New geological and mineralogical data from the Dungash gold deposit, Central Eastern Desert, Egypt Basem Zoheir
aDepartment
a,*,
Pär Weihed
b and
Bernd Lehmann
c
of Geology, Faculty of Science, Benha University, P.O.Box 13518, Benha, Egypt (*corresponding author:
[email protected]) bDivision of Ore Geology and Applied Geophysics, Luleå University of Technology, Luleå, Sweden c Institute of Mineralogy and Mineral Resources, Technical University of Clausthal, Clausthal-Zellerfeld, Germany
INTRODUCTION The Dungash gold deposit, central Eastern Desert of Egypt, is a mesothermal vein-type gold mineralization hosted by island arc volcanic/volcaniclastic rock assemblages, generally metamorphosed under greenschist facies conditions (Fig. 1). The discrete veintype gold localities in the basement complex of the Eastern Desert are mostly the main targets for gold prospection in Egypt since the pre-Dynastic times and until present. It is suggested that formation of this type of gold deposits took place during the Pan-African orogeny coeval with regional metamorphism, associated in several cases with calc-alkaline I-type granitoids. Exploitation of the Dunagsh Au deposit started likely since the Roman times or earlier, based on the abundant mining ruins and millstones spreading everywhere in the mine area, and considering that operation was synchronous with Samut mine where a Roman village is still preserved. Mine activities in the area may have been sporadic during the beginning of the 20th century. Our field observations indicate that a few inclined shafts (10s of meters deep along dip) and an extensive trenching (along strike) for gold panning work were considered the latest workings in the area. Recently, a new exploration program in Dungash concession is carried out by MICA STAR for Mining (from Matz Holdings Ltd.), implying the economic potential of the area which warrants updated genetic information crucial for serious exploration programs. GEOLOGIC AND STRUCTURAL SETTING The Dungash mine area is underlain by strongly folded and contorted sequences of back-arc basin volcano-sedimentary rocks and huge bodies of allochthonous ophiolitic rocks (serpentinite) and is intruded by granitoid-gabbroid intrusions generally of Neoproterozoic age. This rocks pile is part to the so referred to as the “Meulha-Dungash” mélange sequence (cf. El-Gaby et al., 1990), which is made up mainly of remnants of obducted oceanic crust tectonically intermixed with back arc sedimentary and volcanoclastic rocks and later metamorphosed under greenschist facies conditions. The metasedimentary rocks are represented mainly by chlorite schist, mudstones and subordinate volcano-sedimentary tuffs. These rock are common as matrix for mélange blocks (serpentinite, metagabbro and pillowed metabasalt) and less commonly as free bands tectonically interbedded with the metavolcanic rocks. The metavolcanic rocks, dominating the mine area, range in composition from andesite to basaltic andesite. Pyroclastics rocks are intercalated with the metavolcanics sequences along the originally ~ east-west bedding planes. These rocks are mainly volcanogenic tuffs and agglomerates with accidental carbonate fragments. Discrete basic and acid dykes cut the basement complex in the region, mostly in a NW-SE direction (Fig. 2). Structurally, the bulk oreshoots are attenuated and sheared along (a) (b) flexural-slip planes outwards of a major antiform in island arc metavolcanic/volcaniclastic rocks. Ductile deformation in the region is manifested by NW-trending antiforms (F1) superimposed by (E)NE- trending upright folds (F2) and associated large shear zones. The auriferous structures and related alteration halos define a km-scale ENE-trending dilation zone between interlayered andesitic tuffs/agglomerates and turbidities/ volcaniclastic sediments (Figs. 2,3). Faults in (d) (c) different trends dislocate and brecciate most pre-existing rocks, commonly intersecting in the mine area. Field observations indicate that early NW-trending sinistral faults are cut by late ENE- and ~E-W trending dextral fault/fracture zones. Analysis of structural elements including foliation, shear cleavages, intersection lineation and slickenfibers indicates that quartz veining took place synchronous with a dextral shear system by flexural transpression between intensely foliated sediments and coherent, massive metavolcanic blocks (Figs. 3,4). Gold is Fig. (3): (a) Variably developed shear foliation in intermingled massive meta-volcanic blocks and foliated sedimentary matrix, (b) Surface of a gently south-westward dipping NW-fault cutting the ENE-ore zone south of confined to a ~E-, ENE-trending quartz vein system dipping the western ore zone, (c) Three sub-parallel ore bodies stopped out on surface and in the underground through several inclined shafts in the eastern ore zone .Photo looking SE , (d) logitudianl trench work along the main ore steeply to the south and cutting the host rock foliation (WNWbody in teh eastern ore zone (see Fig. 2). Photo looking NW . ESE/55-65ºN). GOLD MINERALIZATION Gold mineralization in Dungash mine is related to a series of grey and milky quartz(±carbonate) lodes, accommodated by a brittleductile shear zone, along which highly strained, intermingled metavolcanic and volcanosedimentary wallrocks are rich in disseminated sulfides (Fig. 5). The mine is divided into eastern and western ore zones over an area of ~1km2. The eastern ore zone, extending for ~800m in an ENE-direction, is characterized by variably deformed milky quartz and quartz-carbonate veins cutting through intensely sericitized, chloritized and sulfidized andesitic agglomerates. The main eastern ore body is furcated into three quartz veins (southern 135m-long, middle 170m and northern 230m) with a maximum vein width ≤150cm. This part of the ore zone (including quartz veins and altered wallrocks) shows a N-S width varying from 100 to 160m. The western ore zone comprises a single, mainly fragmented grey quartz vein extending for 500m in a ~E-W direction. The hydrothermal alteration zones in the eastern and western ore zones grade ≤6g/t Au, whereas, quartz veins contain up to 23g/t Au. The highest gold contents are reported in the grey quartz veins in the western ore zone. (a)
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Fig. (5): Features of the ore bodies in Dungash Au deposit (a) Milky quartz veins in the eastern ore zone (underground) with variable dolomite content, (b) Exposed milky quartz vein in the eastern ore zone cutting intensely carbonatized volcaniclastic rocks with abundant disseminated pyrite. Notice the intense brecciation of the host rock on surface, while ductile deformation is seen in the underground levels, (c) Pinched grey quartz lode (main ore body in the western ore zone) brecciated and sealed by milky quartz in underground work in the western shaft (see Fig. 2), (d) Hand specimen of the main ore body in the western ore zone made up of variably carbonatized/sulfidized tuffaceous sediments seamed with greyish quartz veinlets. ICP-MS analysis revealed Au content of 21g/t in such type of ore bodies, whereas gold grade of the milky quartz veins in the eastern ore zone measures 17g/t. Insets showing locations.
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Fig. (7) : Reflected light photomicrographs showing ore mineralogy and textures of Dungash gold deposit. (a) Intergrowths of pyrrhottie, pyrite and arsenopyrite and interstatial chalcopyrite in grey quartz veins from the western ore zone, (b) Aggregated Sb-bearing arsenopyrite grains in carbonate rich domain in quartz carbonate lode, (c) chalcopyrite filling fractures in pyrite in milky quartz from the eastern ore zone, (d) dispersed gold spcks associated with tetrahedrite and galena in carbonate –rich lode, (e) Disseminated gold blebs associated with pyrite in milky quartz, (f) Fine disseminated of gold/electrum globules associated with large arsenopyrite, (g-i) Backscattered electron images of Sb-bearing arsenopyrite common in Au-rich lodes, gold wire along fractured pyrite and disseminated dolomite with sericite in hydrothermally altered wallrocks.
Fig. (1): Abram et al. (1983)’s composite band ratios image showing the different lithologic units/tectonic terranes and major structural elements in Dungash mine area and surroundings. Notice the occurrence of Dungash Au-deposit peripheral to NEfolded arc metavolcanics (pale green) at the dense intersection of NE and NW-trending faults with ENE-trending shear zones.
Fig. (2): Geological map of the Dungash mine area. Inset showing a location map. Notice the occurrence of the ore zone where foliation is inverted peripheral to major folds and shear cleavage is developed.
The Au-quartz lodes are composed chiefly of quartz with subordinate amounts of carbonate. Vein quartz textures exhibit ample features of ductile deformation, e.g., serrated grain boundaries and sub-grain development. Recrystallization of buck and ribboned quartz and intergrowth with sulfide±carbonate are characterize the western ore zone (Fig. 6a,b), whereas fracture opening and sealing of comb quartz by Fe-Mg-carbonate associated with sulfide crystals/aggregates wrapped by muscovite flakes are common features in auriferous orebodies in the eastern ore zone (Fig. 6c-f). Occurrence of sulfide minerals in primary and secondary sites in the vein quartz imply that sulfide mineralization survived during the bulk quartz veining event.
Fig. (4): Rose diagram showing shear/fault trends and related quartz veins at the Dungash mine area, (b) Equal area stereogram (lower hemisphere) of fault planes great circles and slip lineation, slickensides along the vein walls. Notice that the shear foliation and intersection lineation document a dextral shear system, in which Au-lode occur in the footwall block (northern).
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Fig. (6): Petrographic features of the ore bodies in Dungash gold deposit: (a)&(b) Buck and ribboned vein quartz in the western ore bodies cut by shear planes filled with recrystallized quartz and carbonate mixture, (c)&(d) Comb quartz replaced and sealed by carbonate and sulfide intergrowths in quartz-carbonate lodes from the eastern ore zone, (e)&(f) Ore bodies made up mainly of Fe-Mgcarbonate with abundant disseminations of sulfides scattered or along trails. The large scattered grains are wrapped by muscovite flakes.
ORE MINERALOGY AND PARAGENESIS Ore mineralogy of the investigated deposit comprises ubiquitous several generations of pyrite and arsenopyrite and subordinate chalcopyrite, pyrrhotite, gersdorffite, tetrahedrite, gold/electrum and galena (Fig. 7). Marcasite and covellite are late phases replacing pyrite and chalcopyrite, respectively. Ore textures and electron microprobe studies suggest a three-stage paragenetic sequence of the gold-sulfide mineralization in Dungash deposit, including: (i) an early pyrrhotite+pyrite+aresenopyrite +gersdorffite assemblage, (ii) a transitional arsenopyrite+As-pyrite+chalcopyrite+Sb-gersdorffite+ electrum assemblage, and (iii) a late gold+tetrahedrite+galena+ sphalerite+marcasite+covellite assemblage. Electron microprobe studies indicate that refractory Au is confined to Sb-bearing gersdorffite, Sb-Ni-bearing arsenopyrite and arsenian pyrite. Au/Au+Ag (fineness) is higher, ~5, in gold grains associated with the late sulfides compared with those intergrown with those associated with the transitional sulfides (≤4). Mobilization and re-distribution of early refractory gold by low temperature Sb-rich hydrothermal fluids is likely responsible for abundant high fineness gold specks associated with the transitional and late sulfide assemblages, i.e. tetrahedrite and galena. CONCLUDING REMARKS The dextral transpression system hosting/controlling the Au-oreshoots in Dungash mine shares identical structural and kinematic characteristics with the Idfu-Mersa Alam shear system and the Barramiya-Um Salatit ophiolitic belt. Although high gold grades are limited to the arsenopyrite-rich grey quartz lodes (~23ppm Au: 3479ppm As; 492ppm Sb), gold mineralization is rather dispersed in the sulfidized wallrocks containing abundant Fe-Mg-carbonate (up to 6 ppm Au; 354ppm As; 72ppm Sb), comparable with Au-Sb enrichment in the Barramiya orogenic gold deposit (Zoheir and Lehmann, Ore Geology Reviews 39, 2011). Accordingly, circulation of Au-ore fluids throughout a major postaccretionary tectonic/thermal increment is very likely. Based on the geochemical data given in this study, Sb and to a lesser extent As can be used as pathfinders in hydrothermal alteration zones if primary geochemical dispersion is considered for extensive exploration programs in similar shear zones in the district. MICA STAR team is thanked for help during the course of field work. Inquires regarding methodology, reference list and the complete data set or any further explanations are welcome via B.Zoheir e-mail, Tel: (002) 0162792092
