Constraints on the timing of late-Eburnean metamorphism, gold mineralisation and regional exhumation at Damang mine, Ghana

Abstract

The Damang gold deposit in southwest Ghana is unique among known deposits in Ghana, comprising gold mineralisation in two distinct styles. Early gold, hosted in a stratigraphically controlled, auriferous quartz-pebble conglomerate horizon, is overprinted by later gold mineralisation contained in a sub-horizontal fault-fracture quartz vein array. A multi-system geochronological study is used to constrain the timing of igneous activity, regional metamorphism, gold mineralisation and the thermal history at Damang. U/Pb analysis of zircons from Birimian volcaniclastic and intrusive rocks constrain volcanism and associated intrusive activity at 2178.0 ± 9.3 Ma and 2164.6 ± 8.0 Ma respectively, which is consistent with previous studies. The age of formation of staurolite-grade, amphibolite facies peak metamorphic mineral assemblages at 2005 ± 26 Ma is provided by U–Th–total Pb EPMA analysis of metamorphic monazite grains in the Tarkwa Phyllite. Measured 40Ar/39Ar biotite ages range between 1980 ± 9 Ma and 1898 ± 11 Ma. Argon diffusion modelling with the programme DIFFARG suggests that this age range could be achieved by a period of rapid cooling, at a rate of approximately 17 °C/Ma, followed by a prolonged period of much slower cooling, at a rate of 0.15 °C/Ma. The period of rapid cooling is interpreted to represent localised exhumation of the Damang host rocks during the latest stage of the Eburnean orogeny at the time of hydrothermal gold mineralisation. Given these age constraints, hydrothermal gold mineralisation is inferred to have occurred between approximately 2030 Ma and 1980 Ma. These ages constrain metamorphism, fluid flow and gold mineralisation at Damang and are the youngest currently recognised in the Birimian of SW Ghana.