Mackinaw Mining District, Lemhi County, Idaho, USAi

Regional Level Types
Mackinaw Mining District	Mining District
Lemhi County	County
Idaho	State
USA	Country

The Mackinaw District is located in the northeast part of the Cobalt Ranger District. It is bound by the Mineral Hill and Indian Creek Districts on the north, the Eureka District on the east and northeast, and by Panther Creek on the southwest. Most of the district is drained by Napias, Beaver and East Boulder Creeks.

Placer gold was discovered in Leesburg Basin, an area drained by Napias Creek and its tributaries, during 1866. As word spread about the rich placer deposits during the following several years, the population in the Leesburg Basin grew to 7,000. Placer gold production estimates range from $5 million (Umpleby, 1913) to $16 million (Kirkpatrick, 1934). Lorain and Metzger (1939, p. 7) estimate production from the basin to be $9.5 million. Their estimate, probably the more accurate of the three, would be equivalent to 475,000 troy ounces of gold.

The most important type of mineralization in the Mackinaw District consists of gold-bearing vein/stockwork/disseminated deposits in granitic rock. At the Italian mine, located in 1892, quartz veins and veinlets containing gold, pyrite, sphalerite, pyrolusite, and galena occur in granitic rocks of the Leesburg Stock. A 10-stamp mill, and later a 30-stamp mill, were built to process the ore. Production is estimated to be about $175,000 prior to 1904 (Umpleby, 1913, p. 153). The stockwork mineralization encountered at the Italian extends northwest to the Haidee mine, about 1.3 miles away.

Resources in the Italian-Haidee area (Arnett Hardrock) total about 16.3 million tons; proven and probable reserves are 6 million tons grading 0.033 ounce gold per ton, measured resources are 6.5 million tons grading 0.03 ounce gold per ton, and indicated resources are 3.8 million tons grading 0.025 ounce gold per ton (personal communication, Nick Bertram, American Gold Resources, 1994).

The Beartrack deposit, a vein/stockwork/disseminated deposit, occurs near the intersection of the Panther and Jesse Creek faults. The northeast trending, steeply dipping Panther Creek fault forms the contact between Yellowjacket quartzite and Precambrian quartz monzonite. Gold mineralization occurs along a 10,000-foot-long section of sheared, fractured rock associated with the fault.

The deposit is being mined by conventional open pit methods. Two pits, one near the historic gold mining town of Leesburg and the other about 1.5 miles northeast of Leesburg, supply ore to the mill. The ore is crushed, put on leachpads, and sprinkled with a dilute solution containing cyanide. Gold is recovered from the pregnant solution by carbon adsorption methods. By the end of 1996, 9,261,600 tons containing a recoverable 281,000 ounces of gold had been mined and placed on leach pads. Mine reserves remaining at the end of 1996 totalled 25.6 million tons grading 0.033 ounce gold per ton (Meridian Gold, personal communication, March 7, 1997).