Multielement chemical and statistical analyses from a uranium hydrogeochemical and stream-sediment survey in and near the Elkhorn Mountains, Jefferson County, Montana: Part II, Stream sediments

Abstract

Fifty-two stream-sediment samples, collected from an area south of Helena, Jefferson County, Montana, were sieved into two size fractions (<88 ?m, and between 88 and 149 ?m) and analyzed for 80 different chemical species. Of these species, 43 showed detectable variation over the area, 31 were subjected to correlation analysis for the fine size fraction, and 30 for the coarse-size fraction. Two populations distinguished in the surface water samples of this study (Suits and Wenrich, 1981) are not as clearly evident in the stream sediments. However, stream-sediment samples from streams draining the Boulder batholith and from streams draining volcanics and volcanogenic sediments (referred to only as volcanics) do show differences in their geochemical relationships, especially between SiO2, Al203 and Na2O versus U. Thus, the sediments were also treated statistically as two lithologic populations as well as two populations based on size fraction. In the fine fraction samples, U ranged from 1.3 to 78 ppm, averaging 25 ppm for the Boulder batholith samples and 8 ppm for the volcanics samples. The range for the coarse fraction was 1.9 to 55 ppm with an average of 17 for the Boulder batholith group and 7 for the volcanics group. High U values (>50 ppm for the fine fraction) were encountered in samples from the Warm Springs Creek drainage area, along Prickly Pear Creek near Welmer and Golconda Creeks and along Muskrat Creek. All groups showed a significant correlation at the 99 percent confidence level (r between 0.73 and 0.77) between U and Th. Uranium was found to correlate significantly only with Th (as mentioned above) and with -Ni in the fine fraction of the volcanics group. U correlates significantly with -Al2O3, Ba, organic C, -K2O, -Sr and Y in both size fractions for the Boulder batholith. Correlations between U and each of several elements differ for the fine and coarse fractions of the Boulder batholith group, suggesting that the U distribution in these stream sediments is in large part controlled by grain size. Correlations were found between U and CaO, Cr, Fe203, -Na2O, Sc, -SiO2, TiO2, Yb and Zr in the coarse fraction but not in the fine fraction. U correlates weakly (to the 90% confidence level, crc<.37) with -Co and -Cu in the fine but not the coarse fraction. These results are compared to a previous study in the northern Absaroka mountains. Correlation coefficients between all other elements determined from these samples are also shown in Tables 12 to 15.