Petrogenetic and Metallogenetic Relationships in the Eastern Cordillera Occidental of Central Peru

Ore deposits spatially and temporally related to high-K calc-alkaline intrusions or domes are widespread in the central Peruvian Cordillera Occidental. Geochronology and petrochemistry of intrusive rocks associated with mineralization reveal that most ore deposits (1) were emplaced in the late Middle to early late Miocene and (2) are related to calc-alkaline igneous rocks of granodioritic composition with Sm/Yb > 3, Sr/Y > 40, and Y/ Th < 1.5. The largest deposits are associated with intrusions with Sm/Yb > 4. Strontium isotopic compositions of the igneous rocks vary from Sr-87/Sr-86 = 0.7038 to 0.7089, but the Sr-87/Sr-86 ratio values exhibit only a poorly defined overall trend toward increased crustal contamination through time. Lead isotopic ratios generally correlate with the Sr isotopes and range from Pb-206/Pb-204 = 18.64 to 18.91, from Pb-207/Pb-204 = 15.58 to 15.66, and from Pb-208/Pb-204 = 38.60 to 39.06. The overall Sr and Pb isotopic compositions of the igneous rocks indicate a generally moderate crustal assimilation of ancient radiogenic rocks. The Pb isotopic composition of sulfides from 14 polymetallic ore deposits in the study area lie on parallel arrays between the Pb isotopic compositions of spatially related igneous rocks and higher Pb-208/Pb-204 and Pb-207/Pb-204 sources such as Paleozoic and Mesozoic siliciclastic sedimentary rocks or the Carboniferous and older crystalline basement. The trace element and radiogenic isotope signature of igneous rocks related to mineralization indicates that these rocks have undergone limited midcrustal and shallow crustal assimilation and reflect melt equilibration in the lower crust at a crustal thickness of more than 40-45 km under conditions where hornblende and garnet were stable in the basaltic lower crustal residuum. Mineralization was favored by a change from normal to flat subduction, resulting in crustal thickening as well as broadening and subsequent cessation of the magmatic arc. However, some igneous rocks apparently unrelated to mineralization have high Sm/Yb and Sr/Y ratios as well. Petrographic and Pb isotopic evidence indicates that for some of these rocks, these trace element ratios may be better explained by shallow crustal hornblende fractionation or assimilation of pelitic material.