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.

First Page Preview

First page PDF preview
You do not currently have access to this article.