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Global data for measured Fe 2 O 3 /FeO ratios and Cu contents in unaltered volcanic and intrusive arc rocks indicate that, on average, they are slightly more oxidized than other magmas derived from depleted upper mantle (such as MORB), but contain similar Cu contents across their compositional ranges. Although Cu scatters to elevated values in some intermediate composition samples, the bulk of the data show a steady but gentle trend to lower concentrations with differentiation, reaching modal values of ~ 50–100 ppm in andesitic rocks. These data suggest that Cu is mildly compatible during partial melting and fractionation processes, likely reflecting minor degrees of sulfide saturation throughout the magmatic cycle. However, the volume of sulfides must be small such that significant proportions of the metal content remain in the magma during fractionation to intermediate compositions. Previous studies have shown that andesitic magmas containing ~ 50 ppm Cu can readily form large porphyry-type Cu deposits upon emplacement in the upper crust. A review of the literature suggests that the elevated oxidation state in the asthenospheric mantle wedge source of arc magmas (ΔFMQ ≈ + 1 ± 1) derives from the subduction of seawater-altered and oxidized oceanic crust, and is transmitted into the mantle wedge via prograde metamorphic dehydration fluids carrying sulfate and other oxidizing components. Progressive hydration and oxidation of the mantle wedge may take up to ~ 10 m.y. to reach a steady state from the onset of subduction, explaining the rarity of porphyry deposits in primitive island arcs, and the late formation of porphyries in continental arc magmatic cycles. Magmas generated from this metasomatized and moderately oxidized mantle source will be hydrous basalts containing high concentrations of sulfur, mainly dissolved as sulfate or sulfite. Some condensed sulfides (melt or minerals) may be present due to the high overall f S 2 , despite the moderately high oxidation state. These sulfides may retain some highly siderophile elements in the source, but are unlikely to be sufficiently voluminous to significantly affect the budget of more modestly sulfide-compatible and more abundant elements such as Cu and Mo. These primary magmas can therefore be considered to be largely Cu-Mo-undepleted, although highly siderophile elements such as Au and platinum group elements (PGE) may be depleted unless no sulfides remain in the source. The latter condition seems unlikely during active subduction because of the continuous flux of fresh sulfur from the slab, but may occur during post-subduction re-melting (leading to potentially Au-rich post-subduction porphyry and alkalic-type epithermal systems). Lower crustal differentiation of main-stage arc magmas results in some loss of Cu to residual or cumulate sulfides, but again the amount appears to be minor, and does not drastically reduce the Cu content of derivative intermediate-composition melts. Fractionation and devolatilization affect the oxidation state of the magma in competing ways, but, while crystallization and segregation of Fe 3 + -rich magnetite can cause reduction in reduced to moderately oxidized evolved magmas, this effect appears to be outweighed by the oxidative effects of degassing reduced or weakly oxidized gaseous species such as H 2 , H 2 S, and S IV O 2 , and preferential solvation and removal of Fe 2 + in saline hydrothermal fluids. Consequently, most arc magmatic suites show slight increases in oxidation state during differentiation, reaching typical values of ΔFMQ = + 1 to + 2. This oxidation state is significant, because it corresponds to the transition from dissolved sulfide to sulfate dominance in magmas. It has been shown that Cu and Au solubilities in silicate magma increase up to this level (ΔFMQ ≈ + 1), but while Cu solubility continues to increase at higher oxidation states, Au shows a precipitous drop as sulfide, which solvates Au in the melt, is converted to sulfate. This may explain the somewhat restricted distribution of Au-rich porphyry Cu deposits, but the general association of porphyry Cu deposits with relatively oxidized magmas. Exsolution of a saline, high temperature aqueous fluid enables metals to partition from the magma into a highly mobile volatile phase. Sulfur also partitions strongly into this fluid phase, predominantly as SO 2 at ΔFMQ = + 1 to + 2. However, as the fluid cools below ~ 400 °C, S IV O 2 disproportionates to form reduced H 2 S − II and oxidized H 2 S VI O 4 . The H 2 S bonds with metals in solution to precipitate as Cu- and Mo-sulfides, while the H 2 SO 4 (and HCl) generates progressively acidic wallrock alteration (phyllic, argillic, advanced argillic). Gold may precipitate with early Cu/Mo-sulfides, but some may also stay in solution as bisulfide complexes, eventually reaching the epithermal environment. Thus, three components, [S], [H 2 O], and f O 2 work together throughout subduction and arc magmatic processes to transport chalcophile and siderophile metals from the mantle into the upper crust, where they may be concentrated by hydrothermal processes to form ore deposits. These processes are far from 100% efficient, and metals (especially highly siderophile elements such as Au and PGE) may be left behind at various stages of the passage of arc magmas through the lithosphere, where they may form potentially metalliferous source rocks for partial melts and subsequent magmatic-hydrothermal ore deposits generated during later tectonomagmatic events. [ABSTRACT FROM AUTHOR]

In the Gangdese belt of southern Tibet, Paleocene-Eocene magmas record the final stage of Neo-Tethyan subduction and are associated with few economic porphyry deposits. In contrast, magmas formed during later stages of the India-Asia collision in the Oligo-Miocene are associated with several large porphyry Cu-Mo ± Au deposits, especially in the eastern part of the belt (~89°E-93°E). In a previous study, we showed that these Oligocene-Miocene magmas were more hydrous than the earlier Paleocene-Eocene magmas. In this study, we show that the later magmas were also more oxidized. Paleocene-Eocene rocks from the eastern Gangdese belt are characterized by low zircon Ce4+/Ce3+ ratios (mostly <50; 6.0-66.8; average = 25.7 ± 18.4, n = 26) and low to moderate ΔFMQ values (-1.2 to +0.8, average = -0.2 ± 0.8, n = 5; estimated from ilmenite-magnetite mineral pairs). In comparison, Miocene igneous rocks from the eastern Gangdese belt show higher zircon Ce4+/Ce3+ ratios (mostly >50; 32.3-141.9, average = 74.3 ± 30.1, n = 33) and higher ... values (ΔFMQ 0.8-2.9, average = 1.8 ± 0.8, n = 6; estimated from magnetite-ilmenite pairs). Estimates of magmatic oxidation state from amphibole compositions also show an increase from ΔFMQ 1.2 to 2.1 (average = 1.6 ± 0.2, n = 40) in the Paleocene-Eocene to 2.0 to 2.8 (average = 2.5 ± 0.2, n = 58) in the Miocene. Sparse whole-rock Fe3+/Fe2+ ratios show the same general trend. The amphibole results are systematically shifted to higher ΔFMQ compared to data from magnetite-ilmenite pairs, but their trend is internally consistent. The higher oxidation states (and water contents) of Miocene igneous rocks from the eastern Gangdese belt may explain their unique association with large porphyry deposits in Tibet, because it has been shown from other studies that the potential for hydrous calc-alkaline magmas to transport Au is maximized near ΔFMQ 1.0, and Cu at higher ... (ΔFMQ > 1.0). In comparison, the somewhat less oxidized and less hydrous Paleocene- Eocene magmas would have been less fertile for the formation of such deposits. We suggest that these differences reflect the tectonomagmatic evolution of the Cenozoic Gangdese collisional orogen, from early collision-related magmatism in the Paleocene-Eocene to late collisional magmatism in the Miocene. Asthenospheric upwelling following slab breakoff in the Miocene caused interaction between mantle-derived magmas and previously subduction modified Tibetan lithosphere. The resultant evolved magmas had high oxidation states and water contents, which are favorable properties for the subsequent formation of magmatic-hydrothermal porphyry Cu-Mo ± Au deposits. [ABSTRACT FROM AUTHOR]

Porphyry Cu ± Mo ± Au deposits are commonly associated with intermediate composition arc-related igneous rocks with high Sr/Y (and La/Yb) ratios, and such rocks are equated by some researchers with melts derived from subducted oceanic crust undergoing eclogite fades metamorphism ("adakites"). However, similar geochemical characteristics are readily developed in normal asthenospheric mantle wedge-derived magmas by fractionation of amphibole and/or garnet, and/or by interaction with crustal materials during ascent through the upper plate lithosphere. While there is widespread evidence for such fractionation and contamination processes in porphyry magmas, there is little direct evidence for an origin by slab melting. The enhanced fertility of arc magmas relates primarily to their high water content, which is required for the formation of magmatic-hydrothermal systems upon shallow crustal emplacement. Magmatic water contents >4 wt percent H2O cause abundant amphibole (±garnet) fractionation but suppression of plagioclase crystallization at deep crustal levels, resulting in increasing Sr/Y and La/Yb ratios with differentiation, commonly into "adakitic" ranges, but not reflecting slab melting. Additional factors that affect arc magma fertility are relatively high oxidation states and sulfur content, but these are secondary to the requirement for sufficient water. Thus, arc magmatic suites with high Sr/Y ratios and evidence for the presence of hydrous phenocryst phases (hornblende and/or biotite) are indeed prospective for porphyry Cu ± Mo ± Au deposits, but only because these parameters indicate high magmatic water content. [ABSTRACT FROM AUTHOR]

Abstract: Metals such as Cu, Mo, Au, Sn, and W in porphyry and related epithermal mineral deposits are derived predominantly from the associated magmas, via magmatic–hydrothermal fluids exsolved upon emplacement into the mid- to upper crust. Four main sources exist for magmas, and therefore metals, in convergent and collided plate margins: the subducting oceanic plate basaltic crust, subducted seafloor sediments, the asthenospheric mantle wedge between the subducting and overriding plates, and the upper plate lithosphere. This paper firstly examines the source of normal arc magmas, and concludes that they are predominantly derived from partial melting of the metasomatized mantle wedge, with possible minor contributions from subducted sediments. Although some metals may be transferred from the subducting slab via dehydration fluids, the bulk of the metals in the resultant magmas are probably derived from the asthenospheric mantle. The most important contributions from the slab from a metallogenic perspective are H2O, S, and Cl, as well as oxidants. Partial melting of the subducted oceanic crust and/or sediments may occur under some restricted conditions, but is unlikely to be a widespread process (in Phanerozoic arcs), and does not significantly differ metallogenically from slab-dehydration processes. Primary, mantle-derived arc magmas are basaltic, but differ from mid-ocean ridge basalt in having higher water contents (~10× higher), oxidation states (~2 log fO2 units higher), and concentrations of incompatible elements and other volatiles (e.g., S and Cl). Concentrations of chalcophile and siderophile metals in these partial melts depend critically on the presence and abundance of residual sulfide phases in the mantle source. At relatively high abundances of sulfides thought to be typical of active arcs where fS2 and fO2 are high (magma/sulfide ratio=102–105), sparse, highly siderophile elements such as Au and PGE will be retained in the source, but magmas may be relatively undepleted in abundant, moderately chalcophile elements such as Cu (and perhaps Mo). Such magmas have the potential to form porphyry Cu±Mo deposits upon emplacement in the upper crust. Gold-rich porphyry deposits would only form where residual sulfide abundance was very low (magma/sulfide ratio >105), perhaps due to unusually high mantle wedge oxidation states. In contrast, some porphyry Mo and all porphyry Sn–W deposits are associated with felsic granitoids, derived primarily from melting of continental crust during intra-plate rifting events. Nevertheless, mantle-derived magmas may have a role to play as a heat source for anatexis and possibly as a source of volatiles and metals. In post-subduction tectonic settings Tulloch and Kimbrough, 2003, such as subduction reversal or migration, arc collision, continent–continent collision, and post-collisional rifting, a subducting slab source no longer exists, and magmas are predominantly derived from partial melting of the upper plate lithosphere. This lithosphere will have undergone significant modification during the previous subduction cycle, most importantly with the introduction of large volumes of hydrous, mafic (amphibolitic) cumulates residual from lower crustal differentiation of arc basalts. Small amounts of chalcophile and siderophile element-rich sulfides may also be left in these cumulates. Partial melting of these subduction-modified sources due to post-subduction thermal readjustments or asthenospheric melt invasion will generate small volumes of calc-alkaline to mildly alkaline magmas, which may redissolve residual sulfides. Such magmas have the potential to form Au-rich as well as normal Cu±Mo porphyry and epithermal Au systems, depending on the amounts of sulfide present in the lower crustal source. Alkalic-type epithermal Au deposits are an extreme end-member of this range of post-subduction deposits, formed from subduction-modified mantle sources in extensional or transtensional environments. Ore formation in porphyry and related epithermal environments is critically dependent on the partitioning of metals from the magma into an exsolving magmatic–hydrothermal fluid phase. This process occurs most efficiently at depths greater than ~6km, within large mid- to upper crustal batholithic complexes fed by arc or post-subduction magmas. Under such conditions, metals will partition efficiently into a single-phase, supercritical aqueous fluid (~2–13wt.% NaCl equivalent), which may exist as a separate volatile plume or as bubbles entrained in buoyant magma. Focusing of upward flow of bubbly magma and/or fluid into the apical regions of the batholithic complex forms cupolas, which represent high mass- and heat-flux channelways towards the surface. Cupolas may be self-organizing to the extent that once formed, further magma and fluid flow will be enhanced along the weakened and heated axes. Cupolas may form initially as breccia pipes by volatile phase (rather than magma) reaming-out of extensional structures in the brittle cover rocks, to be followed immediately by magma injection to form cylindrical plugs or dikes. Cupola zones may extend to surface, where magmas and fluids vent as volcanic products and fumaroles. Between the surface and the underlying magma chamber, a very steep thermal gradient exists (700°–800°C over <5km), which is the primary cause of vertical focusing of ore mineral deposition. The bulk of metals (Cu±Mo±Au) that forms porphyry ore bodies are precipitated over a narrow temperature interval between ~425° and 320°C, where isotherms in the cupola zone rise to within ~2km of the surface. Over this temperature range, four important physical and physicochemical factors act to maximize ore mineral deposition: (1) silicate rocks transition from ductile to brittle behavior, thereby greatly enhancing fracture permeability and enabling a threefold pressure drop; (2) silica shows retrograde solubility, thereby further enhancing permeability and porosity for ore deposition; (3) Cu solubility dramatically decreases; and (4) SO2 dissolved in the magmatic–hydrothermal fluid phase disproportionates to H2S and H2SO4, leading to sulfide and sulfate mineral deposition and the onset of increasingly acidic alteration. The bulk of the metal flux into the porphyry environment may be carried by moderately saline supercritical fluids or vapors, with a volumetrically lesser amount by saline liquid condensates. However, these vapors rapidly become dilute at lower temperatures and pressures, such that they lose their capacity to transport metals as chloride complexes. They retain significant concentrations of sulfur species, however, and bisulfide complexing of Cu and Au may enable their continued transport into the epithermal regime. In the high-sulfidation epithermal environment, intense acidic (advanced-argillic) alteration is caused by the flux of highly acidic magmatic volatiles (H2SO4, HCl) in this vapor phase. Ore formation, however, is paragenetically late, and may be located in these extremely altered and leached cap rocks largely because of their high permeability and porosity, rather than there being any direct genetic connection. Ore-forming fluids, where observed, are low- to moderate-salinity liquids, and are thought to represent later-stage magmatic–hydrothermal fluids that have ascended along shallower (cooler) geothermal gradients that either do not, or barely, intersect the liquid–vapor solvus. Such fluids “contract” from the original supercritical fluid or vapor to the liquid phase. Brief intersection of the liquid–vapor solvus may be important in shedding excess chloride and chloride-complexed metals (such as Fe), so that bisulfide-complexed metals remain in solution. Such a restrictive pressure–temperature path is likely to occur only transiently during the evolution of a magmatic–hydrothermal system, which may explain the rarity of high-sulfidation Cu–Au ore deposits, despite the ubiquitous occurrence of advanced-argillic alteration in the lithocaps above porphyry-type systems. [Copyright &y& Elsevier]

Porphyry Cu ± Mo ± Au and some epithermal Au deposits are formed from hydrothermal fluids exsolved from cooling, water-rich, calc-alkaline magmas emplaced in volcanoplutonic arcs above subduction zones. These magmas originate by partial melting of the metasomatized asthenospheric mantle wedge. However, there is increasing evidence for the existence of a suite of porphyry Cu-Au and epithermal Au deposits related to magmas generated after subduction beneath the arc has ceased. Associated magmas tend to be mildly alkaline, relatively sulfur poor, and emplaced as isolated complexes rather than in voluminous volcanoplutonic arcs. They are likely formed by remelting of previously subduction-modified arc lithosphere, triggered by postsubduction lithospheric thickening, lithospheric extension, or mantle lithosphere delamination. Metasomatized mantle lithosphere or hydrous lower crustal cumulates residual from first-stage arc magmatism contain small amounts of chalcophile and siderophile elementrich sulfides, and constitute a fertile source for hydrous, Au-rich, but relatively sulfur-poor magmàs during later remelting. The recognition that porphyry Cu-Au and related epithermal Au systems can also form in postsubduction and collisional tectonic settings expands the range of geological environments and geographical terranes that are prospective for such deposits. [ABSTRACT FROM AUTHOR]

The article presents a study focusing on Porphyry deposits in several Subduction and Post-Subduction tectonic settings including lithospheric delamination, post-collisional stress and back-arc extension. It mentions that exsolution of magmatic hydrothermal fluids during subduction leads to formation of porphyry and epithermal mineral deposits and discusses the impact of magmatic oxidation state on metallogenic potential of the subduction process.

The article reports on role of collisional tectonism, magmatism, and post-subduction porphyry deposits on origin of Cenozoic Porphyry copper deposits present in Gangdese belt, Tibet, China. Topics discussed include petrographic and mineralogic analysis of Paleocene?Eocene magma rocks, overview by research scientist, E. A. Wang and colleagues, on Oligo-Miocene rocks by petrography, magmatic geochemistry, and porphyry-type mineralization and base of Cenozoic evolution by tectonomagnetism.

• Porphyritic intrusions at Yulong were emplaced over a short period of time, with the mineralized Yulong intrusion being the youngest one. • Magmatic oxidation and hydration states increase as magmas evolve. • All the intrusions are relatively oxidized, hydrous and S-rich, and have the potential to form porphyry deposits. • Magmatic recharge events might be the trigger for extensive fluid exsolution and mineralization at Yulong. The Yulong porphyry Cu-Mo ore district in eastern Tibet features a series of Eocene felsic porphyry intrusions, only one of which is intensively mineralized. In this study, zircon U-Pb ages, and major and trace element compositions of whole rock samples and in-situ accessory minerals (zircon, apatite and titanite) from these various intrusions were analyzed to determine the factors controlling Cu mineralization. New zircon U-Pb dating, together with published zircon U-Pb ages, suggest that these porphyry intrusions were emplaced over a relatively short period of time (43.9 ± 0.6 Ma to 40.9 ± 0.3 Ma), with the mineralized Yulong intrusion emplaced at a late stage of the magmatic activity (41.1 ± 0.3 Ma to 40.9 ± 0.3 Ma). These intrusions have indistinguishable whole-rock major and trace element compositions, but the mineralized Yulong intrusion has higher apatite and titanite La/Yb ratios (average apatite La/Yb = 115.1 ± 28.5, n = 52; average titanite La/Yb = 22.4 ± 4.9, n = 38) than the earlier subeconomic intrusions (average apatite La/Yb = 50.4 ± 12.3, n = 72; average titanite La/Yb = 12.4 ± 3.5, n = 58), suggesting more fractionation of amphibole from the magmas that sourced it. Additionally, a negative relationship between La/Yb and Ce N /Ce N * ratios in apatite and titanite are interpreted to reflect increasing oxidation states through magma evolution. Indistinguishable zircon Eu N /Eu N * values and apatite core SO 3 contents from the mineralized (zircon Eu N /Eu N * = 0.69 ± 0.06, n = 25; apatite SO 3 = 0.65 ± 0.24 wt%, n = 21) and subeconomic intrusions (zircon Eu N /Eu N * = 0.62 ± 0.05, n = 64; apatite SO 3 = 0.61 ± 0.27 wt%, n = 15) suggest that the entire magmatic suite was relatively oxidized, hydrous and sulfur-rich, and was therefore fertile for ore-formation. However, apatite crystals from the mineralized Yulong intrusion have significantly lower Cl contents (0.08 ± 0.03 wt%, n = 72) and higher F/Cl ratios (47.80 ± 21.97, n = 72) than those from the subeconomic intrusions (Cl = 0.29 ± 0.29 wt%; F/Cl = 12.87 ± 5.16; n = 78). These data may reflect more extensive fluid exsolution from the parental magma chamber at the time of emplacement of the mineralizing Yulong magmas. Sudden and voluminous release of volatiles coeval with emplacement of the Yulong intrusion was triggered by an as yet unidentified process, but possibly including magmatic recharge. [ABSTRACT FROM AUTHOR]

The recent discovery of large porphyry copper deposits (PCDs) associated with Miocene (22–12 Ma) granitoid magmas in the eastern section of the Paleocene-Eocene Gangdese magmatic arc in the Himalaya-Tibetan orogenic belt raises new questions about the origin of water-rich (≥4.5 wt.%), oxidized (ΔFMQ 1–3) magmas in continental collisional settings and their mineralization potential. We review the literature and compile available data on whole rock and isotope geochemistry for Cenozoic igneous rocks from Tibet, and add new zircon Ce 4+ /Ce 3+ and Ti-in-zircon thermometry data to better understand variations in oxidation state and thermal evolution of these suites, which are key controls on Cu mineralization. Six distinct Cenozoic igneous suites are defined: Paleocene-Eocene syn-collisional Gangdese magmatic arc rocks (ΔFMQ = −1.2 to +0.8) (suite I), and five broadly contemporaneous Miocene suites. A distinct change in magmatism along the length of the belt occurs at around 88°E in the Miocene suites: to the east, porphyry copper mineralization is associated with a moderately oxidized, high-Sr/Y granitoid suite (suite II, ΔFMQ = +0.8 to +2.9) with minor occurrences of transitional (hybrid) monzonitic (suite III) and trachytic rocks (suite IV; both with zircon Ce 4+ /Ce 3+  > 50–100, Eu N /Eu N * = ~0.5, and ΔFMQ = ~+1 to +2). To the west of 88°E, trachytic volcanic rocks (suite V) are more voluminous but more reduced (zircon Ce 4+ /Ce 3+  < 50, ΔFMQ < +1), and are associated with sparse, poorly mineralized high-Sr/Y granitoids (suite VI) which are moderately oxidized (zircon Ce 4+ /Ce 3+  = 20–100, ΔFMQ = ~+1 to +3). The Miocene high-Sr/Y granitoids have many compositional and isotopic similarities to the Paleocene-Eocene Gangdese arc rocks, and are interpreted to have been derived by melting of the hydrated arc root, with minor mantle input. In contrast, the highly evolved isotopic signatures of the Miocene trachytic rocks, combined with deep seismic profiles and a xenolith-derived geotherm, suggest their derivation from the underthrust Indian Proterozoic subcontinental lithospheric mantle (SCLM) or old fore-arc Tibetan SCLM during phlogopite breakdown at temperatures of ~1100°C. Based on published geophysical data and tectonic reconstructions, we develop a model that explains the origin of the various Miocene magmatic suites, their spatial differences, and the origin of related PCDs. Following the early stages of continental collision (Eocene–Oligocene), shallow underthrusting of the Indian continental lithosphere and subcretion of Tethyan sediments (including oxidized carbonates and possibly evaporites) under eclogite facies conditions promoted the release of aqueous fluids, which hydrated and oxidized the base of the overlying Tibetan plate. This metasomatism rendered the Tibetan lower crust fusible and fertile for metal remobilization. During the mid-Miocene, the Indian slab steepened in the eastern sector (east of ~88°E). In this eastern belt, deeply derived trachytic magmas were trapped in melt zones at the base of the Tibetan crust, and variably mixed with the crustally-derived, high Sr/Y granitoid magmas. They may also have released water that contributed to fluid-fluxed melting of the lower crust, producing voluminous high-Sr/Y granitoid magmas, which were associated with significant PCD mineralization. Hybridization between the trachytic magmas and lower crustal partial melts is indicated by intermediate isotopic compositions, enriched Cr and Ni contents, and high Mg # in some intermediate-to-felsic (56–70 wt. % SiO 2 ) high-Sr/Y granitoids. Trapping of the trachytic melts in deep crustal melt zones explains the relatively small volumes of trachytic magmas erupted at surface in the east. In contrast, to the west of ~88°E, subduction of the Indian plate has remained flat to the present day, preventing incursion of hot asthenosphere. Consequently, cooler conditions in the deep Tibetan lithosphere resulted in limited crustal melting and the production of only small volumes of high-Sr/Y granitic magmas. Trachytic melts ascending from the underthrust Indian or Tibetan plate were able to pass through the cooler lower crust and erupted in greater volume at surface, whereas only small volumes of high-Sr/Y granitoid magma were generated and are not associated with significant PCD mineralization. [ABSTRACT FROM AUTHOR]