DOI: 10.5066/p99ybjlx
title: Whole-rock geochemical data for alkaline intrusive rocks in the Wet Mountains area of Custer and Fremont Counties, south-central Colorado, USA

description: This data release contains the whole-rock major and trace element analyses of 51 samples of intrusive igneous rocks from the Wet Mountains area of Custer and Fremont counties of south-central Colorado, collected by U.S. Geological Survey (USGS) geologists. The samples were collected from breccias, veins and thin dikes, and a variety of carbonatite, felsic, mafic, and ultramafic intrusions across the area. The first 41 samples listed in this data release were collected in July 2007, originally as part of a reconnaissance study of the thorium deposits of the area (Van Gosen and others, 2009). The samples are grab samples from outcrops, shallow open-pit excavations, and mineral prospect trenches. The last 10 samples listed in this data release were originally collected and geochemically analyzed in 1976 as part of a USGS study of carbonatites in this area (Armbrustmacher, 1976, 1979; Armbrustmacher and Brownfield, 1978). These 10 carbonatite samples were reanalyzed by modern analytical methods in 2007, and the new data are included in this data release. The Wet Mountains area hosts a variety of alkaline intrusions (Armbrustmacher, 1984), which includes three Cambrian-age alkaline complexes (Olson and others, 1977) that intruded the surrounding Precambrian terrane. These are (1) the McClure Mountain Complex (Shawe and Parker, 1967; Armbrustmacher, 1984), (2) the Gem Park Complex (Parker and Sharp, 1970), and (3) the complex at Democrat Creek (Armbrustmacher, 1984). In the Wet Mountains area, elevated concentrations of thorium and rare earth elements (REEs) occur in veins, syenite dikes, fracture zones, breccias, and carbonatite dikes (Armbrustmacher, 1988). These thorium-REE deposits are distal to the alkaline complexes but are thought to be genetically associated. Characteristics of the thorium and REE deposits in the area, as well as typical concentrations and resource estimates, are detailed in the publications listed in the supplementary file "Wet Mountains area publications.txt". Armbrustmacher (1988) determined that vein and fracture zone deposits contain most of the thorium and REE resources in the area. These are linear features, typically 1-2 meters thick, but a few are as much as 15 meters thick. Some individual thorium veins can be traced in outcrop for 1,500 m and some radioactive fracture zones for as much as 13 kilometers. Most of these vein- and fracture-zone deposits occur within a 57 square kilometers tract of Precambrian gneiss and migmatite (Scott and others, 1976) located south and southeast of the quartz syenite complex at Democrat Creek; in this area Christman and others (1953, 1959) mapped nearly 400 veins. Most of the samples in this data release are examples of unaltered alkaline igneous rocks of the intrusive complexes rather than the mineral deposits. These samples were selected in the field to study possible relationships between the magmatic complexes and the thorium-REE deposits. All samples included in this data release were analyzed by laboratories contracted by the USGS. Major and trace element concentrations were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES) and inductively coupled plasma-mass spectrometry (ICP-MS). An acceptable criteria for the data has been identified based on (1) if recovery of each element is within a designated percentage at five times the lower limit of determination, and (2) the calculated relative standard deviation of duplicate samples is no greater than that percentage. The reported laboratory percentages for the acceptance criteria are +/- 15 percent for ICP-AES and ICP-MS. Ten carbonatite samples were additionally analyzed by wavelength dispersive X-ray fluorescence (WDXRF) to determine the concentrations of major elements as oxides. The reported laboratory percentages for the acceptance criteria are +/- 5 percent for WDXRF. Data are reported in a comma-separated values (CSV) file that lists the samples that were analyzed, latitude/longitude location information, brief sample descriptions, and relevant publications. The column headings and abbreviations are explained in the accompanying data dictionary. References cited above: Armbrustmacher, T. J., 1976, Thorium deposits in the Wet Mountains area, Fremont and Custer Counties, Colorado: U.S. Geological Survey Open-File Report 76-284, 18 p., https://doi.org/10.3133/ofr76284 Armbrustmacher, T.J., 1979, Replacement and primary magmatic carbonatites from the Wet Mountains area, Fremont and Custer Counties, Colorado: Economic Geology, v. 74, no. 4, p. 888-901, http://doi.org/10.2113/gsecongeo.74.4.888 Armbrustmacher, T.J., 1984, Alkaline rock complexes in the Wet Mountains area, Custer and Fremont Counties, Colorado: U.S. Geological Survey Professional Paper 1269, 33 p., https://doi.org/10.3133/pp1269 Armbrustmacher, T.J., 1988, Geology and resources of thorium and associated elements in the Wet Mountains area, Fremont and Custer Counties, Colorado: U.S. Geological Survey Professional Paper 1049-F, 34 p., 1 plate, https://doi.org/10.3133/pp1049f Armbrustmacher, T.J., and Brownfield, I.K., 1978, Carbonatites in the Wet Mountains area, Custer and Fremont Counties, Colorado-Chemical and mineralogical data: U.S. Geological Survey Open-File Report 78-177, 6 p., 3 sheets, https://doi.org/10.3133/ofr78177 Christman, R.A., Brock, M.R., Pearson, R.C., and Singewald, Q.D., 1959, Geology and thorium deposits of the Wet Mountains, Colorado - A progress report: U.S. Geological Survey Bulletin 1072-H, p. 491-535, https://doi.org/10.3133/b1072h Christman, R.A., Heyman, A.M., Dellwig, L.F., and Gott, G.B., 1953, Thorium investigations 1950-52, Wet Mountains, Colorado: U.S. Geological Survey Circular 290, 40 p., 5 plates, https://doi.org/10.3133/cir290 Olson, J.C., Marvin, R.F., Parker, R.L., and Mehnert, H.H., 1977, Age and tectonic setting of lower Paleozoic alkalic and mafic rocks, carbonatites, and thorium veins in southcentral Colorado: U.S. Geological Survey Journal of Research, v. 5, no. 6, p. 673-687, https://doi.org/10.3133/70007423 Parker, R.L., and Sharp, W.N., 1970, Mafic-ultramafic igneous rocks and associated carbonatites of the Gem Park Complex, Custer and Fremont Counties, Colorado: U.S. Geological Survey Professional Paper 649, 24 p., https://doi.org/10.3133/pp649 Scott, G. R., Taylor, R. B., Epis, R. C., and Wobus, R. A., 1976, Geologic map of the Pueblo 1 degree x 2 degree quadrangle, south-central Colorado: U.S. Geological Survey Miscellaneous Field Studies Map MF - 775, scale 1:250,000, https://doi.org/10.3133/mf775 Shawe, D.R., and Parker, R.L., 1967, Mafic-ultramafic layered intrusion at Iron Mountain, Fremont County, Colorado: U.S. Geological Survey Bulletin 1251-A, 28 p., https://doi.org/10.3133/b1251A Van Gosen, B.S., Gillerman, V.S., and Armbrustmacher, T.J., 2009, Thorium deposits of the United States - Energy resources for the future?: U.S. Geological Survey Circular 1336, 21 p., https://doi.org/10.3133/cir1336