Evaluation of strontium isotopes as a geochemical tracer in the middle Fork Mineral Creek basin, southwestern Colorado

Sources and hydrologic flow paths need to be determined to evaluate remedial options in miningaffected basins. The 87Sr/86Sr ratios of a suite of water and rock samples from the Middle Fork Mineral Creek basin in the upper Animas River watershed, Colorado, were determined to investigate their possible use as a geochemical tracer for sources and flow paths. Leaching experiments were performed on the dominant lithologies in the study area to determine the more easily weathered constituents, including strontium. Variations in whole-rock 87Sr/86Sr ratios correlate with lithology and hydrothermal alteration intensity. For a given alteration assemblage, the porphyritic quartz monzonite has a lower 87 Sr/86Sr ratio than the surrounding San Juan Volcanics, and for a given lithology the 87 Sr/86Sr ratio is lower for propylitically altered rocks than for quartz-sericite-pyrite altered rocks. The 87 Sr/86Sr ratios of waters draining different lithologies and alteration assemblages have correspondingly different strontium isotopic ratios. The age of magmatism and alteration is relatively young (28 -25 million years) compared to the half-life of 87Rb, so that the isotopic variation is not great enough to determine mixing ratios for waters derived from multiple sources. In this study area, mine drainage does not have a unique strontium isotopic composition because the mined areas do not have a strontium isotopic composition distinctly different from the unmined, mineralized host rocks.