Listvenite-lode association at the Barramiya gold mine, Eastern Desert, Egypt

Several occurrences of gold-bearing quartz veins are situated along the east-northeast-trending Barramiya-Um Salatit ophiolitic belt in the central Eastern Desert of Egypt. In the Barramiya mine, gold mineralization within carbonaceous, listvenized serpentinite and adjacent to post-tectonic granite stocks points toward a significant role of listvenitization in the ore genesis. The mineralization is related to quartz and quartz-carbonate lodes in silicified/carbonatized wallrocks. Ore minerals, disseminated in the quartz veins and adjacent wallrocks are mainly arsenopyrite, pyrite and trace amounts of chalcopyrite, sphalerite, tetrahedrite, pyrrhotite, galena, gersdorffite and gold. Partial to complete replacement of arsenopyrite by pyrite and/or marcasite is common. Other secondary phases include covellite and goethite. Native gold and gold-silver alloy occur as tiny grains along micro-fractures in the quartz veins. However, the bulk mineralization can be attributed to auriferous arsenopyrite and arsenic-bearing pyrite (with hundreds of ppms of refractory Au), as evident by electron microprobe and IA-ICP-MS analyses. The mineralized quartz veins are characterized by abundant carbonic (CO2 +/- CH4 + H2O) and aqueous-carbonic (H2O-NaCl-CO2 + CH4) inclusions along intragranular trails, whereas aqueous inclusions (H2O-NaCl + CO2) are common in secondary sites. Based on the fluid inclusions data combined with thermometry of the auriferous arsenopyrite, the pressure-temperature conditions of the Barramiya gold mineralization range from 1.3 to 2.4 kbar at 325-370 degrees C, consistent with mesothermal conditions. Based on the measured delta S-34 values of pyrite and arsenopyrite intimately associated with gold, the calculated delta S-34(Sigma s) values suggest that circulating magmatic, dilute aqueous-carbonic fluids leached gold and isotopically light sulfur from the ophiolitic sequence. As the ore fluids infiltrated into the sheared listvenite rocks, a sharp decrease in the fluid fO(2) via interaction with the carbonaceous wallrocks triggered gold deposition in structurally favorable sites. (C) 2010 Elsevier BM. All rights reserved.