Geochemistry and geochronology of Late Jurassic and Early Cretaceous intrusions related to some Au(Sb)deposits in southern Anhui:a case study and review

Qing Hu?Huangling Gu?Xiaoyong Yang?Yisu Ren?Ergen Gao?Zhangxing Nie

1 Introduction

The Jiangnan Orogenic Belt(JOB)is a Neoproterozoic collisional zone that is situated between the Yangtze Block and the Cathaysia Block(Zhao 2015).It spans several provinces,including the provinces of Guangxi,Guizhou,Hunan,Jiangxi,Anhui,and Zhejiang.The total reserve of more than 970 t of Au makes it one of the most important gold producers in southern China(Gu et al.2012;Ni et al.2015;Wang et al.2015;Liu et al.2016;Wen et al.2016;Deng and Wang 2016;Xu et al.2017;Deng et al.2017).Southern Anhui is located at the northeastern part of the JOB.It has not been extensively studied because the Au deposits(or occurrences)found there have been small(reserve lower than 1 t).Despite the lack of large Au deposits,southern Anhui is known to have widespread Au mineralization,indicating a bright Au metallogenic prospect(Wang et al.2013a,b).Recent studies have revealed that some of this mineralization is closely associated with and is considered genetically related to the Late Mesozoic intrusions(Duan et al.2011;Li et al.2014;Nie et al.2017).Examples include the Huashan Au(Sb)deposit(Yang et al.1993a,b;Nie et al.2016,2017;Xiao et al.2017)and the Zhaojialing(Yang et al.2015b),Tianjingshan(Duan et al.2011),Wuxi(Li et al.2014,2015)and Liaojia(Qian et al.2008;Cheng et al.2013)Au deposits(Table 1).However,the tectonic and genetic constraints relating these Mesozoic intrusions to the Au deposits are still unclear.

In this paper,we summarize new findings of Au and Aupolymetallic mineralization,and we discuss the relationship between the Au mineralization and the magmatism presented in southern Anhui.New data from the Huashan Au(Sb)deposit were supplemented by data from previous studies to discuss the genesis of the Au-related intrusions.Finally,we propose a model for the genesis of the magmatism related Au deposits in southern Anhui,in order to guide future mineral exploration.

2 Geological setting

Southern China consists of two tectonic blocks,the Yangtze Block and the Cathaysia Block(Fig.1a).The Jiangshan-Shaoxing Fault separates the Yangtze Block and the Cathaysia Block(Zhang et al.2005).It is considered to have the Archean-Paleoproterozoic crystalline basement(e.g.,Kongling and Dongling complexes)surrounded by Mesoproterozoic to Early Neoproterozoic low-grade metamorphic fold belts,which are unconformably overlain by Neoproterozoic Sinian cover(Zhao and Cawood 2012;Zhao 2015).Two Neoproterozoic igneous rock assemblages that formed under arc systems are exposed in the periphery margins of the block,in the western-northern Panxi-Hannan arc and southeastern Jiangnan arc(Zhou et al.2002;Zhao and Cawood 2012;Zhao 2015)(Fig.1a).The Jiangnan arc developed on the southeastern margin of Yangtze Block and subsequently incorporated onto the Jiangnan Orogenic Belt(JOB)asaresult of the Proterozoic collision between the Yangtzeand Cathaysia Blocks(Zhou et al.2002).

The Jiangnan Orogenic Belt(JOB)is located on southeastern margin of the Yangtze Block.It formed during the collision of the Yangtze and the Cathaysia blocks(Zhao 2015;Xu et al.2017).The JOB consists of Early Neoproterozoic(970–825 Ma)greenschist facies metamorphosed volcanic-sedimentary strata,which are intruded by Middle Neoproterozoic (825–815 Ma) peraluminous granites,Middle Neoproterozoic(815–750 Ma)weakly metamorphosed strata, and Late Neoproterozoic(＜750 Ma)unmetamorphosed Sinian cover(Fig.1a)(Wang and Mo 1995;Li et al.2003;Wang and Li 2003;Wang et al.2011;Zhao and Cawood 2012;Yao et al.2014;Zhao 2015).Two ophiolite belts outcrop in the Neoproterozoic stratums along the southeastern margin of the belt(Zhao 2015).The JOB containsmore than 250 Au deposits or occurrences,such as the Jinshan Au deposit and the world-class Dexing porphyry Cu-Mo-Au deposit at Jiangxi Province,and the Huangjindong,Mobin,Woxi and Wangu Au–Sb-(W)deposits at Hunan Province(Fig.1a).

The southern Anhui Province is located at the northeast JOB(Fig.1),adjacent to the Lower Yangtze River Belt(LYRB).The basement rocks are the low-grade metamorphic volcanic-sedimentary strata,which can be classified into the Mesoproterozoic Shangxi Group and the Neoproterozoic Likou Group.The upper cover consists of Nanhua system,Sinian,Cambrian,Ordovician and Silurian strata.The Neoproterozoic magmatic rocks distribute in the southern part of southern Anhui and can be divided into earlier marine volcanic rocks and later granites.The Early Cretaceous magmatic rocks form the large granitic batholith,with the intrusive agesat ca.140 Ma and ca.120 Ma.The Au depositsin the region are located near the southern Tianjingshan(Duan et al.2011),Liaojia(Qian et al.2008;Cheng et al.2013),the northern Zhaojialing(Yang et al.2015),and Huashan-Zhaceqiao(Nie et al.2016)(Fig.1b).Some Au deposits show close spatial and temporal association with the ca.140 Ma intrusive rocks(Duan et al.2011;Cheng et al.2013;Shen et al.2016;Nie et al.2017).

3 Geological characteristics of regional Au(-polymetallic)mineralization

3.1 Huashan Au(Sb)deposit(case study in this paper)

The Huashan Au(Sb)deposit is located on the eastern side of the Dongzhi Fault(Fig.1b).From south to north,the Nanhua system-Lower Silurian strata distributed from old to young,including till conglomerate,black shale,black silicalite,argillaceous-striated limestone,and carbonate rocks(Fig.2a).The regional EW-trending tectonic structures control the distributions of the magmatism in the Huashan area.The magmatic rocks are dominated by granodiorite porphyries,with minor granodiorite,quartz diorite and mafic dikes.Phenocrysts of granodiorite porphyries are composed of plagioclase(20 vol.%–40 vol.%),quartz(5 vol.%–10 vol.%),biotite(3 vol.%–5 vol.%)and amphibole(5 vol.%).Matrix consists of quartz(15 vol.%–20 vol.%)and feldspar(20 vol.%–35 vol.%).Accessory minerals(3 vol.%–5 vol.%)are zircon,apatite and pyrite.The granodiorite porphyries have undergone strong alterations which in turn caused the transformations of biotite and plagioclase phenocrysts into kaolinite or sericite pseudomorphs and the existence of melting corrosion structures in some quartz phenocrysts(Fig.3).

Two regional EW-trending fault zones with high–angle dipping cross the ore district(Fig.2b).One is a normal fault and the other is a thrust fault.The normal fault controls the ore bodies.The ore bodies were mainly hosted by a fault fracture zone that cuts the Cambrian strata(e.g.,black shales and silicalite,argillaceous-striated limestone)and alternated granodiorite porphyries.The ore bodies are tabular or strata-bounded in shape,with breccia,and they have a disseminated or massive structure.The ore minerals are antimony,pyrite,and arsenopyrite.Gangue minerals are quartz,sericite,carbonate,and clays.

3.2 Zhaojialing Au deposit

The Zhaojialing Au deposit islocated on the eastern side of the Dongzhi fault.It consists of the Changling,Zhaojialing and Yangjiashan Au ore sections(Fig.4).The ore bodies are mainly occurred in the EW trending faults and their secondary fault zones,and they are hosted by Xiuning and Dengjia Group sandstone.Ore minerals are hematite,magnetite,pyrite,arsenopyrite and nature gold,which are characterized by medium-coarse grained texture,fragmental porphyritic texture,and disseminated structure.Two different types of mineralization can be identified.First,high sulphidation type deposits in earlier shear zone and extensional fracture zone are present,where gold is hosted in auriferous quartz vein.It is characterized by strong silicification and sericitization,and nature gold granule can be observed.Second,low sulphidation type deposits controlled by lithology and alteration are also present,whereno obviousfracture zonecan beobserved.It is characterized by alterations like carbonatization,sericitization and pyritization.

3.3 Wuxi Au deposit

The Wuxi Au(polymetallic)porphyry deposit is located in Jingxian County.It was formed during Cretaceous and is hosted by Silurian siltstone.This deposit contains seven Au mineralized belts,one Ag mineralized belt and one Au metallogenic prospective area.Numerous granite porphyries are exposed in the mining area,which show a close spatial relationship with the mineralization.The tectonic structures are controlled by the regional stress field which caused the formation of many band-like fault zones(Fig.5).The magmatic hydrothermal fluids can emplace along these faults and deposit to form ore bodies.Typical porphyry type alteration can be observed,from the central part to outside,and they are silicification,potassic alteration,phyllic alteration and propylitization.Ore minerals are pyrite,marcasite,arsenopyrite,which have breccia texture and banding and a porphyritic and disseminated structure.The 138 Ma Langqiao granodiorite with NE trending is located at the northern part of mine district,in which numerous granite porphyry veins can be identified.

Fig.1 a Geological map showing the distribution of Precambrian rocks in the Yangtze Block(modified from Zhao 2015);b Map of distribution of magmatic rocks and some of the Au deposit in Southern Anhui Province

3.4 Liaojia Au deposit

The Liaojia Au deposit,a middle-low temperature hydrothermal deposit,is located in northwest Shitang County and Liaojia County(Fig.6)(Qian et al.2008;Cheng et al.2013).The regional compressional tectonic stressfield caused the formation of inverted fold beltsand a series of schistosity zones,which provided ideal ore-containing zones.Magmatism in the mine area are mainly gabbro diorite,granodiorite,and diorite porphyry,distributed along NE and EW trending faults.The ore bodies are hosted in Huansha Group phyllite and are mainly controlled by the NNE-trending Dabeiling fault.Cheng et al.(2013)suggest that the regionally widespread granodiorites belong to I-type granite and have a close relationship with Au mineralization.

4 Analytical methods

4.1 Whole-rock major and trace element analysis

Six granodiorite porphyry samples of Huashan(15HS2-1,15HS2-2,15HS2-3,15HS2-4,15HS2-5,15HS2-6,)were selected and analyzed at Guangzhou ALS Geochemistry Laboratory.The samples were powdered to＜200 mesh size using an agate mill.X-ray fluorescence spectrometry(XRF)was used to determine the major elements,with the standard deviations within 5%.Determination of loss of ignition(LOI)was conducted after igniting the sample powders at 1000°C for 1 h.An ignite or calcined sample(0.9 g)was added to 0.9 g of Li2B4O7-LiBO2between 1050 and 1100°C,mixed well,and fused in an auto fluxer.From the resulting melt,a flat molten glass disk was prepared.An AXIOS Mineral spectrometer was used to analyze the disk by wavelength-dispersive X-ray fluorescence spectrometry(XRF).

Trace elements and REE were determined on an Elan DRC-II instrument(Element,Finnigan MAT)by inductively coupled plasma mass spectrometry(ICP-MS)analysis of solutions,which were digested in a closed beaker for two-daysin Teflon screw-cap bombsusing amixture of HF and HNO3acids.The detection limit,which is defined as 3 s of procedural blank,for some elements is as follows(ppm):Th(0.05),Nb(0.2),Hf(0.2),Zr(2),La(0.5)and Ce(0.5).On the basis of replicate analyses of international standard reference material(SRM)and analytical results,the precision and accuracy of the data are better than 5%and 10%for major and trace elements,respectively.

4.2 Zircon U–Pb dating and trace elements

Fig.2 a Structure map of the Huashan-Zhaceqiao deposit(Modified from Nie et al.2016).b Cross-section of the Huashan Au(Sb)deposit

Fig.3 Microphotographs of Huashan granodiorite porphyries,a and b under plane-polarized light,c and d under cross-polarized light.Biotite and plagioclase underwent strong alterations like argillzation and sericitization.(Qtz-quartz,Pl-plagioclase,Bt-biotite,Mus-muscovite)

Zircons were selected from three granodiorite porphyry samples(15HS2,15HS3,15HS4,)using standard density and magnetic separation techniques.The zircon grains were hand-picked for a representative one under a binocular microscope,which was then mounted in epoxy resin and polished to half sections.Cathodoluminescence(CL)image technique was utilized to exam the internal structure of zircons at the CAS Key Laboratory of Crust-Mantle Materials and Environments at the University of Science and Technology of China,Hefei.

In-situ U–Pb dating and trace element analyses of zircons proceeded simultaneously with LA-ICP-MS at the School of Resources and Environmental Engineering,at the Hefei University of Technology.A 4.51 mj/cm-2power energy of pulsed 193 nm ArF Excimer(COMPex PRO)at a repetition rate of 8 Hz,and a spot diameter of 45μm,coupled to a Agilent 7500 s quadrupole ICP-MS was used for ablation.Helium was used as the carrier gas to enhance the efficiency of the transportation of ablated aerosol.

For isotope analysis,all measurements were conducted with the external standard of zircon 91500,which recommended a206Pb/238U age of 1065.4±0.6 Ma(Wiedenbeck et al.1995).For trace element analysis,all quantitative results were calibrated to relative element sensitivitiesby using the NIST-610 astheexternal standard and zircon SiO2as the internal standard.The standards were analyzed for every 10 analysis.The precision of simultaneous NIST-610 analyses for REE,Sr,Nb,Ta,Th and Uareat theppm level,and for Mn,P,Ti are better than 5%.The detection limit for REEs varies from 0.02 to 0.09 ppm.The detailed analytical procedure was described by Zong et al.(2010).Off-line selection,integration of analysis signals,background time-drift correction,trace element analyses,and U–Pb dating quantitative calibration was all performed with ICPMS Data Cal(Liu et al.2008,2010b).Isoplot/Ex_ver3(Ludwig 2003)was utilized to make concordia diagrams and weighted mean calculations.

Fig.4 Sketch map of the Zhaojialing Au deposit(Modified from Yang et al.2015)

4.3 Zircon Lu–Hf isotopes

The in situ Lu–Hf isotopes analyses were conducted at the laboratory of the Tianjin Institute of Geology and Mineral Resource,Chinese Academy of Geological Sciences.The Lu–Hf isotopes were measured by a NEPTUNE multiplecollector inductively couple plasma mass spectator,quipped with NEW WAVE 193 nm laser-ablation system.A 10–11 mj/cm2power energy repetition rate of 8–10 Hz and a spot diameter of 55μm was used for ablation.The ablated materials were transferred into MC-ICP-MS by purified He gas.Detailed introduction of the analyses method and isotope fractionation correction are described by Geng et al.(2011).Off-line date processing was conducted by ICPMSDataCal(Liu et al.2010b).

To calculateεHf(t),parameters likeλ=1.865×10-11year-1, (176Hf/177Hf)CHUR.0=0.282772 and(176Lu/177Hf)CHUR=0.0332 are adopted(Blichert-Toft and Albarede 1997). (176Hf/177Hf)DM=0.28325 and(176Lu/177Hf)DM=0.0384 are used as the parameters to calculate Hf model age(Vervoort and Blichert-Toft 1999).

4.4 Apatite composition

Zircons were selected from three relatively fresh granodiorite porphyry samples(15HS2,15HS3,15HS4,).The methods used to separate and purify apatite are similar to that used with zircons.The selected apatite were mounted in epoxy resin and polished into half sections,then pictured under transmitted and reflected light.Major and trace elements of apatite were determined by Shimadzu EPMA 1600 electron microprobe and LA-ICP-MS respectively,at the CAS Key Laboratory of Crust and Mantle Materials and Environments at the University of Science and Technology of China.

For major elements,the EPMA analyses were conducted under an accelerating voltage of 15 kV,a low beam current(15 nA),and a defocused beam(5μm).A suite of mineral standards and oxide standards from the American Standard Committee were used as calibration.For trace elements,the spot size of the laser beam was 30μm.Two silicate glass reference materials(NIST SRM610,NIST SRM612)were used as calibration,and Ca was used as the internal standard(Danyushevsky et al.2003;Pearce et al.2010).The details of the procedure were described by Danyushevsky et al.(2003),Zhang et al.(2011),Flem and Bédard(2010).

5 Results

The chemical compositions of the magmatic rocks from the Huashan area are listed in Supplement Table 1.

5.1 Whole rock major and trace element analysis

5.1.1 Major elements

The Huashan granodiorite porphyries have a relatively high content of SiO2(69.4 wt%–70.6 wt%) and LOI(6.14 wt%–6.59 wt%),which implies that the magmas underwent strong alterations.Relatively low content of Na2O (0.05 wt%),CaO (2.09 wt%–2.36 wt%),MgO(1.54 wt%–1.78 wt%)and K2O (4.33 wt%–4.58 wt%)can be identified.The magmatic rocks are classified into quartz-monzonite and quartz-rich granitoid on the QAF diagram(Fig.7a)and belong to the calc-alkaline and high-K calc-alkaline series on the SiO2–K2O diagram(Fig.7b).Relatively high content of SiO2could be caused by the strong alteration.In the granite discrimination diagram,the magmatic rocks are characterized as I-type granite affinities,corresponding with previous studies(Fig.7c,d).In Harker diagrams(Fig.8),the contents of Al2O3,TFe2O3,P2O5,and TiO2have negative correlations with those of SiO2(Fig.8),supporting the existence of a fractional crystallization process involving the separation of plagioclase,biotite and accessory minerals.

Fig.5 Sketch map of the Wuxi Au deposit(Modified from Li et al.2015)

5.1.2 Trace elements

The Huashan granodiorite porphyries have relatively low contents of REEs(ΣREE is about 101.7–127.4 ppm).The patterns of REE(Fig.9a)exhibit moderate enrichment of LREEs relative to HREEs,and (La/Yb)Nis high(14.4–18.2),in which slightly negative Eu anomalies can be observed(δEu=0.72–0.86).The REE patterns are similar with other synchronous granites nearby(Song et al.2014;Shen et al.2016).

The patterns of trace elements of Huashan igneous are similar with those of other synchronous magmas in southern Anhui(Fig.9b),except for thestrong depletion of Sr.Previous studies show a slightly positive anomaly of Sr(Song et al.2014;Shen et al.2016),which implies that the depletion of Sr in Huashan magmatic rocks could be the result of strong alteration.The enrichments in LREEs(U)and depletions in HFSEs(Nb,P,Ti,Y)can be interpreted by the involvement of continental crust materials(Condie 1982).The partial melting process is supported by the Ta–Ta/Sm diagram(Fig.10a).

Fig.6 Sketch map of the Liaojia Au deposit(Modified from Cheng et al.2013)

5.2 Zircon U–Pb dating and trace elements

Three samples of Huashan granodiorite porphyries(15HS02,15HS03 and 15HS04)are used to select zircons for experiments.These zircons are generally euhedral,prismatic,transparent,and colorless.Cathodoluminescence(CL)images show clear micro-scale oscillatory zones of zircons(Fig.11),indicating a magmatic origin.Few inherent cores can be identified,and some of these cores have oscillatory zones(Fig.11).

The zircon LA-ICP-MS U–Pb analysis results of the Huashan magmas are listed in Supplement Table 2(only the data with concordant degree above 90%were used).Zircons from 15HS02 showing high concordance yield weighted average age of 145.9±2.0 Ma(Fig.12b).However,most zircons from 15HS03 and 15HS04 show low concordant degree,and few data can be used(Fig.12c and d).The average age of 15HS03 and 15HS04 are 148.3±9.5 Ma and 144±11 Ma,respectively.The206Pb/238U age of inherited cores in all samples ranges from 790 to 900 Ma,with an average of about 803±29 Ma(Fig.12a).The cores with planer and oscillatory zonal structure have Th/U ratios of about 0.5–0.6,while the cores without zonal structures have Th/U ratios of about 0.3,suggesting a magmatic origin and a metamorphic process,respectively.Furthermore,the age of the Neoproterozoic magmas in the northeast JOB are similar with those of the inherent zircons in the Huashan area(Wu et al.2006;Zheng et al.2003,2008),implying the assimilation of the Neoproterozoic magmatic basement during the generation or migration processes of Huashan magma.

The trace elements of zircons of Huashan magmas are shown in Supplement Table 3.The zircons have relatively high contents of REE(ΣREE of young and inherent cores are 3088–5051 ppm and 4909–9571 ppm,respectively),with enrichment of HREE and depletion of LREE.The young zircons(144–148 Ma)have relatively strong positive anomaly of Ce and slightly negative anomaly of Eu(δEu=0.52–0.70),suggesting a high oxidized state of Huashan granodiorite porphyries(Fig.13).

5.3 Zircon Lu–Hf isotopes

The Lu–Hf isotopic data of Huashan granodiorite porphyries are listed in Supplement Table 4.The176Lu/177Hf ratios of most zircons are lower than 0.002,indicating the low radiogenic Hf accumulation after formation of zircons.Thus,the176Hf/177Hf ratios can represent the origin Hf composition of the studied zircons(Amelin et al.1999;Wu et al.2007).

Fig.7 Classification diagram of ore-related magmatic rocks in southern Anhui.a QAP diagram of ore-related magmatic rocks in southern Anhui.After Streckeisen(1976);b K 2O–SiO2 diagram of ore-related magmatic rocks in southern Anhui.Solid lineis from Peccerillo and Taylor(1976),dashed line is from Middlemost(1985).c FeO/(FeO+MgO)versus SiO2 plot,after Frost et al.(2001);d P2O5 versus SiO2 plot,after Chappell et al.(1999).Data sources:Shen et al.(2016),Li et al.(2015),Song et al.(2014),Yang et al.(2015)

The εHf(t)values of the 144–148 Ma aged zircons and inherited zircon coresrange from-11.48 to 1.08 and 5.51 to 12.69,respectively.The two stage Hf model age(tDM2)of Mesozoic zircons and inherited zircon cores are 1130–1928 Ma and 903–1360 Ma,respectively(Fig.14).

5.4 Apatite composition

The major and trace elements of apatite from Huashan granodiorite porphyries are shown in Supplement Table 5.These apatites have 54.67%–56.84%CaO and 42.12%–43%P2O5.The apatites have high contents of F(2.17%–4.05%),which vary between those of sedimentary apatite(～ 2.21%)and volcanic apatite(～ 4.06%)(Wang,1987),but they have low contents of Cl(0.08%–0.54%).The ΣREE and δEu values range from 1311 to 3992 ppm and 0.48 to 0.78,respectively(Fig.15).

6 Discussion

6.1 Constraints on the origin of the Au-related magmatism

As the Au-related magmatic rocks are altered more or less,their mobile element contents such as K,Na,Rb,and Sr could actually be affected by the hydrothermal alteration(Hastie et al.2007).In diagrams(Fig.16),there are some correlations between LOIand mobile element contents for the Au-related magmatic rocks.The K2O(Fig.16a)contents show negative correlation to the LOI contents while Na2O(Fig.16b)contents show positive correlation to the LOI contents,implying different degrees of alternation,such as albitization.For the magmatic rock from the certain area,the K2O(Fig.16a)contents of these rocks do not vary much,suggesting that the K2O-SiO2can be used here.However,the total alkali contents of these rocks show strong negative correlation with LOI,and the variation is large(Fig.16c).Therefore,the TAS diagram is not suitable to be used here,so we used the QAP diagram(Fig.7a)instead.The CaO(Fig.16d)contents show positive correlation with LOI,indicating the effect of carbonatization.Besides,the correlations between mobile trace elements and the LOI are also obvious(Fig.16e and f),implying that these elements are not reliable enough to be used to discuss the formation of Au-related magmatic rocks.Thus,we mainly use the immobile elements to discuss the genesis of the magmatism.

Fig.8 Harker diagram of Au-related magmatic rocks in southern Anhui

Fig.9 a Rare earth element patterns(normalized by chondrite);b Trace element spider diagrams(normalized by N-MORB).Data sources:Li et al.(2015),Shen et al.(2016),Song et al.(2014)

Fig.10 a Ta–Ta/Sm diagram.b(La/Yb)N-(Gd/Yb)N diagram.Data sources:Shen et al.(2016),Li et al.(2015),Song et al.(2014),Yang et al.(2015)

Fig.11 Representative zircon CL images with U–Pb age and εHf(t)values for Huashan granodiorite porphyries

In eastern China,the strong association between the Late Jurassic-Early Cretaceous magmatism and mineralization has been reported by numerous studies(Chen et al.2005;Sun et al.2003;Mao et al.2003,2005;Li et al.2010a,b;Xie et al.2015,2016,2017a,b;Xu et al.2012,2014a;Yang et al.2017;Wu et al.2017;Fan et al.2017;Hu et al.2017;Gu et al.2017a).However,the forming mechanism of the ore-related magmatism is still controversial.Subduction-related and intracontinental models were proposed,including partial melting of thickened or delaminated lower continental crust by basaltic underplating(Wang et al.2004,2006,2007),fractional crystallization from mantle-derived basaltic magmas(Li et al.2009a,b,2013),partial melting of subducted paleo-Pacific Plate(Ling et al.2009;Liu et al.2010a,b;Sun et al.2010;Gu et al.2017b),mixing of mantle-and crustalderived magmas(Wang et al.2003;Xie et al.2009;Chen et al.2016),partial melting of Neoproterozoic crustal rocks(Yang and Zhang 2012;Song et al.2014),and remelting of Neoproterozoic subduction-modified lithosphere mantle(Wang et al.2015).

The Au-related magmatic rocks in southern Anhui mainly formed during 138–148 Ma(Fig.17).However,no synchronous basaltic rock has been found here.The basaltic igneous rocks consisting of gabbros and alkali volcanic rocks from the nearby LYRB were formed at 131–125 Ma(Zhou et al.2008),which are younger than the Au-related magmatic rocks in southern Anhui.Furthermore,the relatively low contents of MgO(approximately 1 wt.%)suggest that the magmas are not mantlederived.Therefore,the fractional crystallization from mantle-derived basaltic magma model and the remelting of Neoproterozoic subduction-modified lithosphere mantle model can be excluded.

The partial melting of the oceanic plate model is difficult to apply here since the adakitic signature(high Sr/Y and La/Yb ratios)isnot shown on the magmas(Defant and Drummond 1990).Although the existence of garnet has been supported by the(La/Yb)Nand(Gd/Yb)Ndiagram(Fig.10b)(Blundy and Wood 1994;Klein et al.2000;Perterman et al.2013;Heet al.2011),theanomaly of Eu of whole rock(δEu=0.72–0.86)and accessory minerals like zircon(δEu=0.52–0.70)and apatite(δEu=0.48–0.78),and the relatively low Sr/Y(3–5)support that the plagioclase isalso stable in source,which does not correspond to a deep melting and the existence of dense eclogitic lower continental crust.Thus,the magmas are not derived from the thickened or delaminated lower continental crust.

Therefore,the partial melting of Neoproterozoic crustal rocks model and the mixing of mantle-and crustal-derived magmas model are supported.Here,we prefer the latter since it has been widely supported that the crust-mantle interaction played an important role during the formation of Late Jurassic to Early Cretaceous ore-bearing magmatism in eastern China(Zhou et al.2008;Xie et al.2009,2011b,c,2012;Wang et al.2015).The mantle derived high-Mg magmatic rocks with Late Mesozoic age are reported in both Jiangnan Orogen and the northern adjacent Lower Yangtze River Belt(Liu et al.2010a,b;Wang et al.2015).

Fig.12 Concordia diagrams of zircons from Huashan granodiorite porphyries(inherited zircons,15HS02,15HS04 and 15HS03)

The zircons of Huashan granodiorite porphyries have εHf(t)values ranging from-11.48 to 1.08,first stage model ages(tDM1)ranging from 765 to 1638 Ma(with centralized valuesfrom 800 to 1000 Ma),and second stage model ages(tDM2)ranging from 1130 to 1928 Ma.The 790–900 Ma inherited zircon cores show an oscillatory zone(Fig.11)and haveεHf(t)and tDM2values ranging from 5.51 to 12.69 and 903 to 1360 Ma,respectively,which indicate a Neoproterozoic magmatic origin.Besides,the existence of quantitative inherited magmatic zircons with Neoproterozoic age in southern Anhuiis also reported by other research(Yang and Zhang 2012;Xu et al.2014b),which indicates that the Neoproterozoic magmatic rocks could contribute to the magmas,either as source material or through contamination(Yang and Zhang 2012;Song et al.2014).The zircon Lu–Hf isotopic characteristics of Huashan and those of Wuxiarecomparable(Fig.14a).The zircons from Huashan and Wuxi intrusions plot in the evolution direction of Neoproterozoic magmatic rocks in the northeast JOB(Fig.14a).Based on the Sr–Nd–Pb isotopic data,the～140 Ma magmas in southern Anhui are proposed to derived from lithosphere mantle(He 2013)or lower Yangtze crust(Song et al.2014),and are contaminated with upper crust materials(e.g.,Shangxi Group,Likou Group volcanic rocks in southern Anhui and volcanic rocks in northwest Zhejiang Province(Wu et al.2006;Wang et al.2012;Xu 1994).These results indicate that the intrusion have a relatively young source,which is different from the Archean-Paleoproterozoic crystalline basement in the central part of the Yangtze Block.

Fig.13 Diagrams of rare earth elements in zircons.a REE patterns of zircons;b Ce4+/Ce3+ratios versus Eu/Eu*oxygen fugacity discrimination diagram.The data of ore bearing and barren rocks in Chile are referred from Ballard et al.(2002);The Dexing orebearing rocks data referred from Zhang et al.(2013);Wuxidata from Liet al.(2014,2015)

During the Jurassic-Cretaceousperiod,thesubduction of the Paleo-Pacific Plate affected tectonic stress field in eastern China(Xie et al.2007,2008,2011a;Zhou et al.2008;Li et al.2015).It reactivated the pre-existing faults(e.g.,Jiangnan Deep Fault)(He et al.2010;Zhou et al.2006)and caused the upwelling and decompression melting of mantle,forming the basaltic melts.The basaltic mantle-derived melts typically stall at the base of the lower crust due to density contrasts(Hildreth and Moorbath 1988).In a MASH(melting,assimilating,storage,and homogenization)process(Hildreth and Moorbath 1988),heat released from the basaltic melts can cause partial melting of crustal rocks.The mixing and differentiation of these melts form hybrid,intermediate-composition magmas with low enough density that they can ascend through upper crust(Richards 2009).In our study,the Au-related magmatism probably formed through this process.

6.2 The arc-magma feature and oxidized magma source

According to the tectonic discrimination diagram proposed by Pearce et al.(1984),the Au-related magmatic rocks in the study area plot in the areas of VAG and syn-COLG(Fig.18a,b and c).In the YbN-(La/Yb)Ndiagram,the magmatic rocks plot in the area of normal arc island rock(Fig.18d).Furthermore,the relatively high contents of Th,U,and REE but the low content of Nb,Ta,and Ti suggest that the magmatic rockshave typical geochemical affinities of arc magma.

It is suggested that the Jiangnan arc had developed on the southeastern margin of the Yangtze Block and subsequently incorporated onto the Jiangnan Orogenic Belt(JOB)as a result of the Proterozoic collision between Yangtze and Cathaysia Blocks(Zhou et al.2002).The existence of Neoproterozoic magmatic arc at the southeastern margin of the Yangtze Block hasbeen supported by much evidence,such as Neoproterozoic ophiolitic assemblages,the arc-geochemical featured Shuangxiwu group(Li et al.2009a,b),and the arc-related metamorphosed volcanic-sedimentary strata at the southeastern margin of the Yangtze Block(Zhao et al.2015).Meanwhile,from late Jurassic to Cretaceous,eastern China was closely associated with the subduction of the Pacific plate(Maruyama et al.1997;Sun et al.2007)and became an active continent margin before the Jurassic(Maruyama et al.1997;Zhou and Li 2000;Sun et al.2007,2010;Liu et al.2010a,b;Deng et al.2016;Ling et al.2009).Therefore,we cannot ruleout thepossibility that the Au-related magmatic rocksin southern Anhui derive arc-magma feature from the Pacific subduction.

Thus,we suppose that the arc-like characteristics of magmas are both likely inherited from the juvenile lithosphere formed by the Neoproterozoic subduction between the Yangtze and Cathaysia blocks(Wang et al.2015)and caused by the subduction of Pacific plate.

It has been widely accepted that the fluids/melts with high oxygen fugacity are beneficial to the Cu-Au mineralization(Mungall 2002;Audétaet al.2004;Oyarzun et al.2001;Ballard et al.2002;Mungall 2002;Kelley and Cottrell 2009; Lee et al. 2012; Sun et al.2011,2013b,2015,2017).Because the chalcophile elements(Cu and Au)are highly compatible in magmatic sulfide phases while incompatible in silicate and oxide minerals(Ballard et al.2002).Thus,the removal of chalcophile elements from the mantle can only happen in oxidized conditions where the sulfate phases are dominant(Ballard et al.2002;Mungall 2002).The redox state(represented by oxygen fugacity)of a magmatic rock can be measured by the contents of multiple valence trace elements in the refractory accessory minerals like zircon and apatite(Trail et al.2012;Ballard et al.2002;Miles et al.2014).Here,we use the zircon Ce4+/Ce3+and EuN/EuN*values to evaluate the mineralization potential of the Au ore-related magmas in the study area(Supplement Table 4).The Ce4+/Ce3+and EuN/EuN*ratio of Huashan and Wuxi granodiorite porphyries are projected in the area defined by ore-related oxidized magmas in Chile and Dexing(Fig.13b),indicating an oxidized feature of the granodiorite porphyries.In REE patterns of apatites(Fig.15),a slightly Eu negative anomaly(δEu ranging from 0.48 to 0.78)also suggests a relatively high oxygen fugacity of the magmas.

Fig.14 Zircon Lu–Hf isotopic compositions.a The εHf(t)versus age diagram.b Histogram of Hf two stage model age.Data sources:Li et al.(2015),Song et al.(2014),Yang et al.(2012),Wu et al.(2006),Zheng et al.(2008)

Fig.15 Rare Earth Element patterns of apatite.The Wuxi apatite data referred from Li et al.(2015)

6.3 Implication for Au mineralization

The ages of Huashan granodiorite porphyries(zircon U–Pb ages of 144–148 Ma)are corresponding to the mineralization age(39Ar/40Ar ore-stage sericite ages of ca.142 Ma)(Nie et al.2017),indicating that both porphyry emplacement and Au mineralization occurred simultaneously.In addition,the H–O isotopic data of fluid inclusions in quartz indicates that magmatic water has been involved in the mineralization process(Nie et al.2017;Ji 1991).Furthermore,the existence of hidden intrusions in the Huashan area is supported by the annular magnetic anomaly(Nie et al.2013).Therefore,the granodiorite porphyries have a close relationship with the Au mineralization.

The lower crustal cumulates residual formed during ancient subduction can contain small amounts of chalcophile and siderophile element-rich sulfides and act as a metal source for Au-rich magmas during later remelting(Richards 2009;Lee et al.2012).The link between the ancient subduction and the post-subduction ore deposits have already been identified in the world,such as in the southwestern Pacific area near North America(Solomon 1990;Core et al.2006;Shafiei et al.2009;Pettke et al.2010),and in the north China Craton (Sun et al.2007,2013a;Zhu et al.2015).

As mentioned before,during Proterozoic era,the existence of oceanic subduction between the Yangtze and Cathaysia blocks has been widely accepted(Li et al.2009a,b;Zhou et al.2009;Zhao 2015).The volcanogenic massive sulfide(VMS)type Pingshui Cu deposit and VMS type Tieshajie Cu deposit(Wang et al.2015)in the southeast margin of the Yangtze Block support the Cu(Au)enrichment during the Neoproterozoic subduction(Li et al.2009a,b;Zhang et al.2009).In the latest study,the finding of Au(Cu)-rich lower continental xenoliths proves that the lithosphere beneath the southwestern margin of the Yangtze Block is Au(Cu)fertilized(Hou et al.2017).In southern Anhui,in the southeastern margin of the Yangtze Block,the same scenario could also exist,suggesting that the fertilized lithosphere could be the potential metal source for the Au deposits.

Fig.16 Correlations between contents of mobile elements(K 2O,Na2O,Na2O+K2O,CaO,Sr and Rb)and LOI

Liang(1992)systematically analyzed the compositions of the Proterozoic-Cambrian stratums in Southern Anhui and suggested that the Shangxi,JingTan,Xiuning,Leigongwu and Lantian groups have relatively high contents of Au.Zhang(1999)posited that the Precambrian strata in Huashan area have elevated contents of Sb.In particular,the Sb contents in the Cambrian Huangboling Group are ninety times higher than the Clark value.Yang(1993a)studied the syn-sedimentary exogenic pyrites of Mesoproterozoic strata in southern Anhui and revealed that the pyrite has high contents of Au,Sb,and Asin the core while low contents of those in the edge.The hydrothermal metasomatism could be the potential reason for the heterogeneous distributions of the elements in the pyrites,which could bring the elementsout of the pyrite and enrich them to form the Au(-polymetallic)deposits.Therefore,we can speculate that,in addition to Late Jurassic-Early Cretaceous magmatism,the Au-and Sb-bearing strata are the other possible providers of ore-forming metals for the deposits in the study area.

Finally,we propose a multiple-stage genetic model that explains the intrusion-related Au and Au-polymetallic deposits in southern Anhui(Fig.19):

Stage 1 During the Proterozoic oceanic subduction between Yangtze and Cathaysia blocks,the lithosphere beneath southern Anhui was produced and fertilized with Au and other economically important elements.These elements are preserved in the lithosphere(Fig.19a).

Fig.17 Chronological histogram of U–Pb agesobtained in thisstudy and from the literature

Stage2 In the Mesozoic era,the subduction of the paleo-Pacific Plate beneath eastern China caused the partial melting of the Au-rich lithosphere in southern Anhui,forming the primary magma.The primary magmas then underwent fractional crystallization and crustal assimilation.Finally,the Au-related magmatic rocks that we have observed in southern Anhui were formed.

Stage 3 The hydrothermal fluids derived from the magmas have relatively high contents of Au.The emplaced magma acted as a heat engine to drive the circulation of hydrothermal fluids,which further extracted elements of economic interest from the surrounding successions.The fluids ascended along passageways such as fault zones and formed deposits where conditions were favorable(Fig.19b).

Fig.18 Tectonic discrimination diagrams of Au-related magmatic rocks in southern Anhui.a Y-Nb discrimination diagram;b Y-Ta discrimination diagram;c Hf–Ta*3-Rb/30 discrimination diagram;d YbN-(La/Yb)N discrimination diagram.WPG within plate granites,VAG volcanic arc granites,ORG ocean ridge granites,syn-COLG syn-collision granites(after Pearceet al.(1984)and Defant and Drummond.(1990)).Data sources:Li et al.(2015),Song et al.(2014)

Fig.19 Schematic cross section from Asthenosphere to Crust.a Formation of fertilized subduction modified lower crust(modified from Wang et al.(2015)and Zhao et al.(2015));b Formation of Au-related magmatism in southern Anhui

7 Conclusions

Based on geochemical and geochronological data from the Huashan Au(Sb)deposits,as well as a database from previous studies, this study draws three primary conclusions:

(1) Au-related magmas in southern Anhui were emplaced during the Late Jurassic and Early Cretaceous periods(138–148 Ma).

(2) Au-related magmas are characterized by arc-magma features and high oxygen fugacity and are rich in inherited zircons.

(3) Zircon U–Pb ages and zircon Lu–Hf isotopes suggest that Proterozoic juvenile lithosphere was the main source of Au-related magmas in southern Anhui.

AcknowledgementsThis study is supported by the National Key R&D Program of China(No.2016YFC0600404),the National Natural Science Foundation of China(Nos.41372087,41673040,41174043),and the Project of Geological Science and Technology of Anhui Province(2014-K-04,2016-K-1).We wish to thank Dr.Fangyue Wang for his assistance during the zircon U–Pb dating analyses and Dr.Lei Liu for his help on the zircon Lu–Hf analyses.

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