CHROMIUM, MANGANESE, NICKEL, AND COBALT MOBILITY FROM MAFIC-TO-ULTRAMAFIC MINE SPOIL WEATHERING IN WESTERN MASSACHUSETTS, USA

Fragmented ultramafic bodies in New England were important mineral resources until the early 20th century, yet few studies have addressed their potential to release trace metals to terrestrial and aquatic environments. Here, we evaluate the release of four trace metals (Cr, Co, Mn, Ni,) from a historic serpentine-talc “soapstone” quarry in Blandford, MA, USA. Soil pits, sediment and rock samples, and stream water samples were collected from upslope undisturbed areas, within the mine spoils and mine face, and downslope of the mine. In order to provide a bottom-up approach for understanding metal release, careful petrographic analysis, electron dispersive spectroscopy (EDS), and wavelength dispersive spectroscopy (WDS) were employed to provide first-order insight into the mineralogy of the deposit and a determination of potential metal-bearing phases. Trace metals were primarily observed in ultramafic sheet silicates, primary Fe-oxides, and interstitial weathering-related sulfates. Bulk rock concentrations were enriched in Cr (1550 mg kg-1), Co (230 mg kg-1), Mn (1100 mg kg-1), and Ni (1960 mg kg1). However, soils and sediments total concentrations were comparable to background soil concentrations: Cr (119 mg kg-1), Co (73 mg kg-1), Mn (894 mg kg-1), and Ni (65 mg kg-1). Moreover, < 0.5 % of the total concentrations were bioavailable (0.1 M ammonium acetate extraction), implying that metals are present as insoluble forms. However, ~20% of the total Cr, Mn, Ni, and Co concentrations were strong-acid extractable, suggesting mobilization over the coming decades. Stream water concentrations of Mn and Cr were < 50 µg L-1, below concentrations outlined by USEPA drinking water standards, and WHO water guidelines for Ni. These results suggest that transport of Cr, Mn, Ni, and Co from the serpentine-talc as dissolved compounds or sediments is limited by retention within silicate and oxides.