GEOCHEMICAL AND SULFUR ISOTOPE ANALYSIS OF TACONIC SLATES: IMPLICATIONS FOR ARSENIC SOURCE AND MOBILITY IN A BEDROCK AQUIFER SYSTEM

In western Vermont, elevated arsenic levels are found in groundwater of the Taconic region and the arsenic source is believed to be Cambro-Ordovician slates of the Taconic Allochthons. Numerous positive correlations of aqueous components, including Fe vs. sulfate and As vs. sulfate, suggest that pyrite is the source of the elevated arsenic. The purpose of this study is to evaluate whole-rock geochemistry and mineralogy of Taconic slates, including chemical analyses of the varied lithologies present in the region as well as examination of arsenic concentration as a function of metamorphic grade.

Field observations indicate that organic-rich black slates contain the greatest amount of pyrite; grey and green slates contain lower amounts of pyrite and purple slates contain little or no pyrite. Preliminary whole-rock sulfur isotope data from black and grey slates yield extremely high δ³⁴S values (+38 to +53 ‰) which likely reflect pronounced bacterial reduction of sulfate combined with limited or non-existent exchange with the open sea during deposition, i.e. highly anoxic, restricted marine sedimentary conditions conducive to formation of sulfide minerals that would have sequestered arsenic from terrestrially-derived run-off or decomposing detrital minerals.

The general increase in metamorphic grade (slate to phyllite) from west to east in the Taconic Allochthons occurs in tandem with an increase in superposed cleavage generations. This subtle change in metamorphic grade may have controlled the abundance of arsenic in the different allochthons (and groundwater). The dominant rock type in the relatively low-grade Giddings Brook slice (GB) is slate, whereas the Bird Mountain (BM) and Dorset Mountain (DM) slices to the east are dominated by phyllite. Groundwater from the GB is extremely enriched in arsenic relative to the higher-grade BM and DM. In the GB, 34 % of bedrock wells (24/71) contain As > 10 ppb, whereas in the BM and DM, only 3 % of wells (1/34) contain > 10 ppb As. Our proposed interpretation is consistent with As release during metamorphism, where high As can occur in low-grade to non-metamorphosed rocks but is driven out during prograde metamorphism. We will test this hypothesis by analyzing whole-rock slate and phyllites as well as individual pyrite crystals from the low-grade GB and the higher-grade BM.