Genesis of the Hamash gold deposit, eastern desert, Egypt: Inferences from new studies on mineral chemistry and fluid inclusions

Abstract

In the South-Eastern desert of Egypt, where gold mineralization is hosted by granites, the Hamash gold deposit is recognized as one of the pharaoh's mines. In this study, the present authors aimed to study the paragenesis of sulfide minerals and the genesis of the related gold deposit based on field observation, ore mineralogy, electron probe analysis, and fluid inclusion data. The result was that the granitoid rocks of the Hamash area, composed mainly of granite and granodiorite, covered the center and southern portions of the study area and intruded into the metavolcanics. The Hamash gold deposit is hosted by intensely deformed granitic rocks that are dissected by numerous gold-bearing sulfide-quartz veins. The abundant ore minerals include pyrite, sphalerite, galena, and chalcopyrite (±tetrahedrite and ±gold disseminations), which occur in crosscutting quartz veins. Gold is associated with sulfides in quartz veins and alteration zones. The results of electron probe microanalysis (EMPA) reveal three varieties of pyrite with low As content (Py1 and Py2) and high As content (Py3) that were replaced by galena ± gold. Gold occurs either as clusters of tiny gold grains included in pyrite or as streak-like small inclusions within goethite. The new microthermometric and laser Raman spectroscopy studies indicate that the gold-bearing quartz veins are characterized by two types of fluid inclusions: H₂O-NaCl and H₂O-CO₂-NaCl. The lithostatic and hydrostatic pressure gradients in the isochores of the two types of fluid inclusions indicate fluid entrapment and gold deposition at temperatures between 235 °C and 270 °C at pressures ranging from 1.6 kbar to 2.7 kbar with low to moderate salinity of ∼2–7 eq. mass % NaCl of aqueous-carbonic fluid during the transition from lithostatic to hydrostatic conditions. Based on all the author's observations and conditions of ore formation, we suggest that the Hamash gold deposit is related to porphyry-type and epithermal sulfide deposits.