Ore-forming Porphyry Research Articles

Mercury isotope evidence for the importance of recycled fluids in collisional ore systems.

The sources of fluids and metals in porphyry systems of continental-collision settings are poorly constrained. Mercury isotopes display unique mass-independent fractionation (expressed as Δ199Hg) and may provide important constraints on metal and volatile sources given that Hg is a highly volatile metal. Here, we report Hg isotope data on ore-forming porphyries, barren magmatic rocks, and mantle-derived mafic magmas from southern Tibet. The fertile porphyries and coeval mafic magmas display mainly positive Δ199Hg values (up to +0.25 per mil), while Δ199Hg values in barren magmatic rocks and mafic magmas are largely negative (-0.54 to 0.00 per mil). The positive Δ199Hg values observed here are consistent with seawater and marine sediments, suggesting that the ultimate source of fluids involved in the genesis of post-subduction porphyry copper deposits was the mantle lithosphere metasomatized by previous oceanic plate subduction. Our Hg isotope data provide an alternative view to current metallogenetic models on collisional porphyry systems that focus on melting of the lower continental crust.

Quantifying the Criteria Used to Identify Zircons from Ore-Bearing and Barren Systems in Porphyry Copper Exploration

Abstract Zircon is a common mineral in igneous rocks, which is resistant to both chemical weathering and physical abrasion. Its chemistry can potentially be used to distinguish ore-forming porphyry magmas from barren magma systems. This study compiles >23,000 zircon analyses from >30 porphyry deposits, barren intrusions, and rivers to determine the principal geochemical characteristics of fertile zircons using predictive modeling, and compares them with traditional geochemical thresholds. The results show that the Eu/Eu* and Dy/Yb ratios, P content, and the curvature at the end of rare earth element (REE) patterns (λ3) are the most diagnostic characteristics of fertile zircons. The use of geochemical thresholds, as Boolean conditions, reach their maximum performance for Eu/Eu* and Dy/Yb (sensitivity [sens] = 0.73, specificity [spec] = 0.90), but it is outperformed by the random forest model (sens = 0.91, spec = 0.93) in the testing set. Explanatory analysis of the models shows that the fertility signal in zircons becomes stronger as the porphyry system evolves and is accompanied by an overall decrease in the middle to light REE and P content, characteristics that are absent in barren zircons. We attribute the observed difference in λ3 to the co-crystallization of other accessory phases, suggesting that the changes in the zircon Ce anomaly is controlled by the depletion of light and middle REE. The low P content in fertile zircons is caused by extensive crystallization of apatite. Fertile zircons have an excess of (REE + Y)3+, which we attribute to charge-balance by H+ in hydrous magmas. Simple machine learning algorithms outperform the traditional geochemical discriminators in their predictions and provide insights into characteristics that have not previously been considered for evaluating porphyry copper fertility using zircon geochemistry. We propose simplified methods that can be easily incorporated into exploration workflows.

Zircon compositional systematics from Devonian oxidized I-type granitoids: examination of porphyry Cu fertility indices in the New Brunswick Appalachians, Canada

Zircon is a common, widely distributed accessory mineral in most igneous rocks and its refractory nature records magmatic evolution in terms of oxygen and U-Th-Pb isotopes, and trace-element contents all of which reflect the intrinsic physio-chemical evolution of the magmatic systems in which it crystallized. Zircon compositions can be used as an indicator of relative fertility of hypabyssal intrusions in terms Cu ± Mo ± Au porphyry mineralization. To further characterize syn- to post-collisional adakitic Devonian oxidized I-type granitoids in the New Brunswick (specifically, those with Cu ± Mo ± Au porphyry-style mineralization), LA-ICP-MS analyses (guided by µXRF-EDS mapping and SEM-BSE imaging of polished thin sections) of zircons from 13 granitoids was conducted. The zircons studied were similar in terms of their textures (homogenous cores, patchy zoning, oscillatory zoning, and some unzoned zircon); however, they have a wide range of trace- and minor-element (Hf, HREE, Y, Th, U) compositions. Specifically, Zr/Hf ranges between 24–60, whereas Th/U ranges between 0.15 and 5.37. The presence of inherited zircon affects the concentrations of Th and U, as well as other key elements. Estimated crystallization temperatures of granitoids, ranging from 737 to 899°C, were calculated via Ti-in-zircon geothermometry assuming reduced TiO2 and SiO2 activities. The calculated log fO2 values for zircons from some of these granitoids indicate a highly oxidized magmatic signature. Zr/Hf, Eu/Eu⁎, and (Eu/Eu⁎)/Y in zircon, as well as zircon (Ce/Nd)/Y are some of the best indicators of porphyry fertility. The Ce/Ce* in zircon exhibit a large range (1.1–590), with higher Ce/Ce* reflecting more metallogenically favourable oxidizing conditions. If Eu/Eu⁎ in zircon is ≥0.4 (relatively oxidized conditions), it indicates a high potential for an ore-forming porphyry Cu mineralizing system. Lower Eu contents reflect relatively reducing conditions, as Eu anomalies vary with oxygen fugacity as well, and the relative abundance of Eu2+ is higher, but does not substitute into the zircon lattice. The evidence extracted from analyzing the zircon composition within New Brunswick’s I-type granitoids indicates the fertility of these hypabyssal intrusions.

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.

The lithospheric cycle of ore-forming ingredients and the resultant recurrent ‘flare-ups’ of porphyry Cu deposits: Examples from the Yidun Arc, eastern Tibet

The porphyry Cu deposits with distinct mineralisation ages frequently occur as recurrent ‘flare-ups’ in a narrow region; however, the reasons controlling their occurrence are still debated. The eastern Yidun Arc in eastern Tibet produced a cluster of porphyry Cu deposits of the late Triassic and late Cretaceous ages, making it the ideal location for conducting this research. As a result, we use the late Triassic Xuejiping Cu–(Au) and late Cretaceous Hongshan–Hongniu Cu–Mo deposits as research objects, compiling new and published geochronological, elemental, and Sr–Nd–Hf isotopic data from their ore-forming porphyries to shed light on the above issue. The zircon U–Pb ages of the Xuejiping and Hongshan–Hongniu porphyries are 217.8 – 217.9 Ma and 78.7 Ma, respectively, and display adakitic characteristics. The Xuejiping porphyries present enriched Sr–Nd–Hf isotopes (0.70529 – 0.70590, −3.30 – −1.09, 0.40 – 2.20, respectively), low (Gd/Yb)N (2.24 – 3.20) and Nb/Ta (12.8 – 15.9) ratios, relatively high Nb (14.9 – 20.0 ppm) and Zr/Sm (23.4 – 41.5) values, indicating their origin in the partial melting of enriched mantle that is metasomatised by slab-melts and fluids in subduction zone. Furthermore, the Hongshan–Hongniu porphyry is adakitic, like the late Triassic igneous rocks in this area. Its Sr–Nd–Hf isotopes (0.69873 – 0.70996, −10.7 – −6.72, −14.9 – −0.50, respectively) are close to the isotopic evolution line of the late Triassic Xuejiping intrusions and show a trend towards the regional crystalline basement, which signify that the Hongshan–Hongniu porphyry mainly originated from the late Triassic juvenile crust, with some contributions from crystalline basement. One highly fractionated sample (i.e. HZK17-11-76) has extremely evolved Sr–Nd isotopes (0.70996 and −10.7, respectively), indicative of assimilation of sedimentary rocks. The new genetic models of the late Triassic and late Cretaceous porphyries in the Geza Arc show that the westward subduction of the Garzê–Litang oceanic crust in the late Triassic triggered the occurrence of the highly oxidised and Cu–S–H2O rich adakitic melts in the middle–lower crust, which evolved to amphibole cumulates and porphyry Cu–(Au) deposits (i.e. Xuejiping) in the upper crust. In the late Cretaceous, the extensional environment triggered the re-melting of late Triassic juvenile cumulates and Mo-rich crystalline basement, resulting in Cu–Mo–S–H2O-rich, highly oxidised magma. Through fractional crystallisation and assimilation of Mo-rich sedimentary rocks, the magma eventually transformed into Hongshan–Hongniu porphyry-(skarn) type Cu–Mo deposits. In summary, we conclude that the periodic recycling of the ore-forming ingredients on a lithospheric scale from slab subduction to post-subduction periods controls the recurrent ‘flare-ups’ of PCDs in this confined area.

Molybdenum isotope heterogeneity of metal sulfides from magmatic hydrothermal systems

The sensitivity of molybdenum (Mo) stable isotopes to redox changes enables innovative geochemical means of tracing the evolution of medium- to high-temperature magmatic hydrothermal processes. However, the Mo isotope geochemical behaviors and fractionation mechanisms in magmatic-hydrothermal systems are still unclear. This study performed high-precision Mo isotope research on well-characterized hydrothermal metal sulfides (pyrite, chalcopyrite, molybdenite and pyrrhotite) from different metallogenic stages and the ore-forming porphyries and surrounding rocks from the giant Pulang porphyry copper deposit in Yunnan, Southwest China, to better understand Mo isotope behaviors in magmatic hydrothermal systems. The results show that compared to the high-temperature magmatic system with homogeneous Mo concentrations (1.44 ppm to 11.3 ppm) and δ98Mo values (−0.41‰ ± 0.05 to −0.08‰ ± 0.03), medium- to high-temperature hydrothermal stage molybdenite displays slightly lighter δ98Mo values (−0.90‰ ± 0.04 to 0.08‰ ± 0.04), and pyrite, chalcopyrite and pyrrhotite, which have low Mo concentrations (0.13 ppm to 28.5 ppm), feature significant δ98Mo variations from −0.34 ± 0.04‰ to 3.01 ± 0.03‰. The absence of fractionated but low δ98Mo values in the magmatic system, combined with the lack of any correlation between the δ98Mo and geochemical index values (e.g., Co/Ni value) of the metal sulfides, implies that the Mo isotope signatures of sulfides in the hydrothermal stage were not inherited from component signatures of the magmatic phase or contamination of surrounding rocks. Our study proposes that heterogeneous partitioning and transportation of heavy/light Mo isotopes into progressively precipitated metal sulfides as a result of changes in the redox-driven metallogenic setting and differential geochemical behaviors of Mo species contribute to the observed δ98Mo variations and that the heavier and strongly variable Mo isotopic signatures correspond to the dominant metallogenic stage of the deposit. Such fractionation may be explained by the Rayleigh fractionation model, whereby 95Mo is preferentially partitioned into isotopically light molybdenite, and the subsequent precipitation of pyrite, chalcopyrite and pyrrhotite from the residual 98Mo-rich ore-forming fluids manifests heavier isotopic compositions. We therefore highlight that the Mo isotopes of metal sulfides provide valuable insights into the precipitation of metal-rich fluids, the identification of favorable mineralization zones, and the ability to trace the evolution of sophisticated magmatic-hydrothermal systems.

Formation of the Chalukou High Fluorine-Type Mo (–Zn–Pb) Deposit, NE China: Constraints from Fluorite and Sphalerite Rare Earth Elements and Sr–Nd Isotope Compositions

Fluorite is a widespread mineral in porphyry and hydrothermal vein Mo-polymetallic deposits. Here, fluorite is utilised as a probe to trace the fluid source and reveal the fluid evolution process in the Chalukou giant Mo (Pb–Zn) deposit, Northeast China, which is characterised as early porphyry Mo and later vein-style Zn–Pb mineralisation. A detailed rare earth element (REE) and Sr–Nd isotope study of fluorite combined with Sr isotopes of sphalerite is conducted for the Chalukou deposit. The chondrite-normalised REE patterns of fluorites from molybdenite veins show light REE (LREE)-enriched patterns, with negative Eu anomalies (δEu = 0.60) and weakly negative Y anomalies (Y/Y* = 0.72). The fluorites associated with sphalerite veins exhibit rare earth element (REE)-flat patterns with negative Eu anomalies (δEu = 0.65 to 0.99) and positive Y anomalies (Y/Y* = 1.37 to 3.08). In addition, during the progression from Mo to Zn–Pb mineralisation, the total concentration of REEs decreases from 839 ppm to 53.7 ppm, and Y/Ho ratios increase from 22.1 to 92.5. These features may be explained by the different mobilities of REE complexes during fluid migration. The Eu anomalies are considered to be inherited from source fluids. All the initial 87Sr/86Sr ratios of fluorite and sphalerite are between those of ore-forming porphyries and wall rocks (rhyolite), with fluorite ratios ranging from 0.706942 to 0.707386 and sphalerite ratios varying from 0.705221 to 0.710417. The majority of εNd(t) values of fluorite varying from −6.4 to −3.6 are also located between the ratios exhibited by ore-forming porphyries and rhyolite, whereas three εNd(t) values of fluorites ranging from −0.26 to 0.36 are close to those of ore-forming porphyries. All the isotopic features indicate that the Sr-Nd isotope ratios of hydrothermal fluid are derived from porphyries and disturbed by fluid–rock reactions. Together with a two-stage Sr–Nd isotope mixing model, we suggest that different sources and fluid–rock interactions (syn-ore intrusions and strata) finally influence the Sr–Nd isotopes of the ore-forming fluids, which are recorded by the majority of fluorite and sphalerite.

Constructing the deep-spreading pattern of tectono-geochemical anomalies and its implications on the Huangshaping W–Sn–Pb–Zn polymetallic deposit in southern Hunan, China

Huangshaping large W–Mo–Pb–Zn polymetallic deposit is a typical representative of the Cu–Sn metallogenic system in Nanling Range, South China. This deposit located in the superimposed tectonic setting of the Qinzhou–Hangzhou and Nanling metallogenic belts. Skarn-type W–Sn orebodies and hydrothermal vein-type Pb–Zn–Cu orebodies located in the faults and contact zone between late Jurassic porphyry and late Paleozoic limestone. Many studies have been conducted on the geological and geochemical characteristics of the deposit, but the comprehensive understanding of the deep-spreading pattern of the tectono-geochemical anomalies remains unclear. The geochemical data of tectonites in deep level adits (−136 m, −176 m, −256 m and −296 m) were studied the distribution characteristics of ore-forming trace elements and the depth index of tectonic geochemical anomalies. Finally, four element combination anomalies were extrated from the cluster analysis and factor analysis. They are generally consistent with the known Cu–Pb–Zn and W–Sn–Mo–Bi metallogenic systems. In addition, from altered porphyry to limestone of the Shidengzi Formation, the plane and profile anomaly contour maps delineated by different factors have regular variation characteristics. High anomalous values usually occur in the part where ore-forming porphyry changes from convex to concave. Comprehensive study trace element combination anomalies shows that the connection line of high value anomalous zones corresponds to the swelling and shrinking characteristics of porphyry, showing periodic wave-like changes. Thus, the deep index and deep-spreading pattern of ore-controlling structure were constructed. The length, width and height of the deep-seated orebodies indicated by the main anomalies are evaluated. The anomalous vertical variation indicates the migration of ore-forming fluid from SE to NW to the shallow. The strike of orebodiesy are consistent with the ore-controlling faults and dip to SE. Overall, the tectonic geochemical anomalies are consistent with the locations where the orebodies were identified, indicating that the method is feasible and effective. This study has an important significance for the layout of deep-prospecting and exploration projects in the Huangshaping deposit and even in southern Hunan.

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.

Exhumation of the Late Cretaceous Ore‐forming Porphyries in Zhongdian Area, Northwestern Yunnan: Evidence from Fission Track Analysis

Acta Geologica Sinica - English EditionVolume 95, Issue 5 p. 1786-1787 Research Advance Exhumation of the Late Cretaceous Ore-forming Porphyries in Zhongdian Area, Northwestern Yunnan: Evidence from Fission Track Analysis Xuelong LIU, Corresponding Author Xuelong LIU xuelongliu@foxmail.com Kunming University of Science and Technology, Kunming, 650093 China Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037 ChinaCorresponding author. E-mail: xuelongliu@foxmail.comSearch for more papers by this authorZhiming YANG, Zhiming YANG Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037 ChinaSearch for more papers by this authorYingxiang LU, Yingxiang LU Kunming University of Science and Technology, Kunming, 650093 China Yunnan Geological Survey, Kunming, 650051 ChinaSearch for more papers by this authorNa ZHANG, Na ZHANG Kunming University of Science and Technology, Kunming, 650093 ChinaSearch for more papers by this authorLeilei DONG, Leilei DONG Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037 ChinaSearch for more papers by this authorQianqian JIAO, Qianqian JIAO Kunming University of Science and Technology, Kunming, 650093 ChinaSearch for more papers by this authorJianhang CHEN, Jianhang CHEN Kunming University of Science and Technology, Kunming, 650093 ChinaSearch for more papers by this authorFucheng YANG, Fucheng YANG China University of Geosciences, Beijing, 100083 ChinaSearch for more papers by this author Xuelong LIU, Corresponding Author Xuelong LIU xuelongliu@foxmail.com Kunming University of Science and Technology, Kunming, 650093 China Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037 ChinaCorresponding author. E-mail: xuelongliu@foxmail.comSearch for more papers by this authorZhiming YANG, Zhiming YANG Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037 ChinaSearch for more papers by this authorYingxiang LU, Yingxiang LU Kunming University of Science and Technology, Kunming, 650093 China Yunnan Geological Survey, Kunming, 650051 ChinaSearch for more papers by this authorNa ZHANG, Na ZHANG Kunming University of Science and Technology, Kunming, 650093 ChinaSearch for more papers by this authorLeilei DONG, Leilei DONG Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037 ChinaSearch for more papers by this authorQianqian JIAO, Qianqian JIAO Kunming University of Science and Technology, Kunming, 650093 ChinaSearch for more papers by this authorJianhang CHEN, Jianhang CHEN Kunming University of Science and Technology, Kunming, 650093 ChinaSearch for more papers by this authorFucheng YANG, Fucheng YANG China University of Geosciences, Beijing, 100083 ChinaSearch for more papers by this author First published: 31 May 2021 https://doi.org/10.1111/1755-6724.14752Citations: 1Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article.Citing Literature Volume95, Issue5October 2021Pages 1786-1787 RelatedInformation

Magmatic Water Content and Crustal Evolution Control on Porphyry Systems: Insights from the Central Asian Orogenic Belt

Abstract Porphyry systems, the most important reserves of Cu and Mo with significant Au, are genetically linked to the emplacement of hydrous and oxidized intermediate to acidic magmas, in response to temporal and geochemical evolution of crust in orogenic terranes. In this study, comprehensive whole-rock and zircon geochemical and isotopic datasets of intermediate to acid igneous rocks were integrated to characterize the crustal evolution and metallogeny of porphyry deposits in the Central Asian Orogenic Belt (CAOB). The ore-forming porphyries of Cu ± Au ± Mo and Mo deposits have higher but largely overlapped ΔFMQ (the proxy for oxygen fugacity) with those of the barren igneous rocks. However, the ore-forming porphyries of Cu ± Au ± Mo deposits are characterized by distinctly higher whole-rock V/Sc and zircon Eu/Eu* (both are proxies for water content) than barren rocks. Furthermore, the V/Sc ratios positively correlate with the Cu tonnages of calc-alkali porphyry Cu deposits, suggesting that magmatic water contents may yield the first-order control on metal endowment. The general decrease of V/Sc from the Paleozoic to Mesozoic, combined with the negative correlation of V/Sc with K2O and SiO2, also indicates the gradual evolution of crust in CAOB clearly controls the end members of porphyry-type systems (i.e., Cu‒Au and Mo deposits). The crustal residence age (TRes, the time difference between Nd depleted-mantle model age and the crystallization age) of c. 500 Ma from the Nd isotopes is proposed as the threshold distinguishing porphyry Cu ± Au ± Mo (TRes < 500 Ma) and Mo deposits (TRes > 500 Ma) in the CAOB. The coupled zircon Hf isotopes and crustal thickness reveal that the fundamental crustal architecture in the eastern and western CAOB had been built by the Late Permian and Late Carboniferous, respectively, highlighted by the converging trends of εHf(t) commencing at c.250 Ma in the eastern segment and c.300 Ma in the western segment of CAOB, indicating reworking and homogenizing of juvenile crust after collision. In the eastern CAOB, porphyry Cu ± Au ± Mo deposits were formed by juvenile materials in thin island arcs, while porphyry Mo deposits were formed by reworked materials in the thickened orogenic crust after c.250 Ma. In the western CAOB, porphyry Cu deposits in the Balkhash region during the Late Carboniferous were formed in thickened continental crust (generally > 40 km), genetically linked to the culmination of world-wide magmatic addition rates (MARs) triggered by accelerated production of the juvenile crust, in contrast to the porphyry Cu ± Au ± Mo deposits formed in thin island arc (generally < 40 km) during the Early Paleozoic. This study tests the zircon ΔFMQ as proxy for fO2, and zircon Eu/Eu* and whole-rock V/Sc ratios as proxy for water content. It highlights that whole-rock V/Sc ratio is a favorable index for the Cu tonnages of porphyry Cu ± Au ± Mo deposits, and that the distinct porphyry-type mineralization in the CAOB is controlled by the crustal evolution reflected by crustal composition and thickness.

Molybdenum isotope systematics of subduction-related magmas from the Zhongdian region: Assessing the Mo fractionation behavior in magmatic-hydrothermal processes

The behavior of molybdenum isotopes in magmatic–hydrothermal systems is poorly known. Late Triassic andesitic rocks from the Disuga and Lannitang areas, and contemporaneous porphyry–skarn Cu polymetallic deposits (e.g., Pulang, Xuejiping, Yaza, Hongshan, and Langdu), from the Zhongdian region, southeastern Tibetan Plateau, China, present an ideal opportunity to study Mo isotope fractionation in magmatic–hydrothermal systems. In this study, we report Mo isotopic compositions, analyzed by multi-collector–inductively coupled plasma–mass spectrometry, of ore-forming porphyries and hydrothermal chalcopyrite, pyrite, and molybdenite from the Pulang deposit, and coeval barren andesites from the Disuga and Lannitang areas. The δ98Mo values of the Pulang porphyries (−0.59‰ to −0.05‰) are lower than those of the Disuga and Lannitang andesites (−0.29‰ to −0.03‰). In the Pulang deposit, whole-rock δ98Mo values are lower (−0.59‰ to −0.05‰) than those of pyrite (+0.73‰ to +2.09‰). Chalcopyrite has lower δ98Mo values (−0.16‰ to +1.06‰), than pyrite, but similar δ98Mo values to molybdenite (−0.18‰ to +0.06‰). We infer, tentatively, that fractional crystallization was not a major control on the Mo isotopic composition of the Pulang magmatic–hydrothermal system. Heavy Mo isotopes partitioned into pyrite relative to molybdenite, and 98Mo is thought to partition into Mo6+-bearing species relative to Mo4+-bearing species. Therefore, if MoO42− was the predominant Mo species in exsolved hydrothermal fluids, then formation of pyrite from these fluids explains the high δ98Mo values of pyrite. We infer that the chalcopyrite from the Pulang deposit contains micron-scale molybdenite inclusions, which partially explains the comparable δ98Mo values of chalcopyrite and molybdenite. Pyrite has a strong influence on Mo isotope fractionation, at least in the magmatic–hydrothermal system described here.

Geology and petrogenesis of the Sungun deposits: Implications for the genesis of porphyry-type mineralisation in the NW Urumieh–Dokhtar magmatic Arc, Iran

The Miocene Sungun porphyry Cu–Mo systems, located in Azerbaijan belt, NW Iran, is associated with the high-Sr/Y magmas, which were engendered in a post-collisional setting. However, the petrogenesis of these fertile magmas and their mineralisation are still unknown. In this study, we present whole-rock major-trace elements, zircon U–Pb dating and the Lu–Hf isotopes in the ore-forming porphyries of the Sungun porphyry systems. The corresponding results showed that the samples were rich in large-ion lithophile elements and poor in high-field-strength elements; moreover, they presented both high K2O (2.4–3.5 wt%) and Sr (727–770 ppm) and low Y (11–14 ppm) contents. The zircon U–Pb age of the Sungun porphyry was ~19 Ma, with positive εHf(t) ratios (5.5–11.8) and young TDM2 ages between 322 and 705 Ma. Additionally, the Sungun high-Sr/Y porphyries contained anhydrites (CaSO4); and had high zircon Ce4+/Ce3+ ratios (33–728), relatively low zircon saturation temperatures (620–804 °C), and plagioclases with excess Al content. Overall, these evidences suggest that a water-fluxed melting of the subduction-modified juvenile lower crust might have generated a high oxygen fugacity, leading to the formation of the hydrous and S-rich Sungun fertile magmas in the NW Urumieh–Dokhtar magmatic Arc.

Porphyry Cu deposits linked to episodic growth of an underlying parental magma chamber

Saindak is one of the typical porphyry Cu deposits (PCDs) in the Chagai magmatic arc in Pakistan. Ore-forming porphyries at Saindak PCD are mainly composed of tonalite. Here, we use geochemistry of apatite enclosed in plagioclase phenocrysts from the ore-forming tonalite to constrain the releasing and recharging processes of S and Cl in the underlying parental magma chamber during PCD mineralization. Although apatite inclusions have homogeneous intra-grain S and Cl compositions, there is significant inter-grain S and Cl variations in apatite inclusions located from core to rim in the hosting plagioclase. Such inter-grain S and Cl variation in apatites are coupled with the core-to-rim trends of An, FeO and Mg contents of the hosting plagioclase phenocryst. It indicates that the Saindak PCD likely formed by episodic injection of primitive magmas during the growth of an underlying magma chamber, rather than by one major injection or by addition of mafic melt derived from different source region. Each primitive melt injection introduced essential ore-forming materials such as S and Cl, which were rapidly and effectively released to the coexisting fluids, causing mineralization. Once primitive melt injection stops, signaling the end of growth of underlying magma chamber, mineralization will cease quickly although the hydrothermal system can still survive for a long time. However, the later released fluids are relatively depleted in ore-forming materials, and thus have lower capability to generate mineralization. Accordingly, predominant porphyry-type mineralizations occurred during the growth rather than waning stage of a magmatic system.

Assessment of magmatic fertility using pXRF on altered rocks from the Ordovician Macquarie Arc, New South Wales

This research presents a new method for assessing magmatic fertility using the portable X-ray fluorescence spectrometer (pXRF) in altered rocks from porphyry terranes. A study of Northparkes and global Cu--Au porphyries highlighted low Zr and Y trends associated with mineralising intrusions. Zircon saturation occurs at low Zr concentrations owing to low-temperature crystallisation depressed by high dissolved H2O in the melt. Yttrium depletion indicates hydrous magmatic conditions that cause hornblende to crystallise early. Previously geochemical differentiation of hydrous, potentially ore-forming (fertile) porphyries has relied on ratios of Sr/Y and Sr/MnO from geochemical and/or pXRF analysis of least altered rocks. Strontium is highly mobile and limits its use in altered terranes. Finding samples in porphyry terranes where Sr can be demonstrated to be retained is difficult given the ubiquity of alteration associated with porphyry emplacement. This precludes the widespread use of pXRF, as quantitative assessment of alteration is beyond the capabilities of pXRF technology. Using immobile elements overcomes issues with alteration and provides more reliable indicators of magmatic fertility. Using a systematic pXRF workflow, plots of Zr vs Y and Zr*Y vs Al2O3/TiO2 were successful in identifying 100% of the mineralising intrusions in the Northparkes intrusive complex and provide an indication of magmatic fertility and a proxy for melt hydration state. The ratios of Zr/Nb vs Y/Nb are insensitive to alteration including weathering and are indicative of evolution and dissolved H2O content of magmatic sources. Moving from bivariate plots to ratio plots shifts the data into a real-number space and provides a linear function to assess against magmatic fertility. Application of the fertility indicators to a regional dataset from New South Wales highlights areas of known porphyry mineralisation indicating that the method has the potential to be a useful indicator of fertility beyond the scope of the pilot study. KEY POINTS The fertility of porphyry intrusions is directly related to their dissolved H2O content and oxidation state. High dissolved H2O content in magmas causes early crystallisation of hornblende, and temperature depression. As a result Zr saturation occurs at low Zr concentrations that results in depletion of Y and the MREE in hydrous rocks compared with normal arc rocks. Magmatic fertility can be rapidly and cheaply assessed in the field using Zr and Y concentrations from pXRF analyses. The use of immobile elements facilitates magmatic fertility assessment in altered rock, which was previously impossible owing to the use of mobile elements Sr and MnO.

Flat-slab subduction and formation of “intraplate” porphyry deposits: Insights from the Jurassic high and low La/Yb ore-forming porphyries along the Qin-Hang belt, South China

The Middle-Late Jurassic porphyry deposits with different element associations in the interior of South China Block are over 1000 km away from subduction zone, which conflicts with the traditional belief that the porphyry Cu-Au-Mo deposits were formed in the convergent margin. However, their genesis and geodynamics are still under debate. To address the above issues, this paper reports new geochronological, mineralogical, elemental and isotopic data for the Yuanzhuding porphyry deposit from Qin-Hang belt, and compared with the published data of porphyry deposits in this belt. The Yuanzhuding porphyry deposit has zircon U-Pb and molybdenite Re-Os ages of 154.6–155.2 Ma and 156.3 Ma respectively, which suggest that it is contemporaneous with the Middle-Late Jurassic Dexing, Baoshan and Dabaoshan porphyry deposits in the belt. The Yuanzhuding and Dexing porphyries have adakitic signatures and Nd-Hf isotopes similar to the contemporaneous EMI-type-mantle-derived mafic rocks featuring high Nb, P2O5 and LILEs/HFSEs values, suggestive of their roots in EMI-type lithospheric mantle hybridized by basaltic-slab-derived melts and pelagic-sediments-derived fluids. Combined with the slightly lower Nd isotopes of Yuanzhuding porphyry than Dexing porphyry, the fractionation modeling suggests that they were formed via AFC and FC processes of basaltic magma respectively and that their adakitic signatures can be attributed to hornblende fractionation. In contrast, the Baoshan and Dabaoshan porphyries have high Yb contents, low (La/Yb)N ratios and Nd-Hf isotopes similar to the Proterozoic crust in the region. Mineral and whole-rock elemental plots reveal that they were stemmed from FC processes of partial melts of the Proterozoic crust, with involvement of voluminous and a little EMI-type-mantle-derived basaltic magma respectively. Integrated with the spatio-temporal evolution of Triassic-Jurassic magmatic, metamorphic, sedimentary, tectonic and structural records in the region, the petrogenesis of ore-forming porphyries indicates that the porphyry deposits were controlled by the northwestward flat-slab subduction and related slab break-off. The basaltic magma sourced from the EMI-type mantle had high Cu, H2O and S contents and oxidized signature, which are favorable for the formation of Dexing porphyry Cu-Au-Mo mineralization via differentiation and volatile exsolution in a closed magma chamber. Also, it can incorporate crustal materials via AFC processes, thus forming Yuanzhuding porphyry Cu-Mo mineralization. In addition, the thick crust caused by flat-slab subduction facilitated the input of voluminous and a little basaltic magma into crustal Baoshan and Dabaoshan porphyries, thus generating Pb-Zn-Cu-Mo and Mo-W-Pb-Zn mineralization, respectively. We, therefore, propose that the “intraplate” porphyry deposits with different element associations result from complicated crust-mantle interaction in the thick “intraplate” crust, and that their metallogenic elements will change from Cu-Au-Mo- to Mo-W-Pb-Zn-dominated mineralization with the transformation of magma sources from enriched-mantle- to continental-crust-dominated materials.

Zircon of the No. 782 deposit from the Great Xing’an Range in NE China: Implications for Nb-REE-Zr mineralization during magmatic-hydrothermal evolution

Niobium, rare earth elements (REE = lanthanides + Y), and zirconium, are widely used in high-tech applications. The Nb-REE-Zr mineralization in the No. 782 deposit of NE China developed in a fine-grained porphyry (porphyry) that occurs as a ring-shaped stock. The ore-forming porphyry is hosted in a coarse-grained granite (granite) that dominates the study area. Zircon is the only Zr-bearing mineral in both the porphyry and the granite. In contrast to the granite that hosts only magmatic zircon, both magmatic (type I) and metasomatic (type II) zircon crystals or aggregates are present in the porphyry. Most type I zircon occurs as individual crystals and are hosted mainly in interstitial quartz, with evident oscillatory zoning. Most type II zircon occurs as aggregates rather than individual crystals and exhibits vague zoning. Distribution of the type II zircon in the porphyry is heterogeneous, with great abundance in places where there are substantial albitization and snowball quartz crystals. Textural and chemical features of the type II zircon and of the associated albite and snowball quartz crystals suggest that all these minerals are metasomatic. Nb is hosted mainly by fergusonite-(Y) and, REE by fergusonite-(Y) and to a lesser extent, by bastnäsite-(Ce) and parisite-(Ce), where parisite-(Ce) occurs as a replacement of bastnäsite-(Ce). The Nb-REE mineralization occurs either as a replacement of the type II zircon or in chlorite-/quartz-dominant veinlets that postdated both the type I and the type II zircon. Most type I zircon crystals in the porphyry were intact from the replacement or alteration by the postdated Nb-REE minerals, possibly because the type I crystals were encased earlier by other magmatic minerals. The chronological and oxygen isotopic data of zircon suggest that the porphyry (453.8 ± 3.1 Ma) is younger than the granite (487.0 ± 4.0 Ma), and that the two granitic phases derived from different magma chambers. The subtle age difference between magmatic type I and metasomatic type II zircon cannot be shown by SHRIMP U-Pb data of zircon. The mineral assemblages associated with type II zircon and the chronological data suggest that the medium from which type II zircon precipitated is likely an internally-derived hydrosilicate liquid (HSL) that lies compositionally in between a silicate melt and a hydrothermal fluid during the magmatic-hydrothermal transition. Textural features of the Nb-REE minerals, combined with the fact that Nb-REE mineralization developed only inside the porphyry, but not in surrounding country rocks (i.e. the granite) suggest that the Nb-REE mineralization originated from an aqueous fluid that was constrained to the porphyry.

Magmatic-hydrothermal evolution of the Yuanzhuding porphyry Cu-Mo deposit, South China: Insights from mica and quartz geochemistry

The Yuanzhuding porphyry Cu-Mo deposit, located in the southern section of the Qinzhou-Hangzhou belt, is characterized by successive generations of quartz-bearing veins with complicated crosscutting relationships. Multiple analytical methods have been applied to different mica types and quartz-bearing veins to reveal the evolution history of the Yuanzhuding porphyry system.Four types of micas have been identified in the Yuanzhuding porphyry Cu-Mo deposit, including magmatic biotite (Bt-I), metamorphic biotite (Bt-II), hydrothermal biotite (Bt-III), and hydrothermal sericite (Ser), based on detailed petrological observations. All three types of biotite belong to the Mg-biotite group but can be compositionally distinguished, whereas Ser lies in the transition zone between Li-phengite and Li-muscovite. Generally decreasing crystallization temperatures with increasing distances from the center of the porphyry have been revealed in Bt-II and Bt-III based on their compositional variations, documenting the outward cooling trends of the Yuanzhuding porphyry system.Early quartz-bearing veins with potassic alteration are characterized by granular quartz with homogeneous CL textures, in combination with their wavy vein walls which indicate the host rock was ductile during vein formation, we infer that they were formed under dominantly lithostatic pressure. Late quartz-bearing veins with sericitic alteration are typically euhedral quartz with growth zones of oscillating CL intensity, and they are inferred to be formed under dominantly hydrostatic pressure. Intersection of fluid inclusion isochores with Ti-in-quartz isopleths revealed that early quartz veins with potassic alteration located close to the ore-forming porphyry were formed at up to ∼575 °C. Late quartz veins with sericitic alteration were formed at ∼310 °C.Formation depth of the Yuanzhuding porphyry deposit is inferred to be ∼5.3 km. Hydrogen and oxygen isotopes of Bt-III and quartz indicate that the ore-forming fluid was mainly magmatic fluid. The fluid pressure was high enough to remain stable in single phase through the entire fluid evolution history. All fluid inclusions are vapor-liquid inclusions. The fluid salinity and the Cl fugacity remained constantly low, which may have limited the efficiency of Cu extraction and transportation.The hornfels host significant amounts of Bt-II and may have served as a reservoir of ferrous iron to reduce the oxidized ore-forming fluid and generate reduced sulfur. Effective metal precipitation occurred in the Yuanzhuding porphyry system owing to the coexistence of the generation of reduced S2- and the drop of metal solubility caused by the decrease of fluid temperature.

Petrogenesis and mineralization of the porphyry and skarn mineralization-related Husite intrusion in the Boluokenu metallogenic belt, Western Tianshan Orogenic Belt, NW China

Many porphyry and skarn Cu (Fe, Mo, Au, Zn) deposits have been discovered along the western Tianshan Orogenic Belt within the Central Asian Orogenic Belt. Generally, these Cu polymetallic deposits are considered to be linked to porphyries that formed in an arc setting, e.g., in a continental magmatic arc and island magmatic arc, but an agreement on their source region has not been reached. The Husite granitic complex and associated Kekesala skarn Fe-Cu deposit from the Boluokenu metallogenic belt provide significant insights into a comprehensive study of the ore-forming porphyries in Boluokenu. The ore-forming porphyries are mainly granodiorites and belong to calc-alkaline and high-K calc-alkaline series. LA-ICP-MS zircon U-Pb dating on the granodiorites yields ages of 368 Ma. The trace element compositions of the granodiorites are characterized by enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE), depletion in high field strength elements (HFSE) and heavy rare earth elements (HREE), and slightly negative Eu anomalies and are similar to rocks that form in a continental arc setting. These rocks also show large variations in their εHf (t) (+4.1 to +9.7) and Mg# (38–69) values, alongside low Ba/Th (4–230) and high Th/Yb (3.77–7.41) and Th/La (0.21–0.71) ratios, suggesting that the granodiorite magma originated from the partial melting of mafic lower crust with a mantle wedge component and involved signals of subduction-zone sediments and fluids. The Husite mafic microgranular enclaves (MMEs) exhibit high Mg# (50–62), MgO (3.25–6.93 wt%) and low SiO2 (48.8–59.5%) contents, with positive εHf(t) from +3.3 to +7.4. The zircon LA-ICP-MS U-Pb ages of the MMEs (373 Ma) indicate nearly coeval formation between the MMEs and HMPs during the Late Devonian. These features with the petrological and mineralogical characteristics (such as needle-like apatite and disequilibrium feldspar xenocrysts) suggest that the Husite MMEs formed from the mixing of felsic magma from the partial melting of mafic lower crust and mafic magma that was derived from mantle wedge, rather than restite, xenoliths or fractional crystallization of magma. Thus, the skarn Fe-Cu mineralization were probably related to hybrid intermediate-acid magma that is characterized by high water and oxygen contents and high sulfur fugacity. It can be inferred that the response to ongoing slab roll-back during the Junggar plate’s subduction beneath the Yili Block, including the underplating of mantle-derived mafic magmas from the metasomatism of subduction-zone melts, led to the partial melting of the mafic lower crust to form felsic magma. Then, the continuous reaction between the mafic and felsic magmas further increased the Fe-Cu concentrations and was closely related to the subsequent Fe-Cu mineralization.

Physicochemical Processes in the Magma Chamber under the Black Mountain Porphyry Cu-Au Deposit, Philippines: Insights from Mineral Chemistry and Implications for Mineralization

The Black Mountain porphyry Cu-Au deposit in the Baguio district, Northern Luzon (Philippines) is associated with late Miocene to Pliocene intrusive rocks that retain primary crystallization signatures, providing an excellent natural laboratory to study the details of magma chamber processes. Each igneous phase contains disequilibrium textures, including resorptive textures in plagioclase and distinct core rim textures in amphibole. High crystallization temperatures and pressures in felsic (~880°C and ~2.8 kbar) and mafic (~1,030°C and ~7.7 kbar) rocks recorded by amphibole phenocrysts reflect crystallization in the magma chamber. Amphibole in felsic rocks aged 6.39 ± 0.49 to 2.83 ± 0.23 Ma have similar crystallization temperatures and pressures, suggesting a long-lived and hot felsic magma chamber under the Black Mountain deposit. Four independent oxybarometers, including amphibole, coexisting magnetite-ilmenite, magmatic anhydrite, and zircon, consistently show a high oxidation state of >NNO+1 for the felsic magma and ~NNO+0.7 for the mafic magma. Element profiles across phenocrystic plagioclase display repeated oscillatory variations of Ca, Fe, and Sr contents with large anorthite variations and a positive correlation between CaO and FeO. The amphibole is classified as group 1 Mg hornblende to tschermakite-hornblende in felsic rocks and group 2 Mg hastingsite in mafic rocks. The group 1 amphiboles show a variety of major and trace element variations from core to rim in disequilibrated grains and transitional compositions to group 2 Mg hastingsite. Both mineral textures and chemistry recorded by phenocrysts of plagioclase and amphibole indicate repeated mafic magma recharge in the felsic magma chamber. The common occurrence of rounded or elongated sulfide inclusions in the megacrystic amphibole from the mafic rocks (2.81 ± 0.15 Ma) suggests a greater enrichment of Cu in mafic rocks relative to the ore-forming porphyries (2.91–2.83 Ma). This is also consistent with higher Cu contents (3.4 ± 0.9 ppm) in the megacrystic amphibole from the mafic rocks relative to amphibole in felsic rocks (1.0 ± 0.2 ppm) and may indicate up to 3.4 times higher Cu in the mafic magma relative to felsic magma. Therefore, large-scale mafic magma recharge (particularly at ~2.8 Ma), likely introduced ore-forming metals to the long-lived felsic magma chamber, further contributing to the fertility of the felsic rocks and ultimately to Cu-Au mineralization at the Black Mountain deposit.