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 ab ........