Deposit Type
- Porphyry
- Breccia pipe / Stockwork
Summary:
The KSM intrusive complex demonstrates many features characteristic of giant diorite or monzonite hosted gold-copper porphyry systems.
The Kerr deposit is a strongly-deformed copper-gold porphyry, where copper and gold grades have been upgraded due to remobilization of metals during later and/or possibly syn-intrusive deformation. Alteration is the result of a relatively shallow, long lived hydrothermal system generated by intrusion of monzonite. Subsequent deformation along the Sulphurets Thrust Fault (STF) was diverted into the Kerr area along pre-existing structures.
The Kerr Zone is centered on a north-south trending, steep westerly dipping, tabular intrusive complex that drilling demonstrates has a horizontal extent of 2,400 m and a vertical extent of at least 2,200 m. The complex includes an east and west limb that may coalesce near the current surface. The west limb is up to 500 m thick, and the east limb is up to 300 m thick. There are several distinct intrusive phases, the earliest of which are fine grained diorites with 5% to 60% quartz-sulphide vein stockworks, and these appear to contribute the majority of metals. Later phases envelope and sometimes invade the earlier phase, and are characterized by coarser textures, less veining, and lower metal contents. The intrusions are hosted by an Early Jurassic sequence of rhythmically bedded siltstones, sandstones, conglomerates, and debris flows that have been altered adjacent to the intrusions, but generally contain marginal metal grades.
Various lithology and alteration types were modeled as 3D wireframes for the Kerr deposit and are summarized in Table 7.1. Mineralization occurs mainly in the PAND1 intrusion and IBX breccia and wall rock complex; however it may extend into the adjacent late PAND2 intrusion and wall rock sediments. The dominant copper mineral is chalcopyrite, which typically occurs as isolated grains about 0.2 to 2 mm across, disseminated and clustered in quartz veins, fractures, and surrounding haloes. Bornite is present almost exclusively in the north half of the east leg, within a phase containing over 50% crackled quartz veins, accompanied by coarse grained chalcopyrite and minor tennantite. Tennantite is rare, but widely distributed in late quartz-carbonate veins, mostly in wall rocks, along with minor sphalerite, rare galena, and arsenopyrite. Dark, arseniferous pyrite is associated with these minerals. Molybdenum is a minor constituent, mostly contained within the PAND1 and IBX units and closely associated with copper. This is distinct from the Mitchell zone, where molybdenum is distributed mainly in a shell in wall rocks peripheral to the copper zone. Visible gold has not been observed except under microscopic examinations, where it is observed as less than 100 µm inclusions within sulphides, mainly chalcopyrite, and sulphide grain boundaries.
The Sulphurets Zone is comprised of two distinct zones—Raewyn and Breccia Gold. The Raewyn Copper-Gold Zone hosts mostly porphyry-style disseminated halcopyrite and associated gold mineralization in moderately quartz stockworked, chlorite-biotite-sericitemagnetite altered sediments and volcanics. The alteration and mineralization are centred on meter scale; sills and dykes of altered porphyritic diorite are believed to tap a stock at depth. It has an apparent north-northeast strike and dips about 45° to the northwest, with approximate true dimensions of 1,000 m in strike, 550 m down dip, and up to 250 m in thickness. It remains open down dip and along strike to the northeast at depth. It may be offset in an echelon style by several north-northeast trending vertical structures. The mineralization is open down-dip. It crops out at surface on the cliff face above Sulphurets Lake and its upper surface follows or is clipped by the Sulphurets Thrust Fault (STF). The Breccia Gold Zone hosts mostly gold-bearing pyritic mineralization with minor chalcopyrite and sulfosalts in a potassium-feldspar-siliceous hydrothermal breccia that apparently crosscuts the Raewyn Zone. It comprises altered intrusive clasts in a matrix of mainly silica, potassium-feldspar and sulphides. An intense structurally controlled phyllic overprint nearly masks all earlier alteration phases over parts of the zone. Above the STF, low-grade, but widespread disseminated copper-gold mineralization, occurs in hornfels and skarned sediments and volcanics that have been intruded by an nonmineralized porphyritic monzonite.
The Mitchell Zone (Figure 1.5) is underlain by foliated, schistose, intrusive, volcanic, and clastic rocks that are exposed in an erosional window below the shallow north dipping Mitchell Thrust Fault (MTF).
The zone is a moderately dipping, roughly tabular gold-copper deposit with approximate true dimensions of 1,600 m in strike, 1,500 m down dip, and up to 850 m in thickness. It remains open down dip and along strike to the northeast at depth. It consists of a foliated, schistose or mylonitic zone of intensely altered and sulphide bearing rocks, with a variably distributed stockwork of deformed and flattened quartz veinlets. The schistosity generally follows an east-southeast direction, and dips moderately steep to the north. In general, the core area of mineralization has a moderate plunge to the north or northwest, and is lineated in an east-southeast direction.
Recent glacial melt back has provided exceptional surface exposure of a relatively fresh gold-copper porphyry system. A zone of intense quartz and sulphide veining (“High Quartz”) forms resistant bluffs in Mitchell Valley. However, the higher grade core area is mostly covered by talus and moraine west of the bluffs. Active oxidation and leaching of sulphides has produced prominent gossans and extensive copper sulphate precipitates at the surface.
The “Bornite Breccia” is found in the center of the Mitchell Zone towards the hanging wall side. It was only intersected in three holes (including one interval of 86 m with 1.42% copper and 0.23 g/t gold), and the interpreted dimensions are about 400 m long down dip, 60 m thick, and 250 m along strike. Its geometry roughly aligns with the northwest plunging trend of the Mitchell deposit. The breccia is composed of a chaotic, swirly mix of crackled and milled light grey quartz, anhydrite and clay, with disseminated and interstitial pyrite, chalcopyrite, bornite, and minor tennantite and molybdenite. In deeper intersects the breccia transitions to a mostly quartz, anhydrite, pyrite, and chalcopyrite hosting structure with only traces of bornite.
A small portion of the Mitchell Resource (less than 2%) is found in the hanging wall of the MTF, where disseminated and veinlet chalcopyrite occur in magnetite skarn and hornfels altered sediments and volcanics adjacent to a non-mineralized porphyritic monzonite. This style is identical to the Main Copper Zone above the Sulphurets Zone.
The Iron Cap deposit is a separate but related mineralized system within the KSM district, and occurs structurally above the Mitchell deposit, in the panel of rocks between the MTF and STF. It is now considered to be hosted by a fine-grained porphyritic diorite similar to the Mitchell and Kerr zones. It has similar geometry; a northwest dipping tabular body with approximate true dimensions of 850 m in strike, 1,200 m down dip, and up to 500 m in thickness. It remains open down dip and along strike to the north-northeast. Intense alteration has obliterated most original textures. Stockwork veinlet density is generally low, and no high quartz volumes phases have been identified as at Kerr and Mitchell. Intrusive breccia phases are more common, but are not necessarily well mineralized. Host rocks are mostly sandstones, siltstones, and conglomerates of the Hazelton Group. There is a high degree of silicification in the wall rocks and peripheral phases, which overprints earlier potassic and chloritic alteration.
Mining Methods
- Truck & Shovel / Loader
- Block caving
Summary:
The proposed mine uses conventional large-scale open pit and block cave underground mining methods.
Ore is mined from Mitchell open pit from Years 1 to 24. Mitchell transitions to block cave mining as the Mitchell pit is mined out. Ore is mined from Sulphurets open pit from Years 1 to 17. Kerr open pit supplements block cave mining from Year 25 to Year 34, and during these years, ore will be transported by an overland conveyor and rope conveyor system starting at the Kerr pit. Mitchell block cave is estimated to have a production ramp-up period of six years, steady state production at 20 Mt/a for 17 years, and then ramp-down production for another 7 years. Iron Cap is estimated to have a production ramp-up period of four years, steady state production at 15 Mt/a for 10 years, and then ramp-down production for another 9 years. The underground pre-production period would be six years, with first underground ore production from Mitchell and Iron Cap in Years 23 and 32, respectively.
The ore production rate is maintained at 130,000 t/d up to Year 35. At the introduction of underground mining at Mitchell in Year 23, and continuing through Iron Cap ore production commencing in Year 32, the open pit production is adjusted so that the combined open pit and underground production matches the maximum mill throughput.
The KSM pit designs are based on the digging reach of the large shovels (15 m operating bench) with double benching between high wall berms; therefore, the berms are separated vertically by 30 m. Single benching will be employed, if required, to maximize ore recovery and maintain the safety berm sequence as warranted.
Previous studies and similar projects in the area have shown that the lowest cost per tonne fleet of cable shovels and haul trucks that are currently being used for large hard rock open pit mines are the 100-t bucket class shovel matched with the 360-t truck. These sizes of units are proven in operating mines around the world. Diesel hydraulic shovels (85 t bucket class) are added to the fleet when a more mobile loading unit is needed. Suitable drill sizes (311 mm hole size) are chosen to match this size of truck/shovel fleet.
The Iron Cap deposit appears to be composed of strong, moderately fractured rock. Rock quality variations are most commonly attributed to variations in fracture frequency, as the strength of the rock mass does not vary significantly within the deposit. The fracture frequency is higher for Iron Cap than for the Mitchell deposit, resulting in a corresponding lower predicted median in situ block size of 2.5 m3, as compared to the Mitchell deposit. A significant proportion of the rock at the Mitchell deposit is predicted to have block sizes greater than 6 m3.