High CO2 content of fluid inclusions in gold mineralisations in the Ashanti Belt, Ghana: a new category of ore forming fluids?
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Fluid inclusions were studied in samples from the Ashanti, Konongo-Southern Cross, Prestea, Abosso/Damang and Ayanfuri gold deposits in the Ashanti Belt, Ghana. Primary fluid inclusions in quartz from mineralised veins of the Ashanti, Prestea, Konongo-Southern Cross, and Abosso/Damang deposits contain almost exclusively volatile species. The primary setting of the gaseous (i.e. the fluid components CO2, CH4 and N2) fluid inclusions in clusters and intragranular trails suggests that they represent the mineralising fluids. Microthermometric and Raman spectroscopic analyses of the inclusions revealed a CO2 dominated fluid with variable contents of N2 and traces of CH4. Water content of most inclusions is below the detection limits of the respective methods used. Aqueous inclusions are rare in all samples with the exception of those from the granite-hosted Ayanfuri mineralisation. Here inclusions associated with the gold mineralisation contain a low salinity (<6 eq.wt.% NaCl) aqueous solution with variable quantities of CO2. Microthermometric investigations revealed densities of the gaseous inclusions of 0.65 to 1.06 g/cm3 at Ashanti, 0.85 to 0.98 g/cm3 at Prestea, up to 1.02 g/cm3 at Konongo-Southern Cross, and 0.8 to 1.0 g/cm3 at Abosso/Damang. The fluid inclusion data are used to outline the PT ranges of gold mineralisation of the respective gold deposits. The high density gaseous inclusions found in the auriferous quartz at Ashanti and Prestea imply rather high pressure trapping conditions of up to 5.4 kbar. In contrast, mineralisation at Ayanfuri and Abosso/Damang is inferred to have occurred at lower pressures of only up to 2.2 kbar. Mesothermal gold mineralisation is generally regarded to have formed from fluids characterized by H2O > CO2 and low salinity ( ± 6 eq.wt.%NaCl). However, fluid inclusions in quartz from the gold mineralisations in the Ashanti belt point to distinctly different fluid compositions. Specifically, the predominance of CO2 and CO2 >> H2O have to be emphasized. Fluid systems with this unique bulk composition were apparently active over more than 200␣km along strike of the Ashanti belt. Fluids rich in CO2 may present a hitherto unrecognised new category of ore-forming fluids.
KeywordsFluid Inclusion Gold Deposit Gold Mineralization Raman Spectroscopic Analysis Fluid Component
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