My reading also indicated SE of Mexico City, perhaps in the Izucar de Matamoros general area, but more than a small area.
The use of metallurgy by the Aztecs is a controversial topic. Somewhere on here is an ancient drawing of what looks like a bellows system used on gold by Aztec workers. This has been a year or two so my memory is not perfect.
A man I know told me in that general area local indigenous folks were still mining gold in the early 20th Century. He said they used that black rock as wedges, obsidian? And, it will break the somewhat soft rock. He told me where they get obsidian, I forget, isn't that volcanic?
Travertine marble is common in that part of Mexico, though I have not been to that exact location. Travertine can be very hard and brittle, or it can be soft. He also told me there is a plant or herb which can be placed on stone to soften it up, but I know no details.
Of course, modern quarrying in that general part of Mexico uses nitrogen fertilizer and fuel oil, excited by small quantities of dynamite. It makes a lovely thump when it goes off. (N is slow, so it is more of a thump than a BANG. At least, that is what it sounds like to me, though I do not have a strong musical skill.)
The Aztecs also received gold as tribute from conquered tribes.
I do not understand about safety now. Do you mean close to Popo and safety issues resulting from eruptions?
Or, do you mean Zetas? They do not usually roam the hills looking for people digging in the ground. Nor do druggies have a strong presence in that area. In fact, an educated man told me they put their families in that part of Mexico, to keep them safe from the violence.
In truly rural areas in that part of Mexico, you have two problems. A lot of that land is ejido, or communal land, and the people who belong to the ejidos are somewhat aggressive about running intruders off. If you go there, you can be sure someone is watching you, even if you don't see them.
Likewise, some of the land where gold was found by digging into rocks is now Federally protected archeological zone. Think Federal prison, hint, hint.
Of course it depends up exactly where you mean. I don't tell where I live, except that local legends insist that Moctezuma's treasure was buried here. Which helps you not at all, heh, heh. So, I don't expect you to tell where you wish to dig.