SOURCE OF ARSENIC-BEARING PYRITE IN SOUTHWESTERN VERMONT: SULFUR ISOTOPE EVIDENCE

Sulfur isotope analysis of pyrite in Taconic slates from southwestern Vermont yield a very large range of δ³⁴S values, including some extremely ³⁴S-enriched values that are associated with elevated arsenic content. Dissolution of arsenic-bearing pyrite is considered to be a source of arsenic in drinking water. Pyrite porphyroblasts in black and green slate have the lowest δ³⁴S values, -5.2‰ and 3.2‰, respectively. These pyrite crystals cut across the foliation, and do not have associated quartz or calcite. Pyrite of sedimentary/diagenetic origin has a much heavier sulfur isotopic signature. This pyrite occurs in three distinctive forms: (1) bedding-parallel layers (up to 3 cm thick) in black and gray slate, (2) large framboids in carbonaceous black slate, and (3) fractured concretions (?) and nodules in black, gray and green slate. In all of these cases, the pyrite is associated with quartz and calcite of presumed metamorphic/hydrothermal origin. The bedding-parallel layers contain extension fractures filled with calcite. The framboids (up to 4 cm in diameter) are flanked by quartz-filled pressure shadows. The fractures in the concretions and nodules contain calcite and quartz; in some cases it is evident how the fractured pieces once fit together, and some of the fragments appear to be flanked by calcite-filled pressure shadows. The sulfur isotopic composition is as follows: (1) pyrite layer (2 samples): 17.3‰ and 17.4‰; (2) framboids (2 samples): 31.4‰ and 31.6‰; (3) concretions/nodules (3 samples): 39.7‰, 53.5‰ and 63.0‰. The ³⁴S-enriched values indicate sulfide precipitation in an oceanic sedimentary environment that became increasingly closed with respect to sulfate and increasingly anoxic. δ³⁴S values of pyrite are generally proportional to arsenic concentrations; the pyrite porphyroblasts have arsenic content from 8 to 50 ppm, while the arsenic content of the sedimentary/diagenetic pyrite ranges from 200 to over 1000 ppm As. This is consistent with pyrite mineralization in increasingly anoxic waters, as this chemical environment would likely fix arsenic into early diagnetic sulfides.