LIMITING FACTORS OF PYRITE FORMATION IN THE APPEKUNNY FORMATION OF THE BELT SUPERGROUP, MT

Pyrite forms from bacterial reduction of sulfate to sulfide in an iron-rich, oxygen-poor environment. Numerous studies and observations of the 1400 Ma Appekunny Formation, located in the Belt Supergroup of western Montana, show that it is rich in early stage framboidal pyrite. However, we found a surprising absence of framboidal pyrite was found in 8 thin sections of 6 hand samples from the formation. This study investigates bulk rock concentrations of sulfur and total organic carbon (TOC) to determine limiting factors of pyrite framboid formation in these hand samples. We use X-ray diffraction (XRD) to determine what, if any, sulfur- or iron-bearing minerals exist in place of pyrite. Preliminary results show extremely low levels of total sulfur (<0.1wt%) while containing standard levels of TOC (1.5—2.8wt%) as compared to previous studies of related formations in the Belt. The S/C ratio is about 0.007, compared to a “normal” ratio of 0.31 in modern anoxic sediments, showing a substantial deficiency in available sulfur. Oxidation of the pyrite over the last 1400 Mya may have diminished available sulfur, however. XRD patterns show slight peaks corresponding with magnetite and detrital magnetite is visible in thin section, suggesting iron is not a limiting factor.