Treasure of the Sierra Nevada
One of the seminal events in US history was the discovery of gold in California and the massive amounts of the yellow metal that were infused into the US economy (not to mention the upheavals in California itself). There has been some mystery about the source of all this gold; a few things about this cropped up at GSA last week.
First off, Erin Marsh gave an initial exploration of the source of some of the paleoplacer gold in the Auriferous Gravels. Although many workers had noted that the Eocene deposits only had gold downstream of lode gold deposits, there has bent he suspicion that some of this gold had come in from the east, perhaps even derived from the Carlin-type gold deposits in eastern Nevada. The limited work to date tends to confirm that the Auriferous Gravel gold is locally sourced, having about the same chemical composition and with similar kinds of associated quartz.
Ryan Taylor spoke on the tectonic conditions associated with the emplacement of the lode gold. Dating gold deposits has always been a bit tricky as the gold itself is not datable, so dates are coming from associated minerals in the veins with the gold. In some instances the phase is monazite, other places the mariposite within the quartz veins can be dated. Although the dates range from 160-110 Ma, most of the dates land in the 130-120 Ma range. Much of this talk was focusing on the nature of the source fluids in the Grass Valley area, where there is a pluton of about the right age for those gold deposits, but the conclusion here is that the fluids were (broadly speaking) metamorphic and regional and not magmatic in origin. Emphasis here was that the emplacement of the main gold deposits was presumably tectonic as the 130-120 Ma range is in a classic magmatic lull; the main event described in the talk was the separation of the Klamath Mountains from the Sierra.
However, the story of a magmatic lull from 140-120 Ma took a blow when Jason Saleeby reported on ages from basement cores from under the Central Valley. The mafic plutonic rocks from under the sediments of the Great Valley Group fall right into that 140-120 Ma gap. So there was a magmatic arc, but it was to the west of the metamorphic rocks of the Sierran foothills. This means that the gold was being emplaced while the region was in the backarc, where heat flow is high, instead of within a quiescent arc. This changes the way we would look at the emplacement of gold; the added heat and the activity of some large fault zones in this time period appears to be the unique combination that led to most of the gold emplacement of the Mother Lode.