ALTERATION OF BASALT OF THE HARTFORD BASIN, EAST GRANBY, CT

The Harford basin is one of several rift basins in the eastern United States that formed during the breakup of Pangaea. Over millions of years, the Hartford basin accumulated clastic sediments as well as mafic lava. The three major basalt units within the basin have been named from oldest to youngest: Talcott, Holyoke, and Hampden basalts. In one particular area of East Granby, Connecticut the Hampden basalt is well exposed and highly fractured. The fracture zone is approximately 100 meters wide but the overall length and depth of the fracture zone is not well constrained. The fractured basalt is enriched with vugs containing minerals that are not found in the surrounding rock.

Common minerals found in vugs and fractures in the study area include euhedral crystals of calcite, quartz, anhydrite, gypsum and pyrite. Less common minerals include celestite, sphalerite, galena, chalcopyrite, strontianite, barite, flourite and datolite. In general, the presence of quartz, calcite, anhydrite and sulfide minerals indicate a relatively early stage of hydrothermal alteration in the basalts. However, relationships between minerals within the vugs indicate a complex hydrothermal history where multiple episodes or pulses of hydrothermal alteration were likely. In some cases, gypsum is replacing anhydrite while pyrite has been oxidized leaving a red stain of hematite coating the minerals within the vugs. The replacement of anhydrite by gypsum and the oxidation of sulfide minerals likely represent a later stage, lower temperature phase of alteration.

Some of the vugs contain trapped water. The water samples within the vugs are typically calcium bicarbonate or calcium sulfate dominated. The trapped water samples are generally consistent with the lower temperature mineral assemblage dominated by gypsum and hematite and most likely represent meteoric water circulating through the fracture zones after hydrothermal alteration stopped. Computer simulations, however, indicate that alteration of fresh basalt with relatively pure meteoric water does not replicate the observed hydrothermal mineral assemblage well. It is likely that the water responsible for the hydrothermal alteration stage was lake water from one of the closed basin lakes contained within the rift valley.