Origin and Characteristics of Ore-Forming Fluids of the Abu Gaharish Orogenic Gold Deposits, Central Eastern Desert, Egypt: Evidence from Alteration Geochemistry, Mineral Chemistry and Fluid Inclusions

Abstract

AbstractThe Abu Gaharish mineralization comprises Au-rich NNE-trending quartz veins, and their surrounding shear zone is found hosted within a Neoproterozoic Sn-W-bearing monzogranite pluton and intersects the metavolcano-sedimentary assemblage at its eastern contact. The ore mineralogy consists of pyrite, galena, chalcopyrite, sphalerite, and gold, with secondary vanadinite occurring after galena and hematite after pyrite. Paragenesis reveals two generations of pyrite and galena, where gold formed mainly during the early sulfidation stage and mobilized upon later deformation events and incipient recrystallization. Mass balance calculations on the alteration zones reveal that the inputs upon alteration included Al2O3, Fe2O3, K2O, MgO, P2O5 W, Rb, Cu, Zn, Pb, V, Cr, Mo, Co, Sn, As, and U, whereas the outputs comprised SiO2, Na2O, CaO, MnO, Sr, Ba, Zr, Y, Th, Nb, and Ni, respectively, in decreasing order. The low contents of granitophile elements (such as Mo, Sn, and W) in the quartz veins at Abu Gaharish suggest that the gold-sulfide-quartz veins have no linkage with granitoid intrusion. However, the enrichment of these elements in the proximal alteration zone is attributed to their leaching by hydrothermal fluids from anomalous zones. The fluid inclusion data observed in quartz showcases the existence of CO2-bearing low-salinity fluids mixed with Mg, Na, and K chlorides under mesothermal conditions during the ore deposition process. This aligns with the formation of orogenic gold deposits as a consequence of the metamorphic devolatilization of the adjacent metavolcano-sedimentary succession. The characteristics of mineralizations at Abu Gaharish display resemblances to orogenic gold deposits and refute the intrusion-related model.