The Schwarzwalder uranium deposit, III: Alteration, vein mineralization, light stable isotopes, and genesis of the deposit

The Schwartzwalder uranium deposit formed at 69.3 + or - 1.1 m.y. in a complex fracture system during the inception of Laramide uplift of the Front Range in Colorado. Geologic and isotopic evidence demonstrates that the ore-forming fluids were in chemical equilibrium with the Proterozoic metavolcanic and metasedimentary host-rock terrane at depth and that the metals, sulfur, and carbonate deposited in the fractures were derived from the metamorphic rocks. The data are not consistent with chemical contributions from an unrecognized magma or from the overlying Phanerozoic sedimentary rocks. The protoliths for the metamorphic rocks were submarine volcanic rocks and related volcanogenic exhalative iron-formations and chert. Water trapped along the basement faults and in the regolith between the basement and the overlying Paleozoic sedimentary rocks interacted with the metavolcanic rocks to produce isotopically heavy fluids containing high concentrations of dissolved metals and carbonate. Calculated delta ¹⁸ O values for this fluid range from 4.3 to 8.2 per mil, indicating a low water/rock ratio in the source terrane.Two stages of alteration and three stages of vein mineralization are recorded in the Schwartzwalder deposit. At the onset of Laramide faulting, fluids migrated along the fracture systems to zones of low hydraulic potential. These fluids contained CO ₂ and had a metastably large K/Na ratio; they altered the gneissic wall rocks to a carbonate-sericite assemblage, adding K (super +) and CO ₂ and removing SiO ₂ with little or no change in volume. As the fractures continued to open, CO ₂ was evolved from the fluids, increasing the pH and superimposing a hematite-adularia alteration assemblage on the earlier alteration.The veins record three stages of mineralization, the second of which generated the high-grade uranium veins. Evidence for the stage I sulfide-carbonate mineralization is poorly preserved, but isotopic and temperature data from this stage are consistent with a trend in fluid composition culminating in stage II pitchblende deposition. Sudden, large movements along the faults caused episodic evolution of CO ₂ from the fluid. This loss of CO ₂ decreased the solubilities of carbonates and adularia and the stabilities of sulfur species in solution. Uranyl carbonate complexes dissociated and sulfur species in solution likely reduced the uranyl ions to produce stage II pitchblende. Carbonate, adularia, and sulfides dominated the vein mineralogy after deposition of pitchblende. Progressively lower delta ¹⁸ O values in vein carbonates suggest the mixing of cooler, less evolved, perhaps meteoric, waters during the later stages of mineralization. However, the fluid pressure remained high, as indicated by explosion breccias and inward collapse features which formed as fault movements produced sudden decreases in the confining pressure. Only the stage III carbonate-iron disulfide assemblage in the major postore segment of the Illinois fault may be the product of meteoric water alone.