Identification and implications of fight hydrocarbon fluid inclusions from the Proterozoic Bidjovagge gold-copper deposit, Finnmark, Norway

Abstract

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 graphitic schists. A primary fluid inclusion zonation was observed with C0 2-rich fluid inclusions in the structural footwall of the deposit, and CH 4-rich inclusions within the ore zone in the oxidation zone. Microthermometry of the primary hydrocarbon inclusions revealed 2 groups; (1) a group which homogenized between −125°C and the critical temperature of CH 4 (−82.1°C), which indicated the presence of pure CH 4, and (2) a group which homogenized between the critical temperature of CH 4 and −42°C, which indicated the presence CH 4 and higher hydrocarbons (HHC). Raman microprobe analyses of the first group confirmed the presence of CH 4. The second inclusion group were fluorescent, and Raman spectra clearly displayed CH 4,C 2H 6, and rarer C 3H 8 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 (CH 4) indicated the presence of CH 4 and low concentrations of C 2H 6 and C 3H 8. Gas chromatography analyses of samples containing the second group (CH 4 and higher hydrocarbons) confirmed the presence of CH 4, and higher hydrocarbons such as C 2H 6 and C 3H 8 and also butanes. Based on the spacial zonation of hydrocarbons and the estimated P T conditions of 300 to 375°C and 2 to 4 kbars, 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.