"artisanal mining" = desperate people trying to earn a few coppers however they can. I this case they're picking over mine tailings, much as in older times wives and kids in the UK would pick over coal slag heaps, or like in India where people pick over rubbish tips.
The only way to stop it is to shoot those doing it............and if you do stop it you end up with starving kids. Whats the bigger evil? Children dying from hunger? Or children working in poor dangerous conditions?
The other point is that you haven't a hope of trailing the origin of the ores - there is so much secrecy in metals trading, and so many middlemen that trying to audit the supply line is impossible, especially given how easy it is to decieve