Moonlight-Superior Project is a historically operated copper mine located in the US state of California.
The project is owned by US Copper Corp, a junior copper exploration and development company. The company is working to explore the potentiality of redeveloping the mine.
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US Copper announced the results of a Preliminary Economic Assessment (PEA) on Moonlight-Superior Copper Project in January 2025. The assessment concluded a substantial economic opportunity at current copper prices.
The PEA demonstrated a Net Present Value (NPV) of $1.075bn after tax.
The development of Moonlight-Superior Project will entail an initial cost of $956m. Over its 14-year life, it is expected to produce 1.61 billion lbs of copper, 13.2 million ounces of silver and 78,660 ounces of gold.
US Copper plans to seek a joint venture partner for the development of the mine, as it proceeds with the pre-feasibility study.
Background
Copper deposit exploitation in Plumas County dates back to 1860s.
During this time period, copper was mined and shipped from the Lights Creek District (LCD). Henry A. Engels and Sons acquired the Superior Mine in 1880 and discovered the Engels Mine in 1883.
The Superior and Engels Mines were operational between 1915 and 1930 producing more than 161 million pounds of copper.
Activity at the LCD was largely limited to exploration from 1930 through 1961.
American Exploration and Mining (Placer-Amex) conducted exploration in the LCD from 1961 through 1981. This led to the discovery of the Moonlight deposit.
Several Canadian junior companies reassembled the property from 2004 to 2012.
US Copper acquired the claims/patents including Superior and Engels from the Trustee in Starfield Resources’ bankruptcy in 2013. The company optioned the Moonlight claims from Canyon Copper in 2016 and finalised the purchase in 2018.
Moonlight-Superior Project Location
The Moonlight-Superior Project is situated within the Lights Creek District, around 10 miles northeast of Greenville, California.
It comprises 270 unpatented claims covering 5,578 acres, 36 patented lode claims totalling 736 acres, and 162 acres of fee lands.
It includes three advanced stage copper deposits– Moonlight, Superior and Engels.
Geology and Mineralisation
The Moonlight-Superior Project includes majority of the historic LCD area. The LCD is situated at the northern end of the 25-mile-long and 5-mile-wide, N20W trending Plumas Copper Belt.
The copper deposits are primarily located in the early Jurassic (178 Ma), multi-phased, quartz monzonite Lights Creek Stock (LCS), which is a near circular intrusive formation composed of fine- to medium-grained quartz monzonite to granodiorite, enriched with tourmaline, and spans an exposed area of about seven square miles.
Historically, the LCD copper deposits were categorised as porphyry copper deposits, with additional gold and silver credits.
The Superior deposit is located within the Lights Creek Stock, near its south-eastern margin, just south of Engels. Hosted within quartz monzonite, copper mineralisation at Superior is disseminated across a roughly circular area with a diameter of approximately 610m.
The Engels deposit, positioned outside the Lights Creek Stock and adjacent to its eastern margin, is associated with gabbroic-phase intrusives and roof-pendant metavolcanics. Mineralisation at the Engels Mine occurs within a pipe-like zone measuring about 390m by 200m. Copper mineralisation at Engels is strongly oxidised to depths of up to 70m.
Moonlight deposit features bornite and chalcopyrite as primary copper-bearing minerals, with smaller amounts of covellite and chalcocite. Magnetite and hematite (particularly specularite) are the dominant iron species present, along with minor pyrite. The deposit also features an acid-soluble copper component that overlies the sulphide mineralisation in three distinct areas: the North, Central, and South Oxide Zones.
Moonlight-Superior Project Mineral Resource Estimate
Moonlight-Superior Project hosts an indicated resource of approximately 402.83 million tonnes at an average of 0.31% copper (Cu), 1.85ppm silver (Ag), containing 2.5 billion pounds of copper and 21 million ounces of silver.
Inferred Resource total 64.59 million tonnes containing 394 million pounds of copper and 1.4 million ounces of silver.
Mining and Recovery
The Moonlight-Superior Project is expected to be mined using conventional open pit mining methods. The open pit areas were found to be suitable for phased designs.
The mine plan is based on delivering 60,000 tonnes of sulphide material to the mill and 10,000 tonnes of oxide and transition material to the heap leach per day.
The mineralised high-grade sulphide material would be transported to the primary crusher, while the low-grade sulphide material would be hauled to a stockpile. The oxide material would be hauled to the leach pad, with mineralised transition material would be sent to either the leach pad or a temporary stockpile.
Separate waste storage facilities would store the waste material.
The project will produce a saleable high-grade copper-silver concentrate.
It will feature a concentrator based on a modern copper sulphide process. The concentrator will be designed to process a nominal 54,348 tonnes per day of copper-silver mineralised material.
The treatment plant will comprise crushing and grinding circuits, followed by a flotation process to extract and enhance copper from the feed material. The resulting flotation concentrate will be thickened, filtered, and stored in a stockpile before being transported to smelters.
Final flotation tailings will be directed to a tailings management facility (TMF) for storage.
Key Infrastructure
Several key facilities must be built for the project, including administrative offices, a warehouse, ammonium nitrate/fuel oil (ANFO) storage, an equipment shop, a fuel station, a processing plant, a leach pad, and waste storage areas.
Process water will be reclaimed from the tailings pond, while fresh water—sourced from a well field—will be reserved for gland service and reagent preparation.
The project’s estimated power demand includes approximately 50MW for the concentrator and 11MW for the heap leach facility.
Electricity will be supplied from an existing transmission network in Westwood, California, about 12km northwest of the site. A new high-voltage transmission line will be constructed to deliver power to the project location.
A 4MW, 4,160 V standby diesel generator will be installed at the concentrator building to support critical process loads in case of power disruptions.
Contractors Involved
The PEA Technical Report on the Moonlight-Superior Mine Project was completed by Global Resource Engineering.
Boyles Brothers Drilling was selected for LCD drilling programme in 1964, while Arizona’s Heinrichs Geoexploration Company (HGC) conducted several Induced Polarization (IP)-Resistivity surveys over the most promising soil anomalies in the area.
