Pyrite discs in coal: Evidence for fossilized bacterial colonies

Discs of pyrite from 1 to 3 mm in diameter and similar to 100 mum thick wer e observed within fracture planes in coal from the Black Mesa coal deposit in northeastern Arizona. The pyrite discs were composed of aggregates of cr ystals, which suggested that sulfide mineral diagenesis had initiated at mu ltiple nucleation sites and occurred prior to the compaction farces occurri ng during coal formation. Stable sulfur isotope analysis of the discs (delt a S-34 = -31.7 parts per thousand) supports a bacterial origin resulting fr om dissimilatory sulfate reduction. Fossilized bacteria on the disc surface s (average = 27/100 mum(2)) appeared as halos when viewed using reflected l ight microscopy, but were lenticular by scanning electron microscopy, each microfossil being 2-3 mum in length. A fossilized bacterial colony (pyrite disc), 1 mm in diameter, would contain similar to2.1 x 10(7) microfossils. These microfossils were not observed on hydrothermal pyrite. Coating and in -filling of sulfate-reducing bacteria with iron disulfide during in vitro s ulfide mineral diagenesis provide mechanisms to explain the preservation of the three-dimensional lenticular microfossils observed on the pyrite discs .