The La Libertad Complex is held 100% by Calibre, through its Desarrollo Minero de Nicaragua, S. A. subsidiary.
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Summary:
The epithermal gold-silver deposits at La Libertad and Pavón can be assigned to the low sulphidation subclass of deposits.
La Libertad Mine
Gold mineralization at La Libertad is contained within multiple vein systems emplaced along zones of extensional dilation within the district scale fault network described above. The most productive vein systems in the district to date include the northeast trending Mojón – Crimea, Santa María – Esmeralda, San Francisco – Los Angeles, San Juan – Mestiza systems, and the east-west trending Jabalí and Santo Domingo vein systems. The following provides a generalized description of mineralization in the Jabalí and Mojon – Crimea vein systems which have been the principal sources of production at La Libertad during the past 15 years.
The Jabalí vein system occurs along an east-west trending fault zone that has been traced on surface over a distance of more than six kilometres. To date, exploration drilling has tested more than 3,950 m of the Jabalí vein system. The vein system dips to the north, varying from 60° to 80° north. Mineral Resources have been developed in the near surface Jabalí Antena open pit and Jabalí West underground mines, which are the primary sources of current production at La Libertad.
Gold mineralization occurs primarily as electrum in association with pyrite and lesser sphalerite within massive to banded quartz veins, vein stockworks and localized breccias developed along the east-west trending mineralized structure. Quartz veins consist of milky white to light grey quartz with minor amounts of adularia. Epithermal textures comprise crustiform and colloform banding, vuggy and drusy quartz, cockscomb, and bladed silica pseudomorphs after low temperature calcite.
The vein structure is commonly oxidized up to 60 m below surface. Gold values within this oxidized portion of the vein are commonly associated with increased limonite, jarosite, and manganese oxides within vuggy textured quartz breccia veins. Sulphides are rare near surface due to moderate to strong oxidation within the structure.
The Mojón-Crimea trend is nearly four kilometres long, strikes 65°, and dips on average 80° to the southeast. The massive quartz veins and adjacent stockwork/stringer zones range in width from 2.0 m to 140 m averaging 30 m and often narrowing at depth. Numerous hanging wall vein splays are present that are generally narrower and less continuous than the main zone. They are oriented at 75° and have vertical to slightly north-northwest dips. As currently defined by exploration drilling, the down-dip dimension of gold mineralization along the Mojón-Crimea trend is on the order of 200 m to 250 m.
The sub-parallel steeply north dipping Santa Mariá-Esmeralda vein system is located approximately 500 m to the southwest and follows a secondary structure developed in the structural hanging wall to the Mojón-Crimea fault system. Veining along the trend is segmented, with the Santa Mariá and Esmeralda veins separated by approximately 1,000 m along strike. The Santa Mariá vein averages 10 m wide and is approximately 450 m long; the Esmeralda vein has been mined out.
Gold appears to be a late-stage phase in the mineralization history of the Libertad district, occurring as electrum in association with pyrite. Higher gold grades on the order of 1 g/t Au to >10 g/t Au are associated with vuggy, drusy, and discrete adularia-bearing bands within quartz veins. Gold grades in the stockwork zones are generally 0.1 g/t Au to 0.5 g/t Au with occasional spikey values. Pyrite and its oxidized products are closely related to gold mineralization but are present in small volumes, generally less than one percent.
Host rocks are moderately altered immediately adjacent to the stockwork and veining zones. Alteration types are typically silica and argillic with minor amounts of propylitic. Surface saprolite alteration is developed to a depth of approximately 15 m to 20 m.
Silicification is often intense within the vein zones. Partial silica replacement and rimming of breccia clasts is widespread throughout the veins and can extend into both the hanging wall and footwall. Host rock alteration is characterized by phyllic (illite) and argillic vein selvages along the main vein structures transitioning outward to pervasive propylitic alteration over a few tens of metres into the surrounding andesites.
Pavón
All the major veins identified on the Pavón concessions are hosted by intermediate to felsic rocks within the Matagalpa Group sequence. Rhyolite tuffs and flows overlying the sequence appear to be syn- to post-mineral and have been mapped regionally as part of the Lower Coyol Group.
Gold-silver mineralization at Pavón is hosted within quartz veins, and stockwork veinlets, and quartz vein breccia with textures and alteration assemblages typical of formation in a low sulphidation epithermal environment. Many of the veins display multiple stages of quartz deposition and both tectonic to hydrothermal brecciation. Brecciated veins are more common than massive fissure veins in the Pavón area.
Quartz vein textures vary both within individual veins, and between veins across the concession. Common quartz textures include granular (locally gray with fine grained disseminated pyrite), massive, and banded clear, grey, and blue chalcedonic. Coarsely crystalline or massive quartz, cockscomb, and cockade textures are less common suggesting most of the multi-phase quartz was deposited at lower epithermal temperatures (Hawksworth, 2005).
Adularia is an important vein component of the Pavón Norte, Pavón Central, and Pavón Sur deposits. It occurs as millimetre wide growth rims with banded massive, granular, or chalcedonic quartz, and locally as radiating crystals extending up to 1.0 cm into the quartz bands. Examination of drill logs shows a general correlation of gold with total quartz volume percent and adularia percent.
Sulphides within the quartz veins are rare. Pyrite occurs within grey silica/quartz that forms the late stage hydrothermal breccia matrix, which is generally the last vein event within the major structures. Trace amounts of base metal sulphide have been observed within select holes.