T8 The Environmental Geology and Geochemistry of Mineral Deposits: Best Practices for Effective Prediction, Mitigation, Closure, and Remediation I

Sunday, 28 October 2007
8:00 AM - 12:00 PM

ARSENIC CONTAMINATION FROM THE KELLY SILVER AND YELLOW ASTER GOLD MINE TAILINGS, CALIFORNIA; A POTENTIAL HEALTH CONCERN IN THE NORTH-CENTRAL MOJAVE DESERT

Paper number: T8-5
Program: Technical Sessions
Time: 9:00 AM – 9:15 AM

Authors

Abstract

Silver and gold mines in the Randsburg mining district are a significant source of arsenic-contaminated tailings in the north-central Mojave Desert of California. The Kelly silver mine in the town of Red Mountain, is California's largest silver deposit with over 20 M oz produced since 1919 from quartz-carbonate-chalcedony veins. Silver ores consisted of Ag-Cu-Sb sulfosalts and Ag chlorides. Ore concentrates contained up to 6% As as arsenopyrite. The Yellow Aster gold mine, 3 km west of the Kelly mine, produced along with adjacent mines over 1 M oz of gold from low-sulfide gold-arsenopyrite-quartz veins. Stamp mills and gravity methods were initially used to recover gold prior to recent mining using heap leach methods.

Arsenic contaminated mine wastes have been fluvially and atmospherically transported from both mine sites. At the Kelly mine, 0.5 M tons of mill tailings and an equal amount of waste rock have arsenic concentrations averaging 1500 ppm and 2000 ppm, respectively. Breaches in the tailings pond have released tailings into “Red Mountain Wash” and Cuddeback Lake, 15 km from the mine site. Tailings from the Yellow Aster and adjacent gold mines have similarly high As levels, (3,800 - 13,000 ppm) and tailings have migrated from a breached tailings pond into Fiddler Gulch and Fremont Valley, 20 km from the mine site. Wind-blown tailings have formed dune fields up to 6 feet high (As concentration 4,000 ppm). On the Kelly mine tailings, a surface crust cemented by Mg-Al- and K-Al sulfate, gypsum, and barite, serves to mitigate wind dispersal.

Most of the primary arsenopyrite has oxidized to form secondary Fe-bearing mineral and sorbed phases. As K-edge X-ray absorption near-edge spectroscopy indicates, As (V) phases predominate; only minor arsenopyrite remains based on SEM analysis. Linear combination fits of As K-edge EXAFS spectra of ore, tailings, efflorescent salts, and soils indicate the presence of variable amounts of scorodite (FeAsO4 2H2O), amorphous ferric arsenate, arseniosiderite (Ca2Fe3(AsO4)3O23H2O), arsenian jarosite [KFe3[(S,As)O4)](OH)6 , and As(V) sorbed to FeOOH. All of these phases are potentially soluble in lung or intestinal fluids.