Something to consider - that boulder moved into that position during a flood - a MIGHTY BIG flood.
The incredible water flow needed to move and then drop it there washed the streambed free of all loose material (except maybe inside any cracks, nooks or crannies). When the boulder finally stopped in that spot (chances are on the "tail end"of the flood) only then did loose material begin to fill in, around and then over it. Yes, there will be 'after it stopped' gold on the upstream and downstream sides of it - but the hidden bonanza that everyone always assumes is directly under it will (probably) not actually be there.
It's really nice to see how well you're doing in this area - I'm anxiously looking forward to more pictures - especially the nooks and crannies under the boulder with all that flood gold.
All the best!
- Joe -