Supergene alteration of zinc and lead deposits in limestone

An attempt is made here to describe occurrences and paragenetic sequences of the supergene Zn and Pb minerals in 3 districts representing varying mean temperatures and humidities. The facts observed in field and laboratory are interpreted in the light of physical chemistry, assuming equilibrium conditions. The stability relationships between smithsonite, hydrozincite, and zinc hydroxide are defined in terms of pH and total carbon dioxide concentrations in the aqueous solution system, or of the partial pressure of carbon dioxide in the dry system. The stability fields of hemimorphite and willemite are also discussed. The study explains the common occurrence of hydrozincite in arid regions, and the scarcity of this mineral in humid regions. The common occurrences of smithsonite and hemimorphite in the southern Appalachian district, and the predominance of smithsonite over hydrozincite and hemimorphite in the Upper Mississippi Valley district can be explained by the pH of the ground waters and soils. The stability fields of supergene lead oxide minerals, wulfenite, cerussite, anglesite, and Cd minerals are calculated in terms of pH, Eh, and total S concentration or total carbon dioxide concentration in solution. The paragenesis of the Pb minerals and the mobilization of molybdate and Cd ions are discussed. The results of the spectrochemical analyses indicate that Mo and As in the supergene facies were derived from galena, whereas V and P were derived from the limestones and shales.