EXTENSIONAL DISMEMBERMENT OF THE HILLTOP MINING DISTRICT, NORTHERN SHOSHONE RANGE, NORTH-CENTRAL NEVADA

Recent mapping of rock types, structure, and hydrothermal alteration and associated geochronology shed additional light on possible relationships between intrusions and hydrothermal systems prior to structural dismemberment. The area, located along the Battle Mountain-Eureka trend and between the Cortez and Battle Mountain districts, exposes rocks of the Roberts Mountains allochthon, windows into the autochthon, the Antler overlap sequence, sedimentary rocks of various Cenozoic ages, ~41-37 Ma intrusive rocks and ore deposits, ~36 Ma dacitic ash-flow tuff of Mount Lewis, and ~16.5-15 Ma basaltic andesite of the northern Nevada rift.

The Hilltop district, in the center of the range, consists of historic small vein mines (Ag ± Au-Cu-Pb-Zn), the structurally controlled Hilltop gold deposit, and weak disseminated porphyry-style mineralization. Total tiling in the district is ~35-40° as indicated by flattened pumice fiamme of the tuff of Mount Lewis. New mapping has identified a previously unrecognized system of west-dipping, ~25° normal faults that have contributed to extending the district >50% and tilting the ore deposits, their host rocks, and the nearby tuff of Mount Lewis. Preliminary structural restorations show relatively small offsets (<500 m on average) with ~20-30° of tilting related to the low-angle normal faults. Previous work focused on ore deposits in the northern Shoshone Range has suggested that <10-20° of tilting is due to higher angle faults, such as the Corral Canyon fault and its splays in Whirlwind Valley and the Malpais Rim.

Recent U-Pb geochronology (C. A. King, unpublished) demonstrates that the major intrusive bodies in the range (Granite Mountain [40.6 ± 0.7 Ma], Park Saddle [37.8 ± 0.8 Ma], Tenabo [39.9 ± 0.7 Ma]) are the result of temporally discrete magmatic events, and not one prolonged magmatic event. The present hypothesis is that the Hilltop gold deposit (⁴⁰Ar/³⁹Ar: ~38.5-39.7 Ma of Kelson et al., 2008) is unrelated to the nearby Park Saddle pluton. This is supported by: 1) structural restorations, coupled with the new U-Pb geochronology of the igneous rocks, and 2) the Park Saddle pluton is dominantly altered to Na-Ca assemblages, whereas the Hilltop gold deposit is sericitically altered, and would likely restore to different structural levels in the paleo-hydrothermal system.