1986 Field Trip to the Fortitude Gold-Silver Mine, NV

In May 1986, I was working for Tenneco Minerals Company as an exploration geologist and visited the Fortitude gold skarn deposit in the Battle Mountain Range in Nevada. The deposit was owned by Battle Mountain Gold Corp. (BMG). The mine visit was part of a field trip part of GEOEXPO/86. The trips were add-ons to a symposium held in Vancouver, BC. The trip and symposium were sponsored by The Association of Exploration Geochemists and Geological Association of Canada. The field trips were lead by Harold (Hal) F. Bonham of the Nevada Bureau of Mines and Geology.

View northerly of the Fortitude open pit from August 1986 (Emmons).

One month earlier, July 1986, three Battle Mountain geologists: P.R. Wotruba, R.G. Benson, and K.W. Schmidt published a brief but informative description of the Fortitude deposit (Battle Mountain describes the geology of its Fortitude gold-silver deposit at Copper Canyon: Mining Engineering, p. 495-499). The benches are 20 feet high for scale.

This cross section of the Fortitude is from the above cited paper by Wotruba, et al. and the following description of the geology is summarized from the paper.




Fortitude gold-silver, sulfide-skarn deposit cross section looking due north by Battle Mountain Gold Corp. 1986.

The reserves reported by BMG in January 1985 were 11 million tons grading 0.154 opt gold and 0.83 opt silver containing 1.7 million ounces of gold and 9 million ounces of silver.

The gold-silver skarn deposit is located in the Copper Canyon area of the Battle Mountain Range. The center of the mineralized zone is occupied by an altered granodiorite pluton (Eocene, radiometric date of 38.5 Ma). An altered dike extends to the north from the Copper Canyon pluton and fills the Virgin fault. Hydrothermal alteration and metal zoning form concentric shells around the intrusion. The inner zone of alteration is potassic altering the granodiorite. The limit of this alteration is defined the limit of secondary biotite. Further out is phyllic and then propylitic alteration. Metal zoning outwards from the intrusion is: Cu+Au+Ag > Au+Ag > Pb+Zn+Ag. Fluid inclusion data indicate temperatures of >500 to 200 degrees centigrade.

The sulfide skarn is hosted by the Antler Peak Limestone (Pennsylvanian).  The deposit is stratabound (manto-like) and is distal to the source intrusion in Copper Canyon. The limestone occurs within other carbonate and clastic rocks of Permian and Pennsylvanian age. Thrust over these strata is the Havallah (previously called the Pumpernickel) sequence. The Golconda thrust is related to the Sonoma orogeny.

The mineralogy of the skarn consist of andraditic garnet, diopside, epidote, actinolite, tremolite, quartz and calcite. Sulfide mineral consist of pyrrhotite, chalcopyrite, pyrite, sphalerite, and galena. Sulfide content is locally of a massive nature. Owing to the pyrrhotite content the mineralization is magnetic and was reflected in a small aeromagnetic anomaly. This was part of the discovery story of the Fortitude deposit.