High Sulfidation and Low Sulfidation Porphyry Copper/Skarn Systems: Characteristics, Continua, and Causes
In the past decade, multi-disciplinary research on active and fossil hydrothermal systems in volcano-plutonic arcs has resulted in important new information on the physical and chemical evolution of hydrothermal fluids of diverse origin, on the sources of metals, sulfur and other dissolved constituents, and on the possible genetic transitions between different ore-forming environments. Among the most studied magma- hydrothermal systems are those linked to felsic magmatism, whose products extend from plutonic porphyry-Cu deposits to volcanic epithermal-Au deposits. In-between these endmembers can be skarns or massive sulfide replacement bodies and veins of both base- and precious-metals. Here I focus on transitions between ore types in porphyry copper systems and use the the sulfidation state of hydrothermal fluids as a framework. I adopt the sulfidation state as a means of classification of ore-forming environments because this variable spans all deposit types and is independent of host rocks, metals contained, and textures exhibited (in contrast with terms such as "acid-sulfate" which is restricted to quartzo- feldspathic host rocks, or "epithermal" which is restricted to one class of deposit).
The sulfidation state of hydrothermal fluids can be classified on a continuous scale on the basis of key sulfidation reactions, as defined in Table 1 below (based on Skinner and Barton, 1979). In bold letters are minerals or assemblages that span only two defined sulfidation states; in italic bold are minerals or mineral assemblages that occupy only one defined state.
Only a few minerals or mineral assemblages are diagnostic of a given sulfidation state. For example, although covellite is diagnostic of very high sulfidation states, enargite is less diagnostic, being stable from upper intermediate, through high, and very high sulfidation states. Because of the chemical links between sulfidation, oxidation, and ionization states of hydrothermal fluids (Meyer and Hemley, 1967), covellite would be expected to be associated with acid-sulfate fluids and advanced argillic alteration containing alunite. Enargite, on the other hand, could be deposited from hydrothermal fluids of intermediate oxidation-sulfidation state and be associated with less advanced degrees of base-cation leaching of wall rocks (e.g., sericitic alteration).
Low- to intermediate sulfidation environment (A): Early and/or deep stages are characterized by potassic alteration and anhydrous skarn with disseminated/veinlet chalcopyrite-bornite-(magnetite) related to refluxing magmatic brines (saline, and hypersaline if a vapor plume is released) at 600-400 C, lithostatic pressure, and intermediate sulfidation-oxidation states (arrow 2, Fig. 1). This early stage is succeeded by late, superimposed and high-level sericitic alteration of porphyry and retrograde alteration of skarn, accompanied by pyrite-chalcopyrite- (hematite), in through-going veins. Late fluids are dominantly meteoric, boiling at 350-250 C under hydrostatic pressures, and are characterized by moderate acididity, low-salinity, and high sulfidation-oxidation states (arrows 5, Fig. 1). The degree of development of sericitic alteration varies significantly at present levels of exposure in porphyry copper districts (e.g., minor at Bingham, Utah; major at Ely (Robinson), Nevada).
High- to very high-sulfidation environment (C): Some porphyry deposits contain very late, high-level advanced argillic alteration (encased in sericitic) with pyrite-alunite, in some cases accompanied by digenite, covellite, and/or enargite (e.g., Butte, Montana; Chuquicamata, Chile), in other cases barren of copper (e.g., El Salvador, Chile). Acid-sulate alteration is localized in faults, hydrothermal breccias and around pebble dikes. Acid-sulfate fluids are of meteoric water (arrow 7) and/or magmatic-vapor plume origin (arrow 6), at 350-200 C, near-hydrostatic pressure, and high to very high sulfidation-oxidation states (arrow 8). In skarn or carbonate wall-rocks, these fluids generate silica-pyrite Cu- (Au) fissures and replacement bodies (e.g., Bisbee, Arizona; Yauricocha, Peru).
The present conceptual model that ties porphyries with high-sulfidation epithermal systems (in contrast with high-sulfidation copper lodes), based on the studies cited above, is paraphrased here from Sillitoe (1989, Fig. 9) and Rye (1993, Figs. 1 & 33). The porphyry copper environment that occupies a position between the water- rich carapace of the magma and the overlying transition from plastic to brittle rock, also may be of critical importance to epithermal deposits. This volume, characterized by the presence of saline magmatic water, quasiplastic behaviour, and low water:rock ratios, and by the absence of long-lived fractures and of meteoric water, may be the reservoir for evolved magmatic fluids that generate epithermal ores. At the ductile-brittle transition, saline magmatic fluids encounter open fractures and hydrostatic pressures; boiling of these fluids results in a hypersaline brine that remains at the ductile-brittle transition (arrow 2, Fig. 1) and a vapor plume (arrow 1, Fig.1 ) that rises to high levels where it generates barren acid-sulfate alteration following condensation (arrow 3, Fig. 1). As the ductile-brittle transition withdraws to deeper levels with time, metal-bearing saline and hypersaline liquid-phase fluids that have been refluxing within the stock (arrow 2) are tapped (arrow 4) and may ascend rapidly to high levels. These are the epithermal ore fluids (environment B, Fig. 1). If the deep environment (A, Fig. 1) fails to evolve to environment (C), then the high-sulfidation deposit (B) is separated from its roots (A) by a rock volume with little or no signs of hydrothermal activity.
The present challenge to students of porphyry systems is to distinguish, within individual districts, between the model endmember processes that generate acid-sulfate fluids (vapor plume or liquid-phase mixing?), the causes of mineralized versus barren acid-sulfate zones, and the potential continuum between copper-rich and gold- rich high-sulfidation deposits.
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