Summary:
The Mercur Gold Project is located in north-central Utah near the northeastern boundary of the Great Basin, an area of high elevation and internal drainage occupying much of Nevada and western Utah. The distance between successive mountain ranges averages about 12 to 18 miles. The Oquirrh Mountains are the first range west of the Wasatch Mountains, which bound the Basin and Range from the Colorado Plateau and Uinta Basin provinces to the east.
Deposit Type
Carlin-type gold deposits like those at Mercur and other parts of the northeastern Great Basin occur in a complex geologic setting generally regarded as the late Proterozoic rifted edge of the North American craton. After rifting, a thick wedge of Paleozoic siliciclastic and carbonate sedimentary rocks accumulated upon a passive margin until a series of generally east-vergent orogenic events broadly affected the area and greatly disrupted marine sedimentation. The first deformation event, the early Mississippian Antler orogeny in Nevada, produced highlands that sourced a large amount of sediment in Carboniferous foreland basins in Utah, including in the Oquirrh Mountains.
Carlintype deposits are disseminated, replacement-type gold deposits commonly contained in finegrained silty limestone and calcareous siltstone. Where unoxidized by surface weathering, the mineralized carbonate rocks are commonly carbonaceous. The deposits are characterized by high gold, minor silver, and negligible base-metal contents; ratios of Au:Ag are typically three or greater. Other elements associated with Carlin-type gold mineralization include arsenic, antimony, mercury, and thallium.
Gold in Carlin-type deposits occurs in micron-size particles. In some cases, gold grains are encapsulated in jasperoid quartz or in cases of unoxidized strata, in arsenic-rich rims of pyrite or marcasite grains. Minerals associated with Carlin-type mineralization are stibnite, orpiment, realgar, and barite.
Mineralization
Four areas of significant precious-metal mineralization are identified on the Mercur Property: the Main Mercur area where most historical production was derived, and the North, South, and West Mercur areas. Most precious-metal mineralization from all areas is disseminated in preferred stratigraphic intervals of the Great Blue Limestone, although some mineralization also occurs in a few steeply plunging and irregular breccia bodies that cut other stratigraphic units. Vein or intrusion-hosted mineralization is rarely, if ever, described.
Most deposits, except for those at West Mercur, occur on the east limb of the Ophir anticline, near its crest, in thin-bedded, carbonaceous, and relatively iron-rich Mississippian carbonate strata of the Mercur Member of the Great Blue Limestone. Gold at West Mercur occurs in similar carbonate strata of the Upper Great Blue Member in the west limb of the Ophir anticline. Gold mineralization at Mercur is broadly replacement style, wherein gold-bearing iron sulfides were disseminated in more favorable carbonate-bearing units during hydrothermal alteration of impure carbonate host rocks. Near the surface or along faults, fracture zones, and dikes, post-mineralization oxidation of the gold deposits converted iron sulfides to oxides. Like other major base and precious metal deposits in the Oquirrh, and neighboring Wasatch and East Tintic Mountains, mineralization at Mercur is considered to be of Eocene age based on close relationships with coeval intrusions.
Sedimentary rock-hosted gold mineralization on the Mercur Property is generally consistent with characteristics of large Carlin-type gold deposits in Nevada. Mineralization at Mercur exhibits the typical Carlin-type geochemical assemblage of gold associated with anomalous arsenic, antimony, mercury and thallium. Gold values generally exceed silver values, and base metals have low average concentrations. Gold in unoxidized, carbonaceous strata generally occurs in two forms:
1) as minute grains of irregularly shaped iron sulfides (“filigree” pyrite) disseminated and in extremely fine veinlets distributed throughout the rock;
2) as a component of micron-scale rims on subhedral pyrite in pyrite-marcasite-sulfosalt micro veinlets.
The gold-bearing rims on earlier pyrite, and the micronsized filigree pyrite, also contain high concentrations of arsenic, antimony, mercury, and thallium.
Three main types of primary hydrothermal alteration associated with precious metal mineralization at Mercur are carbonate removal (“decalcification”), silicification of carbonates to form jasperoid, and argillization of the feldspathic component in impure carbonates and igneous rocks. In addition, concentrations of organic carbon are locally evident adjacent to intrusions and in areas associated with intense decalcification. The deposits at Mercur are variably oxidized, although the redox boundary is highly irregular and, in some cases, oxidation occurs below sulfide- and carbon-bearing zones.
Main Mercur Mineralization
The Mercur Gold Project is comprised of five open pits that were mined between 1983 and 1997. The pits, which were largely expansions of the historical underground workings, are from south to north: Sacramento, Mercur Hill, Golden Gate, Marion Hill, and Rover. The Sacramento pit consumed the smaller Jessie Lakin and Mattie pits, the Mercur Hill pit consumed the Walker pit, and the Marion Hill pit consumed the smaller Lady May, Brickyard and Carrie Steele pits. All five areas contain gold-mineralized material that was not previously mined but was identified through drilling and other historical exploration activities.
North Mercur (Lion Hill-Silveropolis)
The north part of the Mercur Property includes the many historical workings at Lion Hill and Silveropolis Hill about 1.2 km south of the town of Ophir. Mineralization at North Mercur was first found in the Silver Chert, which is a largely conformable jasperoid ledge occurring at the base of the Mercur Member of the Great Blue Limestone. Silver chlorides and native silver indicative of near surface enrichment occurred in oxidized and brecciated Silver Chert.
West Mercur (West Dip)
West Mercur refers to the extensive area located west of the Main Mercur area along the western flank of the Oquirrh Mountains and extending west beneath the pediment. In contrast to Main Mercur, known gold mineralization at West Mercur occurs in the Upper Limestone Member of the Great Blue Limestone near its contact with the Manning Canyon Shale in the west-dipping limb of the Ophir anticline. Mineralization at West Mercur was described by Gilbert (1935) as occurring discontinuously over a 2.25 mi strike length and at the Daisy mine, to depths of at least 700 ft along the West Dip fault, which dips 45° to 60°to the west.
South Mercur
The Sunshine and Overland mines were the principal historical underground mines of the South Mercur area. Gold mineralization occurs in the east limb of the Ophir anticline near its axial trace along a narrow, north-northwest-trending, 1.5 mi-long corridor that follows the Mercur Member in and near the bottom of Sunshine Canyon. This mineralization is considered to be a southern continuation of the deposits in the Main Mercur area, with similar styles of mineralization and host strata.