WEATHERING AND CARBON SEQUESTRATION AT NEW YORK WOLLASTONITE MINES

Wollastonite from Mesoproterozoic skarns is actively mined at two locations in New York: the NYCO Oak Hill mine in the Willsboro-Lewis skarn belt and at the Valentine mine in Lewis County. At both locations crushed ore has become well-cemented by calcite, which is evidence for the reaction CaSiO₃ + CO₂ = CaCO₃ + SiO₂, a model reaction for silicate weathering and carbon sequestration. We have studied this process at the closed Fox Knoll and Lewis mines, which operated from 1938–1982 and 1980–2016, respectively. Here we compare these results to preliminary data from Valentine and Oak Hill, and to wollastonite weathering experiments.

At these sites, cementation is mainly observed as 2–30cm thick, hardened slabs on berms and piles of crushed ore. At Lewis and Fox Knoll, texturally-late calcite and opal are highly correlated and are more abundant in cemented material. These sites contain up to 2% and 6% calcite respectively, and δ¹³C varies by deposit, ranging from -6.3 to -11.7‰ at Fox Knoll and -9.3 to -29.5‰ (VPDB) at Lewis. We take this as evidence for a (variable) microbial role in mineralization at these sites. In contrast, at Valentine, calcite ranges from 1% to 28% and δ¹³C ranges from -4.9 to -20.4‰. Some of this range probably reflects a component of marble calcite from the deposit’s host rocks.

A rooftop experiment analyzed in-situ wollastonite dissolution and carbon sequestration using ore stockpile material from the Lewis mine, both unsieved and sieved into different grain sizes and left exposed for two years. Experiments varied drainage rates and included pouches of pure wollastonite in three different grain sizes. Pure wollastonite samples exhibited a maximum calcite precipitation rate of 0.06% per month. δ¹³C for these samples ranges from -8.5 to -21.1‰, similar to newly precipitated calcite in cemented field samples from the Lewis mine. Ore samples from the experiment across grain size and drainage rates showed wollastonite dissolution with respect to controls. Unsieved materials had the greatest loss of wollastonite across samples. Unsieved untreated materials had a range of 1.3 to 2.4% relative wollastonite loss, per month, between fast and slow drainage conditions (respectively). Unsieved autoclaved materials had a similar relative loss,1.3 to 3.0% per month, to fast- and slow-draining experiments.