ALONG-STRIKE VARIATION IN ARSENIC AND OTHER TRACE ELEMENTS IN METAPELITES OF THE CONNECTICUT VALLEY/GASPE SEQUENCE (NE VERMONT AND SE QUEBEC)

To test the hypothesis that soluble trace elements, notably As, are leached out of metapelites with increasing metamorphic grade, the whole-rock geochemistry of a suite of variably-metamorphosed metapelites from the Siluro-Devonian Connecticut Valley-Gaspe sequence (CVG) was studied. Samples range from unmetamorphosed to low-grade shales and slates (≤ chlorite grade) of the Compton, Famine and Frontenac formations of SE Quebec to stratigraphically-correlative chlorite, biotite and garnet grade phyllites and schists of the Gile Mountain and Waits River formations of Vermont. Depositional age and sedimentary features are very similar along strike from low-grade to high-grade rocks of this sequence, and very similar rare earth element (REE) compositions are consistent with similar protolith composition (pre-metamorphism) along strike.

Whole-rock concentrations of As, Mo and Pb decrease with increasing grade as follows (mean values): (1) in the lowest-grade rocks (≤ chlorite grade pelites from Quebec, N = 21), As = 11.7 ppm, Mo = 2.2 ppm and Pb = 12.3 ppm; (2) in middle-grade rocks (upper chlorite and lower biotite grade phyllites, N = 19), As = 6.3 ppm, Mo = 0.78 ppm and Pb = 8.1 ppm; (3) in the highest grade rocks from northeastern Vermont (≥ biotite grade, N = 26), As = 2.1 ppm, Mo = 0.37 ppm and Pb = 5.5. ppm. Furthermore, 95 % of low-grade pelites contain > 5 ppm As whereas only 4 % of biotite and garnet grade metapelites contain > 5 ppm As. Loss of As, Mo and Pb with increasing metamorphic grade are attributed to recrystallization of pyrite during prograde metamorphism.

Elevated naturally-occurring As in shale and slate groundwater wells has been observed throughout the northern Appalachians. Within these areas, often > 20% of wells contain > 10 ppb As; however, the incidence of elevated arsenic in bedrock wells producing from higher grade phyllites and schists (biotite, garnet and staurolite grade) of the CVG of Vermont is very low – only 2-3 % of bedrock wells producing from these rocks contain > 10 ppb As. The apparent decrease in As content of bedrock wells with increasing metamorphic grade indicates that metamorphism is an important control on As concentration of metapelites as well as on As content of groundwater hosted by these rock types.