June/July 2010

Economic Geology

Nevada-type gold deposits (Part 1)

By R. J. Cathro

“The first recorded prospecting in the Mercur area took place in 1869 with the establishment of a small camp named Lewiston at the head of (what became) Mercur Canyon in later years. The prospectors… (found) small pods of rich silver mineralization, without significant base metals… In the 1870s, Lewiston grew to a town of about 1,500 people… By 1881, the silver ores were exhausted and Lewiston had dwindled to a town (with only) one resident.”

~ Mako, 1999

Mercur Mine, Utah It is unlikely that there is another small part of the earth’s crust that enjoys the distinction held by Utah’s Oquirrh Mountains, which hosts two mineral deposits that were the first of their types to be mined in North America (and possibly anywhere). One was the Bingham Canyon porphyry copper mine, where mining began in 1903; the other was the Mercur gold mine, located on the southwest flank of the mountain range about 22 kilometres south of Bingham, where gold production started in 1893. The two camps also share a common ancestry through Joseph R. De Lamar, a prominent mine owner at Mercur, who optioned part of the Bingham Canyon deposit and sent three of his top engineers to examine it in 1898, including Daniel C. Jackling who turned it into a world-class copper mine (Cathro, 2009a).

Although mining of another member of the porphyry family (this one of the tin-rich variety) had commenced along the German/Czech Republic border about 1440 and continued until 1991, it was not well known in North America and the similarities were not recognized (Cathro, 2009b). It was probably more than 25 years after mining began at Bingham Canyon before its geometry and genesis began to be understood. Similarly, the realization that Mercur was a Nevada-type deposit did not become obvious until about 1968, when Newmont Exploration Ltd. began to explore there. That was significant because Newmont had made the first modern discovery of this type of mineralization at Carlin, Nevada, in 1961 and was the acknowledged expert. That was the same year that Hewitt (1968) published a paper pointing out the strong resemblance between mineralization at Mercur and Getchell, Nevada.

The Mercur camp was given its name in 1879 when a German prospector staked the Mercur claim on a cinnabar showing discovered during the silver boom. The first gold occurrence was only discovered in 1883 by means of fire assays because the gold was too fine-grained to be seen visually or recovered in a gold pan. An attempt in 1890 to recover gold using mercury amalgamation from 1350 tonnes of ore averaging about 27.4 grams per tonne (0.8 ounces per ton) ton achieved a recovery rate of only 15 per cent because the pulp formed a thick slimy mud that was impervious. It is hard to imagine how frustrating it must have been for the prospector who was given the exciting news by the assayer that his sample of apparently barren rock had given a good gold assay. After he crushed and panned more samples and still could not find any gold, the assayer kept insisting that he had discovered something valuable. Even after he had started mining and set up an amalgamation plant, there was still very little to show for it. According to Coope (1991), Mercur prospectors brought a legal action in Salt Lake City against an assayer for that very reason.

According to Palmer (1909), the first cyanide mill in the Western Hemisphere was built at the Mercur Mine and, for a time, it was the largest straight cyanide plant in the world. The recovery rate increased substantially after the cyanide mill was converted to the new MacArthur-Forrest cyanide process, becoming the first in the United States to use it commercially. During 1897-98, De Lamar built the 900 tonne-per-day Golden Gate mill, with Jackling serving as his construction and metallurgical engineer. Reputed to be the largest cyanide mill in North America at the time, it employed gravity feed and a flow sheet that involved roasting (when needed), fine grinding and cyanide. Three of the roasters were referred to as “Jackling type.” An AC electrical current was delivered over a 70-kilometre transmission line. This mill recovered about half of the estimated 2.9 million grams (921,000 ounces) that the camp produced between 1890 and 1912, at a recovery rate of 76.7 per cent. If that rate is applied to the entire camp production, it suggests that the gold content of the ore mined during that period was about 37.3 million grams (1.2 million ounces). At its peak in 1890, the camp had a population of about 5,500, most of Italian and Chinese descent.

Production resumed when the official gold price was raised from $20.67 to $35.00 per troy ounce in February 1934, and ended when the government suspended gold mining during World War II. Another 5.9 million grams (190,000 ounces) were produced during this period.

The next phase of exploration was conducted from 1973 to 1975 by Getty Oil Company under an option from Gold Standard, Inc., which had consolidated the complicated claim ownership in the camp. In 1981, Getty began construction of an open pit mine and a 2,700 tonne-per-day carbon-in-pulp mill complex with a targeted production rate of about 2.5 million grams (80,000 ounces) per year. After Texaco, Inc. acquired Getty in 1985, it sold Getty’s mining interests to Barrick Resources (USA) Inc. Barrick, which also had production experience on this type of deposit in Nevada, expanded the mill by 20 per cent, introduced dump leaching of lower grade oxide ore, and stockpiled higher grade, but refractory ore. In 1988, an 825 tonne-per-day autoclave was installed for pressure oxidation of the refractory ore, which was later enlarged about 15 per cent, along with the mill. Mining ended in 1997 and the recovery plants closed in 1998. Total gold production from 1983 to1998 was about 46.5 million grams (1.49 million ounces).

In its 127-year history, more than 37.7 million tonnes of ore containing about 108.9 million grams (3.5 million ounces) of gold were mined in the Mercur gold camp, an average grade of approximately 2.9 grams per tonne (0.084 ounces per ton). About 81 million grams (2.6 million ounces) of gold (at a recovery rate of 74 per cent) and 36 million grams (1.16 million ounces) of silver were recovered, plus 3,470 flasks of mercury. Stripping ratios ranged from 2.0 at the Marion Hill pit to 6.1 at Sacramento, and averaged 3.6. According to Hewitt (1968), the fineness of the gold (not to be confused with grain size) was 833. (Fineness is a measure of the gold content of the native gold, defined as the ratio of gold to gold+silver; pure gold = 1,000.)

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