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We present in‐situ zircon laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) U–Pb ages, whole‐rock geochemistry, and Sr–Nd–Pb–Hf isotopes of the Mugagangri monzogranite in the southern margin of the Qiangtang Block, Tibet, western China. The zircons yield a U–Pb age of ca. 123 Ma. The hornblende‐bearing monzogranite shows metaluminous to weak peraluminous and high‐K calc‐alkaline characteristics exemplified by high silica (SiO2 = 67.57–70.57 wt%), high aluminium (Al2O3 = 14.68–15.78 wt%), high potassium (K2O = 4.00–5.14 wt%), high alkali (K2O + Na2O = 7.88–8.62 wt%), and low calcium contents (CaO = 1.72–2.17 wt%), with the aluminium saturation index (A/CNK) ranging from 0.98 to 1.09, suggesting that the Mugagangri monzogranite is a metaluminous to weak peraluminous I‐type high‐K calc‐alkaline granite. Geochemically, similar to the arc magmas, the monzogranite is enriched in large‐ion lithophile elements, and relatively depleted in high‐field‐strength elements. The monzogranite displays relatively high (87Sr/86Sr)i values (0.70972–0.71240), uniform εNd(t) values (−2.24 to −3.40), variable zircon εHf(t) values (−14.1 to +8.0), and high radiogenic Pb isotopic values (206Pb/204Pb = 18.588–18.790, 207Pb/204Pb = 15.616–15.642, and 208Pb/204Pb = 38.838–39.053). These geochemical characteristics indicate that the monzogranite was derived from a mixed source comprising ancient crustal and mantle materials, and experienced fractional crystallization during emplacement. We propose that the parental magma of the Mugagangri monzogranite was most likely generated during northward subduction of the Bangong Co‐Nujiang Meso‐Tethys Ocean. [ABSTRACT FROM AUTHOR]

The Tibetan Plateau is characterized by the largest crustal thickness on Earth, although the timing of formation of the plateau remains debated. In this study, we present in situ laser ablation‐inductively coupled plasma‐mass spectrometry zircon U–Pb ages and Hf isotopes, and whole‐rock geochemical data on porphyritic granitoids of Sebuta in the central Lhasa block. Our zircon U–Pb data indicate that these rocks emplaced at ca. 89 Ma, in the early Late Cretaceous. Geochemically, the Sebuta biotite granite porphyry can be broadly divided into two subtypes, namely metaluminous porphyry and peraluminous porphyry. The metaluminous porphyry has high SiO2, Al2O3, Sr, low Y and Yb contents, and high Sr/Y and LaN/YbN ratios, showing adakitic features. These rocks have high K2O and Th contents, low Ti/Eu and Nd/Sm ratios, indicative of a continental crust affinity. Positive zircon εHf(t) values, along with high Mg#, Cr and Ni, imply that the Sebuta adakitic porphyry was most likely derived from partial melting of a delaminated thickened juvenile lower crust, and that the initial melts interacted with mantle peridotite during ascent. The field features, petrographic characteristics, formation age, ore‐bearing potential, and Hf isotope of the peraluminous porphyry are similar to those of the Sebuta adakitic porphyry, indicating that the peraluminous porphyry was likely formed by contamination with ancient crustal material and fractional crystallization from the same primary adakitic magmas which generated the Sebuta adakitic porphyry. Residual phases of amphibole + plagioclase + garnet + rutile in the source region, together with previous studies, suggest that the crust beneath the central‐northern Lhasa block experienced thickening during the early Late Cretaceous, and had already been thickened to more than 50 km by the Late Cretaceous (ca. 89 Ma). [ABSTRACT FROM AUTHOR]

Miocene postcollisional potassic and adakitic rocks are widely distributed in the southern Lhasa terrane and western central Lhasa terrane. However, coeval potassic and adakitic rocks in eastern central Lhasa terrane were rarely recognized, and their origins and formation mechanism remain controversial. In this paper, we provide new geochronological and geochemical data for the Miocene postcollisional potassic and adakitic intrusions exposed in the Qingdu area, eastern central Lhasa terrane, southern Tibet. The Qingdu Miocene intrusions consist of quartz monzonite porphyry and biotite granite with coeval zircon U–Pb ages of 14.1 Ma and 14.0 Ma, respectively. They have high SiO2 (67.98–75.32 wt.%), Al2O3 (14.13–14.78 wt.%), and K2O (4.06–6.18 wt.%) and low MgO (0.25–1.46 wt.%) contents. They are both enriched in light rare earth elements (LREEs) and depleted in heavy rare earth elements (HREEs), with high (La/Yb)N (25.37–42.32) ratios. The biotite granite samples have low Y (7.10–9.96 ppm) and Yb (0.61–0.85 ppm) contents, and high Sr (229–384 ppm) contents and high Sr/Y (30–41) ratios, which show adakitic geochemical characteristics, whereas the quartz monzonite porphyry samples show potassic geochemical characteristics. They display initial (87Sr/86Sr)i ratios of 0.7083–0.7103, εHf(t) values of −6.7 to −0.1, εNd(t) values of −8.96 to −7.44, (208Pb/204Pb)i ratios of 39.028–39.110, (207Pb/204Pb)i ratios of 15.662–15.684, and (206Pb/204Pb)i ratios of 18.541–18.577. These signatures indicate that both the potassic and adakitic intrusions are more likely to originate from partial melting of a thickened lower crust, which were mainly the products of the binary mixing between the juvenile and ancient crust components. Based on the spatial distributions and isotopic features of the postcollisional potassic and adakitic rocks in the Lhasa terrane, we suggest that the differences of the lower crustal composition played a crucial role in causing the geochemical variations of the Miocene postcollisional adakitic and potassic rocks in Lhasa terrane. [ABSTRACT FROM AUTHOR]

The Rongna Cu-(Au) deposit is a recently discovered giant low-grade composite porphyry-high sulfidation epithermal system in the Duolong ore district of central Tibet. We present zircon U-Pb ages, bulk-rock geochemical and Sr-Nd isotope data, as well as zircon Hf-O isotope data to constrain the petrogenesis of the Rongna granodioritic porphyry, on which the ore system is centered. Secondary ion mass spectrometry (SIMS) zircon U-Pb data for two samples yielded ages of 118.7 ± 0.9 Ma and 117.1 ± 0.9 Ma, respectively. The granodioritic porphyry is of high-K calc-alkaline and magnetite-series affinity, with enrichment of light rare earth elements (LREEs) and large-ion lithophile elements (e.g., Rb, Pb, and K), and depletion of high-field strength elements (e.g., Ta, Nb, and Ti), typical of the geochemical signature of arc magmas. The Rongna granodioritic porphyry has initial Sr isotopic ratios of 0.7060–0.7099, and ε Nd (t) values of −5.7 to −1.7 with T DM (Nd) of 983–1900 Ma. The zircons have relatively narrow Hf-O isotopic compositions (ε Hf (t) = +3.30– +6.43; δ 18 O = +5.94– +6.96‰), indicating the involvement of old crustal materials. Our data suggest that the Rongna granodioritic porphyry likely evolved by fractional crystallization from a hybrid magma of mantle-derived mafic melts and lower crustal felsic melts during a period of magmatic flare-up following flat-slab subduction. [ABSTRACT FROM AUTHOR]

Porphyry Cu–Au and porphyry–skarn Cu ± Au ± Mo deposits are widely distributed in the Bangong–Nujiang metallogenic belt, central Tibet. Zircon U–Pb dating has revealed that Cretaceous ore-bearing intrusions related to Cu ± Au ± Mo mineralization formed in two periods (118–115 Ma and 90–88 Ma). These primarily high-K calc-alkaline series intrusions show light rare earth element enrichment (La N /Yb N = 6.06–20.2) and negative to no Eu anomaly (Eu*/Eu = 0.39–1.02). On primitive mantle-normalized diagrams, all the intrusions show strong enrichments in large ion lithophile elements (e.g., Cs, Rb, and K), depletions in Nb, Ta, and Ti, and negative Ba anomalies characteristic of arc magma. These intrusions show a wide range of zircon ε Hf (t) values from –6.3 to + 10.9 and δ 18 O values from 5.82 to 10.82 ‰, suggesting variable contributions from mantle and crustal sources. Considering the ~ 100 Ma Lhasa–Qiangtang collision, the 118–115 Ma magmas and related deposits were likely formed by melting of subduction metasomatized mantle wedge in a continental arc setting during northward subduction of the Bangong–Nujiang ocean, and then further evolved in the upper crust as a result of MASH processes. The younger 90–88 Ma ore-bearing magmas were potentially derived from melting of previously metasomatized lithospheric mantle during slab tear and break-off after the Lhasa–Qiangtang collision. [ABSTRACT FROM AUTHOR]

The Duolong porphyry Cu-Au deposit (5.4 Mt at 0.72% Cu, 41 t at 0.23 g/t Au) was recently discovered in the southern Qiangtang terrane, central Tibet. Here, new whole-rock elemental and Sr-Nd-Pb isotope and zircon Hf isotopic data of syn- and post-ore volcanic rocks and barren and ore-bearing granodiorite porphyries are presented for a reconstruction of magmas associated with Cu-Au mineralization. LA-ICP-MS zircon U-Pb dating yields mean ages of 117.0 ± 2.0 and 120.9 ± 1.7 Ma for ore-bearing granodiorite porphyry and 105.2 ± 1.3 Ma for post-ore basaltic andesite. All the samples show high-K calc-alkaline compositions, with enrichment of light rare earth elements (LREE) and large ion lithophile elements (LILE: Cs and Rb) and depletion of high field strength elements (HFSE: Nb and Ti), consistent with the geochemical characteristics of arc-type magmas. Syn- and post-ore volcanic rocks show initial Sr ratios of 0.7045-0.7055, εNd(t) values of −0.8 to 3.6, (206Pb/204Pb)t ratios of 18.408-18.642, (207Pb/204Pb)t of 15.584-15.672 and positive zircon εHf(t) values of 1.3-10.5, likely suggesting they dominantly were derived from metasomatized mantle wedge and contaminated by southern Qiangtang crust. Compared to mafic volcanic rocks, barren and ore-bearing granodiorite porphyries have relatively high initial Sr isotopic ratios (0.7054-0.7072), low εNd(t) values (−1.7 to −4.0), similar Pb and enriched zircon Hf isotopic compositions [εHf(t) of 1.5-9.7], possibly suggesting more contribution from southern Qiangtang crust. Duolong volcanic rocks and granodiorite porphyries likely formed in a continental arc setting during northward subduction of the Bangong-Nujiang ocean and evolved at the base of the lower crust by MASH (melting, assimilation, storage and homogenization) processes. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Triassic granitoids, including the ∼220 Ma Shuanghu and ∼210 Ma Rongma granitoids studied here, are widely distributed around the Longmu–Shuanghu suture and in the Qiangtang terrane, central Tibet. The majority of these granitoids can be classified as high-K calc-alkaline in nature and yield negative Ba and Sr anomalies on primitive mantle-normalized diagrams. In addition, they are: enriched in light rare earth elements (LREE) ((La/Yb) N = 1.61–21.79); strongly enriched in large ion lithophile elements (LILE: e.g., Cs, Rb, and K), and depleted in high field strength elements (HFSE: e.g., Nb and Ti). Magma mixing played a role in the genesis of the Shuanghu granodiorites, as indicated by the occurrence of dioritic enclaves and the wide range in zircon Hf compositions (εHf( t ) = −15.0 to −2.5). The I-type Shuanghu granodiorites and S-type Shuanghu and Rongma granites might have been derived from melting of southern Qiangtang crust given the high initial Sr (0.7131–0.7272), low εNd( t ) (−8.9 to −11.1) and zircon εHf( t ) values (−15 to −7.2). The granitoids may have formed during melting of southern Qiangtang crust, heated by upwelling asthenosphere mantle, a result of break-off and delamination of the Paleo-Tethys slab in a collisional setting. [ABSTRACT FROM AUTHOR]

The Bangong- Nujiang suture ( BNS) between the Lhasa and Qiangtang terranes is an important boundary and its petrogenesis is controversial. Diabase from the accretionary prism in the southern Qiangtang terrane yields a zircon U- Pb age of 181.3 ± 1.4 Ma. All the diabases show tholeiitic basalt compositions, gentle enrichment patters of light rare earth elements ( REE), variable enrichment in incompatible element concentrations (e.g. Th and Rb), and no anomaly in high field strength elements (e.g. Nb and Ta), similar to that of enriched mid-ocean ridge basalt ( E- MORB). They have relatively homogeneous whole rock Nd (ε Nd(t) = 7.3-9.1) and zircon Hf- O isotopic compositions (ε Hf(t) = 14.8-16.1, and δ18 O = 4.57-6.12‰), possibly indicating melting of the depleted mantle and no significant crustal contamination during the petrogenesis. The element variations suggest that the diabases were formed by plume-ridge interaction at a mid-ocean ridge within the Bangong- Nujiang ocean. [ABSTRACT FROM AUTHOR]

[Display omitted] • Skarn-type mineralization of the Cuonadong Sn-W-Be polymetallic deposit occurred at ca. 15 Ma. • Skarn-hosted scheelite can be divided into an older type 1 scheelite and a younger type 2 scheelite. • The ore-forming fluid was exsolved from the Cuonadong Miocene leucogranite. • Geochemically and isotopically, the ore-forming fluid was modified by intense fluid-rock interaction during skarnization and greisenization. The Cuonadong Sn-W-Be polymetallic deposit is the first rare-metal deposit with an extraordinary metallogenic potential that has been discovered in the Tethyan Himalayan metallogenic belt. The deposit shows a wide range of different mineralization types, including greisen-, pegmatite-, skarn-, and hydrothermal vein-type mineralization. Of these mineralization types, the skarn-type has the largest inferred resources. In order to constrain the source and evolution of the ore-forming fluid, we conducted a detailed study of the skarn-hosted scheelite, including cathodoluminescence imaging, in-situ trace elemental analysis, and bulk Sr isotope analysis. In addition, bulk Sr isotope analysis of fluorite, phlogopite 40Ar-39Ar dating, and marble whole-rock geochemical analysis were also conducted. Phlogopite 40Ar-39Ar dating indicates that the skarn was formed at ca. 15 Ma, which is close to the formation age of the Cuonadong muscovite granite reported in previous studies. Cathodoluminescence images indicate that skarn-hosted scheelite can be subdivided into an older type 1 scheelite and a younger type 2 scheelite. Type 1 scheelite is characterized by a negative Eu anomaly, whereas type 2 scheelite shows a positive Eu anomaly. Considering the close temporal and spatial relationship between the Cuonadong Miocene leucogranite and the ore-bearing skarn, and their similar lanthanide tetrad effect as observed in the REE patterns, we propose that the negative Eu anomaly was inherited from the Cuonadong Miocene leucogranite, while the positive Eu anomaly resulted from the addition of Eu2+ to the ore-forming fluid during greisenization. Sr isotope data show that the fluorite (87Sr/86Sr) i value of 0.728885 is within the range of the Cuonadong Miocene leucogranite (87Sr/86Sr) i values, while scheelite (87Sr/86Sr) i values (0.709717–0.713480) are similar to those of the Cuonadong marble (0.709525–0.712146). Since scheelite is intergrown with fluorite, we propose that the hydrothermal fluid that exsolved from the Cuonadong Miocene leucogranite is responsible for formation of both fluorite and scheelite. Water-rock interaction between this magma-derived fluid and the marble is responsible for the loss of the scheelite magmatic Sr isotopic signature. [ABSTRACT FROM AUTHOR]

• Magmatic-hydrothermal water was dominant at the Rongna deposit. • The epithermal mineralization was partially related with the dissolution and re-precipitation of porphyry-style sulfides. • Leaching downward of contracted vapor formed telescoped epithermal alteration and mineralization. • Zinc (Zn) was enriched in the distal fluid of the Rongna deposit. The giant Rongna Cu-(Au) deposit is a recently discovered low-grade composite porphyry-high sulfidation epithermal system in the Duolong ore district of central Tibet. We present fluid inclusion petrography, microthermometry, stable isotope (H, O), and fluid-compositional data (LA-ICP-MS) to constrain the fluid evolution of the Rongna deposit. Hydrothermal quartz and muscovite formed in the porphyry stage yielded fluid values of 8.2 ± 1.0‰ δ18O H2O , and −75 ± 19‰ δD H2O (n = 13). Pyrophyllite, kaolinite, and dickite define the isotopic composition of the epithermal fluid (δ18O H2O = 2.2–8.3‰, δD H2O = −87 ~ −45‰). This epithermal fluid of lighter oxygen isotope and heavier hydrogen isotope composition still has a magmatic-hydrothermal signature and rules out any significant involvement of meteoric water. The very high concentration of Cu in fluid inclusions from epithermal veins and the common sulfide replacement features suggest that the epithermal mineralization may be partially related with the dissolution and re-precipitation of the porphyry-stage sulfides. Combined with fluid inclusion petrography and microthermometric data, it is indicated that a single-phase fluid exsolved from magma at depth and underwent boiling and cooling, forming the porphyry alteration and mineralization. With decrease of temperature and pressure, the ascending vapor contracted into acid aqueous fluid, leaching downward to develop the telescoped high-sulfidation epithermal alteration and mineralization. [ABSTRACT FROM AUTHOR]

Ore-forming fluids ultimately precipitate Fe-bearing sulfides and oxides in hydrothermal ore deposits and the Fe isotopic composition of these minerals can trace magmatic-hydrothermal evolution. Here, we report on the Fe isotopic compositions of a suite of hydrothermal minerals (magnetite, pyrite, and chalcopyrite) and granodiorite porphyry from the giant Duolong porphyry Cu-Au deposit (Bolong and Duobuza section), central Tibet. Most magnetite grains with potassic alteration show only minor δ 57 Fe variation (0.38 ± 0.07‰ to 0.52 ± 0.04‰ in Bolong and 0.68 ± 0.02‰ to 0.77 ± 0.08‰ in Duobuza), consistent with the equilibrium fluid δ 57 Fe (∼−0.3‰ and ∼−0.1‰, respectively) at ∼550–480 °C. The equilibrium fluids have lighter Fe isotope signatures than the Duolong granodiorite porphyry (δ 57 Fe = 0.03 ± 0.06‰ to 0.07 ± 0.02‰), corroborating the hypothesis that exsolved fluids should have a lighter Fe isotopic composition relative to parental magmas. Chalcopyrite from the mineralized Bolong and Duobuza porphyries have relatively consistent δ 57 Fe values of −0.60 ± 0.07‰ to −0.42 ± 0.07‰ and −0.40 ± 0.08‰ to −0.30 ± 0.05‰, respectively, with equilibrium fluids at ∼450–350 °C having lighter δ 57 Fe values of ∼−0.7‰ and ∼−0.6‰. The trend of decreasing δ 57 Fe values in evolved fluids likely reflects Rayleigh fractionation of magnetite enriched in heavy Fe isotopes. This supposition is supported by the lighter δ 57 Fe value (0.25 ± 0.07‰) recorded in magnetite that equilibrated with a lighter δ 57 Fe fluid (∼−0.5‰) at ∼470 °C. Compared to chalcopyrite, pyrite from the Bolong and Duobuza sections have heavier δ 57 Fe values of 0.35 ± 0.06‰ to 0.71 ± 0.06‰ and 0.53 ± 0.06‰ to 0.71 ± 0.07‰, respectively. The pyrite values correlate variably with the Fe isotopic composition of ore-forming fluids (δ 57 Fe = −0.6‰ to −0.7‰), likely due to variations in the degree of Fe isotope exchange between pyrite and fluid. Moreover, combined with previously published Fe isotopic compositions of hydrothermal minerals from oxidized and reduced ore deposits, the results show that chalcopyrite in oxidized hydrothermal deposits always has a lighter Fe isotopic composition than chalcopyrite from reduced hydrothermal deposits. This is likely controlled by melt composition and precipitation of magnetite/pyrrhotite-bearing mineral assemblages. Therefore, the Fe isotopic composition of chalcopyrite could be a useful diagnostic tool for distinguishing oxidized from reduced fluids in hydrothermal systems. [ABSTRACT FROM AUTHOR]

Abstract This paper presents new geochronological and geochemical (whole-rock major and trace elements and Sr–Nd–Pb isotopes) data on intrusive rocks exposed in the Qiongduojiang area, the northern part of the Tethyan Himalaya. The Qiongduojiang intrusive rocks are mainly composed of mafic rocks (diabase and gabbro) with subordinate felsic rocks (monzonite), indicating bimodal component characteristics. In zircon U-Pb dating of monzonite, diabase and gabbro samples, the weighted mean ages of magma emplacement were determined to be 230.5 ± 2.9 Ma to 227.4 ± 4.7 Ma, showing Middle–Late Triassic magmatic activity in the northern part of the Himalayan belt. The monzonitic rocks share most of the geochemical features of A-type granites, and their Nd isotopic compositions are consistent with those of coeval diabase rocks. This suggests that they are likely generated by mantle-derived magmas, which had assimilated significant amounts of continental crust. The gabbro samples exhibit the geochemical composition of ocean island basalt (OIB) features and therefore assumingly originate from a depleted mantle source with insignificant crustal contamination. In contrast, the diabase samples display enriched mid-ocean ridge basalt (E-MORB) features and were derived from spinel bearing peridotite that was metasomatised by OIB-like components with limited crustal contamination. The Qiongduojiang bimodal magmas can be considered as the products of the opening of the Neo-Tethyan ocean. Considering the magmatic petrology, paleomagnetism and paleontology data, we further propose that the Neo-Tethyan ocean began opening before ~230 Ma. Highlights • The Middle-Late Triassic intrusive rocks (230.5–227.4 Ma) are exposed in Tethyan Himalayan belt. • They are characterised by bimodal composition (monzonite, diabase, and gabbro). • The Neo-Tethys ocean initial opening commenced prior to ~230 Ma. [ABSTRACT FROM AUTHOR]

Ore-forming fluid exsolution in a shallow magma chamber is a critical step in the formation of porphyry Cu ± Mo ± Au deposits but one for which direct evidence is rarely found. Here, Cl abundance, major-trace element content and H isotope analysis of amphibole in diorite and barren granodiorite porphyry samples from the Duolong porphyry-epithermal Cu-Au district are presented in order to reveal processes associated with fluid exsolution and magma evolution. Low-Al Mg-hornblende formed in ore-bearing diorite at ∼750–860 °C and ∼80–200 MPa. On primitive mantle-normalized diagrams, these low-Al amphibole grains have slightly convex upward REE patterns with distinctly negative Eu anomalies and negative anomalies in Pb, Sr, Eu, Zr, Hf, and Ti anomalies, suggesting crystallization from the same arc magma after plagioclase and magnetite crystallization. A large variation in estimated melt H 2 O (∼6 to 3 wt%) and Cl content (0.09–0.38 wt%), as well as low δD values (−103 to −113‰) indicate that the magma underwent large-scale fluid exsolution, contributing to the formation of Duolong Cu-Au mineralization. Additionally, amphibole from a barren granodiorite porphyry shows two distinct populations, distinguished by their Al content. This intrusion has low-Al amphibole (Mg-hornblende) which formed at similar conditions to low-Al amphiboles in the diorite (∼790–870 °C and ∼100–230 MPa), whereas high-Al amphibole (tschermakite) formed at ∼880–970 °C and ∼210–400 MPa. Some high-Al amphibole phenocrysts have slightly convex REE patterns with no negative Eu anomalies, a depletion in Nb, Zr, and Hf, and positive Sr, Ba, and Pb anomalies, likely consistent with amphibole crystallization from more mafic basaltic-andesitic melts. Low-Al amphibole in the granodiorite porphyry shows different compositional trends compared to those in the diorite, suggesting that they crystallized from different magmas. Considering the evidence for mafic magma replenishment suggested by the positive correlation between Al IV and Mg# values in high-Al amphibole, the low-Al amphibole in the barren granodiorite porphyry likely crystallized from a new hybrid magma with mafic to intermediate-felsic magma compositions. Meanwhile, smaller variations in Cl content (0.08–0.24 wt%) in low-Al amphibole was dominantly controlled by magma evolution (shown by variable Mg#) rather than fluid exsolution. Thus, the low δD values (−102 to −122‰) were likely inherited from evolved dioritic magmas that underwent fluid exsolution. Importantly, the Cl compositional variation of amphibole has a potential application as a powerful tool to identify ore-bearing and barren intrusions within porphyry Cu ± Mo ± Au deposits. [ABSTRACT FROM AUTHOR]

A new high sulfidation epithermal Cu–Au occurrence (Nadun) has been discovered adjacent to the Cretaceous Duolong porphyry Cu–Au deposit within the Bangong–Nujiang metallogenic belt, central Tibet. The Nadun Cu–Au mineralization is hosted in a tectonic–hydrothermal breccia with advanced argillic alteration, which occurs above sandstone, associated with quartz–pyrite veins. The granodiorite porphyry with strong argillic alteration yields a zircon U–Pb age of 119.1 ± 1.3 Ma, whereas the weakly argillic granodiorite porphyry intruded into the breccia has a younger age of 116.1 ± 1.3 Ma. This indicates that Cu–Au epithermal mineralization likely occurred between ~ 116 Ma and ~ 119 Ma, consistent with the duration of magmatic–hydrothermal activity at Duolong (~ 115–118 Ma), and providing evidence that Nadun and Duolong were formed during the same event. Moreover, the Nadun and Duolong porphyries have similar Hf isotopic compositions (εHf(t) values ranging from − 8.8 to 8.1; mean = 5.0 ± 1.1, n = 32), likely indicating that the deposits are comagmatic. In addition, boiling assemblages in vapor-rich inclusions coexisting with brines occur in early stage quartz–pyrite veins, and likely record phase separation at a temperature of > 550–300 °C and pressure of 700–110 bars. Most liquid-rich fluid inclusions formed at the breccia stage show similar salinity (1.7–19.3 wt.% NaCl equiv) to vapor-rich inclusions from the underlying quartz–pyrite veins, likely indicating vapor contraction during cooling at elevated presssure. This suggests that quartz–pyrite veins may act as conduits for ore-forming fluid traveling from the porphyry to the epithermal hydrothermal system. O and H isotopic compositions (δ 18 O fluid = 0.42–9.71‰ and δD = − 102 to − 66‰) suggest that ore-forming fluids are dominantly from a magmatic source with a minor addition of meteoric water at a later stage. The S and Fe isotope compositions of sulfides (δ 34 S = − 5.9 to 0.5‰ and δ 57 Fe = − 2.15 to 0.17‰) decrease from the quartz–pyrite vein to breccia ore, indicating that ore-forming fluids gradually become SO 4 2 -enriched and relatively oxidized. This body of evidence suggests that the Nadun Cu–Au mineralization may represent the root of a high sulfidation epithermal deposit. [ABSTRACT FROM AUTHOR]

Abstract: Mesozoic intermediate–felsic intrusions are widely distributed in the southern Qiangtang terrane, central Tibet. Zircon U–Pb dating shows that these intrusions formed in two periods in the Jurassic (169–150Ma) and Cretaceous (127–113Ma). They mostly belong to the high-K calc-alkaline series, and show strong enrichments in large ion lithophile elements (e.g., Cs, Rb, and K), depletions in Nb, Ta, and Ti, and negative Ba anomalies on primitive mantle-normalized diagrams. P2O5 contents decrease with increasing SiO2 content, and Th contents increase with increasing Rb content, consistent with the evolution trend of I-type magmas. These intrusions show a wide range of zircon εHf(t) values from −19.4 to 11.2, including in rocks with similar SiO2 contents, suggesting variable contributions from mantle and Qiangtang crustal sources. Fine-grained mafic to intermediate igneous enclaves in Jurassic intrusions have similar zircon U–Pb ages and similar or slightly higher zircon εHf(t) values to the host rocks, suggesting that the enclave magmas were derived from mixed magmas at depth and injected into more evolved magmas in upper crustal magma chambers. Magma mixing is also supported by the wide range of zircon Hf isotopic compositions (εHf(t)=−19.4 to 2.5) from within individual Jurassic and Cretaceous intrusions, and Jurassic enclaves. The Jurassic–Cretaceous magmas likely formed in a continental arc setting during subduction of the Bangong–Nujiang ocean between 170 and 110Ma, and evolved in the upper plate crust by MASH processes. [Copyright &y& Elsevier]

Abstract: The Duolong porphyry Cu–Au deposit (5.4Mt at 0.72% Cu, 41t at 0.23g/t Au), which is related to the granodiorite porphyry and the quartz–diorite porphyry from the Bangongco copper belt in central Tibet, formed in a continental arc setting. Here, we present the zircon U–Pb ages, geochemical whole-rock, Sr–Nd whole-rock and zircon in-situ Hf–O isotopic data for the Duolong porphyries. Secondary ion mass spectrometry (SIMS) zircon U–Pb analyses for six samples yielded consistent ages of ~118Ma, indicating a Cretaceous formation age. The Duolong porphyries (SiO2 of 58.81–68.81wt.%, K2O of 2.90–5.17wt.%) belong to the high-K calc-alkaline series. They show light rare earth element (LREE)-enriched distribution patterns with (La/Yb)N =6.1–11.7, enrichment in large ion lithophile elements (e.g., Cs, Rb, and Ba) and depletion of high field strength elements (e.g., Nb), with negative Ti anomalies. All zircons from the Duolong porphyries share relatively similar Hf–O isotopic compositions (δ18O=5.88–7.27‰; εHf(t)=3.6–7.3), indicating that they crystallized from a series of cogenetic melts with various degrees of fractional crystallization. This, along with the general absence of older inherited zircons, rules out significant crustal contamination during zircon growth. The zircons are mostly enriched in δ18O relative to mantle values, indicating the involvement of an 18O-enriched crustal source in the generation of the Duolong porphyries. Together with the presence of syn-mineralization basaltic andesite, the mixing between silicic melts derived from the lower crust and evolved H2O-rich mafic melts derived from the metasomatized mantle wedge, followed by subsequent fractional crystallization (FC) and minor crustal contamination in the shallow crust, could well explain the petrogenesis of the Duolong porphyries. Significantly, the hybrid melts possibly inherited the arc magma characteristics of abundant F, Cl, Cu, and Au elements and high oxidation state, which contributed to the formation of the Duolong porphyry Cu–Au deposit. [Copyright &y& Elsevier]