Characterization and dating of argillic alteration in the Mercur gold district, Utah

The Mercur gold district of north-central Utah includes several sediment-hosted disseminated gold deposits which are located in the lower member of the Mississippian Great Blue Limestone. Argillic alteration of host limestone consists of illite (R3 illite-smectite <10% S) + kaolinite + quartz ± Fe oxides or pyrite. Argillized limestone has identical clay mineralogy in both oxidized and unoxidized rock. Unlike some other sediment-hosted disseminated gold deposits, variations in the Kubler index and illite/kaolinite ratios show no spatial relationship to faults or to gold distribution within the mineralized areas.

Hydrothermally altered Long Trail Shale, the middle member of the Great Blue Limestone, also occurs within the Mercur district and surrounding region. The shale varies in mineralogy from an assemblage consisting of Rl-type illite-smectite (20-35% smectite) + chlorite + kaolinite + quartz + pyrite or Fe oxides to an assemblage consisting of R3-type illite-smectite (5-10% smectite) + kaolinite + quartz + pyrite or Fe oxides. Shale with R3 illite-smectite is most highly altered and is mineralogically and chemically very similar to the highly altered limestone within the gold deposits. Both the altered shale and the altered limestone contain veins of R3 illite-smectite + kaolinite ± quartz and pyrite or Fe oxides. Additional hydrothermal features exhibited by both shale and limestone are felted and interlocking clay mineral textures, radiating clay mineral habits, crosscutting and replacement clay mineral textures, and fossil replacement by clay minerals and quartz.

Ten K-Ar ages of illite-rich clay-size separates of argiUically altered and mineralized limestone from the Mercur deposits range from 98.4 to 226 Ma. None of the hydrothermal illites in the deposit have Tertiary ages. These ages are similar to a range of Mesozoic ages previously obtained for vein and matrix clay in the Long Trail Shale. Most of these ages are interpreted as mixed ages resulting from overprinted thermal and/ or mass transfer events. Completely argillized and highly mineralized limestone obtained from the Carrie Steele fault yields an age of 152 ± 4 Ma (0.03-0.3-//m-size fraction) that is interpreted as the best estimate of the age of the gold-bearing hydrothermal event on this fault. The age data, the widespread mineralization, and the expected thermal influence of the Tertiary Eagle Hill Rhyolite calculated using published models indicate that gold deposition at Mercur is not related to Tertiary igneous activity. The complex thermal history of these deposits prevents a precise determination of the age of gold deposition. Comparison of age data from the Mercur deposits and the Long Trail Shale suggest gold mineralization may have occurred between 140 and 160 Ma.