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Rare earth elements (REEs) are essential metals for modern technologies. Recent studies suggest that subcontinental lithospheric mantle (SCLM) remelting, previously fertilized by subducted marine sediments, leads to formation of REE-bearing rocks. However, the transfer mechanism of REE-rich sediments from the subducted slab to the overlying mantle wedge is unclear. We present high-pressure experiments on natural REE-rich marine sediments at 3-4 GPa and 800-1000 °C to constrain the phase relations, sediment melting behavior, and REE migration during subduction. Our results show recrystallization into an eclogite-like assemblage, with melting only occurring at 4 GPa, 1000 °C, experiments. Regardless of melting behavior, REE are refractory and mostly hosted by apatite. Buoyancy calculations suggest that most of the eclogite-like residues would form solid-state diapirs, ascending to the SCLM, resulting in the REE-fertilized source. Such flux may be required for substantial REE transport during subduction, as a foundation for economicgrade mineralization. [ABSTRACT FROM AUTHOR]

Recent studies argue that subduction-modified, Cu-fertilized lithosphere controls the formation of porphyry Cu deposits in orogenic belts. However, it is unclear if and how this fertilization process operates at cratonic edges, where numerous large non-arc Au-rich deposits form. Here we report data from lower crustal amphibolite and garnet amphibolite xenoliths hosted by Cenozoic stocks that are genetically related to the Beiya Au-rich porphyry deposits along the western margin of the Yangtze craton, China. These xenoliths are thought to represent cumulates or residuals of Neoproterozoic arc magmas ponding at the base of arc at the edge of the craton that subsequently underwent high-pressure metamorphism ca. 738 Ma. The amphibolite xenoliths are enriched in Cu (383-445 ppm) and Au (7-12 ppb), and a few garnet amphibolite xenoliths contain higher Au (6-16 ppb) with higher Au/Cu ratios (2 × 10-4 to 8 × 10-4) than normal continental crust. These data suggest that metal fertilization of the base of an old arc at the edge of the craton occurred in the Neoproterozoic via subduction modification, and has since been preserved. The whole-rock geochemical and zircon Hf isotopic data indicate that melting of the Neoproterozoic Cu-Au-fertilized low-crustal cumulates at 40-30 Ma provided the metal endowment for the Au-rich porphyry system at the cratonic edge. We therefore suggest that the reactivated cratonic edges, triggered by upwelling of asthenosphere, have the potential to host significant Au ore-forming systems, especially non-arc Au-rich porphyry deposits. [ABSTRACT FROM AUTHOR]

Most known primary Au deposits are produced by hydrothermal processes. In this paper, we report Au mineralization of magmatic origin in the Bilihe deposit, China (8.5 Mt averaging 2.9 g/t Au). At Bilihe, most (>70%) native Au (no detectable Ag) grains are euhedral or subspherical in shape, and occur as trails in dendritic quartz phenocrysts and comb-layered quartz in a moderately reduced, highly fractionated diorite-granite intrusion. The hosting quartz typically has a dendritic core (Ql) and a rim (Q2), with Q1 having concentric zoning and sector zoning in cathodoluminescence (CL) images. Cathodoluminescence petrography and crystallographic modeling reveal that most of the Au trails occur along intersecting crystallographic planes of the host quartz, indicating simultaneous precipitation of both Au and quartz. Abundant melt inclusions are present in Q2 with >950°C homogenization temperatures. Minor Au grains also occur in melt inclusions in quartz. In rare cases, necking of Au melt inclusions is present. Neither Ql nor Q2 contain primary fluid inclusions; only secondary fluid inclusions were found in healed cracks. The above observations indicate a direct magmatic (quartz phenocryst phase) origin for the Au. This defines a new type of Au deposit, and thereby opens new potential for Au exploration. The magmatic origin of Au at Bilihe also implies that enrichment of Au may occur in a source melt prior to volatile escape, which would enhance the possibility of forming a magmatic-hydrothermal Au deposit. [ABSTRACT FROM AUTHOR]

Magmatic-hydrothermal ore deposits in collisional orogens are new targets for modern mineral exploration, yet it is unclear why they preferentially occur in some specific tectonic environments within these orogenic belts. We integrate geologic and geochemical data (especially zircon U-Pb dating and Lu-Hi isotope data) for Mesozoic-Cenozoic magmatic rocks and associated ore deposits in the Lhasa terrane, a highly endowed tectonic unit within the Himalayan-Tibetan orogen, and provide the first example in a continental collision terrane of the application of zircon Hf isotope data to image the lithospheric architecture and its relationship with ore deposits. Three crustal blocks are identified within the Lhasa terrane by the Hf isotope mapping method. They include a central long-lived Precambrian microcontinent with local reworking and two surrounding juvenile Phanerozoic crustal blocks with significant mantle contributions to constituent magmatic rocks. The three crustal blocks are bounded by two E-W-trending terrane-boundary faults, and each block is cut by two N-S-striking concealed faults. Isotopic signatures of zircons from the juvenile crustal blocks indicate that the Phanerozoic continental crust grew from several Mesozoic voleanic-plutonic arcs and by underplating of mantle-derived magmas generated during Mesozoic accretion and Cenozoic collision. Mesozoic subduction-related porphyry Cu-Au deposits and Cenozoic collision-related Cu-Mo deposits are exclusively located in regions with high εHf (>5) juvenile crust. Cu enrichment during differentiation of high fO2 arc magmas is the key for the formation of Mesozoic subduction-related porphyry Cu-Au. By contrast, remelting of the lower crustal Cu sulfide-rich magmatic cumulates within the juvenile crust is interpreted to have played a key role in the formation of Cenozoic collision-related Cu-Mo deposits. Granite-related Pb-Zn deposits cluster in the oldest crustal regions or developed along the margin of the old crustal block bounded by lithospheric faults. The porphyry Mo deposits are localized along the reworked margins of the old crustal block. It is suggested that crustal reworking released Mo from the old crust to form porphyry Mo deposits, whereas leaching of Pb and Zn from the Paleozoic carbonate cover strata by felsic intrusion-driven fluids is critical to the formation of Pb-Zn ore deposits. Skarn Fe-Cu ore deposits are typically localized along a terrane boundary fault, i.e., lithospheric discontinuity, through which crust-derived felsic melt mixed with Cu-rich mantle-derived mafic magmas ascending upward. Associated granitoid rocks usually bear microgranular mafic enclaves and show a zircon Hf isotope array from negative to positive εHf values (-7.3 to +6.7), supporting mixing of juvenile mantle and evolved crustal sources. The Hf isotope maps show temporal-spatial relationships between crustal structure and the location of ore deposits, demonstrating that the structure, nature, and composition of the crust controlled the localization of ore deposits and the migration of ore-forming metals in the terrane. This study shows that the lithospheric architecture of an orogenic terrane can be imaged by Hf isotope mapping to provide mappable units which can be used to explore for ore deposits at the terrane scale. [ABSTRACT FROM AUTHOR]

The eastern Gangdese magmatic belt (between ~88°E and 94°E) in southern Tibet preserves a series of Cenozoic collision-related igneous rocks that have formed since the start of the India-Asia collision at ~55 to 50 Ma. The Paleocene-Eocene intrusive rocks have intermediate [La/Yb]N ratios (average = 12.0 ± 8.6, n = 150, range = 1.15-64.0), intermediate to low Sr/Y ratios (mostly < 40), and negative Eu anomalies (average Eun/Eu* = 0.76 ± 0.19, n = 150, range = 0.23-1.33). They are mainly composed of equigranular pyroxene and plagioclase in mafic intrusions or porphyritic plagioclase and quartz in intermediate-felsic intrusions, with minor interstitial amphibole and biotite. In contrast, Oligo-Miocene intrusive igneous rocks have high [La/Yb]N ratios (Oligocene: average [La/Yb]N = 42.8 ± 17.0, n = 35, range = 23.2-93.2; Miocene: average [La/Yb]N = 28.5 ± 8.8, n = 93, range = 6.7-49.1), high Sr/Y ratios (>40), and weak or absent Eu anomalies (Oligocene: average Eun/Eu* = 0.99 ± 0.27, n = 35, range = 0.67-1.58; Miocene: average Eun/Eu* = 0.95 ± 0.13, n = 93, range = 0.44-1.46), and their mineralogy consists mainly of plagioclase, quartz, and amphibole as phenocrysts. These geochemical and mineralogical characteristics suggest that the Paleocene-Eocene magmas were relatively dry and evolved primarily by fractionation of pyroxene and plagioclase, whereas the Oligo-Miocene magmas were more hydrous and fractionated significant amounts of hornblende and lesser plagioclase prior to upper crustal emplacement. Oligocene and Miocene magmatic rocks crop out as small-volume intrusions and are associated with several large- to giant-sized porphyry Cu-Mo ± Au deposits, as well as numerous smaller porphyry and skarn deposits. In contrast, earlier, more voluminous Paleocene-Eocene magmatism is only known to be associated with three small porphyry deposits. Erosional loss of subvolcanic porphyry systems from the older Paleocene-Eocene sequence is not thought to be a controlling factor in this temporal difference of porphyry deposit distribution, because exhumation rates after 55 Ma were relatively low and coeval Linzizong volcanic rocks are extensively preserved in the Gangdese belt. Instead, we suggest that differences in magmatic history and petrogenesis led to this restricted temporal distribution of porphyry deposits. The Paleocene-Eocene magmas were generated during the onset of collision between India and Asia and were triggered by rollback of the Neo-Tethyan oceanic slab. This magmatism records the final stage of subduction of oceanic lithosphere beneath southern Tibet, and the relatively low water contents of these magmas may reflect final dehydration of the remnant Neo-Tethyan slab. After oceanic slab breakoff at ~40 to 38 Ma, the Greater India slab began to subduct beneath the Gangdese belt until hard collision with the Indian continent at ~35 Ma and breakoff of the Greater India slab at ~25 to 10 Ma. This last event is suggested to have caused asthenospheric mantle upwelling and partial melting of subduction-modified, hydrated Tibetan lithosphere. The resultant partial melts may have remobilized metals residual in the deep lithosphere from previous arc magmatism, giving rise to a suite of postsubduction porphyry-type ore deposits in the Oligo-Miocene. [ABSTRACT FROM AUTHOR]

Major and trace element, Sr–Nd–Pb isotope and mineral chemical data are presented for newly discovered ultrapotassic lavas in the Tangra Yumco–Xuruco graben in southern Tibet. The ultrapotassic lavas are characterized by high MgO, K2O and TiO2, low Al2O3 and Na2O contents, and also have high molar K2O/Al2O3, molar (K2O + Na2O)/Al2O3 and K2O/Na2O ratios. Their high abundances of incompatible trace elements such as large ion lithophile elements (LILE) and light rare earth elements (LREE) reach the extreme levels typical of lamproites. The lamproites show highly radiogenic 87Sr/86Sr (0· 7166–0· 7363) and unradiogenic 143Nd/144Nd (0· 511796–0· 511962), low 206Pb/204Pb (18· 459–18· 931), and elevated radiogenic 207Pb/204Pb (15· 6732–15· 841) and 208Pb/204Pb (39· 557–40· 058) ratios. On the basis of their geochemical and isotopic systematics, the lamproites in south Tibet have a distinct magma source that can be differentiated from the sources of potassic lavas in the east Lhasa and Qiangtang blocks. Their high Nb/Ta ratios (17· 10–19· 84), extremely high Th/U ratios (5· 70–13· 74) and distinctive isotope compositions are compatible with a veined mantle source consisting of partial melts of subducted Tethyan oceanic sediments and sub-continental lithospheric depleted mantle. Identification of the lamproites and the delineation of their mantle source provide new evidence relevant for models of the uplift and extension of the Tibetan plateau following the Indo-Asia collision. Metasomatism by partial melts from isotopically evolved, old sediment subducted on the young Tethyan slab is an alternative explanation for Precambrian Nd and Pb model ages. In this model, differences in isotopic composition along-strike are attributed to differences in the type of sediment being subducted, thus obviating the need for multiple metasomatic events over hundreds of million years. The distribution of lamproites, restricted within a north–south-trending graben, indicates that the initiation of east–west extension in south Tibet started at ∼ 25 Ma. [ABSTRACT FROM AUTHOR]

We present new whole rock trace element and Pb-isotope data for a suite of Neogene adakitic rocks that formed during the post-collisional stage of the India-Asia collision in an east-west- trending array along the Yalu Tsangpo suture. Compared to classic ‘adakites’ that form along certain active convergent plate margins, the Tibetan adakitic rocks show even stronger enrichment in incompatible elements (i.e. Rb, Ba, Th, K and LREEs) and even larger variation in radiogenic (Pb, Sr, Nd) isotope ratios. Tibetan adakitic rocks have extraordinarily low HREE (Yb: 0.34–0.61 ppm) and Y (3.71–6.79 ppm), high Sr/Y (66–196), high Dyn/Ybn and Lan/Ybn. They show strong evidence of binary mixing both in isotopic space (Sr-Nd, common Pb, thorogenic Pb) and trace element systematics. The majority of the adakitic rocks in south Tibet, including published and our new data, have variational Mg# (0.32–0.70), clear Nb (and HFSE) enrichment, the lowest initial 87Sr/86Sr and 206Pb/204Pb ratios, and the highest 144Nd/143Nd ratios of all Neogene volcanic rocks in south Tibet. These results indicate an involvement of slab melts in petrogenesis. Major and trace element characteristics of the isotopically more enriched adakites are compatible with derivation from subducted sediment but not with assimilation of crustal material. Thus, the south Tibetan adakitic magmas are inferred to have been derived from an upper mantle source metasomatised by slab-derived melts. An interesting observation is that temporally coeval and spatially related lamproites could be genetically related to the adakitic rocks in representing partial melts of distinct mantle domains metasomatised by subducted sediment. Our favoured geodynamic interpretation is that along-strike variation in south Tibetan post-collisional magma compositions may be related to release of slab melts and fluids along the former subduction zone resulting in compositionally distinct mantle domains. [ABSTRACT FROM AUTHOR]

We present new Re–Os molybdenite age data on three porphyry Cu–Mo–Au deposits (Yulong, Machangqing, and Xifanping). These deposits are associated with the Himalayan adakitic magmatism that occurred in a continental collision environment, controlled by large-scale Cenozoic strike-slip faults in the eastern Indo–Asian collision zone. Three distinct episodes of Cu–Mo–Au mineralization are recognized. At Yulong, Re–Os isotopic data of four molybdenite samples from sulfide-quartz veins in the quartz–sericite alteration zone yield an isochron with an age of 40.1±1.8 Ma (2σ), coincident to a zircon sensitive high-mass resolution ion microprobe (SHRIMP) age of 40.9±0.1 Ma for the host monzogranite. The molybdenite Re–Os dates, together with K–Ar, Rb–Sr, U–Pb, and 40Ar/39Ar dates on the pre- and intra-ore porphyries, suggest that Cu–Mo–Au mineralization formed during the late stage (∼40 Ma) of regional porphyry magmatism, but hydrothermal activity probably lasted to at least ∼36 Ma. At Machangqing, molybdenite Re–Os data from the K–silicate and quartz–sericite alteration zones yield an isochron with an age of 35.8±1.6 Ma (2σ), which is identical to the zircon SHRIMP and bulk-rock Rb–Sr ages (35∼36 Ma) of the host granite, but older than bulk-rock K–Ar dates (31∼32 Ma) for associated Au-bearing quartz syenite with advanced argillic alteration. At Xifanping, five molybdenite samples from the K–silicate alteration zone yield the youngest Re–Os isochron age in the area, at 32.1±1.6 Ma (2σ). The Re–Os molybdenite dates here are younger than K–Ar ages (33.5∼34.6) for hydrothermal biotite and actinolite. There is a positive correlation between the absolute age of the deposits and their Cu and Au reserves in the eastern Indo–Asian collisional zone. Episodic stress relaxation probably caused multiple magmatic intrusions, which most likely resulted in three episodes of Cu–Mo–Au mineralization in the eastern Indo–Asian collision zone. [ABSTRACT FROM AUTHOR]

The migration and character of magmatism over time can provide important insights into the tectonic evolution of an orogen. We present evidence for three separate stages of compositionally distinct granitoid magmatism associated with the Acadian orogenic cycle in the eastern and southern Newfoundland Appalachians. The interpretations are based on new zircon U-Pb ages, geochemical data, and Sr-Nd-Hf-O isotopic data for 18 samples from 15 Silurian and Devonian granitoid plutons, combined with previously published data. The three stages outline hinterland- and foreland-directed migration trends and represent subduction (435-420 Ma), syncollision (415-405 Ma), and postcollision (395-370 Ma) settings in the Acadian orogenic cycle. The Silurian plutons (435-420 Ma) of the first stage consist mainly of quartz diorite, tonalite, granodiorite, monzogranite, and syenogranite, with high-K calc-alkaline and enriched Sr-Nd-Hf-O isotopic compositions (e.g., eNd[t] = -5 to -2; eHf[t] = -3 to -1; d18O = +6? to +8?). They are interpreted to record the subduction of oceanic lithosphere of the Acadian seaway that separated the leading edge of composite Laurentia, represented by the Gander margin, and Avalonia. Early Devonian plutons (415-405 Ma) of the second stage contain more voluminous monzogranite and syenogranite; they have calc-alkaline to high-K calc-alkaline features, adakite-like compositions, and more depleted Sr-Nd-Hf-O isotopic compositions (e.g., eNd[t] = -6 to 0; eHf[t] = +1 to +3; d18O = +5? to +6?). Plutons of this stage occur mostly to the northwest of the Silurian granitoids, indicating a regional-scale northwestward (hinterland-directed) migration of magmatism with a rate of >9 km/m.y. The migration is interpreted to have been related to the progressive shallow underthrusting of Avalonia beneath the Gander margin (composite Laurentia) at least as far as 90 km inboard. The Middle to Late Devonian plutons of the third stage (395-370 Ma) consist mainly of monzogranite, syenogranite, and alkali-feldspar granite, which are silica- and alkali-rich granites with large negative Eu anomalies. These rocks are concentrated along both sides of the Dover-Hermitage Bay fault zone, which represents the boundary between Avalonia and composite Laurentia, to the southeast of the Silurian and Early Devonian igneous rocks. This stage of magmatism represents a foreland-directed (retreating) migration. The Early Devonian and Middle to Late Devonian episodes of magmatism were separated by a gap between 405 Ma and 395 Ma and recorded an evolution from (high-K) calc-alkaline to alkaline compositions, ascribed to partial delamination of Avalonian lithospheric mantle in a postcollisional setting. [ABSTRACT FROM AUTHOR]

Whereas the genesis of carbonatitic rare earth element (REE) deposits has long been a focus of study, the controls on mobilization and mineralization of REEs during magmatic-hydrothermal processes still remain open to debate. Here, we present our investigation of the dissolution and crystallization of REE (fluor)carbonate minerals in alkaline carbonate brine-melts up to 850°C and 11.6 kbar. Our results show that REEs are soluble in Na2CO3 brine-melts, achieving concentrations exceeding 8 weight % at temperatures above 650°C. The addition of calcium and/or fluoride has minimal impact on REE mobilization, whereas introduction of silica suppresses REE solubilities by half, due to britholite formation above 550°C. Upon cooling, sodium and REEs combine to crystallize in burbankite or carbocernaite in sodium-enriched brine-melts, even at fluoride saturation. However, while the brine-melts contain substantial ferro-or aluminosilicate, REE mineralization in fluorcarbonates occurs after sufficient sodium precipitation in alkaline silicate minerals, hence revealing how silicate and sodium carbonate govern REE mineralization. [ABSTRACT FROM AUTHOR]

The Angouran deposit (19.3 Mt at 23.4% Zn and 4% Pb) is the second-largest Zn-Pb deposit in Iran. The deposit is hosted in a Neoproterozoic-Cambrian marble-schist sequence within a breccia pipe in a domal structure, with sulfide mineralization under low-temperature hydrothermal conditions (<200 °C). The features of the ore-hosting breccias are similar to known halokinetic diapir breccias in the world but evaporite minerals are subtle. The common types of breccia clasts in the Angouran breccia pipe include a matrix-supported angular clast (float breccia) with highly variable sizes and orientations and exotic volcanic clasts. The volcanic clasts were derived from the underlying Miocene volcanic rocks, evidenced by the consistent petrography and zircon U-Pb ages dated at 20-19 Ma. Abundant smithsonite pseudomorphs after anhydrite and anhydrite inclusions within sphalerite and pre-ore marcasite in the breccia matrix indicate that the breccia pipe contains abundant anhydrite prior to the Zn-Pb mineralization. The enrichment of evaporite is also supported by the occurrence of considerable double-terminated quartz crystals that contain spherical and tabular carbonate inclusions and anomalously high Li, Na, and K concentrations, relatively high B concentration, and high d18O values (up to 28.3‰). These observations suggest the Angouran deposit formed in a former halokinetic diapir breccia pipe. The halokinetic diapirism was possibly triggered by thrust loading of the marble-schist sequence over the Miocene evaporite beds during the Arabia-Eurasia continental collision. Halokinetic structures elsewhere in the Angouran region warrant this consideration. Most of the evaporite minerals in the breccia pipe were dissolved and replaced before and/or during subsequent Zn-Pb sulfide and smithsonite mineralization events. This study provides a good example for the identification of vanished evaporites, halokinetic structure, and associated Mississippi Valley-type mineralization. [ABSTRACT FROM AUTHOR]

Crustal growth is commonly associated with porphyry deposit formation whether in continental arcs or collisional orogens. The Miocene high-K calc-alkaline granitoids in the Gangdese belt in southern Tibet, associated with porphyry copper deposits, are derived from the juvenile lower crust with input from lithospheric mantle trachytic magmas, and are characterized by adakitic affinity with high-Sr/Y and La/Yb ratios as well as high Mg# and more evolved isotopic ratios. Researchers have argued, lower crust with metal fertilization was mainly formed by previous subduction-related modification. The issue is that the arc is composed of three stages of magmatism including Jurassic, Cretaceous, and Paleocene--Eocene, with peaks of activity at 200 Ma, 90 Ma, and ca. 50 Ma, respectively. All three stages of arc growth are essentially similar in terms of their whole-rock geochemistry and Sr-Nd-Hf isotopic compositions, making it difficult to distinguish Miocene magma sources. This study is based on ~430 bulk-rock Sr-Nd isotope data and ~270 zircon Lu-Hf isotope data and >800 whole-rock geochemistry analyses in a 900-km-long section of the Gangdese belt. We found large scale variations along the length of the arc where the Nd-Hf isotopic ratios of the Jurassic, Cretaceous, and Paleocene--Eocene arc rocks change differently from east to west. A significant feature is that the spatial distribution of Nd-Hf isotopic values of the Paleocene--Eocene arc magmas and the Miocene granitoids, including metallogenic ones, are "bell-shaped" from east to west, with a peak of εNd(t) and εHf(t) at ~91°E. In contrast, the Jurassic and Cretaceous arc magmas have different isotopic distribution patterns as a function of longitude. The isotopic spatial similarity of the Paleocene--Eocene and Miocene suites suggests that the lower crust source of the metallogenic Miocene magmas is composed dominantly of the Paleocene--Eocene arc rocks. This is further supported by abundant inherited zircons dominated by Paleocene--Eocene ages in the Miocene rocks. Another important discovery from the large data set is that the Miocene magmatic rocks have higher Mg# and more evolved Sr-Nd-Hf isotopic compositions than all preceding magmatic arcs. These characteristics indicate that the involvement of another different source was required to form the Miocene magmatic rocks. Hybridization of the isotopically unevolved primary magmas with isotopically evolved, lithospheric mantle-derived trachytic magmas is consistent with the geochemical, xenolith, and seismic evidence and is essential for the Miocene crustal growth and porphyry deposit formation. We recognize that the crustal growth in the collisional orogen is a two-step process, the first is the subduction stage dominated by typical magmatic arc processes leading to lower crust fertilization, the second is the collisional stage dominated by partial melting of a subduction-modified lower crust and mixing with a lithospheric mantle-derived melt at the source depth. [ABSTRACT FROM AUTHOR]