Summary:
Copper deposits of the Lights Creek District (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.
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).
During an April 2015 field visit to the LCD, Sillitoe categorized Engels, Lambs Ridge, Superior, and Moonlight as IOCG deposits (Cole,2015).
Within the Moonlight-Superior 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 southeastern margin and south of Engels.
The mineralization at Superior is hosted in the Lights Creek Quartz Monzonite and minor generally flatlying diabase dikes. The quartz monzonite is generally more equigranular and less potassium feldspar-rich than that observed at Moonlight.
Alteration at Superior includes both silicification and potassic alteration. As at Engels, magnetite appears to be a significant alteration mineral as well. Also in common with Engels, there is very little pyrite observed at Superior.
Both disseminated and associated copper mineralization with magnetite and tourmaline veinlets are seen at Superior. Disseminated copper mineralization at Superior, revealed by drilling and exposure in underground workings, lies within a roughly circular area about 2,000 feet (610 meters) in diameter. This mineralization consists of finely disseminated chalcopyrite and lesser bornite. This disseminated mineralization typically runs 0.1 to 0.3% Cu, and copper minerals are typically associated with tourmaline. Within this disseminated mineralization are more than ten tabular brecciated structures (veins) that were mined up to 800 feet (244 meters) along strike, 600 feet (183 meters) down dip, and 10 to 23 feet (3 to 7 meters) wide.
Sulfide copper mineralization, mostly chalcopyrite and bornite, is associated with magnetite tourmaline veinlets.
There are two predominant trends to the breccia-veins. Veins trend north-south and dip to the east and there are a number of essentially flat lying veins. Mineralization in the breccia-veins consists of magnetiteactinolite-minor quartz-siderite-bornite-chalcopyrite. The historic mill feed from these stopes averaged about 2.2% Cu. These veins and the stockworks between them define a high-grade core to the Superior deposit.
Lamb Ridge Deposit
The geology and mineralization at Lamb’s Ridge appears to be most similar to Superior and was characterized by Placer-Amex geologists as a porphyry system. The wide-spaced (328- to 656-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 16.5-foot (5-meter) composites exceeding 0.3% Cu over 4,921 feet (1,500 meters) north to south and 1,640 feet (500 meters) east to west.
The quartz monzonite here is the primary host rock, and except for disseminated copper mineralization, copper sulfide minerals such as chalcopyrite are associated with magnetite and tourmaline veinlets..
Engels Deposit
The Engels deposit lies outside the Lights Creek Stock, immediately adjacent to its eastern margin in an area.
Engels is a structurally-controlled tabular shear-zone hosted deposit striking north-east and dipping steeply to the southeast. Mineralized widths range from 16.5 feet (5 meters) to over 66 feet (20 meters). The historically mined total strike length for the main ore shoot ranges from 328 feet (100 meters) to 820 feet (250 meters), while a narrower ore shoot to the south along strike was mined at lengths from 66 feet (20 meters) up to 197 feet (60 meters). The vertical extent mined is approximately 1,903 feet (580 meters). Mineralization in the Engels Mine area occurs in a 1,280-foot (390-meters) by 656-foot (200-meter) pipelike zone.
Mineralization is associated with brecciated zones that exhibit features of both an intrusion breccia and a hydrothermal breccia.
A diorite or quartz diorite has intruded a pendant of plagioclase phenocryst-rich metavolcanic in a complex mass of dikes and dikelets. The fine-grained matrix of the metavolcanic has often been altered to biotite in the mine area. Primary mineralization consists of zones of silica + magnetite +- biotite hornfels alteration with varying amounts of disseminated bornite and chalcopyrite.
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. The relationship of mineralization to zones of breccia and contacts between the quartz diorite and metavolcanic is evident in surface exposures.
Calc-silicate minerals, especially epidote and locally garnet, are also present. Much of the copper mineralization at Engels is strongly oxidized to a depth of 230 feet (70 meters).
Copper oxide minerals consist primarily as malachite with lesser chryscolla and azurite and in copperbearing limonites and clays.
The principal sulfide minerals consist of bornite, and chalcopyrite hosted in a hornblende gabbro body. Younger quartz diorite and quartz monzonite bodies are associated with the gabbro and are considered to have played an important role in the placement of the copper mineralization.
Moonlight Deposit
Several intrusive phases host the Moonlight deposit. A large part of the deposit lies within two phases of the LCS quartz monzonite. Granitic intrusive (Gr V) hosts the southern third of the deposit. Granodiorite carries copper mineralization at the northern tip of the deposit.
The Granitic intrusive (Grv) at Moonlight shows the same evidence of mineralization at Superior and Lambs Ridge, including magnetite tourmaline veinlets and copper oxide minerals at the surface.
There are at least two styles or stages 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. 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 copper sulfides show a vertical zonation, with chalcocite or digenite predominating in the upper levels of the deposit. With increasing depth, bornite predominates and chalcopyrite appears. Bornite is often observed to rim or cut chalcopyrite. Bornite and chalcopyrite may also be cut by chalcocite veinlets. At the deeper levels chalcopyrite typically dominates in fracture hosted mineralization, but bornite is often still abundant. Magnetite can sometimes appear with hematite decreasing in abundance with depth. Rare pyrite may appear in veinlets at depth. Iron or magnesium-rich carbonates are also common in fracture hosted mineralization.
Veinlet-or-breccia hosted mineralization dominates the northern part of the Moonlight deposit, where chalcocite-rich mineralization commonly grades more than 1% Cu.