FORMATION OF A PALEOTHERMAL ANOMALY AND DISSEMINATED GOLD DEPOSITS ASSOCIATEDWITH THE BINGHAM CANYON PORPHYRY Cu-Au-Mo SYSTEM, UTAH--A DISCUSSION

Abstract

Sir: The recent paper by Cunningham et al. (2004) presents reconnaissance apatite and zircon fission track, conodont alteration index, vitrinite and solid bitumen reflectance, and stable isotope data documenting the extent of the paleothermal anomaly associated with the 37 Ma Bingham Canyon porphyry Cu-Au-Mo deposit. Based on these data, the paleothermal anomaly encompasses the Barneys Canyon and Melco sediment-hosted gold deposits. The authors conclude that the gold deposits are genetically related to the Bingham porphyry system and that the maximum sustained temperature was 100° to 140°C at Melco and near 100°C at Barneys Canyon. Cunningham et al. (2004) contend that Barneys Canyon and Melco are, perhaps, distal disseminated Ag-Au deposits rather than Carlin-type deposits. Fluid inclusion data from ore-grade samples at Barneys Canyon (Presnell, 1992; Presnell and Parry, 1996) and Melco (R.D. Presnell, 1991, Constraints on a genetic model for the Melco gold deposit based on mineralogy, alteration, and fluid inclusions, unpub. report for Kennecott Exploration, 12 p.) yield homogenization temperatures in excess of 200°C. We contend that the paleothermal data support our previous conclusion that the temperature at Barneys Canyon during formation of the Bingham system was near ambient (i.e., 70°C, assuming 2 km of cover and a geothermal gradient of 35°C/km), and that the formation of Barneys Canyon was related to a hydrothermal system separate from Bingham. The data presented in Presnell (1992) and Presnell and Parry (1996) are from drill core through the center and periphery of the orebodies at Barneys Canyon, and the data from Melco (R.D. Presnell, 1991, unpub. report for Kennecott Exploration) are also from drill core through the orebody. All of the samples from Barneys Canyon used in the study by Cunningham et al. (2004) were collected from the pit wall after the orebody was mined, and only two samples from their …