Sancho:
Man, that area is so rugged that I'd almost believe it was too tough for the Spanish to work, but then, if the Ancients were the ones to establish the mines, then the Spanish could have made use of THEIR work. With all of those LARGE, detailed carvings that can be easily seen from as far away as this monument, the theory I'd favor is the Ancients were there first and established the working mines and the Spanish came in centuries later and also worked them. Notice I said "mines" with the "s" because I'm also thinking that there's more than one mine in that area. You wrote that there are at least 3 side streams to the main one and they all are active the year round. That kind of river action is what cuts down across motherloads throughout the centuries and deposites the gold particles (nuggets and fines) into the streams where they're flushed down stream for a ways.
The Ancients could easily have traced those placer gold deposits back upstream to locate the sources and then dug in the mines. That area could have produced thousands of tons of gold ore for them and then, later the Spanish. I believe the Ancients would only have covered the mines openings to protect them from natural destruction and not as a way to keep them secret. The Ancients had no competitors in their day. The Spanish vandals would have tried to HIDE the mines and tailings because of their paranoia. PLUS, I do not believe the Spanish could have constructed the trails into and around such rugged places as THIS, no matter HOW many slaves they had. They just did not have that engineering talent and experience. Those German miners they brought over had the hard rock mining knowledge, but that simply does not equate to having the CIVIL engineering knowledge needed to put in trails and roadways in a place like this. I believe this kind of engineering was done by the same people who built Pumma Punku and the other great projects located all over the world.
The true Ancients had knowledge and technologies that we do not have even today. Many of their signs and symbols were adopted by the Spanish for their own code requirements. I believe the Spanish were just a bunch of opportunists who managed to take the greatest advantages of the work done by those Ancients and that is why the Federal Government has locked up so much ground like this, because so much good stuff is still in the ground. The Ancients did not get it all and the Spanish dang sure didn't either.
That huge, 3 dimensional owl statue that I marked on that other cliff face; along with the king with the crown are so large and detailed that no one can miss seeing them from THIS side of the canyon.......as shown from these last photos that you've posted. That daggum owl is FACINATING. That is exactly the kind of carving the Ancients would have put on their site......and IMPORTANT site. They MIGHT be marking a tomb or a schrine of the Ancients.