Characterization of argillic alteration and K/Ar dating of illite at the Mercur gold mine, Utah: further evidence for a Mesozoic age of gold mineralization (MP 93-5)

By: P. N. Wilson and W. T. Parry

Mineralogical and chemical analyses of argillic ally altered rocks sampled on transects of the Carrie Steele and Lulu Graben fault zones in the Mereur gold deposit, indicate that the argillic mineralogy and geochemistry are heterogeneous. Although previous studies have shown large-scale relationships between gold content and faulting, there is no systematic zonation in mineralogy, type of illite, illite/kaolinite ratio, chemical composition, or gold concentration which can be attributed to the fault structures within 67 meters (200 feet) of the fault. The illite mineralogy is the only consistent aspect of the samples, with almost all containing an R3 type illite with < 10% smectite. This heterogeneity suggests that fluid composition was variable within the deposit and/or there are superimposed hydrothermal events.

Fourteen K/Ar age determinations of illite-rich <2 and 0.03-0.3 ?m size fractions from argillically altered limestone rocks hosting gold ore range in age from 98.4-226 Ma This range in ages is interpreted as representing mixtures of hydrothermal illite which may have been coeval with gold deposition (probable age near 152 Ma), burial metamorphic illite (possible age 230 Ma or older, coinciding with maximum development of the Oquirrh Basin), and thermal effects associated with emplacement of the Eagle Hill Rhyolite (30Ma). Thermal effects of the Eagle Hill Rhyolite appear to have extended as far as the Mercur pit; the Marion hill deposits appear to have been unaffected.

The results of this study provide further evidence for a Mesozoic age of the gold mineralization at the Mereur gold deposit However, recognition of mixed ages which have resulted from multiple hydrothermal/metamotphic events requires that more extensive study be done in order to unravel the complete geologic history of the deposit before a precise age for the gold mineralization can be determined.

Other Information:
Published: 1993
Pages: 26 p.
Location: Tooele County
Media Type: Paper Publication