Ore fluid characteristics and depositional processes of the hydrothermal gold mineralisation at Damang, Southwestern Ghana

Abstract

Gold mineralisation at Damang in the southwestern part of Ghana occurs in two types of geological settings: (1) as paleoplacer in the Banket Series conglomerates of the Tarkwaian system and (2) in hydrothermal sheeted quartz vein system that crosscuts all the Tarkwaian lithologies including the palaeoplacer units. This paper focused on the second type because not much research has be conducted on the fluid inclusion and stable isotope characteristics of the Damang deposit to reveal the nature of the hydrothermal ore-forming associated the quartz veins and gold deposition. Microthermometry measurements of fluid inclusions in mineralised quartz veins indicated two sets of temperature-salinity variations: a relatively low temperature (142–292 °C) and moderate salinity aqueous fluids that range from 2.4 to 8.9 wt% NaCl equivalent, and a high temperature (210–393.6 °C) with low salinity aqueous-carbonic fluids with the salinity ranging from 1.0 to 2.6 wt% NaCl equivalent. These fluids are inferred to have unmixed from a homogeneous fluid with an average salinity of 3.85 ± 2.42 wt % NaCl equivalent. Laser Raman spectroscopic data on the fluid inclusions confirmed an aqueous-carbonic (H2O–CO2–NaCl) composition of the fluids with minor amounts of H2S, CH4 and SO2 as volatile species. The temperature of gold precipitation based on overlapped temperatures of chlorite, fluid inclusions and stable isotope geothermometers, indicated a mean temperature of 324.7 ± 20 °C and corresponding pressures from 1.1 to 3.5 kbar. The calculated oxygen isotopic composition of the ore fluids, δ18Ofluid using the estimated mineralisation temperature indicated average values of 9.3 ± 1.2‰. This isotopic range and the low salinity of the fluids (<10 wt % NaCl equiv.) are suggestive of metamorphic origin, which may be attributed to devolatisation reactions in the underlying Birimian greenstones. This is consistent with orogenic lode gold mineralisation in many Precambrian greenstone belts worldwide. The close association of gold with sulphides is consistent with gold transport as bisulphide complexes. A combination of factors including sulphidation and phase separation, concomitant with temperature decrease are inferred to be the principal factors that controlled the hydrothermal gold precipitation at Damang.