Its the same thing that is seen everywhere the Spainish mined gold. A little ore would be put in and a little water, then a wooden staff would be used to grind the ores to dust, then they'd add a little mercury to amalgam the gold to recover it easier and wash the dirt away.
In this area, look for a burned place of charcoal. Might find the melting fire pit.
Or if your into it, dig up the first 6 inches of dirt around this and wash it all.
Normally, grind holes are found on rocks much larger than the holes (see below). If your rock is in a very remote location, look for a mano. A mano is (usually) a rectangular shaped stone that has been worn smooth from the grinding. Some manos are round, but rectangular is most common (see below).
If you hit the area around the rock, and fine flour or flake gold, then yo0u will know it was used for ore grinding, most likely it was used by Indians for grinding food.