Geochemical and geochronological studies of the Um Samiuki deposit (Eastern Desert, Egypt): Metal mobilization in a stratabound massive sulfide ore

Authors:
raw_author_name: Basem Zoheir
raw_affiliation_string: Department of Geology, Faculty of Science, Benha University, Benha 13518, Egypt; Institute of Geological Sciences, University of Kiel, Kiel 24118, Germany
raw_author_name: Stephanie Lohmeier
raw_affiliation_string: Clausthal University of Technology, Mineral Resources, Clausthal-Zellerfeld 38678, Germany
raw_author_name: Yongyong Tang
raw_affiliation_string: State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China

Abstract: The Um Samiuki deposit in the South Eastern Desert of Egypt is a Kuroko-type volcanogenic massive sulfide deposit associated with arc-related bimodal volcanic rocks. SIMS U-Pb zircon ages of the host metavolcanic rocks and a crosscutting granodiorite intrusion are integrated with a newly obtained Re-Os age for the Um Samiuki sulfides, allowing the genetic relationship between sulfide mineralization and host rocks to be determined. Zircon U-Pb dating reveals that the ore-hosted stratum of banded tuffs formed at ∼700.0 ± 7.3 Ma, whereas the granodiorite yielded a concordia age of 624.0±8.5 Ma. Sulfide mineral separates (pyrite, sphalerite, chalcopyrite) and sulfide mixtures from the massive ores have significantly high radiogenic Re-Os concentrations and ratios. Regression of seven 187Re/188Os-187Os/188Os data points with the corresponding uncertainty correlation values yields a 624±46 Ma age (2σ, MSWD = 0.56), with an initial 187Os/188Os ratio of 3.58±0.7, thereby pinpointing a distinctive crustal isotope signature. Evolution towards textural equilibrium by recrystallization, as revealed by grain growth and annealing textures in the ores, suggests that ductile shearing and penetrative deformation of the host metavolcanic/metavolcaniclastic rocks led to the formation of discrete sulfide disseminations and masses. Deformed, rolled and rotated pyrites in veins and tension gashes with or without quartz evidently suggests an intermittently rejuvenated brittle deformation regime superimposed on the early ductile deformation textures. Pyrite, galena, chalcopyrite, and sphalerite contain elevated Ag contents between 5 and 812 ppm. Two pyrite generations can be distinguished with the earlier one (Py1) having the highest Ag contents of all sulfides. Silver was released during replacement of Py1 by a late assemblage (Py2-galena-chalcopyrite-sphalerite) with some Ag incorporated into the late sulfides but most Ag was released and retained in discrete Ag minerals like cerveilleite, hessite and acanthite. Time-resolved analyses of LA-ICP-MS data reveal the presence of micron-scale Ag-bearing inclusions in galena while other base metal sulfides also contribute a notable part to the Ag budget of Um Samiuki ore. The new geochronological data integrated with results of the microscopic and microanalytical analyses of the sulfide ores provide an updated insight into a metal mobilization event in the Um Samiuki volcanogenic massive sulfides, roughly bracketed with shear-associated granodiorite magmatism.