Evidence for temporal relationship between the late Mesozoic multistage Qianlishan granite complex and the Shizhuyuan W–Sn–Mo–Bi deposit, SE China

The world-class Shizhuyuan W–Sn–Mo–Bi deposit is spatially related to the Qianlishan granite complex (QGC) in Hunan Province, China. However, the age and classification of the QGC are still debated, and a better understanding of the temporal genetic relationship between the QGC and the Shizhuyuan deposit is essential. Here, we present chemical compositions the intrusive phases of the QGC and the results of detailed zircon U–Pb dating and muscovite Ar–Ar dating of a mineralized greisen vein. Our new zircon laser ablation inductively coupled plasma mass spectrometry U–Pb age data constrain the emplacement of the QGC to 155–151.7 Ma. According to petrological, geochemical and geochronological data and the inferred redox conditions, the QGC can be classified into four phases: P1, porphyritic biotite granites; P2, porphyritic biotite granites; P3, equigranular biotite granite; and P4, granite porphyry dikes. All phases, and especially P1-P3, have elevated concentrations of ore-forming metals and heat-producing elements (U, Th, K; volume heat-producing rate of 5.89–14.03 μWm−3), supplying the metal and heat for the metalogic process of the Shizhuyuan deposit. The Ar–Ar muscovite age (154.0 ± 1.6 Ma) of the mineralized greisen vein in the Shizhuyuan deposit is consistent with the emplacement time of the QGC, suggesting their temporal genetic relationship.

Magmatic fractionation and exsolution of a fluid phase from a cooling pluton plays an important role in metal enrichment for intrusion-related deposits [1][2][3] . Reheating of preexisting semi-solidified plutons triggered by the input of a new magma may lead to the exsolution of fluid and element transport, thus contributing to incremental extraction of metals from the magma and their precipitation in the cupola of plutons 4,5 . Successive magma inputs led to the repeated extraction and precipitation of metals to form ore at the vantage position 2,6 . The lifespan of the magmatic-hydrothermal system triggered by the emplacement of a set of successive plutons controls the timescale of the ore-forming process and thus the metal grades and tonnages of deposits. Therefore, establishing tight temporal ties between magmatism and its associated mineralization is key to understanding the contribution of magma to mineralization. It is generally considered that lifespans of magmatic-hydrothermal systems is less than 10 million years, and even shorter (< 2 Ma) for porphyry deposits [7][8][9][10][11][12] . Accordingly, a time interval between plutons and the associated mineralization of > 10 Ma is interpreted to indicate that they have no genetic ties.
Skarnization. The skarn located in the contact zone in the southeastern region of the QGC has experienced the most pervasive alteration in the Shizhuyuan deposit (Fig. 2). This skarn is approximately 1.2 km long, 1.0 km wide and 50-500 m thick (with an average thickness of 150-200 m). There are three types of skarns: a. original  www.nature.com/scientificreports/ skarn; b. retrograde skarn; and c. veinlet skarn. The mineral assemblage of the skarn, whose parent rock is marble, comprises mainly garnet, pyroxene, idocrase and wollastonite 22 . The original skarn has been overprinted by a retrograde skarn. In comparison to the original skarn, the retrograde skarn contains much higher contents of fluorite, epidote, wolframite, scheelite, cassiterite, molybdenite, bismuthinite, magnetite, and pyrite 39 . Generally, mineralization occurs within the retrograde skarn rather than in the original skarn. Skarn veins crosscutting the margin of the retrograde skarn are tens to hundreds meters long and 10-50 cm wide. These skarn veins contain ores with grades of 1% to 6% 22 .
Marmarization. The stockwork marble vein, which is located at the contact between the overlying marble and the underlying skarn, is 750 m long, 300-600 m wide and 20-200 m thick. This vein comprises mainly fluorite, mica, tourmaline, and feldspar. The mineral grains are smaller than 0.05 mm in diameter 41 .
Feldspathization. Stockwork feldspar is a light-colored altered rock located in the fractures of the skarn. It contains mainly potassium feldspar and plagioclase and occasional quartz, fluorite, garnet, and pyroxene 42 .

Results
Zircon LA-ICP-MS age and trace elements. The zircons from Sample GL-13 (S 1 ) are typically transparent, colorless to slightly brown, rectangular to prismatic crystals 100-150 μm long, with aspect ratios ranging from 2:1 to 3:1. Oscillatory zoning, with the occasional appearance of inherited cores, is common in these crystals (Fig. 5). The zircons from Sample 315-36 (S 2 ) are mostly transparent, colorless to pale yellow, euhedral to subhedral crystals 100-200 μm long, with aspect ratios ranging from 2:1 to 3:1. The euhedral grains have concentric zoning with relatively bright cores in CL images (Fig. 5). Compared with those from the S 1 and S 2 granites, the zircons from the S 3 granite (Sample 490-21) are similar in shape and color but are smaller (typically 50-100 μm long), with aspect ratios ranging from 2:1 to 1.5:1, and exhibit weak oscillatory zoning. The zircons of Sample 490-10 (S 4 ) resemble those of Sample 490-21 in terms of their shape, color, and size (Fig. 5).
Whole-rock major and trace element chemistry. Twenty representative samples were analyzed for their major and trace element compositions (Table 3). These samples are characterized by high SiO 2 (70.32-78.28 wt%) and K 2 O (3.54-5.92 wt%) contents. Most of the samples plot in the fields of high-K calc-alkaline field, whereas seven samples plot in the shoshone field. The aluminum saturation index (ASI) values of the five phases of Qianlishan granites are 0. 95-1.78, 1.04-1.05, 0.88-1.34, 1.04-1.2, and 1-1.2. The S 1 , S 2 , and S 5 porphyritic granites have higher contents of K 2 O, CaO, MgO, TiO 2 , Zr, Sr, Ba, and P 2 O 5 but lower contents of Na 2 O and Al 2 O 3 than the other granites (Figs. 7, 8). Notably, fluorine concentrations of the S 3 and S 4 equigranular biotite granites (2500-10,400 ppm) are much higher than those of the S 5 granite porphyry (< 2000 ppm), whereas the S 1 and S 2 porphyritic biotite granites have intermediate F contents (2000-6800 ppm). Moreover, S 2 , S 3 and S 4 have relatively high W contents (20-100 ppm), whereas S 1 and S 5 have lower W contents (< 20 ppm). In the primitive mantle-normalized diagrams (Fig. 9), S 3 and S 4 exhibit the strongest negative Ba, Sr, P, and Ti anomalies, whereas S 1 and S 2 show much smaller anomalies. S 5 has trace element patterns similar to those of S 1 and S 2 . As presented in Fig. 10 and Table 3 Table 4. Age spectra and inverse isochrons are plotted in Fig. 11.
Muscovite from Sample YJW-8-B yields a plateau age of 154.2 ± 1.0 Ma (MSWD = 0.65) (Fig. 11a). All errors are quoted at the 2σ level. The plateau age comprises nine steps accounting for 85.9% of the total 39Ar released and agrees with the inverse isochron age of 154.0 ± 1.6 Ma (MSWD = 0.81) (Fig. 11b). The estimated initial 40 Ar/ 36 Ar is 296.9 ± 4.1%, which is identical to the present-day initial 40 Ar/ 36 Ar (295.5%). The characteristics of the spectra suggest the absence of argon loss and excess argon. In other words, the Ar-Ar system of the muscovite remained closed during the geological history of Sample YJW-8-B.

Discussion
Reclassification of granitic phases. Zoned plutons often require numerous magmatic intrusion pulses continuously emplaced over millions of years, because individual magmatic pulses commonly last for less than 100,000 years 7,9,10,24,43 . The QGC exhibits normal zoning with the most differentiated phases (S 3 and S 4 ) in the central part. Each phase can be distinguished by their emplacement age, mineral assemblage and geochemistry. Different classification schemes for the QGC have been suggested. According to some studies, the equigranular biotite granite (S 3 ) and the porphyritic biotite granite (S 1 ) have been classified as the QGC 25,28,37,39 . In contrast, other studies combined the porphyritic biotite granite (S 1 ) with the porphyritic biotite granite (S 2 ) that is located on the southern margin of the QGC 13,14,22,31,38 . Additionally, the porphyritic biotite granite (S 1 ) has been considered a separate phase by Guo et al. (2015) 27 . To clarify the relationship between different rock types with the QGC, we have undertaken a systematic study of petrology, geochronology and geochemistry of all five sections of the pluton.
Previous geochronological investigations have used diverse methods and obtained a variety of results (Table 5, Fig. 12). For the porphyritic biotite granite (P 1   www.nature.com/scientificreports/ Bulk-rock Rb-Sr ages and potassium/mica (K/Ar)-Ar isochron ages are not reliable for dating the emplacement of granitic plutons because they can be affected by thermal disturbances caused either by prolonged fluid convection or tectonic processes 14,44,45 . In contrast, zircon U-Pb dating, which has closure temperatures of 700-800°C 46,47 , is a reliable tool for placing geochronological constraints on plutonic emplacement.
The QGC (Si content > 70%) was formed by a felsic magma and experienced advanced differentiation, with a fractionation index of > 92 22 . During the fractionation process of felsic magma, although major elements have a limited range, trace elements that favor feldspars (Rb, Sr, Ba) and accessory minerals (Th, P, U, Zr, Sn, Ce, Y) show wide variation 48,49 . The variation diagrams (Figs. 7, 8) indicate that the contents of Na 2 O and Al 2 O 3 increase from the S 1 to S 4 granites, whereas Ca and Mg decrease. The decrease in P with SiO 2 content indicates fractionation of apatite. This is consistent with the presence of apatite in Sample GL-2 (S 1 ) and the absence of apatite in Sample 490-10 (S 4 ). The depletions in Sr, Ba, Nd, and Ti and the striking negative Eu anomalies (Fig. 9) indicate fractionation of plagioclase, K-feldspar and Ti-Fe oxides. In comparison with S 1 , S 2 and S 5 , the S 3 and S 4 granites show much more pronounced depletions in Ti, P, Sr and Ba, as well as extremely negative Eu anomalies (Fig. 10). Extensive magmatic fractionation is further supported by the ratios of Zr/Hf, Nd/Ta, Th/U and Rb/ Sr. With the increase in the degree of magmatic fractionation, Zr/Hf, Nd/Ta, Th/U and ratios decrease, but the Rb/Sr ratio increases [50][51][52]  .05, respectively. All the variations in the ratios from S 1 to S 4 suggest an increased degree of magmatic fractionation. It is worth noting that the S 5 granite does not follow the increasing trend of magmatic evolution from S 1 to S 4 . The S 5 granite composition suggests that it experienced the lowest degree of fractionation.
Based on the primitive mantle-normalized diagrams (Fig. 9), the QGC can be classified into three groups: 1) S 1 and S 2 porphyritic biotite granites; 2) S 3 and S 4 equigranular biotite granites; and 3) S 5 granite porphyry. Group 1 shows relatively flat REE patterns characterized by moderate enrichments in light REEs (LREEs) (with La/Y ratios ranging from 2.50 to 3.71) and slightly negative Eu anomalies (Eu/Eu * = 0.144-0.249), suggesting that these samples have been weakly differentiated. In comparison, Group 2 has undergone strong fractional differentiation, as it exhibits flat REE patterns (with La/Y ratios ranging from 0.84 to 1.31) with much stronger Eu anomalies (Eu/Eu * = 0.001-0.011). In contrast, Group 3 is characterized by strong enrichments in LREEs, with La/Y ratios ranging from 13.16 to 13.96 and the weakest Eu anomalies (with Eu/Eu * ratios ranging from 0.31 to 0.222). On the primitive mantle-normalized spider diagrams (Fig. 9), both Group 1 and Group 3 show weak negative Ba, Sr, P, and Ti anomalies, whereas the anomalies of Group 2 are larger. All three groups record positive U and Th anomalies, but positive Ta anomalies occur only in Group 1 and Group 2.
Field observations indicate that a 0.4-to 1-m-wide baked margin is located at the contact between S 2 and S 3 and that NE-striking granite porphyry dikes (S 5 ) cut through the other granites 26 , thus supporting our classification.. Additionally, positive Ce anomalies in zircon from S 2 are strikingly higher than in zircon from S 2 , indicating that it was formed in a geochemical environment distinct from that of S 2 . Thus, S 1 and S 2 should be separated into individual groups. In summary, the QGC may be reasonably classified into four phases: P 1 , which contains S 1 ; P 2 , which is composed of S 2 ; P 3 , which comprises S 3 and S 4 ; and P 4 , which includes S 5 .   40 Ar/ 39 Ar dating to suggest that the timing of greisenization and its associated W-Sn-Mo-Bi mineralization ranged from 145 to 148 Ma 29 . Collectively, the age discrepancy is up to ~ 20 Myr. Based on our field observations, the W-Sn-Mo-Bi mineralization of the Shizhuyuan deposit is intimately associated with greisenization; therefore, muscovite Ar-Ar dating is ideal candidate for assessing the timing of hydrothermal mineralization. According to our analytical results, greisen-type mineralization occurred ca. 154.2 ± 1.0 Ma. Within error, this age is consistent with molybdenite Re-Os dating (151 ± 3.5 Ma) conducted by Li et al. (1996) 31   www.nature.com/scientificreports/ geochronological Ar-Ar dating on a fluid inclusion in quartz and coexisting muscovite 30 . This age also agrees with our dates.
In conclusion, the QGC has two impacts on the Shizhuyuan deposit. (1) Heat supply: According to the thermal model of Mclaren et al. (1999) 5 , the heat derived from the high-heat-producing granites reaches a maximum ~ 10 Myr after its emplacement. Furthermore, the thermal disturbances caused by the high-heat-producing granites can drive hydrothermal fluid convection, resulting in mineralization. For instance, the mineralization in the Mount Elliott Cu-Au deposits, Australia, was produced by hydrothermal convection driven by the heat released from its associated high-heat-producing granite. The volume heat of the QGC estimated by the U, Th, and K contents is 5.89-14.30 μWm −3 (volume heat of high-heat-producing granite > 5 μWm −3 ), indicating their high heat production. Therefore, when mineralization occurred, the heat anomaly resulting from the QGC was quite strong (nearly its maximum strength), which promoted the development of hydrothermal convection around the QGC, leading to the generation of the Shizhuyuan deposit. (2) Metal supply: Field observations reveal that the Shizhuyuan W-Sn-Mo-Bi deposit is located at the endo-contact of the skarn and the porphyritic biotite granite (S 1 , S 2 ) and equigranular granite (S 3 , S 4 ), demonstrating their close spatial relationship 21,37 . In addition, as shown in Fig. 8a, the W contents in the S 1 , S 2 , S 3, and S 4 granites are mostly 40-60 ppm; therefore, the QGC can supply sufficient metal for mineralization. www.nature.com/scientificreports/   www.nature.com/scientificreports/ In summary, the QGC is temporally and spatially associated with the formation of the Shizhuyuan W-Sn-Mo-Bi deposit. Furthermore, the QGC provided heat and metals for these deposits.  Table 6). Samples GL-12 and GL-13, which represent fine-grained porphyritic biotite granite (S 1 ), were collected on the sides of Taipingli Road (25°46′04″ N, 113°09′42″ E). Samples 315-36 and 315-35 were collected from a microfine-grained porphyritic biotite granite (S 2 ) in the main transport tunnel on Level 500. Samples 490-21 and 490-24 are medium-to coarse-grained equigranular biotite granites (S 3 ) that were collected in Tunnel 490. Samples 490-9, 490-10, SZY-490-4a, SZY-490-4b, SZY-490-5a, and SZY-490-5b are fine-grained equigranular In total, sixteen granite samples and one stockwork greisen were collected with a sledgehammer. Each sample weighed 5 to 10 kg. Samples GL-12 and GL-13 were fresh massive rocks sampled from the outcrops of granites www.nature.com/scientificreports/ beside Taipingli Road, and the weathered parts were chipped off using a hammer. All the other samples were fresh rocks collected with sledgehammers from granite outcrops in the tunnels.
Stockwork greisen. Sample YJW-8-B, consisting of massive rock, was collected using a sledgehammer from stockwork greisen in the main transport tunnel, Level 350. It represents the greisen associated with W-Sn-Mo-Bi mineralization.
In summary, we conducted zircon LA-ICP-MS U-Pb dating and whole-rock major and trace element analyses on all five sections of the QGC. Muscovite Ar-Ar dating was carried out on the stockwork W-Sn-Mo-Bi to constrain the ore-forming age of the Shizhuyuan deposit.
Thin-section preparation and optical petrography. Billets with a size of ~ 45 × 25 × 15 mm were cut from fresh field samples using a diamond blade. Then, they were planed and mounted on a 28 × 48 mm standard petrographic carrier glass using epoxy. After polishing with abrasive powders, the thin sections reached a thickness of 30 μm. To identify the mineralogies and textures of the rocks, polished thin sections were studied under a binocular microscope and an Olympus BX 51 polarizing microscope at China University of Geosciences (Beijing) (CUGB).
Zircon LA-ICP-MS U-Pb dating. Five fresh samples weighing approximately 5 kg (Samples GL-13, 315-36, 490-21, 490-10, and 490-2) of the Qianlishan plutonic rocks were processed at the Central Laboratory of China Railway Resources Group. Zircon grains subjected to U-Pb dating were separated from these samples using conventional heavy liquid and magnetic techniques. Then, approximately 100 of the best-quality zircon grains from each sample were handpicked under a binocular microscope. These grains were mounted in epoxy and then polished. After the mounts were prepared, the grains were photographed using optical microscopy and cathodoluminescence (CL) imaging to reveal their internal morphologies, which were used to select grains and choose analytical spots. The CL images were obtained using a HITACHI S3000-N scanning electron microscope equipped with a Robinson backscattered-electron detector and a Gatan Chroma CL imaging system. These samples were analyzed using an Agilent 7900 quadrupole ICP-MS with a 193 nm coherent Ar-F laser and Resonetics S155 ablation cell in CODES (Centre for Ore Deposit and Earth Sciences) of Tasmania University. The NIST610 standard used for Pb correction was analyzed after every 15 unknowns. Th/U and Pb/Th mass bias downhole fractionation and instrument drift were corrected with the 91,500 zircon standard according to Wiedenbeck et al. (1995) 55 . Each zircon analysis comprised 30 s of blank gas measurements and 30 s of analysis time. The analyzed spots were 29 μm in size, and the laser was emitted at a frequency of 5 Hz with an energy density of approximately 2 J/cm 2 . Particles ablated by the laser were carried out by the flow of He carrier gas at a rate of 0.35 l/min into the chamber to be mixed with argon gas. Then, they were carried to the plasma torch. The Temora standard of Black et al. (2003) and the Plesovice standard of Sláma et al. (2008) were applied 56,57 . Data processing was carried out using concordia intercept ages on the Tera-Wasserburg plot utilizing ISOPLOT (Ludwig, v. 3.75, 2012, copyright@ BGC Berkeley Geochronology Center, 2006, available from: http://www.bgc.org/ isopl ot_etc/isopl ot.html). The method of data reduction was described in Halpin et al. (2014) 58 . The estimation of random and systematic uncertainties followed the method in Paton et al. (2010) 59 .
Whole-rock major and trace element analyses. Billets ~ 70 × 50 × 30 mm in size were cut from the selected fresh samples. They were cleaned with tap water and powdered to a grain size of < 200 mesh using an agate mill. Major and trace element analyses were carried out at the Central Laboratory of China Railway  60 . Major element concentrations were measured using a Thermo Fisher ARL Advant'X X-ray fluorescence (XRF) spectrometer, while LOI was measured using a CPA225D electronic analytical balance. The FeO contents of the samples were measured using the conventional wet chemical titration technique. Concentrations of trace elements, including REEs, were determined by ICP-MS (Thermo Fisher X-series 2) after full digestion. A detailed description of the test and the lower detection limit were documented in GB/T14506-2010 61 . The analytical errors ranged from 1 to 3% of the amount present. Additionally, the temperatures of the experiment ranged from 20-27 °C, and the humidity was 30% to 58%.
Muscovite Ar-Ar dating. The stockwork W-Sn-Mo-Bi (Type 3) ore has the highest average ore grade and the largest amount of ore; it thus represents the main stage of mineralization and is an ideal candidate for studying the mineralization time of this deposit.
In this study, one fresh sample (~ 120 × 80 × 30 mm) was collected for muscovite dating from a greisen vein with mineralization (Type 3) at the contact with the skarn in Tunnel 450. As shown in Fig. 3, this sample contained mainly muscovite (~ 92%) and minor quartz (~ 5%) and K-feldspar (~ 2%). This muscovite sample was crushed to a size of 40-60 mesh, and muscovite separates were carefully handpicked to a purity of over 99% under a binocular microscope. After being washed in an ultrasonic bath using methanol and deionized water, the muscovite crystals were wrapped in aluminum foil and stacked in quartz vials. Then, they were irradiated for 1442 min in the B4 position of the swimming pool reactor at the Chinese Institute of Atomic Energy, Beijing. The Fangshan biotite standard (ZBH-25), which has an age of 132.7 ± 1.2 Ma and a potassium content of 7.6%, was used to monitor the neutron flux (2.65 × 10 13 n cm -2 S -1 ). After the samples underwent cooling for approximately 100 days, step-heating 40 Ar/ 39 Ar analyses were conducted using an MM1200B mass spectrometer at the Ar-Ar laboratory, Institute of Geology, Chinese Academy of Geological Sciences, Beijing. The instrumental conditions and analytical methodology were described by 14 .
Step-heating analyses were performed in a double-vacuum resistance furnace; at each temperature step, the muscovite crystals were heated for 10 min and then purified for 30 min. The Ca and K correction factors calculated based on the analyses of K 2 SO 4 and CaF 2 were ( 36 Ar/ 37 Ar o ) Ca = 0.0002389, ( 40 Ar/ 39 Ar) K = 0.004782 and ( 39 Ar/ 37 Ar o ) Ca = 0.000806. A 40 K decay constant of 5.543 × 10 −10 year −1 was used in the age calculations 62 . ISOPLOT software (Ludwig, v. 3.75, 2012, copyright@ BGC Berkeley Geochronology Center, 2006, available from: http://www.bgc.org/isopl ot_etc/isopl ot.html) was used for data processing. The errors in the plateau ages are quoted at the 2σ level.