HIGH CO2 CONTENT OF FLUID INCLUSIONS IN GOLD MINERALIZATIONS IN THE ASHANTI BELT, GHANA - A NEW CATEGORY OF ORE-FORMING FLUIDS

Fluid inclusions were studied in samples from the Ashanti, Konongo-Sou thern Cross, Prestea, Abosso/Damang and Ayanfuri gold deposits in the Ashanti Belt, Ghana. Primary fluid inclusions in quartz from mineralis ed veins of the Ashanti, Prestea, Konongo-Southern Cross, and Abosso/D amang deposits contain almost exclusively volatile species. The primar y setting of the gaseous (i.e. the fluid components CO2, CH4 and N-2) fluid inclusions in clusters and intragranular trails suggests that th ey represent the mineralising fluids. Microthermometric and Raman spec troscopic analyses of the inclusions revealed a CO2 dominated fluid wi th variable contents of N-2 and traces of CH4. Water content of most i nclusions is below the detection limits of the respective methods used . Aqueous inclusions are rare in all samples with the exception of tho se from the granite-hosted Ayanfuri mineralisation. Here inclusions as sociated with the gold mineralisation contain a low (<6 eq.wt.% NaCl) aqueous solution with quantities of CO2. Microthermometric investigati ons revealed densities of the gaseous inclusions of 0.65 to 1.06 g/cm( 3) at Ashanti, 0.85 to 0.98 g/cm(3) at Prestea, up to 1.02 g/cm(3) at Konongo-Southern Cross, and 0.8 to 1.0 g/cm(3) at Abosso/Damang. The f luid inclusion data are used to outline the PT ranges of gold minerali sation of the respective gold deposits. The high density gaseous inclu sions found in the auriferous quartz at Ashanti and Prestea imply rath er high pressure trapping conditions of up to 5.4 kbar. In contrast, m ineralisation at Ayanfuri and Abosso/Damang is inferred to have occurr ed at lower pressures of only up to 2.2 kbar. Mesothermal gold mineral isation is generally regarded to have formed from fluids characterized by H2O > CO2 and low salinity (+/- 6 eq.wt.%NaCl). However, fluid inc lusions in quartz from the gold mineralisations in the Ashanti belt po int to distinctly different fluid compositions. Specifically, the pred ominance of CO2 and CO2 >> H2O have to be emphasized. Fluid systems wi th this unique bulk composition were apparently active over more than 200 km along strike of the Ashanti belt. Fluids rich in CO2 may presen t a hitherto unrecognised new category of ore-forming fluids.