La Fortuna Gold Project — 2018 Technical Report

This report presents the proposed processing route and metallurgical assumptions for the La Fortuna project, using a purchased used plant and incorporating testwork-based ore sorting.

Report context

The 2018 Technical Report for the La Fortuna Gold Project documents a preliminary economic assessment (PEA) of a proposed processing facility. Minera Alamos has purchased a used 2,000 tonnes per day (t/d) processing facility (grinding/flotation/filtration) that has been used as the basis for the La Fortuna Project processing facilities. The size of the major equipment items allows for plant throughput to be increased from the currently assumed 1,100 t/d rate as the size of the project resource increases.

Processing route

Crushing and stockpiling

Portable crushing and screening equipment will be utilised at the mine site to process approximately 1,200 t daily. Crushing operations are planned as a two-stage circuit with a screen to separate the final product size material for grinding. Crushing will be performed on a nominal six days per week schedule. Coarse ore and fine (crushed) ore stockpiles are utilised to separate the mining and milling operation. The two-stage crushing reduces the rock from a maximum feed size of 400 mm down to 80% passing 18 mm. An external contractor will supply and operate the crushing equipment as well as the transfer of crushed coarse material to the stockpiles located at the mill site. All crushing and transfer equipment will be portable in nature and independently powered via diesel drives.

Grinding and gravity concentration

Crushed material is reclaimed from the stockpile on a seven-days-per-week basis at a rate of approximately 1,200 t/d by a front-end loader and is dumped into two parallel reclaim hoppers with feeders feeding to two parallel primary grinding mills. Rock is ground from particle size F80 of 18 mm down to P80 of approximately 300 microns. Discharges from the two ball mills are collected in a common pump box and then pumped to a centrifugal gravity concentrator for free gold recovery. The gravity concentrator tail slurry is pumped to a hydrocyclone for size classification. Cyclone underflow returns to the ball mills which are operated in a wet-overflow-closed-circuit arrangement with a circulating load. Final cyclone overflow with a particle size of 80% passing 250 microns flows by gravity to a flotation conditioning tank where it is mixed with reagents prior to feeding to the bulk flotation stage. Gravity concentrate from the centrifugal concentrators is regularly removed and pumped to gravity concentrate cyanide leach circuit for gold recovery. A semi-batch automatic centre discharge centrifugal concentrator has been assumed.

Bulk flotation

A bulk rougher flotation line is incorporated for the bulk flotation stage which produces a copper/gold/silver rougher concentrate. The slurry from the grinding circuit is pumped to a conditioning tank where lime and flotation reagents are added at pre-determined dosages to adjust pH and prepare for flotation. Mill process water is also added as required to adjust slurry density prior to flotation. Rougher concentrate is collected via concentrate launders to the concentrate pump box from where it is pumped to the concentrate regrinding area. Rougher tails are collected in a pump box and pumped to a bulk flotation tailings thickener for dewatering and filtration prior to being disposed to the tailings containment area. Overall, the mass recovery in the bulk flotation stage is estimated at 8-10% of mill feed with gold/silver recoveries of +95% (combined flotation and gravity recovery).

Regrinding and copper flotation

The bulk concentrate needs to be further reduced in size prior to copper flotation. Concentrate is pumped to a regrinding ball mill operating in closed circuit with an additional centrifugal gravity concentrator for gold recovery and a hydrocyclone for size classification. The size of the bulk concentrate particles is reduced from an F80 of approximately 250 microns to a P80 of less than 80 microns. Following regrinding the concentrate is pumped to a conditioning tank where it is mixed with further flotation reagents added at pre-set doses. The conditioned slurry then passes through a series of cleaner flotation cells where a copper/silver/gold concentrate is produced. Final concentrate flows through overflow launders into a pump box where it is pumped to a Cu concentrate thickener followed by a filter for solid/liquid separation. Wet filter cake is collected and loaded into trucks for transport to be sold. Final flotation tailings from the cleaner separation stages consists of a sulphide (primarily pyrite) containing some residual gold (4-5% of total feed to the mill). In the future consideration can be given to processing this material in a cyanide leaching circuit for additional gold/silver recovery.

Gravity concentrate leaching

Based on test results, the La Fortuna ore gravity concentrate does not require intensive cyanide leaching to recover contained gold. The leach flowsheet is designed to operate in a batch mode, using two tanks alternatively. Gravity concentrates from the primary and regrinding gravity concentrators are pumped to a decant tank in the gravity gold leaching area to drain excess water. Slurry from the decant tank is fed to one of two leach tanks. To start leach, fresh cyanide solution, milk of lime, as well as compressed air are added to the tank. When leaching completed after 24 hours, slurry is pumped to the carbon column circuit for gold recovery. In the future should a pyrite leach area be included this slurry can be pumped to that circuit where residue gold can be further recovered, and remaining cyanide can be consumed for pyrite leaching.

Precious metals recovery

Gold and silver are recovered from the pregnant leach solution through a series of columns filled with activated carbon. Solution is pumped to the first column in series where it flows up through the bed of carbon and then overflows from the top of the column to pass through the next column in series. The process repeats itself until the solution exits the final column, via a carbon safety screen catching fine carbon particles, to a barren overflow tank from where it is pumped to a cyanide process water tank for recycle. When carbon in a column reaches maximum loading capacity (typically containing +5 kg/t of combined gold/silver) this column is bypassed and taken off the circuit and the rich carbon in the column is removed. The carbon is loaded into porous polypropylene bulk bags that are hung from frames to allow the bulk of the liquid to drain so it can be captured and reused. Once no further free liquid remains, the bags of rich carbon are transported off site for final gold/silver recover.

Tailings containment

An allowance has been included for the installation of a filtered tailings system. Flotation tailings are pumped to a conventional thickening system with recovered water returned to the plant process water tank where it is combined with fresh water for plant requirements. Plans have been completed for the design of the tailings containment strategy which includes the installation of vacuum disk filtration equipment for the production of final solids that are then transported by truck to a nearby containment area where they are distributed and compacted. Additional studies are to be completed to look at potential improvements including pressure filtration, staged containment construction, and others.

Ore sorting

DEXTR (x-ray) ore sorting has been included in the overall project plans as a method to upgrade mid-grade (0.8-2.0 g/t Au) mineralised material from the mine (and future potential project resources). Testwork has demonstrated that sorting of this material at normal project crush sizes can recover +80% of the contained gold into a sorted concentrate with gold contents similar to the high grade (3.5-4.0 g/t Au) direct milling material from the mine. It is conservatively assumed that an ore sorting machine will be purchased and installed in Year 3 of mining operations to upgrade this material. During Years 1 and 2, the mined mid-grade material will be stockpiled for processing starting in Year 3. In the current operations plan, only 20% of the life of mine (LOM) contained gold ounces sent to the processing plant have been upgraded in this manner.

Key reported parameters

Technical qualifications

The report states that a simplified base case process was utilised for the La Fortuna PEA plant site. Overall gold recovery for the PEA study has been conservatively estimated at 90%. No final gold refining facilities are to be constructed at the Fortuna site, although this decision can be revisited in the future should site production rates increase. The flowsheet has assumed the potential to cyanide leach the flotation tail; however, this option is currently not likely to be economic as it contains less than 5% of the gold. This decision can be revisited in the future should the project resources expand and the economics surrounding the additional cyanidation stages be positive. The pyrite leach section is identified as being for future consideration only. Additional studies are to be completed regarding tailings containment improvements, including pressure filtration and staged containment construction.

Source: La Fortuna Gold Project , 2018 Technical Report, Recovery Methods section