THE EFFECT OF CALCITE-BEARING VEINS ON GROUNDWATER GEOCHEMISTRY IN A FRACTURED ROCK AQUIFER SYSTEM

The effect of bedrock composition on groundwater geochemistry in fractured rock aquifers (FRAs) has been the subject of much research over the past 15 years in New England, especially regarding naturally occurring trace elements like arsenic and uranium. The focus of this study is to examine the influence of carbonate-bearing vein chemistry on groundwater composition, with a particular emphasis on tracing groundwater flow using Sr anomalies. Previous studies of FRAs have found that anomalously high Sr concentrations occur in groundwater residing in pelitic rocks (slates and phyllites), and anomalously low Sr concentrations in carbonate groundwater. This anomaly in groundwater in juxtaposed pelite-carbonate packages across thrust faults in the Champlain Valley of western Vermont. The metamorphosed pelites in the hanging walls have lower whole-rock concentrations of Sr (88 ppm) than do the relatively unmetamorphosed carbonates in the footwalls (160 ppm), but hanging wall groundwater has very elevated levels of Sr (2363 ppb) compared to footwall groundwater (196 ppb). The high hanging wall Sr has proven useful in documenting flow through these two thrusts, from hanging wall downward across the thrust into footwall in both cases. At first, the groundwater Sr anomaly may seem intuitively incongruous, but a handful of studies have documented very high Sr concentrations in carbonate-bearing veins in host rocks with low Sr, so we hypothesize that vein calcite is the source of the high Sr in hanging wall pelites. To study this experimentally, calcite-(and quartz)-bearing veins were sampled from pelitic rocks in the hanging walls of the Hinesburg Thrust (Pinnacle Fm) and the Muddy Brook Thrust (Skeels Corner Fm). ICPMS analysis of a preliminary suite (N=7) indicates that Sr concentrations in vein calcite are very high (2400 to 4700 ppm mg/kg) and significantly elevated above the host rock (21 to 287 mg/kg, N=46). This indicates that calcite-bearing veins are a potential source of elevated Sr in shales, slates and phyllites, a phenomenon that appears to be a useful tool for tracing groundwater flow in FRAs. Future analysis will focus on stoichiometric comparison of veins vs groundwater (e.g. Ca:Sr) as well as on potential variability in chemistry of veins formed at different times and under different tectonic regimes.