THE GEOLOGY AND MINERALIZATION OF THE PUMPKIN HOLLOW CU-FE-AU DEPOSIT, YERINGTON, LYON COUNTY, NEVADA

The Pumpkin Hollow Cu-Fe-Au deposit (PHD) is located 8 miles southeast of Yerington, in Lyon County, Nevada. Six orebodies of copper-magnetite skarn mineralization have been defined since the deposit was discovered by United State Steel in 1960 using airborne magnetics.

The PHD lies within the Basin and Range Province, and is underlain by a sequence of Mesozoic sedimentary and volcanic rocks, intruded and mineralized by the Jurassic-age Yerington batholith (169 m.y.a.). Tertiary volcanics unconformably Mesozoic rocks.

The PHD lies within the Yerington District where the Yerington Mine (Anaconda) produced over 1 billion tons of copper from 162 million tons grading 0.55% copper. Ore at the Yerington Mine occurs exclusively in quartz monzonitic intrusive, whereas the PHD orebodies are dominantly skarns, hosted by limestones and associated sedimentary rocks. However, it is likely that the quartz monzonitic mineralizers at the Yerington Mine and the PHD are co-magmatic, and mineralization occurred at essentially the same time.

The rocks have been displaced into numerous blocks by easterly dipping normal faults, leaving all units steeply dipping to the west (Proffett & Dilles 1984). It is likely that the six orebodies were originally connected as one entity, but have since been displaced and separated into its present configuration.

The deposits are typically characterized by a prograde and retrograde alteration mineral assemblage. The prograde assemblage is typically diopside-garnet-calcite-dolomite-tremolite-plagioclase, followed by diopside-garnet-magnetite +/- sulfide replacement along fractures. The retrograde assemblage is typically chlorite-actinolite-epidote-garnet-magnetite-calcite-dolomite +/- pyrite, chalcopyrite, and pyrrhotite.

The orebodies are all “blind” and defined solely from drilling results. In general, the west deposits lie within 300-1,000 feet of the surface, and the eastern deposits lie under 1,000-1,500 feet of gravels and barren hornfels. The grades range from 0.3-10% copper and are amenable to combination of open-pit and underground mining methods. The copper deposits are found adjacent to the high-grade magnetite skarn deposits. Measured and indicated reserves are 312 million tons grading 0.44% copper (using a 0.2% cutoff) and 12.3% iron (Gustavson, June, 2006).