The platinum-group elements (PGE), which include platinum, palladium, rhodium, ruthenium, iridium, and osmium, commonly occur together in nature and are among the scarcest of the metallic elements. These metals have become critical to industry because of their extraordinary physical and chemical properties; they are refractory, are chemically inert to a wide variety of solutions even at elevated temperatures, and display excellent catalytic activity. The major workable deposits of PGE occur with nickel-copper and copper lodes associated with mafic and ultramafic igneous rocks. Magmatic processes, such as crystal fractionation and segregation of an immiscible sulfide liquid, lead to the concentration of PGE as ore deposits. Significant resources of PGE of purely hydrothermal origin are yet to be discovered. However, there is accumulating evidence that hydrothermal fluids may play a role in the redistribution of PGE in a variety of environments.
The oldest rocks in Nevada crop out in Clark County as the western extension of the southwestern U.S. Precambrian basement. It is in this general area where hydrothermal PGE deposits occur; the Bunkerville copper-nickel-platinum ores show linkage of hydrothermal activity to hornblendite, whereas the Goodsprings copper-platinum-palladium ores bear no apparent connection to mafic or other igneous activity other than the presence of nearby granite porphyry. Platinum was a minor by-product of mining in the Ruth porphyry copper district in White Pine County. Thus hydrothermal PGE mineralization may occur in different geologic settings. Hydrothermal mineralization typically involves dissolution of source material by a hot aqueous fluid followed by transport and precipitation of the material as ores where and when drastic changes in physicochemical conditions occur.
To understand the hydrothermal behavior of PGE, we undertook a study to determine PGE solubilities under pressure and temperature conditions most likely to be observed in nature. Palladium was the first PGE chosen for this study. A technique of trapping solutions was employed. In this technique two sizes of palladium capsules were used. The inner capsule was perforated and filled with spectrochemically pure quartz grains and sealed in the outer capsule along with 100 to 150 microliters of solution. On quenching, the solution was entrapped in the interstices of the quartz grains.
When a metal dissolves in a solution, it usually does so by forming a complex ion or group of complex ions. The effect of chloride ion on complex formation (such as PdCI4-2) was determined by using a series of freshly prepared solutions with varying NaCl concentrations at a constant pH of 6.5. The effects of pH on complex formation was determined by using a series of freshly prepared solutions at constant low (0.01 molal) NaCl concentration but varying pH. All the experiments were conducted at a fluid pressure of 1 kbar, corresponding to a 3-km-thick rock load, under conditions of fairly constant oxidation state and at temperatures between 300 and 700C. The fluid that equilibrated at a given pressure and temperature condition in the interstices of quartz grains was recovered and analyzed for dissolved palladium by flameless atomic absorption spectrometry.
The solubilities of palladium in the solutions at a constant pH of 6.5 increased with increasing NaCl concentrations at all test temperatures, particularly at lower temperatures. In solutions with high NaCl concentration (1 molal and 3 molal), the solubility of palladium decreased dramatically from 300 to 700C. In solutions with lower NaCl concentration, the palladium solubility was very low regardless of temperature. In solutions with constant low NaCl concentration, there was a general trend of decreasing solubility with increasing pH or alkalinity. Except in the lowest pH solutions (pH 1.5) the solubility of palladium was generally low and indifferent to temperature change.
The experimental results presented above are only preliminary and much more work is needed. The solubilities of palladium obtained in this study were four to six orders of magnitude higher than those calculated and predicted based on recent theoretical extrapolation of standard state solubility data at 25C and 1 bar. However, recent studies of gold, silver, and PGE contents of fluids in active Cordilleran geothermal systems show that moderate-temperature hydrothermal fluids can transport a thousand to a million times more PGE than generally assumed. Our experimental data are thus supported by field observations.
Our interpretation of the experimental results is that, in solutions low in NaCl, the chloride ion activity is too low to exert appreciable effect on palladium complex formation regardless of temperature. On the other hand, in solutions with high NaCl concentrations, there is significant complex formation at relatively low (300 to 400C) temperatures, resulting in high solubilities. At high temperatures (700C) solubilities are lower -- perhaps due to the formation of a neutral aqueous complex. We conclude that palladium could be transported by hydrothermal brines but not by dilute alkaline hydrothermal solutions.
---L.C. Hsu, Geochemist/Mineralogist, and P.J. Lechler, Chief Chemist/Geochemist