Summary:
The Project area covers most of the historic LCD, located at the northern end of the Sierra Nevada physiographic province at the juncture with the late-Tertiary-to-Recent Cascade volcanic province to the north, and the Basin and Range province immediately to the east.
Within the property, four main deposits, including Superior, Lamb’s Ridge, Engels, and Moonlight have been explored.
Superior Deposit
The Superior deposit lies within the Lights Creek Stock near the south-eastern margin and south of Engels. The deposit is hosted within the quartz monzonite. Disseminated copper mineralization at Superior lies within a roughly circular area about 610 meters in diameter. Disseminated mineralization consists of fine chalcopyrite and lesser bornite with typical grades of between 0.1% and 0.3% copper. Within this disseminated mineralization are tabular brecciated structures that were historically mined up to 244 meters along strike, 183 meters down dip and three to seven meters wide. Mineralization in the breccia-veins consists of magnetite-actinolite-minor quartz-siderite-bornite-chalcopyrite.
Lamb Ridge Deposit
The geology and mineralization at Lamb’s Ridge appear to be most similar to Superior and was characterized by Placer-Amex geologists as a porphyry system. The wide-spaced (330- to 660-foot [100- to 200-meter]) drilling indicates disseminated copper mineralization similar to that found at Superior; however, no occurrences of the high-grade breccia-veins mined at Superior have been encountered in the drill holes. That said, the drilling that has been done defines significant copper mineralization with copper grades in 16-foot (5-meter) composites exceeding 0.3% copper (Cu) over 4,900 feet (1,500 meters) north to south and 1,640 feet (500 meters) east to west.
Engels Deposit
The Engels deposit lies outside the Lights Creek Stock, immediately adjacent to its eastern margin in an area represented by both gabbroic-phase intrusives and roof-pendant metavolcanics. Mineralization in the Engels Mine area occurs in a 1,280-foot (390-meter) by 656-foot (200-meter) pipe-like zone. Mineralization is associated with brecciated zones that exhibit features of both an intrusion breccia and a hydrothermal breccia. The relationship of mineralization to zones of breccia and contacts between the quartz diorite and metavolcanic is evident. The disseminated copper minerals are often very abundant and locally coalesce. Copper grades exceeding 15% Cu have been encountered in several 6.5-foot (2- meter) core intercepts.
Copper mineralization at Engels is strongly oxidized to depths of 230 feet (70 meters). Assay analysis for sulfuric acid soluble copper in a portion of samples from the post 2004 drilling indicates copper oxides representing 90% of total copper within these depths. Copper oxide minerals consist primarily as malachite with lesser chryscolla and azurite. The principal sulfide minerals consist of bornite and chalcopyrite.
Moonlight Deposit
Placer-Amex, Sheffield, and Sheffield's successors recognized that there are at least two styles of mineralization at the Moonlight deposit. The paragenetically earlier style is characterized by disseminated copper minerals located interstitial to quartz, feldspar, chlorite and especially disseminated rosettes of tourmaline. This mineralization usually consists of fine-grained chalcopyrite but zones of disseminated bornite are also common. High in the system disseminated hypogene chalcocite has also been occasionally observed. Bornite rims chalcopyrite grains in some places. This style of mineralization shows some association with potassium feldspar, a very strong association with tourmaline and sometimes chlorite. Unless overprinted by second-stage fracture or breccia-hosted mineralization, this earlier style of mineralization typically assays at 0.1% to 0.8% Cu.
The second stage of mineralization is characterized by veinlets, or stockwork breccias, which often have a gangue of tourmaline and lesser quartz with strong hematite. Strong copper mineralization is commonly observed on veinlets trending N20-35W and dipping 15- 35SW southwest. The vein orientation suggests a good exploration target beneath the meta-volcanic rocks to the southwest. In addition to the mineralization in shallow dipping fractures, copper is contained on north-south, steep to moderately east dipping veinlets, N60-75E steeply north-dipping veinlets, and N70-85W steeply south-dipping veinlets. Although fracture-hosted mineralization is widespread and often high grade at Moonlight, drilling to date has not revealed extensive vein-like structures similar to those mined at the Superior Mine. Veinlet-or-breccia-hosted mineralization dominates the northern part of the Moonlight deposit, where chalcociterich mineralization commonly grades more than 1% Cu. In holes 06MN-9, 10, 11, and 12, chalcocite-rich mineralization grades quickly into chalcopyrite with depth, and bornite is not very abundant. In the southern and central parts of the deposit, the chalcocite-bornite-chalcopyrite zonation is well-developed. Fracture-hosted mineralization may grade more than 1% Cu in the central and southern portions of the deposit.
At the Moonlight deposit, the primary copper-bearing minerals are bornite and chalcopyrite, with lesser amounts of covellite and chalcocite. The dominant iron species found within the deposit are magnetite and hematite (especially specularite). The Moonlight deposit also contains minor amounts of pyrite. The copper sulfides show a vertical zonation, with chalcocite dominating in the upper levels of the deposit. With increasing depth, bornite dominates and chalcopyrite appears. At the deeper levels, chalcopyrite typically dominates in fracture hosted mineralization, but bornite is locally still abundant. Limited oxidation and supergene products of copper minerals are observed in surface outcroppings and in the tops of some drillholes. Minor amounts of precious metals are associated with the copper mineralization, but their paragenesis has not been studied in detail.
Mineralization at the Moonlight deposit also includes an acid soluble component that overlies the sulfide deposit in three areas: North, Central and South Oxide Zones. In the 1970s, Placer-Amex estimated an oxide resource of 12.2 million short tons (st) at an average grade of 0.54% Cu. Sheffield drilled 15 shallow reverse circulation (RC) holes at Moonlight in 2007, which appear to support the deposit's potential for economic copper oxide mineralization.
Deposit Types
Copper deposits of the LCD were historically classified as porphyry copper deposits with associated gold and silver credits. Nevertheless, Placer-Amex geologists recognized that the deposits of the LCD copper deposits had many characteristics that were not typical of porphyry copper deposits. L.O. Storey (1978) noted, “Typical porphyry copper-type alteration zonation as illustrated by Lowell and Guilbert is nonexistent.” Recent work, noting the lack of porphyry style veining, the ubiquitous presence of magnetite (Superior), and specularite (Moonlight), and the relative scarcity of pyrite suggest an Iron Oxide Copper Gold (IOCG) affinity for much of the mineralization in the LCD (Stephens, 2011).
Regarding IOCG deposits, Sillitoe (2003) noted, “The deposits…reveal evidence of an upward and outward zonation from magnetite-actinolite-apatite to specularite-chloritesericite and possess a Cu-Au-Co-Ni-AsMo-LREE (light rare earth element) signature…” The high-grade mineralization at Superior is associated with magnetite-actinolitetourmaline-apatite. At Moonlight, copper mineralization is associated with tourmalinespecularite-chlorite-sericite. During an April 2015 field visit to the LCD, Sillitoe categorized Engels, Lambs Ridge, Superior, and Moonlight as IOCG deposits (Cole, 2015).