A geochronological and geochemical study on the granodiorite porphyry and its implication for the mineralization in the Dayaoshan metallogenic belt,Southeastern China

Panfeng Liu •Jiali Zhang•Zhiqiang Wu•Qingwei Zhang•Meilan Wen•Xianrong Luo•Chaojie Zheng•Wenbin Huang

Abstract A systematic study combining U-Pb zircon dating,lithogeochemical and Sr–Nd isotopic analyses was carried out upon the Xinping granodiorite porphyry in the Dayaoshan metallogenic belt to understand its petrogenesis and tectonic significance.LA-ICP-MS U–Pb zircon dating yielded a 442.7±5.8 Ma age,indicating that the granodiorite porphyry was emplaced during the Llandovery Silurian of the Early Paleozoic.The granodiorite porphyry shares the same geochemical characteristics such as Eu negative anomaly as other syn-tectonic granite plutons in the region,including the granodiorite porphyry in Dawangding and granite porphyries in the Dali Cu–Mo deposit and Longtoushang old deposit,indicating a similar magma evolution process.The Xinping granodiorite porphyry has high contents of SiO2(67.871.8%)and K2O(1.78–3.42%)and is metaluminous–peraluminous with A/CNK ratios ranging from 0.97 to 1.06,indicative of high-potassium calc-alkaline to calc-alkaline affinity.It is a I-type granite enriched in large ion lithophile elements Rb,Sr,while depleted in Ba and high field-strength element Nb.Tectonically,a collision between the Yunkai Block from the south and the Guangxi Yunnan-North Vietnam Block from the north during the Early Paleozoic was followed by uplifting of the Dayaoshan terrane.The Xinping granodiorite porphyry was likely emplaced during the collision.Sr–Nd isotopic analyses show that the granodiorite porphyry has initial 87Sr/86Sr ratios(Isr)of 0.7080–0.7104,εNd(t)range from-0.08 to-4.09,and t2DM between 1.19 and 1.51 Ga,well within the north-east low-value zone of the Cathaysia block,indicating a Paleoproterozoic Cathaysia basement source and an involvement of under plating mantle magma.Field observations,geochronological data,and 3D spatial distribution all lead to the conclusion that the Early Paleozoic Xinping granodiorite porphyry does not have any metallogenic and temporal relationships with the Xinping gold deposit(which has a Jurassic-Early Cretaceous age based on previous studies)but a close metallogenic relation to W–Mo mineralization.

Keywords U–Pb zircon geochronology·Lithogeochemistry·Sr–Nd isotopes·Granodiorite porphyry·The Dayaoshan metallogenic belt

1 Introduction

The Dayaoshan metallogenic belt is located in central and eastern Guangxi Zhuang Autonomous Region,South China.Sitting on the southwestern segment of the Yangtze–Cathaysia suture zone,it belongs geotectonically to Dayaoshan uplift in central Guangxi-northeastern Guangxi fold system of South China mobile zone and is one of the important gold producers in Guangxi(Cai et al.2000;Huang et al.2003;Mao et al.2011).There are several gold deposits discovered in the belt,including Gupao,Taohua,Liucen(of quartz vein-type),and Longtoushan(of porphyry-type)deposits.The quartz vein-type gold deposits often contain altered-rock-type ores in fractured rock zones,such as the Gulinao,Dawangchong and Chaode deposits in Gupao,Yunrong deposit in Taohua,and Daiwu deposit in Liucen(Li,2011;Wang et al.2010;Xiao et al.2015).The Xinping gold deposit,one of the alteredrock-type gold deposits in the fractured rock zones(Zhou et al.2016),was found to be surrounded by large-and medium-sized Cu–Mo deposits,for example,the Yuanzhuding,Shedong,and Dali deposits(Chu et al.2013;Wei et al.2012;Zhang et al.2014).Furthermore,lead and zinc deposits were also spotted in Dali deposit—which is quite inspiring for ore prospecting.Lesson learned from previous prospecting activities in the area suggests that it be sensible to look for host granitic rocks first during ore prospecting—quite a demonstration of how significant it is to perform a thorough study of granite rocks before exploring for gold-polymetallic deposits in the Dayaoshan metallogenic belt.Substantive geological work of the belt shows that the magmatic rocks in Dayaoshan uplift were mostly emplaced during the Early Paleozoic(432.0–468.2 Ma),Jurassic(150–170 Ma)and Cretaceous(90–110 Ma)(Qin et al.2015;Ding et al.2017).Part of the magmatic rocks was emplaced in the Late Paleozoic-Triassic(Fig.1).Granites formed during the Early Paleozoic occur largely as dykes and stocks of intermediate-acidic igneous rocks such as diorite or granodiorite(granodiorite porphyry),with syn-tectonic mantle component(I-type).-Those developed during the Jurassic and Cretaceous are mostly stocks and batholith with some dikes of acid rocks or S-type granite such as biotite granite,monzonitic granite,and diorite(Chen et al.2015;Huang et al.2003).

A granodiorite porphyry dike occurs in the Xinping gold deposit.Part of the gold ore bodies(gold mineralization)occurs in the contact fracture zone between granite porphyry and Cambrian silicized sandstone,while a small part occurs in granite porphyry,which has a close spatial relationship with gold mineralization(Zhou et al.2016).Quartz-molybdenite vein within the granodiorite porphyry was first reported by Wang et al.(2013).Chronology studies have shown that most granite-porphyry were Early Paleozoic in Dayaoshan metallogenic belt.For example,the LA-ICP-MS U–Pb zircon dating of the granite porphyry is 436.1–436.4 Ma,and the Re–Os age of the quartzveins tungsten molybdenum ore in the Wandao gold deposit was 436.6±3.8 Ma,which is similar to the granite porphyry(Xiao et al.2015).However,the fluid inclusion Rb–Sr age of the Taohua gold deposit was 148±10 Ma in the same metallogenic belt(Liucen–Taohua–Gupao metallogenic belt)(Cai et al.2000).It can be seen that the formation of the ore body was significantly later than that of granite porphyry,while tungsten molybdenum mineralization and granite porphyry were formed almost simultaneously.Therefore,a correct understanding of the genetic relationship of granite porphyry and gold mineralization and tungsten-molybdenum mineralization will not only provide a new clue to the tectonic evolution of eastern China in the Late Phanerozoic but also have practical significance for regional gold deposit exploration.This paper documents an attempt to determine high-precision age,petrogenesis,and magmatic process of the granodiorite porphyry and understand their implications in mineralization through a systematic geochronological,major and trace elemental,and Sr–Nd isotopic analyses.

2 Geological setting

2.1 Regional geological setting

The study area is located in Guangxi Zhuang Autonomous Region,southern China.Being part of the South China Caledonian fold system,geotectonically,it sits in the south sub-segment of the E-W-trending tectonic belt of the Nanling Mountains.The study area belongs to the Daoyaoshan uplift which consists of the Cambrian basement and its unconformably overlying Devonian rocks.The study area shows a number of regional faults,including Lipu Fault,Longsheng–Yongfu Fault,Dali Fault,Nandan–Kunlunguan Fault,and Linshan–Tengxian Fault(Guangxi Bureau of Geology&Mineral Prospecting 1985)(Fig.1a).

Stratified formations in the region are mostly the Cambrian and Sinian sedimentary rocks.The Cambrian strata are of a neritic flysch suite of epimetamorphic sandstone,siltstone,siliceous rocks,slates,phyllite rocks,and carbonaceous shale,constituting the basement of the Caledonian fold system and being distributed widely in the region.Gold deposits are mostly hosted by rocks of this Cambrian suite in the region.The Sinian strata are made of slightly metamorphosed coarse-to medium-grained sandstone,fine sandstone,and siltstone,with interlayers of shale,siliceous rocks,and siliceous shale.

A number of Early Paleozoic,Jurassic,and Cretaceous stocks and dikes of granitoids occur in the Dayaoshan uplift region.They are generally granodiorite porphyry,masanophyre,and porphyritic granodiorite.The region has experienced intensive tectonic and magmatic activities,which can be divided into five stages:(1)Early Paleozoic,(2)Late Paleozoic-Triassic,(3)Jurassic,(4)Cretaceous,and(5)Cenozoic,with the Early Paleozoic,Jurassic and Cretaceous dominated.Commercial ore deposits most occur in intersections of plutons and tectono-fracture zones(Sheng 2005).

2.2 Geology of the Xinping gold deposit

Fig.1 Spatial-temporal distribution of granitoids in the Dayaoshan belt and geological maps of the Dayaoshan uplift(a)(after Chen and Jahn,1998;Chen et al.2015)and the Xinping gold deposit(b)(Zhou et al.2016).1.Quaternary;2.Lower Devonian Lianhuashan Formation;3.Cambrian Huangdongkou Formation—middle segment of the 4th Member;4.Cambrian Huangdongkou Formation—upper segment of the 4th Member;5.Cambrian Huangdongkou Formation—upper segment of the 3rd Member;6.Cambrian—undifferentiated;7.Sinian Peidi Formation;8.Yanshanian magmatic rocks;9.Haecian-Indosinian magmatic rocks;10.Caledonian granite;11.Granite porphyry;12.Gold veins;13.Quartz veins;14.Geological boundaries;15.Faults;16.Location of the study area

The Xinping gold deposit is located in the south flank of a sub-order anticline of the Changdong–Mabei anticline in the Dayaoshan uplift.The uplift itself is a regional anticlinorium.The deposit occurs in the contact area of a near east–west-striking fault and a granitic pluton.The ore district includes two locations:Daiwu and Jiguanyan.There are several gold deposits and occurrences around the ore district,for example,Liucen in the northeast,Maershan in the southeast,and Liuma in the northwest(Fig.1b).Stratified rocks in the ore districtare mainly the third and fourth members of the Cambrian Huangdongkou Formation.Ore bodies mostly occur in epimetamorphic sandstone and sandy shale in the central and lower segments of the fourth member and the upper segment of the third member of the Huangdongkou Formation.There exist several NE–SW-trending and locally E–W-trending folds and several near E–W-striking and S–N-striking(some are NE-striking)faults.The faults form ore-hosting structures(Fig.1b).

Igneous rocks exposed in the area are generally granodiorite porphyry,masanophyre,porphyritic granodiorite,and crypto-explosive breccia.The granodiorite porphyrypresents as groups of dikes that are several decameters to more than 1000 meters long and 10–100 meters wide.Hydrothermal alterations such as greisenization,chloritization,epidotization,sericitization,and pyritization are intensive with signs of Au mineralization in individual porphyry dikes.Microscopic observation of phenocrysts shows(Fig.2)idiomorphic plagioclase grains of 0.1–3.5 mm long and 0.1–1.5 mm wide unevenly replaced by sericites.Some contain small amounts of zoisite and muscovite that are shown as partly visible fuzzy polysynthetic twins or pseudomorphs.Idiomorphic quartz grains are sized at 0.1–2.5 mm with some showing melted embayment.Scaly biotite grains of 0.1–1.5 mm long are replaced by chlorites,a small number of epidotes,and a minute quantity of muscovite,with precipitates of magnetite and needle-like rutile,and presented as pseudomorphs.A few of idiomorphic orthoclase grains of 0.2–0.5 mm long are present with slight kaolinization and sericitization.Automorphic phosphorite of 0.1–0.3 mm long is also spotted in the phenocrysts.The matrix,made of mostly felsic minerals and a small quantity of biotite,contains cryptocrystalline texture,and shares the same secondary alterations with that of the phenocrysts.The rocks are slightly pyritized.Small amounts of pyrite and quartz are unevenly filling up metasomatic rocks.

There are two gold-bearing fractured alteration zones of 500–2600 m long and 50–700 m wide in the area,the No.I and No.II zones.Gold deposits are generally concealed in the zones.The area has a number of ore bodies,namely I-1,I-2,I-3 I-4,and II-2.The highest grade of the gold ore body is 5.18 g/t,and the average grade is 2.92 g/t.Ore texture is mainly of idiomorphic and hypautomorphic granular crystalloblast,xenoblast granular crystalloblast,microscaly crsytalloblast,and fractured pattern.Ore structure can be described as a disseminated,stock work,and cellular.Metallic minerals are dominated by pyrite and followed by chalcopyrite,arsenopyrite,and small amounts of galena,and sphalerite.Wall-rock alterations include silicification,pyritization,arsenopyritization,and sericitization.Among them,the first three are considered to closely relate to the gold deposits in the area.

2.3 Sampling and analytical methods

Fig.2 Photomicrographs of granodiorite porphyry(a,b),weakly-altered granodioriteporphyry(c),and slightly pyritized and altered granodiorite porphyry(d)Qquartz;Pl-plagioclase;Ororthoclase;Bt-biotite.Planepolarized light for(a)and(b)and cross-polarized light for(c)and(d)

All granodiorite porphyry samples used in the study were taken from drill holes.Based on the distribution of the drill holes and previous thin-section study,we selected 20 samples of non-weathering and fresh granodiorite porphyry from drill hole ZK13201(Fig.1b)to perform a lithogeochemical and geochronological study.Zircon selection was completed by the Langfang Integrity Geological Service Company of Hebei Province,China.The selecting process started with manual shattering and heavy sand and electromagnetic separation and ended with a binocular microscopic selection.Eventually,zircon grains with highdegree purity and perfect crystal form without obvious cracks or inclusions were selected.The samples were sent to the State Key Laboratory of Isotope Geochemistry at Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,for micro-imaging and U–Pb analysis.The ICP–MS(inductively coupled plasma mass spectrometry)used in the study was model Agilent7500a.A Resolution M50 DUV 193 nm ArFexcimer laser ablation system manufactured by Resonetics(an American company)was also employed with a laser beam diameter and frequency setting each at 31μm at 8 Hz.The TEMORA zircon from Australia was used as external calibration.Isotopic data processing was carried out with ICPMS Datacal 8.7(Liu et al.2010).Age calculation was performed with Isoplot 3.00(Ludwig 2003).For a detailed analysis method and procedure as well as the common Pb correction method,please refer to the references(Andersen 2002;Yuan et al.2003).The Th/U values were achieved by using NIST 610 as the external standard and Si as the internal standard.Dating error based on the isotope ratios was 1σ.

Major and trace elemental analyses of the samples were respectively performed by using an X-ray fluorescence spectrometry and an ICP-MS in an ALS Chemex(Guangzhou).The pretreatment of Sr–Nd isotopes was finished in the Super-clean Laboratory of Orogenic Belts and Crustal Evolution(Ministry of Education),Beijing University.A Triton mass spectrometer in Tianjin Geological Mineral Analysis Center was also resorted to testing the samples.For detailed experimental procedure and analysis methods,please refer to the references(Chen et al.2013;Chen and Jahn 2004;Yuan et al.2018).

3 Results

3.1 Zircon U-Pb geochronology

CL images(Fig.3)show long prismatic form or hexagonal bipyramids of zircon crystals of perfect shape.They are generally 80–200μm long and have a length–width ratio of 1.5:1–3:1.The Th/U ratios indicate a typical magmatic zircon with the values ranging between 0.17 and 1.19(Table 1)and averaged at 0.4,substantially large than that of metamorphic zircons(＜0.1).The zircon grains show well-developed oscillatory growth zones,confirming their magmatic origin.

A total of 25 zircons(analysis points)were analyzed during the study.Table 1 lists the analyzing results.Concordia plots(Fig.3)show that,among the 25 analyses points,16 of them yield a weighted mean206Pb/238U age of 442.7±5.8 Ma with MSWD of 0.01,and that the other 9 analyses points are scattering around with an obvious older average age,indicating inherited zircons from source rocks or wall rocks.Zircon U–Pb geochronology analyses reveal that the granodiorite porphyry was crystallized at 442.7±5.8 Ma.

3.2 Major elements

The results(Table 2)show that the granodiorite porphyry has high contents of SiO2(67.8–71.8 %with an average of 70 %)and belongs to acidic rock type.The granodiorite porphyry has TiO2of 0.180.28 %and K2O of 1.78–3.42 %(averaged at 2.98 %).In the K2O–SiO2diagram(Fig.4a),the granodiorite porphyry falls in the high-K calc-alkalinealkaline series.Its K2O+Na2O(total alkali)is between 6.02% and 7.46%,Na2O between 3.23 % and 42 %(averaged at 3.92 %),and Al2O3between 14.34 % and 14.90 %(averaged at 14.66 %).The aluminum content is relatively high with the aluminum saturation index changing between 0.97 and 1.06(averaged at 1.03).In the A/CNK—A/NK diagram(Fig.4b),most samples are plotted in the peraluminous granite domain,with only a few within the metaluminous granite domain.Other measurements include MgO between 0.58 % and 1.03 %,(Mg#)[Mg#=100×Mg2+/(Mg2++0.898×Fe2O3T)]between 25 and 32,Fe2O3Tbetween 1.69 % and 2.73 %,TiO2and P2O5contents 0.18 %–0.28 % and 0.05 %–0.09 %,respectively.In the Harker diagrams(Fig.5),a positive correlation between Na2O,K2O,and SiO2,and a negative correlation between TiO2,MgO,P2O5,and SiO2were observed.

3.3 Trace elements

Trace elements(REE)analysis results of the granodiorite porphyry samples are listed in Table 2.The results show that the total REE contents(∑REE)are between 68.11 ppm and 105.33 ppm and LREE/HREE ratios are between 6.9 and 9.0,indicating LREE are relatively enriched to HREE,which reflects obvious LREE fractionation.The(La/Yb)Nvalues are between 10.5 and 14.9 andδEu values change between 0.2 and 0.26.The chondrite-normalized diagram(Fig.6a)of REEs shows a right-leaning distribution pattern with LREE enrichment and apparent negative Eu anomalies.

The spider diagram of trace elements(Fig.6b)of the samples exhibits that the granodiorite porphyry is rich in large ion lithophile elements Rb(87.2–153.5 ppm)and Sr(206–420 ppm),and compatible elements such as Cr(10–20 ppm)and V(10–29 ppm),while depleted in Ba(385–603 ppm)and in high field-strength element Nb(11.3–18.2 ppm).U,Hf,and Gd are relatively enriched,whereas Pr and Nd are relatively depleted.The Nb/Tb ratios are between 6.8 and 8.5 and averaged at 7.7,obviously lower than that of the primitive mantle,which has an average Nb/Ta ratio of 17.4.

Fig.3 CL images of zircon crystals and U–Pbconcordia plots

3.4 Sr-Nd isotope analysis

Sr–Nd isotopic analysis results of the Xinping granodiorite porphyry samples are shown in Table 3.The87Sr/86Sr and143Nd/144Nd values range from 0.7157 to 0.7211 and from 0.5122 to 0.5124,respectively.A calculation based on an emplacement age of 442.7 Ma of the samples yields the initial ISrvalues(87Sr/86Sr ratios)between 0.7080 and 0.7104 andεNd(t)values varying from-0.08 to-4.09.A further two-stage model calculation shows that the Nd isotopic model age(t2DM)is 1.19–1.51 Ga.

4 Discussion

4.1 Age and petrogenesis of the granitoids in the study area

Existing geochronological data(Table 4 and Fig.1c)show that the granitoids in Dayaoshan uplift were mostly emplaced in the Early Paleozoic,Jurassic,and Cretaceous,with only a few of them Late Paleozoic-Triassic.The EarlyPaleozoic granitoids can be divided,from north to south,into the north belt(Dajin and Lingzu plutons),the central belt(Liucen–Taohua–Gupao plutons)and the south belt(Gulong–Daoshui–Xiaying plutons).The zircon U–Pb geochronological analysis performed in this study suggests that the crystallizing age of the Xinping granodiorite porphyry is 442.7±5.8 Ma.A porphyry dike in nearby Liucen gold deposit has a whole-rock K–Ar age of 403.8 Ma(Qian 1998).Granite porphyry at Wandao deposit of Gupao ore field in the central belt was dated at 436 Ma(Xiao et al.2015).The ages show that the Xinping granite porphyry is emplaced during the Llandovery Silurian and it is part of the Liucen–Taohua–Gupaogranitoids belt.

Table 1 LA-ICP-MS U–Pb data for zircons of Xinping granodiorite porphyry

The P2O5content is negatively correlated to SiO2content in the granodiorite porphyry samples(Fig.5),which is characteristic of I-type granitoids.According to Wolf and London(1994),P-bearing apatite has low solubility in quasi-aluminous and weak peraluminous magmas(A/CNK＜1.1)and can be easily oversaturated and separated from the magmas,resulting in decreased content of P2O5with increasing content of SiO2.S-type granite rocks generally have a higher content of P2O5and show a positive relationship with A/CNKC values(Chappell 1999).The geochemical characteristics of the Xinping granodiorite porphyry as described above indicate that it is a typical I-type granitoid.Trace elemental analyses indicate that the samples are rich in large ion lithophile elements like Rb and Sr,and compatible elements of Cr and V,and depleted in element Ba and high field-strength element Nb with low Nb/Ta ratios(averaged at 7.7).These geochemical characteristics exclude the granodiorite porphyry to beAtype granite(Wu et al.2003).The granodiorite porphyry contains a total REEs(∑REE)ranging from 68.11 to 105.33 ppm,substantially lower than the transformationtype granitoids in South China(Liu 1985).The REE distribution pattern shows enriched light REEs relative to heavy REEs and aprominent negative Eu anomaly(δEu=0.22),indicating possible relict plagioclase or fractionation of plagioclase from primary magma duringevolution—corresponding well to Ba depletion.The(La/Yb)Nvalues are between 10.5 and 14.9(averaged at 12.6),showing a moderate fractionation and strong separation and crystallization.Samples from the surrounding areas,including granodiorite porphyry in Dawangding,and granite porphyries in Longtoushan gold deposit(Fig.7a),were also found to share the similar REE pattern(Chu 2013;Hu et al.2012;Ye et al.2015;Qian et al.2019)and negative Eu anomaly with that of other syn-tectonic granites in South China(Chen 2002),indicating a similar magma evolution history and involvement of a similar magma source.

Table 2 Analytical results of major elements(wt %)and trace elements(ppm)of Xinping granodiorite porphyry

Table 2 continued

Fig.4 SiO2–K2O diagram(a);A/CNK-A/NK diagram(b)(after Zhou and Li 2006)

Fig.5 Harker diagrams of major elements vs SiO2 of Xinping granodiorite porphyry

Fig.6 a Primitive mantle normalized spider diagram and b Chondrite-normalized REE patterns(after Zhou and Li 2006)

Table 3 Analytical results of Sr–Nd isotopics of Xinping granodiorite porphyry

The Xinping granodiorite porphyry has an initial87Sr/86Srratios(ISr)ranging between 0.7080 and 0.7104,similar to the granite porphyries(0.7092–0.7109)at Tianpingshan(Duan et al.2011).Granite porphyry at Wandao has a higher ISrrange(0.7146–0.7336)(Xiao 2014),between the values for the lower crust(0.702–0.705)and the upper crust(＞0.72)(Rollinson 1993).TheεNd(t)values vary greatly from-7.39 to 3.7(Fig.7b),indicating a source dominated by ancient crustal materials.A two-stage Nd model age calculation yieldedt2DMvalues ranging between 0.93 and 1.95 Ga.The Xinping granodiorite porphyry has at2DMof 1.19–1.51 Ga,showing a possible Meso-Paleoproterozoic basement source.Combined with Sr–Nd isotopic data of the Cathaysia craton and adjacent areas by Gilder et al.(1996)and Chen and Jahn(1998),a two-stage Nd model age of 1.84 Ga was suggested for most of the magmatic rocks in the study area.As for the near northeast-trending low-value zone(1.5 Ga)with an initial Sr isotope ratio range of 0.704–0.730 in the area(Fig.1b),we suggest that it caused by involvement of mantle-derivedmagmas at certain degrees.In a crust-mantle Sr isotopic evolutional curve(Fig.8),most samples plot above the continental crustal growth line.However,the granodiorite porphyry samples from Xinping and granite porphyry samples from Tianpingshan fall in the field between the growth line and the‘‘basalt source area’’,indicating that their genetic relationship to the crust and mixed crustmantle material.It is suggested that the Xinping granodiorite porphyry was derived from the Paleoproterozoic Cathaysian basement rocks and the basement would contain pre-existing mantle-derived materials.The latter was likely involved in the formation of the magmas that were derived from the basement.

Table 4 Ages of granitoids in Dayaoshan uplift

Fig.7 Chondrite-normalized REE patterns(a)andεNd(t)versus(87Sr/86Sr)i diagram for comparison(b)(after Carlson 1995)

Fig.8 Strontium isotope evolution in the crust and mantle(after Faure and Powell 1972)

4.2 Magmatic process and mineralization

Multiple lithospheric extension and crust-mantle interaction have shaped the lithosphere in South China and induced several stages of granitic magmatic emplacement(Chen et al.2002;Hong et al.1998).

As a result,the Dayaoshan terrane has successively undergone the Caledonian,Indosinian,and Yanshanian tectonic–magmatic activities that produced a number of mineral deposits of various types of different ages.The predominant Caledonian compression and lower-greenschist-facies metamorphism resulted in E-W-trending tight and linear folds and the Dali deep fault.The subsequent Indosinian orogeny during the Late Middle Triassic period generateda number of S–N-trending refolds.The Yanshanian orogeny was dominated by faulting and produced several graben basins(Shi et al.1993;Yin et al.1999).

Recently,some deposits have been found in the Dayaoshan uplift and vicinity,including The porphyry-type Cu-Mo polymetallic deposit in the Yuanzhuding of Guangdong Province and porphyry-skarn-quartz-vein-type W-Mo polymetallic deposit and other types of ore bodies in the Shedong of Cangwu County of Guangxi(Chen et al.2012a,b;Zhong et al.2000).The W–Mo deposits in the Shedong were dated at 437.8 Ma(molybdenite Re–Os method)and its hosting granodiorite or granite porphyry at 432.0–435.8 Ma(Chen et al.2011).A molybdenum ore occurrence in the nearby Wujie was dated at 438.4 Ma(molybdenite Re–Os method)and its hosting granodiorite at 434.5 Ma(Han et al.2012).Gold ore bodies may crosscut through granite porphyries and enter into the Cambrian wall-rock of sandstone(Fig.9a)(Zhou et al.2016).For example,in the Wandao gold deposit,a 300-meter-long gold-bearing vein occurs within the Cambrian wall rock and the granite porphyry as observed in the No.30 shaft of the 66-meter-level gallery(Fig.9b)(Xiao et al.2015).Cai et al.(2000)dated fluid inclusions of the Taohua gold deposit using the Rb–Sr method and reported age of 148±10 Ma of the Jurassic.Zhao et al.(2017)concluded that there exists no metallogenic relationship between the porphyry dikes and the gold metallization after a 3D geological model construction and ore-controlling factor analyses of the Daiwu deposit in Xinping area.All the field observations and geochronological analyses support the speculation that the Early Paleozoic granite porphyries have a close metallogenic relationship with W-Mo metallization but not with the gold mineralization in the region.

Fig.9 Simplified geological maps showing ore bodies,intrusions,and wall-rocks at Xinping deposit(a)(after Zhou et al.2016)and Wandao deposit(b)(after Xiao et al.2015)

4.3 Tectonic setting of the granitoids

The Caledonian intracontinental orogeny in the South China continent during the Early Paleozoic induced a near south-north collision between the Yunkai Block and the Guangxi Yunnan–North Vietnam Block,and an afterward uplifting of the Dayaoshan terrane in the suturing area of the two blocks.The Dayaoshan anticlinorium was probably formed and accompanied by a large scale of magmatism(Faure et al.2009;Hao et al.2010;Hu et al.2012;Li 2013).In Y-Nb and Y+Nb-Rb discrimination diagrams(Fig.10)the Xinping samples plot in volcanic arc-syncollision granite domain and/or volcanic arc granite domain—same as the samples from Dawangding,suggesting a similar magmatic evolution process.Putting the geotectonic setting of the Dayaoshan area into a whole jigsaw puzzle,we could conclude that the Xinping granodiorite porphyry is a product of magmatism resulted from collision and compression during the early stage of the Caledonian intracontinental orogeny(Hao 2010;Wu 2000).In the R1–R2discrimination diagram for the tectonic setting(Fig.11),the Xinping samples are mainly distributed in the syn-collisional granite domain and the plate pre-collision domain.During the collision,from Late Cambrian to Early Ordovician,the Yunkai Block collided with the Guangxi Yunnan—North Vietnam Block from south to north and then migrated northward until the collided blocks ran into the Yangtze Block.

Fig.11 R1–R2 discrimination diagram showing tectonic setting(after Batchelor and Bowden 1985).①-Mantle fractionates;②Pre-plate collision;③Post collision uplift;④Late-orogenic;⑤Anorogenic;⑥Syn-collision;⑦Post orogenic

5 Conclusions

Fig.10 Y-Nb diagram(a)&Y+Nb–Rb diagram(b)(after Zhou and Li 2006)

(1)LA-ICP-MS U–Pb zircon dating of the Xinping granodiorite porphyry yields a new emplacement age of 442.7±5.8 Ma and indicates that the granodiorite porphyry was emplaced in the Llandovery Silurian and occurs as part of the Early Paleozoic Liucen–Taohua–Gupao granitic series.

(2)The Xinping granodiorite porphyry is metaluminous–peraluminous and high-K calc-alkaline.It is an I-type granite enriched in large ion lithophile elements Rb and Sr,while depleted in Ba and high field-strength element Nb.

(3)Based on its initial87Sr/86Sr ratios(0.7080–0.7104)andεNd(t)values(-0.08 to-4.09),the Xinping granodiorite porphyry was probably sourced from the Cathaysia basement rocks of the Paleoproterozoic and was affected by mantle material trapped within the basement;the mantle material probably resulted from an early mantle under plating event.Based on the tectonic setting of the Dayaoshan uplift and tectonic discrimination diagrams,it is suggested that the Xinping granodiorite porphyry was emplaced during the orogenic collision between the Yunkai Block and the Guangxi Yunnan–North Vietnam Block.

(4)The Early Paleozoic Xinping granodiorite porphyry shows no metallogenic and temporal relationship to the Xinping gold deposit.The latter is part of the Yanshanian Liucen–Taohua–Gupao gold deposit zone.However,the Early Paleozoic granite porphyries have a close spatial and temporal relationship with W–Mo metallization in the region.Any future prospecting shall target at unexposed Early Paleozoic granitoids for W-Mo deposits.

AcknowledgementsThis study is supported by the National Key R&D Program of China(2016YFC0600603),the Guangxi Science Foundation(2014GXNSFBA118230),and the Foundation of Guilin University of Technology(GUTQDJJ2019166).We thank Prof.Guoxiang Chi for his constructive reviews and useful suggestions.