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Article

Petrogenesis and Metallogenic Significance of the Demingding Mo-Cu Porphyry Deposit in the Gangdese Belt, Xizang: Insights from U-Pb and Re-Os Geochronology and Geochemistry

by
Sudong Shi
1,2,
Shuyuan Chen
2,
Sangjiancuo Luo
2,
Huan Ren
3 and
Xiaojia Jiang
1,*
1
State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Resources, Collaborative Innovation Center for Exploration of Strategic Mineral Resources, China University of Geosciences, Wuhan 430074, China
2
Tibet Xianglong Copper Industry Limited Company, Lhasa 850000, China
3
State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(12), 1232; https://doi.org/10.3390/min14121232
Submission received: 5 November 2024 / Revised: 18 November 2024 / Accepted: 29 November 2024 / Published: 3 December 2024
(This article belongs to the Special Issue Mineralogy of Porphyry Systems: Genesis, Exploration, and Applications)
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Abstract

:
The 1500 km-long Gangdese magmatic belt is a crucial region for copper polymetallic mineralization, offering valuable insights into collisional porphyry copper systems. This study focuses on the Demingding deposit, a newly identified occurrence of molybdenum–copper (Mo-Cu) mineralization within the eastern segment of the belt. While the mineralization age, magmatic characteristics, and tectonic context are still under investigation, we examine the deposit’s petrology, zircon U-Pb geochronology, whole-rock chemistry, and Re-Os isotopic data. The Demingding deposit exhibits a typical alteration zoning, transitioning from an inner potassic zone to an outer propylitic zone, which is significantly overprinted by phyllic alteration closely associated with Mo and Cu mineralization. Zircon U-Pb dating of the ore-forming monzogranite porphyries reveals crystallization ages ranging from 21 to 19 Ma, which is indistinguishable within error from the mean Re-Os age of 21.3 ± 0.4 Ma for Mo veins and veinlets hosted by these porphyries. This alignment suggests a late Miocene magmatic event characterized by Mo-dominated mineralization, coinciding with the continuous thickening of the continental crust during the collision of the Indian and Asian continents. The ore-forming porphyries range in composition from granodiorite to monzogranite and are classified as high-K calc-alkaline with adakite-like features, primarily resulting from the partial melting of subduction-modified thickened mafic lower crust. Notably, the ore-forming porphyries exhibit higher fO2 and H2O levels than barren porphyries in this area during crustal thickening, highlighting the significant contributions of hydrous and oxidized fluids from their source to the Mo-Cu mineralization process. Regional data indicate that the Gangdese porphyry metallogenic belt experienced concentrated Cu-Mo mineralization between 17 and 13 Ma. The formation of Mo-dominated deposits such as Demingding and Tangbula in the eastern segment of the belt, with slightly older ages around 20 Ma, underscores the presence of a significant porphyry Mo metallogenic event during this critical post-collision mineralization period.

Graphical Abstract

1. Introduction

Porphyry mineralization system is responsible for 75% of the world’s copper (Cu), 50% of molybdenum (Mo), and significant amounts of gold, making it a major focus of scientific research due to its economic importance [1]. A classic theory of porphyry mineralization has emerged from numerous studies of the Andean Cu polymetallic metallogenic belt. This theory encompasses porphyry deposits, skarn-type deposits in contact with carbonate strata, and epithermal deposits at the tops of porphyry bodies [1,2]. This mineralization model has effectively guided global mineral exploration and has yielded significant success. Traditional theories posit that porphyry Cu deposits typically form in oceanic island arc and continental arc environments, with their genesis linked to the subduction of oceanic lithosphere [3]. However, recent studies have demonstrated that such deposits can also arise in collisional orogenic environments. Notable examples include the Miocene porphyry mineralization belt in southeastern Kerman, Iran [4,5], as well as the Gangdese and Yulong porphyry Cu belts formed in the collision environment of the Tibetan Plateau [6,7,8,9,10,11,12,13].
The Lhasa terrane, situated between the Bangong–Nujiang suture zone and the Yarlung Tsangpo suture zone (Figure 1a), offers a comprehensive record of significant geological events, including the southward subduction of the Central Tethys oceanic lithosphere and the northward subduction of the Neo-Tethys oceanic lithosphere during the India–Asia continental collision [14,15,16,17]. Over the past two decades, significant advancements have been made in mineral exploration, particularly in the eastern segment of this belt, resulting in the discovery of numerous large to giant porphyry deposits, including Qulong, Jiama, Xiongcun, Zhunuo, Chongjiang, Gangjiang, Demingding, and Bangpu [12,13]. Previous research has systematically examined post-collisional porphyry Cu deposits, establishing fundamental theories regarding their formation [1,9,18,19,20]. However, challenges persist in understanding the magmatic origin and evolution of the mineralizing porphyry as well as the mechanisms driving copper enrichment.
The Demingding Mo-Cu deposit is a recently discovered Mo-dominated deposit located in the eastern segment of the Lhasa terrane. Due to its late discovery, systematic studies on the genesis of its mineralization have yet to be conducted, and the lack of geochronological constraints on the formation and mineralization ages has significantly limited research efforts. This paper comprehensively analyzes the major and trace elements in representative magmatic rocks from this deposit. It also employs zircon LA-ICP-MS U-Pb dating and molybdenite Re-Os dating to establish geochronological constraints on the age of magmatism and mineralization. By discussing the genesis and significance of mineralization dynamics, this study aims to provide valuable insights for mineral exploration efforts in the eastern segment of the Gangdese metallogenic belt.

2. Geological Setting

The Gangdese magmatic belt is located in the central-southern part of the Lhasa terrane (Figure 1b). Research indicates that the Lhasa terrane formed a crustal basement of metamorphic rock series during the Precambrian, with the Longger-Gongbujiangda magmatic arc beginning to develop in the Late Triassic. From the Late Triassic to Early Cretaceous, the Lhasa terrane was influenced by the southward subduction of the Central Tethys oceanic lithosphere and the northward subduction of the New-Tethys oceanic lithosphere, resulting in a complex multi-island arc basin system [15,22]. In the Early Cretaceous, as the Central Tethys Ocean declined, an arc–arc soft collision occurred between the Lhasa and Qiangtang terrane [15,23,24]. The New-Tethys Ocean basin continued its northward subduction, leading to the accretion of the southern Gangdese volcanic magmatic arc on the southern side of the Longger-Nienqing Tanggula composite ancient island arc belt. From the Late Cretaceous to the Paleocene, the closure of the New-Tethys Ocean resulted in the merging of the Himalayan orogenic belt with the southern margin of the Asian continent, causing crustal shortening and thickening. This period witnessed main collision (65–41 Ma), late collision (40–26 Ma), and post-collision (25–0 Ma) orogenic processes, each producing distinctive mineral deposit assemblages characteristic of those stages [25,26,27,28].
The Gangdese magmatic belt is primarily known for its porphyry-type, skarn-type, and epithermal deposits, which are widely considered products of post-collisional tectonics at 17–13 Ma. It is recognized as one of the richest Cu provinces within the Tethyan-Himalayan metallogenic domain, hosting some of China’s largest porphyry Cu deposits (Figure 1b). Porphyry deposits are mainly characterized by Cu and Cu-Mo combinations, with fewer Cu-Au combinations. Five distinct phases of mineralization have been identified: ~213 Ma (Luerma, [29]), 173–165 Ma (Xiongcun; [30]), ~45 Ma (Jiru; [31]), ~30 Ma (Mingze-Chengba; [32]), and 17–13 Ma (Qulong, Jiama, Zhuno; [7,16,33,34,35]). Notably, these deposits are primarily concentrated in the eastern segment of the Gangdese belt, with the distribution of mineralization elements transitioning from Mo-Cu-Pb-Zn in the north to Cu-Mo and finally to Cu-W-Mo in the south [16,31].
The Demingding porphyry Mo-Cu deposit, located in the eastern part of the Gangdese magmatic belt (Figure 1), boasts proven reserves of 0.5 million tons (Mt) of molybdenum with a grade of 0.14%, and over 0.5 Mt of copper with an average grade of 0.26% [36] (Figure 1b). The magmatic units at the Demingding deposit consist of ore-forming and barren monzogranite porphyries, which intrude into Jurassic volcanic and intrusive rocks, specifically the Yeba Formation, rhyolite porphyry, and volcanic breccia (Figure 2). The monzogranite porphyries are exposed only in a limited surface area (Figure 3); however, its concealed portion exhibits a lens-like texture oriented east–west, measuring approximately 2.1 km in length and 100 to 400 m in width (Figure 2b,c). This unit is characterized by a porphyritic texture, composed of quartz (20–35 wt%), plagioclase (25–35 wt%), K-feldspar (20–30 wt%), and biotite (15–25 wt%), with minor amounts of titanite, zircon, and apatite (Figure 4). The plagioclase phenocrysts, measuring 2–3 mm, display subhedral and euhedral forms with notable twinning and zoning; some crystals show slight alteration to epidote and sericite (Figure 4). Biotite grains predominantly exhibit subhedral to anhedral textures, with most having altered to sericite, chlorite, and muscovite (Figure 4b).
Drill core logging has identified three Mo-Cu bodies and one Mo body at the Demingding deposit, with mineralization predominantly occurring in quartz veins and only rarely disseminated in the monzogranite porphyries and wall rocks (Figure 2). The ore minerals present are molybdenite and chalcopyrite (Figure 3e–h). The main alteration types observed are potassic and propylitic alterations, both of which are overprinted by sericite alteration (Figure 3e–h and Figure 5). A limited number of potassic alterations have been identified as magnetite + quartz + biotite + Cu-Fe sulfides (Figure 5a–d), with biotite having altered to chlorite (Figure 5c,d). The phyllic alteration assemblages primarily consist of sericite and quartz (Figure 5f). Propylitic alteration is pervasive in the biotite monzonite and granitoid porphyry, characterized by the presence of epidote and chlorite, often associated with vein-bearing minerals or replaced plagioclase phenocrysts (Figure 5g). The mineralization of molybdenite and chalcopyrite is closely associated with both propylitic and phyllic alterations (Figure 5h–l).

3. Sampling and Analytical Methods

3.1. Sampling

Samples were collected from the Demingding deposit, including rhyolite porphyry (samples 26006, 25053, D05016), ore-forming (samples 25028, 9011, ZK005-596.35) and barren monzogranite porphyries (sample DMD17), and molybdenite samples (26023, 25031, ZK003-161.8). Samples 25053, 25028, ZK005-596.35, and DMD17 were used for zircon dating and trace element analysis of zircon. The Re-Os dating of three molybdenite samples primarily came from the western section of the Demingding mining area. Molybdenite generally exhibits anhedral, fine-grained flaky structure, with flake sizes varying, most being less than 1 mm, and displaying a metallic luster. Aggregates often appear in bundles or radiating forms. The analytical work for zircon and molybdenite was mainly conducted at the Hebei Langfang Regional Institute of Geology and Mineral Resources, China.

3.2. Whole-Rock Major and Trace Elements Analyses

Whole-rock major and trace element contents of the Demingding rhyolite porphyries and monzogranite porphyries were determined using X-ray fluorescence (XRF), atomic absorption spectrometry (AAS), and inductively coupled plasma mass spectrometry (ICP-MS) at the Wuhan Sample Solution Analytical Technology Co., Ltd. in Wuhan, China. Major elements were analyzed via XRF and AAS, with an uncertainty of less than 5%. Trace elements were measured using ICP-MS (Agilent 7500a with a shield torch), while rare earth elements (REEs) were separated using cation-exchange techniques, resulting in uncertainties ranging from 1% to 3%. The analytical results for standards BHVO-1 (basalt), BCR-2 (basalt), and AGV-1 (andesite) indicated that trace element precision typically exceeded 10%. For further details on these methods, see Liu et al. (2008) and Rudnick et al. (2004) [38,39]. The major and trace element compositions of Demingding rhyolite porphyries and monzogranite porphyries are presented in Table S1.

3.3. LA–ICP–MS Zircon U–Pb Isotope Analyses

In situ U–Pb dating of zircon was performed using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at Wuhan Sample Solution Analytical Technology Co., Ltd. in Wuhan, China. The operating conditions and data reduction methods were outlined by Chen et al. (2022a, 2022b) [40,41]. Zircon 91,500 and NIST610 glass were utilized as external standards for U–Pb dating and trace element calibration, respectively. Off-line selection and integration of background and analyte signals, along with time-drift correction and quantitative calibration for trace elements and U–Pb analysis, were conducted using ICPMSDataCal [39]. The processed data were analyzed with IsoplotR, developed by Vermeesch (2018) [42]. The oxygen fugacity (ΔFMQ) and H2O content of the Demingding rhyolite porphyries and monzogranite porphyries were determined using the zircon oxybarometer-hygrometer methods described by Loucks et al. (2020) and Ge et al. (2023) [43,44]. The zircon U–Pb isotope data are presented in Table S2, while trace element data, ΔFMQ, and H2O calculations are summarized in Table S3.

3.4. Molybdenite Re–Os Isotope Analyses

Three molybdenite samples were collected and isolated from disseminated quartz-molybdenite within granite porphyries. Re–Os isotope analyses were performed at the Re–Os Laboratory of the National Research Center for Geoanalysis in Beijing, China. The Re isotope ratio was determined using a TJA X-series inductively coupled plasma mass spectrometer (ICP-MS), following the analytical procedures outlined by Du et al. (2004) [45]. The Re–Os isochron age was calculated using the least-squares method, implemented in IsoplotR [42].

4. Results

4.1. Zircon Morphology, U-Pb Age, Oxygen Fugacity, and H2O Content

The cathodoluminescence (CL) images indicate that zircons from various rock types are euhedral to subhedral, exhibiting length-to-width ratios between 1:1 and 2:1, and are characterized as prismatic or short prismatic (Figure 6). These zircons are transparent to translucent, displaying gray or light grayish-white colors, with some grains appearing darker, likely due to higher U and Th content [46]. All zircons show distinct oscillatory zoning, a growth rhythm typical of magmatic zircons [47]. U-Pb dating was performed on zircon surfaces that were free of cracks and inclusions, and exhibited clear zoning, as illustrated in Figure 6.
The U-Pb dating results for zircons from rhyolite porphyries (25,053) are summarized in Table S2. These zircons display low U concentrations (132–661 ppm) and generally low Th levels (85.8–626 ppm), with one exception having a high Th content (1330 ppm), leading to moderate Th/U ratios (0.48–1.60, excluding outliers). The 206Pb/238U ages range from 185 ± 4 to 213 ± 5 Ma, with a weighted mean age and concordia age of 195 ± 2 Ma (Figure 7a and Table S2). The chondrite-normalized rare earth element (REE) patterns of the zircon grains reveal low light REE (LREE) abundances (LREE = 9.94–58.7 ppm), relatively high and nearly flat heavy REE (HREE) patterns (662–2748 ppm), and pronounced negative Eu anomalies (Eu/Eu* = 0.21–0.47, average 0.33) (Figure 8; Table S3). They are located in a relatively low position on the (Ce/Nd)/Y vs. 10,000(Eu/Eu)/Y (Figure 8a) and (Ce/Nd)/Y vs. Eu/Eu* (Figure 8b) diagrams. Calculations based on whole-rock trace element averages and zircon trace element contents [43,44] (Table S3) suggest melt magma temperatures of 690–804 °C (average 744 °C), Ce4+/Ce3+ ratios ranging from 7.93 to 233 (average 91.6), oxygen fugacity (∆FMQ) values of −0.63 to 2.05 (average 0.94), and H2O contents of 7.10–9.95 wt.% (average 8.45 wt.%) (Figure 9; Table S3).
Zircons from the ore-forming monzogranite porphyries (samples 25,028 and ZK005-596.35) generally exhibit high and variable U concentrations (238–1921 ppm, average 718 ppm), and Th concentrations (237–2588 ppm, average 690 ppm), with Th/U ratios of 0.53–1.67. The 206Pb/238U ages range from 19.0 ± 0.4 to 22.1 ± 0.8 Ma, with a weighted mean age of 20.5 ± 0.1 Ma (MSWD = 2.5) for sample 25,028 (Figure 7b and Table S2) and a weighted mean of 19.8 ± 0.2 Ma (MSWD = 2.9) for sample ZK005-596.35 (Figure 7c and Table S2). These zircons also demonstrate relatively high HREE abundances (HREE = 267–1313 ppm, average 552 ppm), steep distribution patterns (LREE/HREE = 0.05–0.35), and negative Eu anomalies (Eu/Eu* = 0.23–1.01, average 0.49) (Figure 8; Table S3). They are located at a relatively high position on the (Ce/Nd)/Y vs. 10,000(Eu/Eu)/Y (Figure 8a) and (Ce/Nd)/Y vs. Eu/Eu* (Figure 8b) diagrams. The calculated melt temperatures range from 548 to 765 °C (average 668 °C), Ce4+/Ce3+ ratios vary widely (25.3–459, average 215), ∆FMQ values are relatively high (1.17–3.62, average 2.22), and H2O content is elevated (8.02–15.0 wt.%, average 10.8 wt.%) (Figure 9).
Twenty-one U-Pb isotopic analyses of zircons from the monzogranite porphyries (DMD17) in the western section of the Demingding deposit show that the zircon grains have moderate Th (398–992 ppm), U (497–1492 ppm), and Th/U ratios (0.36–1.16). The corresponding 206Pb/238U ages range from 14.8 ± 0.5 to 16.4 ± 0.4 Ma, with a weighted mean of 15.7 ± 0.2 Ma (MSWD = 1.7) (Figure 7b and Table S2). In their chondrite-normalized REE patterns, these zircons exhibit relatively high HREE abundances (191–608 ppm, average 330 ppm) and steep distribution curves (LREE/HREE = 0.07–0.39), alongside noticeable negative Eu anomalies (0.38–0.73, average 0.49) (Table S3). The calculated melt temperatures are slightly higher, ranging from 592 to 817 °C (average 700 °C), with Ce4+/Ce3+ ratios remaining wide but generally lower (15.7–254, average 111), ∆FMQ values (0.19–3.12, average 1.86), and H2O content (6.88–12.3 wt.%, average 9.47) being somewhat lower compared to ore-bearing rocks (Figure 9; Table S3).

4.2. Whole-Rock Major and Trace Element Compositions

The major and trace elements of Demingding rhyolite porphyries, along with ore-forming and barren monzogranite porphyries, are summarized in Table S1. Combining these findings with previous studies, the rhyolite porphyries are characterized by high SiO2 content (67.4–80.9 wt.%), low Al2O3 (11.8–14.5 wt.%), and variable but generally low CaO (0.08–3.90 wt.%, average 1.80 wt.%). In the TAS diagram for intrusive rocks, the samples predominantly fall within the granite field (Figure 10a), exhibiting distinct calc-alkaline and andesite–dacite–rhyolite characteristics (Figure 10b–d). The rhyolite samples display uniform chondrite-normalized rare earth element (REE) distribution patterns (Figure 11a), featuring relatively high ΣREE values (72.8–160 ppm), low LREE/HREE ratios (3.16–10.94), and (La/Yb)N ratios (2.47–12.91), with negligible negative Eu anomalies (0.66–0.96) and Ce anomalies (0.9–1.06). The primitive mantle-normalized trace element spider diagrams (Figure 11b) reveal significant enrichment in large ion lithophile elements (LILEs, such as Rb, Ba, Th) and depletion in high field strength elements (HFSEs, such as Nb, Ta, Hf).
The ore-forming and barren monzogranite porphyries exhibit similar whole-rock analytical results, including slightly lower SiO2 content (65.0–70.3 wt.%), higher Al2O3 (13.2–15.5 wt.%), and generally elevated CaO levels (1.30–2.90 wt.%, average 2.21 wt.%) (Table S1). In the TAS diagram, these samples primarily occupy the quartz-monzonite and granodiorite fields (Figure 10a), demonstrating distinct high-K calc-alkaline and adakite-like characteristics (Figure 10b–d). The chondrite-normalized REE distribution patterns for the monzogranite samples (Figure 11a) resemble those of the rhyolite, with relatively lower HREE values (5.30–9.90 ppm), significantly higher LREE/HREE ratios (6.14–19.97), and (La/Yb)N ratios (6.10–40.9). The Eu anomalies (0.60–1.06) and Ce anomalies (0.97–1.09) are also negligible. The primitive mantle-normalized trace element spider diagrams (Figure 11b) indicate that, in comparison to the rhyolite porphyries, the monzogranite porphyries samples exhibit higher concentrations of large ion lithophile elements and slightly lower levels of high field strength elements.

4.3. Molybdenite Re–Os Age

The Re content in three molybdenite samples from the porphyries ranges from 59 to 836 ppm, with Re-Os model ages for the molybdenite falling between 20.1 and 21.9 Ma (Table S4). These Re-Os values can be approximately fitted to an isochron age of 21.5 ± 1.1 Ma (Figure 12), which serves as the mineralization age of the deposit. This age aligns closely with the magmatic activity period of the ore-forming porphyry, estimated at around 20 Ma, falling well within the error range.

5. Discussion

5.1. Age of Magmatism and Mineralization

U-Pb isotopic dating of various magmatic units in the Demingding deposit reveals that magmatic pulses in this region can be categorized into three distinct stages: approximately 195 Ma, 20 Ma, and 16 Ma. Notably, the 20 Ma age is statistically indistinguishable from the mean Re-Os age of 21.3 ± 0.4 Ma for Mo veins and veinlets associated with these porphyries. This correlation suggests a late Miocene magmatic event characterized by Mo-dominated mineralization in the eastern segment of the Gangdese magmatic belt. The tectonic evolution of the Gangdese magmatic belt can also be divided into three primary stages, reflecting the dynamics of the Tethyan tectonic domain: (1) subduction of the Neo-Tethys oceanic lithosphere during the Late Triassic to Middle Jurassic (213–170 Ma), (2) the early Eocene collision between the Indian and Eurasian continents (51–49 Ma), and (3) the late Oligocene to Miocene period during the Indo–Asian continental collision and subsequent post-collision stage (23–12 Ma) [8,16,21,25,29,52,53]. The rhyolite porphyries with an age of 195 Ma in the Demingding deposit reflect their formation during the subduction of the Neo-Tethys oceanic lithosphere. In the Gangdese magmatic belt, there are a few subduction-related porphyry deposits, such as Xiongcun and Luerma [29,52], which are associated with the northward subduction of New-Tethys oceanic lithosphere.
The ore-forming porphyries as well as a number of samples of barren porphyries exhibit crystallization ages ranging from 19 to 20 Ma, aligning with an average molybdenite Re-Os age of 21 Ma. Hou et al. (2015a) [16] propose a novel theoretical framework for continental collision metallogenesis, comprising three key processes: (1) main collision convergence metallogenesis (65–41 Ma), (2) late collision transformation metallogenesis (40–26 Ma), and (3) post-collision extension metallogenesis (25–0 Ma). This framework suggests that different types of mineral deposits can form at various stages of continental collision, with porphyry deposits primarily developing during the post-collision extension phase [6,8,9,10,11]. This correlation highlights the significance of a late Miocene magmatic event characterized by Mo-dominated mineralization in the Demingding deposit, which occurred following the post-collision of the Indian and Asian continents.
When examining the mineralization ages of other porphyry deposits within the Gangdese belt, we find that the Demingding and Tangbula deposits [54], which exhibit Mo-dominated features in the easternmost section, show ages of approximately 20 Ma. In contrast, the Qulong copper deposit, located in the central part of the belt, has ages ranging from 17 to 15 Ma [31,55], while the Jiama deposit is dated at 15–14 Ma [56], and the Chongjiang deposit spans from 14 to 12 Ma [57,58]. The Zhunuo-Beimulang deposit in the westernmost section falls between 14 and 13 Ma [59]. This trend indicates progressively younger mineralization ages from east to west across the Gangdese metallogenic belt, likely influenced by variations in the angle of subduction of the Indian continent and slab tearing and across the eastern and western segments of the belt [8,60]. Regional data indicate that the Gangdese porphyry metallogenic belt experienced concentrated Cu-Mo mineralization between 17 and 13 Ma. The formation of Mo-dominated deposits such as Demingding and Tangbula in the eastern segment of the belt, with slightly older ages around 20 Ma, underscores the presence of a significant porphyry Mo metallogenic event during this critical post-collision mineralization period.

5.2. Petrogenesis of Ore-Forming Porphyries

The Demingding ore-forming porphyries are characterized by high SiO2, Al2O3, and K2O content, a Sr/Y ratio typically exceeding 35, depletions in HREEs and Y, pronounced fractionation of LREE and HREE, and an absence of a significant Eu anomaly. These features suggest a high-K affinity with adakitic geochemical traits [61,62]. Proposed environments for the formation of adakitic rocks mainly include: (1) melting of subducted oceanic crust [61]; (2) high-pressure crystallization differentiation of water-rich basalt [63]; and (3) melting of the thickened lower crust [64,65,66,67,68,69].
There is growing evidence that the India and Eurasian plates entered a phase of collisional thickening around 50 Ma [70,71], transitioning into a post-collisional environment [16]. By 20 Ma, the continental crust in the Lhasa terrane had already thickened by around 50 km [72]. Consequently, while the ore-forming porphyries exhibit compositional characteristics similar to adakitic melts, it is unlikely they originated from the melting of subducted oceanic plates. Additionally, the trace element profiles of these ore-forming porphyries markedly differ from those of rhyolite porphyries associated with the subduction of the New-Tethys oceanic lithosphere. Subduction-related rhyolite porphyries display high Y and low Sr/Y ratios (<40) as well as (La/Yb)N ratios (<20), indicating the absence of garnet and other HREE-bearing minerals (e.g., amphibole) in the source rocks. Partial melting of basaltic amphibolites at intermediate pressures could produce melts in equilibrium with residues devoid of garnet, which is consistent with melting occurring in the mantle wedge beneath a crust of normal thickness (35–40 km) [73]. This aligns with the observation that rhyolite porphyries predominantly underwent amphibole fractional crystallization (Figure 13) within a crust of normal thickness, further supporting the conclusion that the ore-forming porphyries are not a direct product of partial melting of subducting oceanic crust.
Field investigations indicate that the ore-forming porphyry at the Demingding deposit occurs predominantly as isolated small intrusions and dykes, with limited spatial distribution. These intrusions mainly cut into 190 Ma rhyolite porphyries, and no contemporaneous mafic rocks are associated with them. Moreover, compositional analyses in SiO2 and related element diagrams show lack of evolutionary trends associated with fractional crystallization. Thus, this rock suite cannot be the result of contamination from basaltic magma. Furthermore, if the adakitic melt were derived from high-pressure residual separation of water-rich basalt containing garnet, one would expect increasing La/Yb, Dy/Yb, and Sr/Y ratios with rising SiO2 content [63]. However, these element ratios in the Demingding ore-forming porphyry remain largely unchanged, reinforcing the conclusion that it did not form from basaltic magma through high-pressure crystallization differentiation processes.
The Demingding deposit is likely formed through the partial melting of a thickened lower crust. The ore-forming porphyry exhibits notable depletions in HREE, Y, and Yb, along with high field strength elements such as Nb and Ta. These characteristics suggest the presence of residual garnet in the magma source or the differentiation of the magma chamber with garnet crystals and stable rutile (Figure 13). Xiong et al. (2005) demonstrated through experimental geochemical simulations that rutile is stable only at pressures exceeding 1.5 GPa, indicating that the source of adakitic rocks is located at depths greater than approximately 50 km [74]. Regionally, the collision between the Indian and Eurasian plates has contributed to crustal thickening. Zhu et al. (2017) analyzed whole-rock La/Yb ratios of the Gangdese magmatic arc intrusions to assess temporal and spatial variations in crustal thickness, suggesting that the crust reached depths of approximately 50–58 km during the 55–45 Ma period [72]. Geophysical studies reveal that the current crustal thickness of the Lhasa block is approximately 70–80 km. This evidence indicates that the crust in the Gangdese region has been continuously thickening since the Paleogene, driven by the ongoing collision and compression between the Indian and Eurasian plates. Additionally, the Sr/Y and La/Yb ratios of granitic rocks formed by crustal melting are commonly used to reflect the depth of magma sources. The mineralized porphyry has a Sr/Y ratio around 40, while the 16 Ma porphyry exhibits Sr/Y values ranging from 40 to 120. According to the crustal source thickness calculation formula proposed by Profeta et al. (2015) [75], there is a significant increase in the average depth of the Gangdese magmatic belt from 20 Ma to 16 Ma (Figure 14). This gradual thickening is likely a result of long-term continental collision. Notably, the Demingding porphyry deposit is primarily enriched in Mo, aligning with Mao et al. (2011) [76], who found that magmas remelted from the lower crust are more conducive to forming porphyry Mo deposits than Cu deposits.

5.3. Implication for Mineralization Model

Arc magma generated from the subduction of the oceanic crust is characterized by high fO2 and volatile components such as H2O and Cl Figure 14. As the subducting slab melts, it transports significant amounts of Fe3+ and H2O into the mantle wedge, increasing the fO2 and H2O content of the lithospheric mantle [1,2]. This oxidation process leads to the oxidation of metal sulfides in the mantle, facilitating the incorporation of chalcophile elements into the slab melt or island arc magma [77]. These volatiles act as strong complexing agents and fluxes for metals like Cu and Au, forming stable complexes that migrate with the magma [74], thus promoting mineralization as show in Figure 14. However, the mechanisms of mineralization in hydrous, sulfur-rich, and high fO2 magmas resulting from the partial melting of thickened lower crust remain contentious [78].
Generally, the cumulates of arc magmas within the thickened lower crust are considered reduced in nature, particularly in the Gangdese magmatic belt [7,25]. The average oxygen fugacity for the magmatic pulse in the region around 190 Ma is approximately FMQ + 0.94 (Figure 9b), supporting this interpretation. At low oxidation states (e.g., FMQ < 1), most dissolved sulfur in these arc magmas exists as sulfide rather than sulfate, promoting the early saturation of large volumes of sulfide phases at lower-crustal depths. This suggests that rocks formed from the partial melting of the lower crust are less likely to produce magma with high fO2 [79]. Furthermore, since the rocks formed in subduction environments have low oxygen fugacity, the partial melting of cumulates of arc magmas within the thickened lower crust would also have difficulty generating high oxygen fugacity magma. It is possible that an external high-oxygen fugacity fluid needs to be introduced into the source region. In addition, magma evolution can also lead to changes in magmatic oxygen fugacity. During crustal thickening, the crystallization of a large number of Fe2+-bearing minerals can also lead to an increase in the oxygen fugacity of the magma. However, the barren porphyries formed at 16 Ma, despite having a thicker crust compared to the magma formed at 20 Ma, show a lower oxygen fugacity compared to the ore-forming porphyries at 20 Ma. This further supports the idea that materials with high oxygen fugacity contributed to the source region. Analysis of the oxygen fugacity in the investigated samples indicates that ore-forming porphyries exhibit higher fO2 and H2O values (Figure 8 and Figure 9b,c), closely resembling the ore-related porphyries in Qulong [57] rather than the barren porphyries from 16 Ma in the study area (Figure 9b,c). This suggests that a higher magmatic oxidation state and H2O content are crucial for the formation of porphyry Mo-Cu deposits in the Demingding area.
In the context of crustal thickening, we propose that the elevated oxygen fugacity and hydrous characteristics of the ore-forming magma at 20 Ma may be linked to its source region. This could involve the injection of hydrous, high fO2 fluid or melt into the thickened mafic lower crust during this period, leading to Mo-dominated mineralization. Dehydration reactions in the upper portions of the subducting Indian continental plate, along with the degassing of water-rich ultrapotassic mafic magmas mixing with adakite-like melts at crustal depths, may supply the necessary exogenous water for porphyry deposit formation [7,8,80,81].

6. Conclusions

(1)
The Demingding deposit exemplifies a Mo-dominated porphyry system, characterized by distinct alteration zoning that transitions from inner potassic to outer propylitic zones, significantly overprinted by phyllic alteration associated with Mo and Cu mineralization.
(2)
Zircon U-Pb dating of the ore-forming porphyries indicates crystallization ages of 19–21 Ma, which closely align with the mean Re-Os age of 21.3 ± 0.4 Ma for Mo veins and veinlets, suggesting a late Miocene magmatic event marked by Mo-dominated mineralization coinciding with the post-collision period of the Indian and Asian continents.
(3)
The ore-forming porphyries, ranging from granodiorite to monzogranite, are classified as high-K calc-alkaline with adakite-like features, primarily resulting from the partial melting of thickened mafic lower crust.
(4)
These porphyries exhibit higher fO2 and H2O levels compared to barren porphyries and 190 Ma arc magma formed in a subduction environment, underscoring the critical role of hydrous and oxidized fluids from their source in the Mo-Cu mineralization process.
(5)
Regional data indicate that the Gangdese porphyry metallogenic belt experienced concentrated Cu-Mo mineralization between 17 and 13 Ma. The formation of Mo-dominated deposits such as Demingding and Tangbula in the eastern segment of the belt, with slightly older ages around 20 Ma, underscores the presence of a significant porphyry Mo metallogenic event during this critical post-collision mineralization period.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min14121232/s1, Supplementary Table S1. Major and trace elemental abundances of the Demingding rhyolite porphyries and monzogranite porphyries. Supplementary Table S2. LA-ICP-MS U-Pb isotopic compositions of zircon from the Demingding rhyolite porphyries and monzogranite porphyries. Supplementary Table S3. Trace element concentration (ppm) of zircon from the Demingding rhyolite porphyries and monzogranite porphyries. Supplementary Table S4. Re-Os isotopic compositions of molybdenite from the Demingding ore-bearing monzogranite porphyries.

Author Contributions

Conceptualization, X.J. and S.S.; methodology, S.S.; software, S.L.; validation, X.J., S.S. and H.R.; formal analysis, S.S.; investigation, S.L.; resources, X.J.; data curation, H.R.; writing—original draft preparation, S.S.; writing—review and editing, X.J. and S.C.; visualization, H.R.; supervision, X.J.; project administration, X.J.; funding acquisition, X.J. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Key R&D Program of Xizang grant number [XZ202401JD0017] And The APC was funded by [XZ202401JD0017].

Data Availability Statement

Authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We express our sincere gratitude to the science editor and associate editor for their efficient handling and valuable constructive comments. Funding for this study was provided by the National Key R&D Program of Xizang (grant XZ202401JD0017).

Conflicts of Interest

Authors Sudong Shi, Shuyuan Chen, Sangjiancuo Luo were employed by the company Tibet Xianglong Copper Industry Limited Company. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Simplified geologic map of Xizang (a) and map showing the simplified tectonic and regional geologic setting of the Gangdese magmatic belt (b) (modified after Lin et al., 2024 [21]).
Figure 1. Simplified geologic map of Xizang (a) and map showing the simplified tectonic and regional geologic setting of the Gangdese magmatic belt (b) (modified after Lin et al., 2024 [21]).
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Figure 2. Simplified geological map (a) and cross-section (b,c) of the Demingding Mo-Cu deposit (modified after Ren et al., 2024 [37]).
Figure 2. Simplified geological map (a) and cross-section (b,c) of the Demingding Mo-Cu deposit (modified after Ren et al., 2024 [37]).
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Figure 3. Hand-specimen photographs of the porphyries and Mo-Cu mineralization from the Demingding Mo-Cu deposit. (a) The exposed location of the ore body shows the rock with red alteration. (b,c) Monzogranite porphyry; (d) Widely developed quartz veins in the granite porphyry; (e–h) Monzogranite porphyry with chalcopyrite and molybdenite mineralization.
Figure 3. Hand-specimen photographs of the porphyries and Mo-Cu mineralization from the Demingding Mo-Cu deposit. (a) The exposed location of the ore body shows the rock with red alteration. (b,c) Monzogranite porphyry; (d) Widely developed quartz veins in the granite porphyry; (e–h) Monzogranite porphyry with chalcopyrite and molybdenite mineralization.
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Figure 4. Photomicrographs of ore-forming porphyries with mineral assemblages of quartz, biotite, plagioclase, and K-feldspar. (a) Euhedral to subhedral quartz occurs as phenocrysts in the ore-bearing porphyry; (b,c) The phenocrysts in the ore-bearing porphyry are mainly composed of euhedral to subhedral quartz, plagioclase, biotite, and K-feldspar; (d) A small amount of feldspar has been altered to epidote. Abbreviations: Qz-Quartz; Kfs-K-feldspar; Pl-plagioclase; Bi-biotite.
Figure 4. Photomicrographs of ore-forming porphyries with mineral assemblages of quartz, biotite, plagioclase, and K-feldspar. (a) Euhedral to subhedral quartz occurs as phenocrysts in the ore-bearing porphyry; (b,c) The phenocrysts in the ore-bearing porphyry are mainly composed of euhedral to subhedral quartz, plagioclase, biotite, and K-feldspar; (d) A small amount of feldspar has been altered to epidote. Abbreviations: Qz-Quartz; Kfs-K-feldspar; Pl-plagioclase; Bi-biotite.
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Figure 5. Representative photomicrographs of mineralization and alteration from the Demingding Mo-Cu deposit. (a–c) Magnetite and biotite from the potassic alteration stage; (d,e) Quartz + biotite + Cu-Fe sulfides vein in the potassic alteration stage; (f) Sericite intergrow with Cu-Fe sulfides in the phyllic alteration; (g) Chlorite-calcite veins occurred during the propylitic zone; (h,i) Chalcopyrite-quartz vein in the ore-forming porphyries; (j–l) Disseminated chalcopyrite and molybdenite in the ore-forming porphyries. The dashed lines represent the boundaries of the vein in the figure.
Figure 5. Representative photomicrographs of mineralization and alteration from the Demingding Mo-Cu deposit. (a–c) Magnetite and biotite from the potassic alteration stage; (d,e) Quartz + biotite + Cu-Fe sulfides vein in the potassic alteration stage; (f) Sericite intergrow with Cu-Fe sulfides in the phyllic alteration; (g) Chlorite-calcite veins occurred during the propylitic zone; (h,i) Chalcopyrite-quartz vein in the ore-forming porphyries; (j–l) Disseminated chalcopyrite and molybdenite in the ore-forming porphyries. The dashed lines represent the boundaries of the vein in the figure.
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Figure 6. Cathodoluminescence images for representative zircon grains from (a) the 25,053 sample, (b) the 25,028 sample, (c) the ZK005-596.35 sample, and (d) the DMD17 sample in the Demingding rhyolite porphyries and monzogranite porphyries.
Figure 6. Cathodoluminescence images for representative zircon grains from (a) the 25,053 sample, (b) the 25,028 sample, (c) the ZK005-596.35 sample, and (d) the DMD17 sample in the Demingding rhyolite porphyries and monzogranite porphyries.
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Figure 7. Zircon U-Pb concordia diagrams and mean age for the Demingding rhyolite porphyries and monzogranite porphyries.
Figure 7. Zircon U-Pb concordia diagrams and mean age for the Demingding rhyolite porphyries and monzogranite porphyries.
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Figure 8. (Ce/Nd)/Y vs. 10,000*(Eu/Eu*)/Y (a) and (Ce/Nd)/Y vs. Eu/Eu* (b) diagrams for the Demingding rhyolite porphyries and monzogranite porphyries (modified after Lu et al., 2016 [48]).
Figure 8. (Ce/Nd)/Y vs. 10,000*(Eu/Eu*)/Y (a) and (Ce/Nd)/Y vs. Eu/Eu* (b) diagrams for the Demingding rhyolite porphyries and monzogranite porphyries (modified after Lu et al., 2016 [48]).
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Figure 9. Temperatures (a), ∆FMQ (b), and H2O contents (c) of Demingding rhyolite porphyries and monzogranite porphyries.
Figure 9. Temperatures (a), ∆FMQ (b), and H2O contents (c) of Demingding rhyolite porphyries and monzogranite porphyries.
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Figure 10. (a) (K2O + Na2O) vs. SiO2 diagram (Middlemost, 1994) [49]; (b) SiO2 vs. K2O diagram (Roberts and Clemens, 1993) [50]; (c) Sr/Y vs. Y diagram; (d) (La/Yb)N vs. (Yb)N diagram.
Figure 10. (a) (K2O + Na2O) vs. SiO2 diagram (Middlemost, 1994) [49]; (b) SiO2 vs. K2O diagram (Roberts and Clemens, 1993) [50]; (c) Sr/Y vs. Y diagram; (d) (La/Yb)N vs. (Yb)N diagram.
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Figure 11. Chondrite-normalized REE pattern (a) and primitive mantle-normalized spidergram (b) for the rhyolite porphyries and monzogranite porphyries. Data are normalized by values of chondrite and primitive mantle (Sun and McDonough 1989) [51].
Figure 11. Chondrite-normalized REE pattern (a) and primitive mantle-normalized spidergram (b) for the rhyolite porphyries and monzogranite porphyries. Data are normalized by values of chondrite and primitive mantle (Sun and McDonough 1989) [51].
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Figure 12. Molybdenite Re–Os isochron and weighted mean model age plots of the Mo veins and veinlets hosted in these porphyries.
Figure 12. Molybdenite Re–Os isochron and weighted mean model age plots of the Mo veins and veinlets hosted in these porphyries.
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Figure 13. Dy/Yb vs. SiO2 diagram of Demingding rhyolite porphyries and monzogranite porphyries.
Figure 13. Dy/Yb vs. SiO2 diagram of Demingding rhyolite porphyries and monzogranite porphyries.
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Figure 14. Image illustrating the genetic and geodynamic model for the monzogranite porphyries in the Demingding porphyry Mo -Cu deposit.
Figure 14. Image illustrating the genetic and geodynamic model for the monzogranite porphyries in the Demingding porphyry Mo -Cu deposit.
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Shi, S.; Chen, S.; Luo, S.; Ren, H.; Jiang, X. Petrogenesis and Metallogenic Significance of the Demingding Mo-Cu Porphyry Deposit in the Gangdese Belt, Xizang: Insights from U-Pb and Re-Os Geochronology and Geochemistry. Minerals 2024, 14, 1232. https://doi.org/10.3390/min14121232

AMA Style

Shi S, Chen S, Luo S, Ren H, Jiang X. Petrogenesis and Metallogenic Significance of the Demingding Mo-Cu Porphyry Deposit in the Gangdese Belt, Xizang: Insights from U-Pb and Re-Os Geochronology and Geochemistry. Minerals. 2024; 14(12):1232. https://doi.org/10.3390/min14121232

Chicago/Turabian Style

Shi, Sudong, Shuyuan Chen, Sangjiancuo Luo, Huan Ren, and Xiaojia Jiang. 2024. "Petrogenesis and Metallogenic Significance of the Demingding Mo-Cu Porphyry Deposit in the Gangdese Belt, Xizang: Insights from U-Pb and Re-Os Geochronology and Geochemistry" Minerals 14, no. 12: 1232. https://doi.org/10.3390/min14121232

APA Style

Shi, S., Chen, S., Luo, S., Ren, H., & Jiang, X. (2024). Petrogenesis and Metallogenic Significance of the Demingding Mo-Cu Porphyry Deposit in the Gangdese Belt, Xizang: Insights from U-Pb and Re-Os Geochronology and Geochemistry. Minerals, 14(12), 1232. https://doi.org/10.3390/min14121232

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