You have some good Spanish rock trail markers. One of the things you need to remember is the Spanish engineers did not make giant rock markers for people to see up close. You can spend all the time you want using metal detectors around these large rocks, but the chances are slim you will find nothing or little.
The rocks you have identified are meant to be seen at a distance, usually from the bottom of a hill, mountain, or an arroyo. If you were following a treasure trail of rocks in hostile territory you would not be walking along the Crest of a hill, ridge or mountain. The opportunity of staying alive means you stay our of site, which means you are at the lowest possible point looking up.
The closer you get to a mine or treasure site you will find the rock markers get smaller. Finally the closer you get you will find the rock markers may be flat or embedded evenly with the ground. They will not be upright and seen more than 50 to 100 yards distant. Good luck on your search, but you have a hike ahead of you if you want to find something anytime in the near future.