New Mexico Geological Society Annual Spring Meeting — Abstracts

Water quality of the Pecos River has regional and international concerns as the demand for water for agriculture, municipal, domestic, and recreational uses increases in eastern New Mexico, west Texas, and Mexico. An earlier study of the geochemistry of water and stream- and lake-sediment samples from along the entire Pecos River in New Mexico indicated that mercury, copper, lead, and zinc concentrations are elevated in stream-sediment samples above and immediately below the Pecos mine waste dumps, suggesting that the waste dumps may be potential sources. Outcropping zones of mineralization and outcropping rocks in the immediate area are also potential sources (McLemore, et al., 1993). A more detailed study of the upper Pecos River, from the Pecos Wilderness Area southward,to Villanueva, was undertaken to examine the sources of the elevated metal values and to determine the effect on water quality.

The geology of the upper Pecos drainage basin is diverse and rocks range in age from Proterozoic through recent. The largest mining district in the Pecos River drainage basin in the Pecos mining district (also known as Willow Creek). Total production from the Pecos mine from 1927 to 1944 amounted to over 2 million tons of ore containing 0.4% Cu, 2.9% Pb, 9.1 % Zn, and some gold and silver. No mill tailing are at the mine site because the ore was transported to Alamitos Canyon for processing. Mineralization occurs in Proterozoic igneous and metamorphic rocks which are overlain by unmineralized Paleozoic sedimentary rocks. Ore consisted of sphalerite, galena, and chalcopyrite in a gaugue of quartz, chlorite, pyrite, and sericite.

The stream-sediment samples from the tributaries draining into the Pecos River and from the Pecos River contain predominantly quartz, amphiboles, feldspars, and traces of zircon, rutile, magnetite, garnet, sphene, monazite, and mica. Clay minerals are rare, but total clay content, primarily as kaolinite, increases downstream. Traces of pyrite, chalcopyrite, and galena were found only in panned concentrates at the mine site along Willow Creek.

Stream-sediment samples collected from the Pecos River near the mine at Willow Creek, immediately above Willow Creek, and immediately below Willow Creek are elevated in copper, lead, and zinc. The metal concentrations decrease downstream. A second area of elevated metal values was found at the confluence of Alamitos Canyon and the Pecos River (at the town of Pecos). Stream-sediment samples from Alamitos Canyon below the mill site are elevated in copper, lead, and zinc suggesting that the mill tailings may be a source for the elevated levels. Stream-sediment samples collected farther downstream from Pecos are low in copper, lead, and zinc. Preliminary studies to determine the form of the metal ions in the sediments suggest that the metals are concentrated in the crystalline or mineral phase as opposed to adsorption to metal hydroxide, organic, clay, or dissolved phases. Furthermore, the metal concentrations (copper, lead, and zinc) are higher in stream-sediment samples collected during the Fall 1992 compared to stream-sediment samples collected during the Summer 1993. This difference, in part, may be due to scouring of the river channel and dispersion of sediment during spring runoff.

Surface and ground water samples from along Willow Creek and the Pecos River, both above and below the mine site, are similar in chemical composition. However, seeps from the mine waste dump are low in pH (e.g. acidic) and contain elevated levels of sulfate and metals. However, water samples of the Pecos River below the mine waste dumps contain low dissolved levels of copper, lead and zinc.