Taseko Mines Ltd. owns 100% of the Gibraltar mine. The Company’s wholly-owned subsidiaries, Gibraltar Mines Ltd. and Cariboo, now hold 75% and 25% beneficial interests in the joint venture, respectively.
Initially, the Gibraltar mine was operated as an unincorporated joint venture, with Taseko’s Gibraltar Mines Ltd. holding a 75% interest and Cariboo Copper Corp. holding 25%. Cariboo was originally owned by a Japanese consortium (Sojitz 50%, Dowa 25%, Furukawa 25%).
In March 2023, Taseko acquired Sojitz’s 50% stake in Cariboo, increasing its effective ownership to 87.5%. In March 2024, Taseko acquired the remaining 50% of Cariboo from Dowa and Furukawa, consolidating full ownership of the Gibraltar mine.
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Summary:
Deposit Type
The Gibraltar open pit mine is a calc-alkalic porphyry copper-molybdenum deposit entirely hosted by the Late Triassic Granite Mountain batholith. This pluton occurs in a fault-bounded panel of Quesnel terrane rocks that are partially enveloped by rocks of the oceanic Cache Creek terrane (van Straaten et al., 2020). The oceanic Cache Creek terrane that occurs to the west and the Quesnel volcanic arc terrane are part of the accretionsubduction complex that was responsible for generating the Granite Mountain batholith and other Late Triassic plutons such as the Guichon Creek batholith that hosts the Highland Valley porphyry Cu-Mo deposits located 250 km to the south-southeast.
Mineralization in the Pollyanna, Granite, and Connector deposits is hosted in massive to strongly foliated zones. In contrast, mineralization in the Extension deposit is almost entirely hosted in strongly foliated shear zones. At Gibraltar mine, the Granite, Pollyanna and Connector deposits have been described as “porphyry-type ore”, the Extension deposit as “shear zone ore,” and Gibraltar as intermediate between the two (Bysouth et al., 1995).
Mineralization
Primary hypogene copper mineralization in the form of disseminated and vein-hosted chalcopyrite predominates at Gibraltar. Minor bornite occurs typically in the east and northeast portions of the deposit. Molybdenite occurs mainly in quartz veins and fractures, and rarely as matrix disseminations. Sphalerite occurs in the Extension deposit, and is especially abundant in the TK Zinc zone farther west. Mineralization in the Granite, Pollyanna, and Connector deposits is hosted in massive to strongly foliated zones. In contrast, mineralization in the Gibraltar, Extension, and TK Zinc zones is almost entirely hosted in strongly foliated shear zones.
Pyrite and chalcopyrite are the principal primary sulphide minerals of the Gibraltar deposits. Fine-grained chalcopyrite, generally barely visible without magnification, accounts for 60 percent of the copper content and constitutes the single most important form of copper mineralization. Most of this fine fraction is dispersed within the phyllosilicate foliation lamellae and forms the uniformly distributed grades of the Gibraltar porphyry-type ores. Coarser grained chalcopyrite usually occurs in quartz veins and shear zones. Pyrite mineralization generally shows some degree of segregation from chalcopyrite and, in the Pollyanna and Granite deposits, pyrite forms a halo or blanket of waste material above and away from the orebody. Large-scale pyrite zoning is also evident in the Gibraltar deposit but without the formation of a separate halo. The Connector zone displays mineralization features similar to both the Gibraltar and Pollyanna deposits. In the Gibraltar and Extension, pyrite is closely associated with the ore, often as massive zones 3 m to 7 m thick.
Small concentrations of other sulphides are present in the Gibraltar ores. Bornite, associated with magnetite and chalcopyrite, occurs along the low sulphur extremities of the Pollyanna deposit. Molybdenite is a minor but economically important associate of chalcopyrite in the Pollyanna, Granite, and Connector deposits. Small zones of molybdenum mineralization as molybdenite also occur in Gibraltar but are virtually absent in the Extension. Sphalerite is present in the Gibraltar deposit and particularly abundant in parts of the Extension. Both of these deposits also have elevated silver concentrations associated with copper mineralization. The above relationships suggest a metal zonation from Pollyanna to the Extension Zone that involves a westerly decrease of molybdenum and a corresponding increase of zinc and silver. Overall, in terms of large-scale metal zonation Gibraltar ranges from Cu±Mo in the east to Cu±Zn towards the west.
There is a close spatial relationship between sulphide mineralization and alteration in the Gibraltar deposits. The principal alteration minerals are chlorite, sericite, epidote, carbonate and quartz. Ore grade mineralization is associated mainly with sericite and chlorite. Epidote and the carbonate minerals are not common associates of strong sulphide mineralization. Quartz is common throughout the alteration sequence as both a relict host rock mineral and an introduced mineral.
Supergene mineralization and secondary enrichment occurs to varying degrees in the Gibraltar and Connector deposits and is interpreted to be a remnant of a pre- or inter-glacial period of weathering (Bysouth et al., 1995). Supergene enrichment is best developed in close association with pyrite-rich ore (> 3% pyrite). Supergene enrichment occurs directly beneath a leach cap, forming a blanket-like zone about 15 m to 30 m thick containing the supergene copper minerals chalcocite, digenite and covellite. Episodes of Pleistocene glaciation removed most of the Tertiary weathering surface elsewhere. The present zone of oxidation and leaching in other areas is generally confined to the upper 1 m to 3 m of the bedrock surface. Limited zones of oxidation may also occur to depths up to 100 m where structural controls have facilitated significant groundwater percolation.