An evaluation of hydrogen sulfide in orogenic gold fluids and the uncertainties associated with vapor‐rich inclusions

Abstract

Raman spectroscopy has been used to obtain the concentration of H2S in individual fluid inclusions in the Archean Missouri gold deposit in Western Australia. These results indicated that the gas phase of Type III, vapor‐rich fluid inclusions from the Missouri gold deposit contained highly variable CO2/CH4 ratios and mole fractions of H2S up to 0.0018. There is good agreement between the calculated and empirically estimated H2S contents at 300°C and 200 MPa. The measured H2S concentrations can be explained by the proposed equilibrium with pyrite in the vicinity of or along the pyrite‐pyrrhotite sulfide buffer. In this study, optical methods were used for estimating the volume fractions of vapor and liquid (where observable) in inclusions at room temperature. To investigate the uncertainty associated with these measurements, where no aqueous phase was observed in an inclusion, we have calculated its bulk composition both without liquid water and also by assuming that there was a 0.5 μm film of liquid water on the walls of the inclusion. Calculations of bulk densities were found to be considerably higher for inclusions with liquid water when compared with vapor‐only inclusions. To investigate the error associated with undetected water in vapor‐rich inclusions, numerical experiments were completed for a number of fluid compositions in the H2O‐CH4‐CO2‐H2S compositional space, both for a regular compositional grid and for a set of specific fluid compositions derived from the visual observations and Raman measurements. Using this approach, the bulk compositions of fluid inclusions were calculated in equilibrium with the pyrite‐pyrrhotite buffer within the temperature‐pressure range of 270 to 360°C and approximately 40 to 260 MPa. The numerical modeling has also shown that variations in calculated H2S concentrations of up to two orders of magnitude may arise due to errors associated with visual estimates of volume fractions in vapor‐rich inclusions. To reduce this source of error, bulk inclusion compositions should be checked by other analytical methods, where possible.

Proceedings of the Montana Mining and Mineral Symposium 2021

“…Calcite-Graphite-Wollastonite Reaction, Orogenic Gold Deposits, and the Source of Water Intersecting both the organic phase reactions and the daphnite to muscovite or kaolinite reactions is an equilibria involving calcite-graphite-wollastonite, shown in the lower right corner of fi gure 8G cutting the pyrite-pyrrhotite phase boundary. This assemblage of pyrite-pyrrhotite with graphite and wollastonite refl ects the interpreted upper limit of orogenic gold deposits (Tomkins and Grundy, 2009) and the computed log f CO 2 /CH 4 vs log f H 2 S coordinates of both the Missouri gold deposit in Western Australia (Mernagh and Bastrakov, 2013) and the mineral assemblage in the "Reduced" porphyry copper-gold deposits described by Rowins (2000), labeled in fi gure 8G as "OG-IRCG." Finally, notice that in the lower right corner of fi gure 8G, a new label is shown in red: "III: Non-magmatic Graphitic Metasediment to Basinal Brine Hydrocarbon Fluid Spectrum.…”

“…At diff erent log fH 2 S values serial cross-sections of log fO 2 vs log a (Fe 2+ /a 2 Cu + ) depicted the three-dimensional volumes of chalcopyrite, bornite, chalcocite, covellite, pyrite, and pyrrhotite in relation to the biotite-potassium feldspar-Fe oxide oxygen fugacity buff er (fi g. 7). Recently Mernagh and Bastrakov (2013) developed the diagram of log f CO 2 /CH 4 vs log fH 2 S coordinates around the magnetite-pyrrhotite-pyrite assemblage, making it possible to study vapor-rich fl uid inclusions using Raman spectroscopy to estimate H 2 S concentrations directly.…”

Orogenic gold: Common or evolving fluid and metal sources through time

“…These included (1) very late to post-peak metamorphic timing, although, in some locations, still under transpressive stresses of a subduction/active margin setting, and with a few controversial Archean deposits that have been argued by some authors to show a prograde metamorphic overprint; (2) located most consistently in a metamorphosed fore-arc or back-arc location ( fig. 1); (3) formation in broad thermal equilibrium with country rocks as indicated by alteration assemblages and equivalent-temperature wallrock assemblages, and a lack of telescoped zonation such as that exhibited by deposits formed under high geothermal gradients; Miller, 1989;Goldfarb et al, 1989;Yardley et al, 1993;Mernagh and Bastrakov, 2013). These estimates are remarkably similar to those from direct measurement by LA-ICPMS of 57-1840 ppm S (approximately 0.006 to 0.18 mole percent) for individual fluid inclusions in barren metamorphic veins from medium-grade metamorphic facies rocks in the central European Alps (Rauchemstein-Martinek et al, 2014).…”

Lithospheric conductors reveal source regions of convergent margin mineral systems

“…Orogenic gold ore fluids are typically volatile-rich and have a reduced redox state (i.e. H 2 S is the dominant sulfur species), with typically 5–20 mol% CO 2 and 0.01–0.36 mol% H 2 S 15 , 49 , 50 . In many cases the fluids are CH 4 -rich, indicating even more reduced fluids 51 .…”