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The lower Eocene sandstones in the Rocky Mountain region were studied to determine the geochemical setting for the valuable uranium and other mineral deposits they contain. Three pairs of samples were collected from each of 2 sections in 18 different basins and each sample was analyzed for chemical and mineral constituents. The resultant data on 216 samples have been treated statistically. The samples range from arkosic wackes derived from the crystalline cores of mountain ranges to quartz and carbonate arenites derived from the reworking of Mesozoic and Paleozoic strata. The average composition of the 216 samples, expressed as the geometric mean (geometric deviation follows in parentheses), is [in percent]: SiO2 72.9 (1.2), Al2O3 (1.9), t tal iron as Fe2O3 1.2 (2.1), MgO 0.6 (3.0), CaO 2.4 (4.3), Na2O 0.5 (4.6), K2O 1.4 (2.3), H2O+ 1.4 (1.6), TiO2 0.2 (1.9), P2O5 0.05 (2.6), MnO 0.06 (2.1), CO2 0.7 (12.6); [in ppm] B 13 (2), Ba 415 (2), Co 4 (2), Cr 13 (3), Cu 8 (2), Ga 9 (2), La 12 (2), Ni 9 (2), Pb 8 (2), Sr 138 (3), V 24 (2), Y 10 (2), and Zr 113 (2). Compared with the average composition of sandstone as estimated by Pettijohn, these figures are high for Sr, V, La, Y, Ni, and Co, and low for Fe2O3, MgO, Na2O, TiO2 P2O5, MnO, Zr, and possibly B. Geometric mean values for each of the individual sections show considerable variation from the overall ean.
Analysis of variance data suggests that for most constituents the greatest percentage of variance occurs between pairs of samples. This variance also includes that resulting from analytical errors and sample inhomogeneity. For most constituents, significant variance exists between sections; Al2O3, K2O, Na2O, Ba, Cr, and Ga also show significant variance between basins, suggesting that these data are most useful for plotting regional trends.
Factor analysis of correlation coefficients was used to determine 3 principal geochemical groupings of constituents--quartz, carbonate, and feldspar. Among the minor elements, Cr, Ni, Co, Cu, and V tend to correlate with each other and with iron. Ba correlates best with Al2O3 and K2O; Sr with CaO and plagioclase; Ga with Al2O3 and Na2O. Zr tends to correlate with TiO2; B with quartz; and MnO with CaO.
Similar statistical methods were applied to 4 separate color subsets of the 216 samples--identified as predominantly red, orange, yellow, or green. The subset of 11 green samples, which were yellowish green to greenish gray to olive gray, differs significantly from the other color subsets and the whole. The green subset shows higher concentrations of nearly every constituent except SiO2, 14-15 A clays, kaolin, quartz, potassium feldspar, and calcite. They are enriched especially in Co, Cr, Cu, Ni, Sr, V, Ga, and mica.
Individual stratigraphic sections that show high concentrations of some minor elements include the Patmos Head area of Utah, which is enriched in Cr, Cu, V, Ni, Pb, and TiO2, and the Oregon Buttes area of Wyoming, which is enriched in MnO, Sr, Co, Cr, V, and Ni.
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