Porphyry Cu-(Mo-Au) deposits are relatively rare but reproducible products of subduction-related magmatism. No unique processes appear to be required for their formation, although additive combinations of common tectono-magmatic processes, or optimization of these processes, can affect the grade and size as well as the location of the resulting deposits. These various contributing processes are reviewed, from partial melting in the mantle wedge overlying the subducting plate, through processes of magma interaction with the lithosphere, to mechanisms for magma emplacement and volatile exsolution in the upper crust. Specific ore-forming processes, such as magmatic-hydrothermal fluid evolution, are not discussed. Hot, hydrous, relatively oxidized, sulfur-rich mafic magmas (predominantly basalts) generated in the metasomatized mantle wedge above a subducting oceanic slab rise buoyantly to the base of the overlying crust where they stall due to density contrasts. Because these magmas are oxidized, sulfur is dominantly present as sulfate, and chalcophile elements such as Cu and Au are incompatible (i.e., they are retained in the melt). As these magmas begin to crystallize they release heat which causes partial melting of crustal rocks. Mixing between crustal- and mantle-derived magmas yields evolved (andesitic to dacitic), volatile-rich, metalliferous, hybrid magmas, which are of sufficiently low density to rise through the crust. Magma ascent is driven primarily by buoyancy forces and is dominantly a fracture-controlled phenomenon. As such, crustal stress and strain patterns play an important role in guiding the ascent of magma from the lower crust. In particular, translithospheric, orogen-parallel, strike-slip structures serve as a primary control on magma emplacement in many volcanic arcs worldwide. A feedback mechanism operates, whereby preexisting faults facilitate magma ascent, the heat from which further weakens the crust and focuses strain. Certain structural geometries, such as fault jogs, step-overs, and fault intersections, offer low-stress extensional volumes during transpressional strain. Such sites represent vertical conduits of relatively high permeability, up which magmas will preferentially ascend. Large upper crustal plutonic complexes may therefore be localized within these structural settings. Having delivered a sufficient volume of evolved, fertile arc magma to a focused position in the upper crust, magmatic fractionation, recharge, and volatile exsolution lead to the development of ore-forming magmatic-hydrothermal systems. To a first approximation, the size of the resulting deposit will be limited by the magma volume delivered to the upper crustal magma chamber. System-specific details such as magmatic-hydrothermal evolution, the nature of the country rocks, and subsequent erosional and weathering history will ultimately control the value of the deposit, but these factors fall outside the scope of this paper.

Tectono-Magmatic Precursors for Porphyry Cu-(Mo-Au) Deposit Formation

Terra Australis Orogen: Rodinia breakup and development of the Pacific and Iapetus margins of Gondwana during the Neoproterozoic and Paleozoic

The Jiangnan fold belt separates the Yangtze and Cathaysia blocks in South China and has long been considered Grenvillian in age in order to place South China in central Rodinia. It consists of deformed Early Neoproterozoic strata that are unconformably overlain by undeformed Late Neoproterozoic strata and intruded by undeformed and unmetamorphosed granitic plutons. Zircons from the Early Neoproterozoic strata yield U-Pb ages as young as 830 Ma, and one granitic pluton has a zircon U-Pb age of ca. 827 Ma. The ≥830 Ma mafic rocks along the southeastern margin of the Yangtze block have arc-affinity geochemical characters, whereas mafic rocks younger than 830 Ma have typical ocean island basalt (OIB)—like compositions. Thus, we suggest that the Early Neoproterozoic strata were deposited on an active continental margin prior to amalgamation of the Yangtze and Cathaysia blocks at ca. 830 Ma. The overlying Late Neoproterozoic strata were deposited in the intracontinental rifted Nanhua Basin at 820–730 Ma and probably reflect backarc spreading above the long-lived (950–735 Ma) oceanic subduction zone along the northern and western margin of the Yangtze block. This model is consistent with the secular tectonic evolution of South China during the Neoproterozoic. The Jiangnan fold belt is therefore not a Grenvillian feature as previously suggested, and there is no evidence to place South China in central Rodinia. Instead, we believe that South China was located in a marginal position relative to this supercontinent.

Reappraisal of the ages of Neoproterozoic strata in south China; no connection with the Grenvillian Orogeny

Lithotectonic elements of the Grenville Province: review and tectonic implications

Compositional variations of Central Andean subduction-related igneous rocks reflect the plate-tectonic evolution of this active continental margin through time and space. In order to address the effect on magmatism of changing subduction geometry and crustal evolution of the upper continental plate during the Andean orogeny, we compiled more than 1500 major- and trace-element data points, and 650 Sr-, 610 Nd-, and 570 Pb-isotopic analyses of Mesozoic-Cenozoic (190–0 Ma) magmatic rocks in southern Peru and northern Chile (Central Andean orocline), mostly from new data and the literature. This data set documents compositional variations of magmas since Jurassic time, with a focus on the Neogene period, when major crustal thickening developed and its influence on magma composition was most pronounced. We relate the observed variations in Sr/Y, La/Yb, La/Sm, Sm/Yb, and Dy/Yb ratios, as well as in Sr-, Nd-, and Pb-isotopic ratios, to the crustal structure and evolution of the Central Andean orocline. In particular, the evolution of Dy/Yb and Sm/Yb ratios, which track the presence of the higher-pressure minerals amphibole and garnet, respectively, in the lower crust, documents that crustal thickness has grown through time. Spatial variations in trace elements and isotopic ratios further suggest that crustal domains of distinct composition and age have influenced magma composition through some assimilation. The crustal input in Quaternary magmas is quantified to have been between 7% and 18% by simple two-components mixing. When comparing our geochemical data set to the geological record of uplift and crustal thickening, we observe a correlation between the composition of magmatic rocks and the progression of Andean orogeny. In particular, our results support the interpretation that major crustal thickening and uplift were initiated in the mid-Oligocene (30 Ma) and that crustal thickness has kept increasing until present day. Our data do not support delamination as a general cause for major late Miocene uplift in the Central Andes and instead favor continued crustal thickening.

Geochemical variations in igneous rocks of the Central Andean orocline (13°S to 18°S): Tracing crustal thickening and magma generation through time and space

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.

Robert J. Langridge

Papers

Geologic and geochronologic constraints on the metallogenic evolution of the Andes of southeastern Peru

Grenvillian granulite-facies metamorphism in the Arequipa Massif, Peru: a Laurentia-Gondwana link