The McCoy Mining District

(Spring 1995)
The Cove mine, the largest silver producer in North America and third largest in the world, is located in the McCoy mining district, about 25 miles southwest of the town of Battle Mountain in Lander County, Nevada. The McCoy district is in the northern Fish Creek Mountains, which are composed of upper Paleozoic and Triassic sedimentary rocks, Jurassic, Cretaceous, and Tertiary intrusive rocks, and Tertiary volcanic and sedimentary rocks. The Mississippian to Permian Havallah sequence of altered calcareous sandstone and siltstone crops out just north of the district and underlies most of it.

McCoy District Map

Precious metal mineralization in the McCoy district is interpreted to be genetically related to a large, buried porphyry intrusive system of late Eocene age that underlies the district. The McCoy deposit is interpreted as a proximal expression of this system, whereas the Cove deposit is interpreted as a distal expression. The McCoy deposit is similar in geologic setting and age to deposits in the nearby Battle Mountain district. Direct analogs to the Cove deposit have not been recognized; it is an unusual type of precious metal deposit related to a porphyry system.

Gold was discovered in the McCoy district by Joseph H. McCoy in 1914 and the first gold was produced in 1928. Most of the early gold was produced in 1930 and 1931 when about 1,000 ounces was produced from several small underground mines and by placer mining. The most significant mine was the Gold Dome mine, which had a 250-foot shaft and five levels of workings at 50-foot intervals. Gold grades were about 0.25 to 2.0 ounces per ton and some ore was direct shipping. A small amalgamation mill briefly operated in the district. The Gold Dome mine has been mined through by the northeast side of the present McCoy open-pit mine. During the early 1930s the district had a population of about 50, but by 1938 the district was largely abandoned.

Modern exploration of the McCoy district began in 1969 when Summa Corp. (owned by Howard Hughes) acquired the district. Summa conducted extensive exploration of the McCoy skarn deposit until 1977 when it sold its Nevada mining properties to Houston Oil and Minerals Corp., who explored the district from 1977 through 1980. In 1981, Gold Fields Mining Corp. leased the property for four years and explored the McCoy gold skarn. In September 1984, they dropped their lease and returned the property to Tenneco Minerals Corp., which had acquired Houston Oil and Minerals during their lease. In 1985, Tenneco conducted drilling, metallurgical testing, engineering, and feasibility studies. Tenneco started mining the McCoy gold orebody in February 1986 as an open-pit heap-leach operation and poured the first gold bullion in April 1986 . In October 1986, Echo Bay Mines Ltd. purchased the McCoy mine and claim block from Tenneco. Echo Bay continued with the exploration of the McCoy claim block. In January 1987, Echo Bay discovered the Cove gold-silver deposit through a systematic district exploration program, initiated by Tenneco in 1985, of stream-sediment, soil, and rock-chip sampling combined with geologic mapping, and drilling. The area of the Cove deposit was first targeted due to the presence of anomalous gold, silver, antimony, arsenic, and mercury in stream-sediment samples. Little previous prospecting or exploration had been conducted in the area despite its proximity to the McCoy deposit about 1 mile to the southeast. Further exploration revealed a strong soil anomaly of gold, silver, and arsenic. The gold soil anomaly was 2,800 feet long within an area of low relief and poor exposures. Trenching revealed low-grade gold mineralization at the surface, which was subsequently drilled by Echo Bay Mines Ltd.

The McCoy open pit has been mined to a depth of about 600 feet. Small-scale underground mining of the deposit beneath the open pit began in 1988 and averaged 500 tons of ore per day. Average grade from the underground mining was about 0.25 ounces of gold per ton. The underground mining at the McCoy mine ended in 1994.

Production from the Cove deposit began in 1988 as an open-pit heap-leach operation in association with the McCoy mine. In 1989, a 7,500 ton-per-day mill was commissioned. The mill contains gravity, flotation and cyanide leach circuits. Gold and silver are recovered by milling and heap leaching depending on the grade and metallurgy of the ore. The Cove deposit is currently being mined by open-pit methods at the rate of 160,000 tons of ore and waste per day. At this time, the final pit at Cove is projected to be 5,000 feet long, 3,000 feet wide, and up to 1,400 feet in depth. Underground mining of Cove has been done beneath the operating open-pit mine. Underground mining was by the post-pillar cut-and-fill method. Underground mining averaged 500 tons per day. Average grades from the underground mining were 0.22 ounces of gold and 11.0 ounces of silver per ton. Underground mining at Cove ended in 1993. Underground operations at McCoy and Cove account for about 10% of the total precious metal production.

In 1993, McCoy/Cove produced 12.5 million ounces of silver, more than 24% of the total U.S. production, mostly from the Cove mine. Besides being the largest silver producer in North America, McCoy/Cove was also the fourth largest gold producer in Nevada in 1993 and the fifth largest in 1994. From 1986 through the end of 1994, the total recovered production was 2.04 million ounces of gold and 41.5 million ounces of silver. At the end of 1994, total reserves at McCoy and Cove were 1.9 million ounces of gold and 83 million ounces of silver.

For more information about the McCoy district, see NBMG Map 103, a 1:12,000-scale geologic map of the district prepared by David L. Emmons and Tony L. Eng. This newsletter article was excerpted from the text that accompanies that map and describes in detail the history, geology, and mineral resources of the district.

--- Dick Meeuwig, Editor