Mineralization
Copper mineralization is widespread over most of the 28 km2 Moonlight area, occurring as discrete large high-grade veins, veinlets and extensive disseminations in the wallrock between the veins. The wallrock alteration is far less conspicuous than typical porphyry-copper systems due to the paucity of acid-generating sulfide minerals, and many low-grade copper occurrences in the region have not been recognized in the past due to the lack of conspicuous surface alteration.
The primary hypogene copper minerals at Moonlight consist mostly of bornite, chalcopyrite, and chalcocite which are associated with considerable amounts of magnetite and specular hematite. The secondary copper minerals – malachite, azurite, and chrysocolla – occur in the oxide zone, which extends from the surface to a depth of 50 m or more. The secondary minerals appear to be the products of in-situ oxidation of the primary copper minerals and not the products of supergene enrichment, which is of very limited extent. The lack of supergene enrichment is attributed to the lack of acid-generating primary sulfide minerals. The presence of abundant tourmaline, roof pendants, country rock clasts and other features offer evidence that the mineralization exposed on the project occurs near the roof of the intrusive complex, in which case the mineralization can be expected to persist to considerable depth.
The Moonlight Project was advanced by Placer during the 1960s and 1970s during which the company completed geological mapping, geophysical and geochemical surveys and 43,000 m of core drilling. Moonlight holds seven potential Cu-Au-Ag open pit targets, including the most advanced targets – Moonlight Valley, Engels Mine, Superior Mine Area and Copper Mountain.
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Moonlight Valley
The most intensely explored area on the project is the Moonlight Valley deposit, which contains National Instrument (“NI”) 43-101 compliant mineral resources defined by 199 historical core holes (30,300 m) and 14 recent confirmation core holes (3,400 m) as reported in “Technical Report and Resource Estimate on the Moonlight Copper Property, Plumas County, California for Sheffield Resources Ltd.”, by George Cavey, P.Geo., and Gary Giroux, P.Eng, dated April 12, 2007. The current NI 43-101 mineral resources are as follows:
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Cut off |
Indicated Mineral Resource |
Inferred Mineral Resource |
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Tons > cutoff |
Cu (%) |
Au (oz/t) |
Ag (oz/t) |
Tons >cutoff |
Cu (%) |
Au (oz/t) |
Ag (oz/t) |
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0.20 |
161,570,000 |
0.324 |
0.003 |
0.099 |
88,350,000 |
0.282 |
0.003 |
0.089 |
0.25 |
114,570,000 |
0.366 |
0.003 |
0.112 |
48,820,000 |
0.329 |
0.003 |
0.107 |
0.30 |
76,150,000 |
0.413 |
0.003 |
0.124 |
23,720,000 |
0.390 |
0.003 |
0.118 |
These indicated and inferred resources do not take into account the oxide mineralization drilled at the Moonlight Valley or the historic resources defined at the Superior Mine Area.
The 2007 Technical Report notes that Placer recognized significant amounts of copper were lost during their drilling and because of this the copper grades estimated from the drilling – which are the grades used in the NI 43-101 resource estimation – are conservative and apparently understated. Sheffield’s recent drilling recovered 44% more copper in their Moonlight Valley drill holes than in the adjacent Placer drill holes. Reasons offered for the historically understated drill grades include improper orientation of the historical drill holes, small BX-size core in the historical drilling resulting in very poor recoveries, loss of copper minerals in the historical drilling as confirmed by Placer’s own comparison sludge sampling, possible analytical issues, and perhaps other items as well. This means that the Moonlight Valley deposit, which remains open at depth and along strike, is potentially larger than currently known and may also be potentially richer than indicated by the historical drilling.
The Moonlight Valley sulfide deposit, which contains the previously mentioned mineral resources, remains open laterally and to depth. Mineralization extends from the surface to a vertical depth of at least 600 m, which is the length of the deepest hole yet drilled on the target. This hole was mineralized throughout its length (averaging 0.185% Cu) and bottomed in 0.3% Cu. The limits of the deposit are unknown. Additionally, the historical work by Placer, and reconfirmed by Sheffield, indicates that the area also contains substantial near surface oxide copper mineralization, comprising a shallow copper target possibly amenable to low-cost SX-EW extraction. This oxide mineralization was not included in the resource reported in the 2007 Technical Report.
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Engels Mine
The Engels mine, a historically important producer, is located on private patented ground approximately 4.5 km east of Moonlight Valley. It is currently being drilled by Sheffield to define high-grade mineralization, which might be mined as a “fast-track” starter operation. Mineralization consists of high-grade bornite-chalcopyrite-magnetite occurring in a pipe-like body, 50 to 100 m wide, more than 200 m long, and extending over a vertical range of at least 700 m. Sheffield’s recent 32-hole drilling program at Engels demonstrates high-grade oxide copper mineralization (typically 1% to 2%) from surface to depths of more than 200 m over intervals ranging from a few m up to 102 m.
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Superior Mine Area
The Superior mine area, is on private patented ground approximately 3 km southeast of Moonlight Valley. It consists of a large stockwork of parallel high-grade bornite-chalcopyrite-magnetite veins from 2.5 to 6.0 m thick and separated by 30 to 120 m of wallrock containing disseminated copper in the 0.3% to 0.8% range. Placer examined Superior as a possible open-pit target, drilling 96 core holes to an average depth of 160 m and estimating an historical resource of 43 million tons grading 0.56% Cu (with a 0.3% Cu cutoff), as summarized in the 2007 Technical Report. Although these historical resources are considered relevant, they do not follow the requirements for reserve and resource estimations outlined in NI 43-101 and should therefore not be considered current. Not included in these historical resources are substantial quantities of higher grade material remaining in the Superior mine underground workings below open-pit depth. The configuration of mineralization, rock quality and the abundant mine workings could make the Superior deposit amenable to very low cost underground mining methods.
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Copper Mountain
Copper Mountain is one km southeast of Moonlight Valley. Placer drilled 24 vertical holes (3,670 m) on this target, reportedly intersecting significant intervals of 0.3 to 0.4 % Cu to depth.
Other Targets
Other targets partially tested by Placer, but not yet tested by Sheffield, include Moonlight South, a possible high-grade target in or beneath a large pendant of metavolcanic country rocks which overlies the southwest-plunging Moonlight Valley porphyry system. The pendant contains widespread specular hematite, quartz veinlets and scattered copper oxides, possibly representing an alteration-mineralization halo above a high-grade zone trapped within or beneath the pendant, a situation generally analogous to that reported from some important districts such as Pebble, Alaska, and Oyu Tolgoi, Mongolia. Three grab samples collected by Sheffield from the dump of the collapsed Ruby mine located within this pendant averaged 5.28% Cu, 1.87 g/t Au and 211 g/t Ag. Other outlying targets of future interest include Sulphide Ridge, Gossan Ridge, Blue Copper and others.
Moonlight Project Exploration Potential
The Moonlight project is underlain by an intrusive complex of large size, extending to unknown depth, and contains high-grade structurally controlled mineralization associated with an unusual low-sulfide type of disseminated mineralization that could easily be overlooked in reconnaissance examinations. The copper mineralization is widespread over an area of at least 30 km2, within which several targets of promising size and grade have already been defined, and other targets have been identified which warrant additional exploration.
It is Nevoro’s intention to accelerate the work underway by Sheffield in order to rapidly advance and expand the Moonlight Valley resource. The exploration target objective is a 400 to 600 million tonne copper deposit grading 0.4 to 0.6% Cu, however these potential quantities and grades are conceptual in nature. The historical and current exploration work done to date are insufficient to define this target and it is uncertain if further exploration will result in the discovery of any mineralization of economic importance.
Nevoro also plans to: (a) continue defining the potential SX-EW amenable, high-grade oxide copper mineralization on the Moonlight Valley and Engels targets; (b) advance the high-grade Superior target to a possible resource stage; and (c) explore and advance the several other scarcely tested copper targets on the project and in the surrounding region.
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