Judging by your pictures and the fact that it is black and greasy while being found around a volcanic area, then it is possibly copper that has come into contact with Hydrogen Sulfide (H2S) at some point. Hydrogen Sulfide forms a nasty black coating on copper that can be removed with a bit of elbow grease and some light acid such as vinegar.
It is a real pain to deal with Hydrogen Sulfide around Wastewater plants. It can eat out small electrical traces fairly fast even in low concentrations. Hydrogen Sulfide is a colorless and odorless gas in higher concentrations and will stop electrical activity in your nervous system causing death from respiratory failure. It's hard to get high concentrations out of doors in most situations as it tends to dissipate fairly rapidly.
Not too often I get to contribute something here that I know much about.