GEOCHEMISTRY OF WATER AND STREAM SEDIMENTS IN THE UPPER RED RIVER, TAOS COUNTY, NEW MEXICO

The Red River drainage in Taos County, New Mexico, contains a range of rock types from Precambrian schists through Tertiary volcanic rocks, hosting deposits of volcanic-epithermal Au-Ag-Cu vein, and porphyry molybdenum deposits of the Red River and Questa mining districts. In this study, water chemistry and trace element data from stream sediments are combined to determine which elements moving in the stream environment are transported in solution, and which are moving as components of minerals in sediment. There is a change in water chemistry observable in the stream entering the mineralized area. Mg increases from <5 to >7 ppm, Si increases from below detection limits to >13 ppm, F increased from below detection limits to >0.22 ppm, SO4 increases from approximately 15 ppm to 55-90 ppm, and Na increases from 1.5-4.5 ppm to 5.5-9.5 ppm. This same break point marks an increase in non-carbonate hardness, conductivity, and total dissolved solids. Sediment samples were divided into two fractions, a large fraction (2 mm-63 µm), and a fine fraction (>63 µm). In general, sediment geochemistry does not show as distinct a change in composition as do the water samples, but S concentrations in the sediments increased at the same point as the change in water chemistry. The correlation between As and S (>0.80) seem to indicate that As is present in the large sediment fraction as a sulfide. Strong correlation coefficients exist between Fe, Ti, and V (>0.85) in both the fine fraction and the coarse fraction. Cl concentrations in water samples increased downstream, but remained uniformly low (<500 ppm) in the sediments. A sample of the scoria mixture used as a road treatment on local highways contained a high Cl concentration (7,000 ppm), indicating that this treatment was probably adding Cl, and possibly Na and K, to the stream.