Molybdenite Re–Os and Zircon U–Pb Isotopic Constraints on Gold Mineralization Associated with Fine-Grained Granite in the Xiawolong Deposit, Jiaodong Peninsula, East China

Abstract

Molybdenite Re–Os and zircon U–Pb isotopic data are first obtained from the stockwork and disseminated-style gold-bearing ores and the fine-grained granite hosting these ores in the Xiawolong gold mine, respectively, which is located within the Muping–Rushan gold metallogenic belt, eastern Jiaodong Peninsula, so as to illustrate the genesis of gold mineralization and its implication for exploration. Four molybdenite samples yield a well-defined Re–Os isochron age of 118.4 ± 2.5 Ma (2σ), which is identical to the weighted average Re–Os model age of 118 ± 1.7 Ma (2σ). Integration of the new geochronologic data with those reported recently from the other gold mines in the Muping–Rushan gold metallogenic belt suggests that a discrete gold event occurred in Xiawolong ca. 4 m.y. older than that for the other gold mineralization at ca. 114 Ma in eastern Jiaodong. In addition, two fine-grained granite samples, measured using the LA-ICP-MS zircon U–Pb method, produce the first precise ages of 118 ± 2 to 117 ± 2 Ma (2σ), identical to the molybdenite Re–Os ages, within the margin of error and obtained in this study. The fine-grained granite has a similar lithology and emplacement age as those of the medium-grained monzogranite consisting of the marginal facies of the Sanfoshan batholith, and is considered to be the crystallization products of Sanfoshan granitic magma in the late stage. Combined with the previous S-Pb-D-O isotope, fluid inclusion and geological studies, which suggest that the ore-forming fluid of Xiawolong gold mineralization is from magmatic water, and the identification that the magnetite coexists with the gold-bearing pyrite and molybdenite in the gold ores, which indicates a high oxygen fugacity (fO₂) of both the magma and resultant hydrothermal fluids, it is logical to infer that the Xiawolong gold deposit is genetically in relation to the Sanfoshan granitic magmatism, which is high in fO₂ and rich in Au at the magmatic–hydrothermal transition stage, and the change in fO₂ mostly likely makes a significant contribution to the precipitation of Au. This result reveals that the late-stage granitic magma with high fO₂, which is crystallized into the fine-grained granite, probably is also rich in Au, except the W–Mo–Cu–Zn–U–Be–Li–Nb–Ta–Sn–Bi-elements. Therefore, based on the extensional tectonic regime for the early Cretaceous Jiaodong gold deposits, we propose that gold exploration in the Jiaodong should not only focus on the fault-hosted Au but also on the fine-grained granite-hosted Au around the apical portions of the late Early Cretaceous small-granitic intrusions with high fO₂. This model could also be important for prospecting in other gold ore districts, which have a similar tectonic setting.

1. Introduction

The Jiaodong Peninsula, located in the eastern continental margin of East China (Figure 1A), is divided into the Sulu terrane in the southeast and the Jiaobei terrane in the northwest by the crustal-scale ENE-to NE-trending WQYF (i.e., the Wulian–Qingdao–Yantai Fault) (Figure 1B) [1,2,3]. It is considered to result from the collision of the Yangtze Craton (YC) with the North China Craton (NCC) during the Middle–Late Triassic [4]. The Paleo–Pacific subduction beneath East China initiated at ca. 180 Ma [5,6,7] led to the large-scale magmatism in the eastern NCC between the Late Jurassic and the Early Cretaceous [8,9,10], and is associated with a great number of Early Cretaceous gold deposits developed in the Jiaodong Peninsula [1]. So far, over 260 gold deposits, with a total proved reserve of more than 5000 t Au [11], have been reported in the Jiaodong Peninsula, the first largest gold producer in China (Figure 1B) [12,13]. They are clustered in the three gold metallogenic belts, which are Muping–Rushan, Penglai–Qixia and Zhaoyuan–Laizhou, located in eastern, central and western Jiaodong, respectively (Figure 1B) [14].

It has been widely recognized that the gold deposits in the Jiaodong Peninsula occur as the disseminated and stockwork pyrite–sericite–quartz-altered ores (i.e., the “Jiaojia-type” mineralization), represented by the Taishang, Jiaojia and Sanshandao gold mines (Figure 1B), or the gold-bearing quartz-vein ores (i.e., the “Linglong-type” mineralization), represented by the Wang’ershan, Linglong and Rushan gold mines (Figure 1B) [1,16,17]. Both of them are controlled by either the regional or the lower-order NNE- to NE-striking faults and are hosted within mineralogically altered rocks, which consist of the Guojialing-type granitoids (middle Early Cretaceous), the Linglong-type granitoids (late Jurassic) and the high-grade metamorphic rocks (Precambrian) [16,17]. The dominant ore mineralogies are pyrite, chalcopyrite, sphalerite, galena, gold and tetrahedrite, with sericite, quartz, calcite and chlorite being the main altered mineralogies [1,18]. However, the Xiawolong gold mine, newly found in 2012–2017 by the No.6 Geological Exploration Team of Shandong Province, China [19] in the northeast of the Muping–Rushan gold metallogenic belt within eastern Jiaodong, i.e., Sulu terrane (Figure 1B), is hosted within the non-deformed fresh fine-grained granite (late Early Cretaceous) [20]. The ores occur as the disseminated and stockwork-style pyrite, generally associated with molybdenite and magnetite, within the fine-grained granite, where neither fracture nor hydrothermal alteration occurred [19,20].

So far, no other research work on this deposit has been reported except for the basic geological and geochemical research [19,20]. The geochemical study on the stable isotope and fluid inclusion of the Xiawolong reported by Lv et al. [20] suggests that distinct from the traditional gold deposits for the “Jiaojia-type” and “Linglong-type” deposits, the ore-forming process for the Xiawolong gold deposit is the result of the fluid release, which could take place during the magmatic–hydrothermal transition stage. Furthermore, the genetic relationship between the Pb–Zn deposit [21], uranium deposit [22,23,24], copper deposit [25], W–Mo deposit [26,27,28] and the fine-grained granite has been revealed. In order to illustrate whether the fine-grained granite has a genetic relation to the gold mineralization, the LA-ICP-MS zircon U–Pb dating of the fine-grained granite and molybdenite Re–Os systematic analyses of gold-bearing ore from the Xiawolong gold deposit are first carried out.

2. Geological Background and Sampling

The Jiaodong Peninsula comprises two distinct crystalline basements of the Jiaobei and Sulu terranes, which are dominated by the Precambrian high-grade metamorphic rocks of NCC [29] and the Paleoproterozoic and Neoproterozoic basement rocks of YC [30], respectively (Figure 1B). The former is mainly made up of the Jiaodong Group TTG (tonalite–trondhjemite–granodiorite) gneisses (Mesoarchean to Neoarchean) [31], the Fenzishan and Jingshan Groups metasedimentary rocks (Paleoproterozoic) [32] and the Penglai Group metasedimentary sequences (Neoproterozoic) [33], whereas the latter suffered from a ca. 244–220 Ma UHP (ultra-high-pressure) metamorphic event [34], which formed the UHP metamorphic rocks, mainly including the marble, quartzite, coesite-bearing eclogite and granitic gneiss (Figure 1B) [35]. Mesozoic magmatism occurred widely in the Jiaodong Peninsula as a result of the Paleo–Pacific subduction under East China (Figure 1B) [36,37,38], which developed during two main episodes [8,39]: one is represented by the Late Jurassic (166–149 Ma) biotite granite and monzogranite [7,37], and the other is characterized by the porphyritic-like granodiorite, monzonite, monzogranite and syenogranite formed during the Early Cretaceous (130–110 Ma) [40,41].

Regional-scale structures in the Jiaodong Peninsula are characterized by NE- and NNE-trending fault systems (Figure 1B), which developed either in the cratonic interior or along previous suture zones like the crustal-scale Tan–Lu Fault (TLF), Zhaoyuan–Pingdu Fault (ZPF) and WQYF, respectively, from west to east (Figure 1B) [42]. In addition, on both sides of the ZPF and WQYF, the low-order NNE- to NE-striking faults dipping towards either NW or SE are distributed at intervals of about 15–35 km across the Jiaobei uplift and Sulu terranes (Figure 1B). It has been widely suggested that these low-order faults and the crustal-scale WQYF and ZPF have undergone multistage deformation [43,44] and control these gold deposits developed in the Jiaodong Peninsula (Figure 1B) [45,46]. For example, the NNE- to NE-striking Muping–Rushan fault zone dipping steeply to SE in the Sulu terrane (Figure 1B), made up of an array of sub-parallel NNE- to NE-trending faults (Figure 2a) [20], controls the distribution of the “Linglong-type” gold deposits in the Muping–Rushan gold metallogenic belt (Figure 1B and Figure 2a), which is dominated by two stages of Mesozoic magmatism (Figure 2a). The Late Jurassic Kunyushan granitoid batholith, emplaced during 161–141 Ma [47,48], mainly consists of monzogranite and garnet-bearing leucogranite [40], which was intruded by the late-Early Cretaceous Sanfoshan monzonite (Figure 2a). The former generally shows a strong NNE–SSW-aligned ductile foliation fabric [20] and hosts the “Linglong-type” gold deposits (Figure 2a), whereas the latter is undeformed, where almost no gold deposits develop [20]. In addition, a large number of mafic and intermediate dikes developed in both the Kunyushan and Sanfoshan granitoids, mainly striking NNE to NE (Figure 2a).

Distinct from the other “Linglong-type” gold deposits located within the Muping–Rushan gold metallogenic belt, which are situated within the middle–west part of the Kunyushan granitoid batholith and are controlled by the NNE- to NE-trending faults (Figure 2a), the Xiawolong gold deposit, located at the northeast part of the Kunyushan granitoid batholith, is adjacent to the contact on the west side between the Sanfoshan monzonite and the Kunyushan granitoid (Figure 2a), where no faults and alteration zone develop [19]. Similar to the fractionated granitoid-associated deposits, such as the Mt Bischoff and Renison tin–tungsten deposits and the Wolfram tungsten–molybdenum deposits in Australia [49], the gold orebodies in the Xiawolong are all hosted within the fine-grained granitic dikes (Figure 2b–d) [20].

On the basis of the systematic drill holes, four concealed fine-grained granitic dikes, called I#, II#, III# and IV# from the south to north, respectively, are identified within the Kunyushan monzogranite (Figure 2b). The former are usually fresh and non-deformed and have abrupt contact with the latter, where no baked margins, alteration or faults are observed [19]. These dikes are generally several kilometers long with thicknesses varying between 10 and 80 m (Figure 2b). They trend E–W with dip angles of 20~50° towards the south and are considered to be branches of the Sanfoshan granitic pluton to the west [19].

The gold orebodies are mainly concentrated in the central part of the fine-grained granitic dikes, except that a few are located in the upper or lower boundary between the fine-grained granitic dikes and the Kunyushan monzogranite (Figure 2c,d). They are generally vein-like and S-shaped lenses in shape, being tens to hundreds of meters long in strike with a maximum length of 366 m and several meters thick, varying between 0.91 and 2.85 m (average 1.17 m). These orebodies usually trend 270~285° and dip 20~28° (average 25°) SW, which generally extend from +44 m level to −70 m level, with a maximum sloping depth of 200 m and a buried depth ranging from 46 to 132 m (Figure 2c,d). The proven gold reserve is about 2 t, with the Au grade varying from 1.5 to 52.6 g/t with an average of 11.47 g/t).

The gold mineralization in the Xiawolong deposit is represented by the disseminated (Figure 3a) and stockwork-style (Figure 3b) ores occurring within the fine-grained granitic dikes, where no alteration and deformation occur (Figure 3a,b). Four typical gold-bearing ore samples for molybdenite Re–Os isotope analyses are collected from drilled cores, with the locations shown in Figure 2c,d. Samples XWL-g4 and XWL-g9 are disseminated-style ores with sulfides occurring as disseminations in the fine-grained granite (Figure 3a), whereas samples XWL-g1 and XWL1B2 are stockwork-style ores, where sulfides mainly occur in veinlets with only a few in disseminations; the quartz–sulfide veinlet shows a clear curvy boundary with the fine-grained granite (Figure 3b). The gangue minerals mainly include K-feldspar, plagioclase, quartz and biotite (Figure 3c,d), and the ore minerals are dominated by pyrite, pyrrhotite, magnetite, chalcopyrite, molybdenite, galena and minor native gold (Figure 3e–j). The molybdenite is usually intergrown with pyrite and galena, which occur along the grain margin of quartz (Figure 3k,l). The gold grains are mainly medium-fine-grained native, usually coexisting with pyrite, magnetite and molybdenite and occurring along the intersecting crystallographic planes of or as inclusions in the hosted minerals, such as pyrite and magnetite, with different shapes—droplets, angular or dendritic (Figure 3h–j)—indicating that they are precipitated simultaneously.

In addition, two typical samples of the fine-grained granite for LA-ICP-MS zircon U–Pb dating are collected from drilled cores, with the locations shown in Figure 2c,d. Samples XWL4ZK1B2 and XWL5ZK4B3 are monzogranites, which have a fine-grained subhedral equigranular granitic texture with a grain size of 0.4–0.8 mm and are gray-white in appearance (Figure 4a). The main minerals are dominated by quartz (35–39%), K-feldspar (20–26%), plagioclase (18–23%) and biotite (2–5%) with few accessory minerals of zircon (Figure 4b–d), as well as a few metallic minerals (e.g., pyrite, galena and molybdenite) (10–15%) (Figure 4e,f). No deformation or hydrothermal alteration are observed (Figure 4).

3. Analytical Procedures

3.1. Molybdenite Re–Os Isotopic Analyses

Four typical gold-bearing ore samples were crushed with the grains being 40-60 mesh so as to separate the molybdenites, which were handpicked subsequently with a binocular microscope in order to obtain >99% purity. Then, the molybdenite mineral samples selected were carried out for Re–Os isotopic analyses at the Re–Os Laboratory in the National Research Center of Geoanalysis, Chinese Academy of Geological Sciences, Beijing. The procedures of Re–Os chemical separation and mass spectrometry measurement have been summarized in [50,51,52,53,54] in detail, which are described briefly as follows.

The molybdenite mineral sample weighed accurately, followed by the mixed ¹⁸⁵Re and ¹⁹⁰Os spike solution with an appropriate amount of ultrapure HCl by a thin-necked long funnel, was loaded into the bottom of a Carius tube, which was placed in the vacuum cup containing an ethanol–liquid nitrogen slush with a temperature of −50~−80 °C. Subsequently, 3 mL 10 mol/L HCl was added into the same Carius tube, and 5 mL 16 mol/L HNO₃ was also added only after the HCl was completely frozen. Then, the Carius tube was sealed and heated in an oven at 200 °C for 24 h to decompose the molybdenite sample. After decomposition, the Carius tube was again chilled and then was opened in the abovementioned vacuum cup. After the opened tube was tempered in an ice bath, the Milli-Q water with a volume of about 1.5 times that of the sample solution was added. Next, the Os was separated from the Carius tube by distillation in a steam bath at 100 °C for 60 min and trapped by Milli-Q water. The resulting solution of OsO₄ liquid was used for Os isotope measurements. Re was extracted from the remaining Re-bearing solution with 10 mL acetone in the 10 mL 5 mol/L NaOH solution.

Lastly, the Os–Re isotopic composition of molybdenite was determined using a TJA X-series ICP-MS (Thermo Jarrell Ash Corporation, Franklin, Boston, MA, USA). The total blanks for this technique are 4 pg for Re, 0.0002 pg for common Os and 0.09 pg for ¹⁸⁷Os. The certified reference standard material GBW04435 (HLP) (National Geological Testing Center; Beijing, China, 2022) [50], with a measured model Re–Os age of 222.2 ± 3.4 Ma, which corresponds to the recommended 221.4 ± 5.6 Ma [50], was used to monitor the analytical reliability. The equation [t = ln(¹⁸⁷Os/¹⁸⁷Re + 1)/λ] with a decay constant (λ) of 1.666 × 10⁻¹¹ a⁻¹ (±1.02%) [55] was used to calculate the model Re–Os age. The analytical uncertainties on the Re and Os contents of molybdenite include those in the calibration of spike solutions, the weighting of samples and spike solutions, fractionation correction in the mass spectrometry measurement and the measurement of the isotopic ratio of the sample analyzed. The Re–Os model age uncertainty stems from the decay constant of ¹⁸⁷Re. All uncertainties are shown at a confidence level of 95%. The isochronal and weighted average ages were obtained using Isoplot 3.75 software [56].

3.2. Zircon LA-ICP-MS U–Pb Dating

Each fine-grained granite sample was crushed to 80–100 mesh, and more than 500 zircon crystals were obtained by the gravimetric–magnetic separation technique. Subsequently, no fewer than 200 grains with a diameter greater than 40 um, where no visible inclusions and fractures developed, were handpicked with a binocular microscope and then were mounted in epoxy resin to be polished so as to reveal the center of zircon crystals. Based on the cathodoluminescence (CL) images and the reflected and transmitted light micrographs, the detailed observation of the internal structure of zircon was performed in order to obtain the zircon grains, which contain neither microcracks nor fluid/mineral inclusions, used for zircon U–Pb dating.

Zircon U–Pb dating was carried out at Hefei University of Technology, China, using an Agilent 7500a ICP-Mass equipped with a GeolasPro laser (Agilent technology Co., Ltd., La Jolla, CA, USA). The laser spot diameter was 32 um with a frequency being 6 Hz. Harvard zircon 91500 was adopted as the external standard for U–Pb dating, and NIST610 was used as the external standard for the calculations of U, Th and Pb contents. The ICPMSDataCal 11 program was used to obtain the ratios of ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁶Pb/²³⁸U [57], and the common Pb was corrected based on the method of Ref. [58]. The Concordia diagram, histogram and weighted average age were obtained using the Isoplot 3.75 software [56]. Detailed analytical methods and procedures refer to those described by Refs. [59,60].

4. Results

4.1. Molybdenite Re–Os Age

The abundances and isotopic compositions of Re–Os for four molybdenite samples of the Xiawolong gold deposit are shown in

Table 1. Molybdenite Re–Os abundance and isotopic compositions for gold-bearing ores from the Xiawolong gold deposit.

Sample No.	Ore Style	Sample Weight (g)	Re (ppm)		Common Os (ppb)		187Re (ppm)		187Os (ppb)		Model Age (Ma)
			Measured	Error	Measured	Error	Measured	Error	Measured	Error	Measured	Error
XWL-g1	stockwork-style	0.00019	192.6	1.3	0.7217	0.6775	121.1	0.8	234.72	2.39	116.26	1.84
XWL-g4	disseminated-style	0.00241	107.2	0.8	8.1377	0.1126	67.39	0.49	133.26	0.81	118.57	1.62
XWL1B2	stockwork-style	0.00025	33.85	0.34	0.1431	1.2672	21.28	0.21	41.30	0.54	116.40	2.24
XWL-g9	disseminated-style	0.00129	79.72	0.52	0.0043	0.0976	50.11	0.33	99.93	0.67	119.59	1.62

and illustrated in Figure 5. The Re and ¹⁸⁷Os abundances in these samples are 33.85–192.6 ppm and 41.3–234.72 ppb, respectively, which are much higher than the Re and ¹⁸⁷O blanks of 4 pg and 0.09 pg, respectively, in this test, and the ¹⁸⁷Re concentrations are 21.28–121.1 ppm. However, the common Os is also detectable in these molybdenite samples; three samples of which have low common Os contents of 0.0043–0.7217, with ¹⁸⁷Os/common Os ratios of 288.61–23,239.53 much higher than 20, suggesting that common Os have little effect on calculating the model Re–Os age [61]. Although the sample of XWL-g4 contains a relatively higher common Os of 8.1377 ppb with a ¹⁸⁷Os/common Os ratio of 16.38, a little lower than 20, the calculated model age of 118.57 ± 1.62 Ma is consistent with those of the other three samples, which range between 116.26 ± 1.84 Ma and 119.59 ± 1.62 Ma, within uncertainty. All the four model Re–Os ages give a weighted average age of 118 ± 1.7 Ma (MSWD = 0.83, 2σ) (Figure 5a). Furthermore, the regression for the ¹⁸⁷Re and ¹⁸⁷Os data of all the four molybdenite samples yields a well-constrained isochron age of 118.4 ± 2.5 Ma (MSWD = 2.2, 2σ), with a bigger error initial ¹⁸⁷Os abundance of 0.22 ± 1.94 ppb (Figure 5b), which is consistent with the weighted average Re–Os model age within errors. Based on the identical single Re–Os model age, the weighted mean model age and the isochron age within uncertainties, it is suggested that the molybdenite Re–Os analytical results are accurate.

4.2. LA-ICP-MS Zircon U–Pb Age

The obtained LA-ICP-MS zircon U–Pb concentrations and isotopic compositions from the two fine-grained granite samples (XWL4ZK1B2 and XWL5ZK4B3) of the Xiawolong gold deposit are given in

Table 2. LA-ICP-MS zircon U–Pb data of fine=grain granite in Xiawolong deposit.

Spot	Concentrations (ppm)			Th/U	U–Pb Isotopic Ratios						Ages (Ma)
	Pb	Th	U		207Pb/206Pb	1σ	207Pb/235U	1σ	206Pb/238U	1σ	207Pb/206Pb	1σ	207Pb/235U	1σ	206Pb/238U	1σ
XWL4ZK1B2
XWL4ZK1B2-1	9	358	393	0.9	0.063	0.004	0.150	0.009	0.017	0.0005	702	131	142	8	111	3
XWL4ZK1B2-2	21	1123	680	1.7	0.071	0.004	0.184	0.010	0.018	0.0005	959	125	171	8	115	3
XWL4ZK1B2-3	12	478	518	0.9	0.057	0.003	0.134	0.008	0.017	0.0005	478	125	128	7	110	3
XWL4ZK1B2-4	10	375	405	0.9	0.054	0.003	0.137	0.008	0.018	0.0006	387	139	130	7	116	4
XWL4ZK1B2-5	17	112	135	0.8	0.067	0.004	0.932	0.055	0.104	0.0038	828	119	669	29	635	22
XWL4ZK1B2-6	9	501	319	1.6	0.052	0.004	0.131	0.009	0.018	0.0006	272	167	125	8	115	4
XWL4ZK1B2-7	11	507	402	1.3	0.084	0.004	0.200	0.010	0.017	0.0005	1302	108	185	9	111	3
XWL4ZK1B2-8	10	415	431	1.0	0.047	0.003	0.112	0.007	0.017	0.0005	54	167	108	6	108	3
XWL4ZK1B2-9	10	363	392	0.9	0.057	0.005	0.166	0.012	0.020	0.0007	483	193	156	10	126	4
XWL4ZK1B2-10	10	330	368	0.9	0.065	0.005	0.167	0.011	0.018	0.0006	770	146	157	10	115	4
XWL4ZK1B2-11	11	341	459	0.7	0.052	0.004	0.134	0.009	0.019	0.0006	276	172	128	8	122	4
XWL4ZK1B2-12	11	375	424	0.9	0.054	0.004	0.140	0.008	0.019	0.0006	354	150	133	7	120	4
XWL4ZK1B2-13	11	342	425	0.8	0.049	0.004	0.156	0.010	0.020	0.0007	150	194	147	9	129	4
XWL4ZK1B2-14	39	286	228	1.3	0.063	0.003	1.031	0.040	0.119	0.0033	702	89	719	20	725	19
XWL4ZK1B2-15	9	297	376	0.8	0.044	0.004	0.119	0.008	0.019	0.0006			114	8	120	4
XWL4ZK1B2-16	3	179	108	1.7	0.080	0.008	0.239	0.017	0.019	0.0008	1206	200	217	14	122	5
XWL4ZK1B2-17	8	303	345	0.9	0.048	0.004	0.123	0.008	0.018	0.0005	98	174	118	7	116	5
XWL4ZK1B2-18	12	468	497	0.9	0.058	0.004	0.139	0.009	0.018	0.0005	517	139	132	8	114	3
XWL4ZK1B2-19	9	505	329	1.5	0.052	0.004	0.132	0.008	0.018	0.0005	272	165	126	8	115	3
XWL4ZK1B2-20	10	382	420	0.9	0.047	0.003	0.117	0.008	0.018	0.0006	78	154	113	7	114	3
XWL4ZK1B2-21	10	407	365	1.1	0.045	0.003	0.117	0.007	0.018	0.0006			113	6	117	4
XWL4ZK1B2-22	13	681	445	1.5	0.064	0.004	0.154	0.010	0.018	0.0005	728	149	146	9	112	4
XWL4ZK1B2-23	9	330	383	0.9	0.053	0.004	0.129	0.009	0.018	0.0006	339	188	123	8	114	3
XWL4ZK1B2-24	6	193	212	0.9	0.089	0.007	0.218	0.014	0.018	0.0006	1406	150	200	12	117	4
XWL4ZK1B2-25	9	454	326	1.4	0.053	0.005	0.137	0.010	0.019	0.0007	339	160	130	9	118	4
XWL4ZK1B2-26	16	664	654	1.0	0.054	0.003	0.128	0.007	0.018	0.0005	365	136	122	6	113	3
XWL4ZK1B2-27	6	310	196	1.6	0.072	0.006	0.200	0.013	0.019	0.0007	991	161	185	11	120	4
XWL4ZK1B2-28	15	547	562	1.0	0.075	0.005	0.189	0.011	0.019	0.0005	1066	122	176	10	119	3
XWL4ZK1B2-29	19	1295	625	2.1	0.062	0.004	0.163	0.009	0.019	0.0006	681	116	153	8	121	4
XWL4ZK1B2-30	13	720	421	1.7	0.051	0.003	0.129	0.007	0.018	0.0005	220	151	123	6	114	3
XWL4ZK1B2-31	4	198	142	1.4	0.072	0.006	0.191	0.012	0.018	0.0006	989	174	177	10	117	4
XWL4ZK1B2-32	23	1277	672	1.9	0.107	0.005	0.275	0.011	0.019	0.0005	1743	80	246	9	122	3
XWL4ZK1B2-33	10	303	381	0.8	0.060	0.005	0.200	0.013	0.020	0.0007	594	193	185	11	128	4
XWL4ZK1B2-34	15	795	593	1.3	0.057	0.003	0.139	0.007	0.017	0.0005	476	134	132	7	110	3
XWL4ZK1B2-35	5	146	168	0.9	0.096	0.009	0.298	0.019	0.020	0.0008	1554	183	265	15	125	5
XWL4ZK1B2-36	12	680	474	1.4	0.050	0.003	0.116	0.006	0.017	0.0005	211	122	112	5	108	3
XWL4ZK1B2-37	11	482	461	1.0	0.048	0.003	0.110	0.006	0.017	0.0005	76	130	106	6	106	3
XWL4ZK1B2-38	12	505	528	1.0	0.046	0.002	0.113	0.005	0.018	0.0005	13	111	109	5	112	3
XWL4ZK1B2-39	14	686	533	1.3	0.049	0.003	0.112	0.005	0.017	0.0005	200	120	107	5	107	3
XWL4ZK1B2-40	20	1197	828	1.4	0.048	0.003	0.114	0.006	0.017	0.0005	109	118	109	5	108	3
XWL5ZK4B3
XWL5ZK4B3-1	8	295	335	0.9	0.050	0.003	0.126	0.007	0.018	0.0005	187	143	121	7	116	3
XWL5ZK4B3-2	8	172	266	0.6	0.120	0.007	0.332	0.019	0.020	0.0006	1950	104	291	14	126	4
XWL5ZK4B3-3	6	317	234	1.4	0.055	0.004	0.131	0.007	0.017	0.0005	467	144	125	7	109	3
XWL5ZK4B3-4	7	327	235	1.4	0.052	0.003	0.136	0.008	0.019	0.0006	333	144	130	7	122	4
XWL5ZK4B3-5	9	429	394	1.1	0.050	0.003	0.121	0.006	0.018	0.0005	198	119	116	6	114	3
XWL5ZK4B3-6	3	187	105	1.8	0.062	0.005	0.163	0.011	0.018	0.0007	683	177	153	9	115	4
XWL5ZK4B3-7	6	347	201	1.7	0.048	0.004	0.120	0.008	0.017	0.0005	98	235	115	7	111	3
XWL5ZK4B3-8	15	633	639	1	0.052	0.002	0.124	0.006	0.017	0.0005	283	106	118	5	110	3
XWL5ZK4B3-9	4	226	169	1.3	0.048	0.004	0.127	0.009	0.018	0.0006	120	189	122	8	114	4
XWL5ZK4B3-10	7	336	275	1.2	0.049	0.004	0.120	0.007	0.018	0.0005	154	168	115	7	112	3
XWL5ZK4B3-11	10	437	407	1.1	0.060	0.004	0.147	0.008	0.018	0.0005	594	136	139	7	112	3
XWL5ZK4B3-12	15	1002	499	2	0.046	0.002	0.108	0.005	0.017	0.0005			104	5	111	3
XWL5ZK4B3-13	11	407	463	0.9	0.045	0.002	0.106	0.005	0.017	0.0005			102	5	109	3
XWL5ZK4B3-14	7	373	250	1.5	0.053	0.003	0.123	0.007	0.017	0.0005	339	137	117	6	106	3
XWL5ZK4B3-15	4	238	161	1.5	0.051	0.004	0.129	0.008	0.018	0.0006	256	174	123	7	113	4
XWL5ZK4B3-16	10	345	388	0.9	0.047	0.003	0.114	0.006	0.018	0.0005	58	130	110	6	114	3
XWL5ZK4B3-17	5	196	170	1.2	0.050	0.003	0.137	0.009	0.019	0.0006	198	157	131	8	119	4
XWL5ZK4B3-18	6	235	207	1.1	0.101	0.006	0.256	0.015	0.019	0.0006	1644	123	231	12	121	4
XWL5ZK4B3-19	11	432	519	0.8	0.045	0.002	0.105	0.006	0.017	0.0005			102	5	109	3
XWL5ZK4B3-20	10	384	391	1	0.045	0.003	0.108	0.006	0.018	0.0005			104	6	113	3
XWL5ZK4B3-21	3	130	123	1.1	0.048	0.004	0.144	0.009	0.017	0.0007	98	204	137	8	112	4
XWL5ZK4B3-22	3	98.6	102	1	0.119	0.010	0.317	0.023	0.020	0.0007	1944	144	279	17	124	4
XWL5ZK4B3-23	5	218	172	1.3	0.052	0.004	0.133	0.008	0.018	0.0006	333	166	127	7	117	4
XWL5ZK4B3-24	6	297	226	1.3	0.050	0.004	0.117	0.008	0.017	0.0006	191	174	112	7	110	4
XWL5ZK4B3-25	9	282	393	0.7	0.049	0.003	0.119	0.006	0.018	0.0005	200	-71	114	6	114	3
XWL5ZK4B3-26	12	414	492	0.8	0.043	0.002	0.104	0.005	0.017	0.0005			100	5	111	3
XWL5ZK4B3-27	5	203	212	1	0.047	0.004	0.114	0.008	0.017	0.0006	61	174	110	7	109	4
XWL5ZK4B3-28	3	130	129	1.0	0.049	0.005	0.132	0.010	0.017	0.0007	167	198	126	9	110	5
XWL5ZK4B3-29	7	259	262	1	0.055	0.004	0.129	0.008	0.018	0.0005	398	142	123	7	112	3
XWL5ZK4B3-30	12	432	497	0.9	0.048	0.003	0.110	0.006	0.017	0.0005	83	128	105	5	108	3
XWL5ZK4B3-31	14	820	534	1.5	0.043	0.002	0.102	0.005	0.017	0.0005			99	4	108	3
XWL5ZK4B3-32	20	799	814	1	0.042	0.002	0.100	0.004	0.017	0.0005			97	4	111	3
XWL5ZK4B3-33	4	185	133	1.4	0.061	0.005	0.157	0.010	0.017	0.0006	628	161	148	9	110	4
XWL5ZK4B3-34	7	303	267	1.1	0.048	0.003	0.120	0.007	0.017	0.0005	120	144	115	6	109	3
XWL5ZK4B3-35	5	205	172	1.2	0.053	0.004	0.127	0.008	0.017	0.0006	332	161	122	7	111	4
XWL5ZK4B3-36	7	255	289	0.9	0.047	0.003	0.112	0.007	0.017	0.0005	58	152	107	6	110	3
XWL5ZK4B3-37	4	211	150	1.4	0.046	0.004	0.134	0.008	0.018	0.0006			128	7	116	4
XWL5ZK4B3-38	4	163	150	1.1	0.064	0.005	0.164	0.010	0.017	0.0005	743	169	154	9	107	3
XWL5ZK4B3-39	10	428	418	1	0.053	0.004	0.123	0.008	0.017	0.0005	317	157	118	7	109	3
XWL5ZK4B3-40	4	139	143	1	0.092	0.009	0.272	0.019	0.018	0.0007	1458	192	245	15	117	4
XWL5ZK4B3-41	5	229	211	1.1	0.045	0.004	0.121	0.008	0.017	0.0006			116	8	106	4
XWL5ZK4B3-42	23	946	886	1.1	0.045	0.002	0.104	0.005	0.017	0.0005			100	4	106	3
XWL5ZK4B3-43	11	518	428	1.2	0.049	0.002	0.107	0.005	0.016	0.0005	146	111	103	5	102	3
XWL5ZK4B3-44	7	250	315	0.8	0.051	0.003	0.108	0.006	0.015	0.0005	243	135	104	6	98	3

, as well as the U-Pb age normal distribution histograms and Concordia diagrams associated with the typical zircon cathodoluminescence (CL) images; the weighted average ²⁰⁶Pb/²³⁸U ages are illustrated in Figure 6 and Figure 7.

Forty zircon spots were analyzed for sample XWL4ZK1B2 and forty-four zircon spots were analyzed for sample XWL5ZK4B3. All the zircon analyzed was short, prismatic and exhibited a length of 50–200 um and width/length ratios of 1:3–1:1, with the CL images showing the oscillatory zoning (Figure 6 and Figure 7), indicating that they were of magmatic origin [62]. The Th and U abundances vary from 98.6 ppm to 1295 ppm and 102 ppm to 886 ppm, respectively, with Th/U ratios of 0.6–2.1 (Table 2). In the ²⁰⁶Pb/²³⁸U-²⁰⁷Pb/²³⁵U Concordia diagram (Figure 6a–c,e and Figure 7a,c,e), although some data are further away from the Concordia line, they are all on the right of the Concordia line and are mainly concentrated in a horizontal line, forming distribution patterns similar to those described by Refs. [59,63,64], which are different from those for the classic Pb-loss patterns suggested by Ref. [65]. The former is considered to result from the low credibility of the ²⁰⁷Pb measured in younger zircon [59,64], which will not affect the ²⁰⁶Pb/²³⁸U ratios [63,64,66]; therefore, the ²⁰⁶Pb/²³⁸U ages dated in this study are reliable.

For sample XWL4ZK1B2, forty zircon spots have ²⁰⁶Pb/²³⁸U ages between 106 ± 3 and 725 ± 19 Ma (Table 2; Figure 6a), except for two slightly older grains that have ²⁰⁶Pb/²³⁸U ages of 128 ± 4 Ma and 129 ± 4 Ma, and two older grains that have ²⁰⁶Pb/²³⁸U ages of 635 ± 22 Ma and 725 ± 19 Ma, which are considered to be inherited zircon from the Late Yanshanian granitoids and the Precambrian basement in Sulu terrane [15], respectively; the other grains form two populations (Figure 6b). One population has 24 grains and records ²⁰⁶Pb/²³⁸U ages of 114 ± 3 Ma and 126 ± 4 Ma, with a weighted average age of 118 ± 2 Ma (2σ, MSWD = 0.8) (Figure 6c,d), which is used to estimate the crystallization age of the magma; the other has 12 grains and gives ²⁰⁶Pb/²³⁸U ages of 106 ± 3 Ma to 113 ± 3 Ma, with the weighted average age of 110 ± 2 Ma (2σ, MSWD = 0.48) (Figure 6e,f), probably recording the thermal disturbance by the emplacement of younger dikes, which are widely present in the area (Figure 2) [15].

For sample XWL5ZK4B3, forty-four zircon spots produce ²⁰⁶Pb/²³⁸U ages of 98 ± 3–126 ± 4 Ma (Table 2; Figure 7a), except for two younger grains that show ²⁰⁶Pb/²³⁸U ages of 98 ± 3 Ma and 102 ± 3 Ma, likely resulting from a later thermal disturbance. The other grains also form two populations (Figure 7a,b), similar to those of sample XWL4ZK1B2 (Figure 6b). One population has 16 grains and gives ²⁰⁶Pb/²³⁸U ages of 113 ± 3 Ma and 126 ± 4 Ma, with a weighted average age of 117 ± 2 Ma (2σ, MSWD = 1.15) (Figure 7c,d), and the other has 26 grains and shows ²⁰⁶Pb/²³⁸U ages of 106 ± 3 Ma to 112 ± 3 Ma, with a weighted average age of 109 ± 1 Ma (2σ, MSWD = 0.32) (Figure 7e,f).

5. Discussion

5.1. Timing of Au Mineralization

The abovementioned petrographic observations detail that molybdenite coexists with gold-bearing pyrite, galena and native gold (Figure 3), suggesting that the gold and molybdenite were precipitated from the ore-forming fluid simultaneously. Therefore, the molybdenite weighted average Re–Os model age of 118 ± 1.7 Ma in this study should represent the timing of Au mineralization in the Xiawolong gold deposit. However, this molybdenite Re–Os mode age at Xiawolong is about 4 m.y. older than the 114.2 ± 1.5 Ma monazite age for the Rushan gold deposit, about 20 km southwest of the Xiawolong gold mine (Figure 2a), dated by Ref. [67].

As mentioned above, focusing on the perspective of the whole eastern Jiaodong, i.e., the Muping–Rushan gold metallogenic belt, the other gold deposits, except for the Xiawolong gold deposit, i.e., the “Linglong-type” gold deposits, are controlled by a series of sub-parallel NE- to NNE-trending faults, named the Muping–Rushan Fault Zone (Figure 2a), and are composed of gold-bearing quartz veins with the alteration envelopment of silicification in the outer domain and sericitization in the inner [1,68,69,70]. Of them, the Rushan gold deposit, endowed with the largest gold resource hosted by a single fault-controlled gold-bearing quartz vein in China, is the representative one of the “Linglong-type” deposits in eastern Jiaodong [71]. During the last few decades, much age data, including those by pyrite/K-feldspar Rb-Sr [72,73], sericite ⁴⁰Ar/³⁹Ar [74,75,76] and the hydrothermal zircon U–Pb [47], have been presented for the Rushan gold deposit.

However, for the Jiaodong gold ores, due to the complex mineral growth history [77], the low Rb contents in the hydrothermal K-feldspar and pyrite [78,79], as well as the issue of whether the zircon analyzed is crystallized at relatively low temperatures from ore-forming fluids or from the surrounding igneous rocks, is very difficult to identify [67]. A large spread of ages ranging between 121 Ma and 105 Ma was produced for those by pyrite/K-feldspar Rb-Sr [72,73] and the hydrothermal zircon U–Pb [47]. Although the high-precision ⁴⁰Ar/³⁹Ar dating for hydrothermal sericite has been deemed an effective dating technique for Jiaodong gold deposits recently [67,77], the conflicting ⁴⁰Ar/³⁹Ar ages for Rushan based on hydrothermal sericite, including those first reported to be ca. 109–108 Ma [75] and ca. 129 Ma [76], and more recently ca. 112–117 Ma [71] and ca. 120 Ma [77], indicate that, at least in part, it is very difficult to determine truly the syngold hydrothermal sericite without suffering later modification.

Fortunately, with careful microscopic research with SEM and BSE imaging as well as electron probe microanalysis, trace amounts of syngold hydrothermal monazite, which was considered to be the robust chronometer [80,81,82,83], was identified by Deng et al. [67], who obtained the 114.2 ± 1.5 Ma monazite age for Rushan using the LA-ICP-MS U–Pb in situ dating method and proposed that the ca. 114 Ma age is the most robust age for gold mineralization in eastern Jiaodong. In addition, molybdenite Re–Os dating is also a robust method, which can be applied to determine the mineralization age accurately and directly [84,85,86,87]; thus, the molybdenite weighted average Re–Os model age of 118 ± 1.7 Ma for Xiawolong in this study is highly precise and reliable. Furthermore, this giant gold event in Jiaodong, accompanied by rapid crustal exhumation, had a significantly shorter duration of over only a few million years [67,77]. Therefore, the new molybdenite weighted average Re–Os model age obtained in this study suggests a gold event occurring ca. 4 m.y. before the other “Linglong-type” gold deposits in eastern Jiaodong.

In addition, Deng et al. [67] also dated the syngold hydrothermal monazites from both the representative “Jiaojia-type” (the >200 t Au Jiaojia deposit) and “Linglong-type” (the >100 t Au Lingong deposit) deposits in western Jiaodong, obtaining 119.8 ± 2.1 Ma and 119.1 ± 1.4 Ma monazite ages, respectively. Furthermore, the world-class Xiadian deposit (>200 t Au), another representative “Jiaojia-type” deposit in western Jiaodong, was dated at 120.0 ± 1.4 Ma using the monazite LA-ICP-MS U–Pb method [88]. Moreover, for the smaller “Linglong-type” deposits scattered around central Jiaodong, such as the Zhuangzi, Daliuhan and Hushan deposits, the monazite ages of 119.0 ± 3.1 Ma [89], 120.5 ± 1.7 Ma [90] and 120 ± 3 Ma [14], respectively, have been obtained recently. These consistent precise geochronologic dates indicate that the giant gold event in western and central Jiaodong occurred during a very transient period in ca. 120 Ma.

Overall, the gold event in the Xiawolong deposit occurred between those in ca. 120 Ma for western and central Jiaodong and those in ca. 114 Ma for eastern Jiaodong, representing another discrete gold mineralization in Jiaodong.

5.2. Constraints on the Origin of Au Mineralization in the Xiawolong Deposit

As mentioned above, all the gold orebodies in Xiawolong are hosted by the fine-grained granitic dikes, which have abrupt contact with Late Jurassic Kunyushan monzogranite with an emplacement age of 161–141 Ma. The zircon LA-ICP-MS U–Pb dates reported in this paper produce the first precise geochronology on the fine-grained granite and give the emplacement ages of 118 ± 2 to 117 ± 2 Ma, which are consistent with those of molybdenite Re–Os, within the margin of error, obtained for the gold-bearing ores in Xiawolong.

In addition, the fine-grained granitic dikes have a similar lithology as those of the medium-grained monzogranite consisting of the marginal facies of the Sanfoshan batholith [15]. One monzogranite sample SD-11, located about 15 km south of the Xiawolong deposit (Figure 1) from the marginal facies was dated at 118 ± 1 Ma by Goss et al. [15] using the zircon SHRIMP U–Pb method, which is identical to those of LA-ICP-MS zircon U–Pb dated for the fine-grained granite in this study. Although no other zircon U–Pb dates for the central phase of the Sanfoshan batholith have been reported up to now, Wang et al. [39] suggested that resulting from the abrupt change in the subduction direction of Paleo–Pacific plate under East China at ca. 120 Ma, the catastrophic lithospheric thinning accompanied with the late Early Cretaceous igneous magmatism occurred in Jiaodong, and thus the earliest emplacement timing for the Sanfoshan batholith is no earlier than 120 Ma. Therefore, it is reasonable that the fine-grained granite dike in Xiawolong is the crystallization product of granitic magma, related to the Sanfoshan batholith (Figure 8) in the late stage, considering that the fine-grained granite dikes are the branches of the Sanfoshan granitic pluton [19].

The previous research on the fluid inclusion and O isotopic composition suggests that the ore-forming fluid for the Xiawolong deposit is from magmatic water [20], which accords with the geologic characteristics at different scales. For example, the gold-bearing pyrite coexists with the magnetite and molybdenite in the gold ores, and the ores are characterized by the disseminated and stockwork-style. As well, no hydrothermal alteration occurs adjacent to the gold orebodies. Although the S and Pb isotope compositions indicate they mainly come from the Mesozoic Kunyushan granitoids [20], the large temporal gap between the gold mineralization and granite emplacement precludes the direct genetic relation between gold deposition and Kunyushan granite. Nevertheless, the O isotopic compositions indicate that the ore-forming fluid results from Sanfoshan granitic magma degassing during the magmatic–hydrothermal transition stage [20]. Furthermore, the occurrence of magnetite in the gold ores indicates the high oxygen fugacity (fO₂) of both the magma and resultant hydrothermal fluids. During magma migration, the former would be able to scavenge and carry significant amounts of Au [91,92], with the latter facilitating the transport of Au effectively during the magmatic–hydrothermal transition stage [91], which is similar to the Au transportation mechanism for the Zhongshangou gold deposit, hosted within the monzonite, in the Zhangjiakou orefield north of the NCC margin [93]. However, the change in fO₂ would result in significant Au precipitation during the fluid migration, as has been evidenced from the studies on the Panasqueira W–Sn deposit of Portugal [94,95]; the W–Mo–Au deposit in San Judas Tadeo, southeastern Peru [96]; the Saint-Robert W–Bi–Ag deposit in Quebec [97] and the Lujing uranium field hosted within fine-grained granite in the Nanling metallogenic belt, south China [98].

Overall, it is logical to infer that the Xiawolong gold deposit is genetically related to the Sanfoshan granitic magmatism, which occurred during the regional tectonic extensional process shifting from E–W to NW–SE induced by the Paleo–Pacific slab rollback initiated in ca. 120 Ma (Figure 8) [39]. The Sanfoshan granitic magma has high fO₂ and is rich in Au at the magmatic–hydrothermal transition stage, and the change in fO₂ most likely makes a significant contribution to the precipitation of Au.

5.3. Implications for Exploration

This work reveals that the late-stage granitic magma with high fO₂, which is crystallized into the fine-grained granite, is probably also rich in Au, except for the W–Mo–Cu–Zn–U–Be–Li–Nb–Ta–Sn–Bi-elements [22,26,98,99]. Thus, this would also result in an intimate spatial relationship between the gold orebodies and the fine-grained granite, which is comparable to the other gold deposits worldwide. For example, in the Passa Três gold deposit located in southern Brazil, the gold-bearing quartz veins occur along the aplitic dikes [100]. In addition, the syenite-associated gold orebodies are found in the Porgera gold deposit, Papua New Guinea [101]; the Abitibi greenstone belt in Canada [102] and the Timbarra gold deposit in Australia [103]. Notably, these deposits commonly are located within the apical portions of small intrusions [104], i.e., the roof zone of the granitic system [105].

Early Cretaceous gold deposits occurred intensively and widely in the Jiaodong Peninsula in an extensional tectonic regime, as evidenced by the normal movement along the TLF [106] as well as large-scale extension-related magmatism [8,9,10] during ca. 130–110 Ma. It has been traditionally accepted that all the gold deposits, including the “Linglong-type” and “Jiaojia-type”, are controlled by either the NE- and NNE-trending regional-scale or lower-order faults [45,46], with the gold orebodies surrounded by the silicification or sericitization alteration envelop [1,68,69,70]. Over 90% of the gold resources are clustered in the Jiaobei uplift, i.e., central and western Jiaodong, which consists of Penglai–Qixia and Zhaoyuan–Laizhou gold metallogenic belts, whereas less than 10% of those in eastern Jiaodong are clustered in the Muping–Rushan gold metallogenic belt [1,67]. The new and precise geochronologic dates obtained by U–Pb dating on monazite from the gold deposits scattered around Jiaodong indicate that the former was formed during a very transient period in ca. 120 Ma, about 6 m.y. older than the latter [14,88,89,90], suggesting that they are two discrete gold events even though they show a similar geological character and are the same in the genesis. However, this study reveals that the Xiawolong deposit hosted by the fine-grained granitic dikes was formed in ca. 118 Ma, representing another discrete gold event, and is genetically related to the Sanfoshan granitic magmatism, different from the “Linglong-type” and “Jiaojia-type” deposits hosted by the NNE- to NE-striking faults in Jiaodong. Therefore, we propose that gold exploration in the Jiaodong should not only focus on the fault-hosted Au but also on the fine-grained granite-hosted Au around the apical portions of the late Early Cretaceous small granitic intrusions with high fO₂. This exploration model could also be important for prospecting in other gold ore districts that have a similar tectonic setting.

In addition, the late Early Cretaceous granitoids in the Jiaodong Peninsula have strong magnetism, which is two orders of magnitude higher than the late Jurassic biotite granite and monzogranite and the Precambrian basement rocks in Jiaodong [107]. However, due to no gold orebodies occurring in the late Early Cretaceous granitoids, the high-precision magnetometric survey was mainly used to determine the concealed rock mass in the past. Nevertheless, this study reveals that the fine-grained granitic dike hosting the Xiawolong deposit is the late-stage crystallization product of the late Early Cretaceous Sanfoshan granitic magma, and the magnetite coexists with the gold-bearing pyrite in the gold ores; thus, it is suggested that the high-precision magnetometric survey could be carried out to determine the striped high magnetic anomaly surrounding the late Early Cretaceous granitoid so as to find the fine-grained granite-hosted Au.

6. Conclusions

(1)

Our high-precision molybdenite weighted average Re–Os model age of 118 ± 1.7 Ma (2σ) first obtained for the stockwork- and disseminated-style gold-bearing ores indicates that a discrete gold event occurred in Xiawolong deposit, ca. 4 m.y. older than that of the other gold mineralization in ca. 114 Ma in eastern Jiaodong.

(2)

The fine-grained granite hosting the gold-bearing ores, dated using the LA-ICP-MS zircon U–Pb method to be 118 ± 2 to 117 ± 2 Ma (2σ), is the crystallization product of granitic magma, related to the Sanfoshan batholith, in the late stage.

(3)

The Xiawolong gold deposit is genetically related to the Sanfoshan granitic magmatism, which has high fO₂ and is rich in Au at the magmatic–hydrothermal transition stage, and the change in fO₂ is most likely to make a significant contribution to the precipitation of Au.

(4)

The gold exploration in the ore districts formed an extensional tectonic regime that should not only focus on the fault-hosted Au but also on the fine-grained granite-hosted Au around the apical portions of the small granitic intrusions with high fO₂.

(5)

The fine-grained granite crystallized from the late-stage granitic magma with high fO₂ is also rich in Au, except for the W–Mo–Cu–Zn–U–Be–Li–Nb–Ta–Sn–Bi-elements, and the high-precision magnetometric survey could be used to find the fine-grained granite-hosted Au.