Electron probe micro-analysis (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry
(LA-ICP-MS) data of ore minerals from the Um Eleiga deposit (SE Egypt) suggest that primary Au was sequestered
within Ni–pyrrhotite and Co–Ni–Fe sulfarsenide, while refined, free-milling gold grains (~93 wt.% Au)
are associated with late pyrite, or occur along micro-fractures in quartz veins. A magmatic source of gold is
most likely, yet endowment by circulating metasomatic fluid was crucial for Au concentration.
Sulfur stable isotope values (δ34S) of sulfide minerals disseminated in the quartz veins (pyrite: −0.5 to 1.5‰,
chalcopyrite: −0.3 to 0.6‰, and sphalerite: −0.1 to−0.4‰), indicate a generally light sulfur source. Similarly,
measured δ13C of vein calcite (−3.1 to −1.8, n=9) and calculated fluid δ13CCO2 values (−0.6 to 0.6‰, for
the temperature range 350–375 °C of associated hydrothermal chlorite) cluster around 0‰, consistent with a
magmatic source for carbon. Oxygen isotope data for vein calcite (10.3–13‰, n=9) and quartz (11.7–14.3‰,
n=10) indicate average fluid δ18OH2O values of 7.4 and 7.8‰, respectively, which implicate fluids similar to
those in intrusion-related systems.