Abstract
Although most economic palladium deposits occur in mafic igneous rocks and are the products of magmatic processes, there are a number of polymetallic deposits hosted by organic-rich (bituminous) black shales in which the palladium reaches exploitable concentrations. Considering that palladium is rarely concentrated by hydrothermal processes, and as organic-rich black shales are the source beds for the production of hydrocarbons and bitumen commonly records the path taken by these hydrocarbons, it is reasonable to entertain the possibility that liquid hydrocarbons may act as an ore fluid for palladium. In order to test the potential of liquid hydrocarbons to act as ore fluids, Pd (0) metal wires were reacted with three crude oils, A, B and C (A being the lightest and C being the heaviest crude oil), at temperatures of 150, 200 and 250˚C. After reaction at 150˚C, the Pd concentrations in crude oils A, B and C were 60 ± 20 ppb, 19 ± 4ppb, and 128 ± 29 ppb, respectively. The corresponding concentrations at 200 ˚C, were 18 ± 6ppb, 17 ± 6ppb, and 50 ± 15 ppb, respectively, and at 250˚C, were 16 ± 2 ppb, 11 ± 1 ppb and 26 ± 2 ppb, respectively (n=3). The measured solubility of Pd was highest in the heaviest crude oil, C. All three oils reached their maximum Pd concentration at 150˚C, with Pd concentration decreasing progressively as temperature increased to 250˚C. To gain insights into the mechanism controlling the dissolution of Pd in crude oil, we analysed the reacted Pd wires using X-ray Photoelectron Spectroscopy (XPS). The XPS spectrum displays a well-defined thiol (R-SH) peak at a binding energy of 163eV, which indicates that Pd has an affinity for reduced organosulfur compounds. Moreover, Pd concentrations were observed to increase with the total thiol content of the oil, thereby corroborating the results of the XPS analyses. Our results clearly show that Pd solubility in crude oil is promoted by the presence of thiols. As thiol-rich hydrocarbons usually develop in deep, high temperature (100–140°C) carbonate sequences containing abundant sulfate mineralization, it is thought that the thiols are the products of thermochemical sulfate reduction (TSR) (Machel, 2001). We therefore propose that liquid hydrocarbons, which have interacted with sulfate at relatively high temperature, may serve as effective ore fluids for palladium.
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