AN ACROSS-STRIKE SURVEY OF ARSENIC DISTRIBUTION IN LITHOLOGIES AND GROUNDWATER IN THE PRE-SILURIAN VERMONT APPALACHIANS

In most polyorogenic mountain belts, multiple ocean basins have collapsed with their deposits successively accreted to the craton along with intervening arcs, microcontinents, and ophiolites. Initially, within each ocean basin, sediments were deposited with As contents that directly reflect the composition of source areas as well as ambient geochemical conditions; e.g., the reducing conditions and elevated organic content frequently associated with black shales are a common As sink. However, subsequent modification by deformation and metamorphism may then redistribute arsenic. We report on arsenic levels in lithologies and groundwater from Late Proterozoic- Ordovician oceanic realms and associated suprasubduction zone rocks from the Vermont Appalachians that were deformed and metamorphosed during the Taconian (Ordovician) and Acadian (Devonian) orogenies.

The Vermont Appalachians can be generally divided into 6 lithotectonic belts which are (from west to east): 1) Champlain Valley, 2) Taconic Allochthons, 3) Green Mt., 4) Rowe-Hawley Belt (RHB), 5) Connecticut Valley Sequence (CVS), and 6) Bronson Hill. In the Giddings Brook (GB) slice in the Taconic Allochthons, elevated As levels (> 10 ppb) occur in 33% of slate bedrock wells. The As in groundwater is likely derived from pyrite in black slates, and the groundwater with elevated As appears to be confined to the GB slice, which underwent the lowest grade of metamorphism (chlorite grade) of the Taconic slices. Slates of the GB slice contain relatively high average As content (46 ppm). In the RHB, a number of bedrock wells exhibit high As levels; however, when compared to the slates in the Taconic Allochthons, the biotite grade phyllites present in this highly-faulted belt have relatively low As levels (4.7 ppm); these low levels may be a result of the higher metamorphic grade. Given that slates of the Taconics were ejected and thrust westward from the RHB during the Taconian Orogeny, metapelites of the GB slice and the RHB originated in similar depositional environments off the Laurentian Cambro-Ordovician margin, and the main control on the difference in As content may be metamorphic grade. Serpentinites in the RHB contain average As of 93 ppm, and we hypothesize that As was transferred from RHB metasediments into serpentinite during subduction zone metamorphism.