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.

Vein mineralogy depends on the location of veins relative to the Eureka VolcanicSubvolcanic Complex. Veins within the Complex (Eureka, San Marcos and the Marianas) contain higher silver and gold grades, and the Eureka veins contain abundant adularia and ginguro-style banding. Veins outside the dome and hosted by the Cerro Negro Ignimbrite (Bajo Negro and Vein Zone) contain lower silver grades, coarse pyrite rather than fine sulphides in ginguro bands, and a higher percentage of chalcedony and less adularia and carbonate in the gangue.

Vein textures typical of low-sulphidation epithermal systems include colloform and crustiform banding, cockade, and manganese/iron-oxide matrix breccias.

The Cerro Negro Operations host the Eureka, Mariana Norte, Mariana Central, San Marcos and Bajo Negro zones that are, or will be, exploited using underground mining methods, and the Vein Zone, to be mined via open pit methods. Currently, active mining is taking place at the Eureka and Mariana Central zones.

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.

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.

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.

The process encompasses crushing and grinding of the run-of-mine ore, agitated leaching, counter-current decantation, solution clarification, the Merrill-Crowe process (de-aeration and zinc precipitation) and smelting to produce gold/silver bars that are shipped to a refinery for further processing. The counter-current decantation tailings are washed to recover cyanide prior to being detoxified by the INCO process (SO2 and air) and pumped to the lined tailings storage facility. The plant commenced initial feed on July 14, 2014 and the first gold was poured on July 25, 2014. The Process Plant processes 4,000 metric tonnes per day.

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 clarif ........