LONGEVITY OF ACID ROCK DRAINAGE: MINERALOGICAL AND CHEMICAL COMPARISON OF MINE-WASTE PILES AND POST-GLACIAL TALUS ROCK PRODUCING ACIDIC SOLUTIONS

Acid rock drainage may provide an analog to estimate long term pyrite oxidation, acid production, and metal mobility from pyrite-rich mined sites. Pyrite-bearing quartz-sericite altered waste piles in the Sugar Loaf mining district located in Colorado have atmospheric exposure times of about 100y, while the Chattanooga acid-iron fen in the Silverton caldera of Colorado is fed by a natural acid generating, post-glacial talus slope largely formed of phyllic altered rock with 10,000y exposure. The two sites have similar climate and represent suitable endpoints for comparison. Ratios of pyrite to product iron-oxyhydroxide minerals were estimated by systematic sampling across each site. These estimates in combination with textural observations of pyrite resorption and replacement, solution pH of water draining the pyritic rock, and selective leach experiments were used to estimate reaction progress of oxidative pyrite dissolution and to determine the mineralogical setting of metals after different exposure times. Reaction progress estimates fit a first-order kinetic model with the calculated rate constant of field pyrite dissolution between 10^-10 to 10^-9 mol m^-2 s^-1. The median constant is intermediate between abiotic and biotically-mediated rates that have been determined experimentally. Acid production from such rocks is estimated to last a few ten thousand years under the environmental site conditions. Selective leach analyses show the importance of secondary iron-oxyhydroxide dominated precipitates as reservoirs of relatively readily soluble trace elements, even after 10,000 years of weathering.