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Minerals 2019, 9(4), 243; https://doi.org/10.3390/min9040243
Mineralogy and Garnet Sm–Nd Dating for the Hongshan Skarn Deposit in the Zhongdian Area, SW China
School of Earth Resources, China University of Geosciences, Wuhan 4330074, China
State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China
Jiangxi Institute of Geological Exploration for Mineral Resources of Nonferrous Metals, Nanchang 330025, China
Yunnan Geological Survey, Kunming 650051, China
Author to whom correspondence should be addressed.
Received: 23 February 2019 / Accepted: 17 April 2019 / Published: 19 April 2019
The Hongshan deposit is one of the largest Cu-polymetallic deposits in the Zhongdian area, southwest China. Two types of Cu–Mo ores, mainly developed in the skarns, have been recognized in the Hongshan deposit, i.e., massive or layered skarn and vein-type, with the former being dominant. The highly andraditic composition of garnet (Adr100 to Adr64Gr32) and diopsidic composition of pyroxene (Di90Hd9 to Di1Hd99) indicate the layered skarn ores are of magmatic-hydrothermal origin that formed under oxidized conditions. Sm–Nd dating of garnet yield a well-constrained isochron age of 76.48 ± 7.29 Ma (MSWD = 1.2) for the layered skarn ores. This age was consistent with the Re–Os age for the pyrrhotite from the layered skarn ores, and thereby indicated that the layered skarn mineralization was formed in the Late Cretaceous, rather than in the Triassic as was previously thought. The coincidence of the geochronology from the layered skarn ores and vein-type mineralization further indicated that both ores were the result of a single genetic event, rather than multiple events. The recognition of the Late Cretaceous post-collisional porphyry–skarn Cu–Mo–W belt in the Zhongdian area exhibited a promising prospecting potential.
Keywords:skarn mineralogy; high fugacity; garnet Sm–Nd dating; Late Cretaceous; Hongshan skarn deposit
The Zhongdian Cu-polymetallic area, situated in the southern segment of the Yidun arc (Figure 1a,b), is commonly regarded as one of the most fertile regions for porphyry and skarn copper deposits in China [1,2,3]. Many of the porphyry copper deposits in the Zhongdian area (Figure 1c), such as Pulang (803.85 Mt with 0.52% Cu, 0.18 g/t Au, [4,5]), Xuejiping (54.15 Mt with 0.53% Cu, 0.06 g/t Au), Langdu (1.67 Mt with 6% Cu), Chundu, and Lannitang (36 Mt with 0.50% Cu, 0.45 g/t Au) deposits were formed in the Late Triassic as a result of westward subduction of the Garzê–Litang oceanic crust [4,5]. Recently, the recognition of some Late Cretaceous porphyry–skarn deposits, represented by the Hongshan Cu–Mo, Tongchanggou Mo–Cu, Xiuwacu W–Mo, and Relin W–Mo deposits, has drawn much attention on the collision-related metallogeny of the Zhongdian area [6,7,8,9].
The Hongshan deposit is one of the largest Cu-polymetallic deposit in the Zhongdian area (Figure 1c) with contained metals of 0.64 t Cu, 5769 t Mo, 7532 t W, 323 t Ag, and 25262 t Pb + Zn . Generally, two types of ores, namely the major layered skarn Cu-polymetallic ores and later vein-type Cu–Mo mineralization, have been formed [9,11]. Trace elements of pyrite and pyrrhotite from both types display distinct signatures and could plausibly indicate different origins . Previous research proposed a two-stage model for the genesis of the Hongshan deposit with Triassic skarn Cu-polymetallic ores overprinted by Late Cretaceous vein-type Cu–Mo mineralization [10,11]. The age of later vein-type Cu–Mo mineralization has been well-constrained by molybdenite Re–Os and zircon U–Pb ages [9,11,12]. The mineralization age of the layered skarn Cu-polymetallic ores, however, has been poorly constrained. Zu et al. reported a Cretaceous age for the layered skarn ores using Re–Os dating on pyrrhotite ; however, the closure temperature of Re–Os systems for pyrrhotite is as low as 400 °C and might have undergone osmium diffusion or resetting in the presence of a later stage overprint event [13,14]. Thus, the timing of the layered skarn ores is still one key problem with the two-stage model.
A number of studies have indicated that garnets are useful geochemical tracers and can be used in geochronological studies [15,16]. In this contribution, we report systematic petrography and geochemistry of the main skarn mineral and Sm–Nd isotope systematics of garnet for layered skarn ores in the Hongshan deposit. The results, combined with the previous data, allow us to constrain the mineralization age of the skarns and to explain the genesis and geologic setting of the Hongshan Deposit. The implications for Cretaceous metallogeny in the Zhongdian area are also discussed.
2. Geological Setting
The Zhongdian area is located at the southern end of the Yidun island arc, forming a significant metallogeny belt in the Sanjiang Tethyan metallogenic domain (Figure 1b) [15,16]. The main tectonic framework of the Yidun arc is defined by the Garzê–Litang suture belt to the south and east and the major Dege–Xiangcheng fault to the west separates it from Zhongzan massif (Figure 1c) [9,18].
The southern Yidun arc witnessed a complex tectonic evolution history including Triassic oceanic crustal subduction, Jurassic–Cretaceous continental collision, and Cenozoic regional strike-slip faulting [18,19,20]. In the Triassic, westward subduction of the Garzê–Litang ocean led to the formation of the Yidun arc in the eastern part of the Zhongzan block, accompanied by the formation of voluminous volcanic and granitic rocks with ages mainly from 238 to 210 Ma (Figure 1c) [20,21]. These volcanic and granitic rocks host numerous mineral deposits, such as the Gacun volcanic massive sulfide (VMS) Ag-polymetallic deposit and porphyry–skarn deposits that were formed in a compressional setting (Figure 1c), in association with subduction of different angles . When the Garzê–Litang ocean closed completely at the end of the Triassic, the Yidun island arc collided with the Yangtze block, accompanied by the emplacement of sparsely distributed granites from 138 to 75 Ma. Mineral deposits formed in this period of time, such as the Xiuwacu, Relin, Hongshan, and Tongchanggou deposits [8,23]. During the Cenozoic, a remote response to the Indian–Asian continental collision caused intracontinental strike-slip processes with the emplacement of alkaline granite intrusions and associated Au mineralization within the southern Yidun arc, including the Yaza and Bengge deposits (Figure 1c).
3. Geology of the Hongshan Deposit
The Hongshan Cu-polymetallic deposit is located in the central part of the Zhongdian area (Figure 1c). The main country rocks of the deposit comprise mainly a thick Triassic sequence of crystalline limestone, siltstone, gray to very dark gray argillaceous slate, and interlayered andesitic tuff of the Upper Triassic Qugasi formation (Figure 2). These rocks underwent contact thermal and replacement metamorphism with the development of extensive hornfels, marble and skarn throughout the deposit and surrounding areas (Figure 3).
Sparse felsic dikes and offshoots are exposed in the Hongshan deposit area. Diorite porphyries are the most widely distributed, and occur mainly in the southeastern and northern parts of the deposit area with SHRIMP U–Pb ages of 214 ± 2 Ma . However, a direct contact between these porphyries and the orebodies has not been observed in the Hongshan deposit (Figure 2a). Quartz monzonite porphyry outcrops only as three small intrusions in the middle of the Hongshan area; a small part of the quartz monzonite porphyries have been altered to endoskarn near contact zones with marble (Figure 3a). Boreholes CK 17-2 and CK 17-4 have revealed concealed mineralized stocks that may be larger than the outcrops (Figure 2b,c) . Previous dating suggests that the quartz monzonite porphyry and the concealed granite porphyry were formed in Late Cretaceous with zircon U–Pb ages of 77–81 Ma [12,17,23].
The copper orebodies in the study area are sheet-like, tabular, or lenticular; are subparallel to the main NNW-striking structures in the deposit area; and are hosted by skarn and hornfels zones. Individual orebodies generally strike NNW and range from 30 to 1223 m in length along strike, 20 to 420 m in width along the dip direction, and 3.9 to 19.6 m in thickness. Based on mineral assemblages and ore textures, two types of Cu–Mo ores, mainly developed in the skarns, have been recognized, i.e., massive or layered skarn (Figure 3a) and vein-type, with the former being dominant. Some other ore types are also present: hydrothermal Pb–Zn ores in the periphery of the Hongshan deposit (Figure 2a), and Mo–Cu porphyry ores associated with concealed intrusions at depth . The alteration and mineralization in the Hongshan deposit can be divided into three alteration stages. The early stage (stage 1) is characterized by the alteration of wallrocks to albite–biotite–quartz–andalusite hornfels and massive, sage-green diopside hornfels (Figure 3d–f). In the skarn stage (stage 2), the interaction between the magmatic hydrothermal fluids and marble resulted in diverse assemblages of calc–silicate skarn minerals including garnet, diopside, wollastonite, and magnetite. The early retrograde alteration stage (stage 3a) is characterized by hydrosilicate minerals and sulfide minerals overprinting on the calcsilicate minerals, forming the massive and disseminated skarn ores (Figure 3b,c). The quartz–sulfide stage (stage 3b) featured a variety of quartz–sulfide veins overprinting skarns and hornfels. Marble-hosted hydrothermal Pb–Zn mineralization scattered on the periphery of the Cu–Mo skarn orebodies (Figure 2a), exhibiting a clear mineralization zoning.
The spatial zonation of the skarn minerals in the Hongshan deposits are also apparent with at least three associations. The endoskarns are only present within the quartz monzonite porphyry and have a mineral assemblage of garnet, diopside, and vesuvianite (Figure 3a) . Exoskarns are more widely developed with a mineral assemblage of garnet, diopside, fluorite, and actinolite in the massive or layered skarn ores (Figure 3b,c). Minor distal skarns are scattered in the periphery of the main orebodies and associated with marble, hornfels, or wollastonite (Figure 3d–f). Garnet is the most abundant mineral in different assemblages and several different garnet generations can be optically distinguished. Garnets in the endoskarns are reddish, fine-grained, anisotropic, and have dodecahedral and/or polysynthetic twinning (Figure 3a). Garnets in the exoskarns are isotropic and euhedral, anisotropic (Figure 3j,k), and display evident oscillatory zoning (Figure 3g–i). The epitaxial growth with oscillatory zoning exoskarn garnets indicate these garnets might have formed during different stages (Figure 3g). Garnets from the distal skarns are greenish, brown, and rarely reddish in color without apparent oscillatory zonings (Figure 3l).
4. Sampling and Analytical Methods
Polished sections were used to study skarn minerals following by electron microprobe analyses on representative garnet, pyroxene, and tremolite (actinolite). Major element analyses of these minerals were carried out at the China University of Geosciences (Beijing) using a Shimadzu EPMA-1600. An accelerating voltage of 25 kV, a beam current of 20 nA, and a beam diameter of 1 μm were applied. Natural minerals and synthetic oxides were used as standards, and all data were corrected using standard ZAF correction procedures.
Ten skarn-bearing samples from the layered skarn ores in the Hongshan deposit were collected to pick pure garnet separates from high precision Sm–Nd isotope systematic analyses. Pure garnet separates were handpicked under a binocular microscope to select grains similar in appearance to the chips of the 60–80 mesh size crushed by agate mortar, and then powered to <200 mesh size. Sm–Nd isotopic analyses were performed on a ISOPROBE-T thermal ionization mass spectrometry (TIMS) at the Beijing Research Institute of Uranium Geology.
5. Analytical Results
5.1. Mineral Composition
Electron microprobe analyses show that garnets from the deposit form a grossular–andradite solid solution and dominated with andradite (Table 1, Table 2 and Table 3). Forty-nine spots on garnet grains from exoskarn from this study range from almost pure andradite (Adr100) to Adr64Gr32 with less than 5% of other types of garnets (Figure 4a). They contain significant less Al contents than the garnets from endoskarn (Adr22–57Grs78–43) and from distal skarn (Adr14–60Grs86–40) . Oscillatory zoned garnets are generally chemically zoned with fluctuation-ore repeated changes of FeOT content and Al2O3 content (Figure 5). The invariably negative correlation between FeOT and Al2O3 compositions also indicate the common existence of grossular–andradite solid solution (Figure 5).
The pyroxene (12 spots) from the Hongshan deposit are mainly diopside in composition, with minor hedenbergite and rarely johannsenite (Figure 4b). The composition of the pyroxene from the exoskarn has large variations with Di90Hd9 to Di1Hd99 (Table 4).
The prograde garnet, pyroxene, and wollastonite are commonly overprinted by retrograde tremolite, actinolite, epidote, and minor chlorite. The actinolite minerals (6 spots) in this study belongs to ferro-actinolite with high amounts of MgO (19–22 wt %) and FeOT (5–11 wt %) (Table 5).
5.2. Sm–Nd Dating Result
The Sm and Nd concentrations and isotopic ratios of garnets from the Hongshan deposit are listed in Table 6 and plotted in Figure 6. All the 10 analyzed samples showed a roughly linear correlation with an isochron age of 60.91 ± 4.94 Ma (MSWD = 26) (Figure 6a) using IsoplotR . Three garnet samples (D21-5, D48-13, and KT2-4296-6) deviate the isochron line and could plausibly represent the incorporation of other minerals or/and affected by later perturbation despite detailed petrography study before analyzing (Figure 6a). The remaining 7 samples revealed a well-constrained isochron age of 76.48 ± 7.29 Ma (MSWD = 1.2) (Figure 6b), which represents the crystallization time of the garnet. The uncertainty of ±7.29 Ma is due to low 147Sm/144Nd ratios (<0.30) rather than low precision of the Nd isotope ratios.
6.1. Mineralogy and Geochemistry of Skarn Minerals
The identification and classification of the skarn deposits were based on their mineralogy . The Hongshan deposit is featured with layered or stratabound Cu ores with their occurrence in accordance with the host volcano-sedimentary wall rocks of the Qugasi Formation, which has led to the conception of sedimentary or SEDEX origin for these ores [10,11]. However, the pervasively distributed skarn minerals, e.g., andraditic garnet (Figure 4a), diopside–hedenbergite pyroxene (Figure 4a), actinolite, in the layered Cu ores are similar to the typical skarn Cu deposits as a result of interaction between felsic intrusion and surrounding carbonate rocks . The lithologic or bedding contacts may provide highly permeable and/or reactive layers for the infiltration and lateral movement of the skarn-forming fluids . Other evidences including the vertical zonation of skarn minerals, from top to bottom, are biotite hornfels–pyroxene hornfels, marble, wollastonite skarn (Wo > Pyx–Grt), pyroxene skarn (Pyx > Grt), garnet skarn (Grt > Pyx), pyroxene skarn, pyroxene hornfels, and biotite hornfels .
The chemical composition of the skarn minerals can also be used to evaluate the oxidation state of the mineralization fluids . Many skarn Cu deposits contain a high abundance of chalcopyrite, bornite, pyrite, and magnetite ± hematite, which are believed to have formed from a high oxygen fugacity environment . However, the large proportions of pyrrhotite (>60% sulfide) in the Hongshan deposit tends to be much more reduced than other skarn Cu deposits . The highly andraditic composition of garnet (Adr100 to Adr64Gr32, Figure 4a) and diopsidic composition of pyroxene (Di90Hd9 to Di1Hd99, Figure 4b) indicate the prograde skarn minerals are formed under oxidized conditions as other skarn Cu deposits . This suggest that the initial fluids during the prograde stage are inherently oxidized as other skarn Cu deposits and the rapid descent of the fugacity in the retrograde stage could be led by other effective factors.
6.2. Timing of Mineralization
The layered skarn Cu ores and vein-type Mo–Cu ores are two dominant ores in the Hongshan deposit. The age of the vein-type Mo–Cu ores have been precisely constrained by molybdenite Re–Os ages, ranging from 77 to 81 Ma [9,11,12]. However, the age of the layered skarn Cu ores is still in dispute. The existence of Triassic intrusions in the deposit area led some authors to argue the main layered skarn ores were formed in the Triassic , similar with the Pulang and Xuejiping porphyry Cu deposits in the Zhogndian area . Re–Os dating with pyrrhotite, however, yielded a Late Cretaceous age with large error 79 ± 16 Ma . Considering the low closure temperature of Re–Os systems in the pyrrhotite (as low as 400 °C) , a more robust and convincing method should be considered.
Garnet crystals grow between 400 °C and ~700 °C  and the primary growth ages can survive from wide range of P–T metamorphism, such as amphibolite metamorphism [15,16]. The strong preference of the garnet lattice for Sm over Nd makes Sm–Nd dating in garnet as a highly suitable chronometer for more than 30 years . The garnets in the Hongshan deposit showed consistent or typical oscillatory zonations (Figure 5), indicating they were formed from a single hydrothermal system that underwent chemical fluctuation. The low 147Sm/144Nd ratios of 0.07 to 0.26 for the garnet samples (Table 6) in the Hongshan deposit indicated some silicate inclusions, such as epidote and pyroxene, might not be completely removed when compared with the inclusion-free garnet with high 147Sm/144Nd ratios (>1) [16,32]. Since these inclusions grew in isotopic equilibrium with the surrounding garnet and rock matrix during prograde metamorphism, the requirements of isochron dating are fulfilled. Therefore, the well-constrained isochron age of 76.48 ± 7.29 Ma (MSWD = 1.2) (Figure 6b) revealed from seven garnet separates can represent the crystallization time of the garnet. This age was consistent with the Re–Os age for the pyrrhotite from the layered skarn ores, and thereby indicated that the layered skarn mineralization was formed in the Late Cretaceous, rather than in the Triassic as previously thought . The coincidence of the geochronology from the layered skarn ores and vein-type mineralization further indicated that both ores were the result of a single genetic event, rather than multiple events [10,11]. The development of endoskarn in the quartz monzonite porphyry (Figure 3a) and granite porphyry indicated the Cu–Mo mineralization in the Hongshan deposit was plausibly related to the syntectonic intrusions.
6.3. Implication for the Regional Metallogeny
The metallogenic age of the Hongshan deposit is constrained from the Sm–Nd dating of garnet and previous data to be Late Cretaceous. The Hongshan skarn Cu–Mo deposit, associated with the Xiuwacu quartz vein W–Mo, Relin quartz vein W–Mo, and Tongchanggou porphyry Mo–Cu deposit [8,33,34,35], thus defined a roughly N–S trending Late Cretaceous porphyry belts in the Zhongdian area, locally overlapping the Late Triassic porphyry belt (Figure 1c). Numerous geochemical research on the related intrusions revealed that the Zhongdian area in the Late Cretaceous underwent post-collisional extension event, which was also recently recognized as an optimal setting for the formation of porphyry–skarn deposits [36,37,38]. During post-collisional extension setting, remobilization of chalcophile metals from metasomatized mantle lithosphere have led to the generation of fertile magmas capable of forming porphyry–skarn Cu–Mo–W deposits in the Zhongdian area [17,33].
Two types of Cu–Mo ores, mainly developed in the skarns, have been recognized in the Hongshan deposit, i.e., massive or layered skarn (Figure 3a) and vein-type, with the former being dominant. The highly andraditic composition of garnet (Adr100 to Adr64Gr32) and diopsidic composition of pyroxene (Di90Hd9 to Di1Hd99) indicate the layered skarn ores are magmatic-hydrothermal origin that formed under oxidized conditions. Sm–Nd dating of garnet yielded an isochron age of 76.48 ± 7.29 Ma for the layered skarn ores, indicating the Hongshan deposit was formed in the Late Cretaceous in a unified system rather than from multiple mineralization events. The recognition of the Late Cretaceous post-collisional porphyry–skarn Cu–Mo–W belt in the Zhongdian area exhibits a promising prospecting potential.
Data curation, B.Z. and Y.Z.; funding acquisition, B.Z. and C.X.; project administration, B.Z.; software, C.D.; supervision, C.X.; writing—original draft, B.Z.
This work was supported by National Key Research and Development Program of China (2017YFC0601202), National Science Foundation of China (41802108), and Fundamental Research Funds for the Central Universities (CUGL170812).
We would like to thank Sinacili, Baimakangzhu, He Jianwen, and Lurongcili from the Hongshan deposit for the help in the field. The editor and two reviewers are thanked for their constructive and detailed review of this manuscript.
Conflicts of Interest
The authors declare no conflict of interest.
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Figure 1. (a) Tectonic map sketch showing the distribution of principal continental blocks and sutures of Southeast Asia. The location and outline of the Yidun island arc are indicated. (b) Tectonic framework of the Sanjiang Tethyan domain and location of the southern Yidun arc. (c) Geological map showing the distribution of porphyry and skarn deposits in the southern Yidun arc. Modified from Zu et al. and Deng et al. [1,17].
Figure 2. Simplified geological map (a) and cross-sections (b,c) of the Hongshan Cu–Mo deposit. Modified from Wang et al. .
Figure 3. Photographs and photomicrographs showing characteristics of skarn ores in the Hongshan deposit. (a) Garnet in endoskarn in contact with residual quartz monzonite porphyry. (b) Massive or layered skarn ores with dense disseminated pyrrhotite in exoskarn. (c) Euhedral garnet in exoskarn ores with dense disseminated pyrrhotite and chalcopyrite. (d) Greenish-brown garnet in distal skarns along the marble. (e) Reddish-brown distal skarn in pyroxene hornfels. (f) Brown eduhedral garnet along with marble and wollastonite. (g–k) Garnets in the exoskarns are isotropic and euhedral, anisotropic, and display evident oscillatory zoning. (l) Garnet from distal skarn lacks of apparent oscillatory zonings. Ccp: chalcopyrite, Dio: diopside, Grt: garnet, Po: pyrrhotite, Qtz: quartz, Ser: sericite, Wo: wollastonite
Figure 4. Major element compositions of garnets (a) and pyroxene (b) showing their dominance of end members from three selected skarn mineral deposits. Pyralspite–(pyrope + spessartine + almandine + uvarovite).
Figure 5. Variation of FeOT and Al2O3 content from core to margin in the garnet grains from the layered skarn ores.
Figure 6. Sm–Nd garnet from the layered skarn ore in the Hongshan deposit. Age provided in (a) is calculated from all of the plotted data and age in (b) is calculated from seven garnet samples.
Table 1. Electron microprobe analyses of garnet from the Hongshan deposit (wt %).
|Sample||Garnet–Diopside Skarn||Garnet–Diopside Skarn|
|Number of cations on the basis of 12 O|
Table 2. Electron microprobe analyses of garnet from the Hongshan deposit (wt %).
|Sample||Garnet–Diopside, Exoskarn||Diopside–Garnet Skarn|
|Number of cations on the basis of 12 O|
Table 3. Electron microprobe analyses of garnet from the Hongshan deposit (wt %).
|Sample||Garnet–Diopside Skarn||Diopside–Garnet Skarn|
|Number of cation on the basis of 12 O|
Table 4. Electron microprobe analyses of diopside from the Hongshan deposit (wt %).
|Sample||Garnet–Diopside Skarn||Garnet–Diopside Skarn||Diopside Skarn||Diopside Skarn|
|Number of cations on the basis of 6 O|
Table 5. Electron microprobe analyses of actinolite from the Hongshan deposit (wt %).
|Cations on the basis of 23 oxygens||Si||Ti||Al||Fe3+||Fe2+||Cr3+||Mn||Mg||Ca||Na||K|
Table 6. Sm–Nd concentrations and isotopic ratios of the garnet from the Hongshan deposit.
|Sample||Sm (μg/g)||Nd (μg/g)||147Sm/144Nd||143Nd/144Nd||Standard Error||εNd (t)|
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