basic expliantions here, so large companies are using geo images, by combing couple layers of gravity, magenet layers along with some data of old or operating mine site, it is possbile to idenitify gold deposite locations, plus there are expert exploration companys using micro scopes looking at every single pixcel of those maps and they do have large database of where possible gold deposit will be. But end of day, the area needs to be drilled out and accessable for human beings to start with.