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.

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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 ........