New Mexico Mineral Symposium — Abstracts

The Mercur gold deposit is a sediment-hosted disseminated gold deposit in the southern Oquirrh Mountains of north-central Utah. The mine area consists of folded and faulted Paleozoic sedimentary rocks that were intruded by small igneous bodies during the Oligocene. Mineralization occurs in the Mississippian Great Blue Limestone. The upper half of the lower member of the Great Blue hosts most of the gold mineralization and is locally known as the Mercur Mine Series.

The deposit is characterized by the presence of As-, Fe-, TI-, Sb-, and Hg-minerals that occur in the unoxidized ore as sulfides and sulfosalts. Minerals at Mercur include realgar, orpiment, pyrite, marcasite, and the Tl-minerals lorandite, raguinite, gillulyite, and fangite. Both gillulyite, Tl₂(As,Sb)₈S₁₃, and fangite, Tl₃AsS₄, are new species identified at Mercur, and raguinite, TIFeS₂, is known from only one other locality. Cinnabar has been found at the deposit but has not been seen in recent mining activity. The Tl-Hg mineral christite has supposedly been found at Mercur as well. Other minerals found here include calcite, quartz, barite, and various antimony oxide pseudomorphs of stibnite.

Pyrite occurs as (1) euhedral grains, (2) irregular or rounded zoned grains, and (3) fine-grained "filigree" pyrite. Chemically pyrite can be divided into non-arsenian, moderately arsenian, and strongly arsenian categories. Gold is found in highest concentrations (avg. 0.1 wt %) in strongly arsenian pyrite. Realgar is common in the deposit and is usually observed being replaced by orpiment. Orpiment is common and can be divided into two geochemical groups based on relative content of minor elements: orpiment with low Tl-high Sb and orpiment with high Tl-low Sb.

K/Ar dating of hydrothermal illite, including ammonium-illite and tobelite, resulting from argillic alteration of limestone and shale throughout the Mercur district has resulted in dates ranging from 99 to 226 Ma. This age range is interpreted as resulting from a series of overprinted Mesozoic hydrothermal events. The age of gold mineralization is still not adequately determined but current data suggest about 150 Ma. The age data do not support a Tertiary age or a Tertiary intrusive model for gold mineralization. Combination of the age data with structural analysis by Kroko (1992) suggests that mineralization may have occurred during formation of the Ophir anticline.

Supplemental reading
Kroko, C. T., and Bruhn, R. H., 1992, Structural controls on gold distribution, Mercur gold deposit, Tooele County, Utah: Utah Geological Survey, Miscellaneous Publication 92-3, pp. 325-332.
Wilson, J. R., and Wilson, P. N., 1992, Sulfide and sulfosalt mineralogy at the Mercur gold deposit, Tooele County, Utah: Utah Geological Survey, Miscellaneous Publication 92-3, pp. 343-348.
Wilson, P. N., Parry, W. T., and Nash, W. P., 1992, Characterization of hydrothermal tobelitic veins from black shale, Oquirrh Mountains, Utah: Clays and Clay Minerals, v. 40, pp. 405-420.
Wilson, P. N., and Parry, W. T., 1993, 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: Utah Geological Survey, Miscellaneous Publication 93-5, 26 pp.