Trace Elements and Isotope Data of the Um Garayat Gold Deposit, Wadi Allaqi District, Egypt

Trace element composition of sulfides and O, C, Sr and S isotopic data are assessed to constrain the evolution and potential fluid and metal sources of the Um Garayat gold deposit. Ore microscopy and BSE investigations of quartz veins show blocky arsenopyrite and pyrite replaced in part by pyrrhotite, chalcopyrite, sphalerite, galena, and gersdorffite. Free-milling gold occurs commonly in close association with the late sulfides, and along fractures in pyrite. On the other hand, recrystallized pyrite is disseminated in host metavolcaniclastic/metasedimentary rocks that commonly contain carbonaceous material. In situ LA-ICP-MS analysis of sulfides shows the recrystallized pyrite enriched in most trace elements, while blocky pyrite contains only some traces of arsenic. Detected concentrations of gold (up to 17ppm) were only reported in arsenopyrite disseminated in quartz veins. The δ³⁴S values of blocky pyrite and pyrrhotite in quartz veins define a narrow range (1.6 to 3.7 parts per thousand), suggesting a homogenous sulfur source which is consistent with the dominantly mafic host rocks. The recrystallized pyrite has a distinctive sulfur isotope composition (δ³⁴S -9.3 to -10.6 ‰), which is rather comparable to diagenetic sulfides. Hydrothermal carbonate in quartz veins and wallrock have nearly constant values of δ¹⁸O (10.5 to 11.9 ‰) and δ¹³C (-4.2 to -5.5 ‰). Based on constraints from mineral assemblages and chlorite thermometry, data of six samples indicate that carbonate precipitation occurred at ~ 280 °C from a homogenous hydrothermal fluid with δ¹⁸O_{H₂O} 4.4 +/- 0.7 ‰ and δ¹³C =3.7 +/- 0.8 ‰. Strontium isotope values of two samples (⁸⁷Sr/⁸⁶Sr = 0.7024 and 0.7025) are similar to the initial ⁸⁷Sr/⁸⁶Sr ratios of island arc metabasalts (~ 710Ma) in the South Eastern Desert. The generally homogenous sulfur, C, O, Sr isotope data are suggestive of metamorphogenic fluids, likely produced from dominantly mafic volcanic rocks at the greenschist-amphibolite facies transition.