MINERALOGICAL CONTROLS ON ACID-BASE ACCOUNTING OF MINE WASTE FROM THE ABANDONED ELY COPPER MINE SUPERFUND SITE, VERMONT

Acid-base accounting determined by the “modified” Sobek method indicates that mine-waste samples from the abandoned Ely copper mine Superfund site in the Vermont copper belt have negative net-neutralization potentials (NNP) ranging from –14.4 to -421.9 kg CaCO₃/ton. Minerals include: quartz, plagioclase, muscovite, biotite, altered biotite, clays, chlorite, hornblende, goethite, hematite, jarosite, gypsum, pyrrhotite, chalcopyrite, and traces of pyrite, sphalerite, and efflorescent sulfate salts. The weight percentage of each phase was estimated using a Rietveld refinement on a full XRD profile. The neutralization potential (NP) contributed by each sample was calculated using recently published NP values for individual phases weighted according to their abundance. The acid-generating potential (AP) was estimated according to acidity generated by Rietveld-determined weight percentages of pyrrhotite (0 to 30 wt. %) and pyrite (<1 wt. %). The NNP calculated based on the mineralogy, although averaging ~30% higher, correlates (R²=0.85) with NNP from the laboratory-based Sobek procedure. The higher calculated NNP may be due to a combination of factors including the overestimation of the NP contribution for some silicate minerals and grain-size variations between mine waste and monomineralic samples. Also, amorphous ferric hydroxides were not considered in the Rietveld refinement. The calculated NNP does not account for acidity released by the dissolution of trace amounts of efflorescent sulfate salts. Also, significant amounts of jarosite (up to 17 wt. %) present in all but one sample is refractory in 3N HCl used to determine sulfate sulfur in the modified Sobek procedure. For jarosite-rich samples, a more aggressive procedure using concentratrated HCl and longer boiling times is needed to liberate the sulfate sulfur and to accurately assess AP.

These samples lack carbonate; silicates contribute only small amounts of NP, which is further inhibited by their slow dissolution kinetics. Therefore, NNP is mostly dependent on the amount of pyrrhotite, pyrite, jarosite, and efflorescent sulfate salts present. Overall, quantitative mineralogy may be useful in estimating the NNP of mine-waste samples.