If I had to take a stab at those it's this.

The gold was heavier in the quartz solution so it would settle where it would have a place to land, hence less likely to be found in pure quartz and more likely to found next to the diorite.

As for the geochem, that's easy. The geochem only works to a certain depth, so the geochem is giving us a picture of the 'top layer'. If we peeled back 200m of the earth, the geochem would probably look a lot different. So, where we get a weak signal it doesn't mean there isn't gold there, it just means that the bacteria that lives with gold did not make it all the way up the soil at the surface.

If you ask me, the fact that we are finding gold in places where the geochem is weak signifies that this stuff is simply everywhere and right down to the core. The geochem just gives us a better chance of hitting the stuff from the surface.