CONCENTRATIONS OF PLATINUM GROUP ELEMENTS IN 122 U.S. COAL SAMPLES
Oman, C. L., Finkelman, R.B., and Tewalt, S.J.
U.S. Geological Survey Open-File Report 97-53
This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards, and stratigraphic nomenclature
Analysis of more than 13,000 coal samples by semi-quantitative optical emission spectroscopy (OES) indicates that concentrations of the platinum group elements (iridium, palladium, platinum, osmium, rhodium, and ruthenium) are less than 1 ppm in the ash, the limit of detection for this method of analysis. In order to accurately determine the concentration of the platinum group elements (PGE) in coal, additional data were obtained by inductively coupled plasma mass spectroscopy, an analytical method having part-per-billion (ppb) detection limits for these elements. These data indicate that the PGE in coal occur in concentrations on the order of 1 ppb or less.
Since the mid-1970s the U.S. Geological Survey has analyzed more than 13,000 coal samples for their trace element contents. Included in these analysis were determinations for the platinum group elements [iridium (Ir), palladium (Pd), platinum (Pt), osmium (Os), rhodium (Rh), and ruthenium (Ru)]1. These determinations where done by 6-step and optical plate reader emission spectroscopy (Golightly and Simon, 1989). Except for a nine questionable results, all of the data were qualified as less than the limit of detection (2 ppm for Pt, Rh and Ru, 1 ppm for Pd, and 15 ppm for Os and Ir). To generate reliable data on the platinum group elements (Rh, Ru, Pt, Pd, Ir; Os was not determined) in coal we employed inductive plasma mass spectrometry (ICP-MS; Conrad and Krofcheck, 1992) on 122 selected coal samples. The detection limits for these PGE's by ICP-MS is generally a few parts per billion (Jackson and others, 1990).
1. See Finkelman and Tewalt (1994) for a discussion of platinum group elements in coal including possible modes of occurrence.
Several groups of coal samples were selected for analysis. A small number of samples (8 of the 9) analyzed by emission spectrometry and reported by Bragg and others (1994) to have unqualified values (i.e. concentrations above the detection limits) were reanalyzed. A second group of 62 samples, primarily from the major coal producing areas, was selected for broad geographic coverage and to represent various coal ranks and coal-associated materials (coal waste, limestone, claystone, etc.). A third group of 7 coal samples was chosen because of the relatively high concentration of elements (such as Cr, Co, Ni, V, Ag, Pb, total sulfur, pyritic sulfur) that may be associated with PGE due to thier geochemical properties (Goldschmidt, 1954). A fourth group of 8 samples was chosen because of their proximity to the Cretaceous/Tertiary boundary. The K/T boundary is commonly shown to have anomalous high PGE values, particularly Ir (Gilmore and others, 1984). A fifth group of 37 samples was picked on the basis of their carbon, hydrogen, nitrogen, and oxygen ratios representing mostly cannel coal samples (initial results indicated higher concentrations of PGEs in cannel coals than in banded coals).
RESULTS AND DISCUSSION
Analytical results are presented in Table 1 and are discussed below by sample group and element. Table 1 provides the following information: the sample numbers as well as the original laboratory numbers for those samples previously analyzed by OES, field numbers, state, county, type of sample, the reason why the sample was selected, the lab numbers for this study, and the Rh, Ru, Pt, Pd, and Ir values. The PGE values in Table 1 are on an ash basis for the coal samples.
Group 1. (The eight samples reported by Bragg and others  to have Pt or Pd values greater than 1ppm in the coal): None of these samples had PGE contents greater than a few ppb in the ash (Table 1). Thus, the previously reported OES data are spurious.
Group 2. (Comprehensive geographic, rank and sample type coverage): Only one Ru value and no Rh or Ir contents above detection limits. The Pt and Pd contents are generally less than 5 ppb. One coal waste sample from Coleman County Texas (an area that experienced silver mineralization) had 42 ppb Ru and 1900 ppb Pt! Because two other coal waste samples from this site had no more than ppb levels of the PGEs, these exceptionally high values are suspect.
Group 3. (Samples having a relatively high concentration of elements commonly associated with the PGE): None of these samples has PGE contents above a few ppb. Therefore, element associations do not appear to be a reliable indicator of PGE mineralization in coal.
Group 4. (Cretaceous/Tertiary boundary samples): These samples have the highest average concentrations for the PGEs. This group contained the only two samples having Rh contents above the detection limit; 7 of the 8 samples having Ru contents above the detection limit; and all 7 samples having Ir contents above the detection limits. Pt contents ranged from 1.7 to 36 ppb, averaging more than 12 ppb. Pd contents ranged from 1.5 to 20 ppb, averaging more than 7 ppb.
Two of these samples, a coal and a claystone, are the only samples that contain measurable amounts of all 5 platinum group elements determined for this study. It is apparent that these Cretaceous/Tertiary boundary samples exhibit anomalously high contents not only for Ir, but for all of the platinum group elements determined. Moreover, the PGE ratios in these samples are more like cosmic ratios than crustal ratios (see below).
|1. From Mason and Moore (1982)|
Group 5. (Cannel coals): Pt and Pd contents are generally less than 5 ppb with a maximum value of 12 and 13 ppb, respectively.
Platinum Group Elements
Iridium: The data for Iridium show that only seven of the 122 samples have Ir contents above the limit of detection (2-3 ppb). All seven of the samples are from the Raton area of New Mexico, and are associated with the Cretaceous/Tertiary boundary.
Palladium: Eighty-three samples had palladium values above the limit of detection (3-5 ppb). The highest Pd value is 28 parts per billion in a bituminous coal sample from Greenbrier County, West Virginia. A second coal sample from the same location has less than 4 ppb Pd.
Platinum: Thirty-six of the 122 samples had "less than" values for platinum (generally <2 ppb). The highest concentration of Pt (aside from the suspect value from the coal waste sample from Texas) is 33 ppb from the Greenbrier Co., WV sample. The second sample from that location has only 2 ppb Pt.
Rhodium: Rhodium concentrations were above the limit of detection (2-3 ppb) in only 2 samples, both from the Cretaceous/Tertiary boundary in New Mexico.
Ruthenium: Ruthenium is present in amounts greater than the limit of detection (2-3 ppb) in 9 samples. The highest Ru value (42 ppb) is in the bituminous coal waste sample from Texas. Seven of the remaining samples are from the Cretaceous/Tertiary boundary in New Mexico.
ICP-MS analyses of 122 selected samples of coal and associated rocks indicate that the concentrations of PGE are on the order of parts per billion or less. Only samples from the vicinity of the Cretaceous/Tertiary boundary have PGE concentrations in the range of crustal abundances and have measurable amounts of the PGE analyzed for (Rh, Ru, Pt, Pd, IR).
Aside from one sample of coal waste from Texas, no potentially economic concentrations of PGE were detected nor were there any values or trends suggestive of nearby PGE mineralization.
Platinum and palladium have a high correlation coefficient (+0.72). There are insufficient data for the other elements to test for statistical correlation. For the Group 4 samples the correlation between Pt and the other PGE is between 0.92 and 0.99.
Bragg, L. J., Oman, J. K., Tewalt, S. J., Oman, C. L., Rega, N. H., Washington, P. M., and Finkelman, R. B., 1994, U. S. Geological Survey Coal Quality (COALQUAL) Database: Version 1.3. U. S. Geological Survey Open-File Report 94-205.
Conrad, V. B. and Krofcheck, D. S., 1992, ICP-MS determination of trace elements in coal and other geological materials. In Elemental Analysis of Coal and Its By-Products, G. Vourvopoulos, Ed. World Scientific Publ. Co. Singapore. p. 80-123.
Finkelman, R. B. and Tewalt, S. J., 1994, Platinum-group elements in coal, in J. A. Peterson, Platinum-Group Elements in Sedimentary Environments in the Conterminous United States. United States Geological Survey Bulletin 2049-A, p. A12-A14.
Gilmore, J. S., Knight, J. D., Orth, C. J., Pillmore, C. C., and Tschudy, R. H., 1984, Trace element patterns at a non-marine Cretaceous-Tertiary boundary. Nature, vol. 307, p. 224-228.
Goldschmidt, V. M., 1954, Geochemistry. Oxford University Press, London. 730 p.
Golightly, D.W. and Simon, F.O., 1989, Methods for sampling and inorganic analysis of coal: U.S. Geological Survey Bulletin 1823, 72p.
Jackson, S. E., Freyer, B. J., Gosse, W., Healey, D. C., Longerich, H. P., and Strong, D. F., 1990, Determination of the precious metals in geologic materials by inductively coupled plasma-mass spectrometry (ICP-MS) with nickel sulfide fire-assay collection and tellurium coprecipitation. Chemical Geology, v. 83, p. 119-132.
Mason, B. And Moore, C. B., 1982, Principles of Geochemistry, Fourth Edition. J. Wiley & Sons, New York. 344 p.
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