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

Volcanogenic uranium deposits: Geology, geochemical processes, and criteria for resource assessment

“…Mixing of diverse fluid types was proposed for the Aurora deposit , along the caldera-margin fault zone. The crucial interface of two compositionally different solutions was demonstrated earlier by Granger (in Nash and others, 1981) and by Reynolds and Goldhaber (1983). In these mixing zones, uranium (U +6 ) arrives in an oxidized fluid, encounters a solution of different composition or pH that triggers a reaction or reduction that causes uranium and gangue mineral precipitation.…”

“…Pre-ore sulfides in dolomite and black shale may have been important, or biogenic reduction of sulfate and the formation of abundant fine-grained marcasite may have occurred at the time of uranium emplacement. I favor reduction by metastable sulfur compounds (sulfite or thiosulfite) as proposed for some sandstone-type uranium deposits (Granger and Warren, 1969;Reynolds and Goldhaber, 1983). The most likely time of ore formation was in the Oligocene when volcanic rocks covered the fault zone.…”

“…Likewise, the details of sulfidation and uranium precipitation are not established. By analogy to sandstone-type deposits studied in detail by Reynolds and Goldhaber (1983), we know that several forms of sulfide ion are powerful reductants and could be relevant at temperatures up to about 200°C. The Aurora prospect was thoroughly drilled from 1977 to 1980 and found to be at least 3,660 m x 1,070 m, with at least two mineralized horizons (beds, flows) averaging 1.5 to 2.1 m thick .…”

“…Abundant marcasite, weak alteration, and 51 Ma age are consistent with a warm, weakly oxidizing, near-surface flow of groundwater through the pluton and its roof zone (Ludwig and others, 1981). Abundant marcasite suggests formation from metastable sulfur species, which are powerful reductants (Reynolds and Goldhaber, 1983). Some uranium could have come from the local volcanics, although this is less likely than the stock, because the volcanics are intermediate in composition and have not been demonstrated to be unusually radioactive.…”

“…Reduction in volcanogenic uranium deposits generally is controlled by sulfur chemistry; hydrogen sulfide, bisulfide ion, and several metastable sulfur-oxyanions are kinetically effective reductants for U +6 (Granger in Nash and others, 1981;Reynolds and Goldhaber, 1983). Carbon in various forms (logs, coal) and methane are only locally effective.…”