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Article

Magmatic Processes of Granitoids in the Hongniu-Hongshan Porphyry-Skarn Copper Deposit, Southern Yidun Terrane, China: Evidence from Mineral Geochemistry

by
Tianrui Wang
,
Huijuan Peng
*,
Ying Xia
,
Yue Chen
,
Dongjie Yang
and
Qi Zhou
College of Earth Sciences, Chengdu University of Technology, Chengdu 610059, China
*
Author to whom correspondence should be addressed.
Minerals 2022, 12(12), 1559; https://doi.org/10.3390/min12121559
Submission received: 26 September 2022 / Revised: 27 November 2022 / Accepted: 29 November 2022 / Published: 2 December 2022
(This article belongs to the Special Issue Tectono-Magmatic Evolution and Metallogeny of Tethyan Orogenic Belts)
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Abstract

:
The Hongniu-Hongshan porphyry-skarn deposit is located in the southern Yidun terrane in the Sanjiang Tethyan Metallogenic Domain (STMD). Although its metallogenesis has been well constrained in the past decade, the magmatic processes for granitoids in the Hongniu-Hongshan deposit are still poorly understood. Herein, we provide new geochemical data on magmatic minerals (plagioclase, amphibole, and clinopyroxene) in the Hongniu-Hongshan granitoids to get a better insight into these processes. The complex zoning patterns of plagioclase phenocrysts indicate magma recharge and mixing. Physiochemical estimations indicate that clinopyroxenes were crystallized in hotter (919 ± 11 °C) and more mafic (FeO: 2.8–4.6 wt.%, MgO: 0.8–1.8 wt.%) magmas in a deep chamber (18.6 ± 0.9 km) compared with the colder (819 ± 29 °C), more felsic (FeO: 0.9–2.2 wt.%, MgO: 0.3–0.6 wt.%) and shallow magma chamber (13.4 ± 1.6 km) in which amphiboles crystallized. Therefore, we suggest that magmatic minerals in the Hongniu-Hongshan granitoids were produced by multistage magmatic processes within the upper–middle crust range. In this model, the deep-seated magmas recharged into the shallow reservoir and mixed with the shallow magmas therein. The recharged hot magmas may provide heat sources and rejuvenate the shallow magma reservoirs. On this basis, we further infer that ore-forming materials could be pre-concentrated in the crustal range and mobilized by the Late Cretaceous magmatism in the southern Yidun terrane.

1. Introduction

Magmatic-hydrothermal deposits such as porphyry-, skarn-, and epithermal ore deposits, are major sources of many important metals in the world [1]. For porphyry copper deposits, their magmatic systems typically include intrusions near the surface and shallow magma chambers in the crust [2]. The shallow chambers play a “relay” role in providing ore-bearing magmas and fluids to the minerogenic systems [3,4]. To support such minerogenic systems, a large volume of ore-bearing magma in the shallow chamber is commonly required [5,6]. Numerous studies have indicated that the supply of deep-derived magmas make great contribution of ore-bearing magmas and fluids for the shallow magma chambers [7,8]. Because the recharge of deep-derived magmas can bring ore-forming materials such as metals, sulfur, and chlorine, the life of shallow magma chambers is prolonged [9,10,11,12]. Thus, to better understand the formation of porphyry copper deposits, it is crucial to get insights into their magmatic processes. However, the hydrothermal alteration usually happens within the intrusions of porphyry copper deposits which makes it difficult to track their magmatic processes.
The Hongniu-Hongshan deposit is a typical porphyry-skarn Cu deposit in the southern Yidun terrane of China. The time scale of magmatic and metallogenic events of this deposit were well constrained in previous studies, showing that its diagenesis and mineralization both happened in the Late Cretaceous (81−75 Ma for ore bodies; 81−73 Ma for intrusive rocks) [13,14,15,16,17,18]. However, the magmatic processes related to the Hongniu-Hongshan intrusive rocks are poorly understood. Although there is isotopic evidence (whole-rock Sr-Nd isotope) for a hybrid origin [17], the details are still less studied.
Magmatic minerals such as plagioclase, amphibole, and clinopyroxene found in the Hongniu-Hongshan granitoids provide us a great way to study the magmatic processes of this deposit. Because of the slow intracrystalline diffusion of CaAl-NaSi [19], zonation of major elements in plagioclase is usually reserved, which can provide important information of magmatic processes [20,21,22]. Moreover, the compositions of these minerals are also sensitive to the physiochemical parameters of coexisting magmas which allow us to constrain their crystallization conditions [23,24,25,26,27]. Herein, we collected granitoid samples from drill holes in the Hongniu-Hongshan deposit and reported their whole-rock and mineral (plagioclase, amphibole, and clinopyroxene) geochemical data. On this basis, we hope to better understand the magmatic process of this deposit.

2. Geological Background

2.1. Regional Geology

The Sanjiang region, spanning the area where the Jinsha River, Lancang River, and Nujiang River flow in parallel in southwestern China, is geologically located in the eastern Tethyan tectonic-metallogenic domain [28,29,30]. Multistage closures of Tethyan oceans from the Late Paleozoic to the Early Cenozoic formed the complex framework of converged terranes in the Sanjiang region [29]. The Yidun terrane is in the northern Sanjiang region (Figure 1a), restricted by the Garze-Litang suture zone in the east and the Jinshajiang suture zone in the west (Figure 1b). There are two parts of the Yidun terrane in both the west and the east. The western Yidun terrane (also known as the Zhongza Massif) consists of Paleozoic sedimental rocks with mafic volcanic intercalations [31]. The eastern Yidun terrane is composed of weakly metamorphosed Triassic flysch sequence formations with mafic-to-felsic volcanic intercalations [32]. The Yidun terrane is considered an island arc in the Late Triassic, which was produced by the westward subduction of the Garze-Litang ocean slab, a branch of the Paleo-Tethyan Ocean Plate [29,33,34,35].
Magmatism in the Yidun terrane mainly occurred in the Triassic and Cretaceous, which can be divided into two major episodes. The first major episode formed the N-S trending Late Triassic volcanic and plutonic rocks that were distributed in most areas of the Yidun terrane and are considered responses to the westward subduction and closure of the Garze-Litang ocean in this period [28,31,33,35]. The second major episode formed the Late Cretaceous plutonic rocks in the central area of the Yidun terrane, which is related to the post-collision extension of the lithosphere [33]. In the Zhongdian district at the southern edge of the Yidun terrane, these two major episodes of magmatism formed many magmatic-hydrothermal ore deposits (Figure 1c), such as Pulang, Chundu, Xuejiping, and Lannitang of the Late Triassic, and Xiuwacu, Relin, Hongniu-Hongshan, and Tongchanggou of the Late Cretaceous [28,30,34].

2.2. Geology of the Hongniu-Hongshan Deposit

The Hongniu-Hongshan is a typical porphyry-skarn copper deposit located in the Zhongdian district. This deposit covers a total area of 24 km2 and has a proven reserve of 1.4 Mt copper and 40 Kt molybdenum [36]. Its ore-related intrusions are primarily made up of granitic rocks [13,14,15,16,17,18]. The limestone, siltstone, and slate of the Upper Triassic Qugasi Formation comprise the country rocks of this deposit. Sedimentary rocks near the intrusions are mostly metamorphosed into hornfels, skarn, and marble (Figure 2). Major ore bodies are usually hosted in these hornfels, skarn, and marble, and their contact zones with intrusions (Figure 2b–d).
Previous works have well-limited the magmatic and metallogenic ages of this deposit (81−73 Ma for intrusions, [13,14,15,16,17,18]; 81−75 Ma for ore bodies, [14,16,37]), indicating its generation is closely related to the Late Cretaceous magmatism that occurred in the Yidun terrane [13,14,15,16,17,18,37]. Some petrological studies further suggest that the Hongniu-Hongshan intrusive rocks formed in a post-collision extensional environment [13,17].

3. Petrography and Sample Descriptions

Concealed granitoids from the Hongniu-Hongshan deposit were reported by several studies [13,14,15,16,17]. These granitic rocks have porphyritic textures, with phenocrysts comprised of K-feldspar (20−25 vol.%), plagioclase (15−20 vol.%), quartz (10−15 vol.%), minor biotite (~5 vol.%), and amphibole (~5 vol.%). Their matrix is comprised of feldspar, quartz, biotite, and amphibole. Accessory minerals include apatite, zircon, rutile, magnetite, and sphene. Some feldspars therein were altered into sericite.
Herein, we collected the concealed granitoid samples from newly completed drill holes in both Hongniu and Hongshan ore sections (Figure 2b,c). In the exploration line 11, five samples were collected from the 52, 54, and 55 drill holes (Figure 2c), and in the exploration line 7, three samples were collected from the 50 and 52 drill holes (Figure 2b). These rock samples are all from the central part of the Hongniu-Hongshan pluton and near the ore bodies of this deposit. Based on our observations, these rock samples can be classified into granite porphyry (Figure 3 and Figure 4). K-feldspar (5−20 vol.%), plagioclase (5−10 vol.%), quartz (<5−10 vol.%) and minor (<5 vol.%) biotite, amphibole, and clinopyroxene comprise the phenocrysts of these granite porphyries, and their matrix is mainly comprised of the microcrystals of phenocryst-type minerals. Accessory minerals include apatite, zircon, sphene, and magnetite. For phenocrysts, K-feldspars are euhedral crystals with quartz and feldspar inclusions (Figure 4a), quartzes are mostly anhedral crystals with embayed rims (Figure 4d), plagioclases are subhedral to euhedral and commonly zoned (Figure 4c,e,f), and amphiboles are subhedral to euhedral and twinned (Figure 4g). Locally, clinopyroxenes usually coexist with sphene (Figure 4h), and feldspars were later altered to sericite (Figure 4f). Post-crystallization hydrothermal veins comprised of quartz and sulfides are observed in most granitoid samples in this study which are mainly composed of quartz, pyrite, and chalcopyrite (Figure 3a,b,e). Some Fe-bearing sulfides in these veins were later oxidized into limonite (Figure 3f).

4. Analytical Methods

Fresh parts of granite porphyry samples were divided into two sections, one of which was crushed into powder. The other was polished into slices, from which representative minerals are selected. The major and trace compositions of crushed rocks and the minerals in slices are analyzed. The specific analysis methods are described below.

4.1. Whole-Rock Major and Trace Elemental Analysis

Whole-rock major and trace element compositions were analyzed at Guangzhou Tuoyan Analytical Technology Co., Ltd., Guangzhou, China. The detailed process is introduced in Supplemental Methods. Major element analyses were carried out using an Agilent 720 inductively coupled plasma optical emission spectrometer (ICP-OES). GSR-3, GSD-4, GSD-6, OU-6, GSR-12, and GSR-13 are analyzed as standards, and the accuracies of the analyses were better than 5%. Trace elements were measured by a Jena Plasma Quant inductively coupled plasma mass spectrometry (ICP-MS), for which the OU-6, BCR-1, GSD-11, and GSD-12 were analyzed as standards, and the accuracies of the analyses were better than 10%. The results are listed in Supplementary Table S1.

4.2. Electron Probe Mineral Analysis

Major compositions of selected plagioclase, amphibole, and clinopyroxene grains were analyzed using a JEOL JXA-8230 electron probe microanalyzer equipped with four wavelength-dispersive spectrometers (WDS) in the College of Earth Science and Technology, Southwest Petroleum University, Chengdu, China. Quantitative analyses were conducted with an acceleration voltage of 15 kV, a beam current of 10 nA, and a focused beam of 10 μm. For Si, Al, Ca, Na, K, Cr, Fe, Mg, Mn, and Ti, the peak counting times were 10 s and background counting times 5 s prior and after the peak for all. Measurement data were corrected using the ZAF procedure. Mineral major elements data for the analyzed samples are given in Supplementary Table S2.

4.3. LA-ICP-MS Mineral Analysis

Trace element concentrations of selected plagioclase grains were analyzed using a set of laser ablation inductively coupled plasma mass spectrometers (LA-ICP-MS) at Beijing Createch Testing Technology Co., Ltd., Beijing, China. A RESOlution 193 nm laser ablation system coupled with an Analytik Jena PQMS ICP-MS instrument was used for laser sampling and ion-signal intensities acquisition. The analyses were conducted with a beam size of 30 μm, a pulse frequency of 5 Hz, an energy density of 6 J/cm2, and a carrier gas of high purity helium. Before testing, the instrument was debugged with NIST 610 to achieve the optimal state. The background was collected for 20 s followed by 45 s of data acquisition. All the laser spots overlapped with the pre-measured EPMA spots. NIST 610 was used as an external standard for calibration, and BHVO-2G, BCR-2G, and BIR-1G as monitors. The original data were processed by ICPMSDataCal software [38]. The 29Si measured by EPMA was used as an internal standard element. The analyzed standard glasses were all in satisfactory agreement with the reference values given by GeoReM, which gives an accuracy better than 10%. The measured results are listed in Supplementary Table S3.

5. Results

5.1. Whole-Rock Geochemistry

Geochemically, the newly exposed granite porphyry samples are characterized by wide variation in SiO2 (66.80−73.64 wt.%), total alkalis (Na2O + K2O = 5.49−9.38 wt.%), CaO (0.45−3.91 wt.%) contents, and Mg# [100 × Mg/(Mg + Fe2+)] values (20−47) (Table S1). The loss on ignition (LOI) of these samples are relatively low (0.15–2.00 wt.%). In the TAS diagram, these samples are all plotted below the Irvine line (Figure 5a), showing their sub-alkalic compositions. In the QAP diagram, these samples mostly exhibit moderate quartz contents with 20% < Q < 60% (Figure 5b). Their aluminum saturation index values (A/CNK) are mostly <1.1 with one exception of 1.4 (Figure 5d).
In the chondrite-normalized rare earth element (REE) diagram [40], these samples show significant enrichment of light rare earth elements (LREEs) compared with heavy rare earth elements (HREEs; Figure 6a) with LREE/HREE = 18.3–24.3 and (La/Yb) N = 28.6−58.3. A slightly negative abnormality of Eu is observed for all these rock samples with δEu = 0.54−0.89 (Figure 6a). Mostly, these samples exhibit relative depletion in Ba, Sr, P, and Ti, and enrichment in Rb, Th, U, Pb, Nd, Zr, and Hf (Figure 6b).

5.2. Plagioclase

Plagioclase phenocrysts selected from granite porphyry samples are commonly subhedral with a granularity of 1−3 mm. The major and trace compositions of analyzed plagioclase phenocrysts are listed in Supplementary Tables S2.1, S2.2 and S3. These plagioclases are mostly oligoclase and andesine (Figure 7a). Two types of plagioclase phenocrysts are identified according to their textural and compositional properties.
The PL1 crystal exhibits a typical core-mantle-rim pattern in the cross-polarized micrograph (Figure 8a). Its major and trace compositions change significantly through this pattern (Figure 8c,e,g). Specifically, the cores of PL1 are more calcic (An41–49) and enriched in Mg (87.4–261 ppm), Ti (85.0–200 ppm), and Sr (1540–2248 ppm) compared with its mantle and rim (An23–28, Mg: 38.0–48.5 ppm, Ti: 24.2–38.8 ppm, Sr: 1372–1394 ppm) (Figure 8c,e,g). Moreover, there is an obvious resorption surface between the mantle and rim of the PL1 crystal (Figure 8a). The content of anorthite (An32–39), Mg (49.1–83.1 ppm), Ti (38.6–61.4 ppm), and Sr (1536–1833 ppm) in this resorption surface is relatively lower than that in the cores of PL1, but higher than that in its mantle and rim (Figure 8c,e,g).
The PL2 crystal exhibits a serrated oscillation pattern in the compositional profile (Figure 8d,f,h). In this crystal, the normal zoned low-calc zones were interrupted by more calcic resorption surfaces (An32−34) with an increase of 10−13 mol% An (Figure 8d). Regarding trace elements, the resorption surfaces are more enriched in Mg (57.1−73.6 ppm), Ti (63.3–69.7 ppm) and Sr (1508−1612 ppm) compared with the low-calc zones (An, Mg: 20.0–46.1 ppm, Ti: 25.0–56.3 ppm, Sr: 1225–1431 ppm) they interrupted (Figure 8f,h).

5.3. Amphibole

The selected amphibole grains are mostly euhedral and 2−6 mm in length. Their major compositions are listed in Supplementary Table S2.3. These analyzed amphiboles are characterized by high SiO2 (43.24−50.33 wt.%), FeO (14.10−18.07 wt.%), MgO (11.11−14.51 wt.%), and CaO (10.87−11.59 wt.%) contents (Figure 9b–d). Their Mg# [100 × Mg/(Mg + Fe2+)] values range from 52 to 64 with an average of 59. Structural formulas of analyzed amphiboles are calculated on the basis on 13 cations [41]. These amphiboles are calcic with CaB > 1.50 per formula unit (p.f.u), which mostly belongs to magnesiohornblende with exception of minor tschermakite (Figure 7b).

5.4. Clinopyroxene

The selected clinopyroxene grains are subhedral to anhedral with a diameter of 1−3 mm. Their major compositions are listed in Supplementary Table S2.4. Clinopyroxenes in this study exhibit high SiO2 (51.10−53.29 wt.%), FeO (9.11−13.11 wt.%), MgO (10.78−12.89 wt.%), and CaO (22.06−23.58 wt.%) contents (Figure 9h–j). Their Mg# [100 × Mg/(Mg + Fe2+)] values range from 60 to 72 (average = 66). Structural formulas of these clinopyroxenes are calculated based on 6 oxygen atoms. According to the proportions of Ca2Si2O6 (Wo; 46−49), Mg2Si2O6 (En; 32−37), and Fe2SiO6 (Fs; 15−21), these clinopyroxenes are classified into diopside (Figure 7c).

6. Discussion

6.1. High-K Calc-Alkaline to Shoshonite Series I-Type Granites

The genetic types of granitoids are mainly classified into I-, S-, and M-type by the nature of magma sources [42,43]. In addition, A-type granites were proposed as alkaline and anhydrous granites formed in anorogenic settings [44]. It is usually difficult to distinguish the genetic type for granitoids, especially for the more felsic ones [45,46]. In previous studies, the Hongniu-Hongshan granitoids were classified into I-type granites based on their metaluminous compositions, the negative trend in the P2O5 vs. SiO2 diagram, and the occurrence of amphibole phenocrysts [17,18,39].
Our granitoid samples also show the affinities of I-type granites. First, they are mostly metaluminous with A/CNK < 1.1 (Figure 5d), and the peraluminous minerals (e.g., muscovite and iolite) are absent for all these samples. Second, their P2O5 contents decrease with the increase of SiO2 contents, which exhibits the trend of I-type granites [47]. Third, they are mostly magnesian rather than ferroan, and similar to the I-type granites from the Lachlan fold belt [48]. Moreover, these granitoids also contain amphibole in their phenocrysts and matrix, implying that their initial magmas were hydrous. For the above-mentioned reasons, we suggest that these granitoid samples belong to I-type granites. Most of our rock samples are plotted in the domains from shoshonite to high-K calc-alkaline series. Two calc-alkalic samples (11ZK55-993 and 11ZK55-999) in the K2O vs. SiO2 diagram (Figure 5c) show low K2O (2.05–2.29 wt.%) but have high Na2O (3.44–3.92 wt.%) and CaO (3.24–3.29 wt.%) contents (Table S1), which might be attributed to the enrichment of plagioclase. Thus, our granite porphyry samples are high-K calc-alkaline to shoshonite series I-type granites.

6.2. Physiochemical Conditions for Crystallization

Mineral composition reflects the nature of the magma from which it crystallized and is further a function of temperature, pressure, and melt composition under equilibrium condition. In this way, the physiochemical conditions can be estimated by the composition of typical minerals.

6.2.1. Temperature and Pressure

For plagioclase, the equilibrium constant KD(An-Ab)pl−liq for the An-Ab exchange is an initial test for equilibrium between plagioclase and melt, which should be 0.10 ± 0.05 when T is under 1050 °C [25]. Test of An-Ab exchange equilibrium between plagioclase and melt are shown in Tables S2.1 and S2.2. Most crystals are not in equilibrium with their host whole rocks, but it is possible to find the correct liquid composition by comparing whole-rock compositions along a liquid line [49]. Considering the relatively low LOI of rock samples, the influence of hydrothermal alteration on their compositions should not be obvious. Thus, the whole-rock compositions (CaAl/NaSi ratios) are selected for the equilibrium test. According to this test, plagioclase-whole-rock pairs are selected for thermobarometry. We employ the plagioclase-liquid thermometer equation 24a of Putirka [25] for temperature estimation. Our estimates suggest that the high-calc cores (An41−49) in PL1 are crystallized at 875−899 °C, and the low-calc zones (An22−30) in PL1 and PL2 are crystallized at 765−823 °C. However, since the resorption textures in plagioclase are sometimes formed in non-equilibrium conditions [50,51], their crystallizing temperature is difficult to estimate.
Many thermobarometers for amphibole and clinopyroxene also require both the composition of mineral and coexisting melt [24,25,49]. Following these models, tests for the Fe-Mg exchange equilibrium between minerals (amphibole and clinopyroxene) and melt are shown in the Rhodes diagrams (Figure 10a,b), in which the Mg# [100 × Mg/(Mg + Fe2+)] values of melts are also represented by whole-rocks Mg# values following the similar route as for An-Ab exchange equilibrium test above [25,49]. These tests suggest that the compositions of analyzed amphiboles and clinopyroxenes are mostly in equilibrium with the melt composition represented by that of the Hongniu-Hongshan granitoids. However, some parameters (e.g., H2O content) of mineral-coexisting melt are difficult to assess, which obstructs our attempts to these mineral-liquid thermobarometers.
Considering the above problems, we employ the single-phase amphibole and clinopyroxene thermobarometers of Higgins et al. [23], which were developed based on random forest machine learning. Our estimation suggests that the amphibole grains crystallized at 767–875 °C and 2.5–4.3 kbar, and the clinopyroxene grains crystallized at 900–950 °C and 4.3–5.6 kbar (Figure 10c). The uncertainty of our estimations is acceptable compared with that of the mineral-liquid thermobarometers [24,25]. Furthermore, the estimation results exhibit tight clustering with means of 819 ± 29 °C and 3.5 ± 0.4 kbar for amphiboles, and 919 ± 11 °C and 4.9 ± 0.2 kbar for clinopyroxenes (Tables S2.3 and S2.4).

6.2.2. Composition of Mineral-Coexisting Melt

Several studies have demonstrated that mineral composition can be used to estimate the composition of coexisting melts in equilibrium conditions [25,26,27,52,53]. Thus, we also employ the amphibole and clinopyroxene chemometers of Higgins et al. [23] for estimating the major composition of coexisting melt. Our estimations suggest that the coexisting melts for amphibole and clinopyroxene are both felsic but their major compositions are obviously different (Figure 11). The melts coexist with amphibole are estimated to have higher SiO2 (75.3 ± 1.2 wt.%) contents, but lower Al2O3 (13.8 ± 0.6 wt.%), CaO (2.2 ± 0.4 wt.%), FeO (1.6 ± 0.4 wt.%), and MgO (0.4 ± 0.1 wt.%) contents compare with the clinopyroxene-coexisting melts (SiO2: 69.6 ± 1.7 wt.%, Al2O3: 14.8 ± 0.2 wt.%, CaO: 3.7 ± 0.2 wt.%, FeO: 3.7 ± 0.4 wt.%, MgO: 0.9 ± 0.2 wt.%).

6.3. Multistage Reservoirs of Felsic Magmas

6.3.1. Evidence from Plagioclase Phenocrysts

The complex variation of element concentrations in magmatic plagioclase could be attributed to three contrasting mechanisms: (1) inner crystal diffusions; (2) close-system dynamics; and (3) open-system processes [50,54,55]. Intra-crystalline diffusion of elements tends to rebalance their components in adjacent zones of plagioclase crystals [21]. The CaAl-NaSi in plagioclase has a relatively slow diffusion rate [19] which means the proportion of anorthite and albite in plagioclase can hardly change in diffusion. However, those elements that diffuse sufficiently fast in plagioclase could change significantly, and their concentrations may not reflect the nature of melt during crystallization [22]. Close-system dynamics related to small changes of the temperature, pressure, and water content etc. can only slightly influence the compositions of plagioclase [22,56]. There are two open-system processes that may cause significant impacts on the plagioclase compositions. The first one is kinetic processes, which usually happen in non-equilibrium crystallization related to a large crystallization rate, such as crystallization driven by rapid decompression [50,57,58]. The second one is magma recharge and mixing, which usually leads to great changes in the nature of magma, and therefore greatly changes the texture and composition of plagioclase crystallized from it [22,54,56]. It is usually difficult to distinguish the influence of these two mechanisms on major compositions of plagioclase. However, their influence on the concentrations of trace elements during crystal growth can be distinguished by monitoring the relative behavior of compatible and incompatible elements in plagioclase [22,50].
In the PL1 crystal, the cores exhibit much higher concentrations of anorthite, Mg, Ti, and Sr than the mantle zones (Figure 8c,e,g), which can be interpreted as crystallization from different melts [59]. Under the estimated crystallization temperature of PL1, Mg diffuses faster than Sr, and Sr diffuses faster than Ti in plagioclase [21,60]. Therefore, in the PL1 crystal, the influence of intracrystalline diffusion on the Mg, Ti, and Sr concentrations between cores and mantle is not obvious (Figure 8e,g). Second, close-system dynamics only cause small variation (<10 mol% An) in plagioclase compositions [22,56], which is not the case for the cores and mantles of PL1 crystal (Figure 8c,e,g).
The partition coefficients of Sr (DSr) between plagioclase and melt are calculated using the equation: RTlnDSr = 26,800 − 26,700 × XAn [61], where R is 8.314 J/(K × mol), T is the estimated crystallization temperature of plagioclase in kelvin (K), and XAn is the mole fraction of anorthite in plagioclase. DSr values of 4.1−11.3 are calculated (Tables S2.1 and S2.2), indicating that Sr is a compatible element for the analyzed plagioclase phenocrysts. The partition coefficients of Ti (DTi) between plagioclase and melt can be estimated using RTlnDTi = −32.5 × XAn − 15.1 [62], where XAn is the mole fraction of anorthite in plagioclase, T is the estimated crystallization temperature of plagioclase in kelvin, and R is 8.314 × 10−3 KJ/(K × mol). DTi values of 0.04−0.08 are calculated, indicating that Ti is an incompatible element for our plagioclases. The plagioclase-incompatible element Ti [50,63] and the plagioclase-compatible element Sr generally exhibit coupled behaviors between the cores and mantle of PL1, which refutes the kinetic processes. A decrease in temperature can cause the decrease of An mol% [56]. Also, changes in the water content of magmas may not significantly affect the concentrations of trace elements such as Sr and Ti in the plagioclase, as the contents of these trace elements in plagioclase mainly depend on the compositions of coexisting melt [20,55,59]. Hence, the cores in PL1 are crystallized in a Mg-Ti-Sr-rich, more mafic and hotter magma than its mantle zones, and then carried into a colder, felsic magma chamber with lower Mg, Sr, and Ti contents [21].
Resorption textures in plagioclase can be caused by decompression-driven crystallization, or recharge and mixing of magma [22,51,54]. Decompression usually involves the ascent and eruption of magma [64]. However, resorption surfaces with an increase of over 10 mol% An (anorthite) in the PL1 and PL2 crystals require decompression of more than 3 kbar (3 mol%/kbar, [65]). However, decompression is not suggested in our case because the PL2 contains two major resorption surfaces (Figure 8b,f,h). The resorption surfaces in the PL1 and PL2 crystals are more likely explained as recharges of magma. First, the resorption surfaces are wavy, with peaks near the cores of crystals, and accompanied by a significant increase of An (up to 10 mol%; Figure 8d), which conforms to the features of the R1 resorption surfaces of Ginibre et al. [22]. Second, the Mg, Ti, and Sr concentrations are coupled and increased in the resorption surfaces (Figure 8g,h), which is against the kinetic processes [50]. Third, the estimated crystallization temperature for these resorption surfaces is higher than the low-calc zones they interrupt (Tables S2.1 and S2.2). Thus, these resorption surfaces are interpreted as recharges of hotter, Mg-Ti-Sr-rich, more mafic magma. The lower anorthite content and Mg, Ti, and Sr concentrations in resorption surfaces than those of the cores in PL1 suggest the partial mixing of recharged magma with ambient magma in the shallow chamber after recharge [20]. In summary, the crystallization of PL1 and PL2 plagioclase phenocrysts involves magmas with different compositions and reservoirs.

6.3.2. Evidence from Amphibole and Clinopyroxene

The estimated pressure of amphibole and clinopyroxene can be used to predict the depth for crystallization and, further, the depth of magma chambers [8,10,66]. Using the estimated pressure and referring to a general density of 2700 kg/m3 for continental crust [27], the storage depths are calculated to be 13.4 ± 1.6 km for amphibole-coexisting melts, and 18.6 ± 0.9 km for clinopyroxene-coexisting melts, respectively. The estimated crystallization temperature values of the amphiboles (767–875 °C, mean = 819 ± 29 °C) are lower than those of the clinopyroxenes (900–950 °C, mean = 919 ± 11 °C). Thus, we suggest that amphiboles and clinopyroxene in the Hongniu-Hongshan granitoids crystallized in different magma chambers. The deep chamber is located at about 19 km depth, while the shallow chamber is located at about 13km depth. They are without doubt located in the upper–middle crust, and the magmas therein are both felsic based on our estimation (see Section 6.2.2). However, magmas in the deep chamber are estimated to be more mafic in composition (FeO: 2.8–4.6 wt.%, MgO: 0.8–1.8 wt.%) compared with those in the shallow chamber (FeO: 0.9–2.2 wt.%, MgO: 0.3–0.6 wt.%). This confirms our inference about the origin of PL1 plagioclase phenocryst. Moreover, the estimated crystallization temperature for the cores of PL1 is mostly higher than that of the amphiboles, but lower than that of the clinopyroxenes (Figure 10c). It seems that the cores of PL1 crystallized after clinopyroxene.
In summary, we suggest that the granitic magmas forming the Hongniu-Hongshan deposit had experienced processes in multistage magma reservoirs (Figure 12). A more “mafic” felsic magma derived from the middle-crust chamber was injected into a shallow magma chamber and mixed with the more “felsic” felsic magma therein. The high-anorthite cores of PL1 plagioclase phenocryst together with the clinopyroxenes crystallized in the deeper and hotter magma chamber, and then carried into the shallower magma chamber and mixed into the colder magma. The PL1 crystal continued to crystallize its low-calc mantle zones in the shallow chamber, while the resorption textures in plagioclase phenocrysts together with the amphiboles were formed during the mixing of episode-recharged magmas from the deep source with the felsic magma in the shallow chamber.

6.4. Implications for Ore Deposit Formation

The classical magmatic system models of the porphyry copper system include plutons and magma chambers beneath the ore bodies [2,4]. Recent works emphasize the role of recharged deep-seated magma in the formation of porphyry deposits [7,8,10] because the recharged magmas can bring ore-forming materials into the shallow chamber [8,11,12]. Our work suggests that the formation of Hongniu-Hongshan granitoids is related to multistage processes of felsic magmas in the upper–middle crust range. In these processes, the recharged magmas are hot and deep-seated, which may provide heat sources and rejuvenate the shallow magma reservoirs. Previous work has raised the possibility that ore-forming elements are preconcentrated in the Late Triassic metallogenetic events (such as the Lannitang and Pulang deposits) in this region [17]. Our work may provide indirect evidence for this possibility; that is, the ore-related magma chambers in the Late Triassic metallogenetic events were reactivated in the Late Cretaceous and formed the Hongniu-Hongshan deposit. However, more restrictions are needed regarding the source of ore-forming materials.

7. Conclusions

Based on the geochemical data for plagioclase, amphibole, and clinopyroxene from the granitoids of the Hongniu-Hongshan deposit, we reached the following conclusions:
(1)
Magmatic minerals in the Hongniu-Hongshan granitoids were produced by multistage processes of felsic magmas within the upper–middle crust range.
(2)
Magmas in the deeper reservoir are hotter and more “mafic” than the shallowing magmas.
(3)
The deep-seated magmas recharged into the shallow reservoir and mixed with the shallowing magmas therein.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min12121559/s1, Supplemental Methods; Table S1: Whole-rock data; Table S2: EPMA data; Table S3: LA-ICP-MS data; Table S4: Collected whole-rock data of the Hongniu-Hongshan granitoids.

Author Contributions

Conceptualization, T.W. and H.P.; Methodology, Y.X. and Y.C.; Sample preparation, T.W., D.Y. and Q.Z.; Experimental analysis and data Curation, T.W.; Original Draft Preparation, T.W. and H.P.; Funding Acquisition, H.P. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the National Natural Science Foundation of China (41972077) and (41502074).

Data Availability Statement

The dataset of this study is available in Supplementary Materials.

Acknowledgments

We thank Dicheng Zhu and Lei Yang for their constructive suggestions in rock-type classifications and thesis structure. We thank Xi Chen for her help in our EPM analyses. We thank Zuxing Chen for his useful suggestions in the processing of LA-ICP-MS data. We also thank the academic editor and four anonymous reviewers for their helpful comments and suggestions. The author Tianrui Wang would like to thank Shiling Wang for her help in sample preparation.

Conflicts of Interest

The authors declare no conflict of interest.

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Minerals 12 01559 g001 550
Figure 1. (a) Tectonic position of Yidun terrane, modified from Li et al. [34]; (b) Distribution of Yidun terrane structure and igneous rocks, modified from Zhan et al. [32]; (c) Geological map of Zhongdian district showing the location of plutonic rocks and major porphyry ore deposits, modified from Peng et al. [14].
Figure 1. (a) Tectonic position of Yidun terrane, modified from Li et al. [34]; (b) Distribution of Yidun terrane structure and igneous rocks, modified from Zhan et al. [32]; (c) Geological map of Zhongdian district showing the location of plutonic rocks and major porphyry ore deposits, modified from Peng et al. [14].
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Figure 2. (a) Geological map of Hongniu-Hongshan porphyry copper deposit modified according to the geological report and Peng et al. [36]; (b) Profile of the Hongniu-7 exploration line; (c) Profile of the Hongniu-Hongshan-11 exploration line; (d) Profile of the Hongshan-9 exploration line. Note that the sampling positions of granitoids in previous studies [13,15,17] are also shown in these profiles.
Figure 2. (a) Geological map of Hongniu-Hongshan porphyry copper deposit modified according to the geological report and Peng et al. [36]; (b) Profile of the Hongniu-7 exploration line; (c) Profile of the Hongniu-Hongshan-11 exploration line; (d) Profile of the Hongshan-9 exploration line. Note that the sampling positions of granitoids in previous studies [13,15,17] are also shown in these profiles.
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Figure 3. Photographs of the granite porphyry samples collected from the drill holes in the Hongniu-Hongshan deposit. The sampling positions are marked in Figure 2b,c. Large K-feldspar phenocrysts in granite porphyry samples are mostly euhedral and widely dis-tributed (a–f). Quartz grains are anhedral in the phenocrysts (a–e). Plagioclase grains are smaller than K-feldspar in the phenocrysts (b–e). Clinopyroxene, amphibole, and biotite are commonly small and occurred in both phenocryst and matrix (a–c,f). Hydrothermal veins in these porphyries are usually composed of quartz (b), quartz + pyrite (a,e), quartz + pyrite + chalcopyrite (b), and quartz + limonite (f). Kfs = K-feldspar, Pl = plagioclase, Qtz = quartz, Cpx = clinopyroxene, Amp = amphibole, Bt = biotite, Pyr = pyrite, Cpy = chalcopyrite, Lm = limonite.
Figure 3. Photographs of the granite porphyry samples collected from the drill holes in the Hongniu-Hongshan deposit. The sampling positions are marked in Figure 2b,c. Large K-feldspar phenocrysts in granite porphyry samples are mostly euhedral and widely dis-tributed (a–f). Quartz grains are anhedral in the phenocrysts (a–e). Plagioclase grains are smaller than K-feldspar in the phenocrysts (b–e). Clinopyroxene, amphibole, and biotite are commonly small and occurred in both phenocryst and matrix (a–c,f). Hydrothermal veins in these porphyries are usually composed of quartz (b), quartz + pyrite (a,e), quartz + pyrite + chalcopyrite (b), and quartz + limonite (f). Kfs = K-feldspar, Pl = plagioclase, Qtz = quartz, Cpx = clinopyroxene, Amp = amphibole, Bt = biotite, Pyr = pyrite, Cpy = chalcopyrite, Lm = limonite.
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Figure 4. Micrographs of the granite porphyry samples in this study showing typical structures and mineral assemblages. (a) Large K-feldspar phenocryst with plagioclase and quartz inclusions compared with fine-sized matrix; (b) Quartz and K-feldspar in phenocrysts and smaller clinopyroxene and sphene in matrix; (c) Plagioclase and biotite in phenocrysts; (d) Large quartz and K-feldspar crystals in phenocrysts and fine-sized feldspar, quartz, and minor mafic minerals in matrix; (e) Plagioclase crystals with polysynthetic twining and zoning texture; (f) Plagioclase phenocryst with oscillatory zoning were later altered into sericites; (g) Amphibole phenocryst coexists with magnetite; (h) Clinopyroxene phenocryst coexists with quartz phenocryst and encloses small sphene grains. Kfs = K-feldspar, Pl = plagioclase, Qtz = quartz, Cpx = clinopyroxene, Amp = amphibole, Bt = biotite, Sph = sphene, Mag = magnetite, Ser = sericite.
Figure 4. Micrographs of the granite porphyry samples in this study showing typical structures and mineral assemblages. (a) Large K-feldspar phenocryst with plagioclase and quartz inclusions compared with fine-sized matrix; (b) Quartz and K-feldspar in phenocrysts and smaller clinopyroxene and sphene in matrix; (c) Plagioclase and biotite in phenocrysts; (d) Large quartz and K-feldspar crystals in phenocrysts and fine-sized feldspar, quartz, and minor mafic minerals in matrix; (e) Plagioclase crystals with polysynthetic twining and zoning texture; (f) Plagioclase phenocryst with oscillatory zoning were later altered into sericites; (g) Amphibole phenocryst coexists with magnetite; (h) Clinopyroxene phenocryst coexists with quartz phenocryst and encloses small sphene grains. Kfs = K-feldspar, Pl = plagioclase, Qtz = quartz, Cpx = clinopyroxene, Amp = amphibole, Bt = biotite, Sph = sphene, Mag = magnetite, Ser = sericite.
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Figure 5. Compositional discrimination diagrams of the Hongniu Hongshan granitoids. (a) TAS diagram; (b) Q-A-P diagram; (c) K2O vs. SiO2 diagram; and (d) A/NK vs. A/CNK diagram for classification of rock series. (e) FeOT/(FeOT + MgO) vs. SiO2 diagram and (f) P2O5 vs. SiO2 diagram for classification of genetic types. Gray dots show the Hongniu-Hongshan granitoids reported by previous studies [13,15,17,39]. See Tables S1 and S4 in the Supplementary Materials for data in this figure.
Figure 5. Compositional discrimination diagrams of the Hongniu Hongshan granitoids. (a) TAS diagram; (b) Q-A-P diagram; (c) K2O vs. SiO2 diagram; and (d) A/NK vs. A/CNK diagram for classification of rock series. (e) FeOT/(FeOT + MgO) vs. SiO2 diagram and (f) P2O5 vs. SiO2 diagram for classification of genetic types. Gray dots show the Hongniu-Hongshan granitoids reported by previous studies [13,15,17,39]. See Tables S1 and S4 in the Supplementary Materials for data in this figure.
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Figure 6. (a) Chondrite-normalized REE diagram and (b) primitive mantle-normalized trace element patterns diagram [40] for the Hongniu-Hongshan granitoids. Literature data is the same as that in Figure 5.
Figure 6. (a) Chondrite-normalized REE diagram and (b) primitive mantle-normalized trace element patterns diagram [40] for the Hongniu-Hongshan granitoids. Literature data is the same as that in Figure 5.
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Figure 7. Classification of plagioclase (a), amphibole (b) and clinopyroxene (c). All the analyzed minerals are from the granite samples in this study: plagioclase in 11ZK52-1095 and 11ZK55-993; amphibole in 7ZK50-1184, 7ZK50-1209, 11ZK52-1095, 11ZK52-1174, and 11ZK54-964; clinopyroxene in 7ZK50-1184 and 7ZK52-1225.
Figure 7. Classification of plagioclase (a), amphibole (b) and clinopyroxene (c). All the analyzed minerals are from the granite samples in this study: plagioclase in 11ZK52-1095 and 11ZK55-993; amphibole in 7ZK50-1184, 7ZK50-1209, 11ZK52-1095, 11ZK52-1174, and 11ZK54-964; clinopyroxene in 7ZK50-1184 and 7ZK52-1225.
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Figure 8. Cross-polarized micrographs and compositional profiles of the PL1 and PL2 plagioclase phenocrysts. (a) Core-mantle-rim pattern in the PL1 crystal; (b) Oscillational zoning pattern in the PL2 crystal; (c,d) Compositional profiles showing the variation of anorthite in the PL1 and PL2 crystals. Double-Y diagrams showing the variation of magnesium and strontium (e,f), and titanium and strontium (g,h) contents in the PL1 and PL2 crystals. The white line in (b) shows the profile line of EPM and LA-ICP-MS analyses.
Figure 8. Cross-polarized micrographs and compositional profiles of the PL1 and PL2 plagioclase phenocrysts. (a) Core-mantle-rim pattern in the PL1 crystal; (b) Oscillational zoning pattern in the PL2 crystal; (c,d) Compositional profiles showing the variation of anorthite in the PL1 and PL2 crystals. Double-Y diagrams showing the variation of magnesium and strontium (e,f), and titanium and strontium (g,h) contents in the PL1 and PL2 crystals. The white line in (b) shows the profile line of EPM and LA-ICP-MS analyses.
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Figure 9. Harker diagrams show the major compositions of amphibole (a–f) and clinopyroxene (g–l).
Figure 9. Harker diagrams show the major compositions of amphibole (a–f) and clinopyroxene (g–l).
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Figure 10. Rhodes diagrams for testing the Fe-Mg exchange equilibrium of amphibole (a) and clinopyroxene (b). (c) The estimated temperature of plagioclase, amphibole, and clinopyroxene [23]. The estimated pressure of amphibole and clinopyroxene (d,e). The red and blue lines are the density curves of estimated pressure.
Figure 10. Rhodes diagrams for testing the Fe-Mg exchange equilibrium of amphibole (a) and clinopyroxene (b). (c) The estimated temperature of plagioclase, amphibole, and clinopyroxene [23]. The estimated pressure of amphibole and clinopyroxene (d,e). The red and blue lines are the density curves of estimated pressure.
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Figure 11. Al2O3 vs. SiO2 (a), CaO vs. SiO2 (b), FeO vs. SiO2 (c), and MgO vs. SiO2 (d) diagrams show the estimated major compositions of amphibole- and clinopyroxene-coexisting melts [23].
Figure 11. Al2O3 vs. SiO2 (a), CaO vs. SiO2 (b), FeO vs. SiO2 (c), and MgO vs. SiO2 (d) diagrams show the estimated major compositions of amphibole- and clinopyroxene-coexisting melts [23].
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Figure 12. Magmatic system model for Hongniu-Hongshan deposit. Depths of magma chambers are estimated from the calculated pressure values of amphibole and clinopyroxene.
Figure 12. Magmatic system model for Hongniu-Hongshan deposit. Depths of magma chambers are estimated from the calculated pressure values of amphibole and clinopyroxene.
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Wang, T.; Peng, H.; Xia, Y.; Chen, Y.; Yang, D.; Zhou, Q. Magmatic Processes of Granitoids in the Hongniu-Hongshan Porphyry-Skarn Copper Deposit, Southern Yidun Terrane, China: Evidence from Mineral Geochemistry. Minerals 2022, 12, 1559. https://doi.org/10.3390/min12121559

AMA Style

Wang T, Peng H, Xia Y, Chen Y, Yang D, Zhou Q. Magmatic Processes of Granitoids in the Hongniu-Hongshan Porphyry-Skarn Copper Deposit, Southern Yidun Terrane, China: Evidence from Mineral Geochemistry. Minerals. 2022; 12(12):1559. https://doi.org/10.3390/min12121559

Chicago/Turabian Style

Wang, Tianrui, Huijuan Peng, Ying Xia, Yue Chen, Dongjie Yang, and Qi Zhou. 2022. "Magmatic Processes of Granitoids in the Hongniu-Hongshan Porphyry-Skarn Copper Deposit, Southern Yidun Terrane, China: Evidence from Mineral Geochemistry" Minerals 12, no. 12: 1559. https://doi.org/10.3390/min12121559

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