Gold mineralization in the Um El Tuyor area, South Eastern Desert, Egypt

This thesis reports petrographic, mineralogical and geochemical data that constrain the hydrothermal alteration and gold mineralization in the Um El Tuyor area, SE Egypt. In order to distinguish the characteristics and envisage the genesis of gold mineralization in the study area, insights into the geologic context, structural evolution and geochemistry of the country rocks are provided. The geochemical investigations have been integrated with field and petrographic relationships, along with Landsat (TM) imagery interpretations to better constraints on the tectonic setting of the basement complex in the study area. The particular geographic location near the intersection of two major high strain zones, namely the Allaqi-Heiani suture and Hamisana Shear Zone, played a crucial role in the deformation history of the Um Tuyor area. The basement complex cropping out in the Um El Tuyor area is part of the Neoproterozoic Allaqi-Heiani ophiolitic belt, comprising allochtonous ophiolitic thrust slices and detached sheets, island arc volcano-sedimentary-plutonic assemblages, and syn-orogenic and post-orogenic intrusions. The ophiolitic rocks exhibit field and geochemical characteristics that make them akin to the supra-subduction zone ophiolites, formed most likely in a back-arc basin. The island arc assemblage comprises mainly calc-alkaline metavolcanic-plutonic rocks and back-arc pelitic metasediments. Early calc-alkaline granite intrusions tapered along the foliation and thrust planes during the orogenic episodes, whereas less fractionated tholeiitic olivine gabbro and peraluminous monzogranite encompass a course of post-orogenic plutonism evolved in a within plate setting. An early period of crustal shortening (Dm) involved transportation and overriding of huge ophiolitic sheets from the north to south is manifested by major thrust faults and imbricate ophiolitic thrust slices. Regional folds and pervasive foliation cleavage signify a NE-SW compressional regime (D2) superimposed on the thrust fabrics. A third deformation increment is indicated by the presence of abundant NNW-trending major folds and left-lateral faults superimposed on the older structural fabrics (D3). D4 records an episode of transcurrent deformation yielded slip reactivation of the pre-existing NW-trending faults and formation of discrete shear zones, one of which accommodates gold mineralization in the study area. Finally, a weak shear strain (D5) is indicated by the intersecting fault and joint trends traversing the post-orogenic rocks. Regional metamorphism was coeval with deformation, and peaked under conditions of amphibolite facies during D2. Geothermobarometry calculations point to temperatures of 534-561oC and pressure of 5.26-6.20 kbar for the peak of the metamorphic path of Um El Tuyor basement. Gold is mainly confined to the quartz veins and less commonly to narrow domains of the next quartz-sericite alteration zones. Field, microscopic and microprobe observations suggest that hydrothermal alteration in the Um El Tuyor mine area was post-peak metamorphism, and syn-kinematic with local shearing. The main auriferous veins in the Um El Tuyor mine area consist of massive, partially recrystallized, or laminated quartz ±carbonate. The quartz-carbonate veins are essentially fault-fill bodies, which have been fractured and re-filled with milky to grey laminated quartz in later stages of the geothermal system. The laminated quartz veins contain narrow elongate slivers of the host pelitic rocks (composed essentially of chrorite-sericite-sulphides±graphite), assumed to have been peeled off and incorporated during incremental (crack-seal) vein growth. vii Gold occurs as inclusions or within the lattice in arsenopyrite and arsenian pyrite, commonly in association with subordinate sphalerite, chalcopyrite, and pyrrhotite in the auriferous quartz veins. Another, high fineness type of gold fills microfractures in sulphides and quartz, and/or occurs as dispersed blebs and globules in domains of pervasive alteration, particularly where sericite and carbonate are intergrown. In the mine area, a metal zonation extends from an inner Fe-As-Zn-Au ±Pb±Ag±Te bearing veins through an intermediate Fe-As-Cu rich wallrocks to a distal halo enriched in Fe-Cu-Co, and Ni. A three stage hydrothermal alteration model (initial, transitional and advanced) is proposed for the Um El Tuyor ore-forming hydrothermal system, on basis of the field and microscopic observations and the electron microprobe data. The initial stage involved hydrolysis of the wallrocks in presence of a near acid fluid, whose pH was buffered by the wallrock mineralogy. The transitional stage involved also hydrolysis reactions, but was dominated by carbonatization, sulphidation and redox reactions. The latter reduced the interacting fluids, particularly where the fluid : rock ratio was low and provided favourable conditions for gold deposition. It is interpreted that sulphidation has affected the gold solubility via changes in oxygen fugacity through redox reactions. The advanced stage was most likely a phase of intense sericitization (after chlorite), which consumed K+, liberated H+, and buffered the solution pH. Cation-exchange reactions were limited to the time when favoured Na+ activity and temperature conditions promoted deposition of albite. Sulphidation remained operating through this stage, and the un-buffered conditions were locally attained under high fluid/rock ratios. Compositional zoning of the auriferous pyrite and arsenopyrite crystals, along with the presence of patchy pyrrhotite and sphalerite inclusions in these crystals, and the absence of these features in pyrite and arsenopyrite of the late sulphide assemblage may imply incipient low oxygen fugacity conditions during gold deposition. Further, compositional zoning of the large arsenian pyrite crystals, from barren cores to auriferous margins, is considered as a function of redox reactions involving oxidation of Au and reduction of As. Fluid inclusion studies revealed heterogeneous entrapping of immiscible aqueous and carbonic fluids in the Um El Tuyor auriferous quartz veins. Criteria including the primary and secondary modes of occurrence of the carbonic inclusions, coexistence of inclusions with carbonic and aqueous fluids of variable relative proportions, densities, filling degrees, partial homogenization temperatures, and bulk compositions are considered evocative for phase separation as gold deposition mechanism in the Um El Tuyor auriferous quartz veins. The initially homogenous ore fluid was likely a low salinity aqueous-carbonic solution (±1-2 mol% CH4 or N2), which started to separate into two phases and precipitate gold when conditions attained ~340°C at ~1.5 kbar (at depth of ~6 km under lithostatic condition). Destabilization of gold-sulphur complexes through interplay of cooling, redox state variation, pH changes, and decrease in sulphur fugacity should have contributed in gold deposition in the auriferous veins. The entire gold-base metal mineralization and quartz veining event extended over conditions of 170-429oC at 0.9-2.1 kbar respectively, equivalent to depths of 3-8 km and compatible with crustal conditions of greenschist metamorphism and brittle–ductile transition. This wide range of pressure probably represents the total fluid pressure regime within the shear zone from the formation of the auriferous quartz veins to periods of continuous pressure decrease during uplift, including a sudden pressure decrease occurred as a consequence of incremental opening of the fissure, followed by filling by newly deposited quartz.