HYDROTHERMAL ALTERATION OF BASALT IN THE HARTFORD BASIN

The Hartford basin, a Mesozoic allocogen, encompasses the Connecticut River in the central portion of Connecticut and contains three episodes of basalt flooding that intercalate clastic sedimentary units. The three basalt units have been named from oldest to youngest: Talcott, Holyoke, and Hampden basalts. The Hampden Basalt, which underlays the clastic Portland Arkose, is now exposed in many areas in the Hartford Basin after millions of years of denudation. In one particular area of East Granby, Connecticut the Hampden basalt is highly fractured. The fracture zone is about one hundred meters wide by several hundred meters long. The depth of the fracture zone is not well constrained. Within the fracture zone the basalt is broken and stained and contains abundant vugs that include minerals not found in the host basalt.

The most common minerals within the vugs and fractures are euhedral quartz and calcite. Anhydrite and pyrite are also common alteration products. Often, the anhydrite has been replaced by gypsum and the pyrite has been oxidized to hematite. Less common but significant amounts of sphalerite, galena, chalcopyrite, celestite, strontianite, barite and fluorite occur throughout the fracture zone. The mineral relationships indicate a complex sequence of alteration; however, two major alteration conditions can be deduced from the mineral assemblages. An early hydrothermal alteration episode indicated by quartz, calcite, anhydrite, and sulfide minerals, followed by a lower temperature phase indicated by gypsum and hematite. The conversion of anhydrite to gypsum and the oxidation of pyrite most likely reflect circulation of low temperature meteoric water through the fracture zone after hydrothermal alteration had ceased.

Water is sometimes found trapped within the vugs and fractures and is released when the rock samples are broken. Water samples were collected and analyzed by ICP-MS and IC to determine the major and minor cations and anions. In general, water samples can be characterized as dilute calcium bicarbonate fluids. Speciation calculations indicate that the water chemistry is largely consistent with the lower temperature reaction assemblages. Geochemical simulations are being conducted to further assess the conditions of hydrothermal alteration that occurred within the fracture zone.