The Sleeper Gold-Silver Deposit
Authors: Wood John D. , Hamilton Stanley K. 
Published: November 2019

The Sleeper gold-silver deposit, located in northwestern Nevada approximately 30 air miles northwest of Winnemucca, is one of the most significant precious metal discoveries of the 1980s. The deposit was discovered by Amax Exploration, Inc., and is owned and being mined by Nevada Gold Mining, Inc., a wholly owned subsidiary of Amax Gold, Inc. It is an epithermal bonanza deposit being mined by open-pit methods containing more than 2.5 million oz of gold and 20 million oz of silver with an overall waste to ore ratio of approximately 2.3:1.

The deposit was localized by a N 20° E-striking, 77° W-dipping range front fault zone exceeding 3,000 ft in width and 6 mi in length and was buried under an average of 90 ft of alluvial deposits and eolian sand. Most ore-grade mineralization occurs in Miocene volcanic rocks. High-grade visible gold with values exceeding 1 oz Au/ton occur in several banded gold quartz veins and linear breccia bodies. Lower grade gold values occur in hydrothermal breccias peripheral to veins and in stockwork veins in adjacent volcanic rocks. Over 90 percent of the hydrothermally altered and mineralized rocks associated with the deposit were not exposed at the surface.

Through surface observations, mapping, and rock-chip sampling in the Sleeper prospect area, a zone with anomalous gold and gold pathfinder elements was identified, and placer and lode claims were staked in June 1982. From June 1982 through July 1984, 41 reverse-circulation and two core holes were drilled in four drilling campaigns. Work completed by the end of 1983 (27 holes) had delineated a small zone of potentially economic grade (2.6 million tons avg. 0.03 oz Au/ton and 0.79 oz Ag/ton). The highest grade 5-ft intercept drilled in the first 27 holes was 0.17 oz Au/ton. Analytical results from the high-grade discovery hole (hole 34) were received in July 1984. This hole averaged 0.81 oz Au/ton over 335 ft, including 15 ft averaging 11.49 oz Au/ton and 5 ft averaging 25.77 oz Au/ton.

Work on the project was greatly accelerated with the recognition that Sleeper was probably an ore deposit of considerable significance. Evaluation activities included additional drilling, geologic work, geostatistical ore reserve estimation, bench-scale metallurgical tests, conceptual mine planning, facilities site investigations, application for water rights, infrastructure review, environmental studies, permit planning and acquisition, land status review, and alluvium-dewatering planning and implementation. These activities resulted in a recommendation to develop the deposit, which was approved in July 1985. Production began in March 1986, less than 20 months after discovery of the high-grade gold. Initial start-up capital costs were paid back within six months. Significant additional reserves have been defined, and production has increased from the originally planned rate of 53,000 oz Au/yr to a forecast of 250,000 oz in 1989.

Prior to approval of appropriation requests for each phase of drilling from 1982 through early 1984, Amax considered seeking a joint venture partner. Because it was evident that the gold system was large and not totally defined and because the results of each phase of drilling were as good as or better than expected, the decision was made each time to continue the work with Amax funds. The geometry and quantity of the high-grade ore was not well defined at the time of the development decision, but the location of the high-grade ore at the center of the proposed pit was established. Close-spaced drilling sufficient for complete definition of the orebody was considered impractical, costly, and time consuming, given the existing evidence of structural complexity and lack of grade continuity over short distances within the veins. Information from close-spaced drilling would not change the development decision or the basic design of the project and would only delay project start-up and cash generation.