Origin of the Dexing Cu-bearing porphyries,SE China: elemental and Sr–Nd–Pb–Hf isotopic constraints

The Dexing porphyry copper deposit, part of the circum-Pacific porphyry copper ore belt, is the largest porphyry copper deposit in China. We present new LA–ICP–MS zircon U–Pb and molybdenite Re–Os dating, bulk-rock elemental and Sr–Nd–Pb isotopic as well as in situ zircon Hf isotopic geochemistry for these ore-bearing porphyries, in an attempt to better constrain their petrogenesis. LA–ICP–MS zircon U–Pb dating shows that the Dexing porphyries were emplaced in the early Middle Jurassic (～171 Ma); molybdenite Re–Os dating indicates that the associated Cu–Mo mineralization was contemporaneous (～171 Ma) with the igneous intrusion. The rocks are mainly high-K calc-alkaline and show adakitic affinities, including high Sr and low Y and Yb contents, high Sr/Y and La/Yb ratios, and high Mg# (higher than pure crustal melts). These porphyries have initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7044?0.7047, ?_Nd(T) values of –1.5 to?+0.6, and ?_Hf(T) (in situ zircon) values of?+2.6 to?+4.6. They show unusually radiogenic Pb isotopic compositions with initial ²⁰⁶Pb/²⁰⁴Pb ratios up to 18.41 and ²⁰⁷Pb/²⁰⁴Pb up to 15.61. These isotopic compositions are distinctly different from either Pacific MORB or Yangtze lower crust but are similar to the subducting sediments in the western Pacific trenches. Detailed elemental and isotopic data suggest that the Dexing porphyries were emplaced in a continental arc setting coupled with westward subduction of the palaeo-Pacific plate. Partial melting involved the subducted slab (mainly the overlying sediments), with generated melts interacting with the lithospheric mantle wedge, thereby forming the investigated high-K calc-alkaline porphyry magmas.     

Petrogenesis and metallogenic setting of porphyries of the Duobuza porphyry Cu–Au deposit,central Tibet,China

Given that the Duobuza deposit was the first porphyry Cu–Au deposit discovered in central Tibet, the mineralization and mineralized porphyry in this area have been the focus of intensive research, yet the overall porphyry sequence associated with the deposit remains poorly understood. New geological mapping, logging, and sampling of an early granodiorite porphyry, an inter-mineralization porphyry, and a late-mineralization diorite porphyry were complemented by LA–ICP–MS zircon dating, whole-rock geochemical and Sr–Nd isotopic analyses, and in situ Hf isotopic analyses for both inter- and late-mineralization porphyry intrusions. All of the porphyry intrusions are high-K and calc-alkaline, and were emplaced at ca. 120 Ma. The geochemistry of these intrusions is indicative of arc magmatism, as all three porphyry phases are enriched in light rare earth elements and large ion lithophile elements, and depleted in heavy rare earth elements and high field strength elements. These similar characteristics of the intrusions, when combined with the relatively high (⁸⁷Sr/⁸⁶Sr)_i, negative ε_Nd(t), and positive ε_Hf(t) values, suggest that the magmas that formed the porphyries were derived from a common source region and shared a single magma chamber. The magmas were generated by the mixing of upwelling metasomatized mantle-wedge-derived mafic magmas and magmas generated by partial melting of amphibolite within the lower crust.The inter-mineralization porphyry has the lowest ε_Nd(t) and highest (⁸⁷Sr/⁸⁶Sr)_i values, suggesting that a large amount of lower-crust-derived material was incorporated into the melt and that metals such as Cu and Au from the enriched lower crust were scavenged by the parental magma. The relative mafic late-mineralization diorite porphyry phase was formed by the residual magma in the magma chamber mixing with upwelling mafic melt derived from metasomatized mantle. The magmatic–hydrothermal evolution of the magma in the chamber released ore-forming fluid that was transported mainly by the inter-mineralization porphyry phase during the mineralization stage, which ultimately formed the Duobuza porphyry Cu–Au deposit.These porphyritic intrusions of the Duobuza deposit have high Mg^# and low (La/Yb)_N values, and show some high LILE/HFSE ratios, indicating the magma source was enriched by interaction with slab-derived fluids. Combined with age constraints on the regional tectonic evolution, these dating and geochemical results suggest that the Duobuza porphyry Cu–Au deposit formed in a subduction setting during the final stages of the northward subduction of the Neo-Tethyan Ocean.     

Carboniferous porphyry Cu–Au deposits in the Almalyk orefield,Uzbekistan: the Sarycheku and Kalmakyr examples

The Almalyk porphyry cluster in the western part of the Central Asian Orogenic Belt is the second largest porphyry region in Asia and hence has attracted considerable attention of the geologists. In this contribution, we report the zircon U–Pb ages, major and trace element geochemistry as well as Sr–Nd isotopic data for the ore-related porphyries of the Sarycheku and Kalmakyr deposits. The zircon U–Pb ages (Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS)) of ore-bearing quartz monzonite and granodiorite porphyries from the Kalmakyr deposit are 326.1 ± 3.4 and 315.2 ± 2.8 Ma, and those for the ore-bearing granodiorite porphyries and monzonite dike from the Sarycheku deposit are 337.8 ± 3.1 and 313.2 ± 2.5 Ma, respectively. Together with the previous ages, they confine multi-phase intrusions from 337 to 306 Ma for the Almalyk ore cluster. Geochemically, all samples belong to shoshonitic series and are enriched in large-ion lithophile elements relative to high field strength elements with very low Nb/U weight ratios (0.83–2.56). They show initial (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.7059–0.7068 for Kalmakyr and 0.7067–0.7072 for Sarycheku and low ε_Nd(t) values of ?1.0 to ?0.1 for Kalmakyr and ?2.3 to 0.2 for Sarycheku, suggesting that the magmas were dominantly derived from a metasomatized mantle wedge modified by slab-derived fluids with the contribution of the continental crust by assimilation-fractional-crystallization process. Compared to the typical porphyry Cu deposits, the ore-bearing porphyries in the Almalyk cluster are shoshonitic instead of the calc-alkaline. Moreover, although the magmatic events were genetically related to a continental arc environment, the ore-bearing porphyries at Sarycheku and Kalmakyr do not show geochemical signatures of typical adakites as reflected in some giant porphyry deposits in the Circum-Pacific Ocean, indicating that slab-melting may not have been involved in their petrogenesis.     

Geochronology and magmatic oxygen fugacity of the Tongcun molybdenum deposit, northwest Zhejiang, SE China

The Tongcun Mo porphyry deposit in northwest Zhejiang is hosted in three porphyry units: Huangbaikeng, Songjiazhuang, and Tongcun, from southwest to northeast. U–Pb zircon ages of 162?±?3.0 Ma for the Huangbaikeng porphyry, 159.9?±?3.0 Ma for the Songjiazhuang porphyry, and 167.6–155.6 Ma for the Tongcun porphyry indicate that these intrusions formed during the Jurassic and are most likely associated with the northwestward subduction of the Izanagi Plate. Trace element compositions of zircons from the Tongcun deposit constrain the oxygen fugacity (fO₂) of the magma using zircon Ce anomalies and Ti-in-zircon temperatures. The average magmatic fO₂ for the porphyries in the Tongcun deposit is fayalite–magnetite–quartz (FMQ)?+?2.7, which is similar to the Shapinggou (FMQ?+?3.2) and Dabaoshan (FMQ?+?3.5) Mo porphyry deposits, but much higher than that of the reduced Cretaceous ore-barren Shangjieshou porphyry (FMQ-1.1) around 8 km away from the Tongcun deposit. The distinct difference in magmatic oxygen fugacity between the Jurassic and Cretaceous porphyries may help to explain the absence of Mo porphyry mineralization in northwest Zhejiang during the Cretaceous.     

The giant Dexing porphyry Cu–Mo–Au deposit in east China: product of melting of juvenile lower crust in an intracontinental setting

The Dexing porphyry Cu–Mo–Au deposit in east China (1,168 Mt at 0.45 % Cu) is located in the interior of the South China Craton (SCC), made up of two lithospheric blocks, the Yangtze and Cathaysia blocks. The Cu–Mo–Au mineralization is associated with mid-Jurassic granodioritic porphyries with three high-level intrusive centers, controlled by a series of lineaments at the southeastern edge of the Yangtze block. Available age data define a short duration (172–170 Ma) of the felsic magmatism and the mineralization (171?±?1 Ma). The deposit shows broad similarities with deposits in volcanoplutonic arcs, although it was formed in an intracontinental setting. Porphyries associated with mineralization are mainly granodiorites, which contain abundant phenocrysts (40–60 %) and carry contemporaneous microgranular mafic enclaves (MMEs). They are mainly high-K calc-alkaline and show geochemical affinities with adakite, characterized by relatively high MgO, Cr, Ni, Th, and Th/Ce ratios. The least-altered porphyries yielded relatively uniform ε _Nd(t) values from ?0.9 to +0.6, and wide (⁸⁷Sr/⁸⁶Sr)_i range between 0.7046 and 0.7058 partially overlapping with the Sr–Nd isotopic compositions of the MMEs and mid-Jurassic mafic rocks in the SCC. Zircons from the porphyries have positive ε _Hf(t) values (3.4 to 6.9), and low δ¹⁸O values (4.7 to 6.3?‰), generally close to those of depleted mantle. All data suggest an origin by partial melting of a thickened juvenile lower crust involving mantle components (e.g., Neoproterozoic mafic arc magmas), triggered by invasion of contemporaneous mafic melts at Dexing. The MMEs show textural, mineralogical, and chemical evidence for an origin as xenoliths formed by injection of mafic melts into the felsic magmas. These MMEs usually contain magmatic chalcopyrite, and have original, variable contents of Cu (up to 500 ppm). Their geochemical characteristics suggest that they were derived from an enriched mantle source, metasomatized by Proterozoic slab-derived fluids, and supplied a part of Cu, Au, and S for the Dexing porphyry system during their injection into the felsic magmas. The 171?±?1 Ma magmatic-hydrothermal event at Dexing is contemporaneous with the mid-Jurassic extension in the SCC, followed by 160–90 Ma arc-like magmatism in southeastern China. With respect to the tectono-magmatic evolution of the SCC, the emplacement of Cu-bearing porphyries and the associated Cu mineralization occurred in response to the transformation from a tensional regime, related to mid-Jurassic extension, to a transpressional regime, related to the subduction of the Paleo-Pacific oceanic lithosphere.     

Petrogenesis of Cretaceous igneous rocks from the Duolong porphyry Cu–Au deposit,central Tibet: evidence from zircon U–Pb geochronology,petrochemistry and Sr–Nd–Pb–Hf isotope characteristics

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, ε_Nd(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 ε_Hf(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 ε_Nd(t) values (−1.7 to −4.0), similar Pb and enriched zircon Hf isotopic compositions [ε_Hf(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.     

Genesis of the giant Zijinshan epithermal Cu-Au and Luoboling porphyry Cu–Mo deposits in the Zijinshan ore district,Fujian Province,SE China: A multi-isotope and trace element investigation

The Zijinshan ore district occurs as one of the largest porphyry-epithermal Cu–Au–Mo ore systems in South China, including the giant Zijinshan epithermal Cu–Au deposit and the large Luoboling porphyry Cu–Mo deposit. The mineralization is intimately related to Late Mesozoic large-scale tectono-magmatic and hydrothermal events. The Cu–Au–Mo mineralization occurs around intermediate-felsic volcanic rocks and hypabyssal porphyry intrusions. In this study, we summarize previously available Re–Os isotopes, zircon U–Pb age and trace elements, and Sr–Nd–Pb isotope data, and present new Pb–S and Re–Os isotope data and zircon trace elements data for ore-related granitoids from the Zijinshan high-sulfidation epithermal Cu–Au deposit and the Luoboling porphyry Cu–Mo deposit, in an attempt to explore the relationship between the two ore systems for a better understanding of their geneses. The ore-bearing porphyritic dacite from the Zijinshan deposit shows a zircon U-Pb age of 108–106 Ma and has higher zircon Ce⁴⁺/Ce³⁺ ratios (92–1568, average 609) but lower Ti-in-zircon temperatures (588–753 °C, average 666 °C) when compared with the barren intrusions in the Zijinshan ore district. Relative to the Zijinshan porphyritic dacite, the ore-bearing granodiorite porphyry from the Luoboling deposit show a slightly younger zircon U–Pb age of 103 Ma, but has similar or even higher zircon Ce⁴⁺/Ce³⁺ ratios (213–2621, average 786) and similar Ti-in-zircon temperatures (595–752 °C, average 675 °C). These data suggest that the ore-bearing magmatic rocks crystallized from relatively oxidized and hydrous magmas. Combined with the high rhenium contents (78.6–451 ppm) of molybdenites, the Pb and S isotopic compositions of magmatic feldspars and sulfides suggest that the porphyry and ore-forming materials in the Luoboling Cu–Mo deposit mainly originated from an enriched mantle source. In contrast, the ore-bearing porphyritic dacite in the Zijinshan Cu–Au deposit might be derived from crustal materials mixing with the Cathaysia enriched mantle. The fact that the Zijinshan Cu–Au deposit and the Luoboling Cu–Mo deposit show different origin of ore-forming materials and slightly different metallogenic timing indicates that these two deposits may have been formed from two separate magmatic-hydrothermal systems. Crustal materials might provide the dominant Cu and Au in the Zijinshan epithermal deposit. Cu and Au show vertical zoning and different fertility because the gold transports at low oxygen fugacity and precipitates during the decreasing of temperature, pressure and changing of pH conditions. It is suggested that there is a large Cu–Mo potential for the deeper part of the Zijinshan epithermal Cu–Au deposit, where further deep drilling and exploration are encouraged.     

Geochemistry, Geochronology and Lu-Hf Isotopes of Peraluminous Granitic Porphyry from the Walegen Au Deposit, West Qinling Terrane

Walegen Au deposit is closely correlated with granitic intrusions of Triassic age, which are composed of granite and quartz porphyries. Both granite porphyry and quartz porphyry consist of quartz, feldspar and muscovite as primary minerals. Weakly peraluminous granite porphyry(A/CNK=1.10–1.15) is enriched in LREE, depleted in HREE with Nb-Ta-Ti anomalies, and displays subduction-related geochemistry. Quartz porphyry is strongly peraluminous(A/CNK=1.64–2.81) with highly evolved components, characterized by lower TiO_2, REE contents, Mg~#, K/Rb, Nb/Ta, Zr/Hf ratios and higher Rb/Sr ratios than the granite porphyry. REE patterns of quartz porphyry exhibit lanthanide tetrad effect, resulting from mineral fractionation or participation of fluids with enriched F and Cl. LAICP-MS zircon U-Pb dating indicates quartz porphyry formed at 233±3 Ma. The ages of relict zircons from Triassic magmatic rocks match well with the detrital zircons from regional area. In addition, ε_(Hf)(t) values of Triassic magmatic zircons from the granite and quartz porphyries are -14.2 to -9.1(with an exception of +4.1) and -10.8 to -8.6 respectively, indicating a crustal-dominant source. Regionally, numerous Middle Triassic granitoids were previously reported to be formed under the consumption of Paleotethyan Ocean. These facts indicate that the granitic porphyries from Walegen Au deposit may have been formed in the processes of the closing of Paleotethyan Ocean, which could correlate with the arc-related magmatism in the Kunlun orogen to the west and the Qinling orogen to the east.     

Origin of the Yinshan epithermal-porphyry Cu–Au–Pb–Zn–Ag deposit,southeastern China: insights from geochemistry,Sr–Nd and zircon U–Pb–Hf–O isotopes

The Yinshan deposit is a large epithermal-porphyry polymetallic deposit, and the timing and petrogenesis of ore-hosting porphyries have been hotly debated. We present new results from geochemical, whole-rock Sr–Nd and zircon U–Pb–Hf–O isotopic investigations. Zircon U–Pb data demonstrate that the quartz porphyry, dacitic porphyry, and quartz dioritic porphyry formed at ?172.2 ± 0.4 Ma, ?171.7 ± 0.5 Ma, and ?170.9 ± 0.3 Ma, respectively. Inherited zircon cores show significant age spreads from ?730 to ?1390 Ma. Geochemically, they are high-K calc-alkaline or shoshonitic rocks with arc-like trace element patterns. They have similar whole-rock Nd and zircon Hf isotopic compositions, yet an increasing trend in ?_Nd(t) and ?_Hf(t) values typifies the suite. Older (inherited) zircons of the three porphyries display Hf compositions comparable to those of the Jiangnan Orogen basement rocks. In situ zircon oxygen isotopic analyses reveal that they have similar oxygen isotopic compositions, which are close to those of mantle zircons. Moreover, a decreasing trend of δ¹⁸O values is present. We propose that the ore-related porphyries of the Yinshan deposit were emplaced contemporaneously and derived from partial melting of Neoproterozoic arc-derived mafic (or ultra-mafic) rocks. Modelling suggests that the quartz porphyries, dacitic porphyries, and quartz dioritic porphyries experienced ?25%, ?10%, and ?10% crustal contaminations by Shuangqiaoshan rocks. Our study provides important constraints on mantle–crust interaction in the genesis of polymetallic mineralization associated with Mesozoic magmatism in southeastern China.     

Ages and petrogenesis of the late Triassic andesitic rocks at the Luerma porphyry Cu deposit,western Gangdese,and implications for regional metallogeny

The Tibetan Plateau is one of the most significant Cu poly-metallic mineralization regions in the world and preserves important information related to subductional and collisional porphyry Cu mineralization. This study investigates a new occurrence of Cu mineralization-related andesitic porphyries in the western domain of the Gangdese magmatic belt and assesses its petrologic, zircon U-Pb geochronology, whole-rock chemistry, and Sr-Nd-Hf-Pb isotope data. Zircon U-Pb dating of three ore-related porphyries yields crystallization ages of 212–211 Ma. These ages are consistent with previous molybdenite Re-Os dating, indicating a late Triassic magmatic and Cu mineralization event in the western Gangdese magmatic belt. Nb, Ta, and Ti depletion, Th and LREE enrichment, and high La/Yb and Th/Yb ratios in addition to high U/Yb ratios from zircons suggest that the magma was generated in an active continental arc setting. The porphyries have radiogenic isotopic compositions with (⁸⁷Sr/⁸⁶Sr)_i 0.70431–0.70473, ε_Nd(t) +1.1 to +3.8, (²⁰⁷Pb^/204Pb)_i 15.601–15.622, and (²⁰⁸Pb/²⁰⁴Pb)_i 38.450–38.693, as well as high positive zircon ε_Hf(t) values from +6.2 to +10.6 (mean value 8.3), corresponding to model ages (T_DM) ranging from 509 Ma to 819 Ma (mean 646 Ma). This suggests that the andesitic magmatism was dominantly sourced from depleted mantle materials that were modified by subducted oceanic sediment-derived melts during the subduction of the Neo-Tethys Ocean. The mineralization-related porphyries contain amphibole and epidote, as well as high whole-rock Fe₂O₃/FeO and zircon Ce⁴⁺/Ce³⁺ ratios, suggesting hydrous and highly oxidized parent magmas. Considering the existing Cu mineralization and highly oxidized magma of the well-preserved Triassic andesitic igneous rocks in the western Gangdese belt, the subduction-related continental arc magma system is favorable for subduction-related porphyry Cu deposits. The existence of Luerma porphyry mineralization demonstrates that there are at least five generations of porphyry Cu-(Mo-Au) mineralization in the Gangdese magmatic belt, which advances the timeframe of porphyry mineralization to the late Triassic.     

Early Cretaceous adakitic rocks in the Anqing region,southeastern China: constraints on petrogenesis and metallogenic significance

ABSTRACT

The Anqing region in Lower Yangtze River metallogenic belt is one of the important Cu polymetal producers in China. The origin of Cu polymetallic deposits in the region is closely related to Early Cretaceous adakitic intrusions. To constrain the petrogenetic and metallogenic significance of the adakitic rocks, a detailed geochronological, geochemical, and Sr–Nd–Pb–Hf isotopic study was performed. The Anqing adakitic rocks (SiO₂ = 57.4–64.2 wt.%) consist mainly of quartz monzodiorite, formed at 138.2 ± 1.7 Ma (Mean Standard Weighted Deviation (MSWD) = 0.61). They have high MgO, Al₂O₃, Sr, and low Rb, Y, Yb contents, together with high Sr/Y (50.5–222) and La/Yb (31.9–46.9) ratios. They also show negative whole-rock ε_Nd(t) (?9.8 to ?8.5) and zircon ε_Hf(t) (?10.0 to ?5.4), and high oxygen fugacity (mainly ?17.0 to ?8.01) values and radiogenic Pb isotopic compositions with (²⁰⁶Pb/²⁰⁴Pb)_i = 17.692–17.884, (²⁰⁷Pb/²⁰⁴Pb)_i = 15.413–15.511, and (²⁰⁸Pb/²⁰⁴Pb)_i = 37.611–37.943. Coupled with negative Nb–Ta anomalies, low K₂O/Na₂O ratios (0.39–0.62), and high Mg# values (0.44–0.71), these data suggest the adakitic rocks and associated large-scale Cu–Au mineralization of the Anqing region resulted from partial melting of the high oxidized subducted oceanic crust. Addition of mantle-derived magmas and assimilation of crustal materials during emplacement are also possible.     

U–Pb (zircon) and geochemical constraints on the age,origin, and evolution of Paleozoic arc magmas in the Oyu Tolgoi porphyry Cu–Au district,southern Mongolia

Uranium–Pb (zircon) ages are linked with geochemical data for porphyry intrusions associated with giant porphyry Cu–Au systems at Oyu Tolgoi to place those rocks within the petrochemical framework of Devonian and Carboniferous rocks of southern Mongolia. In this part of the Gurvansayhan terrane within the Central Asian Orogenic Belt, the transition from Devonian tholeiitic marine rocks to unconformably overlying Carboniferous calc-alkaline subaerial to shallow marine volcanic rocks reflects volcanic arc thickening and maturation. Radiogenic Nd and Pb isotopic compositions (ε_Nd(t) range from + 3.1 to + 7.5 and ²⁰⁶Pb/²⁰⁴Pb values for feldspars range from 17.97 to 18.72), as well as low high-field strength element (HFSE) contents of most rocks (mafic rocks typically have < 1.5% TiO₂) are consistent with magma derivation from depleted mantle in an intra-oceanic volcanic arc. The Late Devonian and Carboniferous felsic rocks are dominantly medium- to high-K calc-alkaline and characterized by a decrease in Sr/Y ratios through time, with the Carboniferous rocks being more felsic than those of Devonian age. Porphyry Cu–Au related intrusions were emplaced in the Late Devonian during the transition from tholeiitic to calc-alkaline arc magmatism. Uranium–Pb (zircon) geochronology indicates that the Late Devonian pre- to syn-mineral quartz monzodiorite intrusions associated with the porphyry Cu–Au deposits are ~ 372 Ma, whereas granodiorite intrusions that post-date major shortening and are associated with less well-developed porphyry Cu–Au mineralization are ~ 366 Ma. Trace element geochemistry of zircons in the Late Devonian intrusions associated with the porphyry Cu–Au systems contain distinct Th/U and Yb/Gd ratios, as well as Hf and Y concentrations that reflect mixing of magma of distinct compositions. These characteristics are missing in the unmineralized Carboniferous intrusions. High Sr/Y and evidence for magma mixing in syn- to late-mineral intrusions distinguish the Late Devonian rocks associated with giant Cu–Au deposits from younger magmatic suites in the district.     

Late Cretaceous tectono-magmatic activity in the Nize region,central Tibet: evidence for lithospheric delamination beneath the Qiangtang–Lhasa collision zone

ABSTRACT

The results of zircon U–Pb age dating and whole-rock geochemistry for the Late Cretaceous Nize granodiorite porphyries, combined with analysis of near-coeval structural deformation of the Lower Cretaceous Langshan Formation, provide new data to better understand the tectonic evolution of the northern Lhasa subterrane, central Tibet. Zircon U–Pb ages of 89.2 ± 0.3 Ma to 87.8 ± 0.3 Ma indicate emplacement during the Late Cretaceous. Granodiorite porphyry intrusions were contemporaneous with the development of a regional angular unconformity, overlain by the Upper Cretaceous Jingzhushan (or Abushan) Formation, within the collision zone between the South Qiangtang and Lhasa terranes. Geochemical data for Nize granodiorite porphyries indicate that they have a calc-alkaline composition enriched in large-ion lithophile elements and light rare earth elements and depleted in high-field-strength elements and heavy rare earth elements. High Al₂O₃ and Sr contents, low Yb and Y contents, and high Sr/Y ratios are similar to adakitic magmas.

Structural analysis indicates two stages of deformation (D₁ and D₂), with D₁ forming the focus of the present study. The D₁ deformation is represented by large-scale faults and records two periods of faulting. These periods are recognized as early compressional thrust faulting and a dominant late stage characterized by normal faulting and extension, with the latter stages of D₁ being near-coeval with the emplacement of the Nize granodiorite porphyries. The combination of zircon ages, geochemical data, and structural analysis indicates that the Nize granodiorite porphyries formed after collision of the South Qiangtang and Lhasa terranes. Adakitic magma derived from partial melting of the thickened lower or middle crust resulted from lithospheric delamination that may have been promoted by the convective removal of deeper lithospheric mantle.     

High oxidation magmatic evolution in the Naruo porphyry Cu deposit,Tibet, China

The Naruo porphyry Cu deposit is the third largest deposit discovered in the Duolong metallogenic district. Previous research has focused mainly on the geochemistry of the ore-bearing granodiorite porphyry; the metallogenesis remains poorly understood. In the present work, on the basis of outcrops and drilling core geological mapping, phases of early mineralization diorite, two inter-mineralization granodiorite porphyries, and late-mineralization granodiorite porphyry have been distinguished. Furthermore, the alteration zones were outlined, and the vein sequence was identified. The diorite and three porphyry phases were subjected to Laser Ablation Inductively Coupled Plasma Mass Spectrometry (La–ICP–MS) zircon U–Pb dating and in situ Hf isotope analyses as well as bulk major element, trace element, and Sr–Nd isotopic analyses. Molybdenite Re–Os dating was also conducted.The zircon U–Pb dating results show that the diorite and porphyry intrusions were emplaced at about 120 Ma, and the molybdenite Re–Os isochron age is 118.8 ± 1.9 Ma; this indicates that the Naruo porphyry Cu deposit was formed during a continuous magmatic–hydrothermal process. All of the diorite and granodiorite porphyry samples showed arc magmatic characteristics. Moreover, the moderate (⁸⁷Sr/⁸⁶Sr)_i ratios and low ε_Nd(t) and ε_Hf(t) values of the diorite and porphyry intrusions suggest the source region of the juvenile lower crust. The lower (⁸⁷Sr/⁸⁶Sr)_i and (¹⁴³Nd/¹⁴⁴Nd)_i ratios and higher ε_Nd(t) values and incompatible element concentrations than those in the granodiorite porphyry samples indicate a two-stage magmatic generation process for the intrusions. The early mineralization diorite has a high Cu concentration, implying that the source is enriched in Cu. However, the slightly lower Cu content of the late-mineralization granodiorite porphyry samples might imply Cu release from magmas and deposition within the metallogenic stage. The multiple stages of intrusions and subsequent volcanism within the Duolong metallogenic district, together with high Sr/Y features, indicate persistent magmatism during the metallogenic epoch, which is necessary for maintaining the activity of magmatic–hydrothermal and mineralization processes. Thus, the high Cu content in the source region, mantle-derived melt upwelling, and multiple stages of persistent magmatism were favorable for the formation of the Naruo porphyry Cu deposit.The high Fe₂O₃/FeO ratios of the diorite and granodiorite porphyry intrusions show very high oxidation features, which is coincident with estimated magmatic oxidation state calculated by the zircon trace element compositions. The high oxidation facilitates sulfur and chalcophile metals to be scavenged into the magmatic–hydrothermal systems, which is crucial for the metallogenesis of the Naruo porphyry Cu deposit.     

Recycling of palaeo-Pacific subducted oceanic crust related to a Fe–Cu–Au mineralization in the Xu-Huai region of North Anhui-Jiangsu,East China: Geochronological and geochemical constraints

ABSTRACT

In this study, Early Cretaceous skarn deposits and genesis of their host diorite/monzodiorite porphyry in the Xuzhou-Huaibei (Xu-Huai) region, northern Anhui-Jiangsu have been discussed by detailed geochemical work. In-situ zircon U–Pb dating of the diorites related to Fe–Cu–Au deposits shows that they were formed between 131.4 ± 1.5 Ma and 130.8 ± 1.8 Ma. Geochemical data indicate a depletion of high field strength elements (HFSE) in the diorite porphyry with similarity to that of arc-related igneous rocks. The diorite porphyry was probably derived from typical arc magmas related to continental margin subduction characterized by light rare earth elements (LREEs) enrichment and HFSE depletion. REEs compositions of apatite in the diorite porphyry indicate that the dioritic magma was produced from the metasomatized subcontinental mantle by slab-derived fluids. The magma was proven to be a high oxygen fugacity; thus, it was particularly conducive to the precipitation of Fe, Cu, Au and other ore-forming elements. The δ³⁴S values of pyrite and chalcopyrite of Fe–Cu–Au ores range from ?0.2‰ to 2.8‰, indicating that the sulphur in the ore was probably derived from deep-seated magmas. Integrated with geochronological and geochemical analyses, we suggest that the Early Cretaceous igneous suites associated with Fe–Cu–Au deposits in the Xu-Huai region are related to recycling subduction of Pacific oceanic crust.     

Deciphering petrogenic processes using Pb isotope ratios from time-series samples at Bezymianny and Klyuchevskoy volcanoes,Central Kamchatka Depression

The Klyuchevskoy group of volcanoes in the Kamchatka arc erupts compositionally diverse magmas (high-Mg basalts to dacites) over small spatial scales. New high-precision Pb isotope data from modern juvenile (1956–present) erupted products and hosted enclaves and xenoliths from Bezymianny volcano reveal that Bezymianny and Klyuchevskoy volcanoes, separated by only 9 km, undergo varying degrees of crustal processing through independent crustal columns. Lead isotope compositions of Klyuchevskoy basalts–basaltic andesites are more radiogenic than Bezymianny andesites (²⁰⁸Pb/²⁰⁴Pb = 37.850–37.903, ²⁰⁷Pb/²⁰⁴Pb = 15.468–15.480, and ²⁰⁶Pb/²⁰⁴Pb = 18.249–18.278 at Bezymianny; ²⁰⁸Pb/²⁰⁴Pb = 37.907–37.949, ²⁰⁷Pb/²⁰⁴Pb = 15.478–15.487, and ²⁰⁶Pb/²⁰⁴Pb = 18.289–18.305 at Klyuchevskoy). A mid-crustal xenolith with a crystallization pressure of 5.2 ± 0.6 kbars inferred from two-pyroxene geobarometry and basaltic andesite enclaves from Bezymianny record less radiogenic Pb isotope compositions than their host magmas. Hence, assimilation of such lithologies in the middle or lower crust can explain the Pb isotope data in Bezymianny andesites, although a component of magma mixing with less radiogenic mafic recharge magmas and possible mantle heterogeneity cannot be excluded. Lead isotope compositions for the Klyuchevskoy Group are less radiogenic than other arc segments (Karymsky—Eastern Volcanic Zone; Shiveluch—Northern Central Kamchatka Depression), which indicate increased lower-crustal assimilation beneath the Klyuchevskoy Group. Decadal timescale Pb isotope variations at Klyuchevskoy demonstrate rapid changes in the magnitude of assimilation at a volcanic center. Lead isotope data coupled with trace element data reflect the influence of crustal processes on magma compositions even in thin mafic volcanic arcs.     

Characteristics of Rare Earth Elements,Zr, and Hf in Ore‐Bearing Porphyries from the Western Awulale Metallogenic Belt,Northwestern China and their Application in Determining Metal Fertility of Granitic Magma

The western Awulale metallogenic belt in northwestern China hosts a number of small‐ to medium‐sized porphyry Cu deposits that are associated with albite porphyry. The common presence of plagioclase (albite) as phenocrysts and the absence of hydrous minerals (amphibole and biotite) indicate that the water content of albite porphyry is low. Trace‐element compositions of whole rocks and zircon grains from these ore‐bearing porphyries were measured. Zircon grains from albite porphyries exhibit Ce⁴⁺/Ce³⁺ ratios ranging from 7.75 to 95.1, which indicate that these porphyries have a low oxygen fugacity. Trace element compositions of ore‐bearing porphyries exhibit (La/Yb)_N ratios ranging from 1.09 to 11.1 and Eu/Eu^* ratios ranging from 0.10 to 0.66. These ore‐bearing porphyries have Zr values ranging from 171 to 707 ppm and Hf values ranging from 8.30 to 18.9 ppm. Combining these porphyries with other ore‐bearing porphyries that formed in the Central Asian Orogenic Belt (CAOB) and the Pacific Rim metallogenic belt, we found that the (La/Yb)_N and Eu/Eu^* ratios of ore‐bearing porphyries in western Awulale are low, while the Zr and Hf values are high. Specifically, REEs can be used to evaluate the degree of differentiation and degree of partial melting, and Zr and Hf can be used to evaluate the redox conditions and water content of magmatic rocks. Our findings indicate that ore‐bearing porphyries in western Awulale have a lower oxygen fugacity, degree of differentiation, and water content than do others in the CAOB and the Pacific Rim metallogenic belt. Compared to those of ore‐bearing porphyries with lower zircon Ce⁴⁺/Ce³⁺ ratios, the (La/Yb)_N ratios of ore‐bearing porphyries in our study are low, and the Zr and Hf values are high. This finding indicates that, under reducing conditions, the degree of evolution and water content may have an important influence on the metal abundance in magmas. There is also a clear relationship between (La/Yb)_N, Eu/Eu^*, Zr, Hf, and the size of the deposits. Large‐ (>4 Mt Cu) and intermediate (1.5–4 Mt Cu)‐sized porphyry Cu deposits are associated with granitic intrusions that have higher (La/Yb)_N and Eu/Eu^* ratios and lower Zr and Hf values. This finding indicates that, in addition to oxygen fugacity, the degree of evolution and water content are controlling parameters for metal abundance in magmas, especially in low oxygen fugacity porphyry Cu deposits. Such a conclusion may be useful in the exploration for other concealed porphyry Cu deposits.     

Geochemical differences between subduction- and collision-related copper-bearing porphyries and implications for metallogenesis

Porphyry Cu (–Mo–Au) deposits occur not only in continental margin–arc settings (subduction-related porphyry Cu deposits, such as those along the eastern Pacific Rim (EPRIM)), but also in continent–continent collisional orogenic belts (collision-related porphyry Cu deposits, such as those in southern Tibet). These Cu-mineralized porphyries, which develop in contrasting tectonic settings, are characterized by some different trace element (e.g., Th, and Y) concentrations and their ratios (e.g., Sr/Y, and La/Yb), suggesting that their source magmas probably developed by different processes. Subduction-related porphyry Cu mineralization on the EPRIM is associated with intermediate to felsic calc-alkaline magmas derived from primitive basaltic magmas that pooled beneath the lower crust and underwent melting, assimilation, storage, and homogenization (MASH), whereas K-enriched collision-related porphyry Cu mineralization was associated with underplating of subduction-modified basaltic materials beneath the lower crust (with subsequent transformation into amphibolites and eclogite amphibolites), and resulted from partial melting of the newly formed thickened lower crust. These different processes led to the collision-related porphyry Cu deposits associated with adakitic magmas enriched by the addition of melts, and the subduction-related porphyry Cu deposits associated with magmas comprising all compositions between normal arc rocks and adakitic rocks, all of which were associated with fluid-dominated enrichment process.In subduction-related Cu porphyry magmas, the oxidation state (fO₂), the concentrations of chalcophile metals, and other volatiles (e.g., S and Cl), and the abundance of water were directly controlled by the composition of the primary arc basaltic magma. In contrast, the high Cu concentrations and fO₂ values of collision-related Cu porphyry magmas were indirectly derived from subduction modified magmas, and the large amount of water and other volatiles in these magmas were controlled in part by partial melting of amphibolite derived from arc basalts that were underplated beneath the lower crust, and in part by the contribution from the rising potassic and ultrapotassic magmas. Both subduction- and collision-related porphyries are enriched in potassium, and were associated with crustal thickening. Their high K₂O contents were primarily as a result of the inheritance of enriched mantle components and/or mixing with contemporaneous ultrapotassic magmas.     

Ages and Sources of Ore‐Related Porphyries at Yongping Cu–Mo Deposit in Jiangxi Province,Southeast China

Whole‐rock geochemistry, zircon U–Pb and molybdenite Re–Os geochronology, and Sr–Nd–Hf isotopes analyses were performed on ore‐related dacite porphyry and quartz porphyry at the Yongping Cu–Mo deposit in Southeast China. The geochemical results show that these porphyry stocks have similar REE patterns, and primitive mantle‐normalized spectra show LILE‐enrichment (Ba, Rb, K) and HFSE (Th, Nb, Ta, Ti) depletion. The zircon SHRIMP U–Pb geochronologic results show that the ore‐related porphyries were emplaced at 162–156 Ma. Hydrothermal muscovite of the quartz porphyry yields a plateau age of 162.1 ± 1.4 Ma (2σ). Two hydrothermal biotite samples of the dacite porphyry show plateau ages of 164 ± 1.3 and 163.8 ± 1.3 Ma. Two molybdenite samples from quartz+molybdenite veins contained in the quartz porphyry yield Re–Os ages of 156.7 ± 2.8 Ma and 155.7 ± 3.6 Ma. The ages of molybdenite coeval to zircon and biotite and muscovite ages of the porphyries within the errors suggest that the Mo mineralization was genetically related to the magmatic emplacement. The whole rocks Nd–Sr isotopic data obtained from both the dacite and quartz porphyries suggest partial melting of the Meso‐Proterozoic crust in contribution to the magma process. The zircon Hf isotopic data also indicate the crustal component is the dominated during the magma generation.     

Genesis of Luobuzhen Pb–Zn veins: Implications for porphyry Cu systems and exploration targeting at Luobuzhen-Dongshibu in western Gangdese belt,southern Tibet

Porphyry systems are known to form in magmatic arc environment and commonly include porphyry Cu, epithermal Pb–Zn–Au–Ag, skarn polymetallic mineralization, etc. The systems are rarely reported in collisional zones, such as the Gangdese belt in southern Tibet where many postcollisional porphyry copper deposits occurred. In addition, other types of mineral systems are rarely present except porphyry copper mineralization in the Gangdese belt. In this study, we present Pb–Zn-bearing quartz veins at Luobuzhen in the western Gangdese belt. The Luobuzhen Pb–Zn veins cross-cut dacite of the Linzizong Group with zircon U–Pb age of 50.1 ± 0.2 Ma and monzogranite with zircon U–Pb age of 17.1 ± 0.1 Ma. Ore minerals include sphalerite, galena, chalcopyrite, and pyrite; gangue minerals are quartz with minor chlorite and sericite. Primary fluid inclusions of quartz are liquid-rich, aqueous, and two-phase inclusions. The homogenization temperatures of these primary inclusions are moderate to high (267–400 °C), and salinities range from 8.9 to 18.4 wt.% NaCl equiv. Quartz has δ¹⁸O_SMOW values of 6.2–9.3‰, while sulfides have δ³⁴S_V-CDT values of −5.1‰ to 0.1‰, ²⁰⁶Pb/²⁰⁴Pb of 18.722–18.849, ²⁰⁷Pb/²⁰⁴Pb of 15.640–15.785, and ²⁰⁸Pb/²⁰⁴Pb of 39.068–39.560. These data suggest that magmatic fluids with contribution from meteoric water, magmatic sulfur, and lead derived from upper crust and metasomatized mantle by Indian continental materials would be critical for the Luobuzhen base metal mineralization.The Dongshibu area, located at ∼2 km east of the Luobuzhen, is characterized by high concentrations of Cu (up to 1450 ppm) and Mo (up to 130 ppm) of stream sediments, which is quite different from high concentrations in Pb, Zn, Ag, and Au shown in the Luobuzhen area. In addition, porphyry copper mineralization-related alteration and veins/veinlets occur in the Miocene monzogranite at Dongshibu. The monzogranite is characterized by high Sr/Y ratios, which are also shown on ore-forming intrusions in the Gangdese postcollisional porphyry copper deposits, and shows similar zircon Hf isotopes to the ore-related high Sr/Y intrusions from the Zhunuo porphyry copper deposit which is located ∼20 km northeast of the Luobuzhen-Dongshibu. A comprehensive analysis allows us to infer that the base metal veins at Luobuzhen are components of a porphyry Cu system with porphyry Cu mineralization likely present at Dongshibu and epithermal Au–Ag veins possibly occurring at Luobuzhen, which are indicative of the existence of porphyry copper systems in collisional zones. The potential porphyry Cu mineralization and epithermal Au–Ag veins should be targeted in future exploration at Luobuzhen-Dongshibu.