The Escobal deposit formed in an intermediate-sulfidation epithermal quartz vein system of probable Upper Miocene to Lower Pliocene age. These deposits are commonly included in the low-sulfidation epithermal class of deposits. Distinguishing characteristics of an “intermediate sulfidation” environment include mineral assemblages indicating a sulfidation state between those of high and low sulfidation types, relatively high total sulfide content of 5 to 10 percent, low-iron “blond” sphalerite, presence of silver sulfosalts, and association with andesitic to dacitic volcanics. Magmatic- associated fluids are implied.

Epithermal deposits form as high-temperature mineralizing fluids rise along structural pathways and deposit quartz and precious- and base-metal minerals in open spaces in response to boiling, which is usually coincident to a release of pressure within the hydrothermal system. This quartz and metal deposition, followed by resealing of the system, is repeated over the life of the hydrothermal system resulting in crosscutting and overprinted breccia and vein textures. Typically, the largest and highest grade deposits are associated with long hydrothermal systems marked by complex overlapping veins.

These deposits are strongly structurally controlled. Mineralizing fluids are directed along structural pathways with high- grade “ore shoots” typically concentrated in open dilatant zones. These dilatant zones commonly form where inflections occur vertically and laterally along the vein.

Metal deposition and zoning in epithermal deposits are related to the level of boiling. Typically, precious metals deposit at or near the boiling level while base metals precipitate below. Boiling may occur at different levels as the hydrothermal system evolves producing an overprint of various episodes.

The Escobal deposit occurs in a similar geologic setting with host rocks, vein characteristics and mineralogy typical of other intermediate-sulfidation systems. Specific definitive features include banded, cockscomb, and drusy vein textures; massive, stockwork and breccia veins; intermediate argillic and quartz-sericite alteration; appreciable base- metal and silver-sulfosalt mineralogy, and associated arsenic and antimony.

Economic mineralization at Escobal comprises silver, gold, lead, and zinc hosted within quartz veins, stockwork zones and hydrothermal breccias. The deposit predominantly comprises sulfide mineralization. Silver, lead, and zinc sulfide mineralization predominates in the Central and West zones though elevated gold values also occur at depth in the Central Zone. In the East Zone, gold-rich mineralization is associated with the upper mixed sulfide-oxide horizon. Silver mineralization in all zones shows a close association with galena and low-iron sphalerite.

A petrographic study of vein samples indicated a fairly simple and consistent paragenesis. Stage I veining consists of banded to massive chalcedony intercalated with quartz and carbonate. This is the volumetrically-dominant vein event and contains the bulk of sulfide minerals. Volumetrically lesser Stage II consists of sulfide-bearing granular chalcedony. Various episodes of post-sulfide quartz, and late barren calcite veining locally cut and/or overprint the main banded vein.

Active mining areas in the Escobal mine are accessed through two main portals, called the East and West portals. These two primary declines provide access to the Central Zone. A third primary ramp is being driven into the East Zone from the Central Zone. Access ramps are driven from the main ramp system to establish sublevel footwall laterals driven parallel to the vein in transverse mining areas on 25 m vertical intervals, with stopes accessed from the footwall laterals. In longitudinal mining area, development is done on vein also on 25 m vertical intervals.

Production from the Escobal mine is achieved via longhole stoping methods. Two variations of this mining method are utilized. Where the vein dimension across the strike is less than 15 m, longitudinal longhole stoping is applied. This method consists of driving horizontal drifts, spaced 25 m vertically, along the strike of the vein and then blasting the ore vertically from the upper level (“over-cut”) to the lower level (“under-cut”).

Breaking slots are established at the extreme ends of the stope to provide a void space for production blasting. These breaking slots are excavated utilizing Cubex drills equipped with V-30 blind bore reaming heads to bore a 30-inch diameter raise between the upper-cut and under cut for each stope. Once the breaking slots are complete, the stope faces retreat towards the accesses by drilling holes between the over-cut and under-cut, charging the holes with explosives, and blasting a ring or row of holes at the end of the stope.

This process continues until the maximum hydraulic radius or design limit of the opening along strike is reached, at which time longhole mining ceases and the void is filled with paste backfill.

In areas where the horizontal vein width exceeds 15 m, measured perpendicular to the strike of the vein, stopes are developed perpendicular to the strike of the vein. This is commonly known as transverse longhole stoping. In this case, 5 m wide by 5 m high footwall laterals are developed approximately 20 m to the south of and parallel to the vein. Access to the footwall laterals is from the primary declines. Five (5) m wide by 5 m high over-cut and under-cut drifts are developed from the south side of the vein to the north side of the vein spaced 25 m vertically. The south side of the vein is typically the footwall but due to local variations in vein geometry can sometimes become the hanging wall.

In the same manner as the longitudinal stopes, breaking slots are established at the extreme ends of the stope to provide a void space for production blasting. Once the breaking slots are complete, the stope faces retreat towards the accesses by drilling holes between the over- cut and under-cut, charging the holes with explosives, and blasting a ring or row of holes at the end of the stope.

The transverse mining method allows for multiple stopes to be in production along strike simultaneously on any given sublevel. Stopes along strike are split into primary stopes and secondary stopes. Each primary stope is separated along strike by a secondary stope.

Primary Crushing
Ore from the mine is trucked to the surface and dumped on an ore stockpile proximal to the primary crusher. The ore is picked up by a front-end loader and dumped into the crusher feed hopper, where it passes through a stationary grizzly to a variable speed vibrating grizzly for initial separation, with oversized material going to the primary crusher and undersized material by-passing the primary crusher.

The stationary grizzly retains ore larger than 600 mm as oversize. The oversize material is broken with a pedestal-mounted Tramac Model TX62 hydraulic rock breaker until it passes through the grizzly and drops into a 100-t total capacity (50-t live capacity) primary crusher feed hopper. The ore is then drawn from the hopper through a chute onto a vibrating grizzly feeder that separates the undersize minus 100 mm ore from the coarse ore feed to the crusher. The undersize ore falls directly from the vibrating grizzly onto the secondary screen feed conveyor. The vibrating grizzly feeder is a Telsmith Model VGF SD which is equipped with a model 280HF vibration unit driven by a 37-kW electric motor and variable-frequency drive (VFD). The maximum capacity of the feeder is 430 dry metric tonnes per hour (dmt/h).

Material from the vibrating grizzly feeder drops into the primary crusher feed chute that feeds directly into the primary crusher. The ore is crushed to 80 percent minus 150 mm and then discharged onto the secondary screen feed conveyor via the jaw crusher discharge chute. The primary crusher is a Telsmith Model 3055 Standard single-toggle jaw crusher, 9.50 m x 1.25 m in size, which is driven a 160-kW electric motor with a flywheel diameter of 1,370 mm. The crusher feed opening is 762 mm by 1,397 mm. The design maximum operating capacity of the crusher is 430 dmt/h of oversize ore. The process design for the nominal feed rate to the primary crusher is 219 dmt/h.

Secondary and Tertiary Crushing
Secondary/tertiary crushing reduces the ore to minus 9 mm as feed to the ball mill. Ore coming from the primary crusher first passes a triple deck secondary screen with oversized material going to the secondary 1.73 m diameter, HP400 cone crusher. Undersized material from the screen and crushed material are fed to a triple deck screen, with oversized material going to the tertiary 1.47 m diameter HP300 cone crusher. Undersized material and crushed material exit the tertiary crushing area and is transported to one of two 2,000 t fine ore storage bins.

The secondary screen is a Telsmith Triple-Deck Vibro-King TL Model 6x20 Dual TL26 unit, powered by two 18.8-kW electric motors, that also serves as the feeder for the secondary crusher. Each deck is 1,829 mm by 6,096 mm, with each deck passing progressively finer material. The bottom deck passes minus 10 mm undersize ore, which then drops onto the fine ore transfer conveyor. The design nominal feed rate to the screen is 219 dmt/h of ore with an average particle size of 80 percent minus 150 mm. The bottom deck undersize to the fine ore transfer conveyor is approximately 46 dmt/h, while the oversize feed from all decks to the secondary crusher is about 235 dmt/h. The screen panel openings are selected from operating experience to optimize the throughput of the target size and tonnage to the fine ore bins.

The secondary crusher is a Telsmith Model 44SBS cone crusher driven with V-belts by a 225-kW electric motor. The design nominal feed averages 183 dmt/h. The crushed ore discharged from the secondary cone crusher is conveyed to the tertiary screens on the crusher discharge and tertiary screen feed conveyors at a nominal process design rate of 183 dmt/h of ore (of which 80 percent is minus 50 mm). The secondary crusher discharge is combined with 206 dmt/h of ore (of which 80 percent is minus 9 mm) from the two tertiary crushers, which then discharges onto the same conveyor to be returned to the tertiary screens.

The combined crushed ore is then delivered through tertiary screen feed bins that distribute the ore onto the two tertiary screen feeder belts. The feeder belts then discharge the ore onto each of the two tertiary screens. The crushed ore is weighed on the tertiary screen feed conveyor using a belt scale. The weight of crushed ore passing along the conveyor is measured to inform the operator of the actual crushing rate and then totalized for metallurgical accounting. The ore is fed from the bins directly onto two tertiary screen feeder belts. Each bin has a live capacity of 50 t and are equipped with slide gates to shut off the flow of ore to either or both tertiary screen belts. The ore then flows onto two tertiary screens which split the feed into two outlets to choke feed ore into the two tertiary crushers. The feeders each have a variable-speed drive with 22.4-kW motors. The two tertiary screens are triple-deck inclined vibrating screens, similar to the secondary screens described above. The size of the openings on the screen media on each deck is smaller than on the secondary screen, resulting in the bottom deck passing 80 percent minus 9 mm undersize ore. The undersize ore is the final crushing plant product. This product drops onto the fine ore transfer conveyor. The tertiary screens also serve as feeders for the tertiary crushers.

The final step in the secondary and tertiary crushing system is the crushing of the oversize ore from the tertiary screens. Each tertiary screen discharges the oversize ore into its corresponding tertiary crusher. There are two Telsmith Model 44SBS tertiary cone crushers, each driven with V-belts by 225-kW motors. The process design is such that each tertiary cone crusher processes 103 dmt/h of feed, which is 80 percent minus 50 mm; the two crushers deliver a total of 206 dmt/h of 80 percent minus 9 mm ore to the crusher discharge conveyor. The maximum capacity of each crusher is 430 dmt/h.

Conveyance
There are six conveyors serving the secondary and tertiary crushing system—the crusher discharge conveyor, the tertiary screen feed conveyor, the fine ore transfer conveyor, the reversible conveyor, the fine ore bin feed conveyor, and the fine ore bin reversible conveyor. All of these conveyors are standard belt conveyors with the same basic components.

Grinding
Crushed ore is fed to the grinding circuit from the two fine ore bins. Two variable speed belt feeders are located under each fine ore bin. Ore is withdrawn from the fine ore bins by these belt feeders and the ore is discharged onto the mill feed conveyor. The mill feed conveyor delivers the crushed ore to the ball mill for grinding. In addition to the fine ore, the ball mill receives grinding balls (generally 3- to 4.5-inch diameter) which grind the ore as the mill rotates. Process water is added to form a slurry in the mill. Zinc cyanide and zinc sulfate are added to the mill feed chute to prepare the ground ore for flotation.

A ball feeder meters the grinding balls at a rate of up to 15 t/d. The ball storage hopper has a 10-t capacity that is filled twice per day, once per 12-hour shift. The Escobal ball mill operates with a maximum ball charge of 30 percent total volume.

The mill is driven by a single 3,357-kW fixed-speed synchronous motor having a nominal speed of 200 rpm. The ball mill design throughput rate is 203.8 t/h solids, which is sufficiently designed for the increased throughput of 4,500 tonnes per day (187.5 t/h). The grinding circuit was engineered and sized to reduce the ore to 80 percent passing 105 microns (P80105). The only modification to the existing ball mill components necessary to accommodate the expanded throughput is replacement of the existing pinion gear with a larger gear, which the company holds in its current inventory.

Ore from the Escobal mine is treated using conventional differential flotation processes producing lead concentrates with high precious metal (silver+gold) grades and zinc concentrates with a lesser precious metal component.

The original design basis for the processing facility is 3,500 tonnes of ore per day (t/d) or 1.28 million tonnes per year; though the installed crushing, grinding, flotation and concentrate processing components were sized for the contemplated increased throughput rate of 4,500 t/d.

The following items summarize the process operations required to extract silver, gold, lead and zinc from the Escobal ore to create lead and zinc concentrates:
- Primary Crushing, Secondary and Tertiary Crushing.
- Grinding – grinding crushed material in a ball mill circuit to a size suitable for processing in a flotation circuit.
- Flotation – the flotation plant consists of selective lead and zinc flotation circuits. The ore slurry from the bal ........