UNDERSTANDING BLEACHING FLUID FLOW ALTERATION IN THE NEW HAVEN ARKOSE, HAMDEN CT

The New Haven Arkose is predominantly made up of red micaceous feldspathic sandstone and siltstone. However, white sandstone occurs adjacent to a 25-meter wide mafic dike in Hamden, CT. The white color is the result of local bleaching by reducing fluids associated with the intrusion. This study combines fieldwork, petrographic, and geochemical studies for a better understanding of the bleaching fluids and rock alterations. Two alteration sites were examined, both about 1 km west of Sleeping Giant sill.

The white altered sandstone can be traced for more than 100 m from the intrusion. Bleaching intensity diminishes away from the dike and is recognized as three zones: whole bed, partial and patchy. In sandstone layers within the whole bed zone about 15 m from the dike, 3 cm diameter, slightly ovoid-shaped, concretions with a white center and green rims are found. Large calcite crystals, up to 7 cm in diameter, are also found in the white sandstone in the whole bed zone.

Petrographic and x-ray diffraction analyses for the altered sandstones indicate that the igneous dike contains plagioclase feldspar as well as augite and leucoxene, which is likely hydrologically altered ilmenite. The main mineral grains in the sandstones are quartz and albite cemented by hematite, illite and calcite. Albite is likely a result of albitization of potassium and plagioclase feldspars during diagenesis. The concretions are composed of carbonate, likely ferroan calcite. Within the concretions are fine-grain, green, needle-shaped, vesicular epidote. Millimeter-thick chlorite rims appear around the concretions. The color alteration of the sandstones reflects the change from Fe³⁺ in hematite cement to Fe²⁺ which was either transported away or concentrated in ferroan calcite concretions, the chlorite rims and/or the epidote crystals.

Results of stable isotope analysis of the large calcite crystal yielded a ∂¹³C of -8.99 (vPDB) and a ∂¹⁸O of -16.63 (vPDB). The concretion isotope values range from ∂¹³C of -8.37 (vPDB) and a ∂¹⁸O of -13.49 (vPDB). These results indicate that the same fluid system precipitated both the calcite crystals and concretions. Subsequent fluid inclusion analyses will provide additional fluid properties; XRF and SEM analyses will help document the chemical and mineral alterations.