Sulfur, carbon, and oxygen isotope geochemistry of pyrite and calcite from veins and sediments sampled by borehole CCM-2, Creede Caldera, Colorado

Abstract

Pyrite occurs in veins and as wall rock disseminations in three lithologic units in borehole CCM-2. These are, from top to bottom: (1) Creede Formation lacustrine sediments; (2) volcanic megabreccia and, (3) Snowshoe Mountain ash-flow tuff. In the Creede Formation disseminated pyrite is concentrated at the tops and bases of turbidite beds. Pyrite also forms stalactites near travertine bodies of the Creede Formation marking the paleoshoreline of the caldera lake. Pyrite, without calcite, is found over a vertical span of 52 m in fractures and replacing magnetite in wall rock in the upper portions of the volcanic megabreccia. In the Snowshoe Mountain ash-flow tuff and the base of the megabreccia, pyrite occurs in veins with calcite and as replacements of magnetite and biotite in wall rocks up to one centimeter from veins. Framboidal pyrite is common in Snowshoe Mountain veins. Calcite veins without pyrite occur in the Creede Formation whereas veins in the volcanic megabreccia and the Snowshoe Mountain tuff have both calcite and pyrite. Sulfur isotope variations from -18% to +70‰ δ 3 4 S C D T have been measured in situ with an infrared laser, sulfur isotope microprobe. A large range of sulfur isotope values is observed both in pyrite stalactites collected near travertine deposits on the paleoshoreline of Lake Creede and in veins from the Snowshoe Mountain tuff. Spatially resolved analyses demonstrate that sulfur isotope zoning becomes progressively enriched in 3 4 S with decreasing depositional age in individual veins and stalactites. Calcite veins have values of 0 to -17‰ in δ 1 3 C V P D B and +4% to +22‰ in δ 1 8 O V S M O W . Carbon and oxygen isotope values of calcite veins from the Snowshoe Mountain tuff show a broad antipathetic relationship with higher δ 1 3 C correlating with lower δ 1 8 O. Our interpretation of sulfur isotopic systematics is based on two working hypotheses. The first states that sulfur in the Creede Caldera originated as volcanic H 2 S. According to this hypothesis, pyrite precipitated directly from reactions between H 2 S and iron-bearing minerals. The alternative hypothesis considers that sulfate from volcanic eruptions was the primary source of sulfur in the Creede Caldera. The reduction of sulfate to sulfide as required for pyrite precipitation was catalyzed by enzymes of anaerobic bacteria with an accompanying kinetic sulfur isotopic fractionation. On balance, available evidence favors the latter hypothesis.