Sept/Oct 2010

Economic Geology

Nevada-type gold deposits (Part 3)

By R. J. (Bob) Cathro

John LivermoreWell, of course, you have to be an optimist to be a geologist. And if you don’t find what you’re looking for in some place, you always think, well, the next time it will be the one. But plenty of geologists, very good geologists, spend practically their whole careers without coming up with a prospect which developed into a mine…

There’s an awful lot of luck in it. Looking at the (Carlin) program as a whole, we were fantastically lucky because the whole program only lasted three or four months. On a program like this, you might be working on it for a couple of years and end up with nothing. But we got onto this very quickly, and that was - that’s part of the game.

~ John Livermore, 2000 | Photo courtesy of Newmont Mining Corp.


Early prospectors found numerous insignificant gold occurrences throughout northeastern Nevada during the last half of the 19th century, some of which were associated with base metal mineralization. It was not until Newmont discovered the Carlin deposit in 1961 that it became clear that the important gold deposits in this region were of the “invisible,” “noseeum” or “Nevada” type. This is mineralization composed of flakes of gold that are so fine grained that they float on water, cannot be collected in a gold pan and do not concentrate as placer deposits in creeks. The earliest discoveries of this type of gold were at Maggie Creek in 1926, Gold Acres several years before the first production in 1936, Getchell in 1934 (which Newmont mined briefly starting the next year), and Standard in 1934.

This type of mineralization was first recognized by W. O. Vanderburg (1939) of the U.S. Bureau of Mines. Although he predicted that there was probably similar mineralization waiting to be found because it was so hard to recognize, his report generated little interest because the grades were too low and the deposits were small. The Vanderburg report was particularly important to John Livermore of Newmont because, as luck would have it, he had visited both the Getchell and Standard occurrences early in his career and had been intrigued by this type of mineralization. He read the report in the late 1940s, which was fortuitous because R. J. (Ralph) Roberts and other geologists of the U.S. Geological Survey had commenced regional mapping studies in Nevada in 1939 and had covered the area enclosing northeastern Nevada by the late 1940s and early 1950s. They began to publish their results in 1955, culminating in the most important paper (Roberts, 1960). Livermore attended a lecture by Roberts in Elko in the spring of 1961, which gave him an opportunity to have a private discussion about the complicated geological history of this region, particularly the relationship between stratigraphy, the invisible gold mineralization, and a major fault called the Roberts Mountain Thrust (RMT; not named after Ralph).

The RMT is a regional, east-dipping, low-angle fault of Devonian to Mississippian age that was produced by the Antler Orogeny. The RMT had emplaced a low-permeability, fine-grained, siliciclastic assemblage of Ordivician and Silurian age, called the Upper Plate, over a permeable, thin-bedded package of Ordovician to lower Mississippian silty dolomite or limestone (the Lower Plate). Because of lower permeability and reactivity, the Upper Plate has impeded fluid flow and trapped mineralizing fluids within the Lower Plate. The gold is often associated with high-angle faults and replacement zones nearby.

In addition to mapping the RMT, Ralph Roberts and his colleagues recognized that rocks of the Lower Plate had been exposed in places by erosion and that these “windows” through the Upper Plate hosted the known invisible gold occurrences. Livermore became convinced that the USGS mapping had narrowed the best target areas for invisible gold mineralization sufficiently to make it practical to search for larger deposits.

One of the most interesting facets of the Carlin discovery was that it was not accomplished with a large crew and budget and all the newest, most sophisticated equipment. Fred Searls, who had joined Newmont in 1925 and been the chief geologist who pushed the company into gold mining in the California Motherlode in 1929, had become the president of Newmont in 1946 and was now chairman. While he was still keenly interested, exploration was now the responsibility of Robert Fulton, a 1941 mining engineering graduate of the University of Nevada, who (like Searls) had been raised in the Motherlode gold camp.

Kaufman (1992) described the Newmont approach as follows in a chapter titled “The Nevada Noseeum Jackpot”: “Newmont’s exploration budget under Bob Fulton’s guidance was skeletal. … Fulton often used the word ‘parochial’ to describe his desired modus operandi, and in the western U.S. he outdid himself, budgeting for only two full time geologists to cover the whole region. … Newmont of this era was your classic lean and mean organization. There were no frills and no extras; just enough to do the job efficiently.”

John Sealy Livermore was born in the San Francisco area in 1918 and graduated from Stanford University with a geology degree in 1940. After serving in the navy during World War 2, he worked at a number of mines and exploration projects in the southwest United States, including a stint at the Standard Mine. He joined Newmont in 1952, choosing it over other large U.S. mining companies because it was strong on exploration but was not so big that it was bureaucratic. Most of his work for Newmont was spent on foreign assignments until 1960, when he was sent to Nevada and told to scout around for opportunities. Livermore soon convinced Fulton that exploring for noseeum gold mineralization in Lower Plate windows along the RMT could be an important target for Newmont. Fulton agree to a small, cheap program of detailed geological mapping and prospecting in Eureka and Elko counties, supported by fire assays for gold and geochemical analyses for associated metals.

Page 1 of 2. Next
Post a comment

Comments

PDF Version