Why do gold values commonly drop with depth?

[OwenT]

When I read about a lot of mining districts a common thread is that it seems like a bonanza find at first, there’s lots of strikes, the town booms, and then the veins play out at depth and it all goes under. I understand that with sulfides, there will be more free milling gold in the oxidized zone but I don’t understand why gold values would simply vanish at depth.

Perhaps the one reason I can think of is that of a strike is made on the surface, it’s probably because a fairly high grade spot of the vein was exposed to the surface, now it’s not too unlikely that a different part of the vein is less rich than the exposed portion or that it’s just a small vein. Then at that point it’s not an matter of depth, but probability rather.

Ideas?

 

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[Paulo]

It does seem counterintuitive, right?

While there are endless causes and configurations, I'll offer a few...

1. When you have these hydrothermal (epithermal) deposits, there is a lot of energy in the mineral bearing fluid streams rushing between the fissures to the surface. So, given the force remains fairly constant, the denser elements will have more momentum and travel further (up). Once it reaches a point where it spreads out (V-shaped) velocity drops off square to the area and the heavies are more likely to drop out.
1a. Along with this action, the pH of the contact areas will be favorable to precipitating the minerals.

2. Many of the bonanza style deposits (that I've studied) have a faulting system where vertical fissures get 'capped' by lateral faulted blocks, thus creating a dam of sorts and forcing all the goodies to collect in highly concentrated pockets.

Keep in mind, these are examples of what occur in the districts around where i live and prospect. But I hope it at least gets the gears turning.I think what I'm proposing is that geometry is just as important as the geology.chemistry in determining the placements.

 

[Goldwasher]

When I read about a lot of mining districts a common thread is that it seems like a bonanza find at first, there’s lots of strikes, the town booms, and then the veins play out at depth and it all goes under. I understand that with sulfides, there will be more free milling gold in the oxidized zone but I don’t understand why gold values would simply vanish at depth.

Perhaps the one reason I can think of is that of a strike is made on the surface, it’s probably because a fairly high grade spot of the vein was exposed to the surface, now it’s not too unlikely that a different part of the vein is less rich than the exposed portion or that it’s just a small vein. Then at that point it’s not an matter of depth, but probability rather.

Ideas?

Because in the oxidized zone secondary enrichment occurs. Forms pockets.

It's concentrating in that zone along shears and contacts.

Though depending on the region It can go deeper too and have been due to replacement from below during faulting. In an area where there is also secondary enrichment.
with gold formation(mineralization) happening in more than one way.

 

[StreamlineGold]

Totally depends on the deposit. Sometimes near surface its a narrow rich pay layer in the vein and as you get deeper the vein becomes wider and more disseminated. Theres a sweet spot in the depth, and if you follow that along the strike you are in the money.

But in other cases, the grade doesnt drop at all with depth, but eventually it costs too much to remove the ore due to depth.

 

[Mountaineer2020]

Total rookie here but I've read ALOT of mining records from our area dating from 1901 to 1977. Most of the mining was done from 1860s to 1910. I believe that cyanide treatment had not made it out west yet at least in Oregon. If the ore was heavy with sulfides miners would have a tough time extracting it. Oxidation at the closer to surface levels led to free gold which could be extracted by gravity and mercury, usually vaners in this area. The gold was there at the lower levels they just didn't have the method to concentrate economically on site and got ate up by shipping costs. Consequently only very high grade was shipped. The other consistent problem in this area was dewatering the mine. One I'm currently working on has a consistent 70 gpm spewing out 365 days a year. From what I've read of available mine maps with assays most mines hold values to deeper than I'd want to go. Just looked at one at the 800' below level and it still had good ore. On the other hand the one I'm working on the values disappeared on the 300' below level. Who knows? I like Paulo's answer makes sense.

 

[Johnnybravo300]

The bonanza is usually just another inch down.

 

[RTR]

Dunno. Where I dig The deeper I dig ,the better the results

 

[CGC Miner]

Like others have mentioned, it all depends on the district and morphology of the vein.
One of my favorite areas to prospect has the exact opposite.
Near surface veins carry low values, as you get deeper, enrichment occurs from the top oxidized portion leaching values down to the point of the water table.
The water tables zone seems to be the richest but it is the deepest, most difficult and expensive to work.

 

[Goldwasher]

Total rookie here but I've read ALOT of mining records from our area dating from 1901 to 1977. Most of the mining was done from 1860s to 1910. I believe that cyanide treatment had not made it out west yet at least in Oregon. If the ore was heavy with sulfides miners would have a tough time extracting it. Oxidation at the closer to surface levels led to free gold which could be extracted by gravity and mercury, usually vaners in this area. The gold was there at the lower levels they just didn't have the method to concentrate economically on site and got ate up by shipping costs. Consequently only very high grade was shipped. The other consistent problem in this area was dewatering the mine. One I'm currently working on has a consistent 70 gpm spewing out 365 days a year. From what I've read of available mine maps with assays most mines hold values to deeper than I'd want to go. Just looked at one at the 800' below level and it still had good ore. On the other hand the one I'm working on the values disappeared on the 300' below level. Who knows? I like Paulo's answer makes sense.

they definitely used cyanide in Or. in the 19th century.