Meikle may signal end to `easy’ Nevada finds

Mention Nevada’s Carlin Trend today and there is the immediate vision of gold, and lots of it. There’s more to it than that, though; the name also flags a major advance in metallurgy and deriving from that, a whole new way of looking at the exploitation of gold ores.

The Carlin Trend is a major gold producer in its own right, and just as importantly, it has sent a dozen companies scurrying all over the state to find similar deposits. A score of new deposits has been found as a result. The latest project for which a development plan has been announced is the Meikle gold deposit of American Barrick Resources (TSE).

The original “silver state” of the American west, Nevada has transformed itself into the golden spout. The state produced 5.7 million oz. gold in 1990 and of that total, 2.08 million oz. came from the Carlin Trend. For comparison, Canada’s gold production in 1990 was 5.3 million oz. The Carlin Trend is a series of rocks in northeastern Nevada that have been rolled over on top of themselves (to the geologist, an overthrust). Older rocks lie on top of younger rocks: that is opposite to normal. Attention was drawn to peculiarities of the structure by Ralph Roberts of the U.S. Geological Survey, and geologists of Newmont Gold (NYSE), then known as Newmont Exploration, were intrigued.

Newmont staked its first gold claims on the Carlin Trend in 1961 and the newfound Carlin mine went into production in 1965. Five additional large-tonnage deposits were subsequently located in the same area during the next 15 years and Newmont was well launched as a major world gold producer. The company was the first into the area. It owns the lion’s share of the 50-mile structure, but it’s some way from owning all the deposits Two characteristics of the orebodies have had profound implications. First, the gold is submicroscopic. It completely escaped the attention of past prospectors. Outcropping ore grading 1 oz. gold per ton was passed over without a suspicion being aroused. The ore was left for today’s companies to develop.

Second, most of the orebodies are in the multimillion-ton category. They are typically refractory, with the gold resistant to orthodox cyanidation. Large tonnages are located above the watertable, however, and while low grade, this ore is oxidized and yields readily to cyanidation.

These large tonnages of readily minable ore represented a major resource to Newmont but they were too low grade to handle in a conventional mill. The question was, how to handle it?

At this point, the Reno, Nev., research establishment of the U.S. Bureau of Mines came into the picture and what materialized was little short of revolutionary: the answer was cyanide heap leaching technology. Gold recoveries would be lower than from conventional milling but processing costs would also be low.

The first commercial heap leach went into operation at the Carlin mine and it wasn’t long before it was adopted at Battle Mountain, Pinson and then a dozen others. The technology is spreading across the globe wherever there is low-grade gold mineralization to be mined: it is one of the milestones of 20th-century metallurgy.

So far, all the new gold producers in Nevada have been open pit operations. There were 22 companies producing at a level greater than 50,000 oz. gold per year in 1990 and of these, Newmont and American Barrick operated a total of five pits on the Carlin Trend. Eleven of the 22 companies were Canadian and eight were American, with the remaining three either from Britain or South Africa.

According to Robert Smith, president of American Barrick, “all the easy-to-find surface orebodies have been located — from now on, we will be seeing a lot more headframes.” He made this comment while describing the development schedule for the company’s new Meikle mine.

Previously known as the Purple Vein, Meikle is estimated to carry a proven and probable reserve of 7.2 million tons grading 0.63 oz.

Meikle will be the first underground gold mine in Nevada for well over half a century. It will also be the first to tackle the deep, waterlogged strata of the Carlin Trend. At a forecast pumping rate of 65,000 gallons (US) per minute, and with the water at 135-140F, it will be the world’s wettest mine. At the present time, that dubious record is held by the Konkola copper mine in Zambia; the pumping rate there is 58,000 gallons (US) per minute.