The solubility of palladium in chloride solutions and the distribution of platinum, palladium, and related elements in hydrothermal mineralization

Abstract

As many as 87 occurrences of platinum group elements (PGE) enrichment in hydrothermal systems are known worldwide. When compared with the occurrence and distribution of other metals commonly found in hydrothermal systems, this number is very small, compatible with the perceived rarity of PGE. That Pd is the most abundant of the PGE in hydrothermal systems pervades the literature, although it is not strictly true. In order to define the character of PGE hydrothermal mineralization and enhance the understanding of the genesis of these occurrences this research focuses upon three critical areas: 1) the conditions under which Pd is soluble in aqueous fluids, and therefore available for concentration into hydrothermal lodes. Experiments to determine the solubility of Pd in NaCl solutions were undertaken, and this was augmented by a thorough review and synthesis of experimental and theoretical studies of the solubility of PGE in a variety of solutions reported in the literature, 2) a literature compilation and synthesis was completed for the better studied occurrences, and 3) substantial new geochemical data were generated for PGE and related elements in a number of different classes of mineralization. From this broad analysis of the character of this type of hydrothermal mineralization, an understanding of the genesis of these occurrences is developed. Palladium solubility as high as 70 ppm was measured in 3m NaCl solution at a pH of 1.5, temperature of 300°C, and f02 = Ni-NiO. Solubility is retrograde with respect to temperature over the range 300 to 700°C and is higher in solutions of acidic pH and high salinity. Salinities of greater than 3m have been reported m fluid inclusions from some of the better studied hydrothermal PGE occurrences. Theoretical studies indicate that Pt solubility and behavior is similar to that of Pd but that Pd may be somewhat more soluble than Pt. Platinum and Pd are also soluble m bisulfide solutions under conditions of alkaline pH, dissolved S of > 0.001m, and f02 low enough to prevent the oxidation S to sulfite and sulfate; temperature dependence may be either prograde or retrograde. They are also soluble as ammine complexes in solutions of high dissolved NH3, alkaline pH, and f02 low enough to prevent the oxidation of ammonia to nitrite and nitrate. Geologic relationships indicate that the presence of appropriate source rocks may be of fundamental importance in permitting the enrichment of PGE in hydrothermal mineralization. Both mafic/ultramafic rocks and black shales may be suitable source rocks from which to mobilize the PGE. In hydrothermal mineralization Pt/Pd varies from < 1 to > 1. Although a close relationship between PGE and Cu is widely observed, PGE may not be closely related to Cu rich zones in individual occurrences that exhibit strong metal zoning. There is, however, a persistent and close spatial relationship between PGE enrichment and enrichment in one or more of the metalloids As, Sb, Se, Te, and/or Bi. Only the layered mafic intrusions are known to host mineralization which is mined only for PGE and are very large resources. Few occurrences outside the layered mafic intrusions represent large resources of (byproduct) PGE.