Fluid inclusion gas chemistry in east Tennessee Mississippi Valley-type districts: Evidence for immiscibility and implications for depositional mechanisms

Abstract

Analyses of fluid inclusion gases from Mississippi Valley-type districts in east Tennessee reveal the presence of several distinct aqueous solutions and vapors that were part of the mineralizing process. Inclusion contents were released by crushing 5 to 25 mg mineral samples and by decrepitating individual inclusions; all analyses were obtained by quadrupole mass spectrometry. Most analyzed inclusion fluids consist of H 2O with significant amounts of CH 4 (0.3 to 2.9 mol%), CO 2 (0.1 to 4.7 mol%), and smaller amounts of C 2H 6, C 3H 8, H 2S, SO 2, N 2, and Ar. In general, inclusion gas abundances are greatest for sphalerite from the Mascot-Jefferson City district, lower for the Sweetwater district, and lowest for the Copper Ridge district. Compositional similarities in the inclusion fluids from the three districts imply that mineralization probably formed from fluids that permeated the entire region, rather than from completely separate fluids at each site. Saturation pressures calculated for these fluid compositions range from 300 to 2200 bars. Burial depths for the host unit have been estimated to be about 2 to 3 km in the east Tennessee area during Devonian time, the age of mineralization indicated by recent isotopic ages. Pressures at these depths, whether hydrostatic or lithostatic, would not have been adequate to prevent phase separation. Thus, our gas analyses represent either a mixture of vapor-rich and liquid-rich inclusions, or liquid-rich inclusions that trapped excess vapor. A lack of visible vapor-rich inclusions, high gas contents in individual fluid inclusion gas analyses obtained by decrepitation, and a positive correlation between decrepitation temperature and gas content for individual inclusions strongly suggest that the samples contain liquid-rich inclusions that trapped varying amounts of excess vapor. This excess gas probably accounts for the anomalously high homogenization temperatures and the wide range of homogenization temperatures observed in fluid inclusions in these ores. The vapor phase could have formed either by phase separation resulting from over-pressured aqueous fluids migrating into a region of hydrostatic pressure, or by incorporation of a pre-existing gas cap at the sites of deposition into the invading aqueous fluid. Exsolution of a vapor phase from the mineralizing brines should cause precipitation of carbonate and sulfide minerals, but reaction path modelling indicates that the resulting sparry dolomite:sphalerite ratios would be too high to form an ore-grade deposit. On the other hand, if the vapor phase was from a pre-existing sour gas cap that was intercepted by a Zn-rich brine, large amounts of sphalerite would precipitate in a fairly small region. Preliminary mass balance calculations suggest that a gas cap of dimensions similar to the individual districts in east Tennessee could have contained enough H 2S to account for the total amount of sphalerite precipitated.