Possibilities and limits of infrared light microthermometry applied to studies of pyrite-hosted fluid inclusions
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文摘
Pyrites from various occurrences and of different origin have been examined for fluid inclusions using an infrared microscope. Additionally, Fourier transform infrared spectroscopy was performed in order to characterize the infrared transmittance of individual samples in the spectral range of the IR TV camera (λ≤2.5 μm) used. Studied samples of sedimentary pyrite are always opaque for near infrared light whereas pyrite from metamorphic rocks, hydrothermal ore deposits and pegmatites shows infrared transmittance of varying degree. Pyrite-hosted fluid inclusions are often opaque and only a few samples contain fluid inclusions which are suitable for microthermometric investigations. Ice melting temperatures of fluid inclusions in pyrite from three different occurrences were measured by direct observation on a monitor screen. When heating, the infrared transparency of pyrite decreases with increasing temperature due to shifting of the absorption edge to higher wavelengths. Therefore, most of the pyrites became opaque prior to reaching L–V homogenization. The only combined salinity and homogenization temperature data for fluid inclusions were obtained from pyrite samples from an argillic altered shear zone within the Murgul Cu deposit/NE Turkey. These pyrite crystals show up to 40 % transmittance for infrared radiation in the wavelength range λ=0.8 to 2.5 μm. Homogenization temperatures (Th) were measured visually up to temperatures of about 250°C. At higher temperatures the pyrite samples became completely opaque and Th values of fluid inclusions were obtained by a cycling technique. The data obtained from fluid inclusions as well as observed small fractionation of sulfur isotopic composition indicate that pyrite crystals with different morphologies (octahedrons, pentagondodecahedrons and combinations of both) from Murgul were deposited at different temperatures.