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 Hz0 with significant amounts of CH4 (0.3 to 2.9 mol%), COz (0.1 to 4.7 mol%), and smaller amounts of C2H6, C,Hs, H& SO*, N2, 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 preexisting 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 H2S to account for the total amount of sphalerite precipitated.

Composition and evolution of crustal, geothermal and hydrothermal fluids interpreted using quantitative fluid inclusion gas analysis

“…Fluid inclusion gas data from Mississippi Valley-type (MVT) deposits are reported by Jones and Kesler (1992), Norman et al (1985), and Blamey and Norman (2002). Sphalerite from the Daniel's Harbour deposit in Newfoundland is compared with averages for gas data reported by Jones and Kesler (1992), the Hansonburg gas data reported by Blamey and Norman (2002), and gas data from the Ramah deposit in Labrador (see Fig.…”

“…It is not my aim to discuss the methods in detail for conducting quantitative fluid inclusion gas analysis by mass spectrometry as this is already described in Jones and Kesler (1992), , Norman and Sawkins (1987), and Parry and Blamey (2010). The objective is to present a range of applications based on the interpretation of fluid inclusion gas data, supported by examples generated at the New Mexico Institute of Mining and Technology (NMT) by mass spectrometry.…”

“…There are two main advantages to using Raman spectroscopy; the potential to undertake single inclusion analysis, and the non-destructive nature of the method. The alternative is to analyze fluid inclusion gases by mass spectrometry (Jones and Kesler, 1992;Kesler et al, 1986;Norman and Sawkins, 1987;Parry and Blamey, 2010) or gas chromatography (Andrawes et al, 1984;Bray and Spooner, 1992;Bray et al, 1991;Burruss, 1987;de Ronde et al, 2005). Fluid inclusion volatiles are released under vacuum by the act of crushing or heating, and the volatiles analyzed by a quadrupole mass spectrometer, or in the case of gas chromatography are cryogenically trapped and speciated by slowly raising the temperature.…”

Quantification of mixing processes in ore-forming hydrothermal systems by combination of stable isotope and fluid inclusion analyses

“…As a consequence, fluid mixing is considered a significant precipitation mechanism involved in the formation of hydrothermal ore deposits. Many examples of Mississippi Valley-type deposits are believed to be produced by the mixing of at least two fluids (e.g., Anderson, 1975;Ohmoto and Rye, 1979; Barret and Anderson, 1982) or a metalbearing formation water with H 2 S gas (e.g., Haynes and Kesler, 1987;Jones and Kesler, 1992). Mixing between basement brines and meteoric waters accounts for the formation of many vein-type Pb-Zn mineralizations, for example within the European Variscides (e.g., Lü ders and Mö ller, 1992;Lü ders and Ebneth, 1993;Zheng and Hoefs, 1993a,b;Lü ders, 1994;Ritter, 1995;Werner et al, 2000Werner et al, , 2002.…”

Fluids and Ore Formation in the Earth's Crust

“…Based on thermodynamic equilibrium calculations (Sverjensky, 1986;Tornos and Heinrich, 2008;Hennings et al, 2012), we speculate that sediment-hosted Pb-Zn-deposits are formed from deeply circulating fluids buffered by pyrite -magnetite assemblages in basement rocks, which creates mildly reduced brines with variable excess of Pb and Zn compared to their overall low sulfur content. As a result, Pb-Zn deposition in overlying sedimentary basins is dependent on a local sulfur source at the site of ore deposition, such as a H2S-rich gas trap (Jones and Kesler, 1992), or the biogenic or thermogenic reduction of sulfate (Wilkinson et al, 2005;Anderson, 2008;Thom and Anderson, 2008). By contrast, a decisive ingredient for Cu-(-Ag-Co) ore formation may be a fluid that originates more directly from the oxygenated surface environment and maintains mildly oxidizing redox conditions, allowing effective Cu(+I) transport even at low temperatures (Brown, this volume, fig.…”