Are there any sulfides, sulfates, carbonates that cannot be dissolved by aqua regia?
Chemical Book reports that barium sulfate (barite, or baryte) is

slightly soluble in concentrated sulfuric acid and soluble in carbonate alkali metal solution in which it is converted to barium carbonate; it is insoluble in other kinds of acid or base.

It sounds like the barite is your culprit.  If you try grinding it with an eye towards physically separating the denser gold from it, the Mohs hardness of the mineral is 3-3.5, so there appears to be no issue with the physical grinding process.