November 2010

Economic Geology

Nevada-type gold deposits (Part 4)

By R. J. (Bob) Cathro

“One thing we did was kind of reintroduce the idea of prospecting. People in the States didn’t necessarily believe in prospecting. They thought you had to have either a geologist or more sophisticated exploration, but my experience in Canada indicated that prospecting was still very useful… Anyway, we introduced the idea of prospectors, and also the idea of an incentive bonus, which I always thought was a good idea. There again, that came from Canada because they didn’t do that much in the United States. Canada was very strong on that.” ~ Livermore, 2000

“There are many fine geologists who are simply not good prospectors… In Canada, (Newmont) used prospectors extensively and found them to be very effective if they were knowledgeable about a particular area or type of mineralization.” ~ Livermore, 1996

At the end of 2008, nearly 50 years after the Carlin deposit was discovered, Nevada ranked as the fourth largest gold-producing area in the world after China, Australia and South Africa. Moreover, it was the world leader in terms of gold production per unit area (611 tonnes per million square kilometres). Production that year of 5.7 million ounces (54 per cent from the Carlin Trend) came from 19 major mining operations, while at least 68 companies (juniors and majors) drilled at a minimum of 123 projects. At the end of 2008, the mining industry held 70.2 million ounces of gold reserves in Nevada, enough to sustain production at current levels for about 12 years, plus another eight years in the resource category (Price, 2008). The 2008 production was actually 35 per cent lower than the peak production of 8.85 million ounces in 1998 (Driesner & Coyner, 2009).

The Carlin trend is now recognized to be a 60-kilometre long, northwest alignment of predominantly carbonate-hosted gold deposits, which Newmont continued to dominate until the late 1980s and early 1990s. It began to lose the lead after American Barrick Resources, a subsidiary of the Toronto-based company, acquired the Goldstrike property at the north end of the trend from Western States Minerals Corporation and Pancana Minerals in January 1987 for $62 million. Reserves at that time were 19.6 tonnes (0.63 million ounces) in 10.3 million tons of oxide ore grading 1.9 g/t (0.055 oz/ton). Barrick recognized the depth potential and expanded the Goldstrike orebody into a diverse group of Carlin-type deposits, including some of the largest and highest grade examples known. One spectacular hole drilled into the heart of the Meikle deposit in 1989 intersected 136 metres of 16.1 g/t (0.47 oz/ton) between depths of 416 and 548 metres. At the end of 2001, the largest deposit, Betze-Post, contained approximately 1,244 tonnes (40 million ounces), and Meikle contained 198 tons (7 million ounces) at an average grade of 24.7 g/t (0.72 oz/ton). Goldstrike is part of a larger sub-district that had a total gold endowment, at that date, of 1,960 tonnes (63 million ounces) within an area of 8.5 by two kilometres, placing it among the largest concentrations of gold on earth. This success was a product of the evolution in understanding of geology and ore controls, supported by the application of geochemical and geophysical exploration techniques (Bettles, 2002).

Most of the Goldstrike mineralization is hosted by limy to dolomitic mudstones of the Devonian Popovitch Formation and brecciated limestones and dolomites of the Devonian-Silurian Bootstrap Limestone unit. The age of the gold mineralization is approximately 39 Ma, coeval with late Eocene dykes. Gold deposition favours the presence of reactive carbonate, porosity, permeability and the presence of iron, which can be sulphidized to form auriferous pyrite. Dissolution of carbonate has produced collapse breccias, which commonly host high-grade ore. The deposits have strong structural controls, with enhanced fracturing within 300 metres of the contact of a Jurassic diorite intrusion providing a first-order control of the Betze-Post deposit. Most of the gold in unoxidized ore is found in arsenian pyrite overgrowths on pre-ore pyrite and is associated with Hg, Sb and Tl. Marcasite and arsenopyrite are also present and the total volume of these sulphide minerals averages about 5 per cent. Supergene alteration has produced oxide ores to depths of up to 200 metres. At the Betze and Deep Post sub-deposits, mineralization occurs as stacked ore zones that attain a vertical range of 450 metres adjacent to the Post fault system (Bettles, 2002).

Wall-rock alteration is very critical in the formation of Nevada-type deposits. The most salient features are carbonate, in the form of decalcification (calcite removal) and more advanced decarbonatization (calcite/ dolomite removal); silicification, particularly within structural conduits; argillic alteration of primary silicate minerals; and gold-enriched sulphidation of reactive iron to form gold-bearing sulphide minerals. The general alteration progression includes the following major assemblages (from distal to proximal): limestone (calcite + dolomite + illite + quartz + Kspar + pyrite), to weak to moderate decalcification (dolomite halo), to strong decalcification, to decarbonatization (quartz + kaolinite/ dickite + pyrite +/- gold; (Teal & Jackson, 2002).

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