Summary:
The North Pit and South Pit zones, previously known as Ccalla and Azulccacca, respectively, comprise the Cotabambas deposit and are considered to be examples of porphyry copper gold deposits. The two host porphyries cover an area of about 2.5 km long and 1.8 km wide.
The deposit is hosted in the Andahuaylas–Yauri belt, which is dominated by the Andahuaylas–Yauri batholith which is exposed for approximately 300 km between the towns of Yauri in the southeast and Andahuaylas in the northwest, and Mesozoic to Early Cenozoic clastic and marine sediment sequences.
Mineralization
Mineralization occurs in hypogene, supergene enrichment and oxide zones. A well- developed leached cap hosts the oxide mineralization. Sulphide mineralization occurs below the base of the leached cap. This zonation is considered to be typical of porphyry-style copper and porphyry-style copper–gold deposits.
Hypogene Mineralization
Hypogene mineralization in the Project area has been intersected at depths from approximately 20 m from surface to depths of over 500 m from surface.
Hypogene copper–gold–silver mineralization is best developed with pyrite mineralization in quartz– sericite-altered quartz monzonite porphyry dykes running parallel to the north-north-east trending structural corridors at North Pit and South Pit. Mineralization occurs as disseminated chalcopyrite and pyrite, pyrite-chalcopyrite stringers or veinlets and quartz chalcopyrite pyrite veinlets. Local patches of hypogene mineralization are developed in diorite, peripheral to the quartz monzonite porphyry, where the north– northeast-trending structural system passes within 10 to 20 m of the diorite–porphyry contact. Chalcopyrite mineralization intensity decreases and disseminated pyrite mineralization increases distal to the higher-grade parts of the hypogene zone.
Sulphide mineralization consists of chalcopyrite and pyrite and gold grades are strongly correlated to copper grades in the hypogene zone. Some occurrences of bornite have been noted in deeper portions of the hypogene zones. Silver grades are not asstrongly correlated to copper grades asthey are to gold grades but are generally elevated where copper–gold mineralization is present.
Hypogene mineralization occurs as disseminated stringers and in the form of four different types of veinlets:
• A1: quartz, anhydrite, magnetite, chalcopyrite, and pyrite • A2: quartz, magnetite, chalcopyrite, pyrite
• B: quartz, chalcopyrite, molybdenite
• D: quartz, pyrite, galena, and sphalerite.
• Veinlet types A1 and A2 are part of the early potassic alteration phase. Veinlets of type B are part of a transitional phyllic alteration phase. Type D mineralization is interpreted to be part of that late alteration stage.
Supergene Sulphide Enrichment Zone Zones of high-grade chalcocite mineralization with lesser covellite and chalcopyrite occur at the top of the hypogene sulphide mineralization, and at the base of the leached cap. This type of mineralization is interpreted to be a zone of supergene enrichment that typically forms in porphyry copper deposits where low pH argillic and advanced argillic alteration at the top of the porphyry system leach primary copper mineralization above the paleo-water table and re-deposit it as chalcocite at the water-table surface.
Supergene zones occur at North Pit and South Pit and are characterized by high chalcocite content, correspondingly high cyanide-soluble copper assay grades and total copper grades that are generally >1%.
Oxide Mineralization
Copper–Gold Mineralization
Oxide mineralization occurs in the leached cap of the North Pit and South Pit deposits. The leached cap is characterized by abundant limonite, goethite and manganese wad, and a characteristic mottled orange, brown colour. Iron oxides and oxy-hydroxides replace pyrite, and oxide copper–gold mineralization occurs as patches of green copper oxides, typically chrysocolla, malachite and brochantite. Copper oxides occur as coatings on disseminated chalcopyrite grains and as fill in fractures and veinlets.
Lenses of oxide copper–gold mineralization having lateral extents of 100 to 200 m and thicknesses of 10 to 50 m have been mapped in outcrop and intersected in core drill holes. These lenses typically occur over hypogene and secondary sulphide mineralization; however, isolated drill hole intersections indicate that oxide copper–gold mineralization may also overlie low-grade hypogene mineralization. This could be indicative of possible remobilization of copper mineralization in the leached cap.
Oxide Gold Mineralization
High-solubility Oxides
This zone is differentiated by the presence of oxidized minerals such as chrysocolla, malachite, brochantite, with traces of iron oxides such as limonite, hematite and possibly tenorite. This zone is also related to a high solubility with the solubility ratio is higher than 0.5. Low-solubility Oxides This zone is differentiated by the presence of more iron oxides associated with hematite, goethite, limonite, possible tenorite, and in smaller proportion minerals like chrysocolla, malachite. This zone is also related to a low solubility with the solubility ratio is less than 0.5.
Skarn Mineralization
Mineralization at several of the skarn targets consists of massive magnetite, with semi-massive chalcopyrite and pyrite (Guaclle or NW Pit target); veinlets and disseminated chalcopyrite and pyrite (Chaupec target).