Temporal variation of metal concentrations in biogeochemical samples over the Royal Tiger Mine, Colorado, Part II. Between-year variation

Abstract

Samples of lodgepole pine (Pinus contorta Dougl. ex Loud.) were collected every two months for six sampling periods, May 1983 to March 1984. Needle, twig, and wood samples were taken at sixty-five points at 15-m intervals along a 1-km transect line and analyzed for Cu, Zn, Pb, Mo, Au, and Ag. The study area was the Royal Tiger Mine in the Breckenridge Mining District of Colorado, U.S.A. The mine workings were located within a fault-bounded breccia complex which is well mineralized with Cu, Pb, Zn, Au, Ag, and Bi. The sample transect line extended over the complex into unmineralized areas. The mine workings provided underground geologic control. Wood and ashed needle and twig samples over the mineralized area showed significant variation in Au concentrations. In the 12-month study period, wood samples ranged from <0.1 ppb to 35.0 ppb Au, whereas, needle and twig samples ranged from <0.1 ppb to 300 and 900 ppb Au, respectively. Higher Au concentrations occurred in the spring and early summer. Gold anomalies, as measured by Au concentrations in plant organs, were observed in less than half the sampling periods. Gold concentrations in needles were consistently lower, but paralleled Au concentrations in the twigs. Copper concentrations ranged from <0.5 to 8 ppm in wood; up to 191 ppm in needles, and up to 353 ppm in twigs. Mineralized areas were identified by high Cu concentrations in needle and twig samples over all sampling periods. Concentrations of Zn in wood ranged from 3 to 55 ppm and identified background/anomaly contrasts and zones of mineralization, although the concentrations varied over time. Zn concentrations in needles and twigs of up to 7,420 ppm indicated background/anomaly contrasts without variation of peak concentrations. Molybdenum concentrations in wood were mostly below detection limits and not indicative of mineralization. Mean Mo concentrations of 4.7 and 13 ppm in needles and twigs did not identify any background/anomaly contrasts. Similarly, Ag concentrations were frequently below detection limits and were not useful for mineralization identification.