The geology and physico-chemical characteristics of the hydrothermal gold mineralisation at the Damang gold deposit, Ghana.

Abstract

The Damang gold deposit is located in the Tarkwaian within the structural basin of the Ashanti belt of Ghana. The Damang deposit features an anticline with its western limb displaced downward to the south by a major fault and the eastern limb that predominantly host most of the gold. The anticline plunges 15°- 33° NNE. The beddings of the host sediments dip from about 30° to 70° to the east and have a strike length of about 3km and extends 100m. The deposit is hosted in the Tarkwaian rocks where an earlier conglomerate paleoplacer gold in the Banket Series is over-printed by a later hydrothermal gold. The hydrothermal gold exhibits a post-peak metamorphosed and structurally controlled brittle-ductile deformation such as brecciation, fractures and micro-folds observed in the field and in diamond cores. Deformation effect is demonstrated by subgrain textures observed in quartz grains in thin sections. The mineralisation is associated with sub-horizontal quartz – carbonate veins which cut across all the lithologies in the Damang area, and hence, epigenetic.

Two main dolerite dykes and other numerous minor dykes and sills traverse the deposit. These intrusives are inferred to be associated with calc-alkaline tholeiitic basalt magma in an Island Arc tectonic setting, whiles the sediments are derived from recycled Orogeny provenance.

Chemical signatures and mass balance transfer calculations show that major elements associated with the hydrothermal alteration involve the enrichments of Au, Ag, Ba, Rb, MgO, CaO, and significant depletion of SiO2 and minor depletion of Na2O.

Important physico-chemical characteristics associated with the mineralisation such as temperature, trapping pressure, fluid salinity and composition, mechanism of gold transport and deposition, and source of the ore fluids have been determined by this study. The mineralising ore fluid is an aqueous-carbonic fluid with other constituents of H2S, SO2 and minor amounts of CH4. Little or no N2 was detected by Raman spectroscopy and this is contrary to previous literatures on hydrothermal gold in other parts of the Birimian, which reported the presence of N2 gas as constituent of the ore fluid. The CH4 content may be attributed to localised hydrolysis reaction between carbonaceous sediments and the ore fluid. Two sets of salinities were recorded: a low salinity of 1.0 to 2.6 wt % NaCl equiv. and intermediate salinity of 2.4 to 8.9 wt % equiv. of NaCl, which may be attributed to fluid immiscibility from a homogeneous magmatic-metamorphic fluid. Independent temperature estimates from chlorite and Ti-in-biotite geothermometries indicated a temperature range of 200 ºC to 500 ºC as the possible temperature for the hydrothermal gold emplacement, and fluid inclusion gave a temperature range of 250 ºC to 400 ºC and pressure range of 0.70 – 2.00 kbar based on fluid inclusion isochore calcualations using bulk density of 0.70 - 0.89 g/cm3 from CO2 homogenisation temperatures and salinities of aqueous inclusions.

The close association of gold with sulphides (especially pyrite and pyrrhotite) and the low salinities (≤ 10 wt % NaCl equivalent) of the mineralising fluids suggest that the gold was transported in solution as bisulphide complexe (Au (HS)2 – ) under reducing condition.

Stable isotope constraints for the mineralising quartz vein indicate the following range for the hydrothermal fluid; 18 Ofluid values range from 9.1 to 10.3 ‰, which is consistent with fluid derived from regional metamorphic devolatisation (metamorphic fluids) and/or magmatic hydrothermal fluids. However, the low salinity precludes a magmatic source. The range in C for the non-mineralised carbonate and the mineralised carbonates is -2.0 to -1.7‰ and -5.9 to -5.5‰, respectively.

Considering the association of gold with sulphides in the host rocks, and the nature of the mineralising fluid, it can be concluded that the Damang epigenetic gold was introduced by metamorphic fluids of low salinity under reducing conditions generated by devolatilisation reactions in the rocks resulting in the dissolution and transportion of gold as Au-sulphide complexes under favourable conditions. The principal control for gold precipitation is attributed to phase separation concomitant with temperature decrease during episodic pressure release associated with hydraulic fracturing. Phase separation of the hydrothermal fluid would also have led to 1oss of dissolved H2S and CO2 to the vapour phase in concomitant with temperature decrease during hydrofacturing. The loss of H2S was probably responsible for the deposition of gold, as changes in the physico-cehmical conditions influence solubility of the gold complexes.