So I wonder, how much can a mule or horse carry 200 miles? 20 x 2000lbs = 400,000lbs (20tons) and if a mule can carry 300lbs (guessing) you would need 1333 mules to move that.....
lets wrap our collective heads around that number...
...even if a mule could carry 500lbs (doubtful) that requires 800 mules to move...and that is not everyone's share is it? Even if they made 8 trips over the 10 years it would take 100 mules to move just their shares! Like fish stories, I think the amounts and numbers inflate over time with this legend!
How do you feed 100 mules over 3 months or more in the wilderness? How do you contain them? protect them? and water them daily? Not to mention hide them from Indians.... It would be an ordeal just to do these things with 8 - 10 men not considering the mining, charcoal making, smelting, coining and hunting for food chores!
The task with that many animals becomes monumental at the very least IMHO.
Also consider this, charcoal needed to be made from trees. After 'working' a mine(s) in an area over ten years there would be a huge loss of trees and a large number of stumps left...very hard to hide that from anyone. And 25 years later when trying to find the mines again, many of those stumps would still be there and a sapling forest around the same location.