EVIDENCE FOR THE RELATIONSHIP BETWEEN ARSENIC AND METAMORPHIC GRADE AND IMPLICATIONS FOR BEDROCK AQUIFER GEOCHEMISTRY

In northern New England and southern Quebec, elevated naturally-occurring As levels have been reported in bedrock wells completed in slates of the Taconic Allochthons (VT) and Central Maine Belt (ME), ultramafic rocks of the Rowe-Hawley Belt (VT), contact zones for granitic intrusions (NH and ME), and low grade metasedimentary rocks of the Connecticut Valley Trough (CVT) in Quebec. The most common lithologic sources of arsenic in Vermont bedrock aquifers are: 1) authigenic pyrite which initially formed in geochemically-reduced marine sediments that have been lithified into shales or chlorite-grade slates and 2) serpentines and carbonates in ultramafics. Through the plate tectonic cycle, marine shales may either be subducted or thrust onto a continental margin during orogenesis; in either case, As may initially be metasomatically liberated during metamorphism and then resequestered (i.e. in ultramafics) . Through detailed geochemical analysis of rocks of different metamorphic grades in the Taconics and CVT, we will assess the role that metamorphism plays in As mobility.

Low-grade slates (chlorite-illite) of the Giddings Brook slice in the Taconic Allochthons contain an average of 104 mg/kg As and 33 % of bedrock wells contain > 10 ppb As; by contrast, higher grade slates and phyllites (paragonite-chlorite-muscovite) of the Bird Mountain slice contain an average of 2.4 mg/kg As and only 6 % of wells contain > 10 ppb As. In the Rowe-Hawley belt, ultramafic rocks have elevated As levels (93 mg/kg) compared to biotite grade metapelites (22 mg/kg As) suggesting that As was transferred from metapelites to ultramafics during subduction zone metamorphism and serpentinization.

Based on these results, we hypothesize that for two pelitic rocks of initially equal As concentration, the rock exposed to the greater degree of metamorphism will have a lower As concentration. We are currently evaluating the As content of metasedimentary rocks in the CVT (Gile Mountain Fm and Waits River Fm of VT and Compton Fm of QUE) as a function of metamorphic grade and proximity to granites of the New Hampshire Plutonic series. Existing data from biotite-grade rocks of the CVT in Vermont indicate low average As concentration (7.4 mg/kg); however, As concentrations in chlorite, garnet, and staurolite grade lithologies have yet to be determined.