Late Miocene Mineralized Breccias in the Andes of Central Chile: Sr- and Nd-Isotopic Evidence for Multiple Magmatic Sources
M. Alexandra Skewes, Charles R. Stern, 1998. "Late Miocene Mineralized Breccias in the Andes of Central Chile: Sr- and Nd-Isotopic Evidence for Multiple Magmatic Sources", Andean Copper Deposits: New Discoveries, Mineralization, Styles and Metallogeny, Francisco Camus, Richard M. Sillitoe, Richard Petersen
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Large mineralized breccias are prominent features in the three giant late Miocene Andean copper deposits at Los Pelambres, Rio Blanco-Los Bronces and El Teniente in central Chile. The breccias were emplaced into Miocene igneous host rocks by the expansion of high-temperature, metal-rich fluids exsolvedfrom magmas. Minerals (tourmaline, anhydrite, biotite) precipitatedfrom these magmatic fluids in the matrices of different breccias have variable initial 87 Sr/86 Sr ratios, ranging from 0.7040 to 0.7049, and ∈.Nd values, ranging between +0.8 to +3.6. Although the fluids that generated the breccias may have leached some Sr from contained clasts of host rock, the ∈nd values of these breccia-matrix minerals are interpreted as the Nd-isotopic compositions of the magmas from which the breccia-forming fluids exsolved.
The isotopic compositions determined for the breccia-matrix minerals differ from the host plutons. This implies that the fluids that generated the breccias were not derived from these plutons, which were already crystallized at the time of breccia formation as indicated by the angular nature of their clasts in the breccias. The fluids which generated the breccias must have exsolved from magmas crystallizing to form plutons not yet exposed at the surface, consistent with the fact that the roots of the mineralized breccias have not been encountered.
Significantly, the isotopic compositions of the breccia-matrix minerals from different breccias in each deposit are variable. This indicates that the breccia-forming fluids were not derivedfrom a single magma, but from isotopically variable magmas. We suggest that the mineralized fluids that formed the late Miocene Cu-rich breccias in central Chile exsolved from multiple, compositionally variable magma batches cooling during the last stages of long-lived Andean magmatic systems. Cooling of these systems was triggered tectonically in the late Miocene as subduction angle and, as a result, subarc magma supply decreased.
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The copper deposits of Perú consist of porphyry Cu±Mo, Au, Ag, breccia pipe Cu-Mo, enargite vein and replacement Cu±Au, Ag, Zn, Pb, calcic skarn Cu±Fe, Au, Zn, amphibolitic skarn Cu±Fe, volcanogenic massive sulfide Cu-Zn, vein and manto Cu±Ag, Pb, Zn, Sn, W, and sandstone (“red bed”) Cu types. The vast majority of these deposits formed during the Andean Orogeny and are geographically and chronologically distributed in well-defined metallogenic domains. These domains correlate with geochemically distinct magmatic episodes.
The magmatic and metallogenic domains appear to be controlled in part by transverse growth-faults in the Mesozoic and older basement rocks underlying the intensely folded and thrust-faulted Mesozoic and Tertiary rocks of the higher structural levels of the Cordillera. During the Andean Orogeny the extent of magmatism and the corresponding metallogenic provinces were influenced by subducted plate segmentation and by continental margin basement tectonics. In addition, the lithologic nature of the host rocks played an important role in determining the types of copper deposits formed.
Porphyry Cu, breccia pipe Cu-Mo and calcic skarn Cu deposits are related to the Pomahuaca, Coastal and Caldera batholiths, as well as to felsic Cordilleran volcanism between 8° and 12°S. However, the largest and richest porphyry Cu deposits are related to the Caldera batholith. The Cobriza Cu-bearing skarn is the only significant copper deposit of pre-Mesozoic age.
Perú has many ore deposits associated with the Miocene felsic extrusive and intrusive rocks along the Cordillera, forming veins and disseminations in igneous rocks and noncarbonate sedimentary rocks, and replacement mantos, pipes and veins in limestones. Several are large and high-grade enargite-type deposits containing mainly Cu, Ag, Au, Pb and Zn, accompanied by significant amounts of Cd, Te, Se, In, Bi and Tl. Others are veins and mantos containing Cu±Ag, Pb, Zn, Sn, W.
The Mesozoic volcanosedimentary sequences along the coast host volcanogenic massive sulfide Cu-Zn and vein/manto-type amphibolitic skarn Cu±Fe deposits.
Red bed Cu deposits are relatively unimportant in Perú.
The following information on the history of copper mining in Perú has been condensed largely from Samame (1979), Petersen et al.(1990) and Benavides (1990).
In Perú, gold and silver were apparently used before copper. The latter was first mined and processed by the pre-Inca Chimú culture along the northern coast and by the Tiahuanaco civilization in the Lake Titicaca region.
Copper became an important metal during the Inca period,