It is possible that your black material is silver ore. Tuena has historically produced silver as well as gold, and of course the mine at nearby Peelwood was a copper, silver, lead and gold mine.
Silver ore often presents as a sooty grey/black material, and it is possible that the ore in your pic is quite rich.
Great going with the SDC. I am thinking of getting one if high banking goes the way of dredging, and Tuena is one of my favorite locations.
Don't give away your location, but I'd be back giving that area a really good workout. Back in the VHF days, I'm told some quite large nuggets came from up top rather than in the creek.