NEUTRAL TO ALKALINE MINE DRAINAGE, CARBON SEQUESTRATION, AND ARSENIC RELEASE AT THE ABANDONED VERMONT ASBESTOS GROUP MINE, BELVIDERE MOUNTAIN, VERMONT

The abandoned Vermont Asbestos Group mine produced chrysotile asbestos from serpentinitic host rocks from 1900 to 1993. The site consists of three quarries and waste-rock and tailings piles. The waste is dominated by serpentine (antigorite with minor chrysotile) with lesser pyroxene, olivine, chlorite, brucite, phlogopite, magnesite, and magnetite. The surface of the piles is cemented by secondary magnesium carbonates, dominantly hydromagnesite, locally exceeding depths of 1 m. Bulk concentrations of Ni (max. 2,230 mg/kg), Cr (max. 1,570 mg/kg), Co (max. 125 mg/kg), and As (max. 63 mg/kg) in mine waste all exceed soil guidelines even though the concentrations are typical of serpentinites. Surface water includes streams and ponds, seeps draining tailings piles, and a pit lake. The pH of surface waters is neutral to alkaline, ranging from 6.9 to 10.1. Specific conductance ranges from 0.03 to 0.53 mS/cm, total alkalinity from 14 to 320 mg/L, and Mg, the dominant cation, from 4 to 152 mg/L. Dissolved concentrations of Ni (0.001 – 0.031 mg/L), Cr (<0.001 – 0.003 mg/L), Co (<0.001 – 0.003 mg/L), and As (<0.001 – 0.009 mg/L) are generally low. Arsenic concentrations generally correlate with total alkalinity, suggesting that carbonation of the serpentinitic mine waste may play an important role in the release of arsenic. Published studies indicate that arsenic can substitute into antigorite through the coupled substitution As(III) + As(V) = 2 Si(IV). Thus, destruction of antigorite through conversion to carbonate minerals or dissolution, combined with the neutral to alkaline pH and the lack of significant Fe-Mn hydroxides to act as a sorption substrate, could lead to significant concentrations of dissolved arsenic in these settings under favorable conditions. Therefore, proposals to use ultramafic rocks or mine wastes to sequester carbon dioxide should carefully consider the ramifications of mineral carbonation reactions on trace-element geochemistry.