HYDRONIUM-RICH JAROSITE AND ASSOCIATED WATER FROM THE RICHMOND MINE, IRON MOUNTAIN, CALIFORNIA

The inactive Cu-Zn mines at Iron Mountain, California, are well known for producing extremely acidic drainage and a variety of Fe-sulfate efflorescent minerals. Near the contact between the massive pyrite of the Richmond orebody and the host metavolcanic rocks, jarosite and hydronium jarosite accumulate as short, hollow stalactites and a 3-cm-thick layer of yellow, mustard-like mud on the sulfidic wallrock. Water samples were collected in July 1998 by placing beakers under the dripping stalactites and by extracting pore water from the jarositic mud using a centrifuge. The waters had pH values of approximately 2.2, whereas waters reported from elsewhere in the mine have pH values commonly < 1 and locally < 0. Aqueous speciation modeling indicated that the waters are saturated with respect to jarosite–hydronium jarosite solid solutions, amorphous silica, and goethite, all of which are present. The relatively high pH values in the water samples associated with jarosite-group minerals may be caused by interaction with metavolcanic host rocks by mixing with cooler, more dilute groundwater and by less evapoconcentration. Water extracted from underlying, fine-grained massive pyrite mush is also saturated with respect to jarosite–hydronium jarosite. This suggests that, in this locality, the Fe²⁺ liberated by pyrite oxidation rapidly oxidizes to Fe³⁺and forms jarosite-group minerals instead of Fe²⁺-bearing sulfate salts found elsewhere in the mine. Compositions of the jarosite solid solutions, as determined by electron microprobe, follow a trend between K_0.9(H₃O)_0.1Fe³⁺₃(SO₄)₂(OH)₆ and (H₃O)_0.60K_0.25Na_0.15Fe³⁺₃(SO₄)₂(OH)₆. Values of charge balance and saturation index for jarosite-group minerals, calculated from aqueous speciation modeling, are extremely sensitive to small changes in pH. This sensitivity is caused by the marked change in dissolved sulfate-bisulfate (SO₄-HSO₄) speciation with small changes in pH near 2.0 and by the stoichiometry of the jarosite precipitation reaction, which has 6 moles of H⁺ per mole of jarosite. In practice, it is extremely difficult to measure pH accurately enough for reliable determination of the saturation state of jarosite solid solutions in natural systems. The most accurate indicator of pH may be that which accounts for charge balance.