Episode 368: Alaska, 1898

Today’s episode is from my book, What Things Are Made Of and the chapter that includes gold.

THE LAST GREAT GOLD RUSH began in August 1896, when prospector George Washington Carmack and his two Indian companions, Skookum Jim and Tagish Charlie, found gold in the Klondike River basin in Canada’s Yukon Territory. Two years later, thousands of the gold seekers who had climbed the perilous slopes of Chilkoot Pass were gone, either back to the states or on to other diggings in Alaska. But that summer of 1898 saw a rush of another sort: the rush to understand Alaska’s resources.

The 19-year-old United States Geological Survey dispatched four parties that spring, and the War Department sent two more teams accompanied by geologists. Among other things, they were to “observe and note all occurrences of valuable minerals, giving special attention to the presence or absence of gold, whether in placers or veins.” These early scientific expeditions guided later exploitation of Alaska’s mineral wealth, and established the careers of several USGS geologists.

Spurr's 1898 geologic map of southwestern Alaska

Josiah Spurr was just 28 years old when he led a reconnaissance in southwestern Alaska, but he knew what he was looking for: his previous geological experience in the Yukon gold fields prepared him for any exploration dealing with gold. Spurr’s 1425-mile Alaskan journey with a handful of other scientists in three lightweight cedar canoes resulted in new geologic and topographic maps covering a vast territory. Spurr’s report reflects the dangers of exploration as the 19th Century came to a close. The team arrived at Tyonek on Cook Inlet, on April 26, 1898, but could not head upriver until the ice began to break up on May 4. After reaching the Susitna River mouth, their intended portal to the interior, the weather forced a delay until May 20 when the river became sufficiently ice-free for them to travel. Even then, after they “had gone several miles, we were surprised by a solid wall of ice bearing swiftly down upon us, and we had only time to throw our load upon the banks and drag the boats out of the water before the ice jam swept past, piling over upon the banks in places and grinding off trees.” Spurr’s narrative reads more like an adventure story than a scientific document.

The Spurr Expedition coined the term alaskite, a word for a particular light-colored granitic rock. The scientists observed considerable mineralization associated with southwestern Alaska’s granites, though gold occurred only sparingly. Nonetheless, in 2007 Alaska was the second-leading gold producing state in the U.S., with more than 700,000 ounces, mostly from mines near Fairbanks and Juneau. Alaska’s production is a distant second to Nevada, the heavyweight in the U.S. gold-mining industry with around 5,000,000 to 7,000,000 ounces per year. The United States exports gold, in a virtual three-way tie with Australia and South Africa for second, third and fourth place in the world after China.

Josiah Spurr’s work on western U.S. mineral deposits gained him considerable fame, and he wrote a book on economic geology. In the 1940s his work focused on the moon – earning him a crater named Spurr to go with Spurr Volcano in Alaska and the mineral spurrite, a complex calcium silicate. Another 1898 Alaska explorer, Walter Mendenhall, became the fifth Director of the U.S. Geological Survey in 1930 and gave his name to a glacier near Juneau. Alfred Hulse Brooks, chief Alaska geologist for the USGS from 1903 until he died in 1924, explored Alaska’s interior Tanana River valley in 1898 when he was just 27 years old, and was honored by the naming of the Brooks Range in 1925. The rare mineral hulsite, an iron-magnesium-tin borate, discovered at Brooks Mountain on the Seward Peninsula, also bears his name. George Eldridge and Robert Muldrow led an 1898 expedition that accurately pegged the height of Mt. McKinley, or Denali, at 20,464 feet – remarkably close to today’s value, 20,306 feet. Glaciers descending from Denali’s flanks recall their names. In this way, gold set the stage for Alaskan geological investigations that continue into the 21st Century, and pointed ultimately to the United States’ largest oil field, Prudhoe Bay, and the world’s largest known zinc deposit, at Red Dog, in the western Brooks Range of northern Alaska.

—Richard I. Gibson

Spurr's complete report: Spurr, J.E., 1900, A reconnaissance in southwestern Alaska, 1898, in Walcott, C.D., Twentieth annual report of the United States Geological Survey, 1898-1899: Part VII - explorations in Alaska in 1898: U.S. Geological Survey Annual Report 20-VII, p. 31-264.

October 11. Gold in California

Piping four streams from monitors (giants), with aggregate discharge of 2,500 miner’s inches at a hydraulic mine. Material is washed through bedrock cuts to the sluices which are not visible. Previously published in Annual Report of the State Mineralogist, v. 10, p. 122. Nevada County, CA, 1890. SOURCE

The granitic rocks of the Sierra Nevada Batholith that we discussed yesterday contain gold – enough gold to be a driving force in American history. You all know about the Gold Rush to California in the late 1840s and 1850s. It put California on the map, almost literally, and had a seminal influence on the course of America’s story.

Much of the California gold rush gold was found in placers, pockets in stream beds where gold weathered out of the granite had been concentrated by running water, nature’s own sluice box or gold pan. Miners followed the gold-bearing streams into the mountains to discover the outcrops, or lodes from which the eroded gold came. The California Mother Lode is a long belt of gold-rich rocks more than 100 miles long and typically one to three miles wide. The narrow zone that contains the gold appears to be a suture, the join line between two chunks of crust.

In this case, the suture seems to be between two island arcs that were colliding with western North America as part of the subduction process that gave rise to the Sierra Nevada Batholith. The Smartville Terrane might have been something like the modern Caribbean volcanic island arc, or perhaps more complex, like Japan or parts of Indonesia, but as it interacted with the subduction zone along the western margin of North America, it was pushed down deep enough into the earth that hot water and perhaps some actual magma, or molten rock, was given off. As those hot materials rose, they dissolved and concentrated metals including gold from the surrounding rocks. The gold eventually solidified into veins that filled cracks in the country rock.

The east side of the Smartville Block is a complex fault zone, the sort of thing you might expect to find along a suture between two colliding blocks. Most of the gold is concentrated west of the fault zone. It appears that the Smartville volcanic island arc might have been broken by a rift even as it was colliding, or maybe just before it collided. You can have extension, rifting, in an overall collision, or compression zone, in several ways. Sometimes the area between the compressed arc and the continent, or whatever it’s colliding with, is actually under tension, and a thing called a back-arc basin forms. There are other ways to get extension within compression zones too – these are really pretty complex systems. Bottom line, there are some diverse rocks within the Smartville Igneous Complex that point to a pretty complicated history.

Why is there so much gold right there, right along the suture line? There must have been some considerable amount of pre-existing gold either in the crust or the rocks of the island arc, something for the hydrothermal waters to concentrate into the ore veins of the Mother Lode. Exactly why that pre-existing gold was there is a good question, and I don’t think we have a good answer for that. Possibly something as simple as an ancient concentration of gold in the early earth, a plum in a plum pudding where the plums were scattered through the earth in irregular fashion.

At its peak in 1852, California produced 121 tons of gold. Today, the United States produces about 230 tons of gold per year, second to China and Australia. The leading producer by far is Nevada, whose gold comes mostly from the Carlin Trend, where production began in 1965. We talked about that gold deposit back on April 29.

If I find any thing more definitive about the ultimate reason for so much gold in the Mother Lode, I’ll provide an update.

* * *

October 11, 1737, is the date given for a devastating earthquake and resulting tsunami that destroyed 20,000 ships and killed perhaps 300,000 people in Calcutta, India. Contemporary accounts estimated only 3,000 deaths, and recent scholarship suggests that there was neither an earthquake nor a tsunami, but rather that the destruction was the result of a tidal surge pushed up the Hooghly branch of the Ganges River to Calcutta by a strong cyclone offshore in the Bay of Bengal. Historical accounts can support both ideas, but the geometry of the coast and the likely positions of tsunami-generating earthquakes make the earthquake idea less appealing. Calcutta is about 50 kilometers – 30 miles – inland, and while it’s not impossible for a tsunami to reach that far, it’s not likely. One might say that it would be a challenge for a hurricane-generated tidal bore to reach that far up a meandering stream, but something certainly devastated Calcutta back in 1737.

—Richard I. Gibson

Smartville Intrusive Complex

Photo: Piping four streams from monitors (giants), with aggregate discharge of 2,500 miner’s inches at a hydraulic mine. Material is washed through bedrock cuts to the sluices which are not visible. Previously published in Annual Report of the State Mineralogist, v. 10, p. 122. Nevada County, CA, 1890. SOURCE

April 29. Carlin gold

The Roberts Mountains Formation in Eureka County, Nevada, is mostly carbonate, limestone and dolomite that was laid down in a shallow Silurian sea. Interesting rocks, certainly, but geologists and prospectors alike walked over those rocks for decades without realizing it was the host to one of the largest gold deposits on earth. 

They missed the gold because it’s in the rock as tiny tiny grains – often smaller than a micron. A micron is one-one thousandth of a millimeter. Pretty small. Gold was discovered in 1961, and the first mine began production in 1965, near Carlin, Nevada, which gives its name to these “Carlin-type” gold deposits. 

Goldstrike Mine, Carlin Trend, Nevada

What happened is that the limestones and dolomites, which are soluble in even slightly acidic water, were pretty much turned into a really finely porous sponge – many tiny holes were dissolved in the rock. And the water that dissolved the holes – or maybe later water – was mineral rich, and carried gold in solution that precipitated into those little holes. That happened long after the Silurian rocks were laid down around 417 million years ago. The gold mineralization of these rocks probably happened more like 40 million years ago, when Nevada was beginning to be pulled apart and big normal faults were starting to form. Those faults probably served as conduits for the hot, acidic, mineral-rich waters to percolate through the Silurian strata.

Exactly where the mineral-rich waters came from is controversial – did they pick up gold as they passed through older rocks, leaching out the gold and then redepositing it here? Or did the water come out of magma, molten rock, deeper in the earth? There’s geochemical and geophysical evidence to support the magmatic idea, that the stuff came from molten rock deep down in the earth’s crust, but I don’t think this question is fully resolved.

The gold ore at Carlin is typically only 1 to 10 grams of gold per ton of rock. A gram is about the mass of a paper clip, so you can see how the gold must have been really thinly scattered through the tons of rock – but there was a lot of it. The Carlin Trend in Nevada has produced way more gold than the Mother Lode in California, and today The Silver State – Nevada – produces about 80% of all the gold mined in the United States, and more than 10% of world gold production. The Carlin mines passed the 50-million-ounce mark in 2002 and 70 million ounces by 2008, and they’re still going strong. New mines continue to be opened along the 5-by-40-mile zone, whose production is more than $85 billion at 2010 gold prices.

Much of the gold at Carlin is produced from open-pit mines. It’s the second-largest gold district in the world, second to the Witwatersrand in South Africa, where the gold comes from underground mines almost 2½ miles deep, the deepest on earth. With Carlin approaching 100 million ounces of gold, it’s a distant second to Witwatersrand, which has produced about 1.5 billion ounces of gold since it was discovered in 1886. That’s about half the gold ever mined on the planet. The origin of the gold there is about as different as possible from Carlin. The South African gold is related to a huge meteorite impact more than two billion years ago, back in the Precambrian.

The main use – 38% – of gold in the United States is in electronic components, because gold is an excellent non-reactive conductor. Things like computers are among the main consumers. Jewelry takes another third, coins amount to about 19% of consumption, and all the gold in all the new gold crowns and other dental uses adds up to about 5% of US gold each year.

Thanks to Carlin, Nevada, the US is a net exporter of gold, but China produces twice as much. Australia is #2 in world gold production, and the US and Russia are usually about tied for third place. 

—Richard I. Gibson 

Photo of Goldstrike Mine, Carlin Trend, from USGS 

Links:

http://en.wikipedia.org/wiki/Carlin%E2%80%93type_gold_deposit
http://www.geotimes.org/apr06/feature_GoldOrigins.html
http://minerals.usgs.gov/west/projects/nngd.htm
http://goldinvestingnews.com/6070/the-geology-of-the-carlin-trend.html