Magmatic Evolution of the Giant El Teniente Cu-Mo Deposit, Central Chile

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 center dot 2-6 center dot 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 center dot 9 +/- 1 center dot 4 Ma), the smaller (similar to 30 km(3)) Sewell equigranular tonalite complex (7 center dot 05 +/- 0 center dot 14 Ma) and associated andesitic sills (8 center dot 2 +/- 0 center dot 5 to 6 center dot 6 +/- 0 center dot 4 Ma), small dacitic porphyry stocks (< 1 km(3); 6 center dot 09 +/- 0 center dot 18 Ma), the unusual Cu- and S-rich 'Porphyry A' anhydrite-bearing granitoid stock (< 1 km(3); 5 center dot 67 +/- 0 center dot 19 Ma), the Teniente Dacite Porphyry dike (< 1 km(3); 5 center dot 28 +/- 0 center dot 10 Ma), minor latite dikes (4 center dot 82 +/- 0 center dot 09 Ma), and finally a small dacite intrusion (4 center dot 58 +/- 0 center dot 10 Ma). These plutonic rocks are all isotopically similar to each other (Sr-87/Sr-86 = 0 center dot 7039-0 center dot 7042; epsilon(Nd) =+2 center dot 5 to +3 center dot 5) and also to the Teniente Volcanic Complex extrusive rocks, but distinct from both older Late Oligocene to Early Miocene volcanic rocks (Sr-87/Sr-86 = 0 center dot 7033-0 center dot 7039; epsilon(Nd) = +3 center dot 8 to + 6 center dot 2) and younger Pliocene post-mineralization mafic dikes and lavas (Sr-87/Sr-86 = 0 center dot 7041-0 center dot 7049; epsilon(Nd) = +1 center dot 1 to -1 center dot 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 center dot 31 +/- 0 center dot 03 and 4 center dot 42 +/- 0 center dot 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 similar to 100 x 10(6) tonnes of Cu in the deposit requires a batholith-size (> 600 km(3)) amount of magma with similar to 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 center dot 85 +/- 0 center dot 18 to 2 center dot 9 +/- 0 center dot 6 Ma) pass through the system from the mantle to the surface. The significant progressive temporal isotopic evolution, to higher Sr-87/Sr-86 (from 0 center dot 7033 to 0 center dot 7049) and lower epsilon(Nd) (from +6 center dot 2 to -1 center dot 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, whichmay have played a role in producing oxidized volatile-rich magmas and mineralization at El Teniente.