Abstract: Transmission electron microscopy study indicates that mic rocrystalline Fe-sulfide crystals around large pyrite crystals from Carlin mine of Nevada are marcasite. The marcasite contains Au and As. There are band-li ke features in the marcasite. These band-like areas are relatively rich in As with respect to the neighboring areas. It is proposed that the band-like area s are also relatively rich in lattice-bound Au. According to the obtained equat ion for distribution coefficients of trivalent cations in calcite, it is propose d that cationic Au3＋ were incorporated into the fine grain marcasite at the marcasite－solution interface during non-equilibrium (fast) crystallizatio n of the marcasite. The partitioning of Au3＋ in the marcasite is control led by non-equilibrium distribution coefficient (K'd). However, pyrite formed through equilibrium (slow) crysequilibrium distribution coefficient (K d) for Au3＋. Recrystallization of early-formed fine grain crystals will remove REE and Au from the crystals. Incorpor ation of larger cations of Au3+ and Au+ may result in local structural distortion with respect to normal marcasite structure, and shows the form of the band-like fea ture.