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We present coupled textural, elemental, and boron isotopic data of tourmaline from the large Zhunuo–Beimulang collision-related porphyry copper deposits (PCDs) located within the western Gangdese, Tibet. Based on morphology and high-resolution mapping, the tourmaline is classified into three paragenetic generations. The first generation of schorlitic Tur-1 occurs in the monzogranite porphyry as disseminations intergrown with porphyritic K-feldspar and plagioclase. It shows decreasing Fe and Ca and increasing Mg and Al contents from core to rim and has relatively homogeneous δ11B values (− 9.9 to − 8.6‰); low Fe3+/(Fe2+ + Fe3+), Cu, F, H2O, and Sr/Y ratios; and high rare earth elements. These features indicate Tur-1 formed in a low fO2 and metal-poor granitic magma during the pre-mineralization stage. The second generation of porphyritic euhedral Tur-2 is hosted in diorite porphyry enclaves and dikes, where it is intergrown with plagioclase and biotite. It forms part of the schorl-dravite solid solution, with high Fe3+/(Fe2+ + Fe3+), Cu, F, H2O, Sr/Y, and δ11B (− 9.7 to − 5.1‰) values. These features indicate it crystallized from a hydrous, oxidized, metal-, and volatile-rich diorite magma. The third generation of Tur-3 is the most volumetrically important and occurs as veinlets and disseminations in the porphyry, or around Tur-1 and Tur-2. It shows radial and oscillatory zoning and is locally intergrown with chalcopyrite and pyrite within the main mineralization assemblage. It has δ11B values (− 10.5 to − 6.0‰) that overlap with Tur-1 and Tur-2 values. Tur-3 also has variable Fe3+/(Fe2+ + Fe3+), Cu, and volatiles (F and H2O), indicating it crystallized from oxidized to relatively reducing metal- and volatile-rich hydrothermal fluids. Overall, the three generations of tourmaline show a narrow range of δ11B values between − 10.5 and − 5.1‰ that are indicative of a single magmatic source. The high Cu, ferric iron, volatiles, and δ11B values in Tur-2 are interpreted to reflect injection of diorite magma into an open crustal magma storage system that led to the formation of an oxidizing and metal-volatile-rich porphyry system. The three stages of tourmaline formation reflect evolution of the magmatic–hydrothermal system from low fO2 conditions towards more oxidizing, volatile-rich conditions and then a return to more reducing conditions that accompanied Cu precipitation. Overall, the injection of oxidized metal-rich magma into a long-lived magma reservoir is a critical driving force for the development of collision-related PCDs. [ABSTRACT FROM AUTHOR]

The large magma reservoirs underlying world-class porphyry deposits are one of the key factors in their formation, which thus led to the present study focusing on the unmineralized intrusive rocks underlying the porphyry molybdenum-copper orebody of the post-collisional Jiama porphyry-skarn copper-polymetallic deposit in southern Tibet. The Jiama porphyry intrusion comprises intermediate-silica quartz diorite, monzogranite, and granodiorite porphyries, as well a high-silica granite porphyry. The intrusive rocks suite exhibits similar whole-rock Sr-Nd isotopic compositions [εNd(t) = -3.9 to -0.8], suggesting a common, non-radiogenic magma source. Yet, these rocks have distinct geochemical characteristics. The intermediate-silica rocks are relatively enriched in Ba and Sr with minor Eu anomalies, and show adakite-like geochemical characteristics. In contrast, the high-silica granite porphyry is strongly depleted in Ba, Sr, and Eu, and lacks adakite-like geochemical characteristics. We propose that the high-silica granite porphyry represents highly fractionated melt extracted from a silicic mush reservoir (crystallinity of ∼40%-65%), and that the monzogranite and granodiorite porphyries constitute the complementary residual silicic cumulates. High crystallinity facilitates the formation of connected fluid pathways, allowing the efficient removal of volatiles from the remaining silicic melt and a rapid flux of the ore-forming fluids toward the apical parts of the large magma reservoir. Less-evolved mafic melt is believed to have repeatedly intruded the base of the magma reservoir, thereby releasing volatiles and water into the silicic mush systems. The volatiles migrated upward through the fluid channels and accumulated in the apical part of the magma reservoir, subsequently, as a result of the overpressure in the roof zone, ore-forming fluids and successive batches of magma were expelled together, thereby forming the Jiama porphyry-skarn deposit. [ABSTRACT FROM AUTHOR]

Most large-scale porphyry-style mineralization is genetically associated with oxidized, sulfur-rich, and hydrous magmas, the sources of which were enriched in mantlederived or juvenile crust--derived materials. Geochemical indices for these features can distinguish fertile magmas and barren magmas. However, fertile magmas that generate different-sized porphyry deposits are usually geochemically indistinguishable, and the factors controlling the size of porphyry mineralization are poorly understood. Here, we used zircon H2O contents and the compositions of apatite hosted in zircon, phenocrysts, and groundmass to compare the water contents and volatile evolution of fertile magmas in the Yulong ore belt, Tibet, which generated giant-, large-, and medium-sized porphyry deposits. We found that these porphyries have comparable major- and trace-element and zircon Hf-O isotope compositions, with high oxygen fugacity and sulfur contents. A negative correlation between XClAp/XOHAp versus XFAp/XClAp of apatites suggests that the magmas consistently achieved H2O saturation before zircon crystallization, and zircon hydroxyl contents reveal that the water contents of the melt positively correlate with the size of the deposits, consistent with larger deposits achieving H2O saturation at deeper crustal levels. The deeper H2O saturation also caused larger amounts of Cl extraction, as indicated by the trends of increasing XFAp/XClAp and decreasing XClAp/XOHAp with larger deposit sizes. These data sets can be reconciled by magmas forming larger deposits having higher H2O contents and suggest that the H2O content is a key control on their fertility. Our study highlights the utility of integrated hydrous and nominally anhydrous mineral analyses for constraining enigmatic magmatic volatile processes in magmatic ore-forming systems. [ABSTRACT FROM AUTHOR]

The genetic link between subduction- and collision-related porphyry Cu deposits (PCD) is still not well understood. The Gangdese porphyry Cu belt (GPCB) in southern Tibet hosts two parallel E–W-oriented metallogenic subbelts, including a southern Jurassic subduction-related and a northern Miocene post-collision PCD subbelt. In this study, we combine new and published whole-rock major and trace element data, Cu isotopic compositions and zircon trace element data of Jurassic magmatic rocks from the GPCB. Combined Sr/Y and La/Yb crustal thickness proxies confirm the south to north geometry of the Jurassic arc-back-arc system across the GPCB. The southern Jurassic magmatic arc rocks have systematically higher whole-rock V/Yb ratios, δ65Cu values and zircon Eu/Eu* ratios compared to those of the northern back-arc region. This suggests that Jurassic southern arc magmas had higher oxygen fugacity and H2O concentration than the contemporaneous northern back-arc magmas, which was controlled by a steep subduction geometry of the Neo-Tethyan oceanic slab. Hydrous and oxidized magmas resulted in Cu enrichment during magma evolution and generation of Jurassic PCD in the southern magmatic arc by inhibiting early sulfide saturation at deep crustal levels. In contrast, the northern back-arc magmas were less oxidized and less hydrous, which triggered early sulfide saturation, resulting in segregation of Cu-bearing sulfides in lower crustal cumulates that provided a favorable metal source for later Miocene PCD in the northern subbelt. Our study indicates that the Jurassic subduction geometry controlled the formation and distribution of Jurassic subduction-related and Miocene post-collision-related PCD in the GPCB. [ABSTRACT FROM AUTHOR]

Porphyry deposits supply most of the world's Cu and Mo resources. Most porphyry deposits are developed in magmatic arcs above subduction zones. However, abundant Miocene porphyry Cu-Mo deposits have been found in the post-collision stage in Tibet, more than ∼30 Myr after the Indian-Eurasian continental collision. The magma source and the enrichment process of ore-forming elements for these post-collision porphyry deposits remain controversial. Here, we present Sr-Nd-Mo isotope compositions of a suite of mineralized and barren igneous rocks from the Rongmucuola pluton in the Miocene post-collisional Qulong porphyry Cu-Mo deposit in Tibet to reveal their magma sources and ore-forming processes. Our results indicate that the mineralized and barren igneous rocks in the Qulong deposit were derived from a cogenetic source with similar Sr-Nd isotope compositions (87Sr/86Sr (i) = 0.7049–0.7050 and ε Nd (t) = −0.24 to 0.20). However, the mineralized igneous rocks have large variations in δ98/95Mo values (−0.30‰ to 0.74‰; relative to NIST SRM 3134) and Mo/Ce ratios (0.01 to 4.84) compared to those of the barren igenous rocks (δ98/95Mo = −0.74‰ to −0.02‰; Mo/Ce = 0.01 to 0.04). In combination with the higher LOI values, Mo/Ce and Cs/Ta ratios and Cu concentrations and lower Ce/Pb ratios of the mineralized igneous rocks relative to the barren igneous rocks, we propose the mineralized igneous rocks were affected by ore-forming hydrothermal fluids originated from the exsolution of later co-genetic magmas, which significantly changed the isotopic and elemental compositions of intrusions and resulted in Mo mineralization. The fluid-rock interaction modeling supports this interpretation and successfully reproduce the observed δ98/95Mo and Mo/Ce compositions of mineralized igneous rocks. Our study indicates that the exsolved magmatic-hydrothermal fluids have heavy Mo isotopes, and the fluids with a slightly heavier Mo isotope composition have the greatest potential for mineralization. As the water-rock reaction progresses, leading to the precipitation of Mo-rich minerals, ore-forming fluids with a heavier Mo isotope composition may not have significant mineralization potential. This highlights that Mo isotope system is an effective tool to study the ore-forming processes of porphyry deposits. [ABSTRACT FROM AUTHOR]

Explanation: From Lakange Mo (Cu) deposit to Jiama Cu polymetallic deposit to Bangpu Mo mineralization to Qulong Cu-Mo deposit to Bangpu Cu mineralization, the Cl contents in apatite gradually increase, but the F contents in apatite gradually decrease, and the S content is similar but variation. It indicates that the Mo-dominated porphyry deposit formed from F-S-rich magma, whereas the Cu-dominated porphyry deposit formed from Cl-S-rich magma. [Display omitted] • There are two different magmatic-hydrothermal systems that formed the Bangpu Mo-Cu deposit. • Cu and Mo mineralization contain zircon and apatite with different geochemical features. • Cu mineralization was related to Cl-rich porphyry system, whereas Mo-mineralization was related to F-rich porphyry system. The Bangpu deposit, located in the eastern part of the Gangdese Porphyry Copper Belt, Tibet, is a porphyry Mo-Cu deposit in a continental collisional orogenic belt. Cu and Mo mineralization occurred separately in the Bangpu deposit. Mo (Cu) mineralization occurred mainly in the monzogranite porphyry (MP), while Cu (Mo) mineralization occurred mainly in the diorite porphyry (DP). The cause of such spatial differences in Cu and Mo mineralization in the Bangpu deposit remains unclear. In this study, the mineral chemistry of apatite and zircon was investigated to reveal the differences in the nature of ore-forming magma between Cu (Mo) and Mo (Cu) mineralization in the Bangpu deposit. The chondrite-normalized REE patterns of the apatite grains from the MP were enriched in LREEs with negative Eu anomalies, whereas those from the DP and biotite monzonitic granite (BM) had strong negative Eu anomalies with no obvious differentiation between LREEs and HREEs. The DP, MP, and BM have a low T TiZ range from 657 °C to 917 °C and a high AST range from 687 °C to 1079 °C. The zircon grains from the DP and MP had different Th/U ratios and T TiZ values, indicating that these porphyry samples probably experienced different magma crystallization or crystal fractionation histories. The zircon Ce4+/Ce3+ and Eu/Eu* ratios indicate that the ore-bearing DP and MP have higher oxygen fugacity than the BM. The F, Cl, and S contents in the apatite showed that the DP magmas were Cl- and S- rich, whereas the MP magmas magmas were F- and S- rich. The Bangpu porphyry Mo-Cu deposit results from two superimposed magmatic-hydrothermal systems. Mo was mainly partitioned into the F-rich magmatic-hydrothermal system released from the MP, and Cu was mainly partitioned into the Cl-rich magmatic-hydrothermal system released from the DP. Our results show that zircon and apatite geochemistry can be used to explain the spatial distribution of Cu and Mo mineralization in porphyry Cu-Mo systems. [ABSTRACT FROM AUTHOR]

Modern arc adakites with high Mg# values (molar 100 × Mg/(Mg + Fe) ratio) are generally considered products of interaction between melts derived from subducted oceanic crust and/or eroded forearc crust and peridotite in the mantle wedge. An alternative model, in which high-Mg# adakitic rocks are produced by garnet fractionation of mantle-derived magmas, has been proposed based on whole-rock geochemical variations; however, magmatic garnet has not been found in high-Mg# adakitic rocks, and little is known about the physical conditions required for this magmatic differentiation. Here we report geochronological, mineralogical and geochemical data for Late Triassic garnet-bearing high-Mg# (Mg# = 45–56) adakitic diorite porphyries and garnet-free non-adakitic diorite porphyries with Mg# > 62 from central Tibet. Consistent compositional correlation between Ca-rich garnet crystals, their host rocks and zircon autocrysts suggests that the garnet crystals grew in their host magmas. Amphibole, garnet, zircon and the host rocks display increasing Dy/Yb ratios with increasing magma differentiation. Pristine magmas in equilibrium with amphibole that crystallized prior to garnet are not adakitic. The garnet-bearing high-Mg# adakitic rocks were probably generated by the fractionation of pyroxene, amphibole and garnet at ∼1 GPa from a primitive andesitic parent that was geochemically similar to the garnet-free diorite porphyries. The primitive andesitic magmas with enriched isotope compositions ([87Sr/86Sr]i > 0·709, ɛNd[t] < −3·4) may be derived from shallow melting of subduction-enriched lithospheric mantle in a post-collisional, extensional setting resulting from oceanic slab breakoff. The most likely scenario for garnet crystallization is that mantle-derived hydrous (H2O >5 wt %) magmas stalled, cooled isobarically and differentiated at the base of the crust. This study provides direct mineralogical evidence for the generation of high-Mg# adakitic rocks by crystal fractionation involving garnet, rather than by interaction between crust-derived melt and the mantle, although the latter is potentially a frequent occurrence in the mantle wedge. [ABSTRACT FROM AUTHOR]

Zhuno is the only large porphyry copper deposit that preserves the complete magmatic system in the western part of Gangdese porphyry copper belt. Intrusions in the Zhunuo deposit include pre‐ore porphyritic monzogranite, syn‐ore monzogranite porphyry, and post‐ore quartz syenite‐normal granite porphyry, high‐Cr granite porphyry, normal diorite porphyry, high‐Mg diorite porphyry (diorite enclave), potassic lamprophyre, and ultrapotassic lamprophyre. Enriched Sr–Nd isotopic composition and high MgO, LILE, HFSE, LREE, and compatible element (Cr, Ni) contents of the ultrapotassic lamprophyre indicate that the magma derived from the enriched Lhasa lithospheric mantle, which had been metasomatized by fluids and sediment‐derived melts associated with Neo‐Tethyan oceanic subduction and subsequent Indian continental lithosphere subduction. The depletion of the Sr–Nd isotopic composition and MgO (or Mg#), Cr, and Ni content of the normal diorite porphyry manifest that the magma source is juvenile Lhasa lower crust instead of Indian mantle. Geochemical characteristics indicate that the magma sources of the quartz syenite‐normal granite porphyry, high‐Mg diorite porphyry‐diorite enclave, and potassic lamprophyre was derived from ancient Lhasa middle‐upper crust, crust‐mantle transition zone, and enriched Lhasa mantle without Indian lithosphere contribution, respectively. Their similar Sr–Nd isotopic compositions demonstrate a uniform Sr–Nd isotopic reservoir from crust to mantle in Zhunuo area. The major and trace element compositions suggest that the high‐Cr granite porphyry was formed by mixing of quartz syenite‐normal granite porphyry and ultrapotassic lamprophyre magmas. High Sr/Y values and low Y contents of the monzogranite and monzogranite porphyry display adakitic properties. The major and trace element contents and Sr–Nd isotopic composition indicate the monzogranite‐monzogranite porphyry are likely from mixing of the normal diorite porphyry magma with the quartz syenite‐normal granite porphyry magma, rather than with the high‐Mg dioritic (high‐Mg diorite porphyry‐diorite enclave) and/or potassic‐ultrapotassic magmas. According to the spatial and temporal consistency of the adakitic and potassic‐ultrapotassic rocks with the N–S‐trending rifts, Indo‐Asian convergence rate reduction, and uplifting model of Tibetan Plateau, we infer that the magma genesis of the Zhuno intrusions is attributed to decompressional partial melting of Lhasa crust‐mantle triggered by the tearing of the subducting Indian Plate. [ABSTRACT FROM AUTHOR]

The Xietongmen large deposit is the only known subduction‐related porphyry Cu deposit in the Gangdese Belt, Tibet, which is composed of the No. I and No. II deposits. Previous work suggests that the No. I deposit is reduced while the No. II deposit is oxidized. However, it is ambiguous that both oxidized and reduced porphyry Cu deposits occur in close proximity, in the same tectonic setting, and within a short time period. Detailed examination of the ages, whole‐rock geochemistry, zircon and apatite compositions of the Xietongmen No. I and No. II ore‐bearing porphyries are conducted to investigate their magma characteristics and the genetic types of the Xietongmen No. I and No. II deposits. Zircon U–Pb dating results show that the Xietongmen No. I and No. II ore‐bearing porphyries formed at 166.3 ± 2.6 Ma and 175.5 ± 1.0 Ma, respectively. The Xietongmen No. I and No. II ore‐bearing porphyries have similar mineral assemblage (containing amphibole phenocryst), high Sr/Y ratios and negligible negative Eu anomalies, low Ti‐in‐zircon temperatures (633−749 and 633−763°C, respectively), which indicate that their magmas are water‐rich. In addition, the Xietongmen No. I and No. II ore‐bearing porphyries have: (a) high whole‐rock V/Sc ratios (10−15 and 8−16, respectively); (b) high zircon Ce4+/Ce3+ and EuN/EuN* ratios, falling into the field of the ore‐bearing intrusions in northern Chile; (c) low apatite CeN/CeN* ratios (0.97−1.02 and 0.99−1.02, respectively), which suggest that their magmas are oxidized. The estimated magmatic S contents, based on the magmatic apatite SO3 contents, yield 0.0113−0.0507 wt% and 0.0012−0.0118 wt%, respectively for the Xietongmen No. I and No. II ore‐bearing porphyries, which suggest their magma are sulfur‐rich. Hydrous, oxidized and sulfur‐rich magmas are the parent magma of oxidized porphyry Cu deposits, together with the oxidized minerals of hematite and magnetite, therefore, both Xietongmen No. I and No. II deposit are oxidized porphyry Cu deposits, rather than that the Xietongmen No. I deposit is a reduced porphyry Cu deposit. [ABSTRACT FROM AUTHOR]

Demingding is a poorly studied post‐collisional porphyry Cu‐Mo deposit, located in the eastern part of the Gangdese porphyry copper belt. In this study, we present LA‐ICP‐MS zircon U‐Pb dating, whole‐rock geochemistry, and zircon trace elemental data for the mineralization related biotite monzogranite porphyry in Demingding. Zircon U‐Pb dating shows that the weighted mean 206Pb/238U ages of the Miocene biotite monzogranite porphyry is 20.36 ± 0.46 Ma (MSWD = 2.6, n = 21). The Miocene biotite monzogranite porphyry is characterized by high SiO2, K2O, and Al2O3 contents and shows adakite‐like signatures. These rocks have high Sr/Y ratios, fractionated REE patterns with low HREE abundances, enrichment in LILE compared with HFSE, and have (87Sr/86Sr)i values of 0.7059 to 0.7062, εNd(t) values of −2.35 to −1.67 and (206Pb/204Pb)i ratios of 18.50–18.55. These features are similar to other Miocene adakite‐like intrusions in the Gangdese belt. We propose that the biotite monzogranite porphyry shares the same petrogenesis with other Miocene ore‐bearing adakite‐like intrusions in the Eastern Gangdese belt and was derived from remelting of the subduction‐modified, eclogitized Tibetan lower crust and mixing with hydrous mafic magmas from metasomatized Tibetan mantle. The zircons from biotite monzogranite porphyry have only minimal negative Eu anomalies (EuN/EuN* > 0.3), and exhibit large ratios of Ce4+/Ce3+(average value of 113), 10,000*(EuN/EuN*)/Y (3.94–8.14, >1), Ce/Nd (9–58, Average value of 30), (Ce/Nd)/Y (0.008–0.127, >0.003), and low value of Dy/Yb (0.15–0.24, <0.3), indicating that the biotite monzogranite porphyry is relatively hydrous and oxidized. The biotite monzogranite porphyry, therefore, is believed to be a relatively fertile intrusion with certain exploration potential. [ABSTRACT FROM AUTHOR]

[Display omitted] • The Ahadawa deposit is a newly discovered porphyry Cu–Mo system in the North Qaidam. • The Ahadawa formed by more than 25 Ma prolonged magmatic-hydrothermal activity. • The Late Triassic fertile magmas initially had high water and high oxygen fugacity. • Plagioclase-dominated fractionation occurred with magma evolution. Ongoing exploration in the North Qaidam, Northern Qinghai-Tibet Plateau, NW China, has led to the identification of a porphyry Cu-Mo system centered on Ahadawa. In this study, we conducted a detailed geochronological and geochemical study on porphyritic monzogranite from the Ahadawa deposit, which hosts the Cu-Mo mineralization. The weighted mean zircon U-Pb ages for three mineralized porphyritic monzogranite samples from different sites range from 218.8 ± 1.2 Ma (MSWD = 1.5) to 224.7 ± 1.6 Ma (MSWD = 1.8), and finally to 233.1 ± 0.9 Ma (MSWD = 1.4, n = 23). These results, combined with antecrystic grains as old as ∼ 245.7 Ma, collectively indicate more than 25 Ma multi-stage magmatic emplacement activity in the Ahadawa deposit. Corresponding to complex magma episodes, six studied molybdenite samples yielded variable Re-Os model ages ranging from 436.7 ± 6.6 to 232.6 ± 1.2 Ma, with the three youngest dates constraining a weighted mean Re-Os age of 234.0 ± 6.8 Ma. The 234 Ma mineralization age is consistent with the observed Late Triassic magmatism, whereas the Re-Os model ages of 436.7 to 245.7 Ma, if they remain robust through future investigation, may highlight a long-lived hydrothermal activity in the Ahadawa deposit. Whole-rock and zircon compositions indicate that the Late Triassic porphyries occurring at the surface display distinct geochemical features, with the early porphyritic monzogranite of 233.1 Ma characterized by much higher whole-rock Sr/Y, and zircon Eu/Eu* and Ce/Ce* than the younger (218.8 Ma and 224.7 Ma, respectively) porphyritic monzogranite. This suggests that the Late Triassic magmas were initially characterized by high water content and high oxidation state and the differences between the old and two younger groups might be related to plagioclase-dominated fractionation. The finding of the Ahadawa porphyry system highlights the need for ongoing exploration (especially in poorly studied remote areas) and systematically geochronological and geochemical studies to constrain the temporal and spatial evolution of mineralized episodes within a complex, protracted magmatic history. [ABSTRACT FROM AUTHOR]

[Display omitted] • The Yare complex as well as granite units in the Mayum, Pujue, Nanmuqie, and Qiagong areas in the Gangdese magmatic arc were emplaced in Miocene. • The barren adakitic porphyries in Gangdese magmatic arc were derived from ancient crustal sources with little contributions from mantle-derived material. • The history of continuous and relatively complex magmatism prior to emplacement are important factors that discriminate between fertile and barren magmatism. Numerous Miocene porphyry deposits related to adakites have been identified in the eastern Gangdese magmatic arc (east of 87°E), but few of these type of deposits have been found in the western part (west of 87°E), even contemporaneous adakitic porphyries were exposed in the western part. A little research has been undertaken on barren adakitic porphyries in the Gangdese magmatic arc, and the key controls on the metallogenic differences remains unclear. This study presents new whole–rock geochemical, Sr–Nd isotopic, zircon U–Pb and Hf isotopic data for barren adakitic rocks of the Yare complex as well as barren porphyritic and adakitic units in the Mayum, Pujue, Nanmuqie, and Qiagong areas. These data are compared with data for fertile adakitic porphyries, providing insights into the origin and evolution of barren and fertile adakitic magmatism as well as the conditions associated with the porphyry mineralization. The barren adakitic porphyries were formed at 18.03–13.29 Ma. These intrusions have higher (87Sr/86Sr) i ratios (0.708752–0.709785) and lower Ɛ Nd (t) values (−8.4 to −6.5) and zircon Ɛ Hf (t) values (−13.81 to 3.23) than the fertile adakitic porphyries, suggesting the former were derived from ancient crustal sources with a little contributions from mantle-derived material. The fertile adakitic porphyries were formed as a result of partial melting of juvenile lower crust (such as Cu–Mo deposits) and with mixing of material from middle-upper crust (such as Mo–(Cu)–Pb–Zn deposits). The fertile adakitic porphyries also contain more water than the barren adakitic porphyries. Other important factors that discriminate between fertile and barren adakitic magmatism include the involvement of parental magma chambers as well as a history of continuous and relatively complex magmatism prior to emplacement. [ABSTRACT FROM AUTHOR]

[Display omitted] • Multi-stage intrusions from pre- to post-mineralization at Bairong are studied. • They formed at 14.1 to 11.6 Ma and derived from a cogenetic magma chamber in a post-collisional setting. • They are all oxidized and hydrous with comparable magmatic S contents, but have different Cl contents. • Fluid exsolution and buffering effect of magmatic anhydrite possibly result in S-Cl decoupling. • Oxidized and hydrous magma with effective fluid exsolution is the key factor for Bairong Cu-Mo mineralization. Volatiles are considered to play an important role in porphyry Cu mineralization, and high apatite S and Cl contents are often used to distinguish fertile magma systems from barren ones in the subduction zone. However, magmatic volatile evolution and its implications in post-collisional porphyry deposits remain enigmatic. To address this issue, we conducted zircon U-Pb dating and Hf isotope, and zircon and apatite chemical compositions of multi-stage magmatic rocks from the Bairong post-collisional porphyry Cu-Mo deposit in the Gangdese belt, Tibet. The magmatic rocks included pre-mineralized porphyritic monzogranite and biotite monzogranite, syn -mineralized monzogranite porphyry, and post-mineralized dacite porphyry that formed in 14.1–12.7 Ma, ca. 12.1 Ma, and 11.8–11.6 Ma, respectively. All these intrusions have depleted zircon Hf isotope (ε Hf (t) = 1.7–8.0, 3.7–9.0, 3.3–9.7 for pre-, syn-, and post-mineralized intrusions, respectively) together with the research results of contemporaneous magmatic rocks in the adjacent area, suggesting that the multi-stage intrusions at Bairong are derived from a cogenetic magma chamber in post-collisional extension setting. All the intrusions have similar zircon Ce N /Ce N *, Eu N /Eu N * ratios with average ΔFMQ values of 2.1 – 2.6, together with the high apatite Eu N /Eu N * ratios and low Ga concentrations, implying that their magmas are all oxidized. These intrusions have zircons (10,000 × Eu N /Eu N *) /Y all > 1 and Ce/Nd/Y mostly > 0.01, and obtain low average titanium-in-zircon temperatures of 659 to 714 °C, which indicate their magmas are hydrous. Apatite Cl contents decrease, combined with the drop of apatite X Cl /X OH and the surge of X F /X Cl ratios from pre- to syn -mineralized intrusions, possibly indicating that fluid exsolved from magma at the syn -mineralization stage. The apatite SO 3 contents in these different-stage intrusions, however, are almost unchanged. Combined with previous studies, the buffering effect of magmatic anhydrite maintains the stability of magma S content, which together with fluid exsolution leads to the S-Cl decoupling. Consequently, we conclude that the oxidized and hydrous magma with effective fluid exsolution is a critical control on the formation of Bairong porphyry Cu-Mo mineralization. Considering the wide variations of apatite Cl contents and the occurrences of S-Cl decoupling, simple application of high apatite SO 3 and Cl contents to distinguish the ore-forming and barren systems in the post-collisional porphyry system should be cautious. [ABSTRACT FROM AUTHOR]

The Lakange porphyry Cu-Mo deposit within the Gangdese metallogenic belt of Tibet is located in the southern-central part of the eastern Lhasa block, in the Tibetan Tethyan tectonic domain. This deposit is one of the largest identified by a joint Qinghai-Tibetan Plateau geological survey project undertaken in recent years. Here, we present the results of the systematic logging of drillholes and provide new petrological, zircon U-Pb age, and molybdenite Re-Os age data for the deposit. The ore-bearing porphyritic granodiorite contains elevated concentrations of silica and alkali elements but low concentrations of MgO and CaO. It is metaluminous to weakly peraluminous and has A/CNK values of 0.90-1.01. The samples contain low total REE concentrations and show light REE/heavy REE (LREE/HREE) ratios of 17.51-19.77 and (La/Yb)N values of 29.65-41.05. The intrusion is enriched in the large-ion lithophile elements (LILE) and depleted in the HREE and high field-strength elements (HFSE). The ore-bearing porphyritic granodiorite yielded a Miocene zircon U-Pb crystallization age of 13.58 ± 0.42 Ma, whereas the mineralization within the Lakange deposit yielded Miocene molybdenite Re-Os ages of 13.20 ± 0.20 and 13.64 ± 0.21, with a weighted mean of 13.38 ± 0.15 Ma and an isochron age of 13.12 ± 0.44 Ma. This indicates that the crystallization and mineralization of the Lakange porphyry were contemporaneous. The ore-bearing porphyritic granodiorite yielded zircon εHf( t) values between −3.99 and 4.49 (mean, −0.14) and two-stage model ages between 1349 and 808 Myr (mean, 1103 Myr). The molybdenite within the deposit contains 343.6-835.7 ppm Re (mean, 557.8 ppm). These data indicate that the mineralized porphyritic granodiorite within the Lakange deposit is adakitic and formed from parental magmas derived mainly from juvenile crustal material that partly mixed with older continental crust during the evolution of the magmas. The Lakange porphyry Cu-Mo deposit and numerous associated porphyry-skarn deposits in the eastern Gangdese porphyry copper belt (17-13 Ma) formed in an extensional tectonic setting during the India-Asia continental collision. [ABSTRACT FROM AUTHOR]

Processes generating fertile magma and associated porphyry Cu deposits in post-collision tectonic settings remain uncertain. Dabu, located in the Gangdese porphyry Cu belt of southern Tibet, is a typical porphyry Cu–Mo deposit that formed in a post-collision setting. The deposit records two stages of magmatism: an Eocene (46 ± 1 Ma) barren monzogranite (EG); and a fertile Miocene (15 ± 1 Ma) monzogranite porphyry (MMP). These intrusions have similar Sr–Nd ( 87 Sr/ 86 Sr (i) = 0.70514 to 0.70573, εNd(t) = − 2.78 to + 0.77) and zircon Hf (εHf(t) = + 3.46 to + 8.85) isotopic compositions that fall within the range of the Palaeocene–Eocene Gangdese Batholith and the coeval Linzizong volcanics and plot on a mixing line between Tethyan basalts and Indian Ocean pelagic sediments. These features, coupled with the variable Sr/Nd, Sr/Th, Ba/Th, and Rb/Y ratios of the intrusions, are indicative of a common source involving basaltic lower crustal melts underplated during the Palaeocene and Eocene. The basaltic material was sourced from a mantle wedge that was metasomatized by Neo-Tethyan slab-derived fluids and oceanic sediments. The MMP, however, formed under a higher magmatic oxygen fugacity than did the EG, as indicated by their higher zircon Ce 4 + /Ce 3 + ratios (87-1112) and higher f O 2 values (ΔFMQ = 5), determined using ilmenite–magnetite mineral pairs. The decompression and discharge of SO 2 during the Linzizong volcanic event and the fractionation of magnetite could account for the low oxygen fugacity (Ce 4 + /Ce 3 + = 58–164; ΔFMQ = − 0.3) of the EG as well the limited development of porphyry deposits associated with the Gangdese Batholith. The MMP and Miocene mineralization-related intrusions within other Gangdese porphyry Cu deposits have high Sr/Y and V/Sc ratios and a negative correlation between Dy/Yb and SiO 2 , all of which are indicative of the crystallization of hornblende from a hydrous mafic melt. The EG and Gangdese Batholith, however, have low Sr/Y and V/Sc ratios and decreasing Sr concentrations over a wide range of SiO 2 contents, indicating that these intrusions formed from dry melts that had undergone significant plagioclase fractionation. Late-crystallized hornblende is generally present as small crystals within early formed feldspar phenocrysts in the EG, which is in good agreement with a rapid increase followed by a decrease in Y and Dy concentrations at ~ 60 wt.% SiO 2 . In addition, apatite in MMP has F (2.66–3.72%) and Cl (0.06–0.53%) volatile contents that are higher than those of apatite from EG (0.85–1.50% F and 0.02–0.03% Cl). These geochemical and mineralogical characteristics suggest that EG formed from a relatively dry magma with < 4.0% H 2 O, whereas MMP formed from a more hydrous magma with > 5.5% H 2 O. It is here proposed that long-lived subduction caused the high oxygen fugacity conditions of the arc magma by adding oxidized components to the sub-arc mantle in addition to F, Cl, S, and Cu. Long-term metamorphic dehydration resulted from the India–Asia collision, and subsequent crust thickening and shortening was responsible for the high water contents by continuously replenishing the basaltic melts at the base of the lower crust. Both subduction-related metasomatism and late-collision-related metamorphic dehydration controlled the genesis of fertile magma that formed the post-collision porphyry Cu–Mo deposits of southern Tibet. [ABSTRACT FROM AUTHOR]

[Display omitted] • A comparative study between mineralized and coeval barren magmatic suites in the Yulong porphyry Cu belt. • Apatites from the mineralized Yulong suite have atypical Cl contents that are remarkably lower than the coeval barren suites. • Low apatite Cl content at Yulong was caused by early fluid exsolution favored by high magmatic contents and shallow emplacement depth. • Apatite volatiles (F-Cl-OH) show contrasting variations between mineralized and barren suites. • Apatite SO 3 contents remain high and constant during fluid exsolution due to buffering of saturated anhydrite. High apatite Cl and S contents have previously been proposed to be effective in discriminating fertile porphyry Cu systems from infertile ones. However, apatite Cl contents could be very low in syn -mineralization intrusions of some giant porphyry Cu deposits (e.g., <0.1 wt% at Yerington, USA and Yulong, China). The reason for this abnormal phenomenon remains enigmatic, hindering further application of apatite as potential indicator mineral in porphyry Cu exploration. To address this outstanding issue, we conducted a comparative study between mineralized and coeval barren magmatic suites in the Yulong porphyry Cu belt in eastern Tibet, with emphasis laid on their contrasting apatite volatile contents. Apatites from pre- and syn -mineralization intrusions at the mineralized Yulong suite have similar SO 3 contents (0.35 ± 0.26 wt% and 0.34 ± 0.23 wt%, respectively) with but lower Cl contents (0.41 ± 0.37 wt% and 0.08 ± 0.02 wt%, respectively) than the coeval barren suites (SO 3 : 0.20 ± 0.12 wt%–0.95 ± 0.26 wt%; Cl: 0.49 ± 0.08–1.17 ± 0.06 wt%). The Yulong suite displays a sudden and coupled drop of apatite X OH and X Cl values and surge of X F / X Cl ratios from pre- to syn -mineralization intrusions, likely indicating extensive fluid exsolution. However, this trend is not observed in the barren suites. Zircon grains from pre- and syn -mineralization intrusions at the mineralized Yulong suite have higher 10,000*(Eu N /Eu N *)/Y ratios and lower calculated Ti-in-zircon temperatures (691° ± 49 °C and 703° ± 48 °C, respectively) than the barren suites (770° ± 68 °C–790° ± 76 °C), suggesting higher magmatic water contents for Yulong. This is consistent with higher abundances of hydrous minerals such as biotite and amphibole in the Yulong suite. Amphibole phenocrysts from pre- and syn -mineralization intrusions at Yulong have significantly lower crystallizing pressures (2.1 ± 0.6 kbar and 1.9 ± 0.5 kbar, respectively) than the barren suites (4.6 ± 0.4 kbar to 4.7 ± 0.5 kbar). A similar trend is also shown by crystallizing pressures estimated from biotite phenocrysts from these barren and mineralized suites. We propose that the barren suites were more Cl-rich because their source magma chambers have greater depth but lower water contents, leading to limited fluid exsolution and Cl loss. In contrast, the shallowly-emplaced, more hydrous Yulong suite was originally Cl-rich but underwent early fluid exsolution and Cl loss before/during apatite crystallization. As such, apatite Cl contents may not always be good fertility indictor for porphyry Cu systems. We demonstrate that high apatite SO 3 contents are less affected by fluid exsolution due to buffering of saturated anhydrite in oxidized and S-rich magmas, and may be used to aid exploration. [ABSTRACT FROM AUTHOR]

The Miocene Qulong porphyry Cu-Mo deposit, which is located at the Gangdese orogenic belt of Southern Tibet, is the largest porphyry-type deposit in China, with confirmed Cu ∼10 Mt and Mo ∼0.5 Mt. It is spatially and temporally associated with multiphase granitic intrusions, which is accompanied by large-scale hydrothermal alteration and mineralization zones, including abundant hydrothermal anhydrite. In addition to hydrothermal anhydrite, magmatic anhydrite is present as inclusions in plagioclase, interstitial minerals between plagioclase and quartz, and phenocrysts in unaltered granodiorite porphyry, usually in association with clusters of sulfur-rich apatite in the Qulong deposit. These observations indicate that the Qulong magma-hydrothermal system was highly oxidized and sulfur-rich. Three main types of fluid inclusions are observed in the quartz phenocrysts and veins in the porphyry: (i) liquid-rich; (ii) polyphase high-salinity; and (iii) vapor-rich inclusions. Homogenization temperatures and salinities of all type inclusions decrease from the quartz phenocrysts in the porphyry to hydrothermal veins (A, B, D veins). Microthermometric study suggests copper-bearing sulfides precipitated at about 320-400°C in A and B veins. Fluid boiling is assumed for the early stage of mineralization, and these fluids may have been trapped at about 35-60 Mpa at 460-510°C and 28-42 Mpa at 400-450°C, corresponding to trapping depths of 1.4-2.4 km and 1.1-1.7 km, respectively. [ABSTRACT FROM AUTHOR]

The Duobuza porphyry copper-gold deposit (proven Cu resources of 2.7 Mt, 0.94% Cu and 13 t gold, 0.21 g t Au) is located at the northern margin of the Bangong-Nujiang suture zone separating the Qiangtang and Lhasa Terranes. The major ore-bearing porphyry consists of granodiorite. The alteration zone extends from silicification and potassic alteration close to the porphyry stock to moderate argillic alteration and propylitization further out. Phyllic alteration is not well developed. Sericite-quartz veins only occur locally. High-temperature, high-salinity fluid inclusions were observed in quartz phenocrysts and various quartz veins. These fluid inclusions are characterized by sylvite dissolution between 180 and 360°C and halite dissolution between 240 and 540°C, followed by homogenization through vapor disappearance between 620 and 960°C. Daughter minerals were identified by SEM as chalcopyrite, halite, sylvite, rutile, K-feldspar, and Fe-Mn-chloride. They indicate that the fluid is rich in ore-forming elements and of high oxidation state. The fluid belongs to a complex hydrothermal system containing HO - NaCl - KCl ± FeCl ± CaCl ± MnCl. With decreasing homogenization temperature, the fluid salinity tends to increase from 34 to 82 wt% NaCl equiv., possibly suggesting a pressure or Cl/HO increase in the original magma. No coexisting vapor-rich fluid inclusions with similar homogenization temperatures were found, so the brines are interpreted to have formed by direct exsolution from magma rather than trough boiling off of a low-salinity vapor. Estimated minimum pressure of 160 MPa imply approximately 7-km depth. This indicates that the deposit represents an orthomagmatic end member of the porphyry copper deposit continuum. Two key factors are proposed for the fluid evolution responsible for the large size of the gold-rich porphyry copper deposit of Duobuza: (i) ore-forming fluids separated early from the magma, and (ii) the hydrothermal fluid system was of magmatic origin and highly oxidized. [ABSTRACT FROM AUTHOR]

Abstract: Porphyry deposits are usually thought to form from subduction-related calc-alkaline magmas in magmatic arc settings, although some porphyry deposits also occur in post-collisional extensional settings. The post-collisional deposits remain poorly understood. Here we describe the igneous geology, alteration mineralogy and mineralization history of Qulong, a newly-discovered porphyry Cu–Mo deposit in southern Tibet that belongs to the post-collisional class. The deposit is associated with Miocene monzogranite–granodiorite intrusions and is hosted partly by Jurassic andesitic–dacitic volcanic rocks. The deposit contains 7.1 Mt Cu and 0.5 Mt Mo metal. The ~19.5 Ma granodioritic–monzogranitic Rongmucuola pluton with diorite enclaves is the earliest Miocene intrusive unit. It was intruded by a regularly-shaped stock (P porphyry) and then thin dikes (X porphyry) of monzogranite at about 17.7 Ma. The main Cu–Mo mineralization is associated with the P porphyry. A barren diorite porphyry intruded the P and X porphyries around 15.7 Ma. Petrologic trends of the intrusions suggest that the Miocene intrusions have similar origins and probably formed by fractionation of a deeper magma chamber. Emplacement of Miocene porphyries, controlled by the Qulong anticline, is in direct response to the rapid uplift/erosion of the Gangdese arc batholiths in southern Tibet. Miarolitic cavities and unidirectional solidification textures, key evidence for volatile separation, have been recognized in the P and X porphyries, respectively. Early potassic alteration, characterized by quartz–K feldspar (±anhydrite), pervades the P porphyry and Rongmucuola pluton. Laterally, this alteration grades into quartz–biotite–anhydrite (±K feldspar), which affects all Miocene intrusions except the latest dioritic porphyry. Wall rocks of Rongmucuola pluton and Jurassic andesitic–dacitic volcanics within 1–1.5 km of the porphyries are dominated by pervasive potassic alteration. An outer halo of propylitic alteration (epidote–chlorite±calcite) extends up to 2 km away from the deposit. Feldspar-destructive alteration (sericite–chlorite±clay minerals) has overprinted most of the potassic and part of the propylitic alteration. The alteration is strongly pervasive in the interior of the porphyry bodies and occurs as vein halos away from the porphyry bodies. The earliest quartz–K feldspar alteration and veins are barren, whereas approximately 60% of the Cu reserves is associated with slightly later quartz–biotite–anhydrite alteration. Barren assemblages are related to irregular quartz (–K feldspar±anhydrite) veins, which are truncated by the X porphyry. Cu sulfide-bearing assemblages are associated with discontinuous chalcopyrite (±biotite) and continuous quartz–anhydrite–chalcopyrite (±molybdenite) veins. Deposition of Cu–Mo with abundant anhydrite occurred during or between emplacement of closely related porphyries from high temperature magmatically-derived fluids, and was probably caused by the disproportionation of SO2. [Copyright &y& Elsevier]

• Estimated halogen fugacity in the fluids and F-Cl-S concentrations in the pre-ore and ore-related magmas. • Fluid exsolution and magma recharge can be identified by the volatile variations. • Both magma and ore at Sharang are originated from subduction-modified mantle and Lhasa terrane basement. Volatiles in magmatic‐hydrothermal fluids play an essential role in the formation of porphyry ore deposits. Models of F, Cl and S partitioning between silicate melt, fluid and volatile-bearing minerals (biotite and apatite) in the magmatic system provide an opportunity to estimate halogen fugacity in the fluids and F-Cl-S concentrations in the ore-related magmas. This work integrates mineralogical and isotopic studies of multi-stage intrusions in the Sharang porphyry Mo deposit, with the aim of tracing the evolution of magmatic volatile components in the magmatic-hydrothermal system and elucidating the origin of magma and ores. Sharang apatite compositions in pre-ore and ore-forming intrusions were likely formed at different fractional crystallization stages from volatile-undersaturated to H 2 O-saturated melts. Magmatic fluids that equilibrated with pre-ore melts were HCl-rich, while the exsolved fluids from the granite porphyry were enriched in HF. Magmatic Cl-F-S contents varied accordingly, and above 700 °C, the felsic causative melts likely had high F (ca. 2700 ppm), low Cl (ca. 130 ppm) and high S contents (ca. 300 ppm). Fluid exsolution was likely the main control over a decrease in magmatic Cl concentrations, while the deep magma source chamber appears to have been recharged by a more mafic S-Cl rich magma at the time of quartz diorite emplacement, elevating the sulfur content of the magma. The magma and sulfides likely originated from subduction-modified mantle and Lhasa terrane basement, which were also responsible for the generation of a high-F, Mo-mineralizing magma. [ABSTRACT FROM AUTHOR]

• The Yidun arc had a thick crust in the south and a thin crust in the north during the Late Triassic. • Magmas derived from the northern and southern Yidun arc were different in compositions and were associated with different deposit. • Crustal thickness controlled the compositions of magmas and the types of deposit in the Yidun Arc during the Late Triassic. Modern arc-related granitic magmas have variable Sr/Y ratios and metal contents, depending on the thickness of the arc crust on the global scale. However, whether this is true of ancient arcs on a regional scale is still unclear. The Yidun Arc is a Triassic continental arc in Eastern Tibet that has magmatic and mineralization characteristics varying between the northern and southern Yidun Arcs during the Triassic. Several models have been proposed to explain these differences, however, these models are still controversial. To this end, we investigated the petrogenesis of the Late Triassic Xiuwacu intrusion, which is located between the northern and southern Yidun Arcs. The zircon U-Pb dating and Sr-Nd-Hf-O isotopes indicate that the Late Triassic Xiuwacu intrusion was emplaced at 215.6 ± 2.2 Ma and was predominately derived from partial melting of an enriched metastasized mantle combined with amphibole dominated fractionation and a small amount of crustal assimilation. Comparing the Late Triassic Xiuwacu intrusion with other volcanic rocks and intrusions in the Yidun Arc indicates that the Late Triassic Xiuwacu intrusion has a source similar to that of igneous rocks in the southern Yidun Arc. In addition, volcanic rocks and porphyries in the southern Yidun Arc have high Dy/Yb (1.8–2.6, mostly > 2.0) and Sr/Y (24–120) ratios, lack or have slightly negative Eu anomalies, and exhibit calc-alkaline trends, indicating that garnet, amphibole, and magnetite crystalized from their magma source under high pressure. The volcanic rocks in the northern Yidun Arc have low Dy/Yb (1.3–2.2, mostly < 2) and Sr/Y (4–43, mostly < 20) ratios with high negative Eu anomalies and tholeiitic trends, indicating that amphibole and feldspar crystalized predominantly from the source under relatively low pressure. These geochemical features suggest that the crust under the northern Yidun Arc is thin, the crust under the southern Yidun Arc is thick, and the thickness of the crust in the Xiuwacu area is intermediate between them. Several large scale porphyry Cu deposits formed only in the southern Yidun Arc is probably due to the crust of the southern Yidun Arc is thick. The crystallization of amphibole and magnetite reduced the magmas and led to the accumulation of Cu sulfides at the base of the thick southern Yidun Arc at an early stage. Late re-melting of these accumulated Cu sulfides produced these Cu mineralization. The distinct difference in crustal thickness between the northern and southern Yidun arcs is most likely due to the differences in the dip angle of the subducting slab during the Triassic, with these variations controlling magmatism and mineralization. [ABSTRACT FROM AUTHOR]