Back Forty mine is expected to produce 73 million pounds of zinc per year.
Back Forty is a gold-zinc-rich volcanogenic massive sulfide (VMS) property. Image courtesy of James St. John.

Back Forty is a poly-metallic volcanogenic massive sulfide (VMS) project being developed by Aquila Resources, a company based in Canada. The property is located in the Menominee County, approximately 19km west of Stephenson, Michigan, US.

The project received all state and Federal permissions for its construction, by June 2018.

The feasibility study for the project was completed in August 2018, which confirmed the economic viability of an open-pit mining operation for seven years, following 24 months of construction phase with an estimated initial investment of $296m.

The project is expected to produce 67,000 ounces (oz) of gold, 73 million pounds (Mlbs) of zinc, 7.3Mlbs of copper, 637,000oz of silver and 3.45Mlbs of lead a year.

Back Forty mine development history

Originally discovered in 2002, the zinc and gold-rich sulfide deposit sprawling over 1,183 acres is currently 100% owned by Aquila Resources.

Aquila and various joint venture partners, including Hudbay Minerals, had invested more than $90m to explore and advance the Back Forty project.

The preliminary economic assessment (PEA) for the project was completed in 2014, which contemplated a 16-year mine life involving open-pit as well as underground operations.

Aquila updated the mineral resource estimate for Back Forty in February 2018, incorporating data from 22 additional diamond drill holes completed between 2015 and 2017.

The property was explored with a total of 741 drill holes aggregating 128,670m of drilling since its discovery. 

Back Forty geology and ore reserves

Back Forty poly-metallic VMS deposit  is  hosted within the Penokean Volcanic Belt (PVB), which stretches from the upper peninsula of Michigan to the western portion of north central Wisconsin.

The Back Forty deposit comprises three zones of massive sulphide mineralisation hosted within rhyolitic rocks, namely the Main Zone, Tuff Zone and the Deep Zone. The Main Zone is further composed of three separate massive sulphide bodies, namely the East, Hinge and the South Limb zones.

Back Forty mine was estimated to hold 8.12 million tonnes (Mt) of proven ore reserves grading 1.95g/t of gold, 18.4g/t of silver, 3.02% of zinc, 0.13% of lead and 0.35% of copper, as of February 2018.

Probable ore reserves were estimated to be 3.53Mt grading 1.63g/t of gold, 29.3g/t of silver, 1.76% of zinc, 0.41% of lead and 0.10% of copper.

Mining method for Back Forty

Back Forty will be a conventional open-pit mining operation involving drilling, blasting, loading and hauling.

Blast hole drills, hydraulic excavators, 91t haul trucks and other support equipments including dozers, graders and water trucks will be used for the mining operations.

Mining will commence in a small higher-grade starter pit and subsequently expand outwards by pushing back the pit walls in three phases.

The open-pit will finally measure 2,000ft-long, 2,500ft-wide and 750ft-deep.

Ore processing for Back Forty

The Back Forty project includes a hydrological cut-off wall, a 4,000tpd sulphide processing plant and 800tpd oxide processing plant for processing sulphide and oxide ore separately.

The sulphide ore will go through crushing, grinding, flotation, concentrate thickening and filtration for the production of zinc, copper and lead concentrates at the sulphide processing plant.

The oxide ore will undergo three-stage crushing, grinding, pre-leach thickening, cyanide leaching, counter current decantation (CCD) washing, de-aeration, zinc precipitation and mercury removal before being smelted for the production of dore at the oxide processing plant.

Both the processing plants will have separate tailings thickening facilities and a common tailings management facility (TMF).

Infrastructure facilities for Back Forty mine 

The mine site is accessible from Stephenson via the County G12 Road. Water requirement for the mining operation will be met through local groundwater sources as well as the groundwater inflow into the mine pit.

Back Forty mine will be supplied with electricity by a 138kV transmission line that passes through the north-east edge of the property.

Other infrastructure facilities will include waste rock storage facilities, water basins, and a state-of-the-art waste water treatment plant.

Contractors involved with Back Forty project

Tetra Tech carried out the preliminary economic assessment (PEA) for the project. P&E Mining Consultants prepared the updated mineral resource estimate and the technical report for the project in February 2018.

Lycopodium Minerals Canada compiled the feasibility study for the project.