New Mexico Geological Society Annual Spring Meeting — Abstracts

As part of an ongoing study of base metal concentrations in Upper Pecos River stream sediments, 33 samples were collected in May, 1994 during high-flow spring runoff. Analysis by flame atomic absorption (FAAS) and-x-ray fluorescence (XRF) ofthe whole (excluding >2mm), large (<2mm and >63 microns), and small (<63 microns) sediment fractions have shown elevated levels of Zn, Pb, and Cu at sites immediately below the Pecos mine waste dump and below the tailings pile in Alamitos Canyon. In addition, elevated levels of Zn are found near the Lisboa Springs Fish Hatchery, 18 km below the Pecos mine. These data also show that metal concentrations are greater in the smallest size fraction. Particle size analysis ofall samples have shown that the smallest size fraction (<63 microns) typically represents only 10% ofthe whole sample. Neutron activation analyses were performed on 22 samples for a suite ofelements, including Sb, As, Au, Se, and Cd. These metals were typically low except for elevated concentrations immediately below the Pecos mine. Six samples were sieved into six size fractions (1-2mm,0.5-1mm, 0.2S-0.5mm, 0.125-0.25mm, 63 microns-0.125mm, and less than 63 microns) and analyzed by FAAS for Cu, Pb, Zn, Cd, Fe, Mg, and Mn. In three samples exhibiting low total metal values (Windy Bridge, Above Hatchery, Villanueva), the metaI concentrations (Cu, Pb, Zn, Cd) increases with decrease in particle size fraction. In the sample collected at the site labeled Above Hatchery, Zn concentrations are high (740 ppm) in the less than 0.063 mm size fraction. This suggests that precipitates and suspended sediment is transported from the Pecos mine waste dump and is being deposited at the diversion dam above the hatchery (site AH). The suspended sediment, elevated in Zn, may be entering the hatchery during turbulent flows and could be contributing to periodic fish kills. In three samples with elevated metal values (Below Willow Creek, Above Pecos Village, Below Pecos Village), the highest metal concentrations (Cu, Pb, Zn, Cd) typically correspond to the largest (0.5-1mm, 1-2mm) and smallest (0.063-0.125mm, less than 0.063mm) size fractions and further suggests that the metals are traveling as precipitates, suspended material, and as larger grains weathered from the mine waste dump and tailings pile.