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Sulfidation State of Fluids in Active and Extinct Hydrothermal Systems: Transitions from Porphyry to Epithermal Environments

By
Marco T. Einaudi
Marco T. Einaudi
Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115
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Jeffrey W. Hedenquist
Jeffrey W. Hedenquist
Department of Geology and Geological Engineering, Colorado School of Mines, Golden, Colorado 80401-1887
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E. Esra Inan
E. Esra Inan
Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115
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Published:
January 01, 2005

Abstract

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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Contents

Special Publications of the Society of Economic Geologists

Volcanic, Geothermal, and Ore-Forming Fluids: Rulers and Witnesses of Processes within the Earth

Stuart F. Simmons
Stuart F. Simmons
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Ian Graham
Ian Graham
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Society of Economic Geologists
Volume
10
ISBN electronic:
9781629490342
Publication date:
January 01, 2005

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