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.