A rainbow spans the horizon at the site of Nevada Copper’s Pumpkin Hollow East deposit.
(Photo courtesy of Nevada Copper)

On November 19, 2012, Nevada Copper announced positive results for a new feasi-bility study for its 100%-owned Pumpkin Hollow copper project located 8 miles south-east of Yerington that describes two stages of project development. Stage 1 would be a 6,500-mt/d underground mine on Pumpkin Hollow’s East deposit. Stage 2 would be a much larger open-pit operation based on the project’s North and South deposits.

A separate Pumpkin Hollow feasibility study, due for release by early 2013, will optimize a feasibility study published in February 2012 that considered a larger combined open-pit and underground mine development.

On October 24, 2012, Entrée Gold reported the results of a positive prelimi-nary economic assessment (PEA) for its 100%-owned Ann Mason copper-molybde-num porphyry project 4.3 mi west of Yerington. The Ann Mason PEA envisions an open-pit and conventional sulphide flotation milling operation with an initial 24-year mine life. Feed rate to the mill is projected at 100,000 mt/d.

Anaconda operated an open-pit copper mine in the district from 1952 to 1978. Should both the Pumpkin Hollow and Ann Mason projects be developed into active mining operations, Yerington would be transformed into an important new U.S. copper production center.

Nevada Copper and Pumpkin Hollow:
Nevada Copper is preparing to sink a 24-ft-diameter, 2,200-ft-deep production shaft to access the East deposit at Pumpkin Hollow. Subject to a decision to proceed and con-clusion of financing arrangements, detailed engineering and ordering of key long-lead-time mining and processing equipment are targeted to begin by the first quarter of 2013, with actual construction starting on the issuance of key state permits. Ramp-up of underground production is anticipated to begin in the second quarter of 2015.

Initial capital costs for Pumpkin Hollow underground development are estimated at $329 million, including a $25-million con-tingency. Copper cash costs, including site operating costs and copper conversion costs such as smelter charges and concen-trate transport, net of gold and silver rev-enue credits, are estimated to average $1.21/lb for years one–five of operation and $1.63/lb life-of-mine, excluding royalties.

A sample of high-grade ore from the
deposit. (Photo courtesy of
Nevada Copper)

Underground mining will be by long-hole stoping with paste backfill. Under-ground mining methods and the mining sequence were developed to maximize grades in the early production years to the extent possible.

Ore will be crushed underground, hoist-ed to surface, and transported to a nominal 6,500-st/d concentrator located approxi-mately 1,500 ft northwest of the shaft. The concentration circuit is conventional, with a single, semi-autogenous grinding mill; secondary ball mill grinding; flotation; thickening; and pressure filtration to pro-duce a final concentrate grading 24% cop-per and containing payable gold and silver.

Life-of-mine ore production from the East deposit is estimated at 27.6 million st grad-ing 1.49% copper, 0.008 oz/st gold, and 0.17 oz/st silver. Life-of-mine production of copper, gold and silver in concentrates is estimated at 759 million lb, 167,439 oz, and 2,709,187 oz, respectively.

Entrée Gold and Ann Mason:
Open-pit mining at Ann Mason will utilize conven-tional rotary drilling, blasting, and loading, with large cable shovels and 360-mt trucks. The life-of-mine waste-to-mineral-ization strip ratio is 2.16:1.

The processing plant proposed in the Ann Mason PEA consists of conventional unit operations, including gyratory crush-ing, SAG and ball mill grinding, rougher flotation, concentrate regrinding, cleaner flotation, concentrate filtration and tailings thickening. Locked cycle flotation testing has demonstrated that a simple flotation flow sheet with moderate grinds, two stages of cleaning, and low reagent additions is able to generate a saleable copper concen-trate, with no penalty elements identified. Payable by-product levels of gold and silver are present in the copper concentrate.

Metallurgical predictions of 93.5% copper recovery to a concentrate grading 30% copper are based on average values (last four cycles) from locked cycle test data on the main zone composites.

Average annual production of copper in concentrate is estimated at 214 million lb at total cash costs of $1.46/lb net of by-product credits. Life-of-mine metal production is estimated at 5.14 billion lb of copper and 36.4 million lb of molybde-num. The life-of-mine mill feed grade aver-ages 0.31% copper, 0.004% molybde-num, 0.03 g/mt gold, and 0.58 g/mt silver.

Project development capital costs are estimated at approximately $1.28 billion, including contingency.

With the completion of its positive PEA, Entrée now expects to advance to prefeasi-bility work at Ann Mason. Future work will include additional drilling, particularly to the north and west of the Ann Mason deposit, to potentially extend the mineralization within the current pit design and reduce the waste-to-mineralization strip ratio.