The deposits within the Cerro Negro Operations are considered to be examples of lowsulphidation, epithermal gold–silver deposits.

Mineralization characteristically comprises pyrite, electrum, gold, silver, and argentite. Other minerals can include chalcopyrite, sphalerite, galena, tetrahedrite, and silver sulphosalt and/or selenide minerals. In alkalic host rocks, tellurides, roscoelite, and fluorite may be abundant, with lesser molybdenite as an accessory mineral.

Deposits are typically zoned vertically over about a 250 m to 350 m interval, from a base metal poor, gold–silver-rich top to a relatively silver-rich base metal zone and an underlying base metal-rich zone grading at depth into a sparse base metal, pyritic zone. From surface to depth, metal zones grade from gold–silver–arsenic–antimony–mercuryrich zones to gold–silver–lead–zinc–copper-rich zones, to basal silver–lead–zinc-rich zones.

Silicification is the most common alteration type with multiple generations of quartz and chalcedony, which are typically accompanied by adularia and calcite. Pervasive silicification in vein envelopes is flanked by sericite–illite–kaolinite assemblages. Kaolinite illite–montmorillonite ± smectite (intermediate argillic alteration) can form adjacent to veins; kaolinite–alunite (advanced argillic alteration) may form along the tops of mineralized zones. Propylitic alteration dominates at depth and along the deposit margins.

A combination of transverse and longitudinal long-hole sublevel stoping methods with cemented rock backfill are currently being used at the Eureka and Mariana Central mines. The determination of which method is used is made based on geometries and the rock quality. Generally, transverse stoping is used in wider ore zones and areas where high grades along the contact require parallel drilling. Longitudinal stoping is used whenever practical in narrower zones to reduce development requirements. In certain areas, a modified Avoca mining method is used, which constitutes a longitudinal long-hole method with a rolling backfill front following the mining of ore in a single direction along strike. These methods are planned to be used to mine the other underground deposits at Cerro Negro Mine.

Ore extraction is carried out by load–haul–dump vehicles with capacities ranging from 4.0 cubic metres to 5.4 cubic metres. These units muck the ore from the stopes and haul it to a temporary stockpile located in the haulage drift or directly to trucks. Then the ore is loaded into trucks with capacities between 33 tonnes and 40 tonnes to be hauled to the surface and dumped, depending on grade, into high-, medium-, low- or marginal-grade stockpiles. The ore is then transported to the plant in haul trucks with capacities of 35 tonnes to 40 tonnes.

The mine plan for the Vein Zone deposit considers a two-phase open pit operation, in order to allow quicker access to ore and a smoother ore flow and stripping ratio over the life of the pit. Both phases will be mined concurrently with the underground mines. The Vein Zone will be mined using standard open pit mining methods using drilling, blasting, loading and hauling operations at a scale suitable for selective ore mining.

Surface mining is not planned in 2018 and Cerro Negro Mine will continue to evaluate potential surface mining opportunities to provide a supplemental source of additional mill feed.

The mine plan includes maintaining a stockpile of ore on the run-of-mine pad near the crusher. At June 30, 2017, the surface ore stockpile consisted of approximately 38,000 tonnes.

Waste storage has been designed for Eureka, Baja Negro, Mariana Norte, Mariana Central and San Marcos. During backfilling, the waste stockpiles will be totally consumed. Waste from Vein Zone’s LOM will be stored in a single waste dump.

The flowsheet incorporates the following major process operations:
• Primary crushing with the product directly feeding the milling circuit via a covered coarse ore pile;
• Conventional closed circuit milling;
• Pre-leach thickening;
• Leaching;
• Counter-current decant solution washing;
• Pregnant solution clarification;
• Zinc precipitation and doré smelting
• Cyanide destruction and tailings disposal
• Fresh and reclaim water supply;
• Reagent preparation and distribution.

A gravity concentrator is installed on the cyclone underflow stream but it is not currently operating.

The plant commenced initial feed on July 5, 2014 and first gold was poured on July 25, 2014. The plant is expected to process 4,000 t/d once the mines have ramped-up to full production capacity. Due to the ramp-up, insufficient ore is currently available from the mining operations for this capacity, and the plant is currently operating at about 3,590 t/d.

Primary Crushing and Reclaim.
ROM rock is dumped from haul trucks or a front-end loader (FEL) through a 400 mm square-grid grizzly into a 60 t dump hopper. A 1,250 mm wide apron feeder is used to transfer ore from the dump hopper to the vibrating grizzly screen. The grizzly screens material finer than 120 mm, with the oversize reporting directly to the 1,250 x 950 mm single toggle jaw crusher. The jaw crusher is designed to operate with a closed side setting of 125 mm and produce a product with 80% passing (P80) 115 mm. The crusher operates in open circuit with the crushed product combining with the grizzly undersize, and the ensuing product is conveyed to the covered coarse ore pile.

The coarse ore pile has a live capacity of 4,000 t or approximately 24 hours of mill feed. Total capacity is 15,000 t. Two reclaim belt feeders at the base of the coarse ore pilereclaim ore for milling.

Grinding.
The reclaimed crushed ore feeds the mill at a controlled rate. Discharge from the ball mill gravitates through a trommel and into the cyclone sump pump. The trommel undersize slurry is pumped to the mill cyclone cluster, operating in closed circuit configuration with the ball mill. The trommel oversize is removed to a pebble storage where fines and cyanide solution are washed off prior to transfer to the stock pile by skid steer.

Barren solution from the cyanide recovery thickener circuit overflow is added to the cyclone feed sump pump to achieve the required cyclone feed pulp density. Cyclone underflow is directed to the ball mill feed hopper and the cyclone overflow flows by gravity to the leach feed vibrating trash screen.

The grinding circuit includes a single 6.1 m diameter by 10.1 m long (EGL) ball mill in closed circuit with cyclones.

A gravity concentrator is installed on the cyclone underflow stream but it is not currently operating.

Pre-Leach Thickening.
Cyclone overflow slurry gravitates to the leach feed trash screen. The trash screen underflow slurry gravitates to the 25 m diameter high rate pre-leach thickener along with pregnant solution from the counter-current decant (CCD) circuit. Diluted flocculant and barren solution are added to the feed box of the thickener to assist in solids settling and thickening. The flocculant addition rate is adjustable by a variable speed metering pump. The nominal underflow pulp density is 53% solids w/w. Thickener underflow is pumped to the leaching circuit. The pregnant solution overflowing the pre-leach thickener is pumped to the clarifier thickener.

Leaching.
Pre-leach thickener underflow slurry is pumped to the leaching circuit. Feed to the leach circuit is 53% solids w/w. Leaching of precious metals by cyanide occurs in a series of five 16.3 m diameter x 16.9 m high A-36 carbon steel-welded agitated leach tanks, with 3,200 m3 operating capacity, to provide a total leach residence time of 60 hours. Slurry exiting leach tank no. 5 flows by gravity to the CCD recovery circuit.

The CCD circuit recovers precious metals leached into solution via three-stage countercurrent thickener washing. Leached slurry gravitates to the feed box of the first CCD thickener from the leach circuit. The thickened underflow is pumped to the next CCD thickener where it is washed with recovered solution from the third CCD thickener. The CCD thickeners are 25 m diameter.

Thickened underflow slurry from the third CCD thickener is pumped to the cyanide recovery circuit. The wash solution flows counter-current to the solids flow, increasing in precious metal concentration as it proceeds to the second and first CCD thickener. The pregnant solution from the first CCD thickener is pumped to the pre-leach thickener.

Solution Clarification and Zinc Precipitation. Overflow from the pre-leach thickener is pumped to the 30 m diameter clarifier to further remove suspended solids. This solution, combined with flocculant and a recycle stream of clarifier underflow, feed the clarifier. The clarifier underflow is predominantly recycled to increase the solids density in the feedwell. The bed mass is maintained by periodically pumping the underflow to the pre-leach thickener.

Clarifier overflow gravitates to pump column and is pumped to the 1,100 m³ polishing filter circuit feed tank, (10 m diameter x 14 m high). Solution is then pumped from the tank to the polishing filter circuit. The majority of the remaining suspended solids in the clarified solution is removed by the three clarification pressure leaf filters. Diatomaceous earth is metered into the suction piping of the filter feed pumps to maintain filter cake porosity in the polishing filters. Diatomaceous earth is also used for pre-coating the filters prior to the introduction of clarified solution. The filter cake is discharged by an automatic backwash cleaning sequence to the clarification filter sludge sump, and subsequently pumped to the cyclone feed sump pump. The clarified pregnant solution flows from the clarification filter to the de-aeration tower. The de-aerated pregnant solution is then pumped by the variable speed precipitation filter feed pumps to the zinc precipitation filters. Zinc dust is metered from the zinc cone with lead nitrate to precipitate gold and silver from the de-aerated solution. This mix is added to the suction line on the precipitate filter feed pumps carrying the de-aerated solution being pumped through the precipitate filters.

Three recessed plate filter presses are used for the precipitate filtration duty. Precipitate containing the precious metals is emptied into trays and then loaded into the retort oven by electric forklift. Barren solution leaving the precipitate filters is stored in the filter discharge tank. Barren solution is then pumped from the tank to the barren solution tank.

After precipitate filtration, the filter cake is dried with high pressure air prior to discharge. Filter cake discharges from the filters into trays that are loaded by forklift into a mercury retort oven. The mercury is separated from the precipitate, then recovered and stored in sealed containers. To date, minor amounts of mercury have been detected and recovered in the retort oven. Once the mercury is removed from the calcine cake, the calcine is mixed with smelting fluxes and charged to the reverb furnace. The calcine is smelted and the doré is poured into ingots.

Slag from the smelt is crushed first on a jay crusher and then on a roll crusher. Precious metals entrained in the slag recovered by centrifugal concentrator. The concentrate is returned to the smelting furnace, while the centrifugal concentrator tails returned to the grinding circuit.