Lead Phosphates (Pyromorphite and Phosphohedyphane) from the Oxidation Zone of Baryte–Lead Ores in the Ushkatyn-III Deposit, Central Kazakhstan

The paper describes Ca–Pb phosphates—pyromorphite and phosphohedyphane—from the oxidation zone of barite–lead (calcite–barite–galena) ores of the Ushkatyn-III deposit in Central Kazakhstan. Both minerals occur equally frequently within the deposit. Lead phosphates coexist with ore minerals unaltered in the subsurface setting (galena, pyrite, barite, calcite, rhodochrosite, chamosite, etc.), and minerals of highly oxidized ores (cerussite, montmorillonite, kaolinite, goethite, etc.). As the processes of supergene processes develop (during the transition from weakly to highly oxidized ores), the compositions of newly formed phosphates change regularly in the following consequence: phosphohedyphane → rhythmically zonal Ca–Pb phosphate → pyromorphite. At the same time, the habit of mineral crystals changes from dipyramidal-prismatic through elongated prismatic (barrel-shaped) to pinacoidal-prismatic (short-columnar). Crystallization of pyromorphite is possible already at very low contents of lead, phosphorus and chlorine in the solution. The main source of chlorine is groundwater; phosphorus is coming from the organic matter of the soil cover overlying ore-bearing deposits. The features of the chemical composition and crystal structure of Ca–Pb phosphates, as well as the nature of their assemblages, suggest the presence of a discontinuity in isomorphic miscibility in the following series: pyromorphite–phosphohedyphane and phosphohedyphane–chlorapatite.