Origin of mineralising fluids of the sediment-hosted Navachab gold mine, Namibia: constraints from stable (O, H, C, S) isotopes.

The Navachab mine is an open-pit operation with an average Au grade of 1.6-2.0 g/t, and has an estimated total Au production of 1 200 000 oz. The deposit is characterised by a polymetallic Au-Bi-As-Cu-Ag ore assemblage, including pyrrhotite, chalcopyrite, sphalerite, arsenopyrite, Bi, Au, bismuthinite and bismuth tellurides. Au is hosted by quartz sulphide veins and semi-massive sulphide lenses which are developed in a near-vertical sequence of shelf-type metasedimentary rocks including marble, calc-silicate rock and biotite schist. The sequence has been intruded by abundant syntectonic lamprophyre, aplite and pegmatite dykes, indicating widespread igneous activity coeval with mineralisation. Au is interpreted to have precipitated in equilibrium with metamorphic fluid at peak metamorphic conditions of about 550 degrees C and 2 kbars, consistent with isotopic fractionations between coexisting calcite, garnet and clinopyroxene in the alteration halos. The most likely source of the mineralising fluid was a mid-crustal fluid in equilibrium with the Damaran metapelites that underwent prograde metamorphism at amphibolite- to granulite-facies grades. Magmatic fluids may have been important in contributing to the overall hydraulic regime and high apparent geothermal gradients in the mine area.
The Navachab mine is an open-pit operation with an average Au grade of 1.6-2.0 g/t, and has an estimated total Au production of 1 200 000 oz. The deposit is characterised by a polymetallic Au-Bi-As-Cu-Ag ore assemblage, including pyrrhotite, chalcopyrite, sphalerite, arsenopyrite, Bi, Au, bismuthinite and bismuth tellurides. Au is hosted by quartz sulphide veins and semi-massive sulphide lenses which are developed in a near-vertical sequence of shelf-type metasedimentary rocks including marble, calc-silicate rock and biotite schist. The sequence has been intruded by abundant syntectonic lamprophyre, aplite and pegmatite dykes, indicating widespread igneous activity coeval with mineralisation. Au is interpreted to have precipitated in equilibrium with metamorphic fluid at peak metamorphic conditions of about 550 degrees C and 2 kbars, consistent with isotopic fractionations between coexisting calcite, garnet and clinopyroxene in the alteration halos. The most likely source of the mineralising fluid was a mid-crustal fluid in equilibrium with the Damaran metapelites that underwent prograde metamorphism at amphibolite- to granulite-facies grades. Magmatic fluids may have been important in contributing to the overall hydraulic regime and high apparent geothermal gradients in the mine area.