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Oh, a gossan… A gossan is the leached cap above a vein or sulfide orebody, and it is roughly the same effect as this. Acidic rain mixes with the sulfides in the ground and create a stronger acid, that then leaches away all of the sulfides leaving a depleted cap of basically clay above an enriched ‘supergene’ zone, where the contact with the water table causes precipitation of these sulfides in a concentrated way (a great thing to discover.) This leaves a high grade zone, that’s easy to mine and makes big money.
The red iron staining is cause by the same process, but it isn’t considered the same as a gossan. This is just the residual Fe2+ that ends up in water and precipitates out as it can.
See: [Red Mountain, CO](http://en.wikipedia.org/wiki/Red_Mountain_%28Colorado%29)