The formation of goethite, jarosite, and alunite during the weathering of sulfide-bearing felsic rocks

Abstract

Quantities of goethite and jarosite produced by weathering correlate with abundance and composition of sulfide minerals originally in rocks. To characterize processes forming these minerals, chemical weathering of sulfide-bearing rocks was simulated using differential equations which describe mass transfer between aqueous solutions and minerals (Helgeson, 1968).Goethite begins precipitating during reaction between meteoric water (f (sub O 2 ) = 10 (super -06) atm) and sulfides after dissolution of 6.7 X 10 (super -9) g pyrite or 6.0 X 10 (super -9) g chalcopyrite per 1,000 g H 2 O. Reaction of the oxygen dissolved in 1,000 g water that is initially saturated with the atmospheric abundance of oxygen consumes 0.0085 g pyrite or 0.013 g chalcopyrite. Goethite is the only reaction product. The calculations predict that weathering produces jarosite or alunite after goethite when >0.04 moles of oxygen have been consumed per 1,000 g H 2 O and where rates of oxygen replenishment approximately equal or exceed consumption. Simultaneous reactions between K-feldspar, an aqueous solution, and pyrite or chalcopyrite produce alunite and goethite if the irreversible molar flux of Fe/Al is less than 0.5. Reactions with fluxes of Fe/Al between 0.5 and 1.0 produce alunite, goethite, and jarosite; those with Fe/Al fluxes greater than 1.0 generate only jarosite after goethite.Following saturation of the solution with jarosite, the mass ratio, goethite/(goethite + jarosite) decreases smoothly from 1.0 to 0.0 accompanying continuous dissolution of pyrite and chalcopyrite. The interval of reaction progress over which goethite and jarosite coexist increases, relative to pyrite and K-feldspar reactions, when weathering involves muscovite or chalcopyrite. In simulations whose sulfides have been totally oxidized, this ratio varies antithetically with the volume percent sulfide and/or ratio of pyrite/(pyrite + chalcopyrite) initially present, a prediction consistent with geologic observations.