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The Adamello intrusive suite is a composite batholith in Northern Italy, with an estimated 2000 km3 volume, assembled incrementally over a time span of 10 to 12 million years. The history of crystallization has been studied in detail through laser ablation ICP-MS and SIMS U–Pb geochronology of zircon, which records prolonged crystallization of each of the different intrusive units at mid-crustal levels between 43·47 and 33·16 Ma. The magmas were episodically extracted from this storage area and ascended to the final intrusion level at ∼6 km paleo-depth. Each batch of melt cooled very rapidly down to the ambient temperature of 250°C, evidenced by distinct cooling paths recorded by amphibole, biotite and K-feldspar 40Ar/39Ar dates. The magma source area was moving from SW to NE with time, causing increasing thermal maturity in the mid-crustal reservoir. The resulting temporal trend of higher degrees of crustal assimiliation in the course of the evolution of the magmatic system can be traced through Hf and O isotopes in zircon. Rough estimates of magma emplacement rates (‘magma flux’) yield very low values in the range of 10-4 km3/yr, typical of mid-to-upper crustal plutons and increase with time. Although we cannot discern a decrease of magma flux from our own data, we anticipate that a dramatic decrease of magma flux between 33 and 31 Ma along the northern contact lead to cessation of magma emplacement.

The Cerro de Pasco epithermal Cordilleran base metal deposit in central Peru consists of two mineralization stages which are spatially and genetically related to a mid-Miocene diatreme-dome complex. In order to constrain the duration of the magmatic-hydrothermal system at Cerro de Pasco, U-Pb age determinations and Hf isotope determinations have been conducted on single zircon grains. Biotite from intrusive rocks, and sericite and alunite related to the first and second mineralization stages, respectively, have been used for stepwise IR-CO2 laser 40Ar/39Ar age determinations. This study has been complemented by Rb-Sr analyses of sericite and associated pyrite. Single crystal zircon U-Pb analyses yield ages of 15.36 ± 0.03 Ma for an accretionary lapilli tuff from a collapsed block within the diatreme, 15.40 ± 0.07 Ma for a dacite porphyry dome in the diatreme, 15.35 ± 0.05 and 15.16 ± 0.04 Ma for two quartz monzonite porphyry dikes emplaced into the diatreme-dome complex that are within a time span of 350,000 yr. There is no field evidence for any magmatic activity predating the formation of the diatreme-dome complex. Stepwise IR-CO2 laser 40Ar/39Ar ages have been determined on sericite and alunite. The latter formed during the second mineralization stage in advanced argillically altered domes and from halos of oxidized enargite-pyrite veins in the Venencocha and Santa Rosa area, located on the northwestern margin of the diatreme-dome complex and on the southern part of the Cerro de Pasco open pit, respectively. 40Ar/39Ar alunite ages cluster between 14.54 ± 0.08 and 14.41 ± 0.07 Ma. This age consistency suggests that advanced argillic alteration related to the second mineralization stage at Cerro de Pasco formed during a period of about 100,000 yr. Three significantly younger alunite ages of 12.39 ± 0.06, 12.13 ± 0.07 Ma, and 10.94 ± 0.10 M, probably reflect partial resetting by late circulating fluids. Therefore, it appears that magmatic-hydrothermal activity at Ce

Neoproterozoic Palaeogeographic reconstructions of Rodinia conventionally place the western (Proto-Andean) margin of Amazonia against the eastern (Appalachian) margin of Laurentia. Separation and formation of the Iapetus Ocean is generally considered to have occurred later at ~550 Ma. We examine the U-Pb detrital zircon "fingerprint" of autochthonous rocks from the northern and central segments of the Proto-Andean margin, which formed part of the western margin of Amazonia during the Late Neoproterozoic-Phanerozoic. The Proto-Andean margin is clearly the most feasible source region for most of the zircon grains, except for a 550-650 Ma sub-population, broadly age-equivalent to the Brasiliano/Pan-African Orogeny in eastern Amazonia. No obvious source for this detritus is known in the northern and central Andes. Derivation from eastern Amazonia is considered unlikely due to the stark paucity of detritus derived from the core of the Amazonian craton. Instead, we propose that a Late Neoproterozoic magmatic belt is buried beneath the present-day Andean belt or Amazon Basin, and was probably covered during the Eocene-Oligocene. If this inferred Neoproterozoic belt was an active margin, it would record the initiation of Proto-Andean subduction and imply at least partial separation of West Gondwana from its conjugate rift margin of eastern Laurentia prior to ca. 650 Ma. This separation may be linked to the ca. 770-680 Ma A-type magmatism found on eastern Laurentia in the southern Appalachians, and on the Proto-Andean margin in the Sierra Pampeanas and the Eastern Cordillera of Peru. © 2008 Elsevier B.V. All rights reserved.

We investigated the Neoproterozoic–early Paleozoic evolution of the Gondwanan margin of the north-central Andes by employing U-Pb zircon geochronology in the Eastern Cordilleras of Peru and Ecuador using a combination of laser-ablation–inductively coupled plasma–mass spectrometry detrital zircon analysis and dating of syn- and post-tectonic intrusive rocks by thermal ionization mass spectrometry and ion microprobe. The majority of detrital zircon samples exhibits prominent peaks in the ranges 0.45–0.65 Ga and 0.9–1.3 Ga, with minimal older detritus from the Amazonian craton. These data imply that the Famatinian-Pampean and Grenville (= Sunsas) orogenies were available to supply detritus to the Paleozoic sequences of the north-central Andes, and these orogenic belts are interpreted to be either buried underneath the present-day Andean chain or adjacent foreland sediments. There is evidence of a subduction-related magmatic belt (474–442 Ma) in the Eastern Cordillera of Peru and regional orogenic events that pre- and postdate this phase of magmatism. These are confirmed by ion-microprobe dating of zircon overgrowths from amphibolite-facies schists, which reveals metamorphic events at ca. 478 and ca. 312 Ma and refutes the previously assumed Neoproterozoic age for orogeny in the Peruvian Eastern Cordillera. The presence of an Ordovician magmatic and metamorphic belt in the north-central Andes demonstrates that Famatinian metamorphism and subduction-related magmatism were continuous from Patagonia through northern Argentina to Venezuela. The evolution of this extremely long Ordovician active margin on western Gondwana is very similar to the Taconic orogenic cycle of the eastern margin of Laurentia, and our findings support models that show these two active margins facing each other during the Ordovician.

The Kapan mining district in the southernmost Lesser Caucasus is one of the few locations along the central Tethyan metallogenic belt where ore-forming processes were associated with magmatic arc growth during Jurassic Tethys subduction along the Eurasian margin. Three ore deposits of the Kapan district were investigated in this study: Centralni West, Centralni East, and Shahumyan. The mineralisation consists of veins, subsidiary stockwork, and partial matrix replacement of breccia host rocks, with chalcopyrite, pyrite, tennantite-tetrahedrite, sphalerite, and galena as the main ore minerals. Centralni West is a dominantly Cu deposit, and its host rocks are altered to chlorite, carbonate, epidote, and sericite. At Centralni East, Au is associated with Cu, and the Shahumyan deposit is enriched in Pb and Zn as well as precious metals. Both deposits contain high-sulphidation mineral assemblages with enargite and luzonite. The lead isotope composition of sulphides and alunite (206Pb/204Pb = 18.17-18.32, 207Pb/204Pb = 15.57-15.61, 208Pb/204Pb = 38.17-38.41) indicates a common metal source for the three deposits and suggests that metals were derived from magmatic fluids that were exsolved upon crystallisation of Middle Jurassic intrusive rocks or leached from Middle Jurassic country rocks. The preferred model is that the Centralni West Cu deposit is a volcanogenic massive sulphide deposit and the Shahumyan and Centralni East deposits are parts of porphyryepithermal systems, with the three deposits being broadly coeval or formed within a short time interval in a nascent magmatic arc setting., The Kapan mining district in the southernmost Lesser Caucasus is one of the few locations along the central Tethyan metallogenic belt where ore-forming processes were associated with magmatic arc growth during Jurassic Tethys subduction along the Eurasian margin. Three ore deposits of the Kapan district were investigated in this study: Centralni West, Centralni East, and Shahumyan. The mineralisation consists of veins, subsidiary stockwork, and partial matrix replacement of breccia host rocks, with chalcopyrite, pyrite, tennantite-tetrahedrite, sphalerite, and galena as the main ore minerals. Centralni West is a dominantly Cu deposit, and its host rocks are altered to chlorite, carbonate, epidote, and sericite. At Centralni East, Au is associated with Cu, and the Shahumyan deposit is enriched in Pb and Zn as well as precious metals. Both deposits contain high-sulphidation mineral assemblages with enargite and luzonite. The lead isotope composition of sulphides and alunite (206Pb/204Pb = 18.17-18.32, 207Pb/204Pb = 15.57-15.61, 208Pb/204Pb = 38.17-38.41) indicates a common metal source for the three deposits and suggests that metals were derived from magmatic fluids that were exsolved upon crystallisation of Middle Jurassic intrusive rocks or leached from Middle Jurassic country rocks. The preferred model is that the Centralni West Cu deposit is a volcanogenic massive sulphide deposit and the Shahumyan and Centralni East deposits are parts of porphyryepithermal systems, with the three deposits being broadly coeval or formed within a short time interval in a nascent magmatic arc setting.

The results of step-wise infrared-CO2 laser Ar-Ar analyses combine with earlier findings to indicate that long periods of hydrothermal activity existed in the shallow parts of magmatic-hydrothermal systems such as Cerro de Pasco and Colquijirca. The districts together had 400 000 000-500 000 000 tonnes of nearly massive sulphide ores prior to mining, including more than 2 000 000 000 oz silver as well as zinc, copper, gold, lead and bismuth. The deposits are spatially related to two Miocene dacitic diatreme-dome volcanic centres that intruded carbonate rocks. There are two main high-sulphidation ore types: carbonate-hosted, strongly zoned sulphide-rich base metal bodies of varied forms, some 3 km long, and earlier gold-silver epithermal ores or nearly barren alunite-bearing bodies, hosted by volcanic rocks and consisting largely of vuggy silica with advanced argillic alteration haloes, typically sulphide-poor and including Au-bearing disseminated oxides. Ar-Ar dating and field relationships suggest that a short 500 000-year event was probably responsible for all the base-metal deposits, that the earlier epithermal event lasted around 1 m.y. and that there was an additional mineralisation event prior to this., The results of step-wise infrared-CO2 laser Ar-Ar analyses combine with earlier findings to indicate that long periods of hydrothermal activity existed in the shallow parts of magmatic-hydrothermal systems such as Cerro de Pasco and Colquijirca. The districts together had 400 000 000-500 000 000 tonnes of nearly massive sulphide ores prior to mining, including more than 2 000 000 000 oz silver as well as zinc, copper, gold, lead and bismuth. The deposits are spatially related to two Miocene dacitic diatreme-dome volcanic centres that intruded carbonate rocks. There are two main high-sulphidation ore types: carbonate-hosted, strongly zoned sulphide-rich base metal bodies of varied forms, some 3 km long, and earlier gold-silver epithermal ores or nearly barren alunite-bearing bodies, hosted by volcanic rocks and consisting largely of vuggy silica with advanced argillic alteration haloes, typically sulphide-poor and including Au-bearing disseminated oxides. Ar-Ar dating and field relationships suggest that a short 500 000-year event was probably responsible for all the base-metal deposits, that the earlier epithermal event lasted around 1 m.y. and that there was an additional mineralisation event prior to this.

The deposit consists of two mineralisation stages which are spatially and genetically related to a mid-Miocene diatreme-dome complex. The first stage includes a large pyrite-quartz body and pyrrhotite bodies zoned to Pb-Zn ores containing Fe-rich sphalerite replacing Pucara carbonate rocks, and the second consists of Cu-Ag-Au-(Zn-Pb) veins hosted in the diatreme breccia and in the pyrite-quartz body on the western part of the open pit and Zn-Pb-(Bi-Ag-Cu) Pucara carbonate-hosted replacement bodies in the eastern and central open pit. Single crystal zircon U-Pb analyses were carried out and stepwise IR-CO2 laser 40Ar/39Ar ages determined on sericite and alunite. The latter formed during the second mineralisation stage in advanced argillically altered domes and from halos of oxidised enargite-pyrite veins in the Venencocha and Santa Rosa area, located on the northwestern margin of the diatreme-dome complex and on the southern part of the Cerro de Pasco open pit, respectively. The age consistency suggests that advanced argillic alteration related to the second mineralisation stage formed during a period of about 100 000 yr. Three significantly younger alunite ages probably reflect partial resetting by late circulating fluids, and magmatic-hydrothermal activity appears to have lasted about 1 m.y., a period similar to the duration in the nearby mid-Miocene Colquijirca district., The deposit consists of two mineralisation stages which are spatially and genetically related to a mid-Miocene diatreme-dome complex. The first stage includes a large pyrite-quartz body and pyrrhotite bodies zoned to Pb-Zn ores containing Fe-rich sphalerite replacing Pucara carbonate rocks, and the second consists of Cu-Ag-Au-(Zn-Pb) veins hosted in the diatreme breccia and in the pyrite-quartz body on the western part of the open pit and Zn-Pb-(Bi-Ag-Cu) Pucara carbonate-hosted replacement bodies in the eastern and central open pit. Single crystal zircon U-Pb analyses were carried out and stepwise IR-CO2 laser 40Ar/39Ar ages determined on sericite and alunite. The latter formed during the second mineralisation stage in advanced argillically altered domes and from halos of oxidised enargite-pyrite veins in the Venencocha and Santa Rosa area, located on the northwestern margin of the diatreme-dome complex and on the southern part of the Cerro de Pasco open pit, respectively. The age consistency suggests that advanced argillic alteration related to the second mineralisation stage formed during a period of about 100 000 yr. Three significantly younger alunite ages probably reflect partial resetting by late circulating fluids, and magmatic-hydrothermal activity appears to have lasted about 1 m.y., a period similar to the duration in the nearby mid-Miocene Colquijirca district.

The duration of magmatic-hydrothermal events in porphyry systems is instrumental in the development of genetic models necessary to explore a category of mineral deposits that provide most of the copper and significant amounts of base and precious metals to our economy. The problem of determining the absolute duration has been addressed through thermal modelling of cooling intrusions and time needed to precipitate specified metal amounts from active hydrothermal systems with known metal concentrations and fluid fluxes. Both these methods have shown that the likely duration of hydrothermal systems is on the order of a few tens of thousands of years. Isotopic dating in contrast is the only possible way to determine the life span of magmatic-hydrothermal events in fossil porphyry systems. Analytical and methodological developments during the last decade commonly used for absolute dating (U-Pb, 40Ar/39Ar, Re-Os) allow dating minerals with internal precisions less than 0.2%. Internal precisions of these dating methods allow, in theory, comfortable discrimination of events at the 0.1 Ma scale and the combination of U-Pb, 40Ar/39Ar, and Re-Os geochronology is a tool widely used by ore geologists to bracket the duration of cyclic magmatic-hydrothermal events in porphyry systems., The duration of magmatic-hydrothermal events in porphyry systems is instrumental in the development of genetic models necessary to explore a category of mineral deposits that provide most of the copper and significant amounts of base and precious metals to our economy. The problem of determining the absolute duration has been addressed through thermal modelling of cooling intrusions and time needed to precipitate specified metal amounts from active hydrothermal systems with known metal concentrations and fluid fluxes. Both these methods have shown that the likely duration of hydrothermal systems is on the order of a few tens of thousands of years. Isotopic dating in contrast is the only possible way to determine the life span of magmatic-hydrothermal events in fossil porphyry systems. Analytical and methodological developments during the last decade commonly used for absolute dating (U-Pb, 40Ar/39Ar, Re-Os) allow dating minerals with internal precisions less than 0.2%. Internal precisions of these dating methods allow, in theory, comfortable discrimination of events at the 0.1 Ma scale and the combination of U-Pb, 40Ar/39Ar, and Re-Os geochronology is a tool widely used by ore geologists to bracket the duration of cyclic magmatic-hydrothermal events in porphyry systems.