Summary:
Collahuasi comprises two large copper-molybdenum porphyry-type deposits (Rosario and Ujina) with several peripheral vein deposits (Rosario Oeste and Rosario Sur). The Rosario deposit is the focus of current open cut mining operations.
Mineralisation at Rosario and Ujina is mainly associated with veins, especially in Rosario. These major Cu-Ag-As veins include the NNE trending Montezuma and La Grande vein systems and the NW-trending Rosario and Poderosa vein systems (Masterman, et al,. 2005). The two deposits show typical profile of a secondary enrichment process: a leach cap, a copper oxides zone, a secondary enrichment blanket and a primary sulphide core. To the west of Rosario is located Capella, which is an exotic copper deposit with copper oxides associated to gravels, probably derived from the Rosario porphyry.
The Rosario deposit is characterized by a dome-shaped zone of copper mineralization centred on the Rosario and Collahuasi porphyries. The centre of the mineralised zone contains bornite, chalcopyrite, and primary chalcocite and generally lacks pyrite. Copper mineralization occurs as both disseminations and fracture- controlled veinlets. The deposit contains a thin, erratically developed secondary enrichment blanket underlain by a relatively high-grade primary chalcopyrite zone. The lack of a well-developed secondary enrichment blanket is due to both presence of structures and level of oxidization. In addition, the secondary zone is offset and developed along crosscutting faults, resulting in erratic and locally deep oxidization. Oxide mineralisation is concentrated mostly in the northeast portion of the deposit. The oxide consists of chrysocolla, malachite and brochantite in a strongly limonitic matrix. Cuprite, tenorite and native Cu are also locally present.
At Ujina the hypogene mineralization is spatially related to the Ujina porphyry. It s associated to a low sulphide core with chalcopyrite and pyrite, that grades outward to a pyritic shell. The high grade zones correspond to the contact between Ujina and Collahuasi porphyries. Deep oxidization has produced significant tonnages of secondary enriched copper sulphide and oxide minerals that overlie the primary chalcopyrite material.
Molybdenite is found within the primary hypogene mineralisation zone, peripheral to the Rosario porphyry. Molybdenum is enriched at depth, where grades are above 0.03% Mo. Molybdenum mineralization is predominantly restricted to within quartz veins with no significant alteration halo.
Mineralisation at Rosario Oeste denotes two major episodes of hypogene mineralization both responsible for extensive volumes of rock with copper and molybdenum mineralization type "porphyry copper". These events are represented by high sulphidation "veins" with copper mineralization - (arsenic (Silver - Gold). The spatial distribution of the mineralisation is controlled by movement of structural blocks. Events of secondary enrichment occur along structures and lithological horizons.
In Capella Sur, the gravels thicken to 30 to 40 metres depth and are intercalated with clay horizons rich in copper and manganese. Copper wad predominates in this mineralised body and only minor chrysocolla is found. Mineralisation at Capella Sur is lower grade and extends deeper into the basement rocks.
At Capella Este the copper mineralised zones deepen into basement rock forming a more voluminous mineralized zone with dimensions of 550 by 550 metres at depths between 45 and 120 metres below surface. The host rocks include acid volcanics as well as volcaniclastic and calcareous sedimentary sequences. Mineralisation occurs as chrysocolla, copper wad with some minor chalcocite and cuprite. It is unclear whether the Capella Este deposit is of exotic origin or whether it represents in-situ oxidation of weakly enriched sulphides.
The Rosario deposit is hosted within the La Grande unit, which formed the lower sequence of the Collahuasi formation. The La Grande unit is approximately 2,700 m thick and comprises of interbedded rhyolite, rhyodacite, dacite and andesite. The unit strikes NW and dips from 20 to 45 degrees NE. Andesites thin to the southwest across the Rosario deposit and the sequence become dominated by rhyolite. Volcaniclastic rocks and limestones of the Capella Unit, approximately 1,700 m thick, overlie the La Grande Unit on the northeaster side of the Rosario deposit. The rhyolitic Condor Unit outcrops to the west of the Rosario deposit. Both the Capella and Condor units are considered to have Triassic or Jurassic ages (Munchmeyer, et al, 1984). Emplacement of the porphyries appears to have been controlled by the Rosario fault system resulting in a north-westerly trend to both porphyries. The Rosario fault system also acted as a control to the overprinting highsulphidation, massive Cu-Ag vein, which crosscuts the porphyry mineralisation at Rosario
Two major porphyries intrude Rosario, the Collahuasi porphyry and the Rosario porphyry. Collahuasi porphyry has a granodioritic composition, and is between 50 to 300 m wide and up to 100 m long. Masterman (2003) suggests that this porphyry is Permian, with a U-Pb age of 245 Ma ± 12 Ma. Intruding the Collahuasi porphyry is the Rosario porphyry, which is 300 to 500 m wide and up to 1,500 m long. The Rosario porphyry has a 40Ar/39Ar age of 34.4 Ma ± 0.3 Ma (Masterman, 2004). The Rosario porphyry is thought to be the centre of hydrothermal alteration and hypogene mineralisation at Rosario. Third porphyry at Rosario is the dacitic Ines porphyry, although it is interpreted as a pre-mineralisation intrusion at Rosario yet it also contains the presence of hypogene mineralisation (Masterman, 2004).
The emplacement of the porphyries appears to have been influenced by the Rosario fault system thus implied a NW-trending to both porphyries. Furthermore, the Rosario fault system also acted as a control to the overprinting high sulphidation Cu-Ag massive veins, which crosscut the porphyry mineralisation at Rosario.
At Capella Sur, the gravels thicken to 30 to 40 metres depth and are intercalated with clay horizons rich in copper and manganese. Copper wad predominates in this mineralised body and only minor crysocolla is found. Mineralisation at Capella Sur is lower grade and extends deeper into the basement rocks.
At Capella Este the copper mineralised zones deepen into basement rock forming a more voluminous mineralized zone with dimensions of 550 by 550 metres at depths between 45 and 120 metres below surface. The host rocks include acid volcanics as well as volcaniclastic and calcareous sedimentary sequences. Mineralisation occurs as chrysocolla, copper wad with some minor chalcocite and cuprite. It is unclear whether the Capella Este deposit is of exotic origin or whether it represents in-situ oxidation of weakly enriched sulphides.