Abstract

The laser Raman microprobe has been used effectively in the identification of polyatomic gases, complex ions, and daughter crystals in fluid inclusions in a variety of host rocks (e.g., Dhamelincourt et al., 1979a; Rosasco and Roedder, 1979). This is a generally nondestructive, in situ, semiquantitative analytical technique in which a monochromatic laser beam is focused into a sample. The spectrum resulting from the inelastically scattered radiation indicates the chemical speciation of the irradiated sample. Although laser Raman microprobe analysis is not very effective for brines composed of simple salts, the nonelectrolyte components of hydrothermal fluid inclusions can be readily analyzed. Raman spectroscopy also provides important information, on a microscale, for solid phases in the assemblage. Fluid inclusions in quartz and calcite from several types of samples from the disseminated gold deposit at Carlin, Nevada, were studied petrographically and analyzed by microthermometry and Raman spectroscopy. The laser Raman microprobe was used to scan for CO 2 , CO, N 2 , H 2 S, and CH 4 + or - H 2 O. The semiquantitative microprobe analyses confirmed the presence of three-phase inclusions containing water and different amounts of CO 2 , one-phase inclusions of high-density CH 4 -dominated fluid, and very weak aqueous brines. Very minor H 2 S was detected in a CO 2 -H 2 O inclusion and small amounts of CO 2 were detected in some of the CH 4 -dominated inclusions. These gases may have affected ore deposition. In any event, their identification is necessary for correct estimations of pressure to be derived from homogenization temperatures of the fluid inclusions. The laser Raman microprobe provides more than compositional data. For instance, the exact spectral position and shape of the CH 4 peak indicate the internal pressure, even in inclusions too small (<3 mu m) to be analyzed by microthermometry. The laser Raman microprobe was also used to help characterize the structure of the carbonaceous material in the jasperoid host rock at Carlin (disordered graphite, in part) and to distinguish between the TiO 2 polymorphs anatase and brookite (both present) in hydrothermal quartz deposits at Magnet Cove, Arkansas. The ability of the laser Raman microprobe to provide pinpoint analyses has many advantages over bulk extraction methods, which mix the volatiles from separate generations of fluids and may also produce chemical artifacts.

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