THE USE OF PB ISOTOPES, TOTAL METALS ANALYSIS AND TOTAL METALS RATIOS TO CHARACTERIZE PB TRANSPORT AND FATE IN AN INTERRUPTED STREAM, ARAVAIPA CREEK, SE ARIZONA

摘要

The most probable source of Pb contamination in the perennial reach of Aravaipa Creek, Arizona, has been determined using Pb isotope ratios and total metals analyses of samples collected from the ephemeral and perennial reaches and mine wastes located in the Aravaipa Mining District. Previously, elevated levels of Pb, Cd and As in fish in the perennial reach of Aravaipa Creek, a protected wilderness area,, were assumed to be due to off-site transport from abandoned flotation tailings piles located at Klondyke, Arizona upstream of the wilderness area. Our work replaces this inference with isotopic data that allows us to confirm this hypothesis. The Pb/Zn deposits of the district were mined between the late 1890s and the late 1960s, with the most significant ore production coming from one deposit in a rhyolite dike swarm on the Western front of the Santa Teresa Mountains, a granitic pluton of Cretaceous-Tertiary age (～65 ma). Mineralogy has been well constrained across the length of the lode system: Pb and Zn sulfides predominate in the NW sector, and a mixed suite of hypogene/supergene Pb sulfide and Pb oxide minerals comprises the majority of its SE corner. Total metals analyses and comparison of metals ratios serve to constrain the source and transport of certain metals within different parts of the system. Pb isotope ratios in samples, obtained by ICP-MS analysis, define two distinct contributors. The first end-member (~(208)Pb/~(204)Pb = 38.513 to 38.721 and ~(207)Pb/~(204)Pb = 15.550 to 15.598) is characteristic of the mixed, oxide/sulfide ores at the SE end of the deposit. A second end-member (~(208)Pb/~(204)Pb = 38.855 to 38.946 and ~(207)Pb/~(204)Pb = 15.610 to 15.625) characterizes samples taken from the NW, purely Pb sulfide end of the lode. Pb isotope ratios from perennial reach sediments are closely correlated with ratios given by samples from the Klondyke tailings site and by Pb sulfide samples from the NW sector, the primary source of ores processed at Klondyke.