MAPPING STREAM SEDIMENT GEOCHEMISTRY IN CENTRAL COLORADO

The U.S. Geological Survey (USGS) recently completed a series of geologic resource and environmental assessment studies in the Rocky Mountains of central Colorado, from Leadville eastward to the range front and from New Mexico to the Wyoming border. Regional stream sediment geochemical maps, useful for assessing mineral resources and environmental effects of historical mining activities, were produced as part of the study. Nearly 1,200 samples collected in the late 1970s in the National Uranium Resource Evaluation program were re-submitted for geochemical analysis by 42-element inductively-coupled plasma mass spectrometry. These new data were combined with data from recently collected USGS samples to create a geospatial dataset of 1,479 stream sediment samples and an associated suite of 56 geochemical landscape maps.

Each map includes a histogram, summary statistics, a symbolized point layer of the sample distribution, and a prediction surface calculated from the point source values. The point layer is classified by percentiles, a method chosen because it is reproducible and resistant to skewing effects from statistical outliers. The Inverse Distance Weighting interpolation method, used for generating the prediction surfaces, was chosen because it assumes that relationships in elemental values decrease with increasing distance between the sampled points, and is exact; widely differing values in close proximity do not generate false extremes because minimum and maximum values occur only at sample points.

First order geologic features apparent in the geochemical maps include (1) the NE-trending Colorado Mineral Belt (highlighted by the elements Ag, As, Bi, Cu, Hg, Mo, Pb, and Zn), (2) the Proterozoic Pikes Peak Batholith (Be, Th, and the rare-earth elements), (3) gabbros and amphibolites in Proterozoic gneisses in north-central Colorado (Cr, Ni, and V), (4) the 39-Mile Volcanic Field (Ba), and (5) the Boulder County tungsten district (W).

The 56-parameter suite of landscape geochemical maps serve as a geochemical baseline for the region, indicate elemental abundances characteristic of various lithologic terrains, and identify gross anthropogenic effects of historic mining (Hg). The maps, geospatial data, and a geochemical relational database are being prepared for USGS publication.