Journal of Earth Science  2019, Vol. 30 Issue (2): 309-322   PDF    
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Geochronology and Geochemistry of the Granites from the Longtoushan Hydrothermal Gold Deposit in the Dayaoshan Area, Guangxi:Implication for Petrogenesis and Mineralization
Qian Lihua , Lai Jianqing , Hu Lifang , Cao Rong , Tao Shilong , You Bei     
Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring, Ministry of Education, School of Geosciences and Info-physics, Central South University, Changsha 410083, China
ABSTRACT: The gold mineralization in the Longtoushan hydrothermal gold deposit is concentrated within the contact zone of the granitic complex. Whole rock geochemistry and in-situ U-Pb and Hf isotopic data were used to constrain the genesis and age of the granites and related Cu-Au mineralization in the Longtoushan Deposit. The granites mainly consist of the granite porphyry, rhyolite porphyry, porphyritic granite and quartz porphyry. LA-ICP-MS U-Pb dating of zircons from the granite porphyry, rhyolite porphyry and quartz porphyry indicates that they intruded from ca. 94 to 97 Ma. These intrusions exhibit similar trace element characteristics, i.e., right-dipping REE patterns, depletion of Ba, Sr, P and Ti, and enrichment of Th, U, Nd, Zr and Hf. The εHf(t) values of zircons from the granite porphyry, rhyolite porphyry and quartz porphyry range from -26.81 to -8.19, -8.12 to -5.33, and -8.99 to -5.83, respectively, suggesting that they were mainly derived from the partial melting of the Proterozoic crust. The Cu-Au mineralization is mainly related to the rhyolite porphyry and porphyritic granite, respectively. The Longtoushan granites were most likely formed in a post-collisional extensional environment, and the deposit is a part of the Late Yanshanian magmatism related mineralization in the Dayaoshan area and its adjacent areas.
KEY WORDS: Longtoushan gold deposit    rhyolite porphyry    zircon U-Pb dating    Hf isotopes    petrogenesis    Cu-Au mineralization    
0 INTRODUCTION

The Dayaoshan uplift is located in the southwestern margin of the Qinzhou-Hangzhou metallogenic belt, South China (Mao et al., 2011) (Fig. 1a). The Caledonian and Yanshanian igneous rocks are widespread in the Dayaoshan area and accompanied by numerous W, Mo, Cu, Au, Ag, Pb and Zn mineralization, such as the Longshan, Liucen, Taohua and Gupao gold ore fields (Chen et al., 2015; Huang et al., 2003; Liu, 1993). Approximately 100 gold deposits and occurrences have been discovered in the Longshan ore field, including the medium-scale Longtoushan, Shanhua and Fuliuling deposits (Liu and Tang, 2007).

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Figure 1. (a) Sketch map showing the location of the Dayaoshan uplift in the Qinzhou-Hangzhou metallogenic belt, South China; (b) sketch map of the Dayaoshan uplift (modified after Chen et al., 2015); (c) geological sketch map of the Pingtianshan granitic complex.

The Longtoushan hydrothermal gold deposit is located on the western side of the Pingtianshan granite, which is situated in the southwestern region of the Dayaoshan uplift (Fig. 1). This deposit was discovered in the 1980s and has been mined for Au since 1994. Geological exploration in recent years has recognized deep Cu mineralization that occurs predominantly in the sandstone of the middle member of the Lianhuashan Formation. Both the Cu and Au mineralization shows spatial and temporal associations with the Longtoushan granitic complex that was emplaced during the Middle to Late Cretaceous (Duan et al., 2011; Wang, 2011; Chen et al., 2008). The Longtoushan complex is mainly composed of granite porphyry, rhyolite porphyry, porphyritic granite and felsic dykes, and was emplaced in the sandstone of the Lower Devonian Lianhuashan Formation. Petrogenetic models for these granites include mantle-derived volcanic arc magmatism with the assimilation of the upper crust (Zhang and Zeng, 2014) and partial melting of the Paleoproterozoic Cathaysian basement rock series with some input of mantle material (Duan et al., 2011). However, due to the intensive alteration of the granites, their petrogenesis remains ambiguous. Additionally, their relationship with Cu-Au mineralization is still unclear.

In this paper, we present the petrography, elemental geochemistry, zircon U-Pb dating and Hf isotope data of the granite porphyry, rhyolite porphyry and felsic dykes. The new data, combined with previous studies on the adjacent coeval Pingtianshan granites and Longtoushan granitic complex, are used to constrain the genesis of these felsic intrusions and their relationship with the mineralization. This study is also important for the research on the relationship between the late Yanshanian magmatism and mineralization in South China.

1 GEOLOGICAL SETTING 1.1 Regional Geology

The Qinzhou-Hangzhou metallogenic belt in the Neoproterozoic collisional orogen between the Yangtze Craton and Cathaysia Block is a major polymetallic (Cu, Mo, W, Sn, Pb, Zn, Au and Ag) belt in South China (Mao et al., 2011). The Dayaoshan uplift is a part of the Qinzhou-Hangzhou metallogenic belt, and the strata exposed in the Dayaoshan area, comprises Sinian and Cambrian weakly metamorphic rocks (BGMRGZAR, 1985). The main structures in Dayaoshan include the E-W trending Dayaoshan anticlinorium and the Dali deep fault, which are superimposed by N-E, N-W and S-N trending structures (Fig. 1b). The widespread magmatic intrusions in the Dayaoshan area, including granite, biotite granite, monzogranite and granodiorite, formed during four stages: Caledonian (470–430 Ma), Hercynian-Indosinian (270–240 Ma), Early Yanshanian (170–150 Ma) and Late Yanshanian (110–90 Ma). Among them, the Caledonian and Yanshanian granitoids are the most abundant felsic igneous rocks in the Dayaoshan area (Chen et al., 2015).

1.2 Local Geology

The Cambrian Huangdongkou Formation and Lower Devonian Lianhuashan Formation are exposed in the Longtoushan Deposit (Fig. 2a). The former is composed of fine sandstone, argillaceous siltstone, carbonaceous slate and spotted slate, whereas the latter consists of conglomerate, sandy conglomerate, quartz sandstone, fine sandstone and argillaceous siltstone. The faults in this deposit are NW-SE, N-S, NE-SW or E-W striking. The NW-SE striking faults, characterized by long extensions, deep depths and steep dips, are the main ore-controlling structures in the deposit (Xie and Sun, 1993). The Longtoushan granitic complex dipping to the northwest has an irregular oval shape horizontally and a pipe shape vertically (Fig. 2b). Geological and petrological investigation of the complex revealed that it is composed of cryptoexplosive breccia, rhyolite porphyry and porphyritic granite as the main intrusions, quartz porphyry and felsophyre as dykes emplaced in the rhyolite porphyry, and granite porphyry as the earliest intrusions.

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Figure 2. Geological map (a) and cross-section profile (b) of the Longtoushan Gold Deposit (modified after Huang et al., 1999).

The gold orebodies are dominantly hosted in the contact zone between the rhyolite porphyry and cryptoexplosive breccia, and locally in the fault zone of the wall rock (Fig. 2b). Gold orebodies, which are vein-like, lenticular and cystic in the shape with a dip angle of 76°–90°, are mostly N-W trending in the deposit. In addition, porphyry-style Cu mineralization was discovered in the granite porphyry at depth (~70 m above sea level) (Huang et al., 1999).

The main metallic minerals include native gold, pyrite, chalcopyrite, tetrahedrite, chalcocite, arsenopyrite, pyrrhotite and bismuthinite. The major nonmetallic minerals are quartz, tourmaline, sericite and kaolinite. The ores are mostly in veins or disseminated in the cryptoexplosive breccias with euhedral to anhedral granular textures. Wall-rock alteration is pervasive in the Longtoushan gold deposit and has led to silicification and the deposition of tourmaline, K-feldspar, sericite, argillaceous minerals and pyrite. Tourmalinization and silicification are predominantly distributed in the rhyolite porphyry, cryptoexplosive breccia and their wall rocks, while K-feldspar, sericite and argillic alteration are mainly hosted in the granite porphyry and porphyritic granite.

2 PETROGRAPHY

The off-white granite porphyry shows a porphyritic texture, which consists of quartz, feldspar and biotite. The phenocrysts make up approximately 28% of the whole rock, and the groundmass exhibit a microcrystalline texture. The granite porphyry locally contains breccias, which are composed of sandstones, siltstones and quartzites. The granite porphyry partly occurs as subrounded breccias in the rhyolite porphyry (Fig. 3a). Additionally, feldspar is partly replaced by tourmaline and pyrite (Fig. 3b).

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Figure 3. Field photos and photomicrographs of the granites from the Longtoushan Deposit. (a) The rhyolite porphyry contains subangular breccias of the granite porphyry; (b) feldspar is partly replaced by tourmaline and pyrite in the granite porphyry, transmitted light (-); (c) field photo of the rhyolite porphyry; (d) feldspar is commonly replaced by tourmaline in the rhyolite porphyry, transmitted light (+); (e) intensively eroded quartz phenocryst with a proliferation of fine tourmalines, transmitted light (+); (f) quartz phenocryst has subgrain with abundant cracks, transmitted light (+); (g) neither eroded nor deformed quartz phenocryst, transmitted light (+); (h) field photo of the porphyritic granite; and (i) matrix of the porphyritic granite, transmitted light (+).

The rhyolite porphyry is dark grey, and shows a porphyritic texture (Fig. 3c). The phenocrysts mainly consist of quartz and feldspar, which account for approximately 10%–20% of the whole rock, and the groundmass shows a microcrystalline texture. Feldspar is commonly replaced by tourmaline (Fig. 3d). The rhyolite porphyry contains quartz phenocrysts with different characteristics. Some quartz phenocrysts are intensively eroded and have local proliferations of fine tourmalines (Fig. 3e). Some quartz phenocrysts have plastic deformation such as subgrains with abundant cracks (Fig. 3f). Some quartz phenocrysts are neither eroded nor deformed (Fig. 3g). The rhyolite porphyry is located in the middle to the northern part of the deposit and at the border of the Longtoushan complex, and it locally contains breccias that are mainly composed of quartz sandstones.

The porphyritic granite is grey to off-white, occurs as a stock or apophysis, and intrudes into the rhyolite porphyry (Fig. 3h). It shows a porphyroid texture (Fig. 3i), and the phenocrysts and groundmass are composed of quartzs and feldspars with minor biotite.

The quartz porphyry occurs as veins and shows a porphyritic texture. The phenocrysts are composed of quartzs with minor feldspar and biotite, which make up approximately 12% of the whole rock. The quartz phenocrysts are eroded, with variable morphologies, and the groundmass shows a cryptocrystalline texture.

3 SAMPLING AND METHODS

After years of exploitation, the surface of the Longtoushan Deposit collapsed, and the opening at the northern part of the mine was sealed. We have performed a geological survey on levels between 300 and 420 m a.s.l. (above sea level) at the southern part of the Longtoushan Deposit (Fig. 2). In these levels, we observed granite porphyry, cryptoexplosive breccias, rhyolite porphyry, porphyritic granite and quartz porphyry in the Longtoushan complex. Eleven samples were collected from these levels for major and trace element analyses, and three of the samples were used for zircon U-Pb dating and Hf isotopic analysis, including granite porphyry SL-62, rhyolite porphyry SL-124, and quartz porphyry SL-122. The characteristics of the samples are summarized in Table 1.

Table 1 Characteristics of samples
3.1 Major, Trace and Ore-Forming Elements

Major and trace element analyses were performed at the ALS Geochemistry Laboratory in Guangzhou. Lithium borate and lithium nitrate dissolution, and X-ray fluorescence spectrometry (XRF) were used to determine major element analyses, and the analytical uncertainties were generally within 0.1%–1.0% (RSD). Trace elements were detected using the lithium borate dissolution method by ICP-MS (Element, Finnigan MAT). The uncertainties in the analysis for most trace elements were less than ±10%.

Concentrations of ore-forming elements were analyzed at the Guilin Research Institute of Geology for Mineral Resources. W was detected by a Metrohm 797 Computrace (Herisau, SUI). Sn and Ag were analyzed by emission spectroscopy with a Zeiss PQS-2 2-m plane grating spectrograph (Oberkochen, GER). Mo, Cu, Pb and Zn were detected by Thermo Electron iCAP 6300 ICP-OES (Waltham, MA, USA). Sb and Bi were detected by Haiguang AFS-3000 Atomic Fluorescence Spectroscopy (AFS) (Beijing, CHN). Au was analyzed by chemical spectrum with a Rayleigh WP-1 1-m plane grating spectrograph (Beijing, CHN).

3.2 Zircon U-Pb Dating

The zircons were selected by Langfang Integrity Geological Services Ltd. using the single mineral conventional separation method. Cathodoluminescence (CL) images were taken at Chongqing Yujin Technology Co. Ltd. The U-Pb dating of zircon was conducted by inductively coupled plasma mass spectrometry (LA-ICP-MS) at Nanjing FocuMS Contract Testing Co. Ltd. using Teledyne Cetac Technologies Analyte Excite laser-ablation system (Bozeman, Montana, USA) and Agilent Technologies 7700x ICP-MS (Hachioji, Tokyo, JPN). The 193 nm ArF excimer laser, homogenized by a set of beam delivery systems, was focused on the zircon surface with a fluence of 6.0 J/cm2. The ablation protocol employed a spot diameter of 32 μm at an 8 Hz repetition rate for 40 s (320 pulses). Helium was applied as carrier gas to efficiently transport aerosol to the ICP-MS. Zircon 91500 was used as an external standard to correct instrumental mass discrimination and elemental fractionation during the ablation. Zircon GJ-1 was analysed as the quality control for the geochronology analysis. The results of zircon standards in this study are presented in Tables S1 and S2. During analysis, 91500 and GJ-1 yield weighted average 206Pb/238U ages of 1 062.5±2.8 Ma (2σ, n=29) and 600.7±2.1 Ma (2σ, n=15), respectively, which are in good agreement with the recommended ages (Jackson et al., 2004; Wiedenbeck et al., 1995). Lead abundance of zircon was external calibrated against NIST SRM 610 with Si as internal standard, while Zr as internal standard for other trace elements (Hu et al., 2011; Liu et al., 2010). Raw data reduction was performed off-line by the ICPMSDataCal software (Liu et al., 2010). Quantitative calibration for Pb isotope dating was performed by ComPbcorr#3_18 (Andersen, 2002), and concordia diagrams and weighted mean calculations were performed using Isoplot 4.15 (Ludwig, 2003).

3.3 Zircon Lu-Hf Isotope

Based on the cathodoluminescence images and zircon U-Pb dating results, most zircons were further analyzed for Lu-Hf isotopes. The hafnium isotopic ratios of zircon were conducted by LA-MC-ICP-MS at Nanjing FocuMS Contract Testing Co. Ltd. using Teledyne Cetac Technologies Analyte Excite laser-ablation system (Bozeman, Montana, USA) and Nu Instruments Nu Plasma Ⅱ MC-ICP-MS (Wrexham, Wales, UK) were combined for the experiments. The 193 nm ArF excimer laser, homogenized by a set of beam delivery systems, was focused on the zircon surface with a fluence of 6.0 J/cm2. The ablation protocol employed a spot diameter of 50 μm at an 8 Hz repetition rate for 40 s (320 pulses). Two zircon standards (including GJ-1, 91500, Plešovice, Mud Tank, Penglai) were analyzed as quality control after every ten unknown samples. The results of zircon standards are shown in Table S3.

4 RESULTS 4.1 Major and Trace Element Compositions

Major and trace element data for the Longtoushan felsic rocks are listed in Table 2, and the contents of their ore-forming elements are shown in Table 3.

Table 2 Major (wt.%) and trace elements (μg/g) data for rocks of the Longtoushan complex
Table 3 Ore-forming elements (μg/g) for rocks of the Longtoushan complex (*the data for crust are from Li (1976))
4.1.1 Major elements

The rhyolite porphyry has high SiO2 contents ranging from 70.35 wt.% to 75.52 wt.%, whereas the granite porphyry has SiO2 contents varying from 68.31 wt.% to 70.75 wt.%. The porphyritic granite and quartz porphyry are characterized by slightly lower SiO2 contents (65.87 wt.%–69.31 wt.%). The Al2O3 contents of these rocks range from 11.29 wt.% to 13.90 wt.%, belonging to the peraluminous group. The granite porphyry, porphyritic granite and quartz porphyry show limited compositional variations, whereas the rhyolite porphyry has lower K2O contents and slightly higher CaO contents.

4.1.2 Trace elements

The granite porphyry has a total REE (∑REE) of=58.56–128.19 μg/g with LREE/HREE ratios=4.00–13.74 and δEu=0.28–0.72. The rhyolite porphyry has ∑REE=22.28–36.01 μg/g with LREE/HREE ratios=2.32–4.64 and δEu=0.94–1.09. The porphyritic granite has ∑REE=140.28–159.42 μg/g with LREE/HREE ratios=8.75–15.96 and δEu=0.74–0.79. The quartz porphyry has ∑REE=146.63–148.77 μg/g with LREE/HREE ratios=13.32–13.47 with δEu=1.06–1.15. In the chondrite-normalized REE diagrams (Figs. 4a, 4b, 4c), the granite porphyry, porphyritic granite and quartz porphyry have right-dipping REE patterns, whereas the rhyolite porphyry is characterized by LREE depletion.

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Figure 4. Chondrite-normalized REE distribution patterns (a), (b), (c) and primitive-mantle normalized spidergrams (d), (e), (f) for granites of the Longtoushan complex. The chondrite and primitive mantle values are from McDonough and Sun (1995), the trace elements of the Pingtianshan granites are from Duan et al. (2011).

The granites from the Longtoushan complex exhibit similar primitive mantle-normalized trace element patterns (McDonough and Sun, 1995) (Figs. 4d, 4e, 4f). The granite porphyry, rhyolite porphyry, porphyritic granite and quartz porphyry are depleted in Ba, Sr, P and Ti and rich in Th, U, Nd, Zr and Hf. In addition, the quartz porphyry has negative Nb and Ta anomalies.

4.1.3 Ore-forming elements

The W, Sn, Mo, Bi, Sb, Ag and Au contents of the granites from the Longtoushan Deposit are higher than the Clarke values, whereas the Zn contents of the granites are lower than the Clarke values (Fig. 5). In addition, the granite porphyry has the lowest Sn contents.

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Figure 5. Crust-normalized ore-forming element distribution patterns (crustal values are from Li (1976))
4.2 Zircon U-Pb Data

Most zircon crystals collected from samples SL-62, SL-124 and SL-122 have euhedral, columnar shapes, with lengths ranging from 150 to 300 μm and length/width ratios from 1.5 : 1 to 3.5 : 1. The CL images reveal that most zircon crystals have oscillatory zoning, and parts of them have residual zircon cores (Fig. 6). Except for individual spots, the zircons have Th/U ratios varying from 0.19 to 0.80 (Table 4), implying a magmatic origin (Zhang et al., 2017; Xiong et al., 2016). The 206Pb/238U and 207Pb/206Pb ages are used for young zircons (< 1 000 Ma) and old zircons (> 1 000 Ma), respectively. Fourteen spot analyses on the zircons from SL-62 (granite porphyry) plot on or near the concordia and yield a weighted average 206Pb/238U age of 96.4±1.0 Ma (MSWD=3.0) (Fig. 7a). In addition, one spot from a zircon core has a Paleoproterozoic age (2 381±19 Ma).

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Figure 6. CL images of zircon grains for the (a) granite porphyry (SL-62), (b) rhyolite porphyry (SL-124), and (c) quartz porphyry (SL-122). The white solid circles are the locations of the LA-ICP-MS U-Pb analyses, and the white dotted circles are the locations of the LA-MC-ICP-MS Hf analyses.
Table 4 LA-ICP-MS U-Pb data for granite porphyry SL-62, rhyolite porphyry SL-124 and quartz porphyry SL-122
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Figure 7. LA-ICP-MS U-Pb zircon Concordia diagrams for (a) granite porphyry (SL-62), (b) rhyolite porphyry (SL-124), (c) quartz porphyry (SL-122), and (d) their inherited zircons (SL-62, SL-124 and SL-122).

Sixteen spot analyses of sample SL-124 (rhyolite porphyry) fall in a group, yielding a weighted average 206Pb/238U age of 96.7±0.9 Ma (MSWD=2.9) (Fig. 7b). Four analyses on inherited zircon cores yield concordant 207Pb/206Pb ages of 950±5, 969±5, 756±4 and 1 033±20 Ma. Fourteen spot analyses of sample SL-122 (quartz porphyry) plot on or near the concordia, yielding a weighted average 206Pb/238U age of 94.4±0.8 Ma (MSWD=1.9) (Fig. 7c). Additionally, one spot located in a zircon core shows an Archean age (2 861±15 Ma).

4.3 Zircon Hf Isotopic Compositions

Thirty-six zircons from the three samples (SL-62, SL-124 and SL-122) were analyzed for Hf isotopic compositions, and the results are listed in Table 5. These zircons have initial 176Hf/177Hf ratios ranging from 0.281 952 to 0.282 481, 0.282 483 to 0.282 563, and 0.282 460 to 0.282 549, corresponding to initial εHf(t) values from -26.81 to -8.19, -8.12 to -5.33, and -8.99 to -5.83, respectively. In the εHf(t)-age (Ma) diagram (Fig. 8), all spots plot below the CHUR evolutionary line.

Table 5 Zircon Lu-Hf isotopic data for granite porphyry SL-62, rhyolite porphyry SL-124 and quartz porphyry SL-122
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Figure 8. Zircon εHf(t)-age(Ma) diagram of the felsic rocks from the Longtoushan Deposit. Depleted mantle: (176Lu/177Hf)DM=0.038 4 (Griffin et al., 2000); (176Lu/177Hf)CHUR=0.033 2 (Blichert-Toft and Albarède, 1997).
5 DISCUSSION 5.1 Alteration Effects on the Chemical Compositions

Petrographic examination indicates that these samples have suffered variable degrees of alteration, which is corroborated by the variable loss on ignition (LOI) values of 2.00%–7.66%. Some major elements such as Ca, Na and K, and large ion lithophile elements (LILE: Sr and Ba) are generally mobile during alteration in the granite (Pan et al., 2018; MacLean, 1990). According to Alderton et al. (1980), all REEs were lost during tourmalinization. K and Rb are usually concentrated in feldspar and mica, but could not be accommodated in the tourmaline. The chondrite-normalized REE diagrams and K, Rb, Ba of the rhyolite porphyry were intensively disturbed (Fig. 4). Therefore, we avoid using major elements and large ion lithophile elements to investigate petrogenesis, and instead, mainly focus on the REEs and high field strength elements, considering the alteration of the sample, to constrain petrogenesis. However, compared with the primitive mantle-normalized trace element patterns of the adjacent Pingtianshan granites, Rb would be weakly influenced in the granite porphyry, porphyritic granite and quartz porphyry.

Zircon is extremely resistant to later geological processes and can survive post-crystallization thermal disturbances (Kinny and Maas, 2003; Elburg, 1996). It has high Hf concentrations and low Lu/Hf ratios; thus, the 176Hf/177Hf ratios of samples can reflect the initial Hf isotopic compositions of the system (Wu et al., 2007; Knudsen et al., 2001).

5.2 Age and Tectonic Setting of the Longtoushan Granites

The granite porphyry, rhyolite porphyry and quartz porphyry contain inherited zircons with ages of 2 360–2 400, 950–1 030, ~760 and 2 850–2 870 Ma (Fig. 5d). The ages of 2 850–2 870 and 2 360–2 400 Ma are consistent with the existence of Archean continental basement or some ancient materials from adjacent old land recycled in the Cathaysia Block (Zou et al., 2014). The 950–1 030 Ma age group can be related to the Grenville orogenic event (Greentree et al., 2006). In addition, the age of ~760 Ma is consistent with the split time of the Yangtze and Cathaysia blocks that converged in the Jinningian Orogeny (Shu et al., 2011). These older zircons are considered to be inherited from the source region or introduced by contamination during magma ascent through the crust.

The magmatic zircons from the Longtoushan granite porphyry, rhyolite porphyry and quartz porphyry have ages of 96.4±1.0, 96.7±0.9 and 94.4±0.8 Ma, respectively. The age of the rhyolite porphyry agrees with the previous zircon U-Pb ages of 103.3±2.4 and 96.1±3.0 Ma (Duan et al., 2011; Chen et al., 2008). Combined with field features, the rhyolite porphyry intruded slightly later than the emplacement of the granite porphyry, and the quartz porphyry intruded into the rhyolite porphyry at 94.4±0.8 Ma. These data demonstrate that the entire emplacement of the Longtoushan complex occurred over a short time.

The Longtoushan granite porphyry, porphyritic granite and quartz porphyry, and the Pingtianshan granites plot into the post-COLG (post-collisional granite) field in the Rb versus Y+Nb discrimination diagram, with rhyolite porphyry plotting in the VAG (volcanic arc granite) field (Fig. 9) (Pearce, 1996). However, Rb element would be loss due to the replacement of feldspar by tourmaline in the rhyolite porphyry. The ages of the granite porphyry and rhyolite porphyry are consistent with that of the Pingtianshan granodiorite, which has a LA-ICP-MS zircon U-Pb age of 96.2±0.4 Ma (Duan et al., 2011). Thus, the Longtoushan granitic complex was most likely formed in a post-COLG setting. During the Cretaceous (Late Yanshanian) in South China, the sinistral strike-slip faults and related pull-apart basins of Cretaceous age are closely associated with the changing direction of the Paleo-Pacific Plate from oblique subduction to parallel to the continental margin at 80–135 Ma (Mao et al., 2011). These processes can be ascribed to regional large-scale lithospheric thinning and delamination of the thickened lithosphere and thermal erosion, which is the key for the formation of a wide range of mineral deposits (Mao et al., 2011). Late Yanshanian (90–110 Ma) is one of the most intensive periods of the magmatic activity in the Dayaoshan area (Chen et al., 2015). Consequently, the Longtoushan granites were most likely formed in a post-collisional extensional environment, which is consistent with the Late Cretaceous diagenesis and mineralization in the Dayaoshan and its adjacent areas (Bi et al., 2015; Hu et al., 2012; Wang et al., 2005).

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Figure 9. (Y+Nb) vs. Rb diagram of Pearce et al. (1984) showing the field for post-collision granite (post-COLG) from Pearce (1996).
5.3 Sources of Magma

The granites in the Longtoushan Deposit exhibit similar geochemical characteristics for the trace elements, displaying right-dipping REE patterns (Fig. 4). They are depleted in Ba, Sr, P and Ti and rich in Th, U, Nd, Zr and Hf. In addition, these rocks have similar zircon U-Pb ages and mineral compositions, composed mainly of quartz, feldspar and biotite. Therefore, the Longtoushan granites may have a similar source region. The Longtoushan granites plot next to the continental crust and far to the primitive mantle and MORB (mid-ocean ridge basalts) subfield in the Nb/Th vs. Nb plot (Fig. 10a). The Th/Nb ratios of the Longtoushan granites range from 1.0 to 1.5. The Longtoushan and Pingtianshan granites plot around the trend line of Th/Nb=1 and next to the UCC (upper continental crust) in the Th/Y versus Nb/Y diagram (Fig. 10b). The Zr/Hf ratios of the granites vary between 34.7 and 38.6, which are close to the average crust of 36.7 (Rudnick and Gao, 2003). Therefore, the Longtoushan and Pingtianshan granites were mainly derived from the crust. In addition, the Nb/Ta ratio can represent the proportion of crustal components when the magma was formed (Chen et al., 2018; Stepanov and Hermann, 2013). The Zr/Hf and Nb/Ta ratios of the granite porphyry are slightly lower than that of the rhyolite porphyry, porphyritic granite and quartz porphyry (Table 2), which indicates that source of the granite porphyry may be different to the other granites in the Longtoushan Deposit.

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Figure 10. Nb/Th versus Nb (a), and Th/Y versus Nb/Y (b) plots (after Boztuğ et al., 2007) for granites in the Longtoushan Deposit and Pingtianshan Deposit. Primitive mantle after Hofmann (1988); continental crust, MORB (Mid-ocean ridge basalts) and OIB (ocean-island basalts) after Schmidberger and Hegner (1999) in (a). The compositions of MORB, OIB and UCC (upper continental crust) were taken after Taylor and McLennan (1985) in (b).

The granite porphyry, rhyolite porphyry and quartz porphyry have εHf(t) values of -26.81 to -8.19, -8.12 to -5.33, and -8.99 to -5.83 respectively. The εHf(t) values of the rhyolite porphyry are consistent with the results from Wang (2011).

Moreover, the εHf(t) values of the porphyritic granite range from -6.2 to -2.8 (Wang, 2011). The negative εHf(t) values for zircons from the Longtoushan granitic complex indicate that the magmas were dominantly derived from crustal materials. This is also supported by the presence of inherited zircons in these granites.

Previous detrital zircon U-Pb data indicate that the Dayaoshan-Damingshan area possessed a Cathaysian attribute in continental accretion and tectonic evolution during the pre-Devonian (Zou et al., 2014; Li et al., 2009). In western Cathaysia, the Neoproterozoic crust was generated by mixing between magmas derived from a juvenile source, and others derived by remelting of an older basement, which in turn served as the source for Phanerozoic magmas (Li et al., 2014; Xu et al., 2007). The porphyritic granite has a Neoproterozoic inherited core with a positive εHf(t) values of 9.2 (Wang, 2011), suggesting the generation of juvenile crust at this time. Zircons from the granite porphyry show a large range of negative εHf(t) values, which also suggest the heterogeneity of the source materials (Yu et al., 2010). The model-age approach cannot resolve more than one previous event; thus, the single-stage Hf model ages of zircons from Phanerozoic igneous rocks are equivalent to the age of the main crustal source rocks in western Cathaysia (Xu et al., 2007). The single-stage Hf model ages of zircons from the granite porphyry, rhyolite porphyry and quartz porphyry vary from 1 092 to 1 848, 985 to 1 131, and 999 to 1 151 Ma, respectively. Moreover, the rhyolite porphyry and porphyritic granite contain several Early Neoproterozoic inherited zircons. Therefore, the rhyolite porphyry, porphyritic granite and quartz porphyry in the Longtoushan Deposit were mainly derived from Late Mesoproterozoic to Early Neoproterozoic crustal materials, while the granite porphyry may have dominantly originated from the melting of Mesoproterozoic crust.

The εHf(t) values of the porphyritic granite are slightly higher than those of the rhyolite porphyry and quartz porphyry (Fig. 8), and the εHf(t) values of zircons from the Pingtianshan granodiorite range from -2.5 to 0.9 (Wang, 2011). Combined with ages of the Longtoushan granites and the Pingtianshan granodiorite, the earliest magma was dominantly derived from old crust, whereas the late magma may be contaminated with some juvenile materials. In addition, the porphyritic granite and the Pingtianshan granodiorite may intrude simultaneously.

5.4 Implications for Mineralization

The Damingshan tungsten deposit, located in the southwestern side of the Dayaoshan area, has a molybdenite Re-Os isochron age of 95.4±1.0 Ma (Li et al., 2008). The Wangshe porphyry W-Mo-(Cu) deposit in the Damingshan area has a molybdenite Re-Os isochron age of 93.8±4.6 Ma (Lin et al., 2008), and the ore-related biotite granite was emplaced at 93±1 Ma (Li et al., 2008). The Pingtianshan Mo occurrence has a molybdenite Re-Os isochron age of 96.8±1.9 Ma (Wang et al., 2012). Zircon U-Pb dating indicates that intrusive rocks and associated ores of the Baoshan porphyry Cu deposit formed at approximately 91 Ma (Bi et al., 2015). Zircon U-Pb dating indicates that the Longtoushan complex intruded from ca. 96 to 94 Ma, and associated Cu and Au mineralization dominantly occurred during this period. Thus, Late Cretaceous W-Mo-Cu-(Au) deposits, related to the magmatic rocks, are widespread in Dayaoshan and its adjacent areas, which were induced by the extension of the lithosphere. The Longtoushan gold deposit is part of the Late Yanshanian magmatism related-mineralization in the Dayaoshan area.

The rhyolite porphyry displays a considerable variation in the Au contents, which are relatively low in the porphyritic granite, and the granite porphyry has the lowest Au contents. The positive correlation between Cu and tourmaline is striking, which suggests that the Cu mineralization may be overprinted on the granite porphyry. The δ34S values of pyrite range from +1.29‰ to +2.01‰ (Tao et al., 2017), indicating that ore-forming materials are derived from deep magma. According to Dong (1990), the Neoproterozoic Sibao Group is rich in ore-forming elements, such as W, Sn, Bi, Cu, Pb, Sb, Ag and Au. There was a significant generation of juvenile crust at ~1.0 Ga in the Cathaysia Block (Li et al., 2014; Yu et al., 2010), which may lead to the enrichment of Cu and Au in the Early Neoproterozoic crust. Gold mineralization is mainly concentrated in the contact zone between the rhyolite porphyry and cryptoexplosive breccia, whereas porphyry-style Cu mineralization was discovered in the granite porphyry at depth (~70 m above sea level) (Huang et al., 1999). Additionally, a Mo occurrence was found in the Pingtianshan granodiorite, and the ore minerals mainly consist of pyrite, chalcopyrite and molybdenite. The porphyritic granite was formed concurrently with the Pingtianshan granodiorite. Therefore, the Au and Cu mineralization may be mainly related to the rhyolite porphyry and porphyritic granite respectively, which were dominantly derived from the partial melting of the Late Mesoproterozoic to Early Neoproterozoic crust with an involvement of juvenile materials, whereas the granite porphyry may only contribute to the pre-enrichment of the Au and Cu mineralization.

The cryptoexplosive breccia related to the rhyolite porphyry, provided convergent and precipitable space for ore-forming fluids. The emplacement of gold orebody is close to or slightly less than 0.5 km in the Longtoushan Deposit (Huang et al., 1999). Therefore, it is a hypabyssal hydrothermal gold deposit related to the cryptoexplosive breccia and overprinted by porphyry-style Cu (Au) mineralization. In addition, the mineralization process was completed in a short time based on the ages of the Longtoushan granites.

6 CONCLUSIONS

(1) Zircon U-Pb dating of the granite porphyry, rhyolite porphyry and quartz porphyry yields ages of 96.4±1.0, 96.7±0.9, and 94.4±0.8 Ma, respectively, which indicates they were formed in the Late Cretaceous. The Longtoushan granites were most likely formed in a post-collisional extensional environment.

(2) The granites in the Longtoushan Deposit exhibit similar geochemical characteristics for the trace elements, displaying right-dipping REE patterns. They are depleted in Ba, Sr, P and Ti and enriched in Th, U, Nd, Zr and Hf, which exhibit a crustal affinity.

(3) The rhyolite porphyry, porphyritic granite, and felsic dykes in the Longtoushan Deposit were mainly derived from Late Mesoproterozoic to Early Neoproterozoic crust with an input of juvenile materials, whereas the granite porphyry may dominantly originate from the melting of Mesoproterozoic crust.

(4) The main Cu and Au mineralization stage is related to the rhyolite porphyry and porphyritic granite respectively, whereas the granite porphyry may only contribute to the pre-enrichment of Cu and Au mineralization. The Longtoushan gold deposit is a part of the Late Yanshanian magmatism related mineralization in the Dayaoshan area.

ACKNOWLEDGMENTS

This study was supported by the Project of Innovation-driven Plan in Central South University (No. 2015CX008) and the Fundamental Reserch Funds for the Central Universities of Central South University (No. 2015zzts071). Conggao Liang and Rui Huang are thanked for their invaluable help and support during fieldwork. The U-Pb dating and in situ Hf ratio analyses of the zircons were carried out at Nanjing FocuMS Contract Testing Co. Ltd., with support from Liang Li and Jianfeng Gao. We appreciate two anonymous reviewers, who helped to improve the paper greatly. Moreover, we thank Miao Yu, Quan Ou and Wenzhou Xiao for their constructive reviews and useful suggestions. The final publication is available at Springer via https://doi.org/10.1007/s12583-018-1204-7.

Electronic Supplementary Materials

Supplementary materi-als (Tables S1, S2, S3) are available in the online version of this article at https://doi.org/10.1007/s12583-018-1204-7.


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