Paul B. Barton

A thermodynamic study of pyrite and pyrrhotite

Abstract Through the use of the electrum-tarnish method the following equation has been found to interrelate the composition of pyrrhotite, fugacity of sulfur, and temperature: In this equation fs2 is the fugacity of sulfur relative to the ideal diatomic gas at 1 atm, N is the mol fraction of FeS in pyrrhotite (in the system FeS-S2), and T the absolute temperature. The experimental uncertainty in the equation is 0–003 in N. The activity of FeS (aFeS) in pyrrhotite relative to the pure substance at the temperature of consideration follows from the above equation by virtue of the Gibbs-Duhem relation; it is given by: The electrum-tarnish method has permitted us to determine the fs2 vs. T curve for the univariant assemblage pyrrhotite-pyrite-vapor from 743 to 325°C. Our determinations of the composition of pyrrhotite are in excellent agreement with the results of Arnold. The activity of FeS in pyrite-saturated pyrrhotite is very different from unity, a fact that greatly influences the interpretation of some other phase equilibrium studies involving pyrrhotite and their application to sulfide mineral assemblages, but has little effect on the more general calculations of composition of hydrothermal or magmatic fluids. Pressure effects calculated from available volumetric data on the phases are small.

Thermochemical study of the system Fe-As-S

Abstract The results of Toulmin and Barton (1964) for the Fe-S system have been combined with a series of new measurements on As-bearing assemblages in the 500°–850°C temperature range to derive data on the free energies, enthalpies, and entropies of formation for arsenopyrite, loellingite, orpiment, realgar, FeAs, and Fe 2 As. The enthalpies and free energies of formation of orpiment and realgar are only approximately one-half as large as indicated in recent compilations of thermochemical data ( Wagman et al ., 1965). Data are also presented for the covariation of activity of S 2 (g) with temperature and composition of the sulfur-arsenic liquid.

The electrum-tarnish method for the determination of the fugacity of sulfur in laboratory sulfide systems

A new method for the determination of the fugacity of sulfur in laboratory systems consists of visual observation of the development and decomposition of a sulfide tarnish phase on silver-gold alloy (electrum) of precisely known composition. The alloy system is calibrated against pure sulfur. The method has the following advantages: simple apparatus; ability to cover a large range of fugacity of S2; ability to cover a large temperature range by permitting runs of long duration; ability to tolerate other components in the gas phase; and ease of recovery of the quenched charges for determinations of phases and compositions. Results obtained by the electrum-tarnish method are in satisfactory agreement with those obtained by other workers for the fs2 vs. T curves for the assemblage Ni(1–x)S + NiS2. The electrum-tarnish method shows promise for investigating many other reactions. Univariant reactions studied by this method can be represented as lines forming a genetic grid in terms of the environmental parameters fs2 and T, The slopes of such lines can yield valuable thermodynamic data for the phases involved, but activity coefficients must be known for phases of variable composition.

CALCULATION OF THE VAPOR-SATURATED LIQUIDUS FOR THE NACL-CO2-H2O SYSTEM

Abstract The polybaric liquidus surface for the H2O-rich corner of the NaCl-CO2-H2O ternary is calculated, relying heavily on 1. (1) a Henrys law equation for CO2 in brines (modified from Drummond, 1981), 2. (2) the assumption that the contributions of dissolved NaCl and CO2 in lowering the activity of H2O are additive, and 3. (3) data on the CO2 clathrate solid solution (nominally CO2 · 7.3H2O, but ranging from 5.75 to 8 or 9 H2O) from Bozzo et al. (1975). The variation with composition of the activity of CO2·7.3H2O, or any other composition within the clathrate field, is small, thereby simplifying the calculations appreciably. Ternary invariant points are 1. (1) ternary eutectic at −21.5°C, with ice + clathrate + hydrohalite NaCl-·H2O + brine mNaCl = 5.15, mco2 = 0.22 + vaporPtotal ≈ Pco2 = 5.7 atm; 2. (2) peritectic at −9.6°C, with clathrate + hydrohalite + liquid CO2 + brine mNaCl = 5.18, mco2 = 0.55 + vapor (Ptotal ≈ Pco2 = 26.47 atm); and 3. (3) peritectic slightly below +0.1 °C, with halite + hydrohalite + liquid CO2 + brine (mNaCl ≈ 5.5, mco2 ≈ 0.64) + vapor (Ptotal ≈ Pco2 ≈ 34 atm). CO2 isobars have been contoured on the ternary liquidus and also on the 25°C isotherm. An important caveat regarding the application of this information to the interpretation of the freezing-thawing behavior of fluid inclusions is that metastable behavior is a common characteristic of the clathrate.

Low-temperature heat capacity and entropy of chalcopyrite (CuFeS2): estimates of the standard molar enthalpy and Gibbs free energy of formation of chalcopyrite and bornite (Cu5FeS4)

Abstract The heat capacity of CuFeS2 (chalcopyrite) was measured between 6.3 and 303.5 K. At 298.15 K, Cp,mo and Smo(T) are (95.67±0.14) J·K−1·mol−1 and (124.9±0.2) J·K−1·mol−1, respectively. From a consideration of the results of two sets of equilibrium measurements we conclude that ΔfHmo(CuFeS2, cr, 298.15 K) = −(193.6±1.6) kJ·mol−1 and that the recent bomb-calorimetric determination by Johnson and Steele (J. Chem. Thermodynamics 1981, 13, 991) is in error. The standard molar Gibbs free energy of formation of bornite (Cu5FeS4) is −(444.9±2.1) kJ·mol−1 at 748 K.

Evolution of the Creede caldera and its relation to mineralization in the Creede mining district, Colorado

At 25 Ma a major epithermal silver and base metal deposit formed in rhyolitic welded tuff near Creede, Colorado. Nearly 2400 metric tons of silver, appreciable lead, and small amounts of zinc, copper, and gold, have been produced from large, crustified veins under Bachelor and Bulldog Mountains north and northwest of Creede. Prior geologic, hydrologic, and stable-isotope studies showed that ore deposition was associated with the mixing and boiling of waters from diverse sources and suggested that a critical part of the ore-forming fluid may have originated within the ancient lake and sediments of the lacustrine Creede Formation that filled the Creede caldera. Two drill holes that sampled the heretofore hidden lower half of the Creede Formation are the focus of this book. The Creede caldera formed at 26.9 Ma within a high constructional plateau of silicic ashflows that covered and were sporadically interlayered with, intermediate lavas and lahars from large stratovolcanoes. The Creede caldera lake had an inflow evaporation balance that did not permit rapid filling to create a brim-full deep lake. Thus salts were evaporatively concentrated; but, with the exception of possible gypsum, no evaporite minerals are preserved. Cool springs deposited travertine as mounds and contributed to limestone interlaminations within the sediment. The lake bottom was anoxic, and bacterial reduction of sulfate led to extreme sulfur isotopic fractionation in diagenetic pyrite. The caldera gradually resurged, converting the initial equant lake into an arcuate moat. Resurgent doming, alluvial fans, lacustrine sediments, ashfalls, and lava domes displaced water, lifted the lake so that it overlapped what later became the southern edge of the mineralized area, and eventually filled the basin. At ∼25.1 Ma an unseen pluton intruded beneath the northern part of the Creede district and created a convecting plume that drew in brine from the Creede caldera fill, meteoric water from highlands to the north, and possibly a fluid carrying radiogenic lead. These waters mixed and boiled as they approached the surface and moved southward, deposited a zoned epithermal deposit a few hundred meters below the paleosurface, and finally discharged into the top of the Creede Formation. The sulfide in the ores was of igneous derivation, but the sulfate was a mixture of biogenic sulfur from the Creede Formation, oxidized igneous sulfide, and thermochemically reduced and partially oxygen exchanged sulfate. The studies of the Creede caldera provide key observational and conceptual elements for the generalized model of the Creede ore deposit. The relation of the Creede ore deposit to a brine reservoir has broad significance because other brine accumulations (as in the Great Basin, the Green River Basin, or the playas of the Altiplano) offer similar settings and exploration opportunities.