Portland & Gold Panning

[SilverSalmon]

Re: Portland & Gold Panning

lol I can relate! I remember us kids working all day loading up all our backpacks and wagon to bring home gold so we could be rich and then when we got there and Dad saw it, he made us haul it all back out....told us it was fool's gold or iron pyrite. Talk about crushed...we thought we had hit the motherlode and solved all the money problems!! lol

LoL. talk about a heart breaker. It's nice to feel rich for a few seconds though...

 

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[SilverSalmon]

Re: Portland & Gold Panning

Here's a link to a cool Geothermal map for Oregon...

http://www.oregongeology.com/sub/default.htm

 

[cacarr]

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.

 

[Tuberale]

Thanks for posting, cacarr! This is one of your first, too. That pretty much explains the geology of the Portland area.

 

[ArfieBoy]

A great book to read for a general knowledge survey of the geological processes and building of the Pacific Northwest is: "The Restless Northwest: A Geological Story" by Hill Williams. It pretty much agrees with what cacar has written about the geological forces and ages of the mountain ranges of the area. I found the book interesting and informative, but you must remember it is a "general survey" not a deep scientific study. Get it!

 

[Tuberale]

There is flour gold in the Columbia River. You may need a gold dredge to find it though.

 

[SilverSalmon]

This is pretty amazing I just ran across this old thread started by me 12 years ago......hahaha. It came up in a google search...what a surprise. This was the start of my gold journey I've learned so much since 08' and my back isn't so young anymore....lol.

I haven't been to the Molalla in 10 years or so and I'm heading out today to get some flour gold dirt to run through my recirculating sluice til the weather starts breaking up in Eastern Oregon. I couldn't remember where we used to go so I was searching "gold panning on the Molalla".

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cacarr what a great explanation. This is exactly what I've learned since then. Anyone who would like to learn about NW geology I would highly recommend videos by
Nick Zentner on youtube -- he's a fantastic teacher who takes you as a beginner and builds the learning blocks up seamlessly through his lecture.

One thing I've learned since I started & can share with anyone new getting into gold hunting --- the folks who get the richest are the ones selling the shovels & equipment....hahahha. Where everyone gets rich no matter what is the adventures and remote places gold hunting will take you.

Good Luck to all my fellow NW gold hunting Brothers & Sisters -- have fun on your gold adventures

 

[Johnnybravo300]

I sold my bazooka prospector a few years ago to a guy in oregon. He paid me in gold from his claim nice chunky pieces and a couple of nuggets.