The Raton Coal Basin of Colorado and New Mexico is one of the premier coal basins where the Cretaceous/Paleogene (K-Pg) or the Cretaceous/Tertiary (K-T) boundary can be viewed and samples of coals, tonsteins, and shaly materials proximal to this boundary can be readily obtained. Near Trinidad, CO, extensive coal deposits exist that straddle this boundary. Further, the presence of near-by igneous features such as the Sangre de Cristo Mountains and associated dikes and sills provide a possible source of anomalous geochemical features found in the coals. Reconnaissance geochemical investigations utilizing a hand-held X-Ray Fluorescence Spectrometer (XRF) indicate that appreciable amounts of titanium, zircon, yttrium (Ti, Zr, Y), and light rare-earth elements (REE) are present within the coals and coal partings near the boundary of the Cretaceous Vermejo Formation and the overlying Tertiary Raton Formation. In some instances, semi-quantitative determinations indicate elevated quantities of Y and light REE (hundreds of parts per million) are present in select raw coal samples in the area. Earlier studies of the REE content of coal samples in the basin were conducted by the USGS and presented in their CoalQual database. The results of the XRF spectroscopy corroborates the findings reported in that database.
Altered kaolinitic pyroclastic ash is present in the basin that is interpreted to be in-part sourced from the Cretaceous/Tertiary impact. This thin claystone or tonstein is present in at least 20 locations. This thin layer is generally overlain with a thin layer of coal reported to be 4 to 16 cm thick, (Izett, 1987). Interestingly, 2 samples of this material did show detectable and elevated concentrations of iridium, titanium, and other exotic heavy metals, the REE were not detected suggesting possible other sources of the REE in the coals. Further, aside from the purported impact ash fall materials from localized volcanic episodes within the basin, intrusive igneous activity is also reported in the general area. Geochemical studies of dikes, stocks, and other intrusive material in the Spanish Peaks and the Sangre de Cristo Mountains show similarly elevated REE content. There is an apparent proximal relationship between elevated Y and REE in coal samples to these tonstein features. Additional study of REE occurrences associated with tonstein deposits and kaolinized ash within the coal as well as in the coal material itself is warranted. If the REE occurrence is found to be widespread and stratigraphically predictable, then strategies for mining the coal and segregating coal ash and other coal waste products might be developed to exploit and extract REE resources when processing the coal.
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The Raton Basin is a Laramide structural basin (Baltz, 1985). It is an asymmetrical, elongated sedimentary basin formed covering approximately 4,000 mi2 in southwestern Colorado and northeastern New Mexico. The oldest sedimentary units in the basin are marine beds of Devonian and Mississippian age. Units of interest in this paper are the late Cretaceous and early Tertiary (Paleogene) sediments of terrestrial origin. Baltz, 1985 reports these units reached an aggregate thickness of 12,000 feet in the northern portion of the basin. Geographically the basin extends from Ute Park, New Mexico to Huerfano Park, Colorado and is bound on the western side by the Sangre de Cristo Mountains, on the north by the Wet Mountains and the Apishipa Arch, and on the southeast by the Sierra Grande Uplift (Keighin, 1995). Figure 1 (from Flores and Bader, 1999) shows the generalized Cretaceous/Tertiary boundary (K-T or K/T) geology and areal extent of the basin. Thick coal deposits are present in portions of the basin. Flores and Bader, 1999 report that lower coal units in the Raton Formation in the Raton Basin are interspersed over a 100 to 250 foot thick lower unit comprised of coal, carbonaceous shale, mudstone, siltstone, and sandstone. An upper tertiary coal-bearing unit is also present and is comprised of between 600 to 1100 ft. thick unit of sandstone, siltstone, mudstone, coal and carbonaceous shale. Coal units in these sequences can range up to 10 ft. or more in thickness with total coal thicknesses ranging between 10 and 140 feet (EPA, 2004). The underlying Vermejo Formation (upper Cretaceous) contains numerous coal bearing seams range between a few inches to up to 14 feet with total coal thicknesses ranging between 5 to 35 feet (EPA, 2004). Note also that the Spanish Peaks igneous complex lies within the northwestern portion of the Basin. This complex is composed of intermediate to silicic Cenozoic volcanics
Rare Earth Associations in the Raton Basin
Rare earth elements (REE) are a group of 17 elements with similar chemical properties made up of the 15 lanthanides and the elements yttrium and scandium. Figure 2 from Rare Element Resources (2014) shows where they fall within the periodic chart of elements. Because of their particular fractionation behavior, most rare earth deposits appear to occur in rocks that are under-saturated with respect to silica and are believed to be products of differentiation of partial melts derived from undepleted metosomatically enriched mantle material (Sheard, 2010). As such nepheline syenite lamprophyre, and other alkaline rocks have been known to favor REE enrichment. Both the Spanish Peaks and Wet Mountains of Colorado exhibit elevated REE content. Petrographically, the Spanish Peak rocks are a continuous serial gradation from granite to gabbro, with syenodiorite being the most common rock type (Johnson, 1968) and alkaline rocks high in both Th and REE are present in the Wet Mountains (Armbrustmacher, 1984).
When considering possible mechanisms on how REE material can be introduced into coal and coal bearing basins, recent studies by Andersen et al., (2015) suggest three plausible means.
- introduction of REE via widespread volcanic ash falls (or tuff),
- introduction of REE from igneous intrusions (diapirs, dikes, sill, and pipes), and
- introduction of REE with the fluvial-detrital transport of resistate heavy black sands.
Figure 3 schematically shows the expected influence and distribution of REE in a basin using these three mechanisms (Andersen, et al, 2015). Based on the known geology history and character of the Raton basin, the influence of kaolinitic-rich volcanic ash/tuff deposits or tonsteins layers (for example the K-T layer tonstein) plus local igneous activity during the middle Tertiary resulted in the formation of igneous dikes and sills associated with the development of the Spanish Peaks are likely sources for the observed REE content in both the coals and coaly shales. Many of these igneous features “coked” coal in place in portions of the basin. If they contained REE then their influence on the observed REE in sediments may be substantial.
When considering the Raton Basin, certainly ash falls associated with either the K-T impact, as well as earlier and later volcanic activity could be involved as could direct involvement of intruding dikes and sills. While not discounting fluvial influence as a possibility, this mechanism was not investigated in this report. In the Colorado portion of the Raton Basin, elevated occurrences of light REE in coals and coaly shales are found close to K-T boundary. This boundary is defined by a thin claystone (tonstein) layer, (Izett, 1990). In a recent field trip (Taylor, 2015) by the authors to this area, 2 samples of the actual K-T boundary clay were sampled by XRF. Figure 4 shows one site visited located in the Trinidad Lake State Park. The boundary is depicted by the thin white clay layer just beneath the massive sandstone in this figure. Interestingly enough, while samples from both sites exhibited extremely elevated levels of iridium and tungsten, as well as traces of other heavy metals, no rare earth elements were seen in the analysis. This area however, the basin is punctuated by extensive igneous activity associated with the formation of the Spanish Peaks and the associated radial lamprophyre dikes. In fact, coal
has actually been coked naturally in some areas of the basin suggesting that both volcanic ash as well as igneous intrusion could be responsible for the elevated REE content in the basin coals and coaly shales. Published REE data (Knight, 2011) on the rocks of the Spanish Peaks as well as new data obtained by the authors utilizing a portable X-Ray Fluorescence Spectrometer (XRF) on dike material and coal/coaly shale along the K-T boundary of the area. Knight’s data appears in Table 1. These data were obtained utilizing instrumental neutron activation analysis (INAA) along with literature derived values.
Table 1. Rare Earth Elemental Data of Select Spanish Peak Igneous Samples
In the above table, samples beginning with “bj” are from Jahn (1973), and samples beginning with “rj” are from Johnson (1968).
Surface outcrop samples of coal and shaly-coal material from the Colorado portion of the Raton basin were analyzed for their light REE content using a Niton XL3T Gold++ portable X-ray fluorescence (XRF) spectrometer. Table 2 shows the results of several of these samples as well as some from the Florence, Colorado, area. What is interesting about the Raton samples is they were collected near the Cretaceous/Tertiary boundary and are situated next to the Spanish Peaks volcanics and associated lamprophyre dikes. The Florence samples, collected northwest of the Raton Basin are adjacent to the Wet Mountains, which are reported to contain appreciable thorium and REE (Long et al., 2010) in syenitic and carbonatite dikes.
Coal samples were collected by USGS in the Raton Basin as part of their CoalQual Database effort (USGS, 2014). That study sent coal samples for laboratory analysis using inductively coupled plasma/mass spectrometry (ICP/MS) to determine their REE contents. Comparison of XRF results for samples collected by Tetra Tech in the Raton Basin with the reported USGS values shows that the Raton Basin consistently has elevated REE contents. Figure 5 shows the Colorado portion of the Raton Basin with both light REE values from the USGS CoalQual database coal samples (colored squares) and the XRF spectrometer-derived values of surface coal, coaly shale, and igneous dike samples (colored diamonds). As reported, four USGS coal samples had light REE contents greater than 100 ppm, and two samples had light REE contents greater than 200 ppm. The results for several other samples of coal and coaly shale obtained using the field-portable XRF unit found light REE contents in excess of 1,000 ppm. Samples obtained from a Tertiary lamprophyre dike just southwest of the Town of La Veta showed similarly high light REE contents as the coal samples. This implies that, in addition to introducing REE in the basin via tonstein deposits within the coal, igneous injection of dikes and sills could also be a possible sourcing mechanism for introducing REE. In some portions of the basin, coal has actually been converted to coke due to igneous activity. Further study should include obtaining samples of this naturally occurring coke to ascertain its REE content. Pertinent REE data and other elements derived from XRF analysis within the Raton basin in south-central Colorado appears in Table 2.
Table 2. Rare Earth Elemental analysis of Raton Basin samples by XRF Spectroscopy
Coal, shaly coals and tonstein samples collected in the greater Raton basin of Colorado exhibited elevated rare earth content. Several of these samples were collected proximal to the K-T layer inferring that ash derived from this impact which resulted in the formation of a thin tonstein layer could be a plausible source. Volcanic and other igneous activity in the basin could also be responsible and post-dates the K-T boundary, suggesting that hydrothermal fluids and/or direct contact with igneous dikes and sills could introduce REE into the samples. Certainly, samples of Cenozoic lamprophyre dike material collected at the northern edge of the basin exhibited REE contents that were as high as or higher than those observed in the coal and coaly shale samples.
If a systematic survey over the entire basin can be conducted, then the association of REE-rich layers with coal might provide a means to formulate a strategy to exploit these elements as a by-product of coal production. Future work should include careful scrutiny of exploration cores in the basin if available to identify samples of tonstein, the K-T clay, and fresh coal above and below these layers to determine if the source of REE in the coals are from volcanic ash, intrusive, or the K-T impact.
Armbrustmacher, T.J., 1984, Alkaline Rock Complexes in the Wet Mountains Area, Custer and Fremont Counties, Colorado, USGS Prof. Paper 1269, 38 p.
Andersen, H.T, Bryan, R, Gray, T, and Richers, D, 2015, Rare Earth Occurrences and their Associations with Tonsteins, Diapirs, and Igneous Activity, presented at the 2015 Pittsburgh Coal Conference, October 6, 2015, Pittsburgh, PA., 13 p.
Baltz, E. H., 1985,Stratigraphy and history of the Raton Basin and notes on San Luis Basin, Colorado-New Mexico, AAPG Bulletin, Volume 49,No. 11, pp. 2041-2075.
Flores, R.M., and Bader, L.R., 1999, A Summary of Tertiary Coal Resources of the Raton Basin, Colorado and New Mexico, USGS Professional Paper 1625-A, pp. SR-1,SR-28.
EPA, 2004, Attachment 9, “The Raton Basin” in Evaluation of Impacts to Underground Sources of Drinking Water by Hydraulic Fracturing of Coalbed Methane Reservoirs. EPA 816-R-04-003, 19 p.
Izett, G.A, 1987, The Cretaceous-Tertiary (K-T) boundary interval, Raton Basin, Colorado and New Mexico, and its content of shock-metamorphosed minerals: Implications concerning the K- T boundary impact-extinction theory, U.S.G.S Open-File Report 87606.
Johnson, R.B., 1968, Geology of the Igneous Rocks of the Spanish Peaks Region, Colorado, USGS Professional Paper 594-G, 58 p.
Keighin, C.W., 1995, Raton Basin-Sierra Grande Uplift Province (041), USGS, 126p.http://certmapper.cr.usgs.gov/data/noga95/prov41/text/prov41.pdf
Rare Element Resources, 2014, http://www.rareelementresources. com/rare-earth–elements#.VeXBqpRUyMx
Sheard, E.R., 2010, Behavior of zirconium, niobium, yttrium, and the rare earth elements in the Thor Lake rare-metal deposit, Northwest Territories, Canada, Masters Thesis, Department of Earth and Planetary Sciences, McGill University, Montreal, QC, Canada, 151 p.
Taylor, L. H, 2015m Field guide for the Cretaceous-Paleogene Boundary (Formerly the Cretaceous – Tertiary Boundary), Ludlow and Trinidad, Colorado. A guide prepared for the KT Field Tg the coal.