HR: 11:15h
AN: T22A-04    [Abstracts]
TI: Modeling methane production by iron-bearing carbonate minerals in hydrothermal systems
AU: * Park, J
EM: jhpark16@nmt.edu
AF: New Mexico Institute of Mining and Technology, Department of Earth and Environmental Sciences 801 Leroy Place, Socorro, NM 87801 United States
AU: Norman, D
EM: dnorman@nmt.edu
AF: New Mexico Institute of Mining and Technology, Department of Earth and Environmental Sciences 801 Leroy Place, Socorro, NM 87801 United States
AB: We measured methane and other major volatiles in fluid inclusions from gold deposits such as Lone Tree, Getchell, Twin Creeks, and Pipeline Carlin-type gold deposits in Nevada by bulk analysis using quadrupole mass spectrometry. Ore-stage fluids are characterized by CO₂/CH₄ ratios that typically are < 10. Fluid inclusion methane concentrations generally are between 0.1 and 0.5 mol.%; other gaseous species show much wider ranges in composition. Also we commonly measure CO₂/CH₄ ratios of 10 or less in geothermal fluid inclusions. Similar ratios are reported in some black smokers. Few geothermal systems in production have CH4 in other than trace amounts. Giggenbach (1997) demonstrates that CO₂/CH₄ ratio in geothermal fluids in equilibrium with granite should vary positively with temperature, but we see no such variation and some fluid inclusion CO₂/CH₄ in granite-hosted inclusions ratios are an order of magnitude higher than calculated by Giggenbach (1997). This raises the question about the fluid inclusion gas measurements. We have looked at explanations for measurement of fluid inclusion methane that include preferential trapping of hydrocarbon compounds, concentration of methane by boiling, and contamination, but none can explain in all cases the methane concentrations measured. Hence we have modeled geothermal fluid rock reactions using Geochemists Workbench to demonstrate that methane can be a result of fluid-rock reactions. For gold ore wall rocks, we assume that wall rock includes iron-bearing carbonate minerals, epidote, and biotite. For geothermal systems, we assumed fluids with 1 mol.% CO₂ and salinity of 1 %. We modeled the reaction of iron-bearing carbonate minerals, epidote, and biotite with the fluid between 100 and 300 °C. The reaction released ferrous iron, and the released ferrous iron is oxidized and precipitated as hematite or magnetite. The precipitation of hematite or magnetite produced acid and the acid dissolved calcite increasing CO₂ concentration. The oxidation of ferrous iron produced hydrogen gas and the produced hydrogen reacted with CO₂ producing methane. The equilibrium between magnetite and other ferrous-iron bearing minerals controlled the hydrogen fugacity and the hydrogen fugacity controlled the ratio of CO₂ and methane. When the hydrogen fugacity was controlled by the equilibrium of magnetite and iron-bearing calcite, the modeled CO₂/CH₄ was about 10 and the ratio did not significantly vary between 100 and 300 °C. The reaction occurred over a wide range of pH conditions. At a low pH, the modeled ferrous iron concentration in the fluid was higher than that of higher pH. The modeled result suggests that the methane trapped in fluid inclusion can be produced by water-rock interaction in hydrothermal systems.
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
DE: 3616 Hydrothermal systems (0450, 1034, 3017, 4832, 8135, 8424)
DE: 8424 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8135)
SC: Tectonophysics [T]
MN: Fall Meeting 2005