Gold has a density of 19.3 g/cm^3
Rocks typically have a density of around 3 g/cm^3
25% by volume: 7.1 g/cm^3
25% by weight: 3.8 g/cm^3
(by number of molecules would depend on how one defines a rock-mineral "molecule" -- each metal or silicon ion?)
It should be easy to measure the density of those alleged gold-containing rocks. Measure their masses. Measure the masses of the amount of water displaced. Divide.
If it was good enough for Archimedes, it's good enough for me.