Why were the mining companies interested? There are lot of things that can be mined, many of which are not metal. They might also be interested in it because of access or proximity to other properties.
So you're digging several feet underground because you got a hit with a metal detector? Pardon me for being blunt, but I think you're wasting your time. Metal detectors don't see that far down. And the fact that you got a signal in three spots and they form a triangle means .... what? Any three points that aren't in a perfectly straight line will form a triangle. It doesn't necessarily mean anything.
The rocks in your photos just look like natural rocks. I see no evidence of them being purposely shaped.
Perhaps your digging will turn up something fantastic, but so far I don't understand what you're digging for, and I don't see anything that makes me think you're any closer to finding whatever it is.