"back in the 1800's metal detectors did not exist so the old blokes only mined what they thought or saw that had gold in it".
True, but they would simply put a few more tons through the crusher when in doubt, and if it came up dud would stop. They mined tons at a time, they did not crack open every handful of quartz. This bulk crushing was probably at least as sensitive as a metql detector, and commonly removed anything worth having.
While you can occasionally fluke a few specks detecting in this way (I have known it), I consider it so much less productive than alternatives. Good ore would be lost along the rail used by their hand trucks, and around their ore bins where they stored good ore for crushing. If it was a single, well-defined quartz vein not much good material would have gone onto the mine dumps as well, but if a "stockwork" of many quartz veins they had to guess a bit what was ore and what was waste, and gold ended up on the waste dumps.
It can also be more productive to work out where the vein hit surface, and detect along the parts that they did not mine at all.