Summary:
Deposit Geology
The Sierra Gorda is controlled by a block of continuous concessions that cover an area of approximately 7.5 km by 5 km. The SGP is located within the Sierra Gorda mining district (about 15 km by 10 km extension) where numerous mineral occurrences have been identified in hydrothermal alteration zones with copper, molybdenum, and gold.
The Sierra Gorda Mining District is hosted by rocks of Early Cretaceous age consisting of a sequence of andesites. These rocks outcrop around the project’s district except to the west. These rocks occur as coarse-to-fine- grained porphyritic andesites and andesite flows with intercalated agglomerates, tuffs, sandstones, and siltstones. These rocks have been correlated to the Quebrada Seca Formation, which was dated as 111 ± 3 million years old.
The Sierra Gorda Mining District also encloses Paleocene intrusives represented by plutonic rocks of variable composition from monzodiorite, granodiorite to granite which are apparently related to the underlying Batholith.
Late stage intrusives also occur in the Sierra Gorda Mining District; these are represented by irregular stocks, dikes and small plugs intruding both, the batholith and the andesites.
Numerous breccia zones occur within the Sierra Gorda Mining District. The main Sierra Gorda mineral deposits are associated with breccia zones, such as the Catalina or Santiaguina, Salvadora, Olvidada, Copucha, and the newly discovered 281 and 285 zones.
The Sierra Gorda mineralization consists of copper-molybdenum-gold which appears to have been originated from hydrothermal events during the early part of the Tertiary age. The mineralization at Sierra Gorda is commonly associated to Paleocene intrusives and occurs in veins and disseminated within the andesitic rocks.
Mineralization.
The Sierra Gorda deposit consists of a copper- molybdenum-gold deposit occurring principally in dissemination and in association with quartz veining in stockwork, in fractures and faults and in breccia zones hosted by sedimentary rocks such as agglomerates, sandstones and siltstones which were originated from andesites, tuffs, and other volcanic rocks. The mineralization also occurs associated with intrusive rocks of variable composition including dikes, stocks and batholithic rocks.
The Sierra Gorda mineralization occurs as hypogene sulfides or primary mineralization, as leached oxides or secondary mineralization and as supergene or secondary enriched mineralization. The Sierra Gorda deposit is enclosed by a currently known area of about 350 hectares for the Catalina, Isabel, 281, 285 pit areas, and about 64 hectares for the Salvadora area with opened extensions.
Hypogene Sulfide Mineralization forms the bulk of the known mineralization, both in terms of volume, as well as contained metal. Hypogene copper sulfides consist dominantly of chalcopyrite, although bornite has been locally recognized as an accessory. Chalcopyrite- mineralized rock exists from below the leached zone to the current limits of drilling, more than 1,000 m below the surface. Hypogene molybdenite occurs in distinct bodies at Catalina and Salvadora; elsewhere it is weak to absent. Gold typically accompanies the copper sulfides.
The main hypogene component minerals are:
- Chalcopyrite (CuFeS2),
- Pyrite (FeS2),
- Molybdenite (MoS2), and
- Bornite (Cu5FeS4) in minor occurrences observed in deep drilling.
Leached/Oxide Zone is the product of in situ oxidation of the hypogene sulfides. The leached zone extends from the surface to variable depths of up to 200 m and is grouped into copper-rich zones and copper-leached or barren zones. Over large areas, the leached oxide zone still contains a significant portion of its original copper content. Hypogene molybdenite is typically oxidized to ferrimolybdite and powellite, but the primary metal distribution was apparently little affected by the oxidation.
The most important copper oxide minerals are the following:
- Atacamite (Cu2Cl(OH)3), about 70 percent of the copper in the leached zones. In andesites it may contain 30 to 50 percent of the copper.
- Brochantite (Cu4(SO4)(OH)6).
- Chrysocola ((Cu,Al)2H2Si2O5(OH)4.n(H2O)3).
- Vermiculite (Mg,Fe2+, Al)3(Al,Si)4O10(OH)2.4(H2O).
The oxide copper minerals occur mainly as fine- medium-grained fracture filling with sizes of 20 to 200 micros. They also appear as clusters bounded by silicates or inter-grown with silicates and goethite.
The molybdenum does not appear to migrate within the oxidation zone. Previous investigations noted the presence of up to 3.9 weight percent molybdenum in iron hydroxides. It occurs in oxides as:
- Lindgrenite (Cu3(Mo)4)2(OH)2),
- Powellite (CaMoO4), and
- Ferrimolybdenite (Fe3+ 2(MoO4)3.7(H2O)).
Supergene Copper Sulfide Zone is an irregular zone of secondary copper sulfide enrichment, dominated by the mineral chalcocite. It has a variable thickness of 10 m to 150 m and generally occurs at the boundary between the oxidized zone and the hypogene zone, but can exist well down into the hypogene zone or some distance up into the oxide zone depending on structural conditions.
The main minerals that comprise this zone are the following:
- Chalcocite (Cu2S) is the main mineral containing copper in this zone
- Covelite (CuS)
- Digenite (Cu9S5)
- Bornite ((Cu5Fe 2+ S4)
- Yarrowite (Cu9S8)
- Geerite (Cu8S5)
- Anilite (Cu1.75S).
Commonly the copper minerals appear as disseminations and in veinlets. The average grain size of the copper sulphides varies from 20 to 200 µm. In some instances, the supergene copper sulphides are found coating pyrite or chalcopyrite grains 1 to 90 µm in size.
The hypogene sulphide mineralization at Sierra Gorda is the product of multiple stages of mineralization hosted by meta-sedimentary, volcanic, and intrusive rocks. Chalcopyrite is found predominantly as disseminated grains bounded by silicates, with typical grain sizes of 30 to 70 µm. It also forms some massive fillings with dimensions of several millimeters in breccias.
Molybdenite is found mainly as grain clusters and disseminated grains bounded by silicates. The larger grains exceed 1,000 µm in diameter, while the disseminated grains are usually in 10 to 20 µm size range. Molybdenite generally occurs in early potassium silicate veinlets or in the matrix of some breccias, with quartz, pyrite, and/or chalcopyrite.
Oxidation and leaching of sulfide minerals have affected approximately the upper 10 to 200 m of the mineralized bodies in the Sierra Gorda district. Each of the deposits has a leached zone containing pockets of copper oxide minerals. The secondary copper oxides are most abundant in andesites and in the Catalina breccia zone.
The supergene zones at Sierra Gorda are irregular and discontinuous. The Catalina breccia zone has developed a 15 and 150 m thick supergene zone. In the central sector of the Catalina breccia a supergene zone was developed in apparent association with the Etna fault and its associated fractures which extends approximately 150 m long, by 80 m to 120 m wide, and 50 m to 150 m in depth. The associated leached zone exhibits abundant relict pyrite in granitic rocks.
There are two important breccia zones at the Salvadora area; the Olvidada breccia and the Salvadora breccia zones. The Salvadora breccia is hosted mainly by monzodiorite porphyry and it contains irregular pockets of oxide mineralization and minor supergene enrichment zones within an area of about 240 m long, by 60 m wide, and 100 m thick. The Olvidada breccia is hosted by syenogranite; it is rich in pyrite content, and its upper zone is completely leached. Beneath the leached zone is a supergene enrichment zone of about 600 m long, by 80 m wide, and 100 m to 140 m in thickness.