Archetypal Au-bearing silica sinter from the Miocene Milestone deposit, Idaho, USA

The Milestone silica sinter defines a mid-Miocene epithermal paleosurface that is adjacent to the DeLamar Ag–Au deposit in the Silver City District (SCD), Idaho. Milestone, DeLamar, and other SCD deposits (e.g., War Eagle and Florida mountain) are part of the same suite of ~ 17–14 Ma bonanza Au–Ag deposits associated with volcanism that demarcates the onset of the Yellowstone hotspot across the northern Great Basin (USA). Geologic mapping of the Main Knob of Milestone reveals five lithofacies in the paleosurface: matrix-supported breccia, clast-supported breccia (cross-cut by near-vent silica gel), silica sinter and geyserite, and bioformations. The facies are indicative of a paleohot spring, typical of active hot spring environments. Electrum microparticles occur in dark-gray to black bands in the sinter, which resemble high-grade ore-bearing ginguro bands in epithermal veins. Transect and spot trace element analyses by LA-ICP-MS show that these sinter bands are associated with metal(loid) concentrations (0.03–291 ppm Au and 0.19–28,400 ppm Ag). The metal-rich bands show textures that reflect the transportation and deposition of Au-rich particles. These textures, coupled with a lack of compositional variation across the different sinter facies (distance from hot springs vent), suggest that the Au-rich particles formed at depth and were then transported to the surface. This occurrence of precious metals in Milestone sinter likely indicates metal-bearing veins at depth, along faults and other structural weaknesses that channeled the liquid to the surface. Milestone sinter, first recognized by Lindgren, is analogous to those of the Yellowstone and Taupo Volcanic Zone geothermal systems.