Formation Of Porphyry Deposits Research Articles

Mechanisms of fluid degassing in shallow magma chambers control the formation of porphyry deposits

Abstract Magmatic fluid degassing within shallow magma chambers underneath the ore bodies is critical to the formation of porphyry Cu-Au deposits (PCDs). Yet, it remains unclear that how the ways of fluid degassing influence on the development of PCDs. Here, geochemical data of apatite, amphibole and plagioclase from ore-forming and coeval barren porphyries have been analyzed in Sanjiang metallogenic belt, China. The ore-forming porphyries normally exhibit high and wide XF/XCl (31.76-548.12) and XF/XOH (0.779-7.370) ratios of apatites, which are evidently higher than those of the barren porphyries (XF/XCl of 1.03-26.58; XF/XOH of 0.686-3.602). Combined with the continuous variation features of Cl/OH ratios and H2O contents of melts calculated by amphiboles, as well as fluid migration models, we constrained the mechanisms of fluid degassing within shallow magma chambers underneath PCDs. There are three different ways of fluid degassing, while only fluid degassing via fluid channel stage can migrate and focus the metal-rich fluids effectively, conducive to the development of PCDs. The mechanisms of magmatic fluid degassing processes are further controlled by the storage depths of magma chambers and initial H2O contents of the magmas revealed by the compositions of amphibole, plagioclase and thermodynamic modelling. Magmas, with shallower storage depth and higher initial H2O content, are more likely to experience extensive and focused fluid degassing, leading to the generation of PCDs. This study demonstrates the potential utility of integrated mineral analyses, the thermodynamic modelling for investigating the mechanisms of magmatic fluid degassing in porphyry systems, as well as identifying prospective buried PCDs.

Petrogenesis of Variscan and Yanshanian magmatic rocks in the Longjiang region of Heilongjiang province and implications for ore exploration

The Longjiang region is located on the eastern slope of the southern section of the Great Xing'an Range, in the transition zone between the Songliao Basin and Great Xing'an Range uplift. As it is an extension of the Inner Mongolia Linxi-Tuquan Cu-Mo-Pb-Zn metallogenic belt in the Heilongjiang region, it has conditions conducive to the formation of porphyry deposits. We report zircon U-Pb dating and whole-rock geochemistry data of magmatic rocks in representative mineralized points in the Longjiang region. Zircon U-Pb age shows that the age of magmatic rocks is concentrated in the Variscan period (312 ± 5 ∼ 294 ± 2Ma) and Yanshanian period (134 ± 2 ∼ 123 ± 1Ma). According to whole-rock geochemical analysis, the granites have SiO2 contents of 61.2–69.16 % (66.9 % on average), Al2O3 contents of 13.87–15.98 %, MgO contents of 0.6–2.22 %, TiO2 contents of 0.25–0.88 %, K2O contents of 2.70–3.63 %, Na2O/K2O ratios of 1.05–2.05, and high K2O+Na2O contents (5.8–7.17 %), which are consistent with adakitic rocks. Yanshanian granites have A/CNK values of 1.02–2 and an average differentiation index of 84.7, which indicates that they are highly differentiated I-type granites. Variscan I-type granites represent the post-collisional setting of the Paleo-Asian Ocean and are relatively scarce in the region. Yanshanian highly differentiated I-type granites are products of the Paleo-Pacific Ocean's post-collisional retreat and are widely distributed in the region. As the materials and fluids that formed the Liujiushan, Haiyang, and Fendou deposits were derived from magmatic activities, and Variscan and Yanshanian magmas have excellent ore-forming potential. The highly differentiated I-type granites formed by intense Yanshanian magmatism are the most promising candidates for ore exploration in this region.

Sulfide resorption contributes to porphyry deposit formation in collisional settings

Production of Cu-rich melts via melting of sulfide-bearing lower crustal cumulates is thought to contribute to porphyry Cu formation in the post-collisional Gangdese belt (Tibet). We present new whole rock, including platinum group elements (PGE) data, for ore-causative granites (OCG), ore-related granites (ORG) and barren granites (BG) from the Gangdese belt. The OCG, ORG and BG show indistinguishable Cu, PGE and Cu/Ag behavior during differentiation down to ∼2 wt% MgO. These systematics, together with the occurrence of sulfides, demonstrate that the propensity for some Gangdese magmas to be ore-associated is unrelated to melting of sulfide-bearing cumulates or the relative timing of sulfide fractionation. The behavior of chalcophile and siderophile elements during differentiation of the OCG, ORG and BG diverges when melts evolve past 2–3 wt% MgO and exsolve H2O, S and Cl: the BG show a decrease in Cu, whereas the OCG and ORG show highly variable Cu, Au, and PGE trends. Because S degassing causes melts to become sulfur undersaturated and because Cu is highly soluble in Cl-rich fluids, any sulfides in contact with the melt or magmatic fluids following volatile exsolution would dissolve. Continual injection, cooling and ascent of magmas through the Gangdese crust is expected to have resulted in the development of regions of crystal-rich partially molten crust beneath the intrusive granites. We suggest that prior to porphyry mineralization, extension and erosion caused exhumation of these long-lived sulfide-bearing mush zones. This exhumation may have caused a change in magmatism style from volatile exsolution after the melts transited sulfide-bearing mushes (i.e., the BG), to volatile exsolution during magma transit through sulfide-bearing mushes (i.e., the OCG). We suggest that only the magmas and that exsolved fluids during their ascent through sulfide-bearing mushes were able to assimilate sufficient Cu-rich sulfides to fuel the formation of porphyry Cu deposits in the region.

The origin of mafic–ultramafic rocks and felsic plutons along the Clarke River suture zone: implications for porphyry exploration in the northern Tasmanides

The Clarke River Fault in northeast Queensland records an early Paleozoic history of subduction, accretion and continental suturing. Samples of mafic–ultramafic rocks collected proximal to the Clarke River Fault record oceanic geochemical affinities and comprise alteration assemblages consistent with an ophiolitic origin. The ca 456 Ma Falls Creek Tonalite records a continental-arc geochemical signature and was formed in response to long-lived subduction beneath the Thomson Orogen. Ordovician subduction beneath the Thomson Orogen is broadly coeval with arc magmatism documented in the Lachlan Orogen, which has been associated with the formation of several large porphyry ore deposits. The Falls Creek Tonalite yields adakite-like geochemical signatures that reflect a fertile melt source conducive to the formation of porphyry ore deposits. The outcropping plutons record ductile deformation consistent with mid-crustal depths, and they were emplaced during late syntectonic activity. This implies that the Falls Creek Tonalite was emplaced at too great a depth to have formed porphyry ore deposits. The northern Charters Towers Province shares many geological similarities to the Greenvale Province, where the erosional level may be shallower, and the potential for porphyry deposit formation and preservation may be greater.

UNDERSTANDING Cu DEFICIENCY AND Mo ENRICHMENT IN THE JURASSIC ZHANGGUANGCAI-LESSER XING’AN CONTINENTAL ARC (NE CHINA): INSIGHTS FROM THE LUMING PORPHYRY MO DEPOSIT

Abstract Continental arcs are favorable sites for porphyry Cu ± Mo deposits. In contrast, the Jurassic Zhangguangcai-Lesser Xing’an continental arc, northeastern China, hosts numerous porphyry Mo deposits with only minor total Cu endowment. The reasons for this remain elusive. Here, we address the issue by tracking the origin and evolution of a long-lived (ca. 12 m.y.) magmatic suite associated with one of the largest Mo deposits in the Zhangguangcai-Lesser Xing’an range continental arc—the Luming porphyry Mo deposit. In combination with previous geochronological data, our results indicate that the pre- (187.5–186.5 Ma) and syn-ore (178.6–175.6 Ma) intrusions at Luming represent separate magma batches from isotopically similar parental magmas with the signature of subduction-metasomatized mantle-derived magmas mixed with crustal melts. The Sr/Y, La/YbN, Dy/YbN, and Eu/Eu* values, together with fO2 and apatite S and Cl concentrations of the syn-ore intrusions, however, are systematically higher than pre-ore intrusions. These data indicate that there is a distinct change in magma chemistry during the period of magmatism quiescence between the pre- and syn-ore magmatism. The marked change in magma chemistry is coincident with an episode of major compression and crustal thickening in the Zhangguangcai-Lesser Xing’an range. We, therefore, interpret that the chemical changes most likely reflect a deepening of the locus of lower crustal magma evolution, linked to increased crustal thickness and/or melt H2O contents accompanying an increase in orogenic stress. Considering the tectonic setting, magmatic evolution in the deep crust, emplacement depth, fractionation degree, and oxygen fugacity of the porphyry Mo-related intrusions at Luming, they are very similar to porphyry Cu deposits worldwide. Accordingly, we propose that the deficiency in Cu and enrichment in Mo of the porphyry deposits in the Zhangguangcai-Lesser Xing’an range most likely formed due to mantle-derived magmas interacting with reduced crustal materials or melts in the deep crust. This resulted in substantial sequestration of Cu and S, decreasing porphyry Cu potential. Such an inference is supported by the distinctly lower S and Cl concentrations of the Mo-related intrusions in the Zhangguangcai-Lesser Xing’an range compared to those associated with porphyry Cu deposits. Our results confirm the fundamental role of increasing compressive stress, crustal thickening, and transcrustal petrologic processes in the formation of porphyry deposits and highlight the role of crustal components in modulating the Mo/Cu ratio of porphyry deposits in a continental arc setting. In addition, we tentatively propose that magmatic apatite SO3 and/or Cl contents can be used to discriminate porphyry Mo-related intrusions from those associated with porphyry Cu deposits in continental arcs.

Ore-forming fluid evolution of a porphyry Cu-Mo deposit coexisting with porphyry Mo systems in a post-collisional setting, Xiaokelehe, NE China

The Xiaokelehe porphyry Cu-Mo deposit in the Great Xing’an Range contains six stages of quartz-sulfide veins (V1 to V6) including potassic (V1 to V4), chlorite-illite (V5) and phyllic (V6). Two to three types of quartz were identified within each stage, of which the V2Q1, V3Q1, V4Q2, V5Q1 and V6Q2 are spatially associated with sulfides. Three types of fluid inclusions were identified in these veins, i.e., type I aqueous inclusions homogenized to liquid, type II aqueous inclusions homogenized to vapor, and type III aqueous inclusions containing halite and homogenized to liquid. Type I and type II primary inclusions occur in all stages, and type III inclusions are developed in V2 to V4 veins. Microthermometric data show that the maximum formation temperatures and pressures of V2-V6 veins of type I inclusions are 353 to 437 °C and 19.0 to 27.6 MPa, 309 to 415 °C and 15.0 to 30.1 MPa, 330 to 365 °C and 16.7 to 27.6 MPa, 243 to 351 °C and 13.9 to 18.9 MPa and 255 to 347 °C and 13.9 to 17.8 MPa, respectively, which show a decreasing trend. Oxygen isotope data show that the δ18OH2O values of V2-V4 veins (5.3 to 10.4 ‰) are consistent with typical magmatic values, whilst the δ18OH2O values of V5-V6 veins (0.2 to 4.6 ‰) are much lower than those of the magmatic water, which is due to involvement of meteoric water. The fluid inclusion microthermometry and O isotope data suggest an evolving magmatic-hydrothermal system with decreasing temperatures, pressures from V2 to V4 veins (potassic stage) to V5 (chlorite-illite stage) and V6 veins (phyllic stage), and an increasing incorporation of meteoric water in V5 and V6. The Xiaokelehe deposit has lower CO2 contents than the adjacent porphyry Mo deposits formed in the same post-collisional setting, which is mainly due to differences of mineralization depths and magma source. The formation of porphyry deposits with different Mo/Cu ratios highlights the diversity and complexity of the mineralizing systems in post-collisional settings, which has important implications for mineral exploration in such environments.

Sulfur dissolution capacity of highly hydrated and fluid-saturated dacitic magmas at the lower crust and implications for porphyry deposit formation

Arc basalts with high H2O content are expected to evolve into highly hydrated felsic melts saturated with aqueous fluids at lower crustal pressures, which are potentially important in providing sulfur (S) and metals for porphyry deposit formation. However, the role of such highly hydrated and fluid-saturated magmas in the transfer of S (and metals) remains rarely studied. To investigate the S dissolution capacity of highly hydrated felsic magmas saturated with fluids at the lower crustal conditions, this study performed experiments with dacite + 10 wt% FeS at H2O contents of 10, 15, and 20 wt%. All the experiments were performed in piston-cylinder apparatus at 1.0 GPa and 950 °C with fO2 conditions imposed by the following solid-state buffers: cobalt-cobalt oxide (CCO), nickel-nickel oxide (NNO), rhenium-rhenium oxide (RRO), MnO-Mn3O4 (MMO), and hematite-magnetite (HM), resulting in a fO2 range of FMQ−1.5 to FMQ+3.3 log units (FMQ = fayalite + magnetite + quartz oxgen buffer). The experimental results show that the H2O content in the felsic melt, in addition to fO2 and melt composition, has a strong effect on the concentration of S in the silicate melt at sulfide ± anhydrite saturation. At a given fO2, the S concentration in the melt increases with increasing H2O content (hydration), resulting in a S concentration of up to ∼8000 ppm in the highly hydrated dacitic melts at fO2 ∼ FMQ+2. We found that high melt hydration at an intermediate fO2 not only dramatically increases sulfide and anhydrite solubilities in the dacitic melts but also promotes the saturation of S-rich fluids. The S concentration in the fluid that corresponds to the highest measured S concentrations in the melt (∼8000 ppm) are ∼5 wt%. These results show that highly hydrated felsic magmas saturated with fluids have a high capacity for S dissolution. Therefore, injection of such fluid-saturated magmas from lower crustal to shallow magma reservoirs is a potential mechanism for the effective transport of S (and perhaps metals) to facilitate subsequent porphyry ore deposit formation.

Genetic Relationship between Subduction of Slab Topographic Anomalies and Porphyry Deposit Formation: Insight from the Source and Evolution of Rio Blanco Magmas

AbstractThe Rio Blanco deposit, which is one of the largest porphyry Cu–Mo deposits in northern Peru, formed coevally with the subduction of the Inca Oceanic Plateau at 12–10 Ma. However, the genetic relationship between the subduction of oceanic plateaus and the porphyry deposit formation remains unclear. Igneous rocks emplaced at 23–12 Ma in northern Peru, including the Portachuela batholith (which hosts the Rio Blanco porphyry complex), are normal calc-alkaline to weakly adakitic. In comparison, the 12–8 Ma igneous rocks, including the ore-related Rio Blanco porphyry complex, have typical adakitic signatures, such as high Sr/Y ratios (up to 180) and LaN/YbN ratios (up to 32). The Rio Blanco igneous rocks (Portachuela batholith and Rio Blanco porphyry complex) have uniform zircon εHf(t) values (+0.3 ± 1.2) and δ18O values (6.5 ± 0.14‰). These geochemical characteristics indicate that the Rio Blanco igneous rocks evolved from mantle-derived parental melts in a long-lived, stable, homogeneous isotopic reservoir at the crust–mantle boundary. However, whereas both the Portachuela batholith and the Rio Blanco porphyry complex formed from hydrous parental magmas (>5 wt %; based on plagioclase hygrometry), the ones of the Rio Blanco porphyry complex seem to be more oxidized, hydrous, and sulfur-rich compared with the older batholitic rocks. Reverse zoning in plagioclase phenocrysts, with a systematic core–mantle–rim variation in An (anorthite) and Fe (total iron) contents, are common in the intermineralization rocks. The An content of the mantles of the plagioclase phenocrysts correlates positively with the Fe content, but in the rims, the An contents significantly decrease while Fe remains constant. The apatite inclusions in the mantles are richer in S (0.24 ± 0.06 wt %) and Cl (1.42 ± 0.32 wt %) than those in the phenocryst cores (S: 0.09 ± 0.07 wt %; Cl: 1.03 ± 0.56 wt %) and rims (S: 0.14 ± 0.09 wt %; Cl: 0.83 ± 0. 35 wt %). These systemic geochemical variations in the plagioclase phenocrysts suggest recharge by S- and Cl-rich melts followed by fluid exsolution. This magma recharge and subsequent fluid exsolution may have triggered porphyry Cu mineralization at Rio Blanco. The coincidence of timing between the geochemical transition and collision (initial subduction) of the Inca Oceanic Plateau with the South American plate may indicate a change in the tectonic regime to a compressional state of stress and a thickening of the crust during the collision. The tectonic transition would have facilitated the fractionation of mantle-derived magma in a deep crustal hot zone, resulting in oxidized, volatile-rich residual melts. Replenishment of the upper-crustal magma chamber by such volatile-rich magmas and the subsequent discharge of fluids are interpreted to be fundamental for porphyry Cu mineralization at Rio Blanco and plausibly for the formation of Late Miocene porphyry ore deposits in northern Peru in general.

In situ Sr-O isotopic and elemental compositions of apatite and zircon from Pengcuolin granodiorites: Implications for Jurassic metallogenic variation in the southern tibet

The Gangdese belt is among the most significant magmatic and metallogenic belts in the world. The multistage magmatism that generated the deposits in the Gangdese is a major subject of scientific interest and valuable economic resource. However, the metallogenic variability within the Jurassic arc system is still controversial. In this paper, we present a study of granodiorites in the Pengcuolin to investigate the magma source and crystallisation process in the western Gangdese belt. This work includes detailed descriptions of mineral textural features, major and trace elements of bulk rock and apatite, trace element of zircons and in situ Sr isotope of apatite and O isotope of zircon. The granodiorites yielded zircon U-Pb ages of 169 ± 0.7 Ma and 165 ± 0.8 Ma, and all samples are depleted in high-field-strength elements (HFSEs) and enriched in large-ion lithophile elements (LILEs). In situ Sr isotopic compositions of apatites exhibit low 87Sr/86Sr ratios (0.7036–0.7057). The zircon δ18O values range from 4.29‰ to 6.04‰. In addition, the granodiorites lack Eu anomalies. These results indicate that the Pengcuolin magmas were derived from asthenospheric mantle that was metasomatised by slab-derived fluids following intrusion into the active continental margin in the Middle Jurassic. By comparing previously published data for Jurassic arc rocks with the results of this study, we find apparent differences between the western segment (zircon Ce4+/Ce3+ and Eu/Eu* ratios: 40–2220 and 0.07–1.49, respectively; TTi-Zr: 587–804 °C; S and Cl contents in magma: 7–443 ppm and 0.030–1.264 wt%, respectively) and the eastern segment (zircon Ce4+/Ce3+ and Eu/Eu* ratios: 3–1380 and 0.08–0.66, respectively; TTi-Zr: 547–1053 °C; S and Cl contents in magma: 7–138 ppm and 0.006–0.093 wt%, respectively). The eastern segment is characterised by low water and volatile contents and a reduced oxidation state. In contrast, the western segment exhibits a more highly oxidised state with higher water and volatile contents, which provide ideal conditions for the formation of porphyry deposits.

Apatite fingerprints on the magmatic-hydrothermal evolution of the Daheishan giant porphyry Mo deposit, NE China

Abstract Porphyry deposits are the main source for global Cu and Mo production. The generation of hydrous silicate magmas and subsequent separation of volatile-rich magmatic fluids with hydrothermal alteration are significant processes leading to the formation of porphyry deposits. However, a specific understanding of these processes has been limited by a lack of direct mineralogical records in the evolving magmatic-hydrothermal system. In this paper, we present an integrated textural and geochemical investigation on apatite from the giant Daheishan porphyry Mo deposit in NE China, illustrating that apatite can be a potential recorder of the magmatic-hydrothermal evolution of porphyry systems. Apatite from the ore-forming porphyry displays distinctive core-rim textures, with melt inclusions in the resorption cores (Type-A1) and co-existing of melt and fluid inclusions in the euhedral rims (Type-A2), indicating a magmatic-hydrothermal origin of apatite. This is also supported by both chemical and isotopic compositions obtained by in situ analyses using laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) and LA-multi collector-ICP-MS. The late Type-A2 apatite is relatively enriched in incompatible elements, such as rare earth elements (REE) and Th, but slightly depleted in fluid-mobile elements such as Na and S, compared to the early Type-A1 apatite. Relatively homogeneous (87Sr/86Sr)i ratios (0.70436–0.70504) of the Type-A1 and Type-A2 apatites indicate that they were formed in a relatively closed system without detectable contamination. Meanwhile, some apatite in the wall rock (biotite granodiorite) shows characteristics of secondary altered textures, resulting from the intensive alteration by hydrothermal fluids exsolved from the porphyry system. Apatite trapped in mineral phenocrysts of the wall rock is usually unaltered (Type-B1 apatite), with clear oscillatory growth zones in cathodoluminescence (CL) images. In contrast, the intergranular apatite is commonly altered (Type-B2 apatite), with chaotic zoning in CL images, abundant micro-fractures and secondary fluid inclusions. Compositionally, the Type-B2 apatite shows notable tetrad REE patterns, relatively lower light-REE and S contents, and elevated 147Sm/144Nd ratios compared to the Type-B1 apatite. LA-ICP-MS U-Pb dating yields a lower intercept age of 171.4 ± 2.3 Ma for Type-B2, which is consistent with the age of 171.5 ± 2.4 Ma for Type-A2, but is notably younger than the Type-B1 apatite (175.5 ± 1.3 Ma). It is suggested that the Type-B2 apatite has been significantly reset by hydrothermal fluids exsolved from the porphyry system. Therefore, we conclude that the textures and geochemistry of apatite in porphyry systems can be used as a potential proxy for recording fluid exsolution and hydrothermal alteration processes.

Zircon trace elements and oxygen fugacities of parental magma of Maher abad and Khopik Cu porphyries, Lut block

The Maher abad and Khopik porphyry Cu deposits occurred in the Upper Eocene (39-37 Ma) in Lut block. All of them associated with intermediate (mostly monzonite) rocks. Porphyry deposits are closely associated with oxidized magmas. Oxygen fugacity (fO2) is a key factor that controls the formation of porphyry Cu deposits. The composition of the major and trace elements of zircon grains related to several ore-bearing monzonite were measured in Maher abad and Khopik porphyry copper indices. Zircon grains show moderate to low Ce4+/Ce3+ with a range of 19 to 610 and an average of 155. The average of oxygen fugacity (logfO2) values of Meher abad and Khopik ore-bearing magmas, range ∆FMQ -3.2 to MFMQ -1.3 with mean ∆FMQ -2.2, indicate formation under moderate oxidation conditions (between Ni-NiO (NNO) and Faylite magnetite-quartz (FMQ) buffers, but magnetite-hematite (HM) buffer, which ), which is not ideal for the formation of porphyry deposits. This is supported by whole-rock and Sr-isotopic data, and absence of high oxidation minerals such as hematite, and the poor adakitic charactristic of rocks in both deposits, which are due to factors involved in magma origin such as rock type and partial melting rate (possibly peridotite with low participation of slab).

Zircon Petrochronology of the Kışladaǧ Porphyry Au Deposit (Turkey)

Abstract Porphyry deposits typically occur in subduction-related arcs but have more recently also been described in postsubduction, collisional to extensional back-arc settings. These different tectonic environments not only might imply different genetic processes but also seem to result in different metal endowments (e.g., Au rich versus Cu rich). It is therefore relevant, also for exploration purposes, to understand the magmatic processes involved in porphyry formation in these different tectonic environments. This study focuses on the Kışladaǧ porphyry Au (17.4 Moz) deposit in western Anatolia, which is centered on a series of porphyritic monzonite stocks of high-K calc-alkaline to shoshonitic affinity and formed in a continental rifting environment. With 17.4 Moz of Au, Kışladaǧ is of global metallogenic importance and hence a good example for studying the genetic processes associated with porphyry deposits in extensional back-arc settings. We herein combine a comprehensive set of new zircon textural observations, in situ zircon trace element and Hf isotope data, and previously published zircon geochronology to study the magmatic processes associated with porphyry deposit formation at Kışladaǧ. We show that mafic rejuvenation of a slowly crystallizing (between ~15.8 and 14.9 Ma) magma reservoir below Kışladaǧ immediately preceded porphyry deposit formation. Zircon trace elements and geochronology suggest a longer and deeper evolution for the early fertile magmas compared to the later infertile magmas. Magma evolution at Kışladaǧ was accompanied by crustal wall-rock assimilation. Whole-rock Nd and Sr radiogenic isotopes show that increasing asthenosphere-derived melt input under accelerated regional extension caused a loss in fertility of the system over time.

Isotopic spatial-temporal evolution of magmatic rocks in the Gangdese belt: Implications for the origin of Miocene post-collisional giant porphyry deposits in southern Tibet

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

Molybdenum and Boron Isotopic Compositions of Porphyry Cu Mineralization‐Related Adakitic Rocks in Central‐Eastern China: New Insights Into their Petrogenesis and Crust‐Mantle Interaction

AbstractThe voluminous Late Mesozoic porphyry ore‐related adakitic magmatism in central‐eastern China is crucial to understand the deep geodynamic process of the region and controls on porphyry deposit formation globally. Here, we present a novel study of whole‐rock Mo‐B isotopes on the mineralization‐related adakitic rocks from the Dexing (located in the interior of South China and associated with giant porphyry Cu deposits), and Middle‐Lower Yangtze River (MLYR) (host to one of the largest porphyry Cu‐Au deposit belts) areas in central‐eastern China. The Dexing adakitic rocks have δ98/95Mo ranging from −0.31‰ to −0.01‰ and δ11B from −17.65‰ to −11.49‰. The adakitic rocks from the MLYR area have δ98/95Mo values of −0.5‰ to 0.2‰ and δ11B values of −14.48‰ to −9.53‰. The δ98/95Mo values of both the Dexing and MLYR samples negatively correlate with the La/Yb and La/Sm ratios. Together with the high Mg#, Cr, and Ni contents, these results indicate a process of melt‐mantle interaction resulting in hybridization of initial adakitic melts with low δ98/95Mo and mantle‐derived melts/components with high δ98/95Mo. These adakitic rocks have distinct Mo isotope systematics compared with those of oceanic slab‐derived adakites and MORB‐type eclogites. Their relatively low δ11B values and the lack of correlation between δ11B and B contents support a mafic lower crust source for the initial adakitic melts. We thus suggest that these adakitic rocks were generated by partial melting of delaminated lower crust followed by interaction with the deep mantle, and that melt‐mantle interaction may facilitate Cu‐Mo‐Au mineralization.

New 40Ar/39Ar and (U-Th)/He dating for the Zhunuo porphyry Cu deposit, Gangdese belt, southern Tibet: implications for pulsed magmatic-hydrothermal processes and ore exhumation and preservation

Understanding the magmatic-hydrothermal cooling and exhumation history is fundamental for investigating porphyry deposit formation and preservation. In the Zhunuo porphyry copper deposit of southern Tibet, hydrothermal biotite related to early potassic alteration likely formed from 14.7 ± 0.3 Ma (zircon U-Pb age) to 14.23 ± 0.13 Ma (molybdenite Re-Os age) and cooled below the closure temperature for 40Ar/39Ar system at 13.91 ± 0.05 Ma (hydrothermal biotite 40Ar/39Ar plateau age). The 40Ar/39Ar spectra of another hydrothermal biotite sample that was selectively replaced by chlorite record the timing of late phyllic alteration at 13.53 ± 0.08 Ma. Zhunuo hydrothermal alteration and mineralization likely ceased at 13.09 ± 0.07 Ma based on the 40Ar/39Ar plateau age of the fresh magmatic biotite crystals. The biotite 40Ar/39Ar systematics have not been reset by ~ 12.0 Ma post-mineralization granite porphyry within ore district, which is likely due to rapid cooling and/or absence of fluid exsolution for the granite porphyry. In combination with previous zircon U-Pb and molybdenite Re-Os data, two-pulses of magmatic-hydrothermal activity from 14.78 ± 0.11 to 14.44 ± 0.05 Ma and from 14.23 ± 0.13 to 13.53 ± 0.08 Ma can be recognized at Zhunuo. (U-Th)/He thermochronological data on zircon and apatite from the inter-mineralization monzogranite porphyry suggest three cooling events characterized by first-stage magmatic cooling at a rate of ~ 1850 °C/m.y., and two later phases of uplift and exhumation at ~ 70 °C/m.y. and ~ 10 °C/m.y. Approximately 1.4-km thickness of materials have been removed from the Zhunuo Miocene monzogranite porphyry during uplift and erosion, including a large volume of ore.

Resolving the timescales of magmatic and hydrothermal processes associated with porphyry deposit formation using zircon U–Pb petrochronology

Abstract. Understanding the formation of economically important porphyry Cu–Au deposits requires knowledge of the magmatic-to-hydrothermal processes that act within the much larger magmatic system and the timescales on which they occur. We apply high-precision zircon geochronology (chemical abrasion–isotope dilution–thermal ionisation mass spectrometry; CA–ID–TIMS) and spatially resolved zircon geochemistry (laser ablation inductively coupled plasma mass spectrometry; LA-ICP-MS) to constrain the magmatic evolution of the underlying magma reservoir at the Pliocene Batu Hijau porphyry Cu–Au deposit. We then use this extensive dataset to assess the accuracy and precision of different U–Pb dating methods of the same zircon crystals. Emplacement of the oldest pre- to syn-ore tonalite (3.736±0.023 Ma) and the youngest tonalite porphyry to cross-cut economic Cu–Au mineralisation (3.646±0.022 Ma) is determined by the youngest zircon grain from each sample, which constrains the duration of metal precipitation to fewer than 90±32 kyr. Overlapping spectra of single zircon crystallisation ages and their trace element distributions from the pre-, syn and post-ore tonalite porphyries reveal protracted zircon crystallisation together with apatite and plagioclase within the same magma reservoir over >300 kyr. The presented petrochronological data constrain a protracted early >200 kyr interval of melt differentiation and cooling within a large heterogeneous magma reservoir, followed by magma storage in a highly crystalline state and chemical and thermal stability over several tens of thousands of years during which fluid expulsion formed the ore deposit. Irregular trace element systematics suggest magma recharge or underplating during this final short time interval. The comparison of high-precision CA–ID–TIMS results with in situ LA-ICP-MS and a sensitive high-resolution ion microprobe (SHRIMP) U–Pb geochronology data from the same zircon grains allows a comparison of the applicability of each technique as a tool to constrain dates and rates on different geological timescales. All techniques provide accurate dates but with different precision. Highly precise dates derived by the calculation of the weighted mean and standard error of the mean of the zircon dates obtained by in situ techniques can lead to ages of unclear geological significance that are older than the maximum ages of emplacement given by the CA–ID–TIMS ages of the youngest zircons in each sample. This lack of accuracy of the weighted means is due to the protracted nature of zircon crystallisation in upper crustal magma reservoirs, suggesting that standard errors should not be used as a means to describe the uncertainty in those circumstances. We conclude from this and similar published studies that the succession of magma and fluid pulses forming a single porphyry deposit and similarly rapid geological events are too fast to be reliably resolved by in situ U–Pb geochronology and that assessing the tempo of ore formation requires CA–ID–TIMS geochronology.

Metallogenic potential of porphyries in Tulasu basin, Northwest Tianshan: Insight from magma nature and crustal thickness

Tulasu basin in Northwest Tianshan hosts a lot of Late Paleozoic epithermal deposits, such as Axi, Jingxi-Yelmand, and Tawuerbieke deposits. Epithermal deposits are considered as telescoped systems of porphyry deposits. However, porphyry deposits are not discovered in Tulasu basin, though granitoid porphyries are widespread in this region. Tulasu pluton complex, as one of the largest intrusions located in northeast part of Tulasu basin, is composed of the Late Devonian quartz diorite porphyry and pyroxene diorite porphyry, and the Late Carboniferous granodiorite porphyry and granite porphyry. The metallogenic potential of each rock type is evaluated in terms of magma nature and crustal thickness of source. The magma nature including magmatic oxygen fugacity and water content is estimated based on trace elements compositions of zircons, amphibole hygro-barometer and ilmenite-magnetite mineral pairs. The mean Ce4+/Ce3+ ratio, magmatic oxygen fugacity and water content of the Late Devonian quartz diorite porphyry are 291, ΔFMQ = 1.0–1.8, ΔFMQ = −0.2–1.5 and 0.9–2.2 wt%, respectively, while those of pyroxene diorite porphyry are 96, ΔFMQ = 2.6–3.3 and 0.2–1.3 wt%. The Late Carboniferous granodiorite porphyry has a mean Ce4+/Ce3+ ratio of 138. The crustal thicknesses in the Tulasu area are estimated to be 30–35 km at the Late Devonian and ~40 km at the Late Carboniferous, based on whole rock geochemistry of the Late Devonian dioritic rocks and the Late Carboniferous granitic rocks. Considering magmas associated with typical porphyry deposits which have high oxygen fugacity of ΔFMQ = +1.5 to +3 and water content (>4.5 wt%), and are derived from crustal thickness of >30 km, we think both dioritic and granitic rocks of Tulasu porphyries have favorable conditions of oxidation state and crustal thickness for porphyry deposit formation. However, the magmatic water contents of these rocks are relatively low. Therefore, the Tulasu porphyries have low potential in forming porphyry Cu deposits.

Why No Porphyry Copper Deposits in Japan and South Korea?

AbstractThis analysis concludes that there is little realistic likelihood of discovering large, high‐grade porphyry Cu deposits in Japan and South Korea because of an unusual combination of geological factors. First, weak inter‐plate coupling, trench retreat, and extension – a tectonic regime unfavorable for porphyry Cu formation – appears to have characterized the convergent margin during much of Cretaceous through Neogene time. Second, Cretaceous through Quaternary magmatism involved widespread ash‐flow caldera development, a volcanic style that suppresses porphyry Cu formation because the all‐important Cu‐bearing magmatic fluids – if present – would be explosively discharged during silicic ignimbrite eruption and lost to the atmosphere. Third, due to the trench retreat, crustal profiles beneath Japan are dominated by accretionary complexes containing chemically reduced lithologies, which decrease the redox state of subduction‐related magmas during their ascent. The most reduced intrusions belong to the ilmenite‐series, which may lock up chalcophile elements at depth and produce magmatic fluids that are incapable of transporting the Cu required for significant porphyry deposit formation. Fourth, many of the caldera‐related magmatic rocks, including the principal intrusions, are not only derived by partial melting of reduced crust but are commonly also highly fractionated; hence, any associated mineralization tends to be Mo ± W‐rich rather than Cu dominated. Fifth, intrusive rocks with high Sr/Y, typical of hydrous magmas responsible for porphyry Cu belts worldwide, appear to be present only locally. Sixth, two important magmatic and metallogenic tracts in Japan, the Kuroko‐bearing Green Tuff belt and Kitami low‐sulfidation epithermal Au region, are characterized by bimodal magmatism, which worldwide is compositionally unsuited to porphyry Cu generation. Seventh, the Pliocene‐Quaternary andesitic–dacitic volcanoes that could be the surface expressions of porphyry Cu systems are mostly too shallowly eroded to reveal underlying deposits should they exist. Consequently, few magmatic suites in Japan and South Korea provide propitious environments for porphyry Cu exploration, with the last hope believed to lie at depth below late Miocene–Pliocene advanced argillic lithocaps, several of which contain relatively minor high‐sulfidation epithermal Au mineralization. However, even there, the reduced crust may have militated against effective Cu transport into the porphyry environment. The magmatic and metallogenic factors that downgrade the porphyry Cu potential of Japan and South Korea are believed to be equally applicable to other subduction‐related magmatic arc segments worldwide.

Pulsed magmatic fluid release for the formation of porphyry deposits: Tracing fluid evolution in absolute time from the Tibetan Qulong Cu-Mo deposit

The magmatic-hydrothermal evolution of porphyry-style mineralization in the shallow crust that is linked to magmatic processes at depth has been extensively studied using bulk-sample isotopic analysis combined with relative timing constraints. However, a lack of evaluation of the fluid evolution process against an absolute time frame limits further understanding of the ore-forming process. Here, we quantify the fluid evolution process within an absolute time frame for the first time by integrating new in situ oxygen isotope data from the Qulong porphyry Cu-Mo deposit (Tibet) with existing fluid inclusion data and high-precision Re-Os dates of co-precipitated hydrothermal quartz and molybdenite, respectively. We demonstrate that vein quartz records primary oxygen isotopic compositions and reached oxygen isotope equilibrium with ore-forming fluids, and therefore is an archive of the isotopic composition and source of the ore-forming fluids. The δ18Oquartz and δ18Ofluid values, in absolute time, show periodic fluctuations that indicate the presence of three intermittent pulses of magmatic fluid flux, which have been balanced by meteoric water. As such, the flux of magmatic fluid during ore formation was pulsed, rather than being continuous. The overall highest δ18Ofluid in the first pulse of mineralization, with a gradual decrease to the second and third pulses, is suggestive of a progressive reduction in the magmatic component of the hydrothermal fluids and, by inference, the mineralizing potential of the hydrothermal fluids. This view is supported by a decrease in sulfide-bearing fluid inclusions and metal grade through time. Our findings favor multiple fluid-release events from a single cooling magmatic reservoir, although multiple fluid-melt recharge events remain a competitive alternative. An additional implication is that the magmatic reservoir may have a lifespan of hundreds of thousands of years, with fluid release events occurring over tens of thousands of years.

Large-scale porphyry-type mineralization in the Central Asian metallogenic domain: A review

A series of porphyry-type Cu-(Mo)-(Au) deposits including some giant world-class deposits such as Kounrad, Aktogai, Kal’makyr, Oyu Tolgoi and Chalukou occur in the Central Asian metallogenic domain (CAMD). The outline of the CAMD almost overlaps with that of the Central Asian orogenic belt (CAOB), which is located between the East Europe, Siberia, Tarim and North China cratons. The CAMD comprises sparsely exposed tectonic slices of Precambrian high-grade metamorphic basement rocks that are sandwiched between Phanerozoic accretionary complexes and arc rocks. According to geological, metallogenic, geochronological and tectonic setting characteristics, the CAMD comprises three metallogenic provinces: (1) the Kazakhstan Cu-Au-Mo, (2) the Mongolia Cu-Au and (3) the Northeast China Mo-Cu metallogenic provinces. Early Paleozoic (∼490 to 440 Ma) and Late Paleozoic (∼330 to 295 Ma) large scale porphyry-type mineralization in the Kazakhstan metallogenic province is intimately associated with the evolution of the Central Tianshan–Yili–Northern Tianshan–Kokchetav–Chingiz Paleozoic arc system which was terminated by the late Permian closure of the western segment of the Paleo-Asian Ocean. In contrast, the ∼370 Ma and ∼240 Ma porphyry-type mineralization of the Mongolia metallogenic province is related to the Tuva–Mongolia arc collage system that was formed during the evolution of the Paleo-Asian Ocean in the Paleozoic and the Mongol–Okhotsk Ocean in the early Mesozoic. Furthermore, the Northeast China metallogenic province was affected by three distinct geodynamic processes that were associated with the Paleo-Asian, Mongol–Okhotsk and Paleo-Pacific Oceans during the Paleozoic (>250 Ma), the early Mesozoic (>200 Ma) and the late Mesozoic (200–90 Ma), respectively. Except for the Early Paleozoic Cu mineralization at Duobaoshan (∼480 Ma) and Bainaimiao (440 Ma), the porphyry-type mineralization of this metallogenic province is predominantly characterized by widespread Mesozoic Mo mineralization.In general, porphyry-type deposits of the CAMD have geological, structural, metallogenic, alteration and fluid characteristics, which are consistent either with those related to the oceanic subductional arc settings of the Circum-Pacific metallogenic domain or with those of the continental collisional environment in the Tethyan metallogenic domain. The involvement of highly oxidized intermediate to felsic magmas is fundamental for the formation of porphyry deposits. Even though the prevalent MASH and slab melting models can be employed to interpret the genesis of some porphyry Cu deposits in the CAMD, a here newly suggested model, which is based on ‘pre-enriched mafic lower crust and the subduction of a relictic mid-oceanic ridge’, is thought to explain the large scale copper mineralization in the Balkhash–West Junggar metallogenic belt of the Kazakhstan metallogenic province. Nevertheless, most of the porphyritic suites associated with the Mo deposits in the Northeast China metallogenic province have relatively depleted mantle-like neodymium and hafnium isotopic compositions, suggesting that the ore-forming granitic magma may have predominantly been derived from juvenile lower crust. Upwelling of asthenosphere may have triggered the melting of the lower crust during post-collisional extension subsequent to the closure of the Paleo-Asian Ocean in the Triassic, the subduction of the Paleo-Pacific ocean in the Jurassic and the thinning of the lithosphere caused by rollback of the Paleo-Pacific plate in the Cretaceous. Ancient crustal components were only locally found to have been involved in the magma source of several medium-large Mo deposits.