Abstract Phyllic and argillic alteration at Bingham consists of smectite, sericite, and kaolinite. X-ray diffraction and microprobe studies show that sericite in these alteration assemblages is illite interstratified with 10 percent or less smectite. Illite occurs as a replacement of plagioclase phenocrysts, in veins, and as pervasive disseminations in the rock along with smectite and minor kaolinite. The illite/smectite interstratification is a mixture of 1M and 2M polytypes and generally contains more smectite in the finer size fractions. Structural formulae for the illite show less K and Al and more Si than muscovite. The illites also contain octahedral Mg and an octahedral layer charge. The Kubler crystallinity index increases systematically as particle size decreases. Smectite dominates in pervasive alteration of whole rock, plagioclase phenocrysts, and one vein. Illite is the major constituent of vein clays. The smectite replacing plagioclase and whole rock is montmorillonite and smectite in the vein is beidellite. The distribution of smectite, illite, and kaolinite at present mining levels appears to be different than reported in earlier studies in that smectite may be a major component of phyllic and argillic alteration while kaolinite is a minor component. Sample traverses on pit benches through the quartz monzonite porphyry show no systematic variations in mineral abundances. Measurements of clay alteration of phenocrysts in quartz monzonite porphyry in drill hole 50 from an elevation of 5,730 to 3,260 feet show an increase in smectite content downward to the core of the quartz monzonite porphyry, just below present mining levels, and then a decrease to minimum values near the lower contact with monzonite. Copper grades decrease systematically throughout this interval. This pattern of smectite distribution is not observed in horizontal traverses on bench levels across the quartz monzonite porphyry. 40 Ar/ 39 Ar age spectrum analysis on a hydrothermal biotite from altered quartz monzonite porphyry yields an isochron age of 37.07 ± 0.21 Ma. Sericite and K-feldspar phenocrysts from this sample also yield apparent ages of ~37 Ma, but have complex age spectra. The apparent age concordance for these three minerals suggests a hydrothermal event in the quartz monzonite porphyry that produced biotite and sericite at indistinguishable times and may have partially reset the K-feldspar. In comparison, an isochron age for a 10 to 20 mm split of vein sericite in monzonite from the present pit bottom is 37.81 ± 0.20 Ma, whereas a maximum age of a vein sericite in monzonite from the 6440 level is 39.71 ± 0.34 Ma. Coupled with the results of Moore and Lanphere (1971) and the Warnaars et al. (1978) data compilation, these new results allow for a minimum duration of ~1 Ma for the hydrothermal activity at Bingham.

Abstract Sediment-hosted, disseminated gold deposits in the Mercur district and Bameys Canyon in the Bingham district are infolded and thrust-faulted Paleozoic sediments of the Oquirrh Mountains, Utah. Folding and thrust faulting occurred in a Jurassic event and the Cretaceous Sevier Orogeny (Armstrong, 1968; Almendinger and Jordan, 1981; Almendinger et al.,1984; Snoke and Miller, 1988). The Oquirrh Mountain range is allocthonous and is underlain by the east-vergent Oquirrh thrust (Presnell, 1992; Presnell and Parry, 1995) not te Charleston thrust. Mercur is located . in the southern Oquirrh Mountains dominated by a series of northwest trending anticline-syncline pairs. Barneys Canyon and Bingham are in the central Oquirrh mountains where structural complexity results from the east -vergent thrust belt interacting with the Uinta arch.