Lynn A. Brandvold, Virginia T. McLemore, and, Catherine OConnor, New Mexico Bureau of Mines and Mineral Resources, Socorro, New Mexico, Donald K. Brandvold, Chemistry Department, New Mexico Institute of Mining and Technology, Socorro, New Mexico
A reconnaissance study of base metals and mercury in stream-sediments along the Pecos River, in eastern New Mexico showed elevated copper, lead and zinc at two areas; below an old mine waste dump, and below an old mill. Zinc was also elevated in the area of a fish hatchery located above the mill and 14 miles below the mine. Production from the mine, active from 1927 to 1944, amounted to over 2 million tons of ore containing 0.4% Cu, 2.9% Pb, and 9.1% Zn. The ore consisted of sphalerite, galena, and chalcopyrite in a gaugue of quartz, chlorite, pyrite and sericite. No mill tailings are at the site because the ore was transported for processing to a side canyon that drains to the Pecos much further down stream.
An extensive stream-sediment sampling program was undertaken which included additional sites on the main stem of the river as well as sites on the major tributaries. Sediments were sieved to obtain the 2mm to 63 micron and <63 microns sized-particles. Size fractions were analyzed for total copper, lead, zinc and organic carbon. Partial dissolution techniques were employed to determine the following metal forms: (1) exchangeable, (2) organic, and (3) adsorbed to Fe and Mn oxides. Sediments were also examined by XRD and microscope.
The sediments contain predominately 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.
The smallest size fraction (<63 micron), which constituted 3% or less of the sample weight, contained the highest concentration of metals. Partial dissolution techniques suggested that for the sites above the mine waste dump the metals are concentrated in the crystalline or mineral phase as opposed to adsorption to metal hydroxide, organic, clay or dissolved phases. Below the mine waste dump and below the mill tailings there is a marked increase in the amount of zinc adsorbed to clay and hydroxides. This phenomenon changes downstream in both cases to the initial conditions. Copper, lead, and zinc were higher in samples collected during the Fall of 1992 (low flow conditions) compared to samples collected during the Spring and Summer of 1993 (high flow conditions). This difference, in part, may be due to scouring of the river channel and dispersion of sediment during spring runoff.
Keywords: Stream sediment; metal partitioning; particle size fraction; copper; lead; zinc