FeS 2 minerals in host rocks for deposits that contain fossil vegetal (organic) matter differ in abundance, distribution, texture, and sulfur isotopic ratios from FeS 2 minerals in host rocks for deposits that do not contain fossil vegetal matter. In three South Texas deposits lacking such organic matter, preore FeS 2 is dominantly euhedral pyrite which formed in response to solutions emanating from these faults. Ore-stage FeS 2 is dominantly marcasite that occurs as overgrowths on preore pyrite. In three deposits (two in Wyoming and one in Texas) that contain organic matter, preore FeS 2 is also dominantly pyrite, but it occurs commonly as framboids and as replacements of plant fragments and is formed by bacterial sulfate reduction during early diagenesis and may be isotopically distinct from pyrite formed from fault-related fluids. Ore-stage FeS 2 in these deposits is primarily pyrite. Bacterial sulfate reduction provided sulfide for ore-stage pyrite in deposits which contain organic matter. Abiologic sulfur transformations tend to produce ore-stage marcasite in deposits that do not contain organic matter.--Modified journal abstract.