Geochemical and geochronological characteristics of the Um Rus granite intrusion and associated gold deposit, Eastern Desert, Egypt

The Um Rus tonalite-granodiorite intrusion (∼6 km2) occurs at the eastern end of the Neoproterozoic, ENE-trending Wadi Mubarak shear belt in the Central Eastern Desert of Egypt. Gold-bearing quartz veins hosted by the Um Rus intrusion were mined intermittently, and initially by the ancient Egyptians and until the early 1900s. The relationship between the gold mineralization, host intrusion, and regional structures has always been unclear. We present new geochemical and geochronological data that help to define the tectonic environment and age of the Um Rus intrusion. In addition, field studies are integrated with EPMA and LA-ICP-MS data for gold-associated sulfides to better understand the formation and distribution of gold mineralization. The bulk-rock geochemical data of fresh host rocks indicate a calc-alkaline, metaluminous to mildly peraluminous, I-type granite signature. Their trace element composition reflects a tectonic setting intermediate between subduction-related and within-plate environments, presumably transitional between syn- and post-collisional stages. The crystallization age of the Um Rus intrusion was determined by in situ SHRIMP 206Pb/238U and 207Pb/235U measurements on accessory monazite grains. The resultant monazite U–Pb weighted mean age (643 ± 9 Ma; MSWD = 1.8) roughly overlaps existing geochronological data for similar granitic intrusions that are confined to major shear systems and are locally associated with gold mineralization in the Central Eastern Desert (e.g., Fawakhir and Hangaliya). This age is also consistent with magmatism recognized as concomitant to transpressional tectonics (D2: ∼650 Ma) during the evolution of the Wadi Mubark belt. Formation of the gold-bearing quartz veins in NNE-SSW and N–S striking fault segments was likely linked to the change from transpressional to transtensional tectonics and terrane exhumation (D3: 620–580 Ma). The development of N–S through-going fault arrays and dike swarms (∼595 Ma) led to heterogeneous deformation and recrystallization of the mineralized quartz veins. Ore minerals in the auriferous quartz veins include ubiquitous pyrite and arsenopyrite, with less abundant pyrrhotite, chalcopyrite, sphalerite, and galena. Uncommon pentlandite, gersdorffite, and cobaltite inclusions hosted in quartz veins with meladiorite slivers are interpreted as pre-ore sulfide phases. The gold-sulfide paragenesis encompasses an early pyrite-arsenopyrite ± loellingite assemblage, a transitional pyrite-arsenopyrite assemblage, and a late pyrrhotite-chalcopyrite-sphalerite ± galena assemblage. Free-milling gold/electrum grains (10s μm-long) are scattered in extensively deformed vein quartz and in and adjacent to sulfide grains. Marcasite, malachite, and nodular goethite are authigenic alteration phases after pyrrhotite, chalcopyrite, and pyrite and arsenopyrite, respectively. A combined ore petrography, EPMA, and LA-ICP-MS study distinguishes morphological and compositional differences in the early and transitional pyrites (Py I, Py II) and arsenopyrite (Apy I, Apy II). Py I forms uncommon small euhedral inclusions in later Py II and Apy II. Py II forms large subhedral crystals with porous inner zones and massive outer zones, separated by narrow As-rich irregular mantles. The Fe and As contents in Py II are variable, and the LA-ICP-MS analysis shows erratic concentrations of Au (<1 to 177 ppm) and other trace elements (e.g., Ag, Te, and Sb) in the porous inner zones, most likely related to discrete sub-microscopic sulfide inclusions. The outer massive zones have a rather homogenous composition, with consistently lower abundances of base metals and Au (mean 1.28 ppm). The early arsenopyrite (Apy I) forms fine-grained euhedral crystals enriched in Au (mean 17.7 ppm) and many other trace elements (i.e., Ni, Co, Se, Ag, Sb, Te, Hg, and Bi). On the other hand, Apy II occurs as coarse-grained subhedral crystals with lower and less variable concentrations of Au (mean 4 ppm). Elevated concentrations of Au (max. 327 ppm) and other trace elements are measured in fragmented and aggregated pyrite and arsenopyrite grains, whereas the undeformed intact zones of the same grains are poor in all trace elements. The occurrence of gold/electrum as secondary inclusions in deformed pyrite and arsenopyrite crystals indicates that gold introduction was relatively late in the paragenesis. The LA-ICP-MS results are consistent with gold redistribution by the N–S though-going faults/dikes overprinted the earlier NNW-SSE quartz veins in the southeastern part of the intrusion, where the underground mining is concentrated. Formation of the Um Rus intrusion and gold-bearing quartz veins can be related to the evolution of the Wadi Mubarak shear belt, where the granitic intrusion formed during or just subsequent to D2 and provided dilatation spaces for gold-quartz vein deposition when deformed by D3 structures.