Does anybody know why there's no gold around the Portland area
? I was having a debate with my buddy that the southern Oregon and Eastern Oregon Geology must be older and more eroded ...
This is super old, but I reckon people looking for gold around Portland still stumble across this.
First of all, I'm not a geologist -- just a fairly recent, amateur geology enthusiast. This is very simplified and probably gets a couple things wrong, but it's roughly correct:
Your buddy was on the right track. The Blue Mountains and the eastern part of the Klamath mountains are both very old -- pieces of which began accreting to the North American continent as long as 260 million years ago. The slabs came in on the old Farallon plate (Juan de Fuca is the last remnant) and stuck to the continent as the oceanic plate subducted under N. America.
Multiple fragments of rifted continental crust and volcanic mountain ranges rammed into NA over many millions of years. Both the Blue Mountains the Klamaths are related to the Northern Sierra Nevadas, by the way -- they look discontinuous because of clockwise rotation
The subduction of the plate caused magmatism, and associated intrusions and mineralization. Because these mountains are so old, there has been much erosion to expose the mineralized rock -- and unlike the Western Cascades, they haven't had volcanoes spewing material over the top of the exposed mineralization.
About 50 million years ago, an enormous pile of basalt called Siletzia came in on the old Farralon plate and docked to North America. There was no Cascades range at the time (but the rock making up Klamaths and Blue Mountains was already there). This enormous pile of basalt was so large it jammed up the Farallon subduction zone, and stopped subduction (which also shut down the uplift of the Rocky Mountains).
Some time later, subduction reinitiated farther to the West -- the current Cascadia subduction zone, with the Farallon fragment Juan de Fuca plate subducting under NA. This new subduction zone touched off volcanism that began forming the Western Cascades -- around 35 or 40 million years ago.
The Western Cascades are pretty old -- old enough for their to have been enough erosion with uplift and streams cutting down to expose mineralized ground, but not so much as the older Klamaths and Blue Mountains. Also, as the angle of subduction changed, the arc of volcanism shifted to the east, creating the much, much younger High Cascades. The High Cascades have dumped a whole lot of volcanic overburden in parts of the Western Cascades, what with the lava flows and ash fall and pyroclastic tuff. So there are mining districts in the Western Cascades -- Bohemia, Fall Creek, Blue River, Quartzville, N Santiam -- but they don't extend very far east because of the very young and active High Cascades.
But to your question about the Portland area's lack of gold -- some of this I'm a little sketchy on, but the High Cascades to the east of Portland seem to extend farther west than they do in more southerly parts of the range. So you don't have so much of the older, highly weathered Western Cascades-type range up here east of Portland.
Additionally:
Starting around 17 million years ago there started a series of these absolutely gargantuan flood basalt eruptions that covered a big chunk of eastern Oregon and Washington in a very large amount of lava, some of which flowed into the Portland basin (and even down the Columbia all the way to the northern Oregon coast).
Also, around Portland you had this very odd volcanism that occured called the Boring Lava Field, which started erupting around 2 million years ago to as recently as 57,000 years ago. So that covered potentially mineralized ground to some extent also, I suppose.
And then towards the end of the last ice age, a series of enormous floods from a glacial dam breaking came roaring across Washington, into the Columbia drainage, and sent 400 feet of water smashing into the Portland basin, depositing a whole lot of rocks and gravels (some very large, encased in ice chunks). More overburden.
So that's very roughly why there's more gold and platinum to be mined in southern Oregon and eastern Oregon's mountains, and why there's more precious metal farther south in the Western Cascades. Roughly.
I've learned so much since 08' and my back isn't so young anymore....lol.
