|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.
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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