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The Role of Jarosite and Copiapite in the Chemical Evolution of Acid Drainage Waters, Richmond Mine, Iron Mountain, California by Clare Robinson, MSc (co-supervised with H. Jamieson) The Richmond Mine at Iron Mountain, California is host to the most extreme acid mine drainage ever reported. Prior to treatment, pH values of mine water as low as -3.6 with dissolved solid concentrations near 1000 g/L have been recorded. A significant contributing factor to the water quality at this site is the cyclic precipitation and dissolution of secondary sulphate minerals. The exact influence of many of these minerals is not completely understood. Copiapite [FeIIFe4III(SO4)6(OH)2.20H2O] and jarosite [KFe3III(SO4)2(OH)6] minerals, with co-existing waters, have been collected and studied to obtain a better understanding of mineral-water interaction within the Richmond Mine. Electron
microprobe, scanning electron microscopy and x-ray diffraction techniques
identified two phases in the copiapite samples: Pore-water extracted from the copiapite has world-record ferric iron concentrations of 147 g/L, a pH of –1.0 + 0.5, and a measured density of 1.5 g/mL. The predominance of ferric iron and aluminum in this fluid suggest that at the time of analysis it was supersaturated with respect to the Al-rich ferricopiapite, as opposed to the dominant Mg-rich phase. Insufficient data are available to evaluate the activity coefficients for all the elements in acid mine waters, particularly ferric iron. Interaction parameters and temperature dependence for all the constituents, including copiapite, must be defined in order to completely describe the system thermodynamically. The composition of jarosite stalactites and mud determined by electron microprobe and X-ray diffraction is primarily between the K-H3O end-members within a K-Na-H3O solid solution. An amorphous silica mineral occurring with the stalactites was identified, supporting saturation predictions by other authors doing geochemical modeling of the Richmond Mine effluent (Nordstrom and Alpers, 1990). An iron-rich red mineral occurring with the jarosite stalactites was tentatively identified as goethite, following single crystal x-ray analysis. Geochemical modeling with the speciation program WATEQ4F indicates that the drip waters collected from the stalactites are in equilibrium with jarosites that have compositions primarily between the potassium and hydronium end-members of the jarosite solid solution, supporting mineral chemistry findings. Saturation of goethite and an amorphous silica phase were predicted in geochemical modeling of the stalactite drip water. Speciation calculations with jarosite mud pore-water were not reliable due to an apparent anion surplus reported by WATEQ4F. Sensitivity analyses with both data sets revealed pH to be a very significant variable in determining sulphate speciation and jarosite composition. Copiapite is a relatively soluble sulphate and has very acidic pore-water. Dissolution of significant quantities of this mineral during wet seasons likely contributes to a short-term influx of iron and sulphate in the Richmond Mine effluent. Given the relative low solubility of jarosite, it will attenuate certain elements more effectively than more soluble sulphates (i.e. copiapite). |