The March 2014 technical report on the Garrison property repeats a 2011 conceptual plan for an 11,300 tpd concentrator with four crushing stages, a 75 to 120 micrometre grind, pyrite separation and carbon-in-pulp, with no operating results to support recovery.
The Golden Bear Project, on the Garrison property in Garrison Township, Larder Lake Mining Division, Ontario, Canada, is the subject of a Technical Report by A.C.A. Howe International Limited for Northern Gold Mining Inc., report number 975, dated March 3, 2014. Section 17 was extracted from Howe's 2011 NI 43-101 technical report and preliminary economic assessment and repeated in the 2014 report. The report cautions that the 2011 PEA is based on a 2011 mineral resource estimate that has since been replaced by the 2012 Garrcon mineral resource update, and the PEA was not updated for the change. Available test work is limited to two small samples, and there are no operating results on the recoverability of the gold.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Mineable throughput | 4.1 | Mt/y | 11,300 tpd at 93% plant availability |
| Mill cut-off | 0.3 | g/t gold or more | Gravity and vat leaching |
| Heap leach grade range | 0.15 to 0.3 | g/t gold | 8,000 tpd for 9 months a year |
| Indicative mill grade | 0.9 | g/tonne | Test work suggests |
| Blast product | 19,300 | tpd | Crushing circuit feed |
| Crushing | 4 | stages | 4 inch, 2 inch, 1/4 inch and 10 mesh products |
| Stockpile | About 680,000 | tonnes | Two months of mill supply, about 5 ha |
| Liberation size | 75 | micrometers | Sulphide separation assumption |
| Initial grind | 120 | micrometers | Silica and carbonate stream |
| Sulphide stream | 12 | % of flow | About 1,360 tpd, reground and vat leached |
| Grinding media | 1.5 | kg per tonne | About 17 tonnes per day |
| Energy | About 10 | MW | Estimate |
| Water | 5 to 10 | tonnes per tonne of solids | 50,000 to 100,000 m3/day, essentially recycled |
Overview
The operating philosophy is to get the gold out of the rock as early in the process as possible. To do this, a standard gravity and cyanide circuit is proposed for rock with more than 0.3 g/tonne gold. Rock between 0.15 and 0.3 g/tonne would go to a heap leach facility. The company plans, subject to column test confirmation, to process all material between 0.15 and 0.3 g/t in the heap leach system and material at or above 0.3 g/t in the gravity and vat leaching process. The concentrator is sized to make a gold product, which could be a gravity product, a flotation product or gold bars, depending on options chosen.
Key Process Stages
Crushing and stockpiles
The crushing section assumes a feed similar to the blast fragmentation size analysis from Northern Gold, with 22% finer than 1/4 inch, 44% finer than 1 inch, 66% finer than 2 inch, 79% finer than 4 inch, 89% finer than 8 inch and 97% finer than 12 inch. The suggested circuit has four crushers, with the first and second at 16 hours a day and the fourth at 10.6 hours a day. Vertical shaft impact crushers were costed as single rotor impact crushers of 400 t/h, a 32 inch by 44 inch model, two stages of which are needed, and their use needs further investigation. Run-of-mine stockpiles are in three categories: the main mill feed at an average 0.9 g/tonne, a heap leach stockpile of rock between 0.15 and 0.30 g/tonne that can supplement mill feed, and a low grade stockpile of unknown grade.
Gravity and cyanide circuit
The circuit assumes crystals can be liberated at 75 micrometers, and that sulphides, gold-bearing silica and carbonates can be separated by gravity separators. Gold associated with sulphides is exposed on surfaces or in cracks at that size, so it can be leached. Essentially half of the crusher product is assumed to be ground, and the sulphide stream, about 12% of the flow, is reground and vat leached, as is the gravity or flotation underflow. Spirals are included for gravity separation, though a flotation circuit might have to replace them.
Basis of design
The design assumes a grind can be found that liberates the pyrite and exposes the gold enough for cyanidation, with a two-stage grind if the sizes differ. A pyrite separation stage is needed if gold is within pyrite as small particles that cyanide can reach, rather than locked in the sulphide lattice. The size of pyrite liberation must suit gravity techniques, or flotation might be needed. The pyrite is reground to expose gold particles, and the silica and carbonate stream is assumed to need no regrind, with enough gold recovery at 120 micrometers.
Downstream circuit
The processing scenario includes run-of-mine stockpiles, crushing, fine and coarse ore stockpiles, heap leaching, loading and unloading of gold onto carbon, carbon regeneration, cyanide recycling and destruction, electrowinning and gold bar production. Grinding uses ball mills in closed circuit, with consumables of steel media, activated carbon and cyanide. Hydrocyclone overflow would be thickened to reduce percent solids for leaching.
Additional Interesting Data and Summary
The consumable costs in the report are estimates only, since pumps have not been priced or sized, the mill size and grinding media consumption have not been determined, and chemical consumption rates are not known. The report states that many other configurations could result depending on future test work, and tailings were not included in the processing analysis.
Key Processes
- Four-stage crushing with impact crushers
- Run-of-mine stockpiles by grade
- Gravity separation of sulphides
- Ball mill grinding to about 120 micrometers
- Pyrite regrinding and vat cyanidation
- Heap leaching of low grade rock
- Carbon recovery, electrowinning and gold bars
Source: Technical Report on the Golden Bear Project, Garrison Property, Larder Lake Mining Division, Garrison Township, Ontario, Canada, March 3, 2014.
