Search

The Gangdese porphyry Cu-Au belt (South Tibet, China) in the eastern Tethyan metallogenic belt hosts many deposits, which formed in subduction and collisional settings. Several porphyry Cu-Au deposits have been discovered in the Xietongmen region, including a large porphyry Cu-Au deposit at Xiongcun. However, the formation mechanism for the Xietongmen Cu-Au deposits and their igneous host rocks remain enigmatic. This study reports whole-rock major- and trace-element compositions, Sr-Nd isotopes, in situ zircon Hf- and plagioclase Sr-isotope compositions, and major- and trace-element compositions of amphiboles to constrain the magmatic emplacement and evolution related to the porphyry Cu-Au mineralisation in the Xietongmen region. New zircon U-Pb ages show that the Xietongmen diorites formed at 185 Ma, consistent with the age of Cu-Au mineralisation. The Xietongmen diorites are characterised by enrichment in LILEs and LREE, and depletion in HFSEs. These features suggest that the diorites were produced by arc magmatism in an oceanic-slab subduction environment. In situ isotopic compositions exhibit radiogenic ε Hf (t) (+9.8 to +14.4) and low initial 87 Sr/ 86 Sr values (0.7039–0.7043); whole-rock results show positive ε Nd (t) (+5.26 to +5.31) and unradiogenic initial 87 Sr/ 86 Sr values (0.7042–0.7043). These isotopic results are similar to coeval mantle-derived hornblende gabbros and the Sangri volcanic rocks in the Gangdese belt. Amphibole clots in the Xietongmen diorites have higher MgO and Ni contents, with AlT > 1.0, Mg# > 0.5, Ni > 143 ppm, and REE patterns different from those of magmatic and idiomorphic amphiboles in the host diorites, suggesting that the clots are xenocrysts. All the petrological and geochemical evidence suggests that the Xietongmen mineralised diorites were derived from a Jurassic hydrous mantle wedge induced by slab dehydration. These magmas contain higher contents of water and other volatiles and have higher oxidation states. These conditions favored the transfer of metals (such as Cu, Au) to the upper crust, and then formed the Jurassic porphyry Cu-Au deposits. [ABSTRACT FROM AUTHOR]

In-situ analysis of minerals, such as zircon hafnium (Hf) and oxygen (O) isotopes, has been pivotal in investigating magmatic evolution and related ore systems. In order to better constrain the petrogenesis of Cu-porphyries and related ore-forming processes in southern Tibet, we used a combined application of both zircon by Hf-O isotopes and apatite by developing geochemical indicators involving Sr-O isotopes and volatile (chlorine and sulfur) concentrations. Apatite has several advantages over zircon: it carries ore-related volatiles, is present in less-evolved magmas and is sensitive to magmatic processes. Data on major- and trace elements and Sr-O isotopes in gem-like apatite can supplement information from zircon Hf-O isotopes. We have applied these techniques to granitoids from the Jurassic arc in southern Tibet to illustrate their use in tracing volatile evolution and related genesis of porphyry systems. We demonstrate that (1) robust apatite Sr-O isotopes combined with zircon Hf-O isotopes can record recycled oceanic components in the sources of Gangdese Jurassic porphyries; (2) volatile-enriched magmas metasomatised hydrous mantle wedge to form the Jurassic mineralised rocks; (3) the ore-forming magma with higher contents of water, Cl and S, favoured the transfer of metals and volatiles to the upper crust and to form the Jurassic porphyry Cu-Au deposits. [ABSTRACT FROM AUTHOR]

To investigate the behavior of Li isotopes during magmatic differentiation and the petrogenesis of Cu-bearing ore deposits, a suite of post-collisional adakitic rocks from Qulong region, southern Tibet, was studied. Their lithologies range from diorite through granodiorite to granite porphyry with the latter containing giant Cu deposits. Detailed evidence of field observation and geochemical signature suggest that these three sets of rocks were most likely formed by various degrees of partial melting and fractional crystallization from the same source. The dioritic enclave, granodiorite and granite porphyry have δ7Li values ranging from 0.2 to 8.2‰, 3.1 to 6.8‰, and 3.9 to 7.4‰, respectively. Most of these samples are overlapping in Li isotope composition, comparable to other granitoids worldwide, indicating insignificant Li isotope fractionation during partial melting and magma differentiation in adakite-like rocks. By contrast, their Li concentrations are mainly controlled by fractional crystallization as suggested from different modal mineralogy. This process does not lead to further enrichment of Cu although they have initial high concentrations (an average of ∼ 104 ppm). In comparison, granite porphyry has extremely high Cu contents (up to 2000 ppm) and their δ7Li values are positively correlated with Cu content, suggesting the involvement of magmatic fluids that most likely exsolved from deep magma chamber. Such fluids not only modified the Li isotopic compositions of granite porphyries, but also extracted metal elements from the highly evolved magma, eventually resulting in the Cu mineralization. Our work here provides new insight into the formation and evolution of the porphyry Cu-bearing deposit. [ABSTRACT FROM AUTHOR]

Abstract Cenozoic high–Sr/Y shoshonites in western Yunnan, China, are spatially and temporally associated with porphyry deposits and mafic volcanic rocks that formed at a post–collisional setting. However, the petrogenesis of these rocks and the origins of the associated mineralization remain unclear. Here we present new geochemical data for the high–Sr/Y shoshonites and associated mafic microgranular enclaves (MMEs) in the Jinshajiang–Ailaoshan mineralization belt, which formed as a result of India–Asia collision during the Cenozoic. Both fertile and barren shoshonitic porphyries were emplaced during the Eocene–Oligocene (32–37 Ma) and are characterized by enrichment in large–ion lithophile elements (LILE), depletion in high field strength elements (HFSE), high K 2 O (4–6 wt%), Sr contents (674–1370 ppm), and low Y contents (7–18 ppm). The barren Zhanhe, Yongsheng, Songgui, and Jianchuan, and fertile Machangqing (Cu–Au–Mo mineralization) porphyry intrusions have variable zircon εHf(t) values of −6 to +9 and old T DM2 ages of 1433 to 900 Ma. However, the barren Shigu porphyry intrusion yields relatively positive zircon εHf(t) values (−4 to +8) and younger T DM2 ages (903–631 Ma). These features indicate that the Shigu intrusion was derived from late Permian juvenile lower crust, and the other high Sr/Y shoshonites were derived mainly from Neoproterozoic juvenile lower crust. Fertile and barren porphyries have variable whole–rock εNd(t) values (−5 to −0.7) and (87Sr/86Sr) i ratios (0.70595–0.70788) that are within the range of coeval mafic magmas in western Yunnan. The MMEs comprise mafic enclaves and andesitic enclaves that formed during the Eocene (34–35 Ma) and have spheroidal shapes, igneous textures and contain acicular apatites. This indicates that the MMEs are globules of coeval mafic magma that was injected into and mixed with the host shoshonitic magma. The enclaves show low TiO 2 contents (0.7–0.8 wt%), high MgO (6–9 wt%), Cr (184–450 ppm) and Ni contents (98–157 ppm), positive LILE anomalies, marked negative HFSE anomalies, a narrow range of εNd(t) values (−1.2 to −0.6), and (87Sr/86Sr) i values of 0.70556–0.70635. The andesitic enclaves show adakitic geochemical affinities and have similar zircon εHf(t) values (−2.7 to +2.5) to the host porphyries, whereas, the mafic enclaves have relatively negative zircon εHf(t) values (−8.7 to +1.5), similar to enriched mantle. These evidences suggest the high−Sr/Y shoshonites were produced by partial melting of juvenile lower crust mixed with ultrapotassic or potassic mafic magmas as represented by the andesitic enclaves. The mafic enclave melts were derived from enriched mantle metasomatized by slab–derived fluids, which elevated the Mg#, Cr, Ni contents of the host porphyries at Machangqing, Zhanhe and Yongsheng. Zircon saturation temperatures (T Zr) of the high–Sr/Y shoshonitic rocks range from 735 to 777 °C and indicate the shoshonites were derived mainly from water–fluxed melting of lower crust. The Manchangqing fertile intrusion had a high oxidation state (zircon Ce4+/Ce3+ = 38–180, average = 91) and was derived by mixing between juvenile lower–crustal melts and voluminous coeval lamprophyres. In contrast, the barren Jianchuan, Songgui, Shigu, Yongsheng and Zhanhe high–Sr/Y shoshonites formed by partial melting of juvenile lower crust and some mixing with mafic lavas, and are characterized by low oxygen fugacity (zircon Ce4+/Ce3+ = 0.5–77, average = 20). As such, we suggest the mafic magmas triggered water–fluxed, moderate–degree partial melting of the lower crust and caused amphibole breakdown during melting. This process could not provide enough water for the formation of porphyry Cu-Au deposits in western Yunnan, and therefore formed low–fO 2 , relatively low–H 2 O, and barren high–Sr/Y magmas. However, ultrapotassic magmas (e.g., lamprophyres) with high H 2 O are able to trigger intense water–fluxed, partial melting of metal–fertilized lower crust, which generated high–fO 2 , relatively high–H 2 O, fertile, high–Sr/Y shoshonitic magmas in this intracontinental setting. Highlights • The high-Sr/Y shoshonites were emplaced during the Eocene-Oligocene. • The shoshonites were derived from heterogeneous juvenile lower crust. • The MMEs are dominated by mafic enclaves and andesitic enclaves. • Fertile shoshonites were formed by intense water-fluxed partial melting. • Barren shoshonites were originated from moderate water-fluxed partial melting. [ABSTRACT FROM AUTHOR]

The Jurassic Gangdese arc hosts the giant Xietongmen (Xiongcun) porphyry Cu Au magmatic-hydrothermal centre. In order to understand the tectonomagmatic framework of the arc and its controls on the origin and temporal-spatial distribution of porphyry mineralization we conducted extensive major and trace, and isotopic analysis on a suite of samples collected regionally. Geochemical variations were observed across the Jurassic arc from south to north, controlled by northward subduction of the Neo-Tethyan oceanic plate and overlying Lhasa lithosphere. In the frontal arc (latitude 29°–29.55°N), the magmas had high Ba/La and Ba/Th ratios, high εNd (t) (+ 3 to + 7) values, and their zircons have high εHf (t) (+ 10 to + 18) values, and heterogeneous O isotopic compositions (δ 18 O = + 3.6 to + 6.6‰). The low titanium-in-zircon temperature and zircon saturation temperature (both average = ~ 700 °C) suggest that a greater flux of dehydration fluids from the subducting slab triggered partial melting of the mantle wedge in the southern arc. High zircon Eu/Eu* and Ce/Nd ratios (> 0.4 and > 20, respectively) indicate highly oxidized, fertile magmas, from which the Xietongmen porphyry Cu Au deposit formed. In contrast, the interior arc (north of latitude 29.55°), close to the ancient Tibetan basement, shows low Ba/La and Ba/Th ratios, low εNd(t) (− 0.1 to + 2.5) and zircon εHf (t) (+ 2 to + 12) values, and less heterogeneous zircon O isotopic compositions with higher δ 18 O values. Higher titanium-in-zircon and zircon saturation temperatures (mostly > 750 °C) suggest that less water was involved during the partial melting of mantle wedge in this region. Low zircon Eu/Eu* and Ce/Nd ratios (mostly < 0.4 and < 20, respectively) point towards less oxidized magmatic conditions. We conclude that across-arc geochemical and fluid-flux variations controlled the formation and spatial distribution of Jurassic porphyry deposits in the Gangdese belt. [ABSTRACT FROM AUTHOR]

The 1600 km-long Gangdese magmatic belt features extensive Paleocene–Eocene I-type intrusive rocks and coeval volcanic successions, abundant but more localized Oligo-Miocene calc-alkaline to alkaline plutons, and Miocene potassic to ultrapotassic volcanic rocks. These Cenozoic igneous rocks record geodynamic changes related to the India–Asia collision which began at ~ 55–50 Ma. New and published lithogeochemical and multiple isotopic (Os–Sr–Nd–O–Hf) analyses of these Cenozoic igneous rocks reveal that the Paleocene–Eocene magmas have similar compositions to continental arc rocks throughout the belt, but later Miocene magmas show sharp longitudinal contrasts in geochemical and isotopic compositions, which are also correlated with the occurrence of porphyry-type mineralization. Sparse Miocene high-K calc-alkaline to shoshonitic volcanic rocks in the eastern Gangdese belt have low to moderate ( 87 Sr/ 86 Sr) i ratios (0.7057–0.7121), moderately negative εNd i values (− 9.4 to − 3.4), low ( 187 Os/ 188 Os) i ratios (0.154–0.210), highly variable εHf i values (− 5.9 to + 10.1), and low zircon δ 18 O values (+ 5.0–+ 6.7‰), which are interpreted to reflect derivation by partial melting of subduction-modified Tibetan sub-continental lithospheric mantle (SCLM). In contrast, Miocene high-K calc-alkaline to shoshonitic volcanic rocks in the western Gangdese belt have higher ( 87 Sr/ 86 Sr) i ratios (0.7069–0.7263), more negative εNd i values (− 17.5 to − 6.0) and εHf i values (− 15.2 to + 0.7), and crust-like zircon δ 18 O values (+ 6.2–+ 8.8‰), but mantle-like ( 187 Os/ 188 Os) i values (0.156–0.182), and high Ni and Cr contents. These features suggest that potassic to ultrapotassic magmas in the western Gangdese belt were also derived from partial melting of Tibetan SCLM but with ~ 3–25% input of melts ± fluids from the underthrust Indian plate ( 87 Sr/ 86 Sr = 0.74–0.76, εNd = − 18 to − 10, δ 18 O = + 10 − + 14‰). In contrast, Miocene alkaline magmas to the east were unaffected by this source. Oligo-Miocene calc-alkaline to high-K calc-alkaline granitoids related to large porphyry Cu–Mo deposits in the eastern Gangdese belt (east of ∼ 89° E) are geochemically broadly similar to the early Paleocene–Eocene rocks. They are thought to be derived from partial melting of subduction-modified lower crust with mixing of alkaline melts from partial melting of SCLM, and have relatively low ( 87 Sr/ 86 Sr) i ratios (0.7047–0.7076), high εNd i values (− 6.1 to + 5.5) and εHf i values (1.4–8.7), moderate ( 187 Os/ 188 Os) i ratios (0.224–0.835), and low zircon δ 18 O VSMOW values (+ 5.5–+ 6.6‰). These magmas also had high water contents (weak Dy/Yb enrichment, characterized with amphibole fractionation) and oxidation states (ΔFMQ 0.8–2.9), which explain their unique association with porphyry Cu–Mo mineralization. In contrast, Miocene high-K calc-alkaline to shoshonitic granitoids in the western Gangdese belt (west of ∼ 89° E) show differences in geochemical and isotopic compositions to the earlier Paleocene–Eocene magmatism, and are characterized by crust-like zircon δ 18 O values (+ 6.2–+ 8.8‰), high ( 87 Sr/ 86 Sr) i ratios (0.7147–0.7165), negative εNd i values (− 11.3 to − 7.9), crust-like ( 187 Os/ 188 Os) i values (0.550–1.035), and low εHf i values (− 13.0 to 3.9). These magmas are interpreted to reflect involvement of melts ± fluids from the underthrust Indian plate and high degrees of crustal contamination upon emplacement. Only one small porphyry Cu–Mo deposit is known to be associated with these western granitoids. We suggest that this difference reflects the variable extent of underthrusting of the Indian plate continental lithosphere beneath Tibet in the Oligo-Miocene, and diachronous breakoff of the Greater India slab. In the absence of underthrust Indian lithosphere to the east of ~ 89° E in the Oligo-Miocene, slab breakoff triggered asthenospheric upwelling and partial melting of previously subduction-modified Tibetan lithosphere, generating hydrous, oxidized calc-alkaline magmas with the potential to generate porphyry Cu–Mo deposits. In contrast, underthrusting of the Indian plate to the west at this time limited the involvement of asthenospheric melts and the extent of partial melting of subduction-modified lithosphere, with the result that melts ± fluids derived from the underthrust lithosphere were infertile. [ABSTRACT FROM AUTHOR]

[Display omitted] • There are fertile porphyries aged of 19 Ma in Sungun copper deposit. • Sungun fertile magmas are generated by melting of subduction-modified juvenile lower crust. • Sungun fertile magmas have high oxygen fugacity, H 2 O and sulfur content. • Sungun deposit water-fluxed melting is induced by injection of hydrous mafic magma. The Miocene Sungun porphyry Cu–Mo systems, located in Azerbaijan belt, NW Iran, is associated with the high-Sr/Y magmas, which were engendered in a post-collisional setting. However, the petrogenesis of these fertile magmas and their mineralisation are still unknown. In this study, we present whole-rock major-trace elements, zircon U–Pb dating and the Lu–Hf isotopes in the ore-forming porphyries of the Sungun porphyry systems. The corresponding results showed that the samples were rich in large-ion lithophile elements and poor in high-field-strength elements; moreover, they presented both high K 2 O (2.4–3.5 wt%) and Sr (727–770 ppm) and low Y (11–14 ppm) contents. The zircon U–Pb age of the Sungun porphyry was ~19 Ma, with positive ε Hf (t) ratios (5.5–11.8) and young T DM 2 ages between 322 and 705 Ma. Additionally, the Sungun high-Sr/Y porphyries contained anhydrites (CaSO 4); and had high zircon Ce4+/Ce3+ ratios (33–728), relatively low zircon saturation temperatures (620–804 °C), and plagioclases with excess Al content. Overall, these evidences suggest that a water-fluxed melting of the subduction-modified juvenile lower crust might have generated a high oxygen fugacity, leading to the formation of the hydrous and S-rich Sungun fertile magmas in the NW Urumieh–Dokhtar magmatic Arc. [ABSTRACT FROM AUTHOR]

• The juvenile lower crust enriched in sulfides and hydrous minerals is necessary to the formation of collision-related PCDs. • The evolution processes of fertile magmas determine the metal associations of collision-related PCDs. • Contemporaneous regional tectonics can affect the tonnage of collision-related PCDs. Porphyry Cu deposits (PCDs) in continental collision settings are new targets for modern mineral exploration, and the relationships among locations, tectonic environments, tonnages, and metal associations are not yet understood. The Gangdese PCD belt formed in a typical continental collision setting between the Indian and Asian plates. In the present study, new elemental and isotopic data from fertile porphyries (Cu–Mo, Cu–Mo–Pb–Zn, and Cu–W–Mo) and barren coeval and pre-ore porphyries of the Gangdese PCD belt were combined with previously published data to provide new insights into the metallogenesis of PCDs in continental collision settings. Factors that contributed to the formation of collision-related PCDs in the Lhasa terrane, Tibet, include the nature of the magmatic sources, magmatic evolution during ascent through the crust, and regional tectonics. These PCDs were not formed by partial melting of ancient lower crust. Instead, attributed to the subduction of Neo-Tethyan oceanic slab, arc magmas and slab-derived fluids resulted in the accumulation of sulfides and hydrous minerals at the base of the continental crust. Partial melting of this refertilized juvenile lower crust produced oxidized water- and Cu-rich magmas with the potential to produce large PCDs, although only under favorable regional tectonic conditions. Regional compressional stress is not conducive to the formation of large magma chambers, so the collision-related adakite-like magmas that developed during the early stages of India–Asia collision commonly lack alteration and associated mineralization. In contrast, relatively large magma chambers can develop in extensional settings within the continental collision orogen, which facilitates the formation of large and giant PCDs. Small Cu–W–Mo deposits formed during the transition between compressional and extensional settings. Thus, the regional tectonic setting and structural framework of the host orogen influenced the tonnage of PCDs. The metal associations of collision-related PCDs in the Lhasa terrane were controlled mainly by the extent of assimilation of the upper crust and the degree of fractional crystallization of the ore-forming magmas. Lead–zinc mineralization in Cu–Mo deposits is attributed to the assimilation of metapelites by fertile magmas in the upper crust, and highly fractionated fertile magmas are required to produce W mineralization. [ABSTRACT FROM AUTHOR]