Sherwood Copper’s Minto mine
Not many mines can be planning a move into the third phase of expansion, less than four months after reaching commercial production. That is exactly the case at the MINTO COPPER MINE of Vancouver’s Sherwood Copper, making the Yukon’s first new metal mine in a decade a success from the word “go”.
At the mine opening, which CMJ attended in October 2007,Sherwood president and CEO Stephen Quin joked about the project’s just-intime banking. And its just-in-time engineering that had Hatch working flat out.
Ian Bezins, general manager of the Minto project, said it best:”The Yukon is back in business, and we plan to be here for a long time.”
A long time is what it took to create the Minto mine. The discovery hole was drilled in 1971.The deposit that was eventually outlined, the main Minto deposit, boasts one of the highest open pit copper grades–2.1%–in the world. Over the life of the mine, before figuring the savings with Phase 3 of expansion, operating costs are expected to be a very reasonable US$0.81/lb. All this from a Phase 1 capital investment of Cdn$100 million.
Geologists have yet to arrive at a consensus as to the origin of the deposits at the Minto mine, and their theories are many. The only other deposit known to be similar is Williams Creek 50 km to the southeast. Both are thought to have some characteristics in common with iron oxide copper gold (IOCG) deposits.
The mineralization has been thoroughly studied. On the Sherwood website it is described as being “…in sulphide zones made up of chalcopyrite, bornite and minor pyrite with accessory magnetite. Gold and silver are present in association with the bornite. Gold occurs as free gold and the silver comes from the mineral hessite, a silver telluride. Copper oxide minerals, malachite and azurite, are also present. The mineralization is hosted by foliated granodiorite or gneiss.
“The deposit lies under the south-facing slope along the north side of Minto Creek. The deposit does not outcrop. The north part of the deposit is covered by thin overburden and up to 50 m of rock. The south part of the deposit is covered by up to 100 m of overburden, some of which is permafrost.”
Despite the fact that it took more than 25 years to outline the original Minto deposit, Sherwood is having great success exploring elsewhere on the property. The Area 2 deposit was discovered in 2006, and less than 20 months later, converted into a reserve that supports a 45% increase in mill throughput and a 43% increase in total project copper and gold production. Minable reserves, outlined to date and as demonstrated by a recent prefeasibility study, include 395 million lb of copper and 210,000 oz of gold.
The 2007 drilling program also discovered potential new ore sources at Area 118, Ridgetop and the Airstrip-Copper Keel, and new NI 43-101-compliant resource estimates are underway. If additional resources can be delineated at these targets, the Minto project will have a longer life. Within the next month, Sherwood will commence its 2008 exploration program to follow up on the exploration successes of 2007.
The Main and Area 2 deposits contain a total of 16.66 million tonnes (t) of measured and indicated resources averaging 1.54% Cu and 0.56 g/tAu. Included in that total are the proven and probable reserves that total 9.45 million t grading 1.90% Cu, 0.69 g/t Au and 7.54 g/tAg. There is a further 1.47 million t of inferred resources that grade 1.00% Cu and 0.32 g/t Au.
The Minto pit
The Minto orebody is flat-lying, approximately 335 m long in the north-south dimension and 248 m wide in the east-west dimension. The thickness varies considerably, averaging 30 m. Although the orebody does not outcrop, it lies close to the surface and has been developed as an open pit.
The pit is mined using conventional truck and loader methods. The stripping and mining contract was awarded to Pelly Construction. It is using a Caterpillar 385 backhoe, a Cat 992 loader and seven Cat 777 trucks to haul ore and waste. Blastholes are drilled with Ingersoll-Rand equipment with either 125-mm or 250-mm diameters. The holes are loaded with Anfo and/or emulsion depending on how much water is encountered. Blasts are initiated using a Dyno Nobel Nonel system.
Generally speaking, the pit is not too wet, said mine engineer Dan Russell.
The ore is trucked approximately 500 m from the pit and stockpiled according to grade. Three ore grades are segregated: material that is 1-2% Cu, that which is 2-4% Cu and highgrade material over 4% Cu. Earlier last year the ore was blended to create a head grade of 2-3% Cu, but as mining has progressed in the highgrade areas, a head grade of 4% Cu is being achieved. The ore is crushed to -150 mm in a Westpro jaw unit.
The Minto mill
Originally designed for a throughput of 1,563 t/d, the Phase 2 mill expansion brought the tonnage up to 2,400 t/d. Blended and crushed ore travels over a live feeder to a Marcy 9.75-m-diameter by 3.80-m semiautogenous (SAG) mill. The slurry is cycloned and passed though a pair of secondary Marcy 1.52-mdiameter by 3.05-m ball mills. Slurry from the ball mills joins the SAG mill output to be recycloned, and the cyclone overflow feeds the flotation circuit.
The mill has a bank of three roughers and another bank of three scavengers. All six are 14-m3 units. Underflow from the scavenger reports to a bank for four cleaner cells and then to a pair of recleaners. The concentrate is thickened in a high-efficiency, 6.10-m-diameter thickener. All the flotation and thickening equipment was supplied by Westpro Machinery.
The final concentrate was being dried to approximately 9% moisture in a fully automated Larox ceramic disc filter, but underperformance of the Larox unit resulted in a change to IPM Lasta pressure filters for concentrates.
The result is a very clean concentrate grading over 35% Cu plus gold and silver, said mill superintendent Dave Archibald.
Up to two months’worth of concentrate output is stored in a covered warehouse adjacent to the mill. The concentrates are then transported 400 km by road to the port of Skagway, Alaska, for shipping to smelters in Asia.
Tails are filtered with four IPM Lasta pressure filters before being dry-stacked.
Knowing that the way forward is to grow, in mid-December 2007 Sherwood Copper released the results of its prefeasibility study, prepared by SRK Consulting (Canada) Ltd., for expanding the Minto operation to 3,500 t/d capacity. To meet the higher throughput, plans have been made for mining the Area 2 deposit when the Main pit is exhausted.
Several processing improvements are included in the Phase 3 expansion. There will be coarser initial grind and regrinding of rougher cell concentrates. A gravity gold recovery circuit may be installed. Up-front grades (meaning high-grade ore) will be maximized by rescheduling production in the pit. The project will be tied into the territorial power grid. The waste rock and tailings areas will be reviewed. New geotechnical analysis will allow the optimization of the Main pit slopes.
With an experienced mine evaluation and development crew already assembled, Sherwood Copper has recently acquired control of Western Keltic Mines and its high grade KUTCHO CREEK COPPER-ZINC DEPOSIT.
Based on results of the prefeasibility study done for that project, Sherwood is doubling its copper resources.
A September 2007 prefeasibility study prepared by Wardrop Engineering and commissioned by Western Keltic Mines forecast that an investment of roughly $300 million would be necessary to develop the deposits and a mill at Kutcho, 100 km east of Dease Lake, B. C. Both open pit and underground mining were contemplated in the study. In the three identified deposits, there is a total measured and indicated resource of 17.69 million t averaging 1.71% Cu, 2.36% Zn plus silver and gold, or 2.8
1% CuEq. Sherwood intends to re-engineer the Kutcho Creek project, much as it did with Minto, with the objective of developing a more robust, lower-cost mining operation, likely on a smaller scale than outlined in the prefeasibility study. Feasibility work to advance the project will commence shortly.
Quin calls Sherwood Copper “the little company that could.” Sounds like there could be another mine in its near future.