Geochronology and zircon geochemistry of auriferous intrusions in the Bumo deposit, Hainan Province, China

Yan Hai • Bingtao Li • Teng Deng,4 • Deru Xu • Li Wang •Youzhong Xiong • Xiaowen Zhang • Zhiling Wang • Shaohao Zou •Zhengpeng Ding • Qian Qian • Shichao Guo

Abstract Intrusion-related gold deposits(IRGS)are a lowgrade, large-tonnage exploration target.Recently, auriferous magmatic rocks were found in the Bumo deposit of the Gezhen shear zone in Hainan Province, China.However,the geochronology and geochemical characteristics of the intrusions, as well as the mineralization potential, are still unclear.Field and petrographic work show that the sulfidebearing intrusions can be divided into diorite porphyrites,quartz monzodiorites and monzodiorites.Zircon LA–ICP–MS U–Pb dating demonstrates that diorite porphyrites,quartz monzodiorites and monzodiorites were formed at 104 ± 1, 114 ± 1, 114 ± 1 Ma, respectively.In addition,sulfides in Yanshanian intrusion-related gold mineralization have δ34S values of 0.2–4.4‰, lower than those in Hercynian-Indosinian (1.9–9.8‰) orogenic deposits (ca.219–378 Ma) in the Gezhen shear zone.In addition, all these intrusions display close correlations between Eu/EuN* with Th/U, consistent with the differentiation of amphibole, apatite and titanite from a hydrous melt.Moreover,zircon Eu/EuN*in the intrusions are higher than 0.4, demonstrating that the magmatic rocks have high water contents and oxygen fugacity values, favorable for gold mineralization.Consequently, the Yanshanian magmatic rocks can be a new potential gold exploration target in the Gezhen shear zone.

Keywords Hainan ∙Bumo ∙Intrusion-related mineralization ∙Zircon trace elements ∙Gold mineralization

1 Introduction

Intrusion-related gold deposits (IRGS) can divide into reduced and oxidized IRGS (Hart and Goldfarb 2005).Among them, the reduced type is generally characterized by Au–Bi–Te–W assemblages (Hart 2007), and the oxidized ones are Au-rich variants of the porphyry Cu–Au class (Hart 2007; Niroomand et al.2018).The magmatic rocks in oxidized intrusion-related gold deposits commonly have high oxygen fugacity(Helt et al.2014)and high water content (Li et al.2018), favoring Au–Chalcophile mental association (Elshourbagi and Fan 2021).In many deposits,the study of intrusion-related magma based on whole-rock and major mineral geochemistry is hindered by hydrothermal alteration (Large et al.2018).Zircon, a common accessory mineral in intermediate to felsic igneous rocks, can resist hydrothermal alteration and weathering (Cherniak and Watson 2006).Zircon trace element compositions in magmatic rocks such as Eu/EuN*have been used as an effective way to track the water content and oxidation state(Ballard et al.2002;Dilles et al.2015; Lu et al.2016a, b).

The Gezhen shear zone, hosting more than 50 gold ore deposits and total gold reserves of ＞67 t, is the most important gold-producing area in Hainan Province, South China (Xu et al.2017).This zone hosts several gold deposits, including Bumo, Tuwaishan, Hongfumenling,and Beimiu (Ding et al.2020).Previous research shows that these gold deposits are typically an orogenic type(Liu et al.2020), forming at Hercynian-Indosinian (Liu et al.2020).Some studies suggest that Yanshanian magmatic rocks may also contribute to gold mineralization in Bumo and Beiniu (Chen 1996; Zhan et al.1996; Li et al.1999).However, the direct and unambiguous geological,geochronological and geochemical evidence for auriferous Yanshanian intrusions is still lacking.

Recent prospecting discovered magmatic rocks with abundant sulfides that crosscut the lode ore bodies in the Bumo deposit, but the timing and mineralization potential of these rocks are still unclear.Base on the field and petrographic work, this paper aims to constrain the emplacement age of intrusions through zircon LA–ICP–MS U–Pb dating.In addition, the zircon trace element compositions are used to evaluate the mineralization potential and in-situ LA–MC–ICP–MS S isotopes are conducted on pyrites to trace the metal source.

2 Regional geology

Hainan Island,a marginal continental island in South China(Fig.1),is regarded as one part of the Cathaysian Block of South China (Metcalfe 1996; HBGMR 1997; Liu et al.2020).Six stages of the tectonic movement were found at Jinningian, Caledonian, Hercynian, Indosinian, Yanshanian,and Himalayan epoch(Chen 1996;Ding et al.2020).The Precambrian strata, widely exposed in the west of the Island, are successively the Baoban Group, Shilu Group, and Shihuiding Formation (Ma et al.1997).The Mesozoic strata, well developed in Hainan, were mainly deposited at Early Triassic and Cretaceous,lacking Middle Triassic—Jurassic strata (Xu et al.2017; Cai et al.2017).Cenozoic rocks are mainly exposed in the northern island,which are composed of volcanic rocks and clastic sedimentary rocks (HBGMR 1997).The Baoban Group,- 3.5 km thick,can be further divided into Gezhencun and Ewenling formations (Xu et al.2017; Zhang et al.2019),with the former consisting of biotite-plagioclase gneisses intruded by gneissic granites and migmatites.The Ewenling Formation is characterized by quartz-mica schists intercalated with graphite ore beds and quartzites(Ma et al.1997; Zhang et al.2019).The Baoban Group is the main host for gold deposits in the Gezhen shear zone(Yan et al.2017).

The Shilu Group (- 2.7 km thick) controlling the largest Fe-rich ore deposit in China, only exposed at Changjiang area of the western Hainan Island,is a series of neritic siliciclastic rocks and sedimentary carbonates with lowgrade metamorphism (Xu et al.2013; Wang et al.2015;Zou et al.2017).The Shihuiding Formation (- 125 m thick) is predominated by shallow marine siliciclastic and carbonate rocks with greenschist facies, which comprises the Early Neoproterozoic macroscopic algal fossils(houhsiensia,huariaandTachymacrus) (Yao 1999; Zou et al.2017).

The Late Paleozoic–Early Mesozoic granitoid(350–200 Ma) are widely distributed in the central and southern islands.Over forty percent of granitoids in Hainan are medium to coarse-grained monzogranite (Xu et al.2017; He et al.2020).The Late Mesozoic (175–65 Ma)granitoids are mainly exposed in the east and scattered in the west of Baisha fault, which is characterized by biotite granites and granodiorites (Hu et al.2019).The Mesoproterozoic mafic rocks occurred in the southwest of Hainan Island.The Late Paleozoic-Early Mesozoic mafic rocks,including basalt,dolerite,and gabbro are exposed in southern Hainan and Cenozoic basalts were discovered in northern Hainan(Tang et al.2013;Xu et al.2013;He et al.2020).

Two prominent sets of faults are recognized in Hainan(Fig.1b), i.e., the EW-trending Wangwu-Wenchang,Changjiang-Qionghai, Jianfeng-Diaoluo, and Jiusuo-Lingshui faults and NE-trending Gezhen shear zone and Baisha Faults (Yan et al.2017).The Baisha Fault cut Hainan into NW and SW terranes and illustrated a continental arc setting (Hu et al.2019).

Fig.1 a Location map of Hainan Island (modified after Xu et al.2017); b Schematic diagram showing the main strata and magma units, gold deposits, and occurrences in the Hainan Island, South China (modified after Xu et al.2017)

The Gezhen shear zone locates in the west of the island,which is about 55 km long and 0.5–3.0 km wide and has a striking of NE and SE dipping angles of 30°–80° angles(Zhang et al.2011; Liu et al.2020).By using40Ar/39Ar dating on synkinematic mica, previous research suggests that the latest movement of the Gezhen shear zone occurred at ca.227 Ma (Zhang et al.2011).The rocks to the northwest of the Gezhen fault consist of gneisses, migmatites gneisses, and schists of the Proterozoic Baoban Group and schists of the Late Meso- to Neoproterozoic Shilu Group(Liu et al.2020).The units to the southeast of the Gezhen fault are composed of schists of the Late Mesoto Neoproterozoic Shilu Group, phyllite, slate, and quartz sandstone Cambrian to Silurian, Carboniferous to Permian quartz sandstone, slate, and limestone, Cretaceous terrestrial siliciclastic (Xu et al.2017).The Mesoproterozoic granitoids are widely distributed in both sides of Gezhen shear zone, spatially closed to gold deposits (Liu 2018).The Datian intrusion was emplaced in the middle of Gezhen shear zone at 245 Ma, and distinguished by mylonite deformation (HBGMR 1997).

The Gezhen fault is characterized by multiple orogenic gold deposits in the Gezhen shear zone, including Bumo,Beiniu, and Tuwaishan (Xia 2004; Yao et al.2017).Previous geochronological research demonstrates that gold mineralization occurred at roughly 219–378 Ma (Ye and Zhu 1990;Tu and Gao 1993;Chen 1996;Zhan et al.1996;Liu et al.2020).The mineralization styles are mainly characterized by quartz veins,cataclastic altered rocks,and altered mylonites.Carbonaceous material (CM)-bearing host rocks, including carbonic phyllites, black shales, and carbonaceous slates, are well distributed in the Gezhen shear zone and genetically associated with gold mineralization (Ding et al.2020).

3 Ore deposit geology

The Bumo gold deposit lies southwest of the Gezhen shear zone, 25 km away from Dongfang City, Hainan Province(Xu et al.2013, 2017).The ore deposit is hosted in the Mesoproterozoic Baoban Group,which is characterized by amphibolites, migmatites geneisses, and quartz-mica schists (Liu 2018).The Meso-Neoproterozoic Shilu Group and Quaternary sediments are also exposed in the west section of the deposit (Xu et al.2017; HBGMR 1997)(Fig.2).

Fig.2 Geological map of the Gezhen shear zone and associated gold deposits (modified after Xu et al.2017)

Several Yanshanian intrusive rocks with no signs of mineralization are exposed in the northeast and northwest parts of the deposit.The Yanshanian intrusive rocks are diorite porphyry, pegmatite, and diorite (Zhou et al.2007;Liu and Chen 2013).The major structures in Bumo include NW-, NE-, nearly EW-trending (Fig.3).The deposit is situated in the S-shaped mylonites belt,brittle fracture,and schistosity structure in the mining area (Zhan et al.1996).Auriferous quartz veins display NW-,NE-and nearly EWtrending, and the NE-trending orebodies are more fertile(Zhan et al.1996).Previous studies demonstrate that the Bumo gold deposit is an orogenic type,with a gold reserve of about 13t and a grade of 17.9–71.1 g/t (Xu et al.2017).

Fig.3 Geologic map of the Bumo gold deposit (modified after Xu et al.2017)

Ores in the Bumo deposit are mainly present as quartz veins and cataclastic altered rocks (Fig.4a, b).The ore veins are spatially and genetically associated with carbonaceous material (CM) (Ding et al.2020).Ore mineral includes pyrite, sphalerite, arsenopyrite and chalcopyrite,while the gangue minerals are calcite, quartz, and sericite(Fig.4c, d).The main alteration type of wall rocks in the Bumo mining area are silicification, sericitization and pyritization, with minor chloritization and carbonation(Ding et al.2020).Four paragenetic stages have been identified with the mineral compositions of quartz–pyrite,quartz–pyrite–arsenopyrite–native gold,quartz–native gold and quartz–calcite, respectively (Zou et al.2017).

Fig.4 Photographs of orebodies and mineral assemblages in the Bumo deposit.a Auriferous quartz rocks; b cataclastic altered rocks; c carbonaceous material associate with pyrite and chalcopyrite (reflected-light);d arsenopyrite and sphalerite(reflected-light).Py, pyrite;Apy, arsenopyrite; Qtz, quartz;Sp, sphalerite; Pl, plagioclase;Ccp, chalcopyrite; CM,carbonaceous material

Recently,magmatic rocks with extensive mineralization have been discovered in southeast of deposit and cutting through the IV gold-bearing quartz veins at about thirtymeter underground (Fig.5).From fieldwork and microscopic observation, the rocks are composed of diorite porphyrites, quartz monzodiorites and monzodiorites from outer to inside.The diorite porphyrites consist of feldspar phenocryst (- 40%), quartz phenocryst (- 10%), muscovite (- 5%), hornblende (- 10%), the matrix (- 25%)is microcrystalline feldspar with strong chloritization(Fig.6a–c).The quartz monzodiorites commonly show speckled black and white with phaneritic texture,comprise feldspar (- 35%), quartz (- 15%), amphibole (25%),chlorite (- 5%), biotite (- 10%), feldspar with strong sericitization (Fig.6d–f).The monzodiorites, a phaneritictextured, are composed of feldspar (- 35%), quartz(- 5%), hornblende (25–30%), and biotite (5–10%), feldspar with sericitization(Fig.6g–i).All of the intrusions are extensively mineralized with abundant sulfides including pyrite, chalcopyrite, sphalerite, pyrrhotite.The average gold grades of diorite porphyrites, quartz monzodiorites and monzodiorites are 0.5, 0.2–0.3, and 0.2 g/t.Pyrite grains are euhedral to xenomorphic granular with sizes of 0.1–1.0 mm.

Fig.5 Sketch of the magmatic intrusions crosscutting the goldbearing quartz veins

Fig.6 Representative hand specimens and micrographs of magmatic intrusions in the Bumo deposit.a–c Diorite porphyrites, b–f quartz monzodiorites,g–i monzodiorites.b,e and h is cross-polarized light;c,f and i is reflected-light.Py,pyrite;Chl,chlorite;Ser,sericite;Bt,biotite;Qtz, quartz; Pl, plagioclase; Mus, Muscovite; Hbl, hornblende

4 Sampling and analytic method

Samples used in this study were taken from intrusions near the fourth gold veins in Bumo ore deposit.Thirty-five samples are taken from diorite porphyrites, quartz monzodiorites and monzodiorites, respectively.For a better understanding or more accurate evaluation of geochronology and mineralization potential of intrusions, the samples were used for zircon LA-ICP-MS U–Pb dating and trace element analysis.

4.1 Zircon U–Pb dating and trace element analysis

Zircon particles were separated from samples by conventional separation procedures, using a Wilfley vibrating water table, heavy liquids, and magnetic separation at Guangzhou Tuoyan Analytical Technology Co., Ltd.,Guangzhou,China.Individual zircon particles of intrusions were hand-picked under a binocular microscope, installed in epoxy resin trays,and polished to half particle thickness.Grains that are used for the study are euhedral in shape and colorless.There are about 30 zircon particles in each sample taken by cathodoluminescence; CL imaging was performed using an Analytical Scanning Electron Microscope (TESCAN MIRA 3) by setting the working conditions to an electric field voltage of 10.0–15.0 kV.The NWR 193 laser ablation system is used for laser sampling.Ion signal intensity was produced by ICAP RQ ion mass inductively coupled with plasma mass spectrometry.Helium served as a carrier gas.Argon acts as a supplementary gas and combines with the carrier gas by a Y-connector before getting into the ICP.Laser energy and frequency were 5 mJ/cm2and 8 Hz, respectively, and the spot size is 30 μm.Calibration of trace element and U–Pb dating was done utilizing glass NIST610 (Wiedenbeck et al.1995)and Zircon 91,500(Reed 1992)as the external standards, respectively.ICP-MS Data Cal was performed to sort and quantitative calibration of U–Pb dating and trace element analysis (Liu et al.2008).

4.2 In-situ S isotope analysis of pyrite by using LA–MC–ICP–MS

In-situ sulfur isotope (32S,33S, and34S) analysis of pyrite was carried out on a Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, Bremen, Germany) of Wuhan Sample Solution Analysis Technology Co., Ltd., Hubei Province,China.The instrument is equipped with a Geolas HD excimer ArF laser ablation system (Go¨ttingen Coherence Company, Germany).In the laser ablation system, helium was acted as the carrier gas for the ablation cell and was blending with argon (makeup gas) after entering the ablation cell.One analysis experience 100 laser pulses in one spot ablation mode.Working conditions of 44 μm spot size, 2 Hz pulse frequency and 5 J/cm2laser fluence were performed to avoid the effect which have been mentioned by Fu et al.(2016).The Neptune Plus was combined with nine Faraday cups and 1011Ω resistors.S isotopes were collected in Faraday cups in static mode.The Jet sample with X skimmer cone of Neptune Plus were inset to improve the signal intensity.Reducing the polyatomic interferences by adding nitrogen (4 mL/min) into the central gas flow.All the samples were conducted to correct for instrumental mass fractionation by the standard-sample bracketing method, and pyrite standard PPP-1 (Fu et al.2016) was used as reference material for rectifying the natural pyrite.More information about the in-situ S isotopic compositions and the reference values of δ34Sv-CDTwas reported in Fu et al.(2016).Iso-Compass software was employed to calculate the data of S isotopes (Zhang et al.2020).

4.3 LA–ICP–MS multi-element analysis of sulfides

Contents of trace elements in pyrite were measured by laser-ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) at Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitor of Ministry of Education, Central South University, Changsha, Hunan Province, China.The instrumentation employs a Telydyne Cetac HE 193 Nnm laser-ablation system coupled to an Analytik Jena PlasmaQuant MS Ellite plasma mass spectrometry.Spot ablation was carried out using a 35 μm spot size at 1.5 J/cm2laser fluence, 5 Hz repetition rate and 70 s ablation time,Gas flow rates of 13.5 L/min (Ar), and 1.1 L/min (He)were used.The calibrations employed MASS-1 external and Fe internal standards.In addition,NIST SRM610 were employed as the system monitoring sample.The measure elements include33S,57Fe,59Co,60Ni,65Cu,75As,97Mo,107Ag,111Cd,118Sn,121Sb,125Te,197Au,208Pb,209Bi.For more detail see Griffin (2008).

5 Results

5.1 Zircon U–Pb dating

The zircon LA-ICP-MS U–Pb dating results are given in Table 1 and U–Pb concord plots and typical zircon CL images are illustrated in Fig.6.For the diorite porphyries,twenty concordant ages provide a weighted mean206Pb/238U age of 104 ± 1 Ma(1σ,MSWD = 1.8,N = 19;Fig.7).Twenty-four analytical spots on zircon from the quartz monzodiorites show a weighted mean206Pb/238U ages of 114 ± 1 Ma (1σ, MSWD = 0.88, N = 24; Fig.7).Thirty-five zircons of monzodiorites present the concordant206Pb/238U age of presented a weighted mean age of 114 ± 1 Ma (1σ, MSWD = 1.2, N = 35; Fig.7).

Fig.7 Zircon LA–ICP–MS U–Pb concordia diagrams and CL images of zircon grains from the diorite porphyrites, quartz monzodiorites and monzodiorites

5.2 Geochemical signatures of zircon

Ma 1.80855 2.37313 2.66216 2.69361 3.54436 2.46013 2.43141 2.44740 2.72928 2.24473 2.66027 2.48493 1.79588 2.53115 2.81095 3.10719 2.23024 3.27273 2.53062 2.70370 2.39845 3.25946 2.93938 2.63675 2.24285 2.10664 2.30777 2.62523 2.20979 2.44638 1.88947 2.27215 2.28705 206Pb/238U Age (Ma)111.26534 113.58620 113.83325 114.65936 117.54718 110.02529 117.06859 114.90285 114.05917 115.05944 112.04120 109.47689 116.07977 114.35041 116.76006 113.17147 114.81419 119.32462 114.48304 113.65828 117.84954 115.74009 114.88987 113.30353 114.92941 112.72695 118.54465 112.07918 112.66527 114.53846 110.28830 111.78524 117.45865 Ma 6.27622 8.78940 10.65293 8.03615 8.62709 8.18617 9.08680 7.96201 11.04769 12.53693 8.53173 8.02506 6.01403 12.40717 9.15555 10.58814 8.46286 8.10479 9.78527 10.50390 6.58304 9.93999 6.14851 8.43631 12.26666 8.42665 8.94614 8.89607 7.89037 11.64299 8.08192 6.87783 7.60446 Table 1 Zircon LA-ICP-MS U–Pb isotopic dating of diorite porphyrites, quartz monzodiorites and monzodiorites from the Bumo deposit 207Pb/235U 206Pb/238U 207Pb/235U 238U/232Th U Th Pb Spot 1σAge (Ma)1σRatio Ratio Ratio ppm ppm ppm 114.05513 115.60393 115.34212 114.17610 114.06324 107.47262 121.18131 114.73874 112.56602 115.27607 110.26127 109.71476 116.88395 113.28545 106.83103 112.96801 116.30325 111.59749 109.23719 111.79597 115.93375 112.91121 115.13488 114.32062 112.82980 114.30000 116.33663 112.81082 109.94377 111.95471 106.72402 113.99702 114.06810 0.00029 0.00037 0.00042 0.00043 0.00056 0.00039 0.00038 0.00039 0.00043 0.00035 0.00042 0.00039 0.00028 0.00040 0.00044 0.00049 0.00035 0.00052 0.00040 0.00043 0.00038 0.00051 0.00046 0.00042 0.00035 0.00033 0.00036 0.00041 0.00035 0.00039 0.00030 0.00036 0.00036 0.01741 0.01778 0.01782 0.01795 0.01840 0.01721 0.01833 0.01798 0.01785 0.01801 0.01753 0.01713 0.01817 0.01790 0.01828 0.01771 0.01797 0.01868 0.01792 0.01779 0.01845 0.01812 0.01798 0.01773 0.01799 0.01764 0.01856 0.01754 0.01763 0.01793 0.01726 0.01749 0.01839 0.00692 0.00970 0.01175 0.00886 0.00951 0.00896 0.01008 0.00878 0.01216 0.01383 0.00937 0.00880 0.00664 0.01366 0.01002 0.01165 0.00935 0.00891 0.01073 0.01155 0.00727 0.01094 0.00678 0.00930 0.01350 0.00929 0.00988 0.00979 0.00866 0.01280 0.00884 0.00758 0.00838 0.11888 0.12059 0.12030 0.11901 0.11889 0.11165 0.12676 0.11963 0.11724 0.12023 0.11471 0.11411 0.12200 0.11803 0.11095 0.11768 0.12136 0.11617 0.11358 0.11639 0.12095 0.11762 0.12007 0.11917 0.11753 0.11915 0.12140 0.11751 0.11436 0.11657 0.11083 0.11881 0.11889 0.79731 1.06066 1.06395 1.38102 1.16155 1.09980 1.35775 1.13825 1.09268 1.08344 1.19919 0.91342 1.55240 1.02242 1.00496 1.16265 1.04441 1.08226 1.15248 1.18395 1.10316 1.10000 1.30121 1.13359 1.02225 1.13666 1.06625 1.32065 1.17550 1.34992 1.15963 1.13364 1.40923 163.93001 89.93289 83.38907 84.71253 60.64198 99.52359 102.37605 101.09569 81.13592 80.97548 72.99753 128.05854 168.46504 88.93114 96.09292 98.91673 82.61509 128.02068 108.06369 81.15943 99.32665 81.00006 149.86011 98.82133 102.83299 78.72473 122.83132 96.69647 100.66134 107.98997 87.05431 98.83519 116.88682 213.42248 86.50740 79.79708 61.20432 52.27635 90.42526 74.47710 87.63412 72.12435 73.65989 61.38186 144.89870 106.50361 86.03708 94.21227 84.83801 82.46117 114.32894 92.69949 69.88077 92.02255 76.48682 119.70215 92.21605 103.93445 71.10856 114.78937 74.18858 87.10874 80.77020 79.45505 90.95151 85.84914 Quartz monzodiorite 5.26348 2.59773 2.49271 2.27837 1.61598 2.75039 2.78047 3.26201 2.23084 2.32075 1.93364 3.73438 4.55713 2.49056 2.89715 2.82094 2.31799 3.33945 2.82407 2.27058 2.63453 2.04633 3.84110 2.63519 2.88407 2.43108 3.24138 2.77378 2.67516 2.86330 2.35319 2.61696 2.85808 21BM04-1 21BM04-2 21BM04-3 21BM04-4 21BM04-5 21BM04-6 21BM04-7 21BM04-8 21BM04-9 21BM04-10 21BM04-11 21BM04-12 21BM04-13 21BM04-14 21BM04-15 21BM04-16 21BM04-17 21BM04-18 21BM04-19 21BM04-20 21BM04-21 21BM04-22 21BM04-23 21BM04-24 21BM04-25 21BM04-26 21BM04-27 21BM04-28 21BM04-29 21BM04-30 21BM04-31 21BM04-32 21BM04-33 Diorite porphyrites

Ma 1.38805 1.25943 1.28733 1.24665 1.45364 1.22128 1.18016 1.26896 1.33262 1.36481 1.47485 1.37705 1.37817 1.35592 1.38077 1.32422 1.06740 1.17904 1.05938 2.86538 1.90075 1.85330 2.40371 2.78369 2.24878 2.07414 2.27089 2.24093 2.42609 1.76400 1.90799 1.66756 2.52554 206Pb/238U Age (Ma)105.06305 102.30011 106.61447 101.60822 107.92319 103.19107 102.78040 105.68341 108.77913 104.05502 107.66572 105.24896 103.35950 104.20156 104.84581 103.58252 103.35374 104.67668 102.33796 118.02322 115.62138 116.90063 113.37064 118.98143 117.11768 118.06534 113.76656 116.49625 117.22902 113.23504 113.43628 115.49309 118.96060 Ma 2.07736 2.47828 2.47744 1.75866 2.30835 2.78527 2.47652 2.01017 2.80897 1.71476 1.91894 3.14583 3.44816 2.61178 2.38525 2.85724 1.92942 2.60120 2.02307 10.36283 4.42210 4.77548 7.21813 9.01121 9.41116 7.19153 6.34487 7.90451 8.39781 5.50739 5.81245 6.05061 7.82965 207Pb/235U 206Pb/238U 207Pb/235U 238U/232Th U Th 1σAge (Ma)1σRatio Ratio Ratio ppm ppm 107.20146 103.46887 108.32011 105.61966 106.67138 101.47158 105.79804 104.71440 110.57674 107.84472 108.87307 106.71026 109.35951 106.80597 113.45292 104.72536 105.14126 111.81498 103.69151 0.00022 0.00020 0.00020 0.00020 0.00023 0.00019 0.00019 0.00020 0.00021 0.00021 0.00023 0.00022 0.00022 0.00021 0.00022 0.00021 0.00017 0.00019 0.00017 0.01643 0.01600 0.01668 0.01589 0.01688 0.01614 0.01607 0.01653 0.01702 0.01627 0.01684 0.01646 0.01616 0.01630 0.01640 0.01620 0.01616 0.01637 0.01600 0.00227 0.00270 0.00271 0.00192 0.00252 0.00303 0.00271 0.00219 0.00308 0.00188 0.00210 0.00344 0.00378 0.00286 0.00263 0.00312 0.00211 0.00286 0.00221 0.11135 0.10727 0.11258 0.10962 0.11077 0.10510 0.10982 0.10863 0.11505 0.11206 0.11318 0.11081 0.11372 0.11092 0.11822 0.10865 0.10910 0.11641 0.10752 1.49646 2.20800 1.96551 4.24274 1.50092 2.98389 1.55082 1.42065 1.56666 5.17348 5.04067 1.04055 1.03856 1.43785 0.55021 1.59543 3.87978 2.05166 1.56280 1730.91222 1333.43877 1565.99239 8631.48710 2024.91049 1025.10699 1613.26000 2035.04741 1126.50290 10,064.20870 11,151.22280 1105.97101 811.12186 1565.99003 1584.46420 1029.57182 3305.24071 1194.00706 5389.07978 1203.27920 632.49670 827.32995 2322.36084 1459.49234 351.64103 1128.73146 1421.31637 791.31929 2007.97636 2049.24479 1011.12740 746.71341 1089.96749 4174.05850 1035.59591 892.24460 621.65412 3536.46422 122.20450 116.97226 115.41171 113.16379 109.05771 119.83718 116.80026 111.22683 117.77439 116.33616 114.80940 112.57929 113.19808 121.32920 0.00045 0.00030 0.00029 0.00038 0.00044 0.00036 0.00033 0.00036 0.00035 0.00038 0.00028 0.00030 0.00026 0.00040 0.01848 0.01810 0.01830 0.01774 0.01863 0.01833 0.01848 0.01780 0.01824 0.01835 0.01772 0.01775 0.01808 0.01863 0.01151 0.00489 0.00527 0.00795 0.00988 0.01043 0.00795 0.00697 0.00874 0.00927 0.00607 0.00640 0.00666 0.00869 0.12790 0.12210 0.12037 0.11790 0.11339 0.12527 0.12191 0.11577 0.12298 0.12140 0.11971 0.11725 0.11793 0.12692 0.92654 0.56805 0.55381 0.79679 0.50127 0.50987 0.52253 0.37794 0.68478 0.58927 0.83816 0.62303 0.43004 0.54816 89.66533 269.34719 265.60137 121.73060 171.37319 122.48857 225.05720 247.99694 148.77400 112.99378 212.98227 255.90906 255.14254 144.55707 102.13767 485.03591 483.25921 154.79808 339.70641 237.00672 423.69922 647.27393 212.61415 183.68877 249.43711 413.95542 584.19293 255.66584 Table 1 continued Pb Spot ppm 36.88608 23.64606 30.21292 133.04271 42.71071 16.80057 31.04700 42.71058 24.47247 149.08810 168.89480 24.68640 18.29201 31.21271 64.52063 23.79727 51.90392 21.86923 106.79843 2.69781 10.46982 10.65784 4.36376 7.66499 5.47882 9.20095 12.68796 5.43139 4.34511 6.72488 9.27250 11.01283 6.06989 21BM05-1 21BM05-2 21BM05-3 21BM05-4 21BM05-5 21BM05-6 21BM05-7 21BM05-8 21BM05-9 21BM05-10 21BM05-11 21BM05-12 21BM05-13 21BM05-14 21BM05-15 21BM05-16 21BM05-17 21BM05-18 21BM05-19 Monzodiorites 21BM06-1 21BM06-2 21BM06-3 21BM06-4 21BM06-5 21BM06-6 21BM06-7 21BM06-8 21BM06-9 21BM06-10 21BM06-11 21BM06-12 21BM06-13 21BM06-14

Ma 1.69060 1.87942 1.74292 1.54767 1.78213 2.24794 1.87044 2.60610 1.94138 1.86756 1.94980 1.55805 1.93762 2.31150 1.72789 1.59967 1.97959 1.89990 1.75758 1.78697 2.33731 206Pb/238U Age (Ma)114.47767 114.31278 115.01216 116.28442 116.41101 114.41720 113.44339 110.61389 112.92363 111.37249 112.52273 111.60930 111.52258 116.28609 118.08873 116.20301 116.25112 117.21679 114.31159 111.86526 114.88255 Ma 5.06602 6.20575 6.36094 6.75339 7.56197 8.40424 6.12349 5.60192 5.01727 4.83819 6.86885 5.35565 6.97220 6.52735 6.90319 5.19088 5.22159 8.77215 6.23995 5.46897 9.40335 207Pb/235U 206Pb/238U 207Pb/235U 238U/232Th U Th 1σAge (Ma)1σRatio Ratio Ratio ppm ppm 113.21610 115.88585 124.40864 116.93471 115.98964 114.24410 115.00904 115.78850 113.21086 107.90510 118.19781 111.86374 115.33287 114.84825 113.56561 111.72380 117.89616 117.25218 111.14971 104.91690 111.08995 0.00027 0.00030 0.00028 0.00024 0.00028 0.00035 0.00030 0.00041 0.00031 0.00029 0.00031 0.00025 0.00031 0.00036 0.00027 0.00025 0.00031 0.00030 0.00028 0.00028 0.00037 0.01792 0.01789 0.01800 0.01820 0.01822 0.01791 0.01775 0.01731 0.01767 0.01743 0.01761 0.01746 0.01745 0.01820 0.01849 0.01819 0.01820 0.01835 0.01789 0.01750 0.01798 0.00558 0.00685 0.00708 0.00746 0.00835 0.00926 0.00675 0.00618 0.00552 0.00530 0.00760 0.00589 0.00769 0.00720 0.00760 0.00571 0.00577 0.00970 0.00686 0.00597 0.01033 0.11795 0.12090 0.13035 0.12206 0.12101 0.11909 0.11993 0.12079 0.11795 0.11212 0.12345 0.11647 0.12029 0.11975 0.11834 0.11631 0.12312 0.12241 0.11568 0.10885 0.11562 0.56077 0.44181 0.57853 0.43742 0.57435 0.80772 0.53778 0.69695 0.59583 0.48378 0.61346 0.70365 0.86089 0.45385 0.53920 0.44727 0.55434 0.80349 0.69285 0.61826 0.65613 268.32648 225.84147 222.75343 190.96879 143.61451 155.06505 210.55331 180.57587 189.67932 194.95088 182.91691 240.31317 119.70010 203.02755 198.21982 262.65519 237.71100 116.74541 175.58578 260.40440 100.23378 479.31607 512.85493 393.37605 447.10015 261.50935 194.19308 407.12333 272.78025 342.02620 446.01454 317.27527 359.62904 144.36219 460.71512 380.26221 602.57581 437.22422 149.57063 263.02114 395.12655 156.29015 Table 1 continued Pb Spot ppm 9.88759 9.66824 8.51785 9.10372 6.01971 5.41205 9.04441 6.16534 8.24828 8.49174 7.18207 8.35502 3.45070 9.28378 7.73177 11.58611 8.98832 3.68078 5.87877 9.69338 3.50300 21BM06-15 21BM06-16 21BM06-17 21BM06-18 21BM06-19 21BM06-20 21BM06-21 21BM06-22 21BM06-23 21BM06-24 21BM06-25 21BM06-26 21BM06-27 21BM06-28 21BM06-29 21BM06-30 21BM06-31 21BM06-32 21BM06-33 21BM06-34 21BM06-35

All the results of zircon trace element compositions are listed in Supplementary Table 1.The contents of the rare earth element (REE) analysis in zircons of the intrusions were normalized to chondrite values of Sun and McDonough (1989).The REE patterns of zircons in magmatic rocks show similar characteristics.They all have low contents of light rare earth elements and high contents of heavy rare earth elements, with obvious large positive cerium and moderate negative europium anomalies(Fig.8).Eu anomalies(Eu/EuN*)range from 0.4 to 0.8 and Ce anomalies (Ce/Ce*) range from 10 to 4362, where(Dilles et al.2015) and Ce* is calculated by a logarithmic function (Zhong et al.2019).Quartz monzondiorites and monzodiorites have similar high field strength elements (HFSE), such as P content(233.0–488.0 and 195.0–585.0 ppm, respectively), Ta content (0.5–1.8 and 0.6–1.4 ppm, respectively), Nb content (1.3–5.8 and 1.1–5.0 ppm, respectively).For diorite porphyrites, its P content ranges from 102.0 to 440.0 ppm.Ta content ranges from 1.5 to 5.1 ppm and Nb content ranges from 4.1 to 20.6 ppm.

Fig.8 Chondrite-normalized REE patterns for zircons coming from the diorite porphyrites, quartz monzodiorites and monzodiorites.Chondrite values are from Sun and McDonough (1989)

Figure 9a shows the relationship between Th/U and Hf/Y ratios calculated for the study sites.Monzodiorites have relatively larger Th/U ratios (1.1–2.6) and relatively lower Hf/Y ratios (2–10) than quartz monzodiorites and diorite porphyrites.Quartz monzodiorites have moderate Hf/Y ratios (3–11) and narrow lower Th/U ratios (0.6–1.3).Diorite porphyrites are characterized by relatively high Hf/Y ratios (6–21) and low Th/U ratios (0.3–1.0).The zircon Eu/EuN* values of diorite porphyrites range from 0.35 to 0.65, whereas zircons from quartz monzodiorites and monzodiorites show highest Eu/EuN*ratios,with the range of 0.5–0.8 and 0.5–0.7, respectively.Moreover, on zircon Eu/EuN*versus Th/U ratios plot(Fig.9b),most zircon Eu/EuN* ratios are higher than 0.4, serval Eu/EuN* ratios located at 0.30.4.Figure 9c shows Eu/EuN* versus the Hf content (wt%).The diorite porphyrites have a higher Hf content of 1.0–1.4 wt%.The Hf content of quartz monzodiorites and monzodiorites are 0.8–0.9 wt% and 0.7–1.0 wt%, respectively.

On the zircon Eu/EuN*versus(Ce/Nd)/Y plot(Fig.9d),the(Ce/Nd)/Y ratios of diorite porphyrites range from 0.01 to 0.10, quartz monzodiorites and monzodiorites range from 0.01 to 0.05 and 0 to 0.07,respectively.On the zircon Eu/EuN* versus 10,000 × (Eu/Eu*)/Y plot (Fig.9e),10,000 × (Eu/Eu*)/Y ratios of diorite porphyrites range between 2.0 and 9.3, quartz monzodiorites and monzodiorites vary from 1.9 to 7.2 and 1.4 to 8.8, respectively;On the zircon Eu/EuN*versus Dy/Yb plot(Fig.9f),the Dy/Yb ratios of diorite porphyrites vary from 0.1 to 0.3, quartz monzodiorites and monzodiorites range from 0.1 to 0.2;only five Dy/Yb values of diorite porphyrites are larger than 0.3.

Fig.9 Zircon trace element ratios.a Zircon Th/U versus Hf/Y ratios plot,b zircon Eu/EuN*versus Th/U ratios plot,c Eu/EuN*versus Hf ratios plot,d Eu/EuN*versus(Ce/Nd)/Y ratios plot,e Eu/EuN*versus 10,000*(Eu/Eu*)/Y ratios plot,f Eu/EuN*versus Dy/Yb ratios plot.The dashed vertical line separates fertile from the barren suite in magmatic hydrothermal system.Fertile intrusion having high Eu/EuN*ratios(＞0.4),high Hf content (＞1.6 wt%), (Ce/Nd)/Y (＞0.01), 10,000*(Eu/Eu*)/Y (＞1) and Dy/Yb (＜0.3) (Ballard et al.2002; Dilles et al.2015; Lu et al.2016a, b; Loader et al.2017)

5.3 Sulfur isotopes and sulfide trace element compositions

The S isotope compositions of seven pyrite samples from intrusion-related gold mineralization,and the obtained δ34S ratios range from 0.2 ‰ to 4.4 ‰.In comparison, sulfur isotopes of orogenic gold mineralization in Gezhen shear zone are complied, and they have higher δ34S values(1.9–9.8 ‰) (Fig.11; Table 2).Gold concentrations of quartz monzodiorites and monzodiorites range from 0.1 to 0.2 ppm and 0.2 to 0.7 ppm, respectively, and gold contents of diorite porphyrites vary from 0.1 to 5.0 ppm(Table 3).For more detail see Supplementary Table 2.

Table 2 Pyrite δ34S‰ ratios in the gold deposits of the Gezhen shear zone, Hainan Island

Table 3 Summarizing of trace element compositions of pyrite

6 Discussion

6.1 Timing of gold mineralization

Tectonic movements resulted in magmatism and hydrothermal mineralization.By using whole rock and quartz vein of Rb–Sr and sericite of40Ar/39Ar dating(Fig.10) on Bumo, Erjia, Tuwaishan, Hongfumenling deposits, previous research suggests that gold mineralization in the Gezhen shear zone took place at ca.219–378 Ma (Hercynian–Indosinian) (Ye and Zhu 1990;Chen 1996; Liu et al.2020).These mineralization-deformation ages coincide with Permian–Triassic igneous magmatism at post-collisional extensional setting during the closure of Palaeo–Tethys ocean beneath SE China and subsequent slab rollback (Yan et al.2017).In this study,the crystallizing ages of the ore-associated magmatic intrusions in the Bumo deposit date at 114–104 Ma(Yanshanian).Gold mineralization caused by this magmatic stage is coeval with tectonic regime from NS trending Tethys to westward subducting Palaeo–Pacific beneath the Eurasia plate at Jurassic to Cretaceous (Yan et al.2017; He et al.2020).Compared with those in orogenic gold deposits(1.9–9.8 ‰),the δ34S ratios of sulfides in magmatic intrusions are lower (0.22–4.39 ‰; Fig.11),indicative of magmatic sources.Consequently, the goldbearing magmatic rocks present in this paper represent an independent intrusion-related gold mineralization event in the Gezhen shear zone.

Fig.10 Geochronological data of gold deposits and associated host rocks and granitoids in the Gezhen shear zone, Hainan Island

Fig.11 Sulfur isotopes data (δ34S) of pyrites from the intrusions of the Bumo deposit

6.2 Implications for gold mineralization

Gold-bearing magmatic rocks are commonly characterized by high water contents(≥4 wt%)and oxidation state,and ore metals would be transferred from melt to hydrothermal system by crystal fractionation (Richards 2011; Loucks 2014).Zircon trace elements are used to track the water content and oxidation state of magmas(Ballard et al.2002;Dilles et al.2015;Fu et al.2016;Zhong et al.2018).Trace element compositions of zircon have been used as an effective tool for distinguishing mineralized intrusions from barren intrusions(Belousova et al.2002;Ballard et al.2002;Dilles et al.2015;Lu et al.2016a,b).Trace element contents like Hf,Y,U,Th,and REE in zircon can represent magmatic evolution, oxidation state, and water content in the melt (Gagnevin et al.2010; Lee et al.2017; Li et al.2017).Europium anomalies of zircon have been proposed to establish the magmatic oxidation state of melt (Ballard et al.2002; Dilles et al.2015),

The characteristic trace element fingerprints were formed in residual magma by the gradual crystallization of accessory minerals, especially Th/U and Hf/Y ratios are commonly used to track the degree of magma differentiation (Gagnevin et al.2010; Buret et al.2016).For the zircons of monzodiorites and quartz monzodiorites, Hf/Y ratios increase as Th/U ratios decrease(Fig.9a),and the Eu anomalies are higher than 4 (Fig.9b).These trends can be explained by changes in melt composition due to amphibole plus titanite and apatite fractional crystallization.In addition, these two sets of rocks, outcropping in the same place with close zircon206Pb/208U age, share similar characteristics in zircon REE and high field strength elements(HFSE),as well as CL images.Consequently,we concluded that zircons in monzodiorites and quartz monzodiorites were grown in liquids of similar compositions and high-water content.

Previous research suggests that compared with barren magmatic rocks, zircon in fertile rocks are generally characterized by high Eu/EuN* ratios (＞0.4), high Hf content (＞0.8 wt%), (Ce/Nd)/Y (＞0.01), 10,000*(Eu/Eu*)/Y (＞1) and Dy/Yb (＜0.3) (Ballard et al.2002;Dilles et al.2015; Lu et al.2016a, b; Zhong et al.2017).Among them, elevated Eu/EuN* ratios are mainly influenced by co-crystallization of amphibole, apatite, titanite,garnet and plagioclase (Buret et al.2016; Loader et al.2017; Lu et al.2019; Zhong et al.2021).Plagioclase crystallization is inhibited at an early stage with high-water contents but it would crystallize, when magma cools close to solidus (Lu et al.2019; Pizarro et al.2020), so the Eu anomalies in the intrusions of the Bumo deposit are not significantly influenced by plagioclase.As for the diorite porphyrites,monzodiorites and quartz monzodiorites in the Bumo deposit, Eu/EuN* value range from 0.35 to 0.77,with most ratios higher than 0.4 (Fig.9c), suggesting that all intrusions have a high oxidized state.Other zircon geochemistry fingerprint of three intrusions displays high Hf content (0.7–1.4 wt%; Fig.9c), high (Ce/Nd)/Y ratios(＞0.01; Fig.9d), high 10,000*(Eu/Eu*)/Y ratios (＞1;Fig.9e) and low Dy/Yb values (＜0.3; Fig.9f).All these geochemical signatures suggest that the magmatic rocks are fertile.Consequently, the newly found Yanshanian intrusions characterized by abundant sulfide have a great potential for mineralization.

Based on the zircon U–Pb age and trace element compositions mentioned above,all these Yanshanian intrusions in Bumo are auriferous and characterized by high water content and oxidation conditions.Therefore, there are two stages of gold mineralization (orogenic-related and intrusion-related) in the Bumo deposit at difference periods.Consequently,the mineralization model for the Bumo gold deposit (Fig.12) is outlined as: (1) Orogenic gold systems were formed under the subduction of NS-trending Tethys Ocean and slab rollback,and gold is probably sourced from the metamorphic rocks of the Baoban Group (Ding et al.,2020); (2) The second stage of intrusion-related gold mineralization occurred under the westward subduction and subsequent roll-back of Paleo–Pacific plate.More importantly, the newly discovered Yanshanian intrusionrelated gold mineralization reported in this paper could be a new prospecting target in the Gezhen shear zone in the future.

Fig.12 Genetic model depicting the tectonic evolution in the Bumo gold deposit (modified after Yan et al.2017)

7 Conclusions

1.Magmatic intrusions with extensive mineralization in the Bumo ore deposit were formed at Yanshanian(114–104 Ma), representing independent intrusion-related gold mineralization in the Gezhen shear zone.

2.Compared with orogenic gold deposits in the Gezhen shear zone with higher δ34S values, sulfides in Yanshanian intrusion-related gold mineralization have lower δ34S ratios (0.2–4.4 ‰), indicating different metal sources.

3.Petrographic and geochemical research shows that the magmatic rocks are fertile with high water content and oxidation condition, indicating great mineralization potential.

4.The Bumo deposit was formed by two-stage gold mineralization, including Indosinian orogenic and Yanshanian intrusion-related systems, and the Yanshanian intrusions can be newly-discovered prospecting target in the Gezhen shear zone.

Supplementary InformationThe online version contains supplementary material available at https://doi.org/10.1007/s11631-021-00507-w.

AcknowledgementsThis work was co-founded by the National Natural Science Foundation of China (42002090, 41930428), Jiangxi Double Thousand Plan (No.SQJH2019XDR), Project of China Geological Survey (No.DD20190119), National Key Research and Development Program of China (No.2018YFC0604200), Open Research Fund Program of State Key Laboratory of Nuclear Resources and Environment, the East China University of Technology (No.NRE1915), Open Research Fund Program of Jiangxi Engineering Laboratory on Radioactive Geoscience and Big Data Technology (No.JELRGBDT202006), International Geoscience Programme (No.IGCP-675).

FundingNational Natural Science Foundation of China (Grant Nos.42002090, 41930428).

Declarations

Conflict of interestThe author declare that they have no conflict of interest.