MINERALIZATION PATTERNS ON THE MESOZOIC EASTERN BORDER FAULT, HARTFORD BASIN, CONNECTICUT AND MASSACHUSETTS

Mineralization patterns and geochemical variations across a fault damaged zone were analyzed at four exposures of the Eastern Border Fault, which has at least 5000 m throw and forms the eastern margin of the Mesozoic Hartford Basin in central Connecticut and southwestern Massachusetts. Analyses of rock chemistry were performed in outcrop using a field, handheld X-ray fluorescence spectrometer during the summer of 2006. Multiple analysis were performed at each exposure along transects perpendicular to the plane of the fault in order to study the variation in geochemistry between intact, non-deformed rock and the highly deformed center of the fault zone. In addition, analyses were performed on fresh versus weathered faces and in bagged samples of loose fault gouge to test the sensitivity of the instrument to surface effects. Twenty-five samples collected during field XRF analysis were analyzed with conventional laboratory XRF to verify results from the field instrument, and thin sections were made of several samples to study mineralization patterns. GPS locations were measured at each field XRF analysis point. Geochemical patterns in relation to the fault surface were spatially analyzed using XRF and GPS data with ArcGIS Geostatistical Analyst software to visually show the distribution of several trace elements enriched by fluids flowing along the fault.

Initial synthesis of analytical data suggests that copper, arsenic, iron, cesium, barium, nickel, and lead were enriched in gouge, breccia, and slickenside-bearing fractures near the center of fault zone. Titanium enrichment also is observed in some locations, but may also be an artifact of later surface alteration. Enrichment of these metals decreases with distance away from the center of the fault zone toward un-fractured metamorphic rocks of the Eastern Highlands. Syn-tectonic mineralization alternated between phases of calcite and quartz precipitation within fractured rocks. Late-formed veins are less deformed than early veins. Deformation and mineralization affect both basement rocks in the footwall and Mesozoic basalt in the hanging wall of the fault.