Early Silurian Wuchuan-Sihui-Shaoguan exhalative sedimentary pyrite belt,South China:constraints from zircon dating for K-bentonite of the giant Dajiangping deposit

Yingying Zhang•Taiyi Luo •Tian Gan•Mingzhong Zhou•Xinqiao Han

Abstract The Wuchuan–Sihui–Shaoguan(WSS)exhalative sedimentary pyrite belt in the southwestern part of the Qinzhou–Hangzhou(Qin–Hang)belt is the most important sulfur industry base in China.However,a wide range of metallogenetic ages spanning from Ediacaran to Devonian has been reported in the literature.This age range does not support the idea that the typical character of‘‘coeval mineralization’’in an exhalative sedimentary mineralization belt in China and worldwide.Therefore,the precise determination of mineralization ages of representative deposits is necessary to provide guides for exploration and metallogenetic models.The Dajiangping pyrite deposit is a typical example of this kind of deposits and is also the largest deposit with a proven reserve of 210 Mt.This deposit was thought to have formed in Ediacaran or Devonian.In this study,2–3 layers of 10–25 cm thick 2M1-type microcrystalline muscovite slate abruptly embedded in the No.IV massive orebody of the deposit has been identified to be low-grade metamorphic K-bentonite.A Concordia zircon LA–ICP–MS U–Pb age of 432.5±1.3 Ma(mean standard weighted deviation of concordance and equivalence=1.2;N=11)has been yielded for the low-grade metamorphic K-bentonite.This age is distinctly different from the Rb–Sr isochron age of 630.1±7.3 Ma for siliceous rock at the top of the No.III banded orebody and the Re–Os isochron age of 389±62 Ma for pyrites from a laminated orebody.Instead,it is close to the intercept age(429 Ma)of the youngest detrital zircons from sandstone interlayers of the No.III banded orebody.The Concordia age is also coincident with those of the Late Caledonian(400–460 Ma)magmatism-metamorphism events which are widely distributed in Cathaysia Block.Particularly,it agrees well with that of the Early Silurian extensional volcanism(434–444 Ma)which have been revealed in the Dabaoshan,Siqian–Hekou,and Nanjing volcanic basins in northern Guangdong Province and southern Jiangxi Province.Hence,the dating result in this study confirms that the sedimentary time of the ore-host Daganshan Formation is Early Silurian,and implies that the mineralization age of the Dajiangping pyrite deposit should also be Early Silurian.In combination with the Early Silurian age of Shezui pyrite deposit and the Dabaoshan volcanic basin along the WSS pyrite belt,it could be inferred that the WSS pyrite belt provides a record of the northern expanding of Qinzhou–Fangcheng trough in Early Silurian and that the exhalative pyrite mineralization was triggered by the postcollisional extension of the margin of Cathaysia Block after the intracontinental collision between Cathaysia Block and Yangtze Block during Late Caledonian stage.

Keywords Wuchuan–Sihui–Shaoguan exhalative sedimentary pyrite belt·Dajiangping pyrite deposit·Lowgrade metamorphic K-bentonite·Zircon U–Pb dating

1 Introduction

Hydrothermal exhalative sedimentary deposits,including sedimentary rock-hosted exhalative deposits(SEDEX)and volcanic rock-hosted massive sulfide(VMS)deposits,are characterized by distinct spatial and temporal distribution(Haitian 1992).As for the mineralization ages and geological settings,they are mostly concentrated in Paleo-Mesoproterozoic(1.9–1.4 Ga)and Early-Middle Palaeozoic(530–300 Ma)eras(Dergachev and Eremin 2008;Zhai et al.2017)in age,and are known to be accumulated in rift and graben basin settings(Hu et al.1994;Zaw et al.2007;Wang et al.2014;Mortensen et al.2015).Hydrothermal exhalative sedimentary deposits distribute widely in South China,such as the Middle-Late Proterozoic copper belt(Xiqiu,Luocheng,and Tieshajie),the Late Proterozoic Xinyu-type iron-belt(Yingyangguan,Qidong,and Liangshan),and the Carboniferous pyrite belt(Xinqiao,Shuizhuling,Dongguashan,and Tongguanshan)in the Tongling area of the middle and lower reaches of the Yangtze River,and the Wuchuan–Sihui–Shaoguan(WSS)pyrite belt in Guangdong Province(Hu et al.1994).The temporal and spatial distribution characteristics of them have been outlined as‘‘coeval mineralization accumulated as a cluster in tectonic junction,deposits clusters grouped as mineralization belt along tectonic boundary’’(Hu et al.1994;Xu et al.1996).The WSS pyrite belt is situated in the south-western part of the Qinzhou–Hangzhou(Qin–Hang)suture zone.It is one of the most important source bases for the sulphur industry(with a total proven pyrite reserves of ca.300 Mt)in China(Editorial Board of Discovery History of China Mineral Deposit·Guangdong Volume 1996).A wide metallogenetic age range(Ediacaran–Devonian)for the WSS pyrite belt,including the Dajiangping,Xiniu,and Dabaoshan(only the strati-form pyrite orebody)deposits,has been proposed by previous works,which is inconsistent with the coeval mineralization temporal distribution hypothesis mentioned above.This is an enigma to geologists.Thus,reliable mineralization age for individually representative pyrite deposit from the belt is urgently needed to clarify the temporal issue mentioned above.

The Dajiangping pyrite deposit(with proven reserves of 210 Mt)in Yunfu City in western Guangdong Province,is the largest in the WSS pyrite belt and also in China(Yang et al.1997).Its mineralization age is still unclear due to a lack of fossils and suitable dating materials in the lowgrade metamorphic ore-host strata(the Daganshan Formation).Consequently,various mineralization ages including Early Ediacaran,Middle Devonian,and Lower Devonian have been proposed during exploration in the 1960s(Editorial Board of Discovery History of China Mineral Deposit·Guangdong Volume 1996).Since the 1990s,the Daganshan Formation has been considered to be Ediacaran sequences based on a Rb–Sr isochron age of 630.1±7.3 Ma for the siliceous rocks at the top of the No.III banded orebody(Wang et al.1996).Recently,some workers considered it to be Middle-Late Devonian in age based on an imprecise pyrite Re–Os isochron age of 389±62 Ma for the laminated ores(Qiu et al.2018a).

Volcanic zircons preserved in K-bentonites provided most of U–Pb ages in theInternational Chronostratigraphic Chart(Cohen et al.2013).Given the higher closure temperature(＞900 °C)(Lee et al.1997)of the zircon U–Pb isotopic system,metamorphic K-bentonites within metamorphic sedimentary strata would be the most valuable candidates for obtaining reliable U–Pb ages for their host sequences.In this study,according to the geological criteria updated by us,we regarded the 10–25 cm thick pyritized silicified slate in the No.IV massive orebody of Dajiangping deposit as a low-grade metamorphic K-bentonite.Moreover,a reasonable zircon U–Pb Concordia age has been yielded to constrain the mineralization age of the Dajiangping deposit and to develop its significances on the related geological events recorded in the WSS pyrite belt.

2 Geological setting

The research area is located in the Yunkai terrane which is controlled by the Wuchuan–Sihui deep fault and the Bobai–Wuzhou fault(the southern prolongation of the Chenzhou–Linwu fault)(Guo et al.2017;Gan et al.2018)(Fig.1a).The basement is composed of widely exposed Precambrian high-grade metamorphic rocks and some Early Palaeozoic low-grade metamorphic rock series(the Yunkai Group),and is unconformably overlain by nonmetamorphic Devonian strata(Wang et al.2013;Zhou et al.2015).Large-scale Early Palaeozoic igneous rocks are intensively developed in the Cathaysia Block,such as extensive granites in Wuyi,Nanling,and Yunkai areas,and metamorphic intermediate-basic volcanic rocks in Cengxi,Luchuan,and other areas(Wang et al.2013;Qin et al.2017).

The Dajiangping pyrite deposit is located 4.5 km northwestern of Yunfu City in Guangdong Province and is close to the eastern margin of the Daganshan dome in the middle-northern part of the Yunkai uplift terrane(Fig.1b).The mainly ore-controlling structures include the Jianshan inverted anticline,723 inverted anticline,and the 20–50 m thick in each wing)and high grade of average sulfur content of 37.3 wt%(Editorial Board of Discovery History of China Mineral Deposit·Guangdong Volume 1996).In a residual section of the open pit,the bottom of No.IV orebody is composed of pyritised siltstone and slate,the middle is dominated by massive pyrite(12 m)with some thin layers(10–25 cm)of low-grade metamorphic silicified slate,and the upper part is mainly composed of carbonaceous slate(Fig.2).Sample 17DJP-20 with weight ca.25 kg was collected from the middle part of No.IV massive orebody(Fig.2)(22°58′7′′N,112°0′48′′E)(Fig.3a,b).The sample contains quartz(50–60% in volume,the same as below),sericite(30–40%),pyrite(5%),carbonaceous(5%),and occasionally coarse yellow zircons(Fig.3c).

The separation of zircons,target preparation,and photography of crystal morphology and internal textures(transmittance light,reflected light,and cathodoluminescence)were performed by the Langfang Yuneng Mine and Rock Separation Technical Services Co.,Ltd.X-ray diffraction(XRD),bulk rock major and trace element analyses,and laser ablation–inductively coupled plasma mass spectrometry(LA–ICP–MS)U–Pb dating were carried out at the State Key Laboratory of Ore Deposit Geochemistry,Institute of Geochemistry,Chinese Academy of Sciences.The XRD analysis was performed on an Empyrean instrument(PANalytical B.V.,Netherlands)and processed with MDI jade 6.0 software.The bulk rock major elements were determined by the melting flaking method on an Axios(PW4400)X-ray fluorescence(XRF)spectrometer(PANalytical B.V.,Netherlands)with an accuracy of＞5%.Trace elements were analyzed according to the procedure developed by Qi et al.(2000)on a Plasma Quant-MS Elite(Jena Analytical Instruments Co.,Ltd.,Germany)with an accuracy of＞10%.The LA–ICP–MS U–Pb analysis was performed using an Agilent 7700x ICP–MS(Agilent Technologies Co.,Ltd.,USA)with a laser beam spot diameter of 32μm(Xia et al.2004).Zircon standard 91500 was used as an external standard for isotope fractionation(Liu et al.2010),the synthetic silicate glass NIST SRM612 was used as an external standard for elements analyses,and91Zr was used as the normalization to correct element content(Pearce et al.1997).The analytical data were processed offline using ICPMS Data Cal(Liu et al.2008,2010),and the U–Pb Concordia age and weighted mean ages were calculated by Isoplot 4.15(Ludwig 2012).

4 Results

Forty zircon grains with clear growth oscillatory zonings in CL images were selected for LA–ICP–MS U–Pb dating(Table 1,Fig.3e).The selected zircons were subhedral–euhedral,transparent(colorless or yellow),a short prismatic shape,and mostly between 50 and 100μm in size with an elongation(length-to-width)ratio of between 1 and 3.

Fig.2 Schematic stratigraphic column of the Dajiangping pyrite deposit ore-hosting strata(No.IV).The sampling location of for the low-grade metamorphic K-bentonite is also shown(modified from Gan 2017)

Fig.3 a Field photograph of a distant view of the residual section of the No.IV massive orebody in Dajiangping pyrite deposit.b Field photograph of the low-grade metamorphic K-bentonite beds(10–20 cm thick)with late fault.c Photomicrographs of the low-grade metamorphic K-bentonite showing parallel foliation of yellow-white microcrystalline muscovite and occasionally coarse yellow zircons.d XRD patterns of the low-grade metamorphic K-bentonite(the peaks are the eigenvalues and the corresponding minerals).e Cathodoluminescence(CL)images of representative magmatic zircons from the low-grade metamorphic K-bentonite were used for calculating the Concordia age.The circles on the zircons represent the spots for the LA–ICP–MS U–Pb isotope analyses,which are 32μm in diameter

For Phanerozoic tuff,zircon ages are often vulnerably disturbed by both positive(from xenocrysts)and negative(from Pb loss)age biases.Ludwig(2012)has suggested a detailed process to deal with such biases.The youngest population of 400–500 Ma of 18 zircons is the main population which is sharp in the plot of the probability density spectrum(Fig.4b).Getting rid of 5 zircons for heritage or Pb loss biases,a preliminary reliable age of 435.06+2.71-8.35 Ma could be extracted from the youngest population(45% of the total zircons)(Fig.4c).Moreover,after rejecting 2 zircons for a little lower concordance(94%),the remaining 11 zircons yield a Concordia age of 432.5±1.3 Ma(Mean Standard Weighted Deviation of concordance and equivalence(MSWDCE)=1.2)(Fig.4d).Weighted mean of206Pb/238U ages(ranging from 424 to 441 Ma)and207Pb/235U ages(ranging from 424 to 448 Ma)of these 11 zircons is 432.4±3.7 Ma(MSWD=1.6)and 435.0±4.9 Ma(MSWD=0.99),respectively.These three age groups are highly consistent with certain errors.However,the Concordia age is slightly or significantly more precious than any single U–Pb or Pb–Pb ages and therefore represents the most reliable age for zircon crystallization according to Ludwig(2012).

1σCon%206Pb*/238U(Ma)1σ 10.1 99 8.8 99 5.2 98 4.0 99 7.7 99 6.9 98 7.1 97 4.7 98 3.6 82 11.2 99 5.5 97 7.7 98 3.4 94 5.2 99 4.7 98 3.1 94 6.6 99 20.9 99 11.1 99 9.7 99 4.2 97 22.7 98 3.2 99 5.2 99 16.1 99 9.1 99 10.7 97 16.0 98 7.7 98 8.7 99 5.9 93 10.0 99 5.7 99 3.7 94 5.3 97 13.2 1139.2 822.0 432.6 424.0 801.5 743.2 945.4 441.3 414.1 457.6 821.2 438.4 435.1 424.6 426.7 753.7 857.2 420.3 427.8 560.2 962.2 756.4 890.0 457.4 745.8 445.6 536.7 11.9 7.0 8.2 10.7 13.2 9.7 8.0 8.8 12.5 1279.3 8.7 11.7 6.3 8.1 8.0 7.4 11.4 16.6 2444.3 13.4 1051.7 17.2 9.5 17.7 1888.3 8.9 9.4 13.7 1900.2 11.7 12.9 1403.8 15.6 1830.6 11.6 12.3 14.8 11.8 1040.4 10.4 7.8 10.6 Table 1 LA–ICP–MS zircon U–Pb isotope analyses of the low-grade metamorphic K-bentonite from the Dajiangping pyrite deposit orebody IV 207Pb*/235U(Ma)1σ 207Pb*/206Pb(Ma)1σ 206Pb*/238U 1σ 207Pb*/235U 1σ 207Pb*/206Pb*Th/U U(ppm)Th(ppm)Zircon spot 37.0 1140.9 825.1 424.3 425.4 799.8 751.3 966.8 448.8 493.0 468.1 809.8 462.9 437.4 432.5 453.0 760.2 864.8 432.2 425.6 565.1 971.7 767.4 893.6 489.3 750.4 473.1 552.7 45.4 40.7 53.7 41.7 50.0 31.5 51.8 43.5 30.7 1288.4 52.8 44.4 38.9 46.3 52.8 16.7 44.4 28.1 2464.0 39.7 1061.1 59.3 57.4 32.6 1851.3 57.4 45.2 27.2 1919.0 35.2 30.1 1445.5 31.5 1856.8 41.5 41.2 85.2 32.3 1040.0 238.9 40.7 48.1 0.19329 0.00187 1138.9 827.8 372.3 431.5 787.0 764.8 487.1 872.2 522.3 772.2 576.0 438.9 464.9 588.9 772.2 879.6 487.1 466.7 576.0 983.3 783.3 900.0 650.0 766.7 594.5 611.1 0.13599 0.00155 0.06941 0.00086 0.06799 0.00067 0.13239 0.00136 0.12220 0.00119 0.15796 0.00128 1007.1 0.07085 0.00077 0.06634 0.00060 0.21952 0.00211 1300.0 0.07357 0.00091 0.13585 0.00135 0.06982 0.00086 0.06808 0.00078 0.06843 0.00052 0.12403 0.00115 0.46108 0.00473 2472.2 0.17722 0.00203 1075.9 0.14223 0.00172 0.06738 0.00070 0.34035 0.00472 1802.2 0.06861 0.00053 0.09079 0.00088 0.34281 0.00335 1931.5 0.16098 0.00164 0.24329 0.00206 1494.8 0.32839 0.00329 1875.6 0.12450 0.00135 0.14805 0.00154 0.07353 0.00098 0.17515 0.00183 1027.8 0.12266 0.00099 0.07157 0.00062 0.08683 0.00089 0.039810 0.026354 0.023080 0.027308 0.024603 0.043853 0.025501 0.023798 0.18428 0.037380 0.039580 0.107443 0.088888 0.029831 0.052614 0.095586 0.024294 0.029105 0.032294 0.021341 2.0761 1.2537 0.51877 0.010510 0.52030 0.012307 1.1983 1.0958 1.5914 0.55586 0.012215 0.62496 0.014038 2.5570 0.58574 0.013551 1.2201 0.57756 0.0097651 0.07038 0.00057 0.53842 0.012277 0.53109 0.012089 0.56227 0.011306 1.1141 10.321 1.8435 1.3437 0.53065 0.014338 5.1917 0.52066 0.013279 0.74455 0.016077 5.6186 1.6038 3.1521 5.2258 1.1292 1.4111 0.61911 0.023517 1.7850 1.0938 0.59353 0.012252 0.72344 0.018003 0.077699 0.0014325 0.066712 0.0013131 0.054035 0.00097014 0.055488 0.0013481 0.065381 0.0011732 0.064750 0.0015513 0.072750 0.0011214 0.056899 0.0012996 0.068151 0.0016119 0.083902 0.0013269 0.057520 0.0012637 0.064904 0.0013711 0.059273 0.0010458 0.055664 0.0011427 0.056311 0.0012262 0.059320 0.0012147 0.064956 0.0014078 0.0026847 0.0019320 0.0017960 0.0020000 0.16148 0.075245 0.0015045 0.068384 0.0019618 0.056867 0.0014905 0.11017 0.054736 0.0013984 0.059261 0.0012600 0.11831 0.071856 0.0012592 0.093342 0.0014926 0.11475 0.065314 0.0012997 0.068765 0.0013791 0.061263 0.0024603 0.073459 0.0011914 0.064496 0.0013308 0.059750 0.0011539 0.059946 0.0013461 0.88 0.64 0.53 0.35 1.2 0.74 0.15 0.32 0.93 0.57 0.46 0.76 0.61 0.13 0.79 0.56 0.72 0.29 0.29 1.2 0.99 0.62 0.72 1.4 0.61 0.47 0.73 0.46 0.74 0.30 0.68 0.16 0.54 0.89 1.5 339 277 919 463 387 219 629 566 787 770 592 332 1161 661 449 748 367 616 530 155 307 218 421 434 437 547 483 262 588 377 135 777 461 812 457 17DJP-20-01 299 17DJP-20-02 176 17DJP-20-03 485 17DJP-20-04 163 17DJP-20-05 457 17DJP-20-06 16297.1 86.5 91.9 17DJP-20-07 17DJP-20-08 182 17DJP-20-09 734 17DJP-20-10 443 17DJP-20-11 275 17DJP-20-12 254 17DJP-20-13 711 17DJP-20-14 17DJP-20-15 356 17DJP-20-16 416 17DJP-20-17 266 17DJP-20-18 181 17DJP-20-19 155 17DJP-20-21 191 17DJP-20-22 303 17DJP-20-23 135 17DJP-20-24 305 17DJP-20-25 589 17DJP-20-26 265 17DJP-20-27 256 17DJP-20-28 351 17DJP-20-29 120 17DJP-20-30 436 17DJP-20-31 113 17DJP-20-32 17DJP-20-33 121 17DJP-20-34 251 17DJP-20-35 724 17DJP-20-36 696

Table 1 continued 1σCon%206Pb*/238U(Ma)1σ 207Pb*/235U(Ma)1σ 207Pb*/206Pb(Ma)1σ 206Pb*/238U 1σ 207Pb*/235U 1σ 207Pb*/206Pb*Th/U U(ppm)Th(ppm)Zircon spot 4.3 99 4.9 99 3.6 99 4.8 96 4.9 99 435.8 437.8 435.6 431.4 436.3 12.4 10.8 8.2 7.4 8.1 434.0 436.2 436.5 444.6 438.5 79.6 64.8 45.4 44.4 51.8 433.4 420.4 431.5 509.3 455.6 0.06994 0.00071 0.07027 0.00082 0.06991 0.00059 0.06921 0.00079 0.07002 0.00081 0.53324 0.018659 0.53660 0.016302 0.53714 0.012429 0.54943 0.011275 0.54016 0.012305 0.055299 0.0019730 0.055201 0.0016179 0.055440 0.0012536 0.057467 0.0011538 0.055866 0.0012748 0.73 0.79 0.79 0.42 0.39 200 284 694 1011 942 146 224 549 427 364 17DJP-20-37 17DJP-20-38 17DJP-20-39 17DJP-20-40 17DJP-20-41 Pb*indicates the radiogenic portions.Italic shading indicates data rejected from the Concordia age calculations.Spot 17DJP-20-20 spot was not detected due to instrument malfunction

5 Discussion

5.1 From fallout volcanic ash to K-bentonite

Extensively dispersed fallout volcanic ash,long-distance transported as volcanic ash cloud in the stratosphere,could be a stratigraphic marker in various sedimentary environments(Ninkovich et al.1978).Take the largest volcanism in Quaternary—the ca.75 Ka Youngest Toba Tuff(YTT)in Sumatra(Mark et al.2014)as an example,the relative volcanic ash has been discovered in marine sediments from the Indian Ocean(up to 1.5 m thick)(Pattan et al.2010),the Arabian Sea(10–38 cm)(Nambiar and Sukumaran 2002)and the South China Sea(2 cm)(Lee et al.2004),in lacustrine sediments from Malawi Lake in Africa(≤1 cm,7000 km away from Toba caldera)(Lane et al.2013),in terrestrial deposits from India continental(10 cm)(Blinkhorn et al.2014)and loess from China(Rao et al.2007).Based on observations as above,the simulation reveals that ca.8600 km3of magma(ca.2800 km3dense rock equivalents,DRE)had been erupted into the stratosphere within 15 h by a 50–80 km high volcanic ash column,and the volcanic ash clouds gradually deposited in 9–14 days to form a volcanism ash bed with more than 5 mm thick covering ca.40 million km2on the Earth(Lee et al.2004).

Volcanic ash bed is mainly composed of low-density(1.1 g/cm3)volcanic glass,which would be easily altered into clay minerals during the hydrolysis and diagenesis process(Christidis and Huff 2009).Altered volcanic ash bed could be further divided into three types:(1)tonstein mainly composed of kaolinite and hosted in coal measures(Dai et al.2017),(2)bentonite mainly composed of smectite and mostly occurred in the marine strata after the Cretaceous(Christidis and Huff 2009),and(3)K-bentonite mainly composed of illite and illite–smectite(I–S)and mostly occurred in the marine strata before Cretaceous(Huff 2016).

Fig.4 a CI chondrites normalized REE patterns of zircons from the low-grade metamorphic K-bentonite.b The age probability density spectrum of the low-grade metamorphic K-bentonite shows that the 400–500 Ma is the main population.c Zircon age extractor diagram gives a preliminary reliable age of 435.06+2.71-8.35 Ma which could be extracted from the youngest population.d U–Pb Concordia diagram of magmatic zircons from the low-grade metamorphic K-bentonite

K-bentonite is the most investigated object for stratigraphic chronology.The criteria to distinguish K-bentonite from normal claystone has been summarized by Huff(2008)and emphasized again by Huff(2016).In field investigation,K-bentonite is a fine-grained clay-rich band(1–200 mm)with a slippery and waxy feel when wet.In the laboratory,the main minerals of K-bentonite are smectite and I-S determined by XRD.The heavy minerals separated by wet sieving are typical with volcanogenic minerals(quartz,sanidine,biotite,etc.).However,it should be noted that these heavy minerals may both have volcanic and clastic origin due to the complexity of volcanic magmatic evolution or bioturbation of volcanic ash layers after emplacement.

The criteria established by Huff(2008,2016)might need to be rectified when K-bentonite had been transformed to silicified claystone,slate or schist caused by later modification(e.g.hydrothermal alteration and regional metamorphism).According to the simulation of the YTT(Lee et al.2004),the fallout of the volcanic ash cloud formed by giant volcanic eruption may last for several weeks or months.Given the geological time scale,the fallout itself could be depicted as an‘‘instantaneous’’deposition of high flux volcanic ash in an extensive area.Therefore,the most essential geological features of a volcanic ash bed(including K-bentonite)should include two aspects.First of all,the volcanic ash bed occurs as a‘sandwich’-like unconformity in a successive sequence,that is,the ash bed is very abruptly embedded into the section,and the lithology and sedimentary facies of the roof and floor rocks are unchanged.The‘sandwich’-like unconformities of K-bentonite have been observed in many Palaeozoic sections,e.g.‘‘white muddy bed’’in littoral clastic phosphorite of the Cambrian Meishucun section in China(Zhang et al.2004);Osmundsberg,Deicke and Millbrig K-bentonites in platform carbonate of Ordovician–Silurian sections in Europe and America(Huff 2016);light-color K-bentonites in deep basin black shale of the Silurian sequence in China(Hu et al.2009).Besides,the volcanic ash bed dispersed in a broad area could be a synchronous horizon among different sedimentary environments.For example,the YTT could be a stratigraphic marker in sediments from ocean to continental loess in China and even freshwater lake 7000 km away;and Osmundsberg K-bentonite,one of the most widespread ash beds,could be traced from Sweden across Estonia to the British Isles.

5.2 Discriminating of sample 17DJP-20

The massive pyrite orebody in Dajiangping represents a stable and durative hydrothermal sedimentation environment.However,the abrupt occurrence of the clay beds(low-grade metamorphic slate)exhibits‘sandwich’-like unconformity which seems to be an‘‘instantaneous fallout’’of volcanic ash.Laterally,a similar slate with a zircon Concordia age of 433.5±0.64 Ma is also found in the massive pyrite orebody of the Shezui pyrite deposit in Yingde city.Also,a Silurian volcanic ash bed(430.7±4.2 Ma,weighted mean206Pb/238U age,the same below except for the label)has been disclosed in the Lunshan section in Nanjing,Jiangsu Province(Yang et al.2019).Regionally,large-scale volcanism in Early Silurian has been revealed in the Nanjing(442.1±3.9 and 439.9±3.7 Ma,Liu et al.2018),Siqian-Hekou(443.6±5.4 Ma,Wu et al.2012)and Dabaoshan(436.4±4.1,434.1±4.4 Ma,Wu et al.2014;439.1±3.6,437.1±2.3 and 437.3±2.1 Ma,Wang,et al.2019)volcanic basins in southern Jiangxi Province and northern Guangdong Province.These evidences clearly show that the intensive Early Silurian volcanism had erupted along the Wuchuan-Sihui deep fault in southwestern periphery of Cathaysia Block.Therefore,it is reasonable that the instantaneous fallout of relative volcanic ash could be recorded in the narrow and long Qin–Fang trough(Pan et al.2016).

Under microscopic observation(Fig.3c),the slate 17DJP-20 is mainly composed of microcrystalline quartz(about 50–60% in volume),sericite(about 30–40% in volume)which has been identified to be 2M1type microcrystalline muscovite through XRD analysis(Fig.3d)and about 10%carbonaceous plus pyrite.Minerals composition of the slate could also be reflected by the major chemical components of SiO2,Al2O3,LOI and K2O(showing in the appendix table in ESM).Moreover,the molar ratio of Al2O3to K2O is 2.9,which means that Al2O3and K2O exist almost in the standard form of muscovite(molar ratio of Al2O3to K2O is 3).Considering the high SiO2content(73.08 wt%)of initial volcanic magma of K-bentonite(Huff et al.1998)and hydrothermal sedimentation environment,the fallout volcanic ash could be altered to pyritized silicified clay rock in the pyrite mineralization stage.And clay minerals would further transform into 2M1type microcrystalline muscovite to form pyritized silicified slate in later metamorphism.

Zircon is the most stable heavy mineral to resist lowertemperature hydrothermal alteration and shallow metamorphism.Although zircon preserved in ash bed might has complexity sources,the youngest age population of magmatic zircon should represent the relative volcanism(Zhou et al.2018).40 sub-euhedral to euhedral zircons with clear growth oscillation zone have been selected as magmatic zircon(Koschek 1993;Corfu et al.2003),which could be supported by Th/U ratios(Belousova et al.2002)range from 0.13 to 1.52 and typical left-leaning trend REE patterns(Fig.4a)with obviously positive Ce anomalies(δCe=1.1–213)and negative Eu anomalies(δEu=0.01–0.54)(Hoskin and Ireland 2000).These zircons display a206Pb/238U age spectrum(Fig.4b)with a high concordance ratio(＞93%,except No.09 spot of 82%)from 420 to 2444 Ma.The youngest age population(400–500 Ma,n=18)is very sharp in the age spectrum(Fig.4b)and a coherent group(n=11)yields a Concordia age of 432.5±1.3 Ma with suitable MSWDCE.

Therefore,based on the criteria suggested by Huff(2008,2016)and the amendment proposed in this paper,it could be confirmed that the pyritized silicified slate occurred as‘sandwich’-like unconformity in the No.IV massive orebody of the Dajiangping deposit is low-grade metamorphic K-bentonite.

5.3 Caledonian magmatism-thermal event and pyrite mineralization

The siliceous rocks at the top of Dajiangping No.III banded orebody have been dated at 630.1±7.3 Ma using Rb–Sr method(Wang et al.1996).However,the reliability of such an age cannot be evaluated due to the easy resetting of the low closure temperature of Rb–Sr isotopic system(Nebel 2013).A Re–Os isochron age of 389±62 Ma for the pyrite from the laminated orebodies of the Daijiangping deposit has been determined,and thus the Daganshan Formation is inferred to form during the Middle-Upper Devonian(Qiu et al.2018a).Nevertheless,higher error makes the Re–Os age a little depreciated.

Both the intercept ages of the youngest detrital zircons from a sandstone interlayer in the laminated orebody(Qiu et al.2018a)and the Concordia age of magmatic zircons from the modified K-bentonite in the No.IV massive orebody imply that the Caledonian magmatism was recorded in Dajingping pyrite mineralization process.Hence the Daganshan Formation is not an Ediacaran stratum.Expanding with growing abundant zircon chronology data,a huge Caledonian granite belt with ca.200 granite masses dispersed within a total area ca.20000 km2in South China has been identified(Zhou 2003;Shu and Wang,2019).Twostage granites have been distinguished.The early-stage(460–430 Ma)of I-type granite with foliation texture are typical for small masses and scale,and the main and peak stage(430–400 Ma)of S-type granite are significant for large numbers and broad-scale over 90% area(Shu 2006).Extensive ductile shear deformation and metamorphic zones circled and coupled with the Caledonian granite are the most important constituent of the Caledonian intracontinental orogenic belt(Yu et al.2007).Combining granite dating with metamorphic minerals dating(e.g.Ar–Ar dating of white mica,biotite,and hornblende,EPMA dating of monazite),Li et al.(2010)suggest that the Wuyi–Yunkai Caledonian orogeny occurred between mid-Ordovician(＞460 Ma)and earliest Devonian(ca.415 Ma).Therefore,the metamorphic Daganshan Formation should be deposited before 415 Ma.

A curious characteristic of the Caledonian granite in south China,the absence of matching volcanic rocks probably caused by anatexis under compression,has been first summarized by Zhou(2003).Shu(2006)seconded the opinion and emphasized the lack of hydrothermal exhalative sulfide ore deposits in a compression tectonic setting.However,in the last decades,a brief post-collisional extension episode in Early Silurian has been revealed by a series of intermediate-basic volcanic activity in the northern area of Yunkai,including scattered volcanic rocks(420–438 Ma)(e.g.Genzhushan and Chitong)(Xu et al.2019;Qin et al.2017;Liu et al.2018)and a cluster of Caledonian volcanic basins(Nanjing,Siqian-Hekou and Dabaoshan)with zircon U–Pb ages from 434 to 444 Ma occurring along WSS fault at southern Jiangxi Province and Northern Guangdong Province.Based on dating of dacite,the VMS-type pyrite mineralization in the Dabaoshan volcanic basin,which had been determined to Devonian(Ge and Han 1986),has been revised as Early Silurian(Wu et al.2014).

Due to uplifting of the Cathaysia Block and closing of Nanhua ocean in Caledonian orogeny,there is a broad unconformity between Devonian and Ordovician sequences for the absence of Silurian in many areas of the Cathaysia Block,but only the Qin–Fang trough kept successive sedimentation during Silurian–Devonian(Liu and Xu 1994).During the brief post-collisional extension episode,the north-east Qin–Fang trough further extended toward to the Dabaoshan area and then transformed into west–east volcanic basins in the northern Guangdong and southern Jiangxi regions(Liu and Xu 1994;Hao et al.2010;Pan et al.2016).Besides,accompany with activating of the northeast Wuchuan–Sihui deep fault during the extension episode,intensive hydrothermal exhalative sedimentary mineralization occurred(e.g.SEDEX-type Dajiangping and Shezui deposits and VMS-type mineralization in Dabaoshan deposit).Thus,in the Early Silurian of Qin–Fang trough,the low-grade metamorphic K-bentonite in this study represents the matching volcanic event and the giant pyrite mineralization have recorded the brief extension episode of Caledonian orogeny in South China.

6 Conclusions

For modified K-bentonite,the amending geological criteria emphasize the‘‘instantaneous fallout’’of ash cloud and the broadly distributing‘sandwich’-like unconformity in strata sequence.Combining mineralogy with geochemistry,some thin silicified slates abruptly embedded in the No.IV massive orebody of the Dajingping pyrite deposit have been identified as low-grade metamorphic K-bentonites.The main and youngest zircon population in one of them yields a Concordia age of 432.5±1.3 Ma(MSWDCE=1.2,n=11),which is consistent with the weighted mean206Pb/238U and207Pb/235U ages of 432.4±3.7 Ma(MSWD=1.6)and 435.0±4.9 Ma(MSWD=0.99),respectively.Given the Caledonian magmatism-metamorphism event in South China,the mineralization age of the Dajiangping pyrite deposit and the sedimentary time of the Daganshan Formation have been rectified as Early Silurian in this study.In combination with the Early Silurian dating of the Shezui pyrite deposit and the Dabaoshan volcanic basin,the WSS pyrite belt in the south-western part of the Qin–Hang suture zone is inferred to be the Early Silurian exhalative sedimentary mineralization triggered by a transient extensional volcanic episode during the Caledonian orogeny.

AcknowledgementsWe are indebted to the State Key Laboratory of Ore Deposit Geochemistry at the Institute of Geochemistry,Chinese Academy of Sciences,for their technical support,and to anonymous reviewers for their constructive comments and valuable suggestions,which helped to significantly improve this manuscript.This work was supported by the National Natural Science Foundation of China(Grant Nos.41873058 and 41462001)and the Natural Science and Technology Foundation of Guizhou Province,China(Grant No.JZ[2015]2009).

Compliance withethical standards

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