Summary:
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.
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.
Silver minerals are dominantly proustite (+/- pyrargyrite), silver sulfide (acanthite), lesser amounts of other silver sulfosalts (stephanite, polybasite and silver-bearing tetrahedrite) and minor native silver. Gold minerals include electrum and native gold. These minerals and other sulfides occur as aggregates of abundant finely disseminated grains most commonly in chalcedony, and as grains interstitial to quartz in select bands, and as more isolated grains, especially with visible gold, throughout the vein in chalcedony/quartz. Aggregates of grains commonly consist of pyrite, acanthite, proustite, visible gold, ± galena, ± sphalerite, and ± chalcopyrite. Acanthite, proustite, and visible gold commonly are found together as disseminated aggregates exhibiting no, or rare, mutual contacts. In places, gold exhibits mutual contacts with acanthite and proustite.
The Escobal vein is divided into three primary zones.
East Zone:
- In the East Zone, the Escobal vein follows an east-west to N80E normal fault that dips variably (60- 75°) to the south and can be followed for at least 600 m on strike (807,200E to 807,800E).
- The East zone demonstrates a dilational jog similar in character and at roughly the same elevation as found in the Central Zone. The upper reaches of the East Zone vein follows south dipping fault for roughly 500 to 600 m to approximately the 1250 m elevation where it changes dip towards to vertical or steeply towards the north. This deeper extension remains partially open and untested down-dip to the north and laterally to the east and west.
- Widely-spaced step-out drilling in the deep eastern margin of the East Zone has identified mineralization that can be followed for 350 m along an east-west strike and to a 400 m depth. The “East Extension” comprises multiple zones of low-grade, moderate- width (two to 20 m) veins and stockwork zones with near vertical dips. Mineralization extends from 1400 to 1000 meter elevations and remains open to depth. The zone is capped by a 300-m-deep zone of un-mineralized narrow veining that was recognized in earlier drill campaigns.
- Geochemistry in the main East Zone is characterized by a gold-rich sector in the near surface oxide/mixed sulfide-oxide zone that abruptly changes at depth to silver-rich mineralization across the sulfide interface. Lead and zinc concentrations show a strong correlation to silver mineralization in the lower portion of the sulfide zone and increase with depth relative to silver. A gradational zoning pattern is observed with silver giving way to lead and then zinc with depth. The East Extension is characterized by high silver and relatively low grades of lead, zinc and gold.
Central Zone:
- The Central portion of the Escobal vein is the thickest part of the vein system. The zone extends 700 m on strike and covers a nearly 800 m vertical range, from outcrop at 1500 m elevation to the deepest drill intercept at 700 m elevation. The zone strikes east-west with the main portion of the vein dipping moderately (60-70°) to the north. Flexures in mineralization in the upper and lower reaches of the Central Zone are controlled by high-angle faults. The wide, moderately north-dipping main portion of the vein represents mineralization along the dilational jog, or tensional shears between two major normal faults.
- High-grade silver occurs throughout the Central Zone, the bulk of which forms a wide roughly horizontal zone related to the north-dipping dilational structure. The zone narrows towards the east and exhibits greatest vertical extent on its western margin where it abruptly terminates along a barren “gap zone” bounding the western Margarito area. Lead and zinc concentrations correlate extremely well with silver grades in the Central Zone, though silver grades are maintained at depth, contrary to the gradational Ag-Pb-Zn vertical zoning that is evident in the East Zone.
West Zone:
- The West mineralized zone is characterized by surficial gold occurrences that give way to a wide zone of silver, gold, and base metal rich vein stockwork at depth. The deeper “Margarito” mineralization is a discrete shoot as it is separated from the Central Zone and the upper gold zone by a 50 to 100 m wide barren “gap” in mineralization. The zone as currently modeled extends over a 350 meter strike length and spans +550 m vertically, raking down to the east. The top of mineralization is entirely preserved with significant grades commencing 250 m below the surface. The zone is partially open to depth, while the western margin is believed to be down-dropped further west along a normal basin-bounding fault, interpreted through markerbed offset.
- The West Zone follows a semi-arcuate trace with moderate north dips in the upper reaches of the zone giving way to steep, near vertical inclination at depth. The upper portion of the zone is characterized by high gold values in the mixed-oxide zone. The deeper Margarito shoot exhibits very wide (30-50 m) zones of stockworkveining with moderate silver grades and moderate-high gold grades throughout. Base metal values show a marked increase in the lower portion of the zone.