CHALCOPYRITE WEATHERING: LABORATORY VS NATURAL ENVIRONMENTS

Oxidation of sulfide debris in the intertidal zone was investigated near abandoned mine sites in Prince William Sound, AK. At the Ellamar and Threeman sites, clasts within beach gravels  contain massive sulfide composed of pyrrhotite + pyrite + chalcopyrite. During alteration, pyrrhotite and chalcopyrite exhibit contrasting susceptibilities to oxidation.  In most clasts, pyrrhotite is progressively replaced by marcasite, Fe sulfate, sulfur, and Fe oxyhydroxide; alteration proceeds along grain edges and cleavages toward complete pseudomorphic replacement.  Chalcopyrite is considerably more stable with alteration limited to corrosion along grain edges.  Fe oxyhydroxide is deposited along grain margins and in cracks.  Contrasting alteration of chalcopyrite and pyrrhotite could simply be due to dissolution rates.   Previous experimental studies have shown that pyrrhotite reacts ~2 orders of magnitude faster than chalcopyrite at pH 3.  However, variations in pH, solution chemistry, and dissolved O₂ may affect sulfide oxidation processes.  Therefore, chalcopyrite oxidation under varying conditions was further explored with a series of laboratory experiments. Dissolution experiments were conducted with a continuously-stirred, flow-through reactor, using a chalcopyrite with 30% Fe, 26% Cu, and trace concentrations of Au, Bi, Mn, Pb, Sb, Se, Te, and Zn.  Reactions were conducted at pH 8, 3, 2, and 1, in NaCl solutions ranging from 0M to 0.7M.  Dissolved O₂ concentrations were varied from 3 to 7 ppm. These experimental conditions are similar to the chemistry of pore waters in contact with sulfide debris in intertidal gravels described above. Experimental results show that dissolution rates (mol Fe m^-2 sec^-1) in NaCl-free solutions are 3.3e-9 at pH 1 and decrease to 7.3e-11 at pH 3.  Fe is released preferentially to Cu, with Fe/Cu release ratios ranging from 165 at pH 1 to 3 at pH 3.   Sulfur appears to be retained in the reactor.  Variations in dissolved O₂ concentration have no effect on Fe or Cu release rates.  Results suggest that sulfide oxidation processes in the intertidal zone at the Ellamar and Threeman mine are controlled by pH rather than Cl and dissolved O₂ concentrations.