The long-awaited preliminary economic assessment for Osisko Mining’s Windfall deposit in Quebec, Canada, has not disappointed with strong economics and a surprising amount of detail about how the company will move ore and waste at the underground asset.
Windfall, in the James Bay region, looks like becoming a 3,200 tonne per day, long-hole mining operation producing some 218,000 ounces per year of gold-equivalent at an all-in sustaining cost of $704 per ounce over an eight-year mine life. Output would peak in year one at 248,000 oz.
Costing C$397.3 million to build, the project is expected to return a post-tax net present value (5% discount) of C$397.3 million using gold and silver prices of US$1,300 per ounce and US$17/oz, respectively.
The PEA, conducted by consulting firms BBA Inc, InnovExplo, Golder Associates, WSP Canada and SNC-Lavalin, outlined simultaneous exploitation of two separate deposits – Windfall and Osborne-Bell – to achieve the production rate, with both delivering ore to a central processing facility in Lebel-sur-Quévillon (115 km from Windfall and 17km from Osborne-Bell).
Both deposits are set to be mined by long-hole with longitudinal retreat and have mineralised ore zones going from surface down to 1,200 m depth (Windfall) and 520 m (Osborne-Bell).
The panels at Windfall will have minimum length-by-height-by-thickness dimensions of 20 m by 20 m by 3.5-4 m and will be extracted using a fleet of 14-t LHDs and 50-t underground haul trucks at a rate of 2,600 t/d using a ramp.
Mineralised material at Osborne-Bell is expected to be extracted using a fleet of 7 t LHDs and 45 t haul trucks using a ramp operating at 600 t/d.
Mineralised material from both deposits is expected to be transported to the process plant by contractor.
The central processing facility is to consist of a jaw crusher, mineralised material reclaim system and storage (2,100 t live capacity), primary SAG mill (22’ by 11’) and secondary ball mill (15’ by 27’) in close circuit with cyclones producing a product of P80= 45 microns.
“A portion of the cyclone underflow will be fed to the gravity circuit with intensive leach,” Osisko said, with cyclone overflow going onto an eight-tank CIL circuit with 40-hour retention time. This will be followed by cyanide destruction and tailings disposal.
A 6 t/d carbon processing ADR circuit and gold room are expected to recover the gold and produce doré, with payable gold and silver recovery averaging 92.4 % and 68.9 %, respectively.
“The plant also includes a reagent preparation area and two process water circuits (cyanide-bearing and cyanide-free) to service the entire plant,” Osisko said.
With such detail, it is easy to forget the project is only at the PEA stage and will be further refined as it moves to feasibility study stage.
John Burzynski, President and CEO of Osisko Mining, said as much: “This is a very strong start to a project that is growing as inferred resources (2.284 million ounces averaging 6.7 g/t Au, out of a total 2.885Moz indicated and inferred resources) are being converted to higher-grade indicated resources by the ongoing drill programme, with optimised mining methods and with the ongoing exploration success at Windfall.”
The company has recently discovered a new zone, Triple 8, outside of the current resource base at Windfall and has also not yet factored in the Bobcat Zone into this study. These areas are expected to be included in next year’s feasibility study work.
Osisko said last year it planned to carry out 800,000 m of drilling at Windfall, capturing the market’s attention.
