Deposit Type
- Porphyry
- Vein / narrow vein
Summary:
Mineralization
The Red Chris deposit is about 3.4 km in strike length, 0.3 km in width and over 1.3 km in vertical extent. There are two main zones, East and Main. The Main zone is 1,200 m in strike length, 300 m in width and has been drill tested to 1,300 m depth. The East zone is 1,200 m in strike length, 300 m in width and has been drill tested to 1,400 m depth.
The zone of >0.2% Cu%, is >3 km long, oriented west–southwest–east–northeast and can reach 650 m in width. Quartz–magnetite stockworks are overprinted by higher gold and copper grades. The highest copper–gold grades occur in and around the P2 porphyries and associated igneous breccias.
Mineralisation consists of thin wavy or thicker planar quartz veins containing chalcopyrite, bornite and magnetite; these minerals are also disseminated outside the veins. In the upper part of the deposit, the bornite-rich mineralisation was overprinted by sericite and clay alteration and associated sulphidation. Gold occurs as microscopic inclusions in the copper sulphides, and occasionally as free grains in high-grade zones.
Molybdenite occurs locally in quartz veins, especially deeper and outside the high-grade core.
DEPOSIT TYPES
Overview
Red Chris is considered to be an example of a porphyry copper deposit with elevated gold values, characterized by the composition of its host rocks, its alteration, and its copper–gold signature, with only minor molybdenum. It is classified as belonging to the high-potassium calc-alkalic type of porphyry system, which includes several world-class deposits such as Bingham Canyon (Utah).
Porphyry Copper Deposits
The following discussion of the typical nature of porphyry-copper deposits is sourced from Sillitoe, (2010), Singer et al., (2008), and Sinclair (2007).
Porphyry copper systems commonly define linear belts, some many hundreds of kilometres long, as well as occurring less commonly in apparent isolation. The systems are closely related to underlying composite plutons, at paleo-depths of 5 km to 15 km, which represent the supply chambers for the magmas and fluids that formed the vertically elongate (>3 km) stocks or dyke swarms and associated mineralization.
Commonly, several discrete stocks are emplaced in and above the pluton roof zones, resulting in either clusters or structurally controlled alignments of porphyry copper systems. The rheology and composition of the host rocks may strongly influence the size, grade, and type of mineralization generated in porphyry copper systems. Individual systems have life spans of circa 100,000 years to several million years, whereas deposit clusters or alignments, as well as entire belts, may remain active for 10 million years or longer.
Deposits are typically semicircular to elliptical in plan view. In cross-section, ore-grade material in a deposit typically has the shape of an inverted cone with the altered, but lowgrade, interior of the cone referred to as the “barren” core. In some systems, the barren core may be a late-stage intrusion.
The alteration and mineralization in porphyry copper systems are zoned outward from the stocks or dyke swarms, which typically comprise several generations of intermediate to felsic porphyry intrusions. Porphyry copper–gold–molybdenum deposits are centred on the intrusions, whereas adjacent carbonate wall rocks commonly host proximal copper–gold skarns and less commonly, distal base metal and gold skarn deposits.
Beyond the skarn front, carbonate-replacement copper and/or base metal–gold deposits, and/or sediment-hosted (distal-disseminated) gold deposits can form. Peripheral mineralization is less conspicuous in non-carbonate wall rocks, but may include base metal- or gold-bearing veins and mantos.
Data compiled by Singer et al. (2008) indicate that the median size of the longest axis of alteration surrounding a porphyry copper deposit is 4–5 km, while the median size area of alteration is 7–8 km2.
Alteration is typically zoned laterally from the core of the deposit and vertically. Alteration is defined by the alteration mineral assemblages that occur in each alteration zone. In silicate-rich rocks, the most common alteration minerals are K-feldspar, biotite, muscovite (sericite), albite, anhydrite, chlorite, calcite, epidote, and kaolinite. In the core of the deposit, alteration is typically potassic and consists of secondary K-feldspar, biotite, anhydrite, and quartz with possible bornite, chalcopyrite, gold, magnetite, and pyrite. Outward and upward from the potassic core, alteration includes argillic with various clay minerals, pyrite and chalcopyrite, propylitic with actinolite, epidote, chlorite, pyrite, carbonate, pyrite, and albite.
In carbonate rocks, the most common minerals are garnet, pyroxene, epidote, quartz, actinolite, chlorite, biotite, calcite, dolomite, K-feldspar, and wollastonite.
Other alteration minerals commonly found in porphyry-copper deposits are tourmaline, andalusite, and actinolite.
Red Chris
The Red Chris deposit was initially interpreted to be an example of an alkalic or “hybrid” alkalic deposit. Interpretations of chemical data on synmineral porphyry phases completed by Rees et al., (2015) suggests that the host porphyries are high-potassium, calc-alkalic in composition, similar to the monzonitic copper–(molybdenum–gold) class of porphyry deposits, which includes Bingham and Bajo de la Alumbrera. Red Chris is also classified as an A-vein type deposit, due to the strong control on higher-grade copper–gold by A-vein stockworks with abundant disseminated copper sulphides.
Whole rock analyses from early and intermediate stage porphyries in the Red Stock fall marginally on the calc-alkaline side of the lkaline/subalkaline divider, suggesting that Red Chris should be assigned to the high-K calc-alkalic category of Lang et al. (1994). However, late-mineral stage porphyries plot in the alkalic field. The mineralizing (intermediate-stage) quartz monzonite porphyries plot in the high-K calc-alkalic field.
In Seedorff et al.’s (2005) classification scheme, Red Chris conforms with their “monzonitic Cu (Mo–Au) class” of porphyry copper deposits. This class features an uncommon association of copper with both gold and molybdenum credits, although at Red Chris the molybdenum relationship with copper–gold is irregular.
Proffett (2009) recognized two types of porphyry-copper systems, based on differences in mineralization style (copper in A-type quartz veins versus copper in early-formed fracture halos), and the corresponding genetic correlation with a particular porphyry magma phase (a high versus a low correlation, respectively). He attributed the differences to depth of formation, and whether a two-phase or a single-phase fluid was responsible for sulphide behaviour. Red Chris has the hallmarks of an ‘A-vein type’ system, and probably formed at a relatively shallow level in the crust from a two-phase mineralizing fluid.
Summary:
The Red Chris Operations are a copper–gold open pit mining operation located in northwest British Columbia, Canada.
Open pit operations are conducted using conventional methods and a conventional truck and shovel fleet.
Geotechnical Considerations
The pit design parameters for Red Chris are designed to control bench scale stability and rockfall issues more than the global stability of the pit walls. In 2020, the geotechnical parameters were updated based on geotechnical and operating history at the Project.
Golder defined six geotechnical domains within the open pit. Recommended inter-ramp angles ranged from 41.9–52.8º. These were adjusted to take the pit ramps into account, leading to designed slope angles from 34–46º.
The greatest area of concern for stability is on the southern wall and in particular the Bowser Lake Group sediments. The results of the overall slope stability analyses indicated that the proposed south wall design is expected to exhibit adequate factors of safety with respect to overall slope circular type failure that involves failure from the crest of the scope through the South Boundary Domain and South Domain rock mass.
Two areas that show a factor of safety risk are associated with the Bowser Lake Group sediments near the pit crest, and are only problematic with elevated pore water pressures.
The benches are expected to be relatively free draining, and significant pore pressures are unlikely to be generated at the crest of the slope. Consequently, provided that good quality wall-controlled blasting is used along the south wall, and relative pore pressures are appropriately maintained, the Bowser Lake Group sediments are expected to exhibit adequate stability with respect to inter-ramp scale circular failure through the south wall.
Operations
The mine uses rotary blasthole drills (drilling variable diameter holes up to 311 mm) and 28 m³ electric hydraulic shovels loading 230 t capacity haul trucks from 12 m benches. The operation is supported by standard ancillary equipment including an 18 m³ front-end loader, track and rubber-tired-dozers, and graders.
Ore and waste are drilled and blasted together on 12 m benches and mined in a single pass. Where practicable, walls are drilled with a pre-split to assure stable wall rock conditions.
Ex-pit ore is allocated by gold and copper grade and either sent to the mill crusher pocket directly or sent to low-grade or mineralized waste stockpiles (material below Mineral Reserve cut-off grade).
NAG rock is used to line the WRSF and for construction. PAG rock is sent to designated PAG WRSFs.
Stockpiles
The Red Chris Operations use a grade binning ore control system based on NSR value of mineralized material. High-grade (Codee 1) and medium-grade ore (Codee 2) is generally fed to the crusher directly with low-grade ore (Codee 3) stockpiled for later use as required.
Mineralized waste is also stockpiled but is not estimated as part of the Mineral Reserves.
Due to the potentially acid forming nature of the ore, the low-grade stockpiles (Including mineralized waste) overlie NAG material and are segregated from the WRSF. Environmental controls direct contact water away from at-risk catchments.
A mineralized waste stockpile has been retained as a potential buffer for the mill in the event of production interruptions from the mine, should low-grade ore stockpiles become depleted. Mineralized waste treatment would be contingent on sufficiently high metal spot prices to make processing the material economically viable.
Waste Rock Storage Facilities
NAG waste rock will generally be used as base below the PAG rock storage area, as road-topping material, or as general construction material. Depending on the available quantity of NAG rock, the base below the PAG waste will be designed from 1–5 m thick.
Low-grade material is stockpiled on a base of NAG material. PAG waste is hauled to the north WRSF (north of the open pit limits) and placed upon a base of NAG waste material.
Sufficient WRSF space was designed to store 150 Mt of NAG and PAG waste, assuming a loose density of 2.2 t/m3. The open pit schedule requires 120 Mt of waste be stored.
Block Cave
The Red Chris Operations have the potential to transition to block cave underground mining.
The proposed block cave considered in the 2021 PFS envisages development of three successive macroblocks (MB1, MB2, and MB3), using a conventional post-undercut, El Teniente-style layout (an intersecting pattern of parallel extraction and parallel drawpoint drives) with diesel load–haul–dump (LHD) machines to transport rock to a central crusher. From the crusher, rock will be transported to surface, where it will be directed either to the existing coarse ore storage stockpile via a transfer conveyor, or to a new coarse ore storage stockpile feeding a new grinding train. An underground mine dewatering system will pump collected water from sumps and settlers to the process plant for re-use. Primary access to the mine workings will be via an extension of the exploration decline.
The proposed mine plan uses technology conventional to block cave operations, including mine design and equipment. The planned mining equipment is conventional to block cave operations.
Mine Design
Declines
The exploration/access decline portal is constructed, and is located at the lowest accessible point above the ultimate height of the TIA. Access to the mine will be via two declines: exploration/access and conveyor declines. Both declines will have the portals located east of the mine footprint.
The exploration decline started construction in June 2021, and is the critical item required for Project development. It will be developed west towards the orebody, from which an exploration platform will be developed that will be used for drilling and LOM ventilation. From this point the decline will switch back to the east to intercept the third leg of the conveyor decline before switching back to the west towards the footprint.
The access decline gradient will be 1:6.5 because the decline will be used for haulage prior to the conveyor installation. The exploration decline will connect to the exploration level before connecting to the footprint. The exploration level will become a ventilation transfer drift once exploration drilling is completed.
The conveyor decline is designed to house a straight conveyor, minimize conveyor transfers, and have a minimum transfer angle of 11°. It was also designed to keep the transfer stations accessible from surface via a ventilation raise if required, as well as keeping the 1,200 L transfer north of the decline portal due to poorer ground conditions identified at depth south of the decline portal. The conveyor decline is designed at 1:5.3
gradient. There will be a lateral connection between the exploration decline and leg two of the conveyor decline. It is currently planned that traffic in the conveyor decline will be limited to conveyor maintenance and secondary egress.
Ventilation shafts from surface will be developed along the length of the access decline, to provide ventilation during decline development as well as supporting LOM ventilation requirements. The decline strategy is to have multiple touch points between declines allowing for the conveyor decline to be mined from multiple fronts so that the average development rate per heading can be reduced taking it off the project critical path.
The mine access will consist of about 17,400 m of lateral development and approximately 3,700 m of vertical development. The declines were designed with stockpiles and sumps at spacings that will allow for future use of the excavations as refuge chambers, pump cuddies, sub-station cuddies, passing, and storage.