Thats the local metavolcanic slate. It can be a good gold trap, as Lanny pointed out, but it's not a source of gold. Lot's of cracks in this stuff. Take the largest sledge you can find (I use an old 16 pound one) and give the base rock a good wack. Watch for dust or water rising from minute cracks. Keep hitting it in different areas until you see a pattern of crack signs, or identify a single major crack.
You will find good hidden cracks this way. Even though a crack may appear to be hairline on the surface you will be surprised how often the crack "opens up" as you work your way in deeper. These are the most productive cracks in my experience and you already know you are the first one to open them. I keep a high quality rock point and a 3 pound sledge to help open these small cracks. It helps to have a friend with you because the dust or water pops out pretty quick and tends to be pretty narrow and hard to see when you are slamming with your sledge.
Uphill to your East is the Marine Graywacke, it's a kind of course metamorphosed sandstone. Gray in general appearance with a hackly fracture you can see the coarse grains of quartz and mixed minerals in it. The Graywacke doesn't produce any gold either and usually doesn't make a good gold trap.
Just up the hill to your West right about where the road does the sharp switchback is a small serpentine belt. The margins of the Serpentine do produce some gold and the coarser flakes you find on the bar are probably from that source. There is a small old landslide that has moved that gold bearing material down into Mineral Bar.
At Mineral Bar you are on the northern extreme of a good mineralized zone. Upriver from the bar the metavolcanic slate predominates for about the next five miles. There are no other significant gold sources within that stretch of the river. With the exception of a small area of exposed Tertiary gravels, on the divide just North of Colfax, all the significant gold mineralization is far to your East. You may get some small stranded flood deposits on the banks or some crevice material but that stretch of the River is pretty slim pickins until you get below the Iowa Hill diggins.
The northern portion of the campground and the river on both sides for 1/4 mile upstream at Mineral Bar are claimed and are not legally prospectable. It's been claimed there for more than 50 years - long before any withdrawals of the SRA. Don't be fooled into prospecting another man's claim. Always check the land status
before prospecting. It's your legal obligation as a prospector on the public lands.
The lower serpentine belt and the landslide area that is moving the gold deposit onto the bar are part of that claim. The extreme uphill portion of that small serpentine belt (above the road switchback) is unclaimed and would be a good area to look. Although you won't have the advantage of flowing water nearby you will be the first to open up those cracks.
Heavy Pans