In many old mines, the old miners had to sort out the ore to only the highest grade material as they had to haul it miles to the nearest mill so the gold and silver values could be extracted. Sometimes they were not so good at sorting and a lot of very high grade ore was tossed out on the dumps. A VLF detector like the MXT is just the tool for this type of prospecting. Watch out for old mine workings - they can be vary dangerous, and I strongly urge you to stay out of them. Stick to the mine dumps.  Scan the old dumps with your MXT and save and carefully scrutinize any rock that gives a good metallic signal.  Take a bucket for your finds. If you get any high grade samples, with lots of visible ore, there can be most valuable as specimens for collectors. Ore specimens with little visible gold can be crushed and the resulting sand processed with a gold pan - you still  may find some nice gold. Be sure also that you have the legal right to prospect for gold around the old mine. While there are many old abandoned mines, there are also many that are currently under claim.
HUNTING THIS SITE WITH THE MXT:
The soil conditions of these dumps will vary much depending on location. These are not heavily hunted sites, in general. You'll need good shoes as dumps can be steep and they can run as well.  My preferred setting on the MXT here is to use the relic mode, trigger forward with a discrimination setting  of 2, so as to not discriminate out any fine sized gold.  The DD coils are preferred because of their ability to sort through the trash and better handle the mineralization.  I try to set the gain as high as it can go without excessive chatter and noise.