Economic Geology

Economic Geology BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS VOL. 103 March–April 2008 NO.2 Fluid Inclusion Evidence for Magmatic-Hydrothermal Fluid Evolution in the Porphyry Copper-Molybdenum Deposit at Butte, Montana BRIAN G. RUSK†,* MARK H. REED, Department of Geological Sciences, 1272 University of Oregon, Eugene, Oregon, 97403 AND JOHN H. DILLES Department of Geosciences, 104 Wilkinson, Oregon State University, Corvallis, Oregon, 97331 Abstract The porphyry Cu-Mo deposit in Butte, Montana, formed where magmatic hydrothermal fluids, introduced with injections of porphyrytic dikes, fractured and permeated the Butte Quartz Monzonite. These fluids formed a stockwork of quartz and quartz-sulfide veinlets with a variety of styles of potassic and sericitic alteration envelopes. The distribution of vein and alteration types and the distribution of fluid inclusions in these veins record the progressive pressure, temperature, and compositional evolution of the hydrothermal fluids that formed this world-class deposit. Deep drilling and 1,300 m of offset along the Continental fault provide a vertical view of almost 3 km through the Butte deposit. Deep veins within and below the highest Mo grades are quartz dominated with thin K-feldspar or, less commonly, biotitic alteration rims. Fluid inclusions in deep veins trapped a single phase aqueous fluid containing 2 to 5 wt percent NaCl equiv and 2 to 8 mol percent CO2 at temperatures between 575° and 650°C and pressures between 200 and 250 MPa, corresponding to depths between 6 and 9 km. Al- though Cu grades are low in this region, abundant chalcopyrite daughter minerals in fluid inclusions indicate that the fluids were Cu rich. Fluids that formed these veins transported Cu from the magma below, upward into the region of Cu mineralization with only minor Cu precipitation. Over a kilometer above the bulk of deep quartz and quartz-molybdenite veins, the highest Cu grades are in and around chalcopyrite-bearing quartz-sulfide veins with biotitic alteration (early dark micaceous veins), and their upward, equivalent magnetite-chalcopyrite-pyrite-quartz veins with wide K-feldspar, green sericite, and chlorite alteration (pale-green sericitic veins). These veins contain more evidence for brine-vapor unmixing than any other vein type. The upward progression of early dark micaceous veins to pale-green sericitic veins formed where low salinity, CO2-bearing fluids, similar to those trapped in deep quartz veins, ascended, de- pressurized, sometimes unmixed, and cooled from ~650°C at 90 MPa to ~475°C at ~50 MPa. As low salinity, CO2-bearing, aqueous fluids, similar in composition to fluids trapped in deep quartz veins, cooled at shallow depths, they formed late pyrite-quartz veins with sericitic alteration. These veins formed from fluid cooling at temperatures between 370° and 450°C at transiently hydrostatic pressures between 40 and 70 MPa, corresponding to depths of 4 to 7 km. Most pyrite-quartz veins formed at pressures and temperatures above the H2O-NaCl-CO2 solvus, but evidence for brine-vapor unmixing is also present. Pyrite-quartz veins formed at progressively greater depths as the hydrothermal system cooled, overprinting much previous mineralization. Late Cu-Pb-Zn-Ag-As-rich Main stage veins formed from dilute fluids containing <3 wt percent NaCl equiv and <2 mol percent CO2. These fluids were trapped between 230° and 400°C under hydrostatic pressures between 20 and 60 MPa and depths of 2 to 6 km. No evidence of boiling is observed in Main stage veins. Fluid inclusion phase relationships indicate that the Butte porphyry Cu-Mo deposit formed at 5 to 9 km depth, greater than any other porphyry-type deposit. At Butte, the similarity in bulk composition of fluids trapped in early quartz-rich veins with potassic alteration and late pyrite-quartz veins with sericitic alteration implies that an underlying magma continually provided low salinity, CO2-bearing fluids of relatively constant composition during the entire life of the hydrothermal system. We hypothesize that rather than resulting from changes in fluid chemistry due to magma crystallization, the entire suite of vein and alteration types and the ore metal distribution reflect the path of cooling, depressurization, and wall-rock interaction of a parental magmatic-derived fluid of relatively constant initial composition. † Corresponding author: e-mail, [email protected] *Present address: School of Earth and Environmental Sciences, James Cook University, Townsville, Queensland, 4810, Australia. ©2008 by Economic Geology, Vol. 103, pp. 307–334 0361-0128/08/3727/307-28 307 308 RUSK ET AL. Fluid inclusions, vein and alteration relations, and ore metal distribution indicate that Cu and Mo were introduced into the hydrothermal system by the same fluids, but that the mechanisms of precipitation of these metals were decoupled. Early dark micaceous and, to a greater extent, pale-green sericitic, veins have wide alteration envelopes and contain more evidence for fluid unmixing than any other vein type, which suggests that chalcopyrite precipitation was driven by a combination of fluid unmixing, fluid-rock reaction, and fluid cooling between 650° and 475°C. Most molybdenite mineralization, however, is in quartz-dominated veins with little or no alteration that are dominated by low salinity inclusions. These veins formed in response to pressure de- crease rather than cooling. After chalcopyrite and molybdenite precipitation, low salinity fluids cooled, usually at temperatures and pressures above the H2O-NaCl-CO2 solvus, to produce significant acid and voluminous sericitic alteration accompanied by pyrite-quartz vein formation that overprints much of the deposit and con- tains anomalous but noneconomic Cu. Introduction and Einaudi, 2004; Seedorff et al., 2005). Therefore, some ambiguity remains as to the relationship between low salinity PORPHYRY Cu (Mo-Au) deposits form in the upper crust, where fluid overpressures generated in the cupola of a crys- and high salinity fluid inclusions and the origin and evolution tallizing magma hydrofracture the overlying rock (Burnham, of these fluids. For example, previous fluid inclusion studies 1979), allowing magma and fluids to intrude, forming por- in many porphyry Cu deposits suggest that early K-silicate al- phyry dikes surrounded by stockwork fractures. Aqueous flu- teration that accompanies Cu and Mo mineralization is produced by high salinity, high-temperature magmatic fluids. ids permeate the fractured rock where the fluids depressur- However, low to moderate salinity fluids are implicated in K- ize, cool, and chemically react with wall rock, forming veins silicate alteration in some deposits (Redmond et al., 2004; this with alteration envelopes. Magmatic volatiles enriched in in- study). Later sericitic alteration has been interpreted to have compatible elements precipitate sulfides of Cu and Mo in ad- been produced by low salinity, low-temperature meteoric dition to quartz. fluids (Sheppard and Taylor, 1974; Beane and Titley, 1981; Porphyry Cu deposits from around the world and of all ages Beane and Bodnar, 1995; Taylor, 1997); however, a magmatic show similar vein and alteration types and similar patterns of fluid origin of pyrite veins with sericitic alteration has been in- vein and alteration distribution (Gustafson and Hunt, 1975; ferred more recently in many deposits (Hedenquist et al., Beane and Bodnar, 1995; Seedorff et al., 2005). Alteration is 1998; Zhang et al., 1999; Watanabe and Hedenquist, 2001; typically zoned from an inner K-silicate assemblage outward Ulrich et al., 2001, Seedorff and Enaudi, 2004; this study) and to propylitic alteration, with later sericitic alteration cutting high salinity, magmatic fluid inclusions have been identified the K-silicate zone or lying near the potassic-propylitic inter- in sericitic alteration at Henderson, Colorado (Seedorff and face (Lowell and Guilbert, 1970; Gustafson and Hunt, 1975; Einaudi, 2004); Endeavor 26 North, Australia (Harris and Beane and Titley, 1981; Seedorff et al., 2005). Commonly, Golding, 2002); Gaby Sur, Chile (J. Dilles, unpub. data); and early Cu-bearing quartz veins with K-silicate alteration are Butte, Montana (this study). cut by quartz-molybdenite veins, which are cut by pyrite veins Halite-saturated inclusions are ubiquitous in porphyry Cu with sericitic alteration (cf. Gustafson and Hunt, 1975). In deposits and low-density vapors are also extremely common. some deposits, late veins with advanced argillic and sericitic However, in most deposits, it is unclear whether these fluids alteration cut all of the above vein types (Meyer et al., 1968; directly exsolved from a magma or whether they were gener- Lowell and Guilbert, 1970; Corn, 1975; Arribas et al., 1995; ated by fluid unmixing. Where brine and vapor are generated Phillips et al., 1998; Ossandon et al., 2001; Manske and Paul, by fluid unmixing, the composition of the parental hydrother- 2002; Khashgerel et al., 2006). mal fluids is typically unknown. In the vast majority of por- Fluid inclusions in vein quartz from porphyry Cu deposits phyry Cu deposits, it is therefore difficult to determine how trap fluids with a wide range of compositions under a wide the compositions of hydrothermal fluids change with time range of pressure and temperature conditions. In a single de- and how those changes affect ore mineral distribution and al- posit, hydrothermal fluids may originate from magmatic, sed- teration zonation. Models of the physical and chemical imentary,

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