Abstract

The post-Paleozoic metallogenic evolution of southeastern Peru is clarified on the basis of the stratigraphic and lithologic settings of the majority of the known metallic ore deposits and a regional program of K-Ar and 40 Ar- 39 Ar geochronology. This central Andean transect displays a range of mineralization types unparalleled in other regions of the country. Contrasting magmatic, tectonic, and metallogenic relationships are shown by the calc-alkaline--shoshonitic Upper Triassic--Holocene Main Arc magmatic domain, underlying the present Cordillera Occidental and Altiplano, and the more restricted Triassic--Pliocene Inner Arc domain of the Cordillera Oriental, which incorporates a great variety of igneous suites and exhibits a correspondingly diverse metallogeny. Major economic mineralization occurred simultaneously in the two domains only during the late Oligocene to early Miocene interval.The earliest significant Andean W, Cu, Mo, Sn, and Au mineralization is hosted largely by weakly peraluminous granites of the Upper Triassic to Lower Jurassic (190-225 Ma) Carabaya batholith in the Inner Arc; magma genesis resulted from sediment anatexis along the margin of the Permian ensialic Mitu rift. Renewed rifting in the Middle Jurassic (ca. 170-180 Ma) in this area was associated with the emplacement of the volcano-plutonic Allinccapac Peralkaline Complex, syenitic plutons of which host minor Cu, Ag, and Zr(-REE) vein systems. Mesozoic mineralization in the Main Arc, weak in comparison to that in other central Andean transects, comprises Upper Jurassic (145-165 Ma) Cu-Au veins (e.g., Rosa Maria), and mid-Cretaceous (ca. 95-110 Ma) Cu (Santiago, Valparaiso) and Fe (Morritos, Cerro Pelado) veins of the Ilo-Ite district. Restricted Upper Cretaceous (ca. 80 Ma) Cu-Pb-Zn-Ag veins in the Crucero district of the Inner Arc are interpreted as recording a brief episode of arc broadening or bifurcation.Large-scale porphyry Cu(-Mo, Ag) centers were emplaced in the Main Arc in the interval 52.15 (Cuajone) to 57.1 Ma (Toquepala) as the terminal stage in the evolution of the subaerial volcanic succession of the Upper Cretaceous-Paleogene Toquepala Group, which had previously experienced only minor Cu(-Pb-Ag-Au) vein mineralization at ca. 80 (Challatita) and 62 Ma (Lluta district). The early Eocene event represents the metallogenic culmination of the Peruvian Coastal batholith and its extension in northern Chile. In contrast, only the northern extremity of the upper Eocene-lower Oligocene arc of northern Chile occurs in the study area, and the minor Ataspaca Cu-Mo-Pb-Zn-Ag stockwork and skarn mineralization (39-45 Ma) is a pale reflection of the coeval array of giant porphyry copper deposits farther south. Much of south-eastern Peru lacked magmatism immediately before and after the ca. 40-Ma Incaic orogeny and hence experienced a metallogenic hiatus. The Main Arc was resuscitated in the Santa Lucia area at 31 to 32 Ma; at 28.5 + or - 1 Ma it abruptly broadened to a width of ca. 235 km. However, hydrothermal activity remained restricted in both scale and distribution. The moderate-sized Berenguela (ca. 27 Ma) and Santa Barbara (23.5 Ma) epithermal Ag deposits are associated with calc-alkaline subvolcanic centers, in the latter case emplaced in the initial stages of cordilleran uplift. Post-Oligocene mineralization in the Main Arc was also apparently sparse in comparison to the broadly contemporary epithermal Ag-Au-base metal deposits of central and south-central Peru, but it comprises the Au-rich veins of the Manazo camp (19 Ma) and the Ag veins of the Cacachara (6.5-7 Ma) and Compuerta camps (7 Ma).The Inner Arc revived at 28 to 29 Ma, simultaneously with the Main Arc broadening. Anatexis resulting from shoshonitic basalt injection generated strongly peraluminous monzogranite stocks with which are spatially associated major, high-grade, lithophile and base metal lode systems, including San Rafael (23-24 Ma) and Palca 11 (24-25 Ma), now the most productive hard-rock Sn and W deposits of the Western Hemisphere. However, the widespread middle and late Miocene peraluminous magmatism in this region failed to produce Sn polymetallic mineralization of the scale developed in Bolivia at this time; only the small Jesica vein system (17.4 Ma) has been confirmed to contain Sn, and the Sb veins of the area (e.g., Collpa: 12.3 Ma) are also of restricted size. In contrast, the uranium stockworks (6.8-8.0 Ma) associated with the rare element-enriched, rhyolitic Macusani Volcanics are large and apparently of high grade.Although sharing several metallogenic features with contiguous central Andean transects, southeastern Peru differs markedly from other areas of the country in the nature and age of mineralization. Thus, the Inner Arc domain does not persist to the northwest, and the radical and commonly abrupt changes we define in the distribution of magmatism, and hence mineralization, during the middle and late Tertiary are apparently unrepresented in central Peru. The individual metallogenic evolution of this region is ascribed to the inferred occurrence of a marked deflection in the western boundary of the South American plate throughout the Andean orogeny.

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