THE EFFECT OF METAMORPHISM ON ARSENIC CONCENTRATION IN METAPELITE BEDROCK AQUIFERS: A CASE STUDY OF THE CONNECTICUT VALLEY/GASPE SEQUENCE (NE VERMONT AND SE QUEBEC) (Invited Presentation)
To test the hypothesis that As is driven out of metapelites with increasing metamorphic grade, this study is designed to control as many factors as possible, particularly protolith depositional setting and age. Samples have been obtained from a suite of variably-metamorphosed metapelites from the CVG, including unmetamorphosed to low-grade shales and slates of the Compton, Famine and Ayers Cliff formations of SE Quebec as well as stratigraphically correlative medium to high-grade phyllites and schists of the Gile Mountain and Waits River formations of NE Vermont. Preliminary data indicate the following: (1) phyllites and schists from NE Vermont (> biotite zone) contain average whole rock As of 2.1 ppm (31/32 values are < 6 ppm, including many pyrite-rich samples); (2) a literature search of As concentration in Paleozoic shales (N = 281, including USGS standard SDo-1) indicates that average As concentration in unmetamorphosed shales is 53 ppm (range = 8 to 823 ppm); (3) analysis of 30 low-grade shales and slates (< chlorite zone) from Quebec is in progress. Recrystallization of pyrite during prograde metamorphism or desorption of As from sediments during metamorphism are two possible explanations for the apparent decrease in As with metamorphic grade.