Iron disulfide minerals and the genesis of roll-type uranium deposits

Abstract

Iron disulfide (FeS/sub 2/) minerals in host rocks for roll-type uranium deposits that contain fossil vegetal (organic) matter differ in abundance, distribution, texture, and sulfur isotopic ratios from FeS/sub 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/sub 2/ is dominantly euhedral pyrite. This preore pyrite formed in close proximity to fault zones in response to solutions emanating from these faults. In these deposits, ore-stage FeS/sub 2/ is dominantly marcasite that occurs as overgrowths on preore pyrite in the ore zone and as far as 400 m downdip from the altered tongue. In three deposits (two in Wyoming and one in Texas) that contain organic matter, preore FeS/sub 2/ is also dominantly pyrite, but it occurs commonly as framboids and as replacements of plant fragments. This preore pyrite formed by bacterial sulfate reduction during early diagenesis and may be isotopically distinct from pyrite formed from fault-related fluids. Orestage FeS/sub 2/ in these deposits is primarily pyrite. Geochemical conditions favoring pyrite formation (such as bacterial control on pH and sulfur speciation) suggest that bacterial sulfate reduction provided sulfide for ore-stage pyrite in deposits which containmore » organic matter. In contrast, abiologic sulfur transformations (involving elemental sulfur or metastable sulfur oxyanions) tend to produce ore-stage marcasite in deposits that do not contain organic matter. The contrasting origins of ore-stage FeS/sub 2/ minerals in host rocks with and without organic matter are consistent with previously proposed biogenic and nonbiogenic theories for the origin of roll-type deposits.« less