Mobility of palladium chloride complexes in mafic rocks: insights from a flow-through experiment at 25 °C using air-saturated, acidic, and Cl-rich solutions

Abstract

A flow-through chromatographic experiment was carried out to determine the ability of an oxidized, acidic, chloride-bearing solution to transport Pd through a column of crushed Columbia River basalt at 25 °C. An initial plug of solution containing 13.8 mg Pd/kg H2O was loaded onto the column of crushed basalt and eluted with an air-saturated, 1 m NaCl solution of pH 3. Even after 3606 h of elution, corresponding to a fluid-rock mass ratio of 1338, less than 10% of the Pd originally loaded on the column was recovered. On the other hand, in a similar experiment involving relatively unreactive quartz sand, the Pd was almost completely recovered after the passage of only 4 pore masses of eluent (or a fluid-rock mass ratio of approximately one), a behavior similar to that of Rb. The results suggest that unrealistically large amounts of oxidizing and acidic fluids are required to react with a given mass of basalt in order to overcome the acid- and redox-buffering capacity of the rock and to mobilize palladium as a chloride complex. On the other hand, Pd-chloride complexes can be easily transported through a rock with minimal acid- and redox-buffering capacity, such as clean quartz sandstone. The implications of this study for PGE enrichments in sediment-hosted stratiform copper (Kupferschiefer type) deposits are explored briefly.