The Pumpkin Hollow deposit is found within the Yerington copper porphyry district located in the productive Walker Lane mineralized belt in Nevada. The deposit is a high grade IOCG chalcopyrite- magnetite skarn located along the flanks of the copper producing Yerington batholith were the district wide mineral inventory is over 24 billion pounds.
The Pumpkin Hollow deposits are blind, a halo of hornfels and un-mineralized skarn is the only surface expression. The deposits were originally discovered in 1959 by US Steel Corp. using airborne magnetics. Follow-up magnetic surveys and drilling delineated several magnetite-rich skarn bodies, commonly greater than 40% iron, hosted by Mesozoic sedimentary and igneous rocks. Later work by Anaconda and other companies outlined copper rich areas within the system where drilling intercepted grades up to 20 % Cu. A recently completed resource study at a 0.2% copper cutoff reported Measured and Indicated Resources totalling 5.9 Billion pounds of Copper plus additional Inferred Resources of 3.7 Billion pounds of Copper (all using a 0.2% copper cutoff grade), 1.6 million ounces of Gold, 42 million ounces of Silver, and Iron resources at a 20% cutoff total 341 million tons at a grade of 33%.
Lithologic, alteration and assay data from this drilling combined with re-analysis of old drill data has defined manto-like ore bodies and breccia-hosted controls for high grade mineralization. Sediments of the Gardnerville Formation have been metamorphosed to pyroxene-garnet hornfels and later veined and replaced by epidote-garnet-actinolite-calcite skarn accompanied by chalcopyrite, pyrite, magnetite, and pyrrhotite. High grades of copper are found as chalcopyrite filled fractures in skarn breccia. Skarn mineralization within the Mason Valley Limestone follows the contact of a large sill of granodiorite endoskarn. Fine-grain massive magnetite mineralization replaces marble and extends hundreds of feet into underlying calcic endoskarn. Sulfides are disseminated throughout the magnetite and are particularly concentrated; in strata-bound skarn-breccia zones, at the endoskarn-marble contact, and at the front between marble and magnetite. Local late stage talc and extensive chlorite + calcite + pyrite veining overprints the entire system. The associated intrusive bodies, sills and dikes, are the same calc-alkaline suite present in the Yerington Batholith porphyry deposits. Strong calcic alteration of the main stage granodiorite is widespread with secondary albite and potassium feldspar present together with the same skarn assemblage seen in the sedimentary host rocks.
Post-mineral geologic events include regional folding, Cretaceous plutonism, and the deposition of a thick sequence of Oligocene ignimbrites. Great Basin extension has resulted in district-wide low-angle normal faults and westward rotation of pre-Miocene rocks. The Pumpkin Hollow deposits are structurally deeper than the known porphyry and related skarn deposits of the district. The great volume of magnetite mineralization of the Pumpkin Hollow skarns is typical of IOCG deposits. The district and deposit geology indicates the Pumpkin Hollow IOCG deposits are porphyry-related but formed at a deeper level than the other copper deposits in the district.
Locally copper grades at Pumpkin Hollow exceed 4% and iron grades are over 65%. For example in the E-2 deposit USX hole KM44 reported 4.2% copper over a true width of 115 feet. In the North deposit USS hole USX-44 returned 4.54% copper over a true width of 173 feet.
To date exploration has defined five individual deposits (although with additional drilling, some of the deposits will likely merge) the North, South and Southeast; open pittable deposits and the high grade East and E-2 both underground deposits.
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