Sulfidation State of Fluids in Active and Extinct Hydrothermal Systems: Transitions from Porphyry to Epithermal Environments
Marco T. Einaudi, Jeffrey W. Hedenquist, E. Esra Inan, 2005. "Sulfidation State of Fluids in Active and Extinct Hydrothermal Systems: Transitions from Porphyry to Epithermal Environments", Volcanic, Geothermal, and Ore-Forming Fluids: Rulers and Witnesses of Processes within the Earth, Stuart F. Simmons, Ian Graham
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Use of the concept of “sulfidation state,” in parallel with oxidation state, in the study of ore deposits finds its beginnings with the studies of Reno Sales and Charles Meyer at Butte, Montana. Experimental determination of the stability of sulfide minerals in terms of ƒS2 and temperature followed, leading to definition of contrasts in ore-forming environments. More recent studies of vapor compositions in active volcanic and geothermal systems allow direct comparisons to geochemical environments deduced from petrologic study. In this paper, we present a compilation of oxidation and sulfidation states of fresh igneous rocks from arc environments and on sulfidation states of sulfide assemblages in calc-alkalic porphyry copper, porphyry-related base metal veins, and epithermal gold-silver deposits. These data are contrasted with compositions of fluids from active systems by plotting vapor compositions in log ƒS2 − 1,000/T, RH − 1,000/T, and RS − 1,000/T diagrams, where RH ≈ log (XH2/XH2O), RS ≈ log (XH2/XH2O), and X = mole fraction of the gas.
Oxidation states of andesitic arc magmas plot in a tight cluster between fayalite + magnetite + quartz and pyrrhotite + pyrite + magnetite. On equilibrating below the solidus, arc plutons deviate toward higher oxidation states. Sulfidation states of arc magmas are very low to low, lying between fayalite + magnetite + quartz + pyrrhotite and pyrrhotite + pyrite. A plot of RH values versus measured temperatures for volcanic fumaroles reveals close agreement with the isomolar SO2 = H2S curve (sulfur-gas buffer) to temperatures below 500°C. Giggenbach concluded from this observation that the oxidation state of the vapors is controlled by their magmatic sulfur-gas composition, a conclusion consistent with oxidation state trajectories for cooling plutons.
Reactive magmatic-hydrothermal fluids from active systems trend toward lower RH and RS (higher oxidation and sulfidation states) with declining temperature, achieving minima at 200° to 100°C (RS = −1.5 to −3.0). Below 200°C, RH and RS both increase abruptly (RS = 0) through interaction with wall rock. In contrast, geothermal liquids are relatively reduced, near-neutral pH, and their sulfidation state remains low to intermediate (RS = 0) throughout the range 320° to 100°C. This may be caused by a greater degree of fluid-rock interaction at depth, a smaller magmatic component, or a distinct magmatic component. The reduced limit of geothermal compositions has an RH value of about −3, equivalent to Giggenbach's rock buffer, where iron-bearing minerals in fresh rock establish a “floor” to the oxidation state, just as the sulfur-gas buffer acts as a “ceiling.”
The majority of porphyry copper deposits contain magnetite, either without sulfides or as part of oregrade assemblages containing bornite and/or chalcopyrite without pyrite. In some deposits, pyrite + chalcopyrite dominates the ore zone. All of these assemblages are of intermediate-sulfidation state. High-temperature volcanic fumaroles plot largely in the bornite + magnetite field, consistent with the view that porphyry copper assemblages precipitate from magmatic volatiles that cooled along the sulfur-gas buffer. Base metal veins associated with porphyry copper deposits extend this cooling trend and display a range of sulfidation states from very high in central zones (pyrite + digenite + covellite + enargite) to intermediate and low in peripheral zones or latest stages (pyrite + tennantite + chalcopyrite).
In high-sulfidation epithermal deposits the sulfidation state ranges from high for copper-rich enargite-bearing assemblages to intermediate for the later gold-rich tennantite-tetrahedrite + pyrite assemblages, with similarities to and overlap with the base metal veins. In intermediate-sulfidation epithermal deposits the full range of intermediate-sulfidation states is represented by the assemblage pyrite + chalcopyrite + tetrahedrite. The general similarity of assemblages associated with gold in high- and intermediate-sulfi-dation deposits suggests a closer affiliation between these two types than is commonly thought. Low-sul-fidation epithermal deposits appear to be distinct and show little variation from low- and intermediate-sulfidation states. Evidence for transients in sulfidation state, due to boiling, local wall-rock influence, or other factors, exists in all three types of epithermal deposits.
Sulfide mineral assemblages in porphyry copper deposits, porphyry-related base metal veins, and high-and intermediate-sulfidation epithermal deposits, when taken together, describe a cooling path toward increasing sulfidation states from RS = −1 at 600°C to RS = −3 at 300°C, followed by an abrupt increase to RS = 0 as equilibrium with the rock buffer is achieved. This pattern, also evident in fluid compositions from active magmatic-hydrothermal systems, suggests a continuum between these deposit types. Fluid compositions in active hydrothermal systems span the complete range of chemical and physical states that are commonly relegated to changing time in intrusion-centered ore deposits.
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Volcanic, Geothermal, and Ore-Forming Fluids: Rulers and Witnesses of Processes within the Earth
To be honest, I am surprised to find myself addressing a meeting of the Society of Economic Geologists—being neither a geologist nor economic. And looking at the title of my paper, I wouldn’t be offended if people told me that I may be going to talk about something I know nothing about. After listening to some of this afternoon’s talks, however, it is clear to me that I wouldn’t be the only one. With this I don’t mean that the previous speakers were inept but that there are still quite a few basic problems which have to be solved before we may safely say, we know what’s going on in hydrothermal systems. And by basic, I mean basic.
The title of my talk links two processes: magma degassing, something I have been studying now, from the gases’ point of view, for more than 20 years, and mineral deposition, something I had my nose rubbed into by living in close vicinity to some of the biggest gold freaks like Kevin Brown, Jeff Hedenquist, Dick Henley, and Terry Seward. I myself had, quite early on, declared gold a four letter word and had vowed never to use it in any of my papers, together with other uncouthities, such as zinc or lead. Now that the above have dispersed, each into his corner of the globe, I think myself free to reconsider my earlier pledge.