EFFECTS OF GEOMICROBIOLOGY ON ACID MINE DRAINAGE IN A POLYMETALLIC MASSIVE SULFIDE DEPOSIT, BLUE LEDGE MINE, SISKIYOU COUNTY, CALIFORNIA

The Blue Ledge is an abandoned underground copper mine that exploited a polymetallic massive sulfide deposit from 1906 to 1919 in Siskiyou County, California. The massive sulfide zone varies in width from 0.1 m to 3 m and extends laterally for at least 1000 m. The massive sulfides are composed primarily of brecciated pyrite with chalcopyrite and quartz and lesser amounts of sphalerite and pyrrhotite. Standing pools of water within the Blue Ledge Mine have pH values ranging from 2.3 to 2.5, a dissolved solids content of 500 to 600 ppm, and temperatures between 12^o and 14^oC. During the wet season, the acidic water is flushed into nearby Joe Creek, lowering the pH, inducing toxic metal loading, and resulting in the precipitation of numerous iron oxides and hydroxides in the streambed.

Microbial activity plays an important role in catalyzing the oxidation of sulfide minerals and the consequent development of acid mine drainage from the Blue Ledge Mine. Selected polished thin-sections of various sulfide assemblages collected from the Blue Ledge Mine were inoculated under controlled conditions with the sulfide oxidizing bacterium Leptospirillum ferrooxidans. The strain of L. ferrooxidans used was isolated from tailings of the Blue Ledge Mine. Two groups of polished sections were inoculated and monitored weekly for pH over four-week and nine-week intervals respectively, with surface examinations before and after inoculation by scanning electron microscopy. Concentrations of Cd, Cu, and Zn were determined in the control and inoculating medium by flame atomic absorption spectroscopy. Concentrations of Fe²⁺ and Fe³⁺ were determined using visible spectroscopy after complexation with 1,10-phenanthroline. Pyrrhotite was the least stable sulfide, and readily oxidized under both control and inoculated conditions. Pyrite and sphalerite were stable under control conditions, but were readily oxidized by L. ferrooxidans as evidenced by altered surface textures. Chalcopyrite was the most stable of the sulfides exhibiting little to no oxidation under control and inoculated conditions. Total metal loadings in the inoculated samples reached concentrations of 56.2 µg/mL for zinc and 26.0 µg/mL for copper. The concentration for cadmium was below detection limits (1.0 µg/mL).