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Figure 6. Photographs of middle Miocene sedimentary and volcanic units in the Ivanhoe basin. A: Area between Willow Creek Reservoir and the Ivanhoe district, looking east. RR—Rimrock mercury mine; a—andesite flow unit; vt—15.4 Ma vitric tuff unit, both of which were erupted subaerially. Light-colored, ash-rich strata below the andesite were deposited subaqueously, as were strata between the andesite and the vitric tuff. Dark capping units in distance are 15 Ma rhyolite porphyry flow units and domes, which dip 10° less than the underlying units, indicating tilting between ca. 15.4 and 15 Ma. The Rimrock mercury deposit formed in silica replacement bodies in lacustrine sedimentary units that overlie the vitric tuff unit. B: Thin-bedded, ash-rich lacustrine sedimentary units exposed in the north wall of the open pit of the Hollister gold mine, Ivanhoe district. These beds are stratigraphically between the andesite and the vitric tuff unit (see Fig. 6A). Draping near the base of the exposure is above the irregular top of an andesite flow unit that is exposed just below the photo. Note the progressive upward change to planar bedding. The variegated colors are due to hydrothermal and supergene alteration. C: Sinter and silicified lacustrine sedimentary units (light, massive unit in lower part of photo), overlain by unsilicified lacustrine sediments (middle of photo) and a capping 15.4 Ma rhyolite flow unit. The silicified horizon is widespread throughout the Ivanhoe district and formed prior to the deposition of the 15.4 Ma vitric tuff. Photo was taken 2 km east of the Hollister gold mine. D: Tan, pebble-rich sandstone bodies along Antelope Creek south of the Ivanhoe district. The pebbles are subangular to angular and were derived from the ca 15 Ma rhyolite exposures in the left background, although the main transport direction for the sand component was from right to left. Arrows point to soil horizons between sand bodies. E: Tan—epiclastic sedimentary units overlain by a 15 Ma basalt flow in the southwestern part of the Ivanhoe basin just southwest of the confluence of Rock and Antelope Creeks (Fig. 5). The basalt flow was emplaced subaerially. F: Midas district, looking to the north-northwest. Unwelded tuffs and lacustrine sedimentary units (light tan in foreground) are overlain by 15.7 Ma red rhyolite flows at the skyline. A wide, north-northwest–striking dike fed the flows and intruded the tuffs and sedimentary units in that area. The mine workings are along veins that filled north-northwest–striking faults in the tuffs and sediments; the veins formed at ca. 15.4 Ma (Leavitt et al., 2004).

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