|
Damien Gaboury, Université du Québec à Chicoutimi (Canada)
Moussa Keita, Université du Québec à Chicoutimi (Canada)
Jayanta Guha, Université du Québec à Chicoutimi (Canada)
Huan-Zhang Lu, Université du Québec à Chicoutimi (Canada)
|
|
The preliminary results from an apparatus under development are presented here, as well as the technical specifications of the system. The technique involves direct analysis by mass spectrometry of gases released by progressive and controlled heating under vacuum of decrepitating fluid inclusions from a small solid sample.
The Bubble Blaster system is composed of 5 main components:
1) an oil free Pfeiffer Vacuum Turbo pumping station; 2) a heating and sample holder probe; 3) a programmable temperature controller; 4) a MicropoleTM mass spectrometer; and 5) a vacuum-meter. All these components are fully integrated and computer-controlled.
Technical specifications are:
1) mass resolution of 0.8 AMU over the range of 2 to 65 AMU with a scan speed of 0.6 AMU/s;
2) detection limit of 1 ppm or gas at partial pressure of 1E10-10 mbar;
3) programmable rate controlled heating up to 600°C;
4) internal pressure of about 1E10-6 mbar during analysis.
Small samples between 1 to 10 mg of hydrothermal minerals are used as material for fluid inclusion analysis. As the sample is heated under vacuum, the fluid inclusions decrepitate due to internal overpressure. Volatiles released under vacuum are directly analyzed by the mass spectrometer as AMU abundance measured as partial pressure of N2 equivalent or ppm. The following gases can be analyzed: CO2, N2, CO, CH4, SO2, H2S, H2O, NH3, Ar, and some light hydrocarbons of the type CnH2n+2, such as C2H6, C3H8 and C4H10. The data are presented graphically as gas abundance relative to decrepitation temperature. Such graphs permit the definition of families of fluid inclusions with similar composition and decrepitation temperature. Using a quartz sample from the Huaqiao gold mine in China as example, where the gold mineralization occurs as bedding parallel gold-bearing quartz veins developed in the fold hinges of low grade metamorphic clastic sedimentary rocks of Proterozoic age, three families of fluid inclusions were characterized. The first is a family of aqueous (H2O-rich) fluid inclusions with decrepitation from 125 to 300°C and a peak at 200°C.
The second is dominated by H2O with a lower content of N2, CO2 and CH4 and a decrepitation peak at 400°C. The third family started its decrepitation at 420°C up to 450 °C. This familly is different from the second, by being less rich in H2O, although it is the dominant gas, and richer in N2, with about the same content of CH4 and CO2 and enrichment of C4H10. These results are in agreement with petrographic, microthermometric and Raman spectroscopy data previously obtained from this sample. Applications of such a versatile technique are numerous in ore formation research as well as for mineral exploration, since information is gained from generation of inclusions rather than a single inclusion. Finally, the technique has the potential to analyze very small and dilute inclusions that are impossible to analyze by any other method (e.g., LA-ICP-MS).
|
