And, at least where I’m from (area with lots of quartz and creeks), this is where gold is more likely to form. Gold panners would look for “black sand” in creeks, which implies a lot of eroded slate, which means the surface of quartz is likely to be exposed where gold deposits may have been freed.

We call those quartz veins, and I’ve seen many huge examples while hiking that are several inches thick and a dozen feet long, but typically bound by standard grey bedrock. Think: OP’s picture but the rock is scaled to a boulder the size of a van.