Epithermal gold mineralization in a fossil hot spring system, Red Butte, Oregon

Abstract

The Red Butte gold prospect is a well-preserved, sediment-hosted, fossil hot spring system located within the Lake Owyhee volcanic field of eastern Oregon, 10 km west of the Owyhee Reservoir. Its features include siliceous hot spring sinter, a blanket of intense argillic alteration produced by surficial steam-heated waters, and a 50-m-wide hydrothermal eruption crater filled with bedded breccia deposits. Mineralization is hosted by north- and northwest-striking faults in volcaniclastic sediments of the Miocene Deer Butte Formation. Red Butte is capped by 10 m of syngenetic fluvial sedimentary rocks consisting of a basal silicified mudstone overlain by arkosic sandstones and conglomerates interbedded with siliceous hot spring sinter. The sediments at this paleosurface horizon have been cemented by hydrothermal quartz and adularia and form the resistant cap of the butte. A blanketlike zone of pervasive argillic alteration occurs at the base of the fluvial sediments. Numerous hydrothermal veins and breccias located within the lacustrine sedimentary rocks beneath the argillic blanket contain a telescoped mineral assemblage related spatially to the paleosurface. Quartz and adularia are the most abundant hydrothermal minerals in this zone along with minor pyrite, marcasite, electrum, bladed calcite, stellerite (Ca 4 (Al 8 Si 28 O 72 )28H 2 O), illite-smectite, and smectite clays. The veins and argillic blanket are cut by a 50-m-wide hydrothermal eruption crater filled with bedded breccia deposits. The delta 18 O SMOW values for hydrothermal quartz and adularia range from 5.3 to 8.3 and 0.1 to 2.4 per mil, respectively. The delta 18 O SMOW and delta 13 C PDB values for calcite range from -3.6 to 8.9 and -8.9 to -5.2 per mil, respectively. These values are consistent with those of minerals from meteoric hydrothermal systems. A plot of delta 18 O versus delta 13 C values for calcite shows that HCO (super -) 3 was the dominant aqueous carbon species. INAA analyses of altered sedimentary rocks and hydrothermal vein and breccia samples reveal the presence of anomalous concentrations of Au, Ag, As, Sb, Hg, and Mo. Q-mode factor analysis indicates that much of the variation in the geochemical data set can be explained in terms of five end-member components (factors) which reflect the composition and evolution of the hydrothermal fluid and the initial composition of the host rock. Elemental associations for these factors are (1) Au, K 2 O, Ag, FeO, Hg; (2) CaO, Mo, Hg, As; (3) FeO, Na 2 O, Hg, Sb; (4) K 2 O, Zn, Na 2 O, As; and (5) As, Mo, Sb, Ag. The abundance of adularia and stability of stellerite at Red Butte suggests that fluids reaching the surface had experienced a high degree of vapor separation due to boiling. Au mineralization occurred at the peak of hydrothermal activity in response both to boiling and mixing of the boiling hydrothermal fluid with surficial steam-heated waters.