Tectonic and chemical evolution of a late Proterozoic gold deposit, Gebeit Mine, Northern Red Sea Hills, Sudan

Starling, Antony (1990) Tectonic and chemical evolution of a late Proterozoic gold deposit, Gebeit Mine, Northern Red Sea Hills, Sudan. University of Southampton, Doctoral Thesis.

Abstract

The Gebeit Gold Mine in the Northern Red Sea Hills of Sudan is a late-Proterozoic mesothermal, shear zone-hosted, gold-quartz vein deposit. It is situated on the western margin of the accreted oceanic island arc terranes which comprise the central area of the Pan-African, Nubian-Arabian Shield. The deposit is hosted by low-K tholeiite to calc-alkaline, basaltic andesites of the Gebeit Volcanic Group. At Gebeit, four main accretion-related deformation events have been recognised:i) D₁, NE-trending regional upright folding,ii) D₂, the main phase of dextral strike-slip shearing, accommodated along regional, NE-trending ductile phyllonite shear zones which subdivided the region into elongate (≈5x30km) tectonic blocks.iii) D_3, a minor phase of NW-directed folding and thrusting, which was followed by renewed dextral strike-slip shearing, and iv) D_4, a switch from dextral to sinistral strike-slip deformation followed by NE-directed thrusting and folding. The ENE-trending quartz veins which host the gold mineralisation were emplaced during late-D_2 dextral shearing and were subsequently folded and fragmented by D_3 and D_4 deformation phases. Together with three phases of Pan-African dyke-emplacement, fluid flow and vein formation were controlled by secondary extensional Riedel shear zones which were preferentially developed across the Gebeit Block. Gold mineralisation was immediately preceded by a phase of pervasive ankerite metasomatism which sealed the main Riedel shears and restricted mineralisation to secondary, parallel, adjacent shear zones. Formation of the V_1 lode veins was pre-dated by emplacement of calc-alkaline, diorite dykes, and post-dated by a transitional phase of calc-alkaline to alkaline dolerite dykes. Six phases of veining are evident but gold is confined either to gold-only fractures or on the surfaces of vein sulphides within the massive V_1, blue quartz veins. Gold has not been observed in association with wallrock sulphides. Wallrock alteration consists of sulphidation (Fe-As-sulphides) and sericitisation together with varying degrees of carbonatisation (calcite), and is characterised by significant additions of K_2O (± Rb, Ba), MgO, Fe_tot, Cr, Ni, As, Au, ± CaO and MnO, with concomitant depletion of Na_2O and Sr. The alteration assemblages, incorporated with fluid inclusion data, indicate that ore fluids at Gebeit comprised H_2O-CO_2 fluids (X_co2 ≈0.1) of low salinity and low Eh, and near neutral pH, at 250-300^oC. Oxygen isotope analyses indicate that fluid compositions remained fairly constant for most of the accretion-related deformation (fluid δ18O estimated at 2.4-8.7%). The fluids were probably metamorphic in origin but may have had a magmatic input. Fluid pressure estimates of 2.0 to 3.9kb (≈7-12km depth) are consistent with evolution at the brittle-ductile transition at greenschist facies grade. Available evidence, although limited, suggests that phase separation was the main gold precipitation mechanism.

This record has no associated files available for download.

Contributors

Download statistics