Carbonate-sulfide equilibria and “stratabound” disseminated epigenetic gold mineralization: a proposal based on examples from Alleghany, California, U.S.A.

Abstract

“Stratabound” disseminated pyritic Au ore bodies were produced by reactions between wall rocks and through-flowing fluids in Mesozoic epigenetic Au quartz vein systems in the Sierra Nevada metamorphic belt. Equilibrium relations among Fe-bearing carbonate and sulfide minerals were critical in determining which rock types were likely to host disseminated mineralization along portions of discordant veins. The compositions of metasomatic carbonates in hydrothermally altered wall rocks at Alleghany, California, U.S.A., were larely predetermined by the relative proportions of Fe, Mg and Ca in the unaltered wall rocks. Thus, coexisting solid solutions in the magnesite-siderite and dolomite-ankerite series from a variety of different wall rocks yield an empirical phase diagram for a large part of the Ca CO 3 MgCO 3 FeCO 3 system at the temperature of metasomatism (325 ± 50°C). Because Fe,Mg-silicates were unstable in alteration zones adjacent to the veins, wall rock Fe was partitioned between carbonates and sulfides. Pyritization and disseminated Au mineralization occur in a variety of igneous and metasedimentary wall rocks in which the initial molar Fe/(Fe + Mg) ≧ 0.5. In altered wall rocks with initial molar Fe/(Fe + Mg) ≦ 0.5, Fe was incorporated almost entirely within Mg-rich carbonates (X FeCO 3 ≦ 0.6 in magnesite-siderite solutions). It is proposed that the CO 2-rich vein fluid responsible for the alteration and mineralization was partially buffered with respect to H 2S/CO 2/H 2 ratios by equilibrium between pyrite and Mg 0.4Fe 0.6CO 3 (+graphite?) as it traversed and altered intermediate volcanic and sedimentary rocks. This fluid then locally reacted with lower Fe/(Fe + Mg) rocks to form Fe-bearing dolomite + magnesite assemblages, and reacted with higher Fe/(Fe + Mg) rocks to form ankerite + pyrite assemblages. Gold precipitated from saturated solutions of bisulfide complexes partly in response to fluid desulfidation and reduction caused by the pyritization reactions. In terranes dominated by intermediate metavolcanic and metasedimentary rocks, favorable host rocks for this type of mineralization need not have high Fe contents, but do require high Fe/(Fe + Mg) ratios. They may include felsic volcanic and plutonic rocks, Fe-rich tholeiitic differentiates, banded Fe formations, and a variety of siliceous and argillaceous sedimentary rocks. Rocks which tend not to be heavily sulfidized because they have low initial Fe/(Fe + Mg) ratios include ultramafic and mafic igneous rocks, and some argillaceous sedimentary rocks. Exploration guidelines based on these principles may be useful elsewhere in the Sierra Nevada and in other comparable heterogeneous metamorphic terranes, if modified to reflect the dominant buffering rock types in a given fluid flow path. Carbonate-sulfide equilibria are capable of approximately buffering the carbonate-sulfide ratios of CO 2-rich vein fluids (f CO 2≧ 10 2.8 at 325°C, 200MPa or 2000 bar). The Alleghany fluid (f CO 2 ≈ 10 3.2, or ∼ 10 mol % CO 2) had a molar CO 2/H 2S ratio of approximately 10 3, assuming graphite saturation. At lower CO 2 fugacities, Fe-bearing silicates entered the buffering assemblages. Carbonatization reactions could potentially de-sulfidize some wall rocks, releasing S (and associated metals?) to the fluid. This would be most likely to occur in pyrite-bearing mafic and ultramafic rocks and some argillites.