U-Pb dating and geochemistry of granite porphyry dykes in the Xicha gold-(silver) deposit, southern Jilin Province, China,and their metallogenic significance

Jian Wang · Fengyue Sun · Bile Li · Lihui Tian

Abstract We report U-Pb dating of zircon, as well as geochemical and Hf isotope data, in order to constrain the formation time, magma source, and tectonic setting of granite porphyry dykes in the Xicha gold-(silver) district in southern Jilin Province, Northeast China. The zircon grains are euhedral-subhedral, display oscillatory growth zoning and have Th/U ratios varying between 0.11 and 0.78, which together imply a magmatic origin. The dating results indicate the porphyry formed in the Early Cretaceous (122 ± 1 Ma),and it contains SiO2 = 70.64-72.31 wt%, Al2O3-= 13.99-14.64 wt%, K2O + Na2O = 6.96-7.81 wt%,K2O/Na2O = 1.24-2.10, and A/CNK = 1.11-1.41. Chemically, the porphyry belongs to a high-K calc-alkaline S-type granite. Chondrite-normalized rare earth elements (REE)patterns show LREE enrichment, light rare earth elements(LREE)/heavy rare earth elements (HREE) = 9.93-11.97,(La/Yb)N = 11.08-15.16, and δEu = 0.69-0.95. On the trace element spider diagram, large ion lithophile elements such as Rb, Ba, K, Th, and U are enriched, whereas the high field strength elements Ti and P are depleted. The εHf(t) values of zircon from the granite porphyry vary between - 17.1 and - 13.2, and their Hf two-stage model ages vary from 2.01 to 2.26 Ga, implying that the magma was derived from partial melting of old lower crust. The granite porphyry dykes and many A-type granites in the region formed at the same time, suggesting an extensional environment. The combination of the occurrence of strong magmatism, large-scale mineralization, and extensional tectonics throughout much of Eastern China indicate that the Early Cretaceous was a period of significant lithospheric thinning. The southern Jilin Province, therefore, experienced lithospheric thinning during the Early Cretaceous.

Keywords Southern Jilin Province · Syn-mineralization dykes · Jinchanggou · Zircon U-Pb geochronology ·Geochemistry · Hf isotopes

1 Introduction

The study area is situated near the northeast margin of the North China Craton, which was affected by the closure of the Paleo-Asian Ocean toward the north (Sun et al. 2005;Deng et al. 2009; Guo et al. 2009; Li et al. 2009a, b) and the subduction of the Pacific Plate in the Early to Middle Jurassic. As a result of subduction and collision from multiple directions, the North China Craton became activated and experienced large-scale magmatism (Zhai et al.2003; Wu et al. 2005a) and mineralization (Mao et al.2003; Yang et al. 2003; Mao et al. 2005) during the Mesozoic. However, the tectonic setting and temporalspatial range of this activity have yet to be studied in detail.Intense magmatism and mineralization also occurred during the Mesozoic in southern Jilin Province, Northeast China. However, compared with the Jiaodong area, studies from the Jilin Province are lacking; consequently, a better understanding of the timing and geodynamics of metallogenesis is needed. Typical hydrothermal type gold deposits in southern Jilin Province include the Jiapigou gold deposit, the Haigou gold deposit, the Erdaodianzi gold deposit, and the Xicha gold-(silver) deposit. The Jiapigou gold deposit has been extensively studied and was formed at 170-160 Ma (Luo et al. 2002; Li et al. 2003, 2004). But,the rest of the gold deposits have not been studied in detail.The Xicha gold-(silver) deposit, located in Ji’an City,Tonghua, southern Jilin Province, is a medium-sized (about 5.3 t Au) hydrothermal vein-type gold-(silver) deposit that was discovered in the 1960s. Previous studies have considered various aspects of the Xicha deposit, including its geological characteristics and metallogenic conditions(Feng 2000). However, the age of mineralization has yet to be resolved. Feng (2000) suggested that the deposit formed in the Yanshanian, a relatively large time span, which constrains the timing of metallogenesis, the geological setting, and the geodynamic setting of mineralization.Therefore, we have studied the geochronology and geochemistry of granite porphyry dikes that are closely related to ore-forming veins in the Xicha gold-(silver) deposit.The results are combined with previously published data to constrain the age of mineralization and to provide a geochronological framework and information on the tectonic setting of mineralization.

2 Geological setting and sample descriptions

The study area is located near the northeast margin of the North China Craton (Fig. 1a). The area is adjacent to the Xingmeng orogenic belt to the north (across the Xilamulun River-Changchun-Yanji suture), the Korean Peninsula to the east, and the Liaodong Peninsula to the south, while the Dunmi Fault (the northern extension of the Tanlu fracture)and the Yalujiang Fault cut through the northwest and southeast parts of the area, respectively. Crystalline basement in the region is mainly composed of Archean tonalite trondhjemite granodiorite gneisses, the supracrustal rocks of the Paleoproterozoic Ji’an groups, and the Paleoproterozoic granites (Lu et al. 2004). The Ji’an groups consist of marble, leptynite, mica schist, amphibolite, plagioclase amphibolite, quartzite, and quartz schist. The cover rocks consist of thick Sinian-Paleozoic sedimentary strata and Mesozoic-Cenozoic volcano-sedimentary strata. In addition, Mesozoic felsic intrusive rocks occur in the region.Zircon U-Pb age data indicate that magmatic activity occurred primarily in the Late Triassic, Early-Middle Jurassic, and Early Cretaceous (Lu et al. 2003; Pei et al.2005; Sun et al. 2005; Qin et al. 2012).

Paleoproterozoic strata are exposed in the Jinchanggou mine area (Fig. 1b); the main lithologies consist of leptynite, graphite marble, plagioclase amphibolite, leucoleptite, and olivine marble. Notably, boron is enriched in the olive marble. Structural features are strongly developed in the region. The Xiaomenggou-Sidaoyangcha anticline is the main NW-SE-trending fold and NE-SW-trending faults are the main structures that cut early magmatic rocks and folds. The F7 fault (Fig. 1b), which is the major fault in the region, strikes 15°-30° and dips at 80° to the SE. The Xicha gold-(silver) deposit is located along the F7 fault,which controlled the location of mineralization. The intrusive rocks in the region are Triassic biotite diorite,syenite granite, and granitic porphyry. The biotite diorite intrudes Paleoproterozoic strata in the core of the Xiamenggou-Sidaoyangcha anticline, with an exposed area of 16 km2, and is a composite pluton displaying features of multi-stage intrusion. Zircon from biotite diorite yields a U-Pb age of 238 ± 1 Ma (Wang et al. 2016), and zircon from biotite diorite outcropping near the Jinchanggou deposit area yield a U-Pb age of 221 ± 1 Ma (Wang et al.2016). There are many types of dikes in the mine area,including granite porphyry, diorite, and diabase. The granite porphyry dykes and gold-bearing veins both occur along the F7 fault and they are closely related to each other,both spatially and temporally. Gold enrichment occurs where granite porphyry dykes are most commonly developed and these dykes show varying degrees of mineralization and alteration. The granite porphyry dykes should,therefore, be considered as syn-mineralization dykes because they have a close genetic relationship with the gold-bearing veins.

The Jinchanggou ore area is composed of the Jinchanggou gold deposit and the Xicha gold-(silver) deposit.The Xicha mineralization was controlled by the F7 fault and parallel secondary faults located on either side of the F7 fault. The ore body occurs as veins, both lenticular and branching, and these commonly show pinch-and-swell texture (Fig. 2a, b). The ore body varies in thickness between 0.33 and 7.3 m (average thickness, 2.1 m). The average grade for the ore body is 2.01 g/t Au and 17.58 g/t Ag. Ore bodies can be divided into gold-silver ore bodies and lead-silver ore bodies. To date, more than 10 ore bodies have been found, including 6 gold-silver ore bodies,4 lead-silver ore bodies, and numerous other mineralized bodies. The ore is altered-rock type (Fig. 2a, b). The mineralization in the deposit is present as disseminated,veinlet, veined, and brecciated ore. The ore mineralogy consists of pyrite and arsenopyrite (Fig. 2c, d), with minor chalcopyrite, galena, sphalerite, native gold, electrum,native silver, argentite, and pyrargyrite. The gangue consists of quartz, sericite, and calcite. Wall-rock alteration is dominated by silicification (Fig. 2e, f), sericitization(Fig. 2e, g), and carbonatization (Fig. 2f), with minor baritization. According to the mineral assemblage, mineralization of the Xicha deposit can be divided into stages of hydrothermal mineralization and supergene mineralization.

Fig. 1 a Tectonic sketch map of NE China, modified after Wu et al. (2011). (1) Xiguitu-Tayuan Fault; (2) Hegenshan-Heihe Fault; (3)Solonker-Xra Moron-ChangchunFault; (4) Mudanjiang Fault; (5) Yitong-Yilan Fault; (6) Dunhua-Mishan Fault. b The geological map of Xicha mining area, modified after Guan (2005)2

Fig. 2 Photo(micro)graphs of ore body, mineral, alteration and granite pouphyry dykes. a, b Spatial relationships of ore body and granite porphyry dykes; c disseminated arsenopyrite; d disseminated arsenopyrite and pyrite; e. quartz and sericite alteration; f sericite and carbonate alteration; g sericitization granite porphyry; h feldspar polysynthetic twins dissolution in phenocrysts; i quartz dissolution in phenocrysts

The granite porphyry dykes in this study are located along the F7 fault. There is a close spatial relationship between the granite porphyry dykes and gold-bearing veins. The dykes near the ore body are strongly altered and mineralized (Fig. 2g), whereas some dykes located far from the ore body are fresh (Fig. 2i). The dykes and the ore body were possibly coeval or the ore body is slightly younger than the dykes. Samples were collected from the area (41° 23′02′′N, 125° 48′17′′E) around the mineral deposit for the measurement of zircon U-Pb ages (sample XC-3) and whole-rock geochemical analyses. Samples are fresh or weakly altered, gray in color, and are porphyritic.They also show microscopic crystal structure, felsitic structure in the matrix. Quartz phenocrysts (20% of the total phenocrysts) are anhedral and granular, show weak wavy extinction, and are 0.5-1 mm in size. Plagioclase phenocrysts (80%) are euhedral to subhedral, have polysynthetic twinning, and are 0.2-1 mm in size. The plagioclase has a composition of An = 15-20 according to the Np’ ^ (010) maximum extinction method, indicating oligoclase. The dikes show evidence of weak carbonate alteration. Previously, the dykes were described as albite porphyry (Feng 2000). In the present study, the porphyry has plagioclase and quartz phenocrysts (Fig. 2a, b) and the content of SiO2is more than 65 wt%. Therefore, the dykes are defined as granite porphyry.

3 Analytical methods

3.1 Zircon U-Pb dating

Sample XC-N3 was chosen for zircon U-Pb dating of the Xicha granite porphyry. The sample was crushed before zircon were separated using conventional magnetic and heavy liquid methods, with final handpicking of zircon under a binocular microscope for analysis at the Regional Geology Survey, Langfang, Hebei, China. The zircon was mounted in epoxy before being polished to reveal crosssections for analysis. Prior to analysis, the zircon was imaged under transmitted light and using cathodoluminescence (CL) to reveal internal structures. LA-ICP-MS analysis was undertaken at the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an,China. Helium was used as carrier gas to provide efficient aerosol transport to the ICP and minimize aerosol deposition around the ablation site and within the transport tube(Eggins et al. 1998; Jackson et al. 2004). Argon was used as the make-up gas and was mixed with the carrier gas via a T-connector before entering the ICP. The analysis spots were 32 μm in diameter. U, Th and Pb concentrations were calibrated using29Si as an internal standard. All measurements were undertaken using an external zircon 91,500 standard with a recommended206Pb/238U age of 1065.4 ± 0.6 Ma (Wiedenbeck et al. 2004). Analytical procedures used follow those described by Yuan et al.(2004). Raw data were processed using the GLITTER program. Uncertainties of individual analyses are reported with 1σ error; weighted mean ages were calculated at 1σ confidence level. The data were processed using the ISOPLOT (Version 3.0) program (Ludwig 2003).

3.2 Lu-Hf isotope analysis

In-situ zircon Hf isotope analysis was carried out using a NewWave UP213 laser-ablation microprobe, attached to a Neptune multi-collector ICP-MS at the Institute of Mineral Resources, Chinese Academy of Geological Sciences.Instrumental conditions and data acquisition techniques have been comprehensively described by Wu et al. (2006b)and Hou et al. (2007). Lu-Hf isotopic measurements were made on the same zircon grains previously analyzed for UPb isotopes, with ablation pits 44 μm in diameter, repetition rates of 8-10 Hz, laser beam energy density of 10 J/cm2, and an ablation time of 26 s. The analytical procedures were similar to those described in detail by Hou et al.(2007). In order to correct the isobaric interferences of176Lu and176Yb on176Hf, the ratios for176Lu/175Lu(0.02658) and176Yb/173Yb (0.796218) were determined(Chu et al. 2002). For instrumental mass bias correction,the Yb isotope ratios were normalized to172Yb/173Yb = 1.35274 (Chu et al. 2002) and Hf isotope ratios to179Hf/177Hf = 0.7325, using an exponential law.Zircon GJ1 was used as the reference standard during our routine analyses, with a weighted mean176Hf/177Hf ratio of 0.282011 ± 24 (2σ, n = 10). This figure is indistinguishable from the weighted mean176Hf/177Hf ratio of 0.282013 ± 19 (2σ) reported for in situ analysis by Elhlou et al. (2006).

3.3 Whole-rock major and trace element geochemistry

Whole-rock major and trace element concentrations were determined at the Analytical Laboratory of Beijing Research Institute of Uranium Geology. All samples were crushed in a corundum jaw crusher (to 60 mesh) after the removal of weathered surfaces. Then, they were powered in an agate ring mill to less than 200 mesh. Major element concentrations were determined using X-ray fluorescence(XRF) and a PANalytical Axios XRF instrument. Trace element compositions were determined on solutions obtained by sealing and dissolving samples using HF and HNO3acids, before conversion into 1 wt% HNO3media and addition of an Rh internal standard solution. These solutions were analyzed using inductively coupled plasmamass spectrometry (ICP-MS) and a PEElan 6000 instrument. The results of international standard BCR-2(basalt),BHVO-1(basalt) and zAGV-1(andesite) show that analytical precision values better than 5 wt% for major elements and better than 10 wt% for trace elements. The detailed sample-digesting procedure for ICP-MS analyses and analytical precision and accuracy for trace elements are the same as description by Liu et al. (2008).

4 Results

4.1 Zircon U-Pb dating

CL images of zircon separated from granite porphyry indicate that they are euhedral to subhedral, are elongate or granular, and contain minor fractures. They generally have concentric magmatic oscillatory zoning (Fig. 3). A total of 15 analyses (Table 1) yielded U concentrations ranging from 62.52 to 740.26 ppm, Th concentrations from 62.52 to 740.26 ppm, and Th/U ratios between 0.11 and 0.78,which are indicative of a magmatic origin (Li et al.2009a, b). The data plot along or close to the U-Pb Concordia line. Concorde age is 122 ± 1 Ma (MSWD = 1.3,n = 15; Fig. 4a), weighted mean age is 122 ± 1 Ma(MSWD = 0.81, n = 15; Fig. 4b) indicating that the granite porphyry was formed at 122 Ma, during the Early Cretaceous.

4.2 Zircon Lu-Hf isotopes

The zircon Lu-Hf isotopic data are presented in Table 2.The176Lu/177Hf ratios of 0.001139-0.002385 and fLu/Hfvalues of - 0.97 to - 0.93 in the zircon are lower than upper crustal values (176Lu/177Hf = 0.0093, fLu/Hf-= - 0.72; Vervoort and Patchett 1996). These zircons have176Hf/177Hf ratios between 0.282217 and 0.282331,εHf(t) values from - 17.1 to - 13.2, and t2DMmodel ages between 2.01 and 2.26 Ga.

Fig. 3 Cathodoluminescence images of analyzed zircons of Xicha granite porphyry. The small circlets, the Arabic numerals(the number same as Table 1, omission XC-N3-) and the numbers nearby the circlets respectively represent the location of U-Pb in situ erosion,the analytical sequence number and zircon U-Pb age

Table 1 Results of LA-ICP-MS zircon U-Pb dating of Xicha granite porphyry

4.3 Geochemistry

4.3.1 Major elements

The whole-rock geochemical data are given in Table 3.The samples are acidic and contain SiO2concentrations between 70.64 and 72.31 wt% (average of 71.51 wt%).They contain high Al2O3concentrations(13.99-14.64 wt%), high alkali concentrations (total K2O + Na2O between 6.96 and 7.81 wt%), and low concentrations of MgO (0.54-0.83 wt%) and CaO(0.68-1.88 wt%). Their Mg#values range from 35 to 45.The samples belong to subalkaline granite series (Fig. 5a),have K2O = 3.85-4.90 wt%, and belong to high-K calcalkaline series (Fig. 5b), have alumina saturation index (A/CNK) values range from 1.11 to 1.41 and classified to peraluminous (Fig. 5c). They have relatively high K2O concentrations (3.85-4.90 wt%) and relatively low Na2O concentrations (2.33-3.21 wt%), yielding K2O/Na2O values of 1.24-2.10. The samples contain TiO2and Fe2O3Tconcentrations of 0.18-0.22 and 1.95-2.44 wt%,respectively.

Fig. 4 U-Pb Concordia age (a) and weighted average age (b) of Xicha granite porphyry

Table 2 Zircon Hf isotopic compositions of Xicha granite porphyry

4.3.2 Trace elements

The trace element compositions of the samples are presented in Table 3. Total rare earth element (ΣREE) concentrations range between 75.02 and 94.35 ppm, with an average of 84.43 ppm. They have chondrite-normalized REE patterns that decrease to the right (Fig. 6a), and they are characterized by clear fractionation of the REEs, with LREE enrichments, LREE/HREE ratios of 9.93-11.97, and(La/Yb)Nvalues of 11.08-15.16. The samples also show weak negative Eu anomalies (Eu/Eu* = 0.69-0.95) and have low concentrations of HREEs, indicating the granite porphyry was formed by partial melting with plagioclasein-melt.

Table 3 Major (wt%) and trace element (ppm) data for the Xicha granite porphyry

Table 3 continued

Fig. 5 TAS diagram [a after Irvine and Bangmi (1971)], SiO2-K2O diagram [b after Peccerillo and Taylor (1976)] and A/CNK-A/NK diagram[c after Maniar and Piccoli (1989)] for Xicha granite porphyry

The granite porphyry samples have uniform primitivemantle-normalized trace element patterns (Fig. 6b), which demonstrate that large ion lithophile elements (LILEs) and incompatible elements (e.g., K, Rb, Sr, Th, and U) are enriched, whereas the high field strength elements (HFSEs)Ti and P are depleted. These samples have variable element ratios, such as Rb/Sr (0.46-0.87), Nb/Ta (12.44-13.81),Rb/Nb (7.91-10.06), and Nb/La (0.76-0.94).

Fig. 6 Chondrite-normalized REE patterns (a) and primitive mantle-normalized trace element patterns (b) for the Xicha granite porphyry[chondrite REE values from Boynton (1984); primitive mantle values from Sun and Mcdonough (1989)]

5 Discussion

5.1 Magma sourcing and petrogenesis

The results of major element geochemistry indicate that the granite porphyry dykes contain high concentrations of SiO2, Al2O3, and alkali elements, and low concentrations of CaO, MgO, and Fe2O3. The K2O/Na2O ratios are greater than 1 and the alumina saturation index (A/CNK) values are greater than 1.1. The average content of Na2O is less than 3.2. Corundum containing (1.66-4.51%) more than 1% is found in CIPW standard minerals. When SiO2content is 66%, CaO content is less than 3.7%. These features are all characteristic of S-type granites (Sang and Ma 2012). The Mg#value of a rock reflects whether the source magma that formed the rock was derived from crustal material alone or from crustal material that has been contaminated by mantle material (Smithies and Champion 2000). The low Mg#(35-45) values of the samples analyzed from this study do not support any interaction between the crust and the mantle materials. The enrichment in LILEs and depletion in HFSEs also reveal that these rocks formed from magmas derived from the melting of crustal material (Taylor and McLennan 1985; Hofmann 1988). The REE patterns decrease to the right, as LREEs are enriched and HREEs are depleted. This indicates the presence of residual garnet in the source of the magmas that formed the porphyry granite dykes and suggests a crustal thickness greater than that of average crust. The granite porphyry has Nb/Ta ratios of 12.44-13.81, which are consistent with the Nb/Ta ratios of magmas generated by the melting of crustal material (i.e., 11-12; Green 1995).In addition, they have Rb/Sr ratios of 0.46-0.87 and Rb/Nb ratios of 7.97-10.06, which are similar to the ratios expected for crustal material (0.32 and 4.5, respectively;Taylor and McLennan 1985). The samples have negative εHf(t) values, indicating a crustal origin, and they plot along the crustal line on a εHf(t)-t diagram (Fig. 7). The negative εHf(t) values and the t2DMmodel ages of 2.01-2.26 Ga indicate the granite porphyry formed from magmas derived from partial melting of Paleoproterozoic lower crustal material. Sylvester (1998) combined experimental studies with a statistical analysis of the components of typical orogenic granites around the world and suggested that variations in the Al2O3/TiO2ratios of strongly peraluminous granites with SiO2concentrations of 67-77 wt% are indicative of partial melting temperatures.For example, samples with Al2O3/TiO2ratios of ＜100 must have formed from magmas generated by partial melting at temperatures of ＞875 °C. The samples analyzed during this study have Al2O3/TiO2ratios range from 64.4 to 81.3, indicating The porphyry formation temperature is greater than 875 °C. Sr content is from 193 to 288,Yb content is from 0.89 to 1.04, indicating the samples belong to Himalaya-type granite (Zhang et al. 2006)formed at a pressure of 0.8-1.5 GPa corresponding depth of 45-50 km (Zhang et al. 2011).

Fig. 7 εHf-t diagram of Xicha granite porphyry [after Yang et al.(2006)]

5.2 Age of granite porphyry formation

To our knowledge, the granite porphyry dykes have not previously been dated. Results of the present study indicate that the zircon has concentric magmatic oscillatory zoning,high concentrations of U (120.38-2090.03 ppm) and Th(62.52-740.26 ppm), and high Th/U ratios of 0.11-0.78,which are all characteristics of a magmatic origin. The weighted age shows that the granite porphyry was emplaced at 122 Ma. Studies have shown that Early Cretaceous magmatic events are widespread in southern Jilin.For example, the Shanglushuiqiao granite was emplaced at 118 ± 2 Ma (Qin et al. 2012), the Chibosong gabbro was emplaced at 134 ± 7 Ma (Pei et al. 2005), the Sankeyushu Group trachyte was formed at 118 Ma (Sui and Chen 2012), and diorite dykes in this study area were emplaced at 126 ± 1 Ma. Early Cretaceous magmatic events are also widely distributed in eastern China, including the Jiaodong Peninsula, the Liaodong Peninsula, Yanshan and western Liaoning, western Shandong, and the Taihang Mountains of North China (Guo et al. 2004; Wu et al. 2005a, b), as well as in the eastern and southern Da-Hinggan Mountains in northeast China (Guo et al. 2004; Qin et al. 2012). Most of the Early Cretaceous granites in eastern China are classified as A-type granites.

5.3 Early cretaceous lithospheric thinning and mineralization in the Xicha deposit

Analyses of mantle xenoliths from basalts show that the lithosphere in Eastern China underwent a thinning of at least 100 km during the Mesozoic and that the thickness was only 80-100 km in the Cenozoic (Fan and Hooper 1989; Chi 1988). The lithosphere changed from old, lowdensity, cold, and isotopically enriched to young, highdensity, hot, and isotopically depleted (Lu et al. 1991).There are two main mechanisms of lithospheric thinning:delamination (Wu et al. 2006a) and thermal erosion (Zheng et al. 2006, 2007). To identify the timing of lithospheric thinning, we recently proposed the following indicators: (1)widespread intense magmatism; (2) a change in mantle magma from asthenospheric to lithospheric; (3) the generation of metamorphic core complexes and extensional basins; and (4) a change in the elevation of terrain from high to low (Wu et al. 2008). Wu et al. (2005b, 2006a)suggested that large-scale magmatic activity in the North China Craton began in the Middle Jurassic, in two stages at 180-155 and 135-115 Ma. During the earlier stage, magmatism was mainly distributed along the margins of North China, with none inland, and was related to subduction of the ancient Pacific Ocean plate. During the later stage,magmatism was widely distributed throughout North China, and the coexistence of different sources and depths suggests that significant lithospheric thinning occurred during the Early Cretaceous.

Metamorphic core complexes, faulted basins, detachment faults, syntectonic magmatic rocks, and extensional tectonics are widely developed in the Early Cretaceous of North China, and previous studies have indicated that these features are the surface response to lithospheric thinning.Liu et al. (2006) analyzed the Yagan and Hohhot metamorphic core complex in the north of North China, the Waziyu and south Liaoning metamorphic core complexes in the east of North China, and the Xiaoqinling and Yunmengshan core complexes in the south, and reported an average stretching lineation direction of 130°-310°, indicating that the lineations formed in the same stress field;the age of formation was 130-120 Ma. Zhu et al. (2008)studied an Early Cretaceous extensional basin in North China and proposed that lithospheric delamination was the main mechanism of lithospheric thinning. Lin et al. (2011)studied a metamorphic core complex, syntectonic granites,and low-angle detachment faults on the Liaodong Peninsula, and demonstrated that they all have an NW-SE direction at the regional scale and formed between 130 and 120 Ma. The studies above show that large-scale lithospheric thinning occurred in the Early Cretaceous in North China.

Based on the present work and previous studies, the following model is proposed for the mineralization of the Xicha gold-(silver) deposit. During the Early Cretaceous,large-scale lithospheric thinning occurred in North China.The asthenosphere rose upward, heating the lower crust prompting partial melting of various crustal components.Mineral-enriched mantle fluids and magmas derived from the melting of the various components intruded simultaneously and syntectonically. The ore-forming fluid, enriched in CO2and other volatile components, extracted Au,Ag, and other ore-forming elements from ancient basement rocks and rose along NE-SW trending faults. When the fluids reached the shallow crust, changes in temperature and pressure, as well as mixing with meteoric waters, led to the formation of the mineralized veins and the alteration of rocks and dykes.

6 Conclusions

1. Granite porphyry dykes and gold-bearing veins in the Xicha gold deposit are formed at nearly the same time during the Early Cretaceous (122 ± 1 Ma) under the same tectonic background.

2. The granite porphyry dykes formed from magmas that were derived from the partial melting of Paleoproterozoic lower-crustal material.

3. Xicha gold-(silver) deposit and granite porphyry dykes in Northeast China represent the surface response to lithospheric thinning.

Acknowledgements The current research was supported by the Undergraduate Institutions Basic Scientific Research Foundation of Heilongjiang Province (Hkdcx201805), the Natural Science Foundation of Heilongjiang Province, China (QC2017035) and the National Nature Science of Foundation of China (41272093). The authors thank Professor Liu Xiaoming of State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, China, for helping in LAICP-MS dating. The authors would like to thank ALS Chemex Co.,Ltd., Guangzhou, China, for helping in major and trace element analyses. The authors also give their thanks to Institute of Mineral Resources, Chinese Academy of Geological Sciences for assisting in the Hf isotope analyses.