Thursday, June 7, 2018

A Gold-Copper Mine for Wyoming?

The Copper King - a gold mine or just another hole in the ground?
Many people and companies looked at the Silver Crown district west of Cheyenne in a quest to find economic amounts of copper particularly at the Wyoming Comstock and Copper King mines. But after decades of searching, no one found enough metal to make a serious venture in the district. The Comstock vein was too narrow and small for economic consideration, and the Copper King porphyry was too deeply eroded and too low-grade in copper to compete with the likes of the giant porphyry copper deposits in Alaska, Arizona, Montana and Utah. Even so, many companies and consultants tried to find the right combination of ore grade, tonnage and metallurgy.

The Copper King is in section 36, which later became State of Wyoming fee land, and was initially developed by the Adams Copper mining and Reduction Company in 1881, and later worked by Hecla Mining Company. Many companies drilled the property including ASARCO (1938 and 1970), the Copper King Mining Company (1952-1954), Henrietta Mines (1972-1974), Caledonia Mines (1987), Compass Minerals (1993-94), Mountain Lake Resources, (1997), Saratoga Resources (2006-2008), and US Gold (2017-18).

Pervasive propylitic alteration of granitic rock collected in the Silver Crown
district by the author, 1982.
Samples collected by Jamison (1912) while at the Wyoming Territorial Geological Survey contained highly anomalous copper, gold and silver values. Later core drilling by the US Bureau of Mines (McGraw, 1954, Soule, 1955) intersected silicified and hydrothermally altered rock at depths to 1,024 feet (Klein 1974). An estimate of in situ ore was made by Nevin (1973) based on the available drilling and the ore zone was described to include 35,000,000 short tons averaging 0.21% Cu and 0.022 opt Au. Not great, particularly at gold price of $106.48 per ounce in 1973 (Hausel, Blog). But gold prices increased over time.

Personally, I've investigated this property a number of times as a researcher and as a consultant. My initial reconnaissance in 1978 led to grants by the University of Wyoming Mining and Minerals Resource Research Institute (MMRRI) to the Minerals Section of the Wyoming Geological Survey to investigate geophysical and geochemical signatures of a group of diamondiferous kimberlites in the Colorado-Wyoming State Line district, search for new kimberlite deposits, and also search for  disseminated metal deposits in Wyoming for the University of Wyoming to conduct heap leaching and in situ metallurgical research. At the time, geologists with the Wyoming Geological Survey investigated several deposits around the state, but the Copper King was the most favorable for this kind of research. Sampling and mapping of the Rambler Mine was conducted on the Bill Ferguson Ranch in the Silver Crown district, and mapping of the Copper King mine on state land (Hausel and Jones, 1982; Hausel 1997). Ferguson (1965) previously sampled the Rambler Mine and provided us with assay reports. Over the years, I also identified visible gold in rock samples on the property - something previously unrecognized.

Potassic alteration at the Copper King mine
Caledonia Resources Inc. of Canada later leased State Section 36 in 1987 for additional drilling. Preliminary results showed a higher grade zone of 4.5 million tons averaging 0.044 opt Au, or about 200,000 contained ounces of gold. Later, Compass Minerals of Reno (1995) did additional drilling finding more mineralization. Additional investigations by the author and several companies including Saratoga Gold identified what appears to be a copper-gold deposit with a potential of 2 million ounce gold-copper equivalent. Now, US Gold's NI43-101 report indicates that the Copper King contains 926,000 Measured and Indicated ounces of gold with 223 mm pounds of copper and an inferred gold resource of 174,000 Inferred ounces of gold with 62.5 mm pounds of copper. In other words, drilling has identified 1.1 million ounces of gold with some copper.

Hydrothermally altered rock at the Rambler prospect
From recent investigations, it is apparent that there is a geophysical anomaly to the west of the known mineralized body as well as a distinct fault break along the eastern part of the ore body which down-dropped the ore to an unknown depth. Will this disseminated gold-copper deposit ever become a mine? This is unknown as it is very low-grade even though drilling shows it has about $2.6 billion in gold and copper metals at present prices (2018). So, now it depends on how much it will cost to recover those metals from the rock, and prices to build gold mines are not cheap.

But a couple of things to think about - one is that this is a deeply eroded gold-copper deposit, and some of that gold eroded in the past 60 million years must be somewhere downstream. In addition, diamondiferous kimberlites in the nearby State Line district as well as scattered throughout the Laramie Range are also deeply eroded and as much as 2,500 feet of diamondiferous kimberlite pipes also eroded and those diamonds must be somewhere downstream in the Poudre River and other drainages.