Geochronology and Geochemistry of the Granite Porphyry from the Qinglingou Gold Deposit, South Qinling, China: Implication for Petrogenesis and Mineralization

: The Zhashui-Shanyang ore cluster area is one of the most important gold polymetallic ones in northwestern China. The Qinglingou gold deposit is a newly discovered small-scale deposit in the Zhashui-Shanyang area. Gold mineralization closely related to acid intrusive rocks has been found for the ﬁrst time in this area. In this paper, the geochronology, whole-rock geochemistry, and isotope signature of the granite porphyries in the deposit are studied. They are characterized by variable medium SiO 2 (67.22–71.72 wt.%), high K 2 O + Na 2 O contents (6.54–10.34 wt.%), and variable Al 2 O 3 (14.47–15.10 wt.%) values. The A/CNK ratios range from 0.90 to 1.23, and the A/NK ratios vary from 1.14 to 1.48. These rocks also contain biotite and amphiboles and are similar to peraluminous and high-K calc-alkaline I-type granites. The Qinglingou granite porphyries show relative enrichment of Rb, Th, and K, and depletion of Nb, Ta, P, Ti, and other high ﬁeld strength elements. These porphyries are enriched in light rare earth elements relative to heavy rare earth elements with weak negative Eu anomalies ( δ Eu = 0.63–0.89), which is interpreted to be the best age of crystallization of the Qinglingou porphyries. LA–ICPMS zircon U–Pb dating yields an age of 211 ± 1.2 Ma ( n = 16, MSWD = 0.39). The close spatial relationship between the ore bodies and granite porphyry dikes indicates that this age may represent the metallogenic age in this area. The zircons have 176 Hf/ 177 Hf ratios ranging from 0.282625 to 0.282702, the ε Hf (t) values are near 0 and the two-stage zircon Hf model ages range from 1121 to 1296 Ma. These data indicate that the Qinglingou granite porphyries were derived from the interplay between mantle- and crustal-derived magmas. We therefore propose that there is potential gold in carbonate rocks at the periphery of porphyry-skarn copper-molybdenum deposits.


Introduction
The Qinling Orogen is characterized by multiple episodes of subduction, accretion, and collision between the North China Block (NCB) and South China blocks (SCB). It links the Dabie Mountains to the east and the Qilian and Kunlun Mountains to the west (Figure 1). Accompanying the intracontinental orogeny, the Neoproterozoic, Paleozoic, and Mesozoic tectono-magmatic events occurred in the Qinling Orogen, which hosts numerous deposits of different metals, in particular large Cu-Au-Pb-Zn-Mo deposits [1][2][3]. The South Qinling Belt is a significant Cu-Au polymetallic metallogenic belt and hosts abundant mineral resources, including world-class Au deposits. The Zhashui-Shanyang ore district, one of the most representative polymetallic ore clusters within the northern segment of this orogen, hosts 20 gold deposits with different Au ore reserves, such as Ertaizi, Qinglingou, Xiajiadian, Wangjiaping, and etc. The influence of multi-stage tectonic magmatic events in the Qinling orogenic belt led to the Cu and Au polymetallic mineralization. These mineralizations were closely related to Triassic, Late Jurassic, and Early Cretaceous magmatic events that have been strongly developed in the area [4,5]. Previous studies mainly focused on petrology, geochemistry, and geochronology [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19], The results have shown the existence of SEDEX-type deposits and medium-scale, porphyryskarn Cu (Mo) and Carlin-type Au deposits, with examples including the Daxigou Fe deposit, Yindongzi Ag-Pb deposit, Mujiazhuang Cu deposit, Xiaohekou, Chigou and Lengshuigou Cu (Mo) deposits, and Ertaizi Au deposit. The intensive Indosinian (e.g., the Dongjiangkou, Zhashui, Shahewan, and Caoping pluton) and Yanshanian (e.g., Lengshuigou, Chigou, Baishagou, and Shuangyungou pluton) magmatic activity resulted in the emplacement of a number of some granitoids, accompanied by a variety of mineralization [14,[17][18][19]. Several hydrothermal porphyry-skarn Cu, Mo, Au, and Fe deposits or occurrences were discovered within hypabyssal intrusions, such as the Xiaohekou, Lengshuigou, and Chigou [7,9]. However, little has been reported on the comprehensive understanding of the tectonic magmatic evolution and mineralization of the district.
The intrusive rocks in the Qinglingou mining district are granite porphyry and lamprophyre dikes. Granite porphyries, located in the northwest Zhashui-Shanyang district, are widespread in the Qinglingou gold deposit. The timing and magmatic evolution of these plutons and associated deposits, as well as their potential for future mineral exploration, remain equivocal.
With this in mind, in this study, we present the whole-rock geochemistry and in situ zircon U-Pb-Hf isotope signature for the Qinglingou granite porphyries from the Zhashui-Shanyang district to (1) constrain the timing of the granite emplacement; (2) understand the magma source, petrogenesis, and metallogenic implications; and (3) evaluate future exploration potential.

Geological Setting
The Zhashui-Shanyang, located in the western part of the South Qinling Belt ( Figure 1B), is abundant in the Indosinian-Yanshanian granitoids and Au and Cu-Mo polymetallic deposits. The multiple Jurassic to Cretaceous porphyry-skarn Cu (Mo) orebodies were discovered in the Zhashui-Shanyang district in the southern Qinling Orogen within or near the Late Jurassic-Early Cretaceous granitic intrusions, such as Xiaohekou, Chigou and Lengshuigou Cu (Mo) deposits. The area was divided into two gold ore belts along the Shanyang-Fengzhen Fault. The two belts are the southern margin of the Ertaizi-Fengzhen-Xiajiadian Carlin and Carlin-like type gold belt and Qinglingou-Wangjiagou-Miaoliang Carlin and Carlin-like type gold belt, both of which extend from north to south ( Figure 2). The Zhashui-Shanyang area mainly comprises Devonian and Carboniferous quartz sandstone, siltstone, slate, and limestone. The Zhashui-Shanyang area is an important part of the Qinling orogenic belt. The area is bounded by the NWW-trending Shangxian-Danfeng Fault to the north, and the NW-trending Fengzhen-Shanyang Fault to the south. The Shanyang-Fengzhen and Zhen'an-Banyanzhen regional fault zones run through the whole area, controlling the distribution of the main orebody in the area. The structural intersection part is composed of multi-stage EW and NNE, NNW fault. What is more, the SN fault (or NNE fault) has also developed widely in the ore concentration area. These faults are also the main location of deposits, which are conducive to the upward migration of ore fluid. The magmatic activity in the Zhashui-Shanyang area is relatively intensive and is characterized by multi-period and multi-stage. Indosinian to Yanshanian was a significant period of magmatism in the Zhashui-Shanyang area. The Indosinian granitoids were mainly located in the West and North of the area, such as Dongjiangkou, Zhashui, Shahewan, and Caoping pluton with large exposure areas which are mainly spatially and temporally associated with Cu, Au, Mo, Pb, Zn and Fe mineralization, and others. The Yanshanian granitoid intrusions were distributed along both sides of the Fengzhen-Shanyang Fault and occurred as small-scale dikes. They are mainly distributed in the east and middle of the area, such as Miaoliang, Lengshuigou, Xiaohekou, Chigou, Baishagou plutons, etc. The Yanshanian granitoid intrusions mainly host two different types of deposits, such as porphyry type deposit and skarn type deposit [17,[21][22][23][24][25][26].
The Qinglingou gold deposit is located in the northwest of Zhashui-Shanyang area (109 • 10 E, 33 • 38 N) ( Figure 3). The rocks exposed in this ore district are mainly Middle Devonian sedimentary units belonging to the Niuerchuan Group. A few scattered felsic magmatic dikes (mainly granite porphyries) intrude into the Niuerchuan Group. The major fault of this ore district is the Shanyang-Fengzhen Fault, controlling the ore-forming process of Au and accompanying several NNW-and NNE-trending secondary faults. There are eleven main orebodies with a grade of 0.75 × 10 −6 to 6.2 × 10 −6 Au within the Qinglingou deposit, which developed in both carbonate rocks and granite porphyry dikes, as well as host-rock sedimentary units. The orebodies are usually stratigraphically controlled and show stratabound, lenticular, and vein-like shapes. Granite porphyry dikes near the ore show pyritization and silicification. The geological characteristics of the Qinglingou gold deposit are close to those of the Carlin-type gold deposit, and the geochemical composition of the deposit is similar to that of the Yangshan Carlin-type gold deposit. According to the mineral assemblages and cross-cutting relationships of veinlets, three stages of mineralization can be identified from early to late. The first is the quartz-pyrite stage, during which the quartz is milky to gray in color with small particle sizes. Pyrite is present as euhedral to subhedral crystals with disseminated, veinlet, and irregular massive forms. The second is the quartz-multimetal sulfide stage, actually the main mineralization stage, during which ore-related alteration includes sulfidization (arsenopyrite and pyrite), sericitization, silicification, and carbonatization. The third is the quartz, and carbonate stage, during which carbonate veinlets have in-filled open-space fissures and cross-cut the earlier veins. There are a large number of pyrite, arsenopyrite, pyrrhotite, and chalcopyrite, of which pyrite is the main gold-bearing mineral in the Qinglingou gold deposit. The gangue minerals include quartz, biotite, dolomite, sericite, feldspar, and epidote, with accessory zircon, and monazite ( Figure 4).

Major and Trace Element Analysis
The fresh portions of samples were powdered to 200 mesh and then dried for analysis. Major and trace elements were carried out at the Mineralization and Dynamics Laboratory of the Ministry of Land and Resources, Chang'an University, Xi'an, China. Major elements were determined on flux glass discs by XRF with analytical precision generally better than 5%. Trace elements, including rare earth elements, were determined using ICP-MS (Agilent 7500a). A set of USGS and Chinese national rock standards include AGV-2, AMH-1 and GBPG-1. The accuracy for all trace elements is mostly between 1% and 5% [24].

Zircon U-Pb Dating
The representative samples QL-13 were selected for in situ zircon U-Pb, trace elements, and Lu-Hf isotope analyses. These analyses were conducted at the Mineralization and Dynamics Laboratory of the Ministry of Land and Resources, Chang'an University, Xi'an, China.
Sample QL-13 was sieved for separating zircon grains using conventional heavy liquid and magnetic techniques at the Langfang Tuoxuan rock mine Testing Service Co., Ltd., China. Representative zircons were handpicked under a binocular microscope. Then zircon grains were mounted in epoxy. The polished mounts were imaged under cathodoluminescence (CL). Zircon U-Pb ages and trace elements were obtained using an Analyte Excite 193 nm Laser Ablation coupled to an Agilent 7700E ICP-MS. The diameter of the laser ablation crater was 32 µm. The detailed operating conditions and analytical methods are based on Liu et al. [27]. The U-Pb age was calculated using Isoplot 3.7 software [28]. The results were achieved by repeated analyses in accordance with the zircon standards 91500 and GJ-1, with a precision of the U-Pb ages of 1%.

Zircon Lu-Hf Analysis
In-situ Lu-Hf analysis for zircons was carried out using a Nu PlasmaHR MC-ICPMS Laboratory of Beijing Kehui Testing Technology Co., Ltd., Beijing, China. Details of instrumental conditions and data acquisition were described by Yuan et al. [29]. The measurements were conducted on the same zircon grains previously analyzed for U-Pb isotopes with a 43 µm ablation spot size. The laser repetition rate was 5 Hz. Calculations of zircon isotope ratios were performed by ICP-MS DataCal [29]. Harvard 91500 and GJ-1 were used as reference standards during analyses. The primary reference material for Hf isotope measurement used for monitoring accuracy and precision of internally corrected Hf isotope ratios was zircon 91500 and Mudtank.

Whole-Rock Geochemistry
The major and trace element compositions of samples from the Qinglingou plutons are listed in Table 1.
All samples have similar REE patterns characterized by right-inclined on the chondritenormalized rare earth element (REE) diagram (Figure 6a) and relatively low ΣREE  (Figure 6b), most of the Qinglingou granite porphyry show pronounced relative enrichment of large ion lithophile elements (LILEs; e.g., Rb, Th, and K), and depleted in high field strength elements (HFSEs; e.g., Nb, Ta, and Ti) and P.

Zircon U-Pb Ages
The analytical results of LA-ICPMS zircon U-Pb dating are presented in Table 2. The representative CL images of the dated zircons and Concordia diagrams are shown in

Zircon Lu-Hf Isotope Analysis
The results of in situ Hf isotope analysis for the zircon grains from sample QL-13 are listed in Table 3. Most zircons have low 176 Lu/ 177 Hf ratios, ranging from 0.000825 to 0.002327 (all are less than 0.002), which indicate little radioactive Hf accumulation after the emplacement of the rock mass, except zircon QL13-11, which provided slightly positive 176 Lu/ 177 Hf values of 0.002327. The zircon 176 Hf/ 177 Hf ratio can, therefore, be used to explore the genetic information on petrogenesis [35,36]. Sixteen representative magmatic zircons from the Qinglingou granite porphyry sample were analyzed in this study. The initial 176
The Zhashui plutons, which are located in the north of Qinglingou plutons, yield zircon LA-ICPMS U-Pb ages of 234-213 Ma. However, the new zircon LA-ICPMS U-Pb analysis of the Qinglingou granite porphyry in the study yielded a weighted mean 206 Pb/ 238 U ages of 211 ± 1.2 Ma. This age is similar to the previously reported zircon LA-ICPMS U-Pb ages for the Zhashui plutons, which suggests that both the Zhashui plutons and Qinglingou granite porphyries formed during Late Triassic (Table 5). In addition, granitoids from Dongjiangkou, Zhashui, Shahewan and Caoping (Figure 2) also gave zircon LA-ICPMS U-Pb ages of 246-209 Ma [8][9][10]14], 234-214 Ma [9,12], 212-205 Ma [11,13] and 224-220 Ma [6,9], respectively. Hence, the Qinglingou granite porphyry in the area has an affinity with the Dongjiangkou, Zhashui, Shahewan, and Caoping plutons, and all of them were the products of the Late Triassic magmatic event. Although the precise metallogenic age of the Qinglingou gold deposit was not well constrained, it can be inferred that the age of Au mineralization in the Qinglingou is later than that of the dikes according to the geological characteristics that the Au ore body occurs in the altered dikes. On the one hand, most scholars consider that the genesis of Carlin-type gold deposits is related to magmatism [55,56]. The granite porphyry from the Qinglingou deposit gave zircon LA-ICPMS U-Pb ages of 211 ± 1.2 Ma. Meanwhile the Yangmugou (203.6 ± 8.7 Ma) Mo deposit was developed at a similar time in the Zhashui-Shanyang area (Table 4). It is suggested that the intrusive and metallogenic ages of Qinglingou gold deposit are consistent within the error range, which further reveals that there are Indosinian intrusive and metallogenic events in the Zhashui-Shanyang area of the South Qinling Belt.

Source of the Granitoid Stocks and Its Implications
The zircon Lu-Hf isotopic system has a high closure temperature, and the 176 Hf/ 177 Hf values of zircon is less affected by the uncertainty of age. Therefore, zircon in situ Hf isotopic analysis can effectively reveal the magmatic evolution and the source area [57]. When a magmatic zircon initial ε Hf (t) values are positive, it usually indicates that the source area is a depleted mantle or Cenozoic crust, while the negative ε Hf (t) values indicate that the source area is ancient crust [58]. The heterogeneity of the zircon Hf isotope signature suggests that it has experienced a relatively significant process of crust-mantle magmatic mixing [59]. The range of ε Hf (t) values is mainly concentrated between −0.08 −+1.98, close to 0, indicating that both a crustal source and lithospheric mantle components were involved in the magmatic source [60]. This result is supported by the Indosinian Shahewan, Caoping and Zhashui granites in this area, which also have homogeneous ε Hf (t) values close to 0 ( Figure 8). We argued that the depleted mantle material and the basic crustal material of the South Qinling basement may be the source material of the Indosinian granites in Zhashui-Shanyang area. Regionally, many studies have confirmed that the Shahewan, Caoping and Zhashui plutons from the Zhashui-Shanyang area are all of mixed crust-mantle magma origin. In summary, it can be reasonably inferred that the source of the Indosinian granites in the Zhashui-Shanyang area was derived from the thickened continental crust as a result of the underplating of mantle material.
Accompanying frequent tectonic movement, mineralization and magmatic activity, copper-gold deposits, closely related to Indosinian to Yanshanian magmatism, are widely developed in the Zhashui-Shanyang area. According to the relationship of deposits, some researchers have proposed that the disseminated gold deposits and porphyry−skarn copper-gold deposits in sedimentary rocks were formed in the same metallogenic system [61]. Importantly, Large et al. [62] have argued that the Carlin-type gold deposit was the distal product of the porphyry-skarn copper mineralization system. In terms of time, space, and genesis, the Yanshanian plutons in the district are related to the metallogenic ones of porphyry−skarn copper (molybdenum) deposits, such as Lengshuigou, Chigou, and Xiaohekou. As described above, previous studies have carried out an important magmatic-metallogenic event that resulted in the discovery of Cu-Mo-Au orebodies in the Late Jurassic-Early Cretaceous Chigou-Lengshuigou porphyries and their contact zones. The intensive Late Jurassic-Early Cretaceous magmatic activity also resulted in the emplacement of some granitoids in the form of small hypabyssal intrusions, such as the Xiaohekou, Lengshuigou, and Chigou granitic stocks in Zhashui-Shanyang area [63][64][65][66]. The Xiajiadian, Wangjiaping, and other Carlin gold deposits are 50 km away from the porphyry skarn Cu-Mo deposits, such as Chigou and Lengshuigou in Zhashui-Shanyang area [67][68][69][70][71][72]. These ages (150-145 Ma) of the Chigou and Lengshuigou Cu-Mo deposits in the Zhashui-Shanyang district are consistent with the dolomite Sm-Nd age of 139.6 ± 0.98 Ma, which represents the age of Au mineralization in Xiajiadian Carlin-type Au deposit. The result is in correspondence with the views of previous studies [61,62].
Moreover, the Yangmugou porphyry molybdenum deposit is situated around 10 km west of the Qinglingou deposit and the molybdenite Re-Os is 203 Ma. This information indicates that the Xiajiadian, Wangjiaping, and Qinglingou deposits may belong to the distal Carlin-type gold deposit. The porphyry-skarn-Carlin-type deposits were distributed in the Zhashui-Shanyang area. Similarly, we reasonably inferred that it is great potential for discovering Au deposits at the periphery of the porphyry-skarn Cu-Mo deposit in the studied area.

1.
The Indosinian and Qinglingou granite porphyry have similar geochemical characteristics.The Indosinian granitoids are composed of quartz monzonite, granodiorite and granite series. Geochemical data show that the Qinglingou pluton is peraluminous and high-K calc-alkaline granite, similar to that of I-type granite. Isotopic characteristics of zircon Hf and regional geological evidence suggest that they resulted from the interplay of mantle-and crustal-derived magmas. 2.
LA-ICPMS zircon U-Pb ages indicate that the Qinglingou granites were emplaced at 213±2.1 Ma. The close contact relationship between the Au ore body and the vein rock suggests that they may be the same products of the Late Triassic magmaticmetallogenic activity.

3.
The comprehensive comparative analysis indicates that it is needed to strengthen the prospecting for the Carlin-type gold deposit in the periphery of the porphyry-skarn Cu-Mo deposits in the Zhashui-Shanyang area.