we must remember that not all mines were successful. The Spanish had the opportunity to explore an entire portion of an untouched continent, no competition and they had slave labor in the form of the natives and their own soldiers.
When miners find an area that appears to hold minerals they will follow the indicators, a vein of quartz eg and if it happens to be on a level area they go down in the form of a shaft. If in a hill or mountain side they follow it in, a tunnel or "adit" is nearly horizontal. Passages off this tunnel are known as drifts where they explore further for more veins.
The most common shafts going straight down are made by placer miners attempting to get through the gravel, rocks, and other overburden to get to the bedrock of a stream or river course. That is where the concentration of gold is, then they "drift" following the bedrock. The Yukon and Alaska gold miners have to deal with permafrost and used to work in tunnels where they had to keep fires burning to thaw the frozen ground. Imagine being down there with all that smoke!
In summer they had to keep the entrance closed or the permafrost would melt and collapse the gravel onto them.
They have found whole frozen Mammoths and prehistoric Horses in the frozen ground but no Spanish miners
Those Spanish pussy cats were not fond of cold weather and did little work north of my area and certainly not in Alaska
There have been Spanish mines and artifacts found here on the south coast of BC.
Fred this article of your is interesting and right up my alley! Thanks!
Rude comes to mind 
