I live in Utah and I roll my eyes every time I hear about “Spanish Mines”. I have not done much research on the subject so I could be wrong I admit but based on all my common sense and knowledge of mining and history in this county, as well as the current situation, no significant gold has ever been found in the Unitas. If it had, we would still be finding some today. It would have been impossible for the Spanish to have gotten it all and very unlikely that there were only a few pockets of gold on the surface and no other gold in nearby areas. The same goes for everywhere else.
Millard county is large and there is gold in the Drum and House range. If the Spanish found gold there then it was in places that were mined again during the mining booms or are currently being mined still. Truth is there is nothing special about a “Spanish mine”. It’s either an old mine and has gold in it or it doesn’t. Lost mine stories are much more likely to be myth than fact and the details are so skewed over the years that it would be impossible to match up for certain with anything.
People may say that the gold was so good that the Spanish had to remove every trace of it and bury the entrance and all that but it just isn’t realistic. They didn’t leave a pile of gold at the end, buried for however was going to come and unbury it. If there’s gold there you will be able to find it in the surrounding dirt or rocks, even if small amounts. Remember, if it was there, they were only after the rich stuff. The idea that they took anything with gold off to another location is a little far fetched. Even if they did take the richest or to another location for processing.
Hope you do find the gold but as others have said, if there’s is a mine behind that dirt, you’ll just have to dig it out which will take forever and it’s probably going to dangerous judging by the fact that it’s currently full of dirt. The most likely reason would be that nature has filled it in over time. How many people go through the effort of backfilling a mine like that?