Difference between tertiary and intervolcanic ancient river channels

[wjbell]

I've got the big 10 gold map for my area (map 5) that shows where gold was found and also the tertiary and intervolcanic river channels. As far as I can gather, the intervolcanic channels happened after the tertiary.

Can anyone explain the meaning of the two?

[wes]

There are some dead rivers that are mainly quartz and are absent of any volcanic gravels. These flowed before a volcanic era. I believe these are the ones often referred to as tertiary. Then there are old channels mixed together with quartz and volcanic rocks, formed after the volcanic era. There are also volcanic flows of cement and mudflows of sediment all of which can also carry gold but were never true rivers.

[Sean.C]

The Tertiary time period is a broad period ranging from about 65 - 2.5 million years ago. During the Eocene epoch ( 55-34 million years )is when the "Ancient" auriferous river channels formed, which contained mainly quartz materials. So when they refer to a Tertiary gold bearing river they usually are referring to the Eocene or earliest rivers of the period.

Later in the Tertiary during the Miocene (23-5 million years) volcanoes east of the Sierra's erupted covering the ancient rivers in an ash flow. With the old channel covered, the water began carving out a new path through the volcanic ash flows and sometimes back down through a small portion of the ancient channel. This is what is referred to as an intervolcanic channel.

This is my understanding of it at least maybe one of the experts can chime in.

[wes]

Yes the volcanic channels never were really rivers as you would see today. They were mad raging lava flows of cement, ash, mud etc, that sometimes buried or mixed with the ancient true rivers. My opinion

[wjbell]

The Tertiary time period is a broad period ranging from about 65 - 2.5 million years ago. During the Eocene epoch ( 55-34 million years )is when the "Ancient" auriferous river channels formed, which contained mainly quartz materials. So when they refer to a Tertiary gold bearing river they usually are referring to the Eocene or earliest rivers of the period.

Later in the Tertiary during the Miocene (23-5 million years) volcanoes east of the Sierra's erupted covering the ancient rivers in an ash flow. With the old channel covered, the water began carving out a new path through the volcanic ash flows and sometimes back down through a small portion of the ancient channel. This is what is referred to as an intervolcanic channel.

This is my understanding of it at least maybe one of the experts can chime in.

Thanks Sean and wes. Really good explanations. I have no idea if areas in the paths of these ancient rivers are worth searching out for placer gold, but the intervolcanic paths of the rivers are the latest and might have a better chance at being exposed over time?

[wes]

Both are good places to prospect!!! Many volcanic channels were drift mined as they were very deep.

[wes]

Heres a pic I have, inside a volcanic cemented channel that was drift mined. This one had a lot of quartz bolders on slate bedrock.

[wes]

pic:

[wjbell]

Heres a pic I have, inside a volcanic cemented channel that was drift mined. This one had a lot of quartz bolders on slate bedrock.

Two questions. Say there aren't any mines at the volcanic channel, put the modern day river crosses it's path. Any chance erosion from the river could expose part of the volcanic channel? Or are all the channels hundreds of feet deep and have to be hard rock mined?

How can I find out where old productive mine shafts are in or around the Placer county area? And which ones are safe enough to enter. (I know there's a lot of dangers in old mines, I wouldn't be taking any chances...)

[wes]

Definitely. Thats how the old prospectors found them. Generally if a piece of old channel was found exposed at the surface, where it was cut by the new rivers, they would hydraulic as much as they could if possible. If the overburden became to deep, or a good water supply was not readily available, they would start drift mining into the channel, trying to follow the pay streaks on bedrock.

[Guest bedrock bob]

Two questions. Say there aren't any mines at the volcanic channel, put the modern day river crosses it's path. Any chance erosion from the river could expose part of the volcanic channel? Or are all the channels hundreds of feet deep and have to be hard rock mined?

How can I find out where old productive mine shafts are in or around the Placer county area? And which ones are safe enough to enter. (I know there's a lot of dangers in old mines, I wouldn't be taking any chances...)

You are taking chances when you enter any mine..period. Many are patented and many are claimed...none are safe unless they are operational, and even then they might be a disaster. I know nothing about the mines in your area but I know a lot about mines in general...STAY OUT! Even an experienced hardrock miner is always cautious and never bold. Without ventilation, the proper equipment, and a line of communication your head should never go below the surface.

Bob

[wjbell]

You are taking chances when you enter any mine..period. Many are patented and many are claimed...none are safe unless they are operational, and even then they might be a disaster. I know nothing about the mines in your area but I know a lot about mines in general...STAY OUT! Even an experienced hardrock miner is always cautious and never bold. Without ventilation, the proper equipment, and a line of communication your head should never go below the surface.

Bob

Yeah, I agree. Not only that, I'd be afraid of what kind of animals decided to make it their home.

I was thinking more along the lines of detecting the entrance of the mine and around the area of the entrance, nearby tailings, etc. When I said enter above, i meant only a few feet it it looked sound.

Speaking of that, There's a small mine at the Yankee Jims bridge at the NF American. It's only cut into the mountain about 10 feet back and then stops. I always wondered about that. I guess they thought there was something there but ended up not finding anything?

[Bunk]

I have seen lots of animals come running out of adits, would not want to be in there way.

Bunk

[Hoser John]

Looks ARE deceiving ESPECIALLY lava capped channels-remove the wrong rock and a whole bunch can very easily come down on your head. We have many MANY lava capped channels here in shasta county. One of the biggest is just up from Lamoine and runs UNDERNEATH the freeway and you can go in and listen to the semis rumble along right over your head. Another standout was Feather river outta Homegarden,UNDER the feather river but when I was underwater to my armpits and leaned on a HUGE 2' x 2' pillar my hand went right through the mushy thang and scart my ol'arse outta there pronto. Always with at least a friend,with a hardhat,tied off to a rope,with 3 light sources and BE CAREFUL-John

[Bunk]

So, here in AZ, would that be intervolcanic activity that covered up the old rivers? Being self taught is rough!

Bunk

[nvchris]

Another drift mine, this one is near BLANK CA. I did a survey here a few years ago. Nice gold, safer than most hard rock mines I've been in.

[David Wiseman]

http://books.google.com/books?id=cpYNAAAAYAAJ&pg=PA176&lpg=PA176&dq=sierra+deep+blue+lead&source=bl&ots=2zb9H_-Xwe&sig=CHikH76J4R9SKeiu11TNYo5MY8A&hl=en&ei=0MfQS8UPkoiyA6LfgN4J&sa=X&oi=book_result&ct=result&resnum=3&ved=0CA0Q6AEwAg#v=onepage&q=sierra%20deep%20blue%20lead&f=false Lindgren is the #1 source for this stuff,however every county in California has a mines and mineral report issued by the state plus numerous special bulletins etc, The main state library is in Sac,as well as BLM libraries,U.S. geological survey reports,maps etc.Everything is right in your area.