Geochemistry and mineral chemistry of lode gold mineralisation, SE Egypt: implications for ore genesis and exploration
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ORIGINAL PAPER
Geochemistry and mineral chemistry of lode gold mineralisation, SE Egypt: implications for ore genesis and exploration Basem A. Zoheir & Mohamed G. Abdel-Fattah & Sameh M. ElAlfy
Received: 14 July 2012 / Accepted: 16 October 2012 # Saudi Society for Geosciences 2012
Abstract Orogenic, lode gold mineralisation in the South Eastern Desert of Egypt is related to quartz veins spatially and temporally associated with conjugate NW- and NEtrending brittle–ductile shear zones. These structures are assumed to be linked to a regional transpression deformation which occurred late in the tectonic evolution of the area. In the Betam deposit, gold is confined to quartz(±carbonate) veins cutting through tectonised metagabbro and metasedimentary rocks in the vicinity of small granite intrusions. The ore bodies contain ubiquitous pyrite and arsenopyrite, in addition to minor disseminated chalcopyrite, pyrrhotite, galena, tetrahedrite and rare gold/electrum. New ore microscopy and electron microprobe studies indicate that most free-milling Au is intimately associated with the lateparagenetic galena–tetrahedrite–chalcopyrite assemblage. An early Fe–As sulphide assemblage, however, shows minor traces of refractory gold. New mineralogical and geochemical data are used to better constrain on possible element dispersions for exploration uses. This study indicates that parameters that most consistently define primary dispersion of gold in the mine area include pervasive silicification, sericite and carbonate alteration. The trace element data of gold lodes reflect a systematic dispersion of gold and certain base metals. Low-cost, extensive exploration programs may use elevated concentrations of Ag, Sb, Cu and Pb as tracers for Au ore zones in the Betam mine area and surroundings. B. A. Zoheir (*) Department of Geology, Faculty of Science, Benha University, 13518, Benha, Egypt e-mail: [email protected] M. G. Abdel-Fattah : S. M. ElAlfy Nuclear Materials Authority of Egypt, P.O. Box: 530, El-Maadi, Cairo, Egypt
Keywords Lode gold . Geochemistry . Electron microprobe . Element dispersion . Betam deposit . Egypt
Introduction A clear association has been recognised between lode gold deposits and accretionary orogens, where several gold deposits are related to a broadly consistent tectonomagmatic setting within the evolving orogen (e.g. Robert et al. 1997; Hagemann and Brown 2000; Poulsen et al. 2000; Groves et al. 2003; Robert 2004; Goldfarb et al. 2005; Miller et al. 2006; Bierlein et al. 2009; Hronsky et al. 2012). Several important orogenic, lode gold ores occur either within or close to large-scale compressional or transpressional structures and crustal discontinuities (e.g. Hodgson 1993; Robert 1996; Phillips et al. 1996; Groves et al. 1998; Sillitoe 2000; Haynes 2002). In the crustal continuum model of Groves (1993), gold-bearing quartz veins form over a broad range of upper to mid-crustal pressures and temperatures (200–650 °C and 1–5 kbar). The formation of orogenic gold deposits over a metamorphic continuum is
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