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digger68
10-12-2012, 02:17 PM
While running my 5" dredge on some of my claims this summer I found a decomposed layer. At about 10' down I found a cemented layer of gravels that were very hard and mineralized. Very rusty layer. There was good gold on top of the cemented layer and alot of it was course gold. Above this cemented layer was a foot to foot and a half of very decomposed round rocks. There was 2' round granite boulders that I would hit with the dredge nozzle and POOF they would suck right up the house. The granite was the most decomposed but the other rocks were as well. These rocks were so decomposed that I couldnt bring a 1 foot rock to the surface without it falling apart. It was really interesting watching these large rocks just turn to sand when disturbed. I am wondering what was the cause of this? I know I am down to an area that is really old but do not have enough geological knowledge to know what I am finding. This is a clear water stream but at 6' or so there is a 6" to 8" layer of glaicial silt. This layer also had really nice gold in it. On one side of my creek the mountains are new and on the other side they are wore down and old. It seems to me that the creek is a crack in the earth between the two mountain ranges. I haven't found bedrock yet but there is good pay in some of the layers and I wonder if bedrock might hold alot of gold. I thought I would post an here and pick the brains of some folks that have more knowledge than I do about such things. THanks for your thoughts.

geowizard
10-12-2012, 02:41 PM
digger,

Here's a good explanation:

http://en.wikipedia.org/wiki/Decomposed_granite

An added component in Alaska is the freeze-thaw cycles. Freeze-Thaw thousands of freeze-thaw cycles. Then try tens and hundreds of thousands of freeze-thaw cycles on top of the natural chemical decomposition and you begin to get the idea.

In Arizona, granite tombstones begin to decompose in a few years. The cycle is due to moisture and heating. The heat causes the water to expand!The water contained in the pore spaces - is working to disassemble the rock!

I hope this helps! :)

- Geowizard

digger68
10-12-2012, 03:02 PM
Wizard if the freeze-thaw cycle was responsible then why are the rocks in the top 8' not affected? In this part of central ALaska the ground does not freeze to 10'. I thought it might have something to do with the highly mineralized layer under it? It is not only the granite rocks it is all of the material at that layer that is decomposed. The only exception is these black fist sized rocks that were still hard. I appriciate your input but I dont think the freeze-thaw cycle is what is causing this decomposed layer.

geowizard
10-12-2012, 03:28 PM
digger,

You are probably right. Since we don't "know" what the rocks were exposed to we only "know" that granite decomposes as shown in the reference given above.

Another thought just occurred to me. Rain water is naturally acidic. It contains a very miniscule amount of Carbonic acid. Remember the acid rain hysteria? Acid rain works on rocks to chemically disassemble the grains. Question: Were there periods where more carbon was emitted from volcanic activity that increased the levels of carbonic acid? It is certainly reasonable to think so. The result would be an increase of the chemical break-down in rocks that are susceptible. Some rocks as you mentioned are not porous or not composed of chemicals that can be readilly attacked by chemical action.

Oxidation of sulfide rocks causes decomposition and the sulphides decomposing into sulphur - then recombining with water would create another form of acid called sulphuric acid.

Volcanos also emit sulphur dioxide (SO2) combined with water (H2O) = H2SO4 = sulphuric acid.

- Geowizard

shaftsinkerawc
10-12-2012, 05:48 PM
Did you bring a sample of your "cemented layer" back so you can crush it up and make sure it doesn't carry good pay?

digger68
10-12-2012, 07:43 PM
shaftsinker, no I didnt bring a sample back but I think that there is probally going to be pay in it. There is good pay on top of it, I dont know about what is actually in the cemented layer. I did make a hole through the layer and found more river gravels under it. Next season I am planning on taking out some heavy equipment and doing some mining. We found enough gold in our testing to show that it is worth mining and I am looking forward to getting a pit opened up and seeing what the layers look like out of the water. Once I get the pit dug it will be alot easier to determine just where the gold is laying and hopefully get to bedrock where there hopefully will be large amounts of gold. With the amount of gold we are getting off of the silt and cemented layers I have high hopes for large amounts of gold on the bedrock. Unless the gold was deposited later and has beed trapped by the cemented layer? Some of the gold is quite rough and doesnt appear to have traveled far.

Gold Seeker
10-13-2012, 08:15 AM
In the southeastern USA we have what is called Saprolite, i.e. chemically weather rock most of it is granite and it does exactly what you described just turns into dust when handled or using a dredge nozzle and it holds a lot of gold down here, whether the gold is contain within the Saprolite or is deposited within the cracks of the decomposing rock I don't know but I think it a little of both, so I think the granite rock you're finding were at one time as Geowizard mentioned was exposed to acid rain before the stream was or beginning to be formed and they were starting to get chemically weathered which continued after they were covered by the other overburden that came afterwards.

The only difference is that Saprolite is formed when acid rain seeps down to the bedrock and chemically decompose it, but in your case the rocks you're finding aren't actually bedrock, but otherwise they sound just like the Saprolite that I find here in the southeast.

http://en.wikipedia.org/wiki/Saprolite

Bill Bohan
10-14-2012, 06:05 PM
Digger you are discribing the same situation I am having at Ottertail Ck.
1. The mineralized zone is giving up gold as is crumbles but alot of the sulfy- dide slate is passing through the box with out breaking up. It leaves the box as tailings.

2. I also have rust "moldings" from pod cast that are holding gold and yet it rolls out of the box as rust balls
3.My third gold loss is micron sliding gold along the bottom of the box.

Geo Jim
10-14-2012, 08:03 PM
Hi digger68,
I believe the reason the granite boulders are crumbling before your eyes is due to them being very, very old. I think it is a paleo placer which means they may have been deposited at least 100,000 years ago or older. The granite could be more susceptible to physical decomposition as well. Not all granites are created equal. Actually larger crystalline granite decomposes faster than fine textured granitic rocks. (I dont remember why.) Whatever the reason, it is unusual for granite to crumble in stream valley sediments. Stream valleys are generally too young for that sort of nonsense. So the age most be greater than normal.

As far as what it means as to whether you will discover a great bonanza when you punch down through it hard pan, it is mostly neutral as an indicator. Go ahead and mine through the bottom sediments with the expectation of a bonanza, but I can offer little re-assurance of such great luck. If it was me, i would keep mining till I got to bedrock.

Geowizard, your granite in the hot desert decomposes by expansion heating and cooling contraction. Water is too scare to do the work. Water in warm climes causes chemical decomposition more than physical decomp. At least this is what the physical geology text books have written in them.
Geo Jim

digger68
10-14-2012, 11:21 PM
Thank you for all of the great information gentlemen. Bill I hadnt thought about the mineralized material not breaking up in the tromell. That is something that I can see being a problem because it is cemented together tightly. Geo nice to hear that you think that this is a very old deposit as well. The glacial silt layer had me thinking that I was into a very old area. I believe that at one time the Susitna river ran through this valley and that is when the glacial silt was deposited. By looking at a topo map I can see where the river could have easily ran where the creek is now before shifting to its present day location. I plan on going to bedrock and looking for the old channel. I just hope it is not 100' down to bedrock like someplaces in this area are. Im about 15 miles from Valdez Creek and there the bedrock is deep. I have dug some large pits for other mines in Valdez Creek and have never found a layer like this. I find it all very interesting and I cant wait to start digging next season. With the gold we have found I think that there is a good chance that bedrock could be very rich but I wont know till I get there.

Diamond digger
10-22-2012, 11:47 AM
You must be very lucky!
That cemented layer is a Conglomerate formed long ago and if you can crush and wash it you will find gold and possibly diamonds in it like the old days. River flows change not only in path but also direction so anything can be in that conglomerate.
I used to move around 80 000 tons of overburden to get to such a layer and boy did it pay! Took a 20 CT E color diamond out of one spot and many smaller ones.
Enjoy!
Andy

digger68
10-22-2012, 01:43 PM
Diamond digger, I dont know that I am very lucky but the cemented layer does look like it is going to pay well. As far as finding diamonds that would be very unlikely. I dont know of any diamonds that have been found in the Valdez Creek mining district. As far as I know there has only been a few found in the whole state of Alaska. I know some miners that wish they only had 80,000 tons of overburden to move before they hit the pay. Moving that much material is not that difficult if you have the equipment and men to do it. It looks like I am going to have to sell a couple of my claims to help pay mining the rest of them. If anyone is interested PM me.

Diamond digger
10-22-2012, 01:50 PM
Never underestimate an ancient riverbed I mean look at South Africa. The big Vaal River flows east to west, meaning no crocodiles or hippos in it. However when I opened an ancient conglomerate river bed, I found fossilized Hippo teeth and crocodile teeth in it. That means the river used to flow west to east at some point in time. The big companies are still looking up-river for the kimberlite pipe that deposited the diamonds in the Vaal, the more I say look down river the more they ignore me.
But time will tell.
That same ancient river bed gave me lots of stone age tools little statues and stuff. Very strange but great fun.
Andy

bill-costa rica
10-22-2012, 02:30 PM
diamond digger do you have a photos, sure would be nice to see.

bill-cr

Diamond digger
10-23-2012, 12:56 AM
Bill-cr,
I had some but it was misplaced in the move London All my mine pictures my video camera and tapes (8mm remember them?) and my camera never arrived this side. I was pretty annoyed and all they ever said was Oh ah claim from insurance. As if that was any good.
Andy

FedFire
10-26-2012, 12:57 PM
Not to throw a monkey wrench into everyone's theory about your deposit being very very old... I find decomposed granite and cemented hard-pack as close as 8" to the surface of the overburden. Difference is, I don't have the heavy oxydization, I personally agree that the heavy oxidation could be a layer of sulfides that have helped to decompose the rock. On the flip side of that, I have found these heavily oxidized zones in areas that have been dredged as recently as 1993. I have a claim that was dredged by Peacock in 1985-1987, there isn't much more than fine flood gold on or below it. I found a spade and a wire crevice tool made out of what appears to have been a toilet lever. Anyway, point is, evidently, it doesn't take granite very long to decompose and for "hardpack" to form, especially with the presence of kaolin. One more thing, gold, being soft, appears to really get mashed by the material around it, making it appear younger than it is. I don't believe much of the gold on my claim comes from granodiorite or the surrounding carbonaceous/mica schist.