IDENTIFICATION AND IMPLICATIONS OF LIGHT-HYDROCARBON FLUID INCLUSIONSFROM THE PROTEROZOIC BIDJOVAGGE GOLD-COPPER DEPOSIT, FINNMARK, NORWAY

Carbonic fluid inclusions were observed in quartz-bearing veins at the Proterozoic Bidjovagge Au-Cu deposit within the Kautokeino greenstone belt in Norway, where mineralization occurred in oxidation zones of g raphitic schists. A primary fluid inclusion zonation-was observed with CO2-rich fluid inclusions in the structural footwall of the deposit, and CH4-rich inclusions within the ore zone in the oxidation zone. Mic rothermometry of the primary hydrocarbon inclusions revealed 2 groups; (1) a group which homogenized between -125 degrees C and the critical temperature of CH4 (-82.1 degrees C), which indicated the presence of pure CH4, and (2) a group which homogenized between the critical temp erature of CH4 and -42 degrees C, which indicated the presence CH4 and higher hydrocarbons (HHC). Raman microprobe analyses of the first gro up confirmed the presence of CH4. The second inclusion group were fluo rescent, and Raman spectra clearly displayed CH4, C2H6, and rarer C3H8 peaks. A typical feature of the Raman spectra were elevated baselines at the hydrocarbon peaks. Carbon peaks were also usually detected in each inclusion by Raman analysis. Bulk gas chromatography analyses of samples containing the first group (CH4) indicated the presence of CH4 and low concentrations of C2H6 and C3H8. Gas chromatography analyses of samples containing the second group (CH4 and higher hydrocarbons) c onfirmed the presence of CH4, and higher hydrocarbons such as C2H6 and C3H8 and also butanes. Based on the spacial zonation of hydrocarbons and the estimated P-T conditions of 300 to 375 degrees C and 2 to 4 kb ars, the authors suggest an abiotic origin for the hydrocarbons. It is suggested that the hydrothermal fluids oxidized the graphitic schist, precipitated Cu and Au and formed light gas hydrocarbons. Copyright ( C) 1996 Elsevier Science Ltd.