, ANS）内的蛇绿岩碎片（ophiolitic fragments）中超镁铁质岩体（ultramafic portions）普遍发育碳酸盐蚀变（carbonate alteration），形成不同蚀变程度的碳酸盐化蛇纹岩（carbonated serpentinites）与利斯特文岩（listvenitic rocks）。尽管蚀变规模可观，但学界对其成因过程、二氧化碳（CO₂）来源及蚀变条件仍知之甚少。本研究针对埃及中东部沙漠（Central Eastern Desert, CED）美提克（Meatiq）地区不同碳酸盐蚀变程度的超镁铁质岩套，开展矿物学、稳定（氧、碳）及放射成因（锶）同位素组成与地球化学研究。本次研究的样品包括弱蚀变利蛇纹石（lizardite）蛇纹岩、叶蛇纹石（antigorite）蛇纹岩，以及伴生碳酸盐脉与石英脉的利斯特文岩。脉体样品的碳、氧、锶同位素组成分别为δ¹³C值-8.1‰~-6.8‰、δ¹⁸O值+6.4‰~+10.5‰，⁸⁷Sr/⁸⁶Sr比值为0.7028~0.70344，投影点均落在亏损地幔（depleted mantle）组分区间内。蛇纹岩的同位素组成呈现出一条介于亏损地幔与沉积碳酸盐岩区间之间的混合趋势。碳酸盐脉中发育大量富二氧化碳（CO₂±CH₄±N₂）及水-二氧化碳（H₂O-NaCl-CO₂±CH₄±N₂）型低盐度流体包裹体（fluid inclusion），其捕获温度为270~300℃，捕获压力为0.7~1.1 kbar。相较于原始地幔与亏损地幔，该类蛇纹岩富集金（Au）、砷（As）、硫（S）及其他流体活动性元素。其中，强碳酸盐化叶蛇纹石蛇纹岩的上述元素含量显著低于利蛇纹石蛇纹岩，表明这些元素在碳酸盐蚀变过程中发生了淋失。中东部沙漠金矿床的流体包裹体与稳定同位素组成与本研究中碳酸盐脉的特征高度相似，据此我们提出：幔源含CO₂流体的注入引发阿拉伯-努比亚地盾蛇绿岩发生碳酸盐化，导致含Au矿物（如镍黄铁矿pentlandite）发生分解，将Au与S释放至热液流体（hydrothermal fluids）中，最终形成金矿床。本研究首次报道了在利蛇纹石-叶蛇纹石转变（lizardite–antigorite transition）过程中，金发生了从岩石中的活化迁移。