ALBERT

Description: El Teniente, the world’s largest Cu deposit, is hosted in Late Miocene and Pliocene plutons that intrude the older Teniente Volcanic Complex (or Farellones Fm; 14·2–6·5 Ma). The Late Miocene and Pliocene plutonic host rocks of the deposit include, sequentially, the relatively large (〉50 km 3 ) Teniente Mafic Complex laccolith (8·9 ± 1·4 Ma), the smaller (~30 km 3 ) Sewell equigranular tonalite complex (7·05 ± 0·14 Ma) and associated andesitic sills (8·2 ± 0·5 to 6·6 ± 0·4 Ma), small dacitic porphyry stocks (〈1 km 3 ; 6·09 ± 0·18 Ma), the unusual Cu- and S-rich ‘Porphyry A’ anhydrite-bearing granitoid stock (〈1 km 3 ; 5·67 ± 0·19 Ma), the Teniente Dacite Porphyry dike (〈1 km 3 ; 5·28 ± 0·10 Ma), minor latite dikes (4·82 ± 0·09 Ma), and finally a small dacite intrusion (4·58 ± 0·10 Ma). These plutonic rocks are all isotopically similar to each other ( 87 Sr/ 86 Sr = 0·7039–0·7042; Nd =+2·5 to +3·5) and also to the Teniente Volcanic Complex extrusive rocks, but distinct from both older Late Oligocene to Early Miocene volcanic rocks ( 87 Sr/ 86 Sr = 0·7033–0·7039; Nd = +3·8 to + 6·2) and younger Pliocene post-mineralization mafic dikes and lavas ( 87 Sr/ 86 Sr = 0·7041–0·7049; Nd = +1·1 to –1·1). Multiple Cu-mineralized magmatic–hydrothermal breccia pipes were emplaced into these plutonic rocks during the same time period as the felsic porphyry intrusions, between at least 6·31 ± 0·03 and 4·42 ± 0·02 Ma. These mineralized breccia pipes, which formed by exsolution of magmatic fluids from cooling plutons, have their roots below the deepest level of mining and exploration drilling and were derived from the same magma chamber as the felsic porphyries, 〉4 km below the paleosurface. To produce the ~100 x 10 6 tonnes of Cu in the deposit requires a batholith-size (〉600 km 3 ) amount of magma with ~100 ppm Cu. We suggest that both the mineralized magmatic–hydrothermal breccias and the progressively smaller volumes of more fractionated, but isotopically equivalent, Late Miocene and Pliocene felsic plutonic rocks that host the deposit were derived from the roof of a large, long-lived, thermally and chemically stratified, open-system magma chamber, or magmatic plumbing system, recharged from below by mantle-derived magmas. Only when this system fully solidified did post-mineralization mafic olivine-hornblende-lamprophyre dikes (3·85 ± 0·18 to 2·9 ± 0·6 Ma) pass through the system from the mantle to the surface. The significant progressive temporal isotopic evolution, to higher 87 Sr/ 86 Sr (from 0·7033 to 0·7049) and lower Nd (from +6·2 to –1·1), that occurred between the Late Oligocene and Pliocene in the vicinity of El Teniente for mafic mantle-derived magmas, and by implication their sub-arc mantle-source region, was due in part to increased mantle-source region contamination by subducted crust tectonically eroded off the continental margin. The post-mineralization olivine-hornblende-lamprophyres also imply extensive hydration of the mantle below this portion of the Andean arc by the Pliocene, which may have played a role in producing oxidized volatile-rich magmas and mineralization at El Teniente.