Petrogenesis of Ore-Hosting Diorite in the Zaorendao Gold Deposit at the Tongren-Xiahe-Hezuo Polymetallic District , West Qinling , China

The newly discovered Zaorendao gold deposit is in the Tongren-Xiahe-Hezuo polymetallic district in the westernmost West Qinling orogenic belt. The estimated pre-mining resource is approximately 13.6 t of Au at an average grade of 3.02 g/t. Mineralization is predominantly controlled by NW-trending and EW-trending faults within diorite intrusions and surrounding sedimentary rocks. In the present study, in situ zircon U–Pb geochronology and Lu–Hf isotopic analyses of the ore-hosting diorite at Zaorendao were measured using LA-ICP-MS. The data suggest that the diorite was emplaced at ca. 246.5 ± 1.9 Ma. The large variation of zircon Hf isotopic composition (εHf(t) values ranging from −12.0 to −1.8) indicates a two-stage model age (TDM2) that ranges from 1.4 Ga to 2.0 Ga. Such Lu–Hf isotopic compositions indicate that the diorite was dominantly derived from a Paleoto Meso-Proterozoic continental crust. The wide range of εHf(t) and the presence of inherited zircon can be interpreted to suggest the mixing of Paleoto Meso-Proterozoic continental crust with a mantle component. Combining such characteristics with the geochemistry of coeval rocks that are associated with the diorite, we therefore proposed that the gold-hosting Triassic diorite in the Zaorendao gold deposit formed in an active continental margin that was associated with the northward subduction of the paleo-Tethyan ocean.


Introduction
The WNW-trending West Qinling orogen, the western half of the Qinling orogen, is connected to the Kunlun Orogen and the Qaidam terrane in the west [1,2].The West Qinling orogen evolved over a protracted period from the Proterozoic up to the late Mesozoic.The tectonic history evolved in response to subduction and closure of the paleo-Tethys ocean and subsequent continental collision between the South China block and North China block during Late Triassic (Figure 1) [1][2][3][4][5][6].This region contains abundant Cu-Au, Cu-Mo, Au, Au-Sb, and Pb-Zn deposits, and is known to be one of the most shows the location of the study area (modified after [12]).(B) Simplified geological map of the Tongren-Xiahe-Hezuo district of the West Qinling orogen, showing distributions and ages of Early Triassic granitoids and ore deposits (modified after [12] and [21]).
The Tongren-Xiahe-Hezuo district, located to the northwestern section of the West Qinling, contains numerous sediment-and/or dike-hosted disseminated and lode gold deposits, as well as many Cu-Au-W-Fe skarn deposits, and has been one of the most popular districts for exploring metal resources over the last decade (Figure 1) [12,19,22].The relationship between the disseminated gold shows the location of the study area (modified after [12]).(B) Simplified geological map of the Tongren-Xiahe-Hezuo district of the West Qinling orogen, showing distributions and ages of Early Triassic granitoids and ore deposits (modified after [12] and [21]).The Tongren-Xiahe-Hezuo district, located to the northwestern section of the West Qinling, contains numerous sediment-and/or dike-hosted disseminated and lode gold deposits, as well as many Cu-Au-W-Fe skarn deposits, and has been one of the most popular districts for exploring metal resources over the last decade (Figure 1) [12,19,22].The relationship between the disseminated gold deposits to the south of Xiahe-Hezuo fault and magmatism is controversial and poorly understood, due to the rare constraints on the emplacement age and petrogenesis of stocks and dikes.
The newly discovered Zaorendao gold deposit is one of largest in the Tongren-Xiahe-Hezuo polymetallic district.The diorite stock and surrounding sedimentary rocks that are cut by NW-trending and nearly EW-trending fracture belts host the dominant gold mineralization.However, no petrography, geochronology, or geochemistry research on the diorite has been conducted until now.Based on systematic field investigations and petrographic observations, we present zircon LA-ICP-MS U-Pb geochronology, and Lu-Hf isotopic composition of the ore-hosting diorite in the Zaorendao deposit, to: a) establish the time of its emplacement, b) understand the petrogenesis of the diorite.

Regional Geology
The E-W-trending Qinling orogen in central China, which extends for more than 1500 km and is ~200-250 km wide, is tectonically situated between the North China and the South China blocks (Figure 1A) [1,23].The Lingbao-Lushan-Wuyang fault separates the West Qinling orogen from the Qilian Orogen and North China block to the north.The West Qinling orogen is separated from the Songpan-Ganzi Orogen and the South China block in the south by the Mianlue-Bashan-Xiangguang fault [2,23,24].The Qinling orogen is further divided into Southern North China block, North Qinling and South Qinling blocks by the Kuanping and Shangdan sutures [1,[25][26][27].
The West Qinling orogen, the western part of the Qinling orogen, is traditionally separated from the East Qinling orogen by the Huicheng basin or Foping dome, which can be observed as roughly marked by the Baoji-Chengdu railway line (shown as blue dash line in Figure 1A) [25,28].The West Qinling orogen is bounded by the Linxia-Wushan-Tianshui fault (Shangdan suture) on the north and the A'nimaque-Mianlue suture on the south (Figure 1A) [24,29,30].The West Qinling had a protracted and complex evolution history from the Proterozoic up to the late Mesozoic, with the final collision between the South China block and the South Qinling block along the A'nimaque-Mianlue suture zone [1,3,23,[30][31][32].The West Qinling orogen is dominantly covered by Paleozoic to early Mesozoic marine sedimentary rocks that were deformed in the early Mesozoic [2,33], and intruded by numerous Mesozoic granitoid intrusions.The widespread Mesozoic igneous rocks are grouped into subduction-related granitoids with ages ranging from ca. 250-235 Ma, syn-collision granitoids with ages ranging from ca. 228-215 Ma and post-collisional granitoids with ages ranging from ca. 210-190 Ma [4,11,13,[34][35][36].This is interpreted to be due to the northward subduction of paleo-Tethys ocean and subsequent syn-collision to post-collision between the North China and South China blocks.
The Tongren-Xiahe-Hezuo district is located to the northwestern section of West Qinling (Figure 1B) [12,19,21,37].The exposed strata are dominated by weakly metamorphosed to un-metamorphosed marine sedimentary rocks of Carboniferous to Triassic age (Figure 1B) [12,19,21,37].The Carboniferous rocks consist mainly of sandstone, siltstones, thin-to thick-bedded limestones and clastic limestone, whereas the Permian slope to deep-marine basinal facies rocks are mainly composed of carbonaceous/argillaceous siltstone, sandstone, conglomerate, limestone and calcirudite.The Lower to Middle Triassic strata comprise interbedded sandstones, siltstones, calcareous siltstones, and mudstones, with minor amounts of siliceous nodules hosted by siltstone layers [12,19,21].The Carboniferous carbonates and siltstones, and Permian marine clastic and carbonates are exposed mainly in the core and both flanks of the Xinpu-Lishishan anticline, respectively, and cover the central to southeast part of the district.The Triassic strata are separated from Carboniferous and Permian rocks by the Guanyindazhuang-Lishishan fault to the north and Xiahe-Hezuo fault to the south [12], and are exposed in the west of the district.Several dioritic to granitic plutons, represented by the Tongren, Jiangligou, Shuangpengxi, Xiekeng, Shehaliji, Xiahe, Ayishan, Daerzang, Dewulu, Laodou, Meiwu and Yeliguan plutons/batholiths, intrude the strata.These intrusions were emplaced between 250 and 234 Ma (Table 1) [12,[37][38][39][40][41], and form a discontinuous NW-trending magmatic belt between the axial trace of the Xinpu-Lishishan anticline and the Xiahe-Hezuo fault.The magmatic belt extends northwest to the Tongren district [19,39].Numerous stocks and dikes occur mainly to the south of the Xiahe-Hezuo fault, and these stocks and dikes are similar in composition to the plutons in the north.The emplacement age and petrogenesis of stocks and dikes is poorly constrained, but recent studies on zircon U-Pb age of dikes in the Zaozigou gold deposit indicate that they were emplaced in the 238-249 Ma interval, coeval with the plutons in the north [20,41].A few tens of hydrothermal ore deposits and occurrences have been identified in the Tongren-Xiahe-Hezuo district, and these deposits are temporally and/or spatially associated with the Triassic granitoids (Figure 1B).Mineralization to the south of the Xiahe-Hezuo fault is mainly hosted by Triassic marine sedimentary rocks and, less significantly in the intermediate to felsic stocks and/or dikes.These deposits are characterized by disseminated gold deposits [42][43][44].In the north part of the district, dozens of lode gold deposits and Cu-Au-(W)-(Fe) skarn deposits have been discovered hosted within or close proximity of intrusions [12,18,19].

Geology of the Zaorendao Gold Deposit
The Zaorendao gold deposit (102 • 50 59"E, 35 • 01 57"N, also referred to as Jiademu deposit), is located about 7 km northwester of Hezuo in Gansu Province (Figure 1B).The estimated pre-mining resource of the Zaorendao deposit is approximately 13.6 t Au at an average grade of 3.02 g/t [51].
The stratigraphic sequence in the Zaorendao area is dominantly composed of slate, sandy slate and arkose of late Triassic age.The sedimentary sequence is up to 3047m in thickness and represents a shore to shallow sea facies.These sedimentary rocks are intruded by two diorite stocks and numerous intermediate to felsic dikes (Figure 2).The dioritic stocks, named the West rock and the East rock, are exposed in the central and northwestern part of the mining area.The East rock outcrops have a rectangle-shape with approximately area of 0.19 km 2 , while the square-shaped West rock has an exposed area of 0.12 km 2 .Another barren, small diorite stock is also exposed to the northeast of the East rock.The dikes mainly consist of porphyritic diorite, porphyritic granodiorite, and porphyritic quartz diorite.These dikes are 20~250 m long, 0.3~16 m wide, and cut across bedding of the Early Triassic slate.

Geology of the Zaorendao Gold Deposit
The Zaorendao gold deposit (102°50′59″E, 35°01′57″N, also referred to as Jiademu deposit), is located about 7 km northwester of Hezuo in Gansu Province (Figure 1B).The estimated pre-mining resource of the Zaorendao deposit is approximately 13.6 t Au at an average grade of 3.02 g/t [51].
The stratigraphic sequence in the Zaorendao area is dominantly composed of slate, sandy slate and arkose of late Triassic age.The sedimentary sequence is up to 3047m in thickness and represents a shore to shallow sea facies.These sedimentary rocks are intruded by two diorite stocks and numerous intermediate to felsic dikes (Figure 2).The dioritic stocks, named the West rock and the East rock, are exposed in the central and northwestern part of the mining area.The East rock outcrops have a rectangle-shape with approximately area of 0.19 km 2 , while the square-shaped West rock has an exposed area of 0.12 km 2 .Another barren, small diorite stock is also exposed to the northeast of the East rock.The dikes mainly consist of porphyritic diorite, porphyritic granodiorite, and porphyritic quartz diorite.These dikes are 20~250 m long, 0.3~16 m wide, and cut across bedding of the Early Triassic slate.One hundred and ninety-nine orebodies have been delineated in the Zaorendao mine, among which are six major orebodies, including Au1, Au2, Au5, Au6, Au7 and Au8 orebodies.These orebodies contain approximately 85% of the total gold reserves.Mineralization is controlled predominantly by the NW-trending faults cutting through the diorite and surrounding sedimentary rocks.These orebodies, striking NW 310° to 330° and dipping NE, and are a few meters thick and hundreds of meters long.The downward extent of the orebodies dip from 3227 m to 2366 m above sea level.
Hydrothermal alteration mineral assemblages in the mining area includes hydrothermal pyrite, arsenopyrite, stibnite, sericite, silicification, hematite and carbonate.Ore minerals are dominated by pyrite, arsenopyrite, and stibnite, with lesser sphalerite, chalcocite, galena, and hematite.The gangue minerals include sericite, quartz, epidote, biotite, plagioclase and calcite (Figure 3).One hundred and ninety-nine orebodies have been delineated in the Zaorendao mine, among which are six major orebodies, including Au1, Au2, Au5, Au6, Au7 and Au8 orebodies.These orebodies contain approximately 85% of the total gold reserves.Mineralization is controlled predominantly by the NW-trending faults cutting through the diorite and surrounding sedimentary rocks.These orebodies, striking NW 310 • to 330 • and dipping NE, and are a few meters thick and hundreds of meters long.The downward extent of the orebodies dip from 3227 m to 2366 m above sea level.

Sampling Strategy
One representative, minimally altered and relatively pristine sample of diorite (17ZRD01) was collected from surface on the Zaorendao gold mine premises (Figure 4A,B).This samples were used for zircon separation and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb dating and Lu-Hf isotope analyses.The sampling locations are shown in Figure 2. The grey to sage-green diorite has medium-grained texture, and is mainly composed of plagioclase (~65 vol.%, 0.5~1.5 mm), biotite (~20 vol.%), and quartz (5-10 vol.%) with minor hornblende and pyroxene (Figure 4C,D).Accessory minerals are titanite, apatite and zircon.Plagioclase shows polysynthetic twinning and oscillatory zoning, and is locally altered to sericite (Figure 4C).

Sampling Strategy
One representative, minimally altered and relatively pristine sample of diorite (17ZRD01) was collected from surface on the Zaorendao gold mine premises (Figure 4A,B).This samples were used for zircon separation and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb dating and Lu-Hf isotope analyses.The sampling locations are shown in Figure 2. The grey to sage-green diorite has medium-grained texture, and is mainly composed of plagioclase (~65 vol.%, 0.5~1.5 mm), biotite (~20 vol.%), and quartz (5-10 vol.%) with minor hornblende and pyroxene (Figure 4C,D).Accessory minerals are titanite, apatite and zircon.Plagioclase shows polysynthetic twinning and oscillatory zoning, and is locally altered to sericite (Figure 4C).

Zircon LA-ICP-MS U-Pb Dating and Trace-Element Analyses
Zircon crystals were separated from crushed diorite sample through standard magnetic and density separation techniques.The crystals were then carefully handpicked under a binocular microscope, mounted in epoxy, and polished down to approximately half sections to expose internal structures.To identify the internal structure and texture of the zircon grains and to select potential locations for U-Pb analysis, the mount was photographed in transmitted and reflected light, and cathodoluminescence (CL).The CL images were taken on a JXA-880 electron microscope (JEOL Ltd., Akishima, Tokyo, Japan) and an image analysis software was used under operating conditions of 20 kV and 20 nA at the Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing, China.
Zircons were analyzed in situ for U, Th, and Pb isotope composition by a LA-ICP-MS system at the Isotopic Laboratory, Tianjin Institute of Geology and Mineral Resources of China Geological Survey, using a Neptune multiple-collector inductively coupled plasma-mass spectrometer (Thermo Fisher Ltd., Waltham, Massachusetts, USA) to a NEW WAVE 193 nm-FX ArF Excimer laser-ablation system (Elemental Scientific, Inc., Omaha, NE, USA).The analytical procedures are described in Reference [4].The analyses were carried out with a pulse rate of 8 Hz, beam energy of 11 J/cm 2 , and a spot diameter of 30 μm.Helium was used as a carrier gas.The NIST SRM 610 glass standard was used as an external standard to normalize U, Th, and Pb concentrations of the unknowns.Plešovice zircon, a new natural reference material for U-Pb isotopic microanalysis, and 91500 zircon were used as internal standards for U-Pb dating, and were both analyzed twice every 8 unknown zircons analyses, to normalize isotopic fractionation during isotope analysis.Common-Pb corrections were made using the method of Anderson [52]. 207Pb/ 206 Pb, 206 Pb/ 238 U, 207 Pb/ 235 U and 208 Pb/ 232 Th ratios were calculated from measured ion intensities using ICPMSDataCal 8.4 [53].Concordia diagrams and weighted mean U-Pb ages were processed using ISOPLOT 3 [54].Age data and concordia plots were

Zircon LA-ICP-MS U-Pb Dating and Trace-Element Analyses
Zircon crystals were separated from crushed diorite sample through standard magnetic and density separation techniques.The crystals were then carefully handpicked under a binocular microscope, mounted in epoxy, and polished down to approximately half sections to expose internal structures.To identify the internal structure and texture of the zircon grains and to select potential locations for U-Pb analysis, the mount was photographed in transmitted and reflected light, and cathodoluminescence (CL).The CL images were taken on a JXA-880 electron microscope (JEOL Ltd., Akishima, Tokyo, Japan) and an image analysis software was used under operating conditions of 20 kV and 20 nA at the Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing, China.
Zircons were analyzed in situ for U, Th, and Pb isotope composition by a LA-ICP-MS system at the Isotopic Laboratory, Tianjin Institute of Geology and Mineral Resources of China Geological Survey, using a Neptune multiple-collector inductively coupled plasma-mass spectrometer (Thermo Fisher Ltd., Waltham, MA, USA) to a NEW WAVE 193 nm-FX ArF Excimer laser-ablation system (Elemental Scientific, Inc., Omaha, NE, USA).The analytical procedures are described in Reference [4].The analyses were carried out with a pulse rate of 8 Hz, beam energy of 11 J/cm 2 , and a spot diameter of 30 µm.Helium was used as a carrier gas.The NIST SRM 610 glass standard was used as an external standard to normalize U, Th, and Pb concentrations of the unknowns.Plešovice zircon, a new natural reference material for U-Pb isotopic microanalysis, and 91500 zircon were used as internal standards for U-Pb dating, and were both analyzed twice every 8 unknown zircons analyses, to normalize isotopic fractionation during isotope analysis.Common-Pb corrections were made using the method of Anderson [52]. 207Pb/ 206 Pb, 206 Pb/ 238 U, 207 Pb/ 235 U and 208 Pb/ 232 Th ratios were calculated from measured ion intensities using ICPMSDataCal 8.4 [53].Concordia diagrams and weighted mean U-Pb ages were processed using ISOPLOT 3 [54].Age data and concordia plots were reported at 1σ error, whereas the uncertainties for weighted mean ages are given at 95% confidence level.

In situ Zircon Lu-Hf Isotope Analyses
The same domains or the same zircon zones that had been analyzed for U-Pb were analyzed for their Lu-Hf isotopic composition at the Isotopic Laboratory, Tianjin Institute of Geology and Mineral Resources of China Geological Survey, following the method described in Reference [4] and using a Thermo Finnigan Neptune MC-ICP-MS system coupled to a New Wave UP193 nm laser-ablation system.The analyses were carried out with a laser repetition rate of 11 Hz at 100 mJ, and a spot diameter of 50 µm.Helium was used as a carrier gas.During the analyses, the zircon GJ-1 was analyzed as quality control every 8 unknown samples, and yielded 176 Hf/ 177 Hf ratios of 0.282023 ± 30 (2σ, n = 12).Results excellently coincide with the recommended 176 Hf/ 177 Hf ratios of 0.282015 ± 19 [55] for GJ-1.
For calculation of initial epsilon Hf (ε Hf (t)) values, the decay constant of 1.865 × 10 −11 [56] and the values for the chondritic uniform reservoir (CHUR, 176 Lu/ 177 Hf = 0.0332, 176 Hf/ 177 Hf = 0.282772 [57]) were used.The initial 176 Hf/ 177 Hf ratios and ε Hf (t) for all analyzed zircon domains were calculated by the corresponding 206 Pb/ 238 U age.The single stage depleted mantle model ages (T DM ) were determined for each sample by calculating the intersection of the zircon/parent-rock growth trajectory with the depleted mantle evolution curve [58] calculated with a present-day 176 Hf/ 177 Hf ratio of 0.28235 (similar to mid-ocean ridge basalts values [59] and 176 Lu/ 177 Hf of 0.0384 [60] for the depleted mantle (DM).The two stages DM Hf model age (T DM2 ) was calculated for the source rock of the magma from the initial 176 Hf/ 177 Hf of each zircon at the time of crystallization (in terms of the apparent 206 Pb/ 238 U age) by using 176 Lu/ 177 Hf= 0.015 for the average crust [61].

Zircon U-Pb Morphology and Geochronology
Cathodoluminescence images of representative analyzed zircons for U-Pb and Lu-Hf analyses are illustrated in Figure 5.The LA-ICP-MS zircon U-Pb analytical data for diorite sample 17ZRD01 are presented in Table 2 and graphically illustrated in the concordia diagrams (Figure 6).reported at 1σ error, whereas the uncertainties for weighted mean ages are given at 95% confidence level.

In situ Zircon Lu-Hf Isotope Analyses
The same domains or the same zircon zones that had been analyzed for U-Pb were analyzed for their Lu-Hf isotopic composition at the Isotopic Laboratory, Tianjin Institute of Geology and Mineral Resources of China Geological Survey, following the method described in Reference [4] and using a Thermo Finnigan Neptune MC-ICP-MS system coupled to a New Wave UP193 nm laser-ablation system.The analyses were carried out with a laser repetition rate of 11 Hz at 100 mJ, and a spot diameter of 50 μm.Helium was used as a carrier gas.During the analyses, the zircon GJ-1 was analyzed as quality control every 8 unknown samples, and yielded 176 Hf/ 177 Hf ratios of 0.282023 ± 30 (2σ, n = 12).Results excellently coincide with the recommended 176 Hf/ 177 Hf ratios of 0.282015 ± 19 [55] for GJ-1.
For calculation of initial epsilon Hf (ɛHf(t)) values, the decay constant of 1.865 × 10 −11 [56] and the values for the chondritic uniform reservoir (CHUR, 176 Lu/ 177 Hf = 0.0332, 176 Hf/ 177 Hf = 0.282772 [57]) were used.The initial 176 Hf/ 177 Hf ratios and ɛHf(t) for all analyzed zircon domains were calculated by the corresponding 206 Pb/ 238 U age.The single stage depleted mantle model ages (TDM) were determined for each sample by calculating the intersection of the zircon/parent-rock growth trajectory with the depleted mantle evolution curve [58] calculated with a present-day 176 Hf/ 177 Hf ratio of 0.28235 (similar to mid-ocean ridge basalts values [59] and 176 Lu/ 177 Hf of 0.0384 [60] for the depleted mantle (DM).The two stages DM Hf model age (TDM2) was calculated for the source rock of the magma from the initial 176 Hf/ 177 Hf of each zircon at the time of crystallization (in terms of the apparent 206 Pb/ 238 U age) by using 176 Lu/ 177 Hf= 0.015 for the average crust [61].

Zircon U-Pb Morphology and Geochronology
Cathodoluminescence images of representative analyzed zircons for U-Pb and Lu-Hf analyses are illustrated in Figure 5.The LA-ICP-MS zircon U-Pb analytical data for diorite sample 17ZRD01 are presented in Table 2 and graphically illustrated in the concordia diagrams (Figure 6).Zircons separated from sample 17ZRD01 are mostly colorless to pale yellow, transparent, and commonly euhedral prismatic to elongated prismatic.The length of zircons varies from 90 to 250 μm with aspect ratios of 5:1 to 2:1.Most zircon grains show oscillatory zoning in CL images (Figure 5), consistent to that of igneous zircons [62].Some grains display a core-rim structure consisting of an inherited core with distinct oscillatory and patchy zoning (Figure 5).
Thirty-nine analyses were carried out for U-Pb dating on sample 17ZRD01, yielding U-Pb ages ranging from 237.2 ± 2.3 Ma to 1823.8 ± 17.9 Ma.Thirty-seven analyses were obtained from the zircons with oscillatory zoning, among which, two analyses (spot 17ZRD01.10,17ZRD01.17)give 206 Pb/ 238 U dates of 285.0 ± 3.0 Ma and 306.4 ± 3.2 Ma, and are excluded because they are statistical outliers.The remaining 35 analyses are concordant, with 206 Pb/ 238 U dates ranging from 237.2 ± 2.3 Ma to 256.1 ± 2.9 Ma with a weighted mean of 246.5 ± 1.9 (MSWD = 4.6, Figure 6).The Th/U ratios of the zircons are between 0.15 and 0.55, with an average of 0.34.The high Th/U values for the zircons suggest that they are magmatic in origin.The Th content ranges from 25.17 to 324.02 ppm, with an average of 104.43 ppm.The U content ranges from 97.60 to 1404.46 ppm, with an average of 344.91 ppm.The U-Pb age is therefore interpreted as the crystallization age of the diorite.One analysis (17ZRD01.27)on the zircon core yield 206 Pb/ 238 U age of 1707.3 ± 17.4 Ma is excluded because it is a statistical outlier.The remaining one analysis (spot 17ZRD01.36),gives a 206 Pb/ 238 U dates of 1823.8 ± 17.9 Ma with Th/U ratio of 0.17 (Table 2, Figures 5 and 6A).This is concordant and interpreted to be inherited during its emplacement.

Zircon Lu-Hf Isotopic Composition
In situ Hf isotope analyses have been carried out on zircons at the same spots used for zircon U-Pb dating.Zircon Lu-Hf isotopic data are presented in Table 3, and shown in Figure 7.The Hf isotopic data, expressed as values of epsilon Hf (ɛHf(t)) at the time of emplacement, from the 17ZRD01 are plotted versus time in Figure 7. Zircons separated from sample 17ZRD01 are mostly colorless to pale yellow, transparent, and commonly euhedral prismatic to elongated prismatic.The length of zircons varies from 90 to 250 µm with aspect ratios of 5:1 to 2:1.Most zircon grains show oscillatory zoning in CL images (Figure 5), consistent to that of igneous zircons [62].Some grains display a core-rim structure consisting of an inherited core with distinct oscillatory and patchy zoning (Figure 5).
Thirty-nine analyses were carried out for U-Pb dating on sample 17ZRD01, yielding U-Pb ages ranging from 237.2 ± 2.3 Ma to 1823.8 ± 17.9 Ma.Thirty-seven analyses were obtained from the zircons with oscillatory zoning, among which, two analyses (spot 17ZRD01.10,17ZRD01.17)give 206 Pb/ 238 U dates of 285.0 ± 3.0 Ma and 306.4 ± 3.2 Ma, and are excluded because they are statistical outliers.The remaining 35 analyses are concordant, with 206 Pb/ 238 U dates ranging from 237.2 ± 2.3 Ma to 256.1 ± 2.9 Ma with a weighted mean of 246.5 ± 1.9 (MSWD = 4.6, Figure 6).The Th/U ratios of the zircons are between 0.15 and 0.55, with an average of 0.34.The high Th/U values for the zircons suggest that they are magmatic in origin.The Th content ranges from 25.17 to 324.02 ppm, with an average of 104.43 ppm.The U content ranges from 97.60 to 1404.46 ppm, with an average of 344.91 ppm.The U-Pb age is therefore interpreted as the crystallization age of the diorite.One analysis (17ZRD01.27)on the zircon core yield 206 Pb/ 238 U age of 1707.3 ± 17.4 Ma is excluded because it is a statistical outlier.The remaining one analysis (spot 17ZRD01.36),gives a 206 Pb/ 238 U dates of 1823.8 ± 17.9 Ma with Th/U ratio of 0.17 (Table 2, Figures 5 and 6A).This is concordant and interpreted to be inherited during its emplacement.

Zircon Lu-Hf Isotopic Composition
In situ Hf isotope analyses have been carried out on zircons at the same spots used for zircon U-Pb dating.Zircon Lu-Hf isotopic data are presented in Table 3, and shown in Figure 7.The Hf isotopic data, expressed as values of epsilon Hf (ε Hf (t)) at the time of emplacement, from the 17ZRD01 are plotted versus time in Figure 7.

Petrogenesis of Ore-Hosting Diorite
The diorite from the Zaorendao deposit yields relatively low 176 Lu/ 177 Hf and 176 Hf/ 177 Hf ratios (0.000093 to 0.001847 and 0.282278 to 0.282575), and has negative zircon ε Hf (t) values varying from −12.0 to −1.8 with an average of −6.7 (Figure 7).The Lu-Hf isotopic composition is a sensitive tracer to detect the evolutionary history of the crust and mantle [58], by virtue of the fact that Hf partitions more strongly into the melt than Lu during mantle melting [63].During the production of granitoid magmas, according to Belousova et al. [63], high values of 176 Hf/ 177 Hf (ε Hf 0) indicate a 'juvenile' mantle input, either directly via mantle-derived mafic melts, or by remelting of a young mantle-derived mafic lower crust.The low values of 176 Hf/ 177 Hf (negative ε Hf (t) values) provide evidence for crustal reworking.Mixing of crustal-derived and mantle-derived magmas during granite production can also be detected by inhomogeneity (including zoning) or wide variations in the Hf isotope composition and trace-element abundances in zircon populations [61,63].The Hf isotopic composition therefore indicates that the source material of the diorite in the Zaorendao deposit is mainly the ancient crust-derived material.
Zircon Hf model ages represent a time when isotopic composition of zircon was the same as that of the parental magma that precipitated zircon.The two-stage Hf isotope model age (T DM2 ), from which we assume that the sample was derived via melting of an average continental crust ( 176 Lu/ 177 Hf = 0.015) following derivation from the DM [63], range from 1.4 Ga to 2.0 Ga, with an average of 1.7 Ga.This indicates that the source material is mainly the Paleo-to Meso-Proterozoic crustal component.This is generally consistent with the analyses on the core (spot 17ZRD01.36) that yields a 206 Pb/ 238 U age of 1823.8 ± 17.9 Ma, and has been interpreted to be inherited.This indicates that this zircon grain could be inherited from a Paleoproterozoic basement during magma crystallization at ca. 246.5 Ma.
Another important feature of the Hf isotope data is the wide range of ε Hf (t) values over 10 units.Such variations within a single sample can only be reconciled by the operation of open-system processes, such as magma mixing and/or assimilation [64][65][66].The Hf isotope composition of the diorite from the Zaorendao deposit is very similar to the widespread coeval igneous rocks in the Tongren-Xiahe-Hezuo district (Figure 7).The Tongren granodiorite exhibits ε Hf (t) values of −5.8 to −0.6, with a corresponding T DM2 age of 1.31 to 1.64 Ga [37].The Shuangpengxi granodiorite shows ε Hf (t) values of −4.7 to −3.4 and T DM2 values of 1.49 to 1.57 Ga [39].The Ayishan granodiorites have ε Hf (t) values that range from −15.8 to −5.6, with a corresponding T DM2 of 1.63 to 2.27 Ga [48].The Da'erzang granodiorite exhibits ε Hf (t) values ranging from −22.0 to −4.0, with a corresponding T DM2 of 1.38 to 2.18 Ga [40,48].The Dewulu granites have ε Hf (t) values of −8.2 to −4.2 [67], while the dioritic MME have negative ε Hf (t) values in the range of −8.0 to −3.3 with two-stage model ages that range from 1.48 to 1.78 Ga [12].These granitoids turned out to be generated by melting of a lower crustal component with additional input of a mafic component derived from the mantle, indicating a regional magma mixing process [12].Furthermore, the high Mg# values (58.25 and 60.00 [51]) of the diorite from the Zaorendao deposit are significantly higher than the melts produced by melting of metabasalts and eclogites (usually, Mg# < 45 [68]), indicating that mixing of a basic magma is required for the origin.Thus, taking into account the regional occurrence of coeval igneous rocks (250-235 Ma), we propose that the Zaorendao diorite was derived mainly from the partial melting of ancient Paleo-to Meso-Proterozoic crustal material and has been mixed with a lesser number of mantle-derived components.

Geodynamic Implications
The widespread early to middle Triassic magmatism provide a window to better understand the geodynamic evolution of the Tongren-Xiahe-Hezuo polymetallic district.Li et al. [37] proposed that the Tongren granodiorite shows geochemical features of arc-related granitoids, and formed in a subduction-related regime in response to slab roll-back of the northward-subducting A'nimaque-Mianlue oceanic lithosphere.Combining zircon U-Pb dating with geochemical study of Shuangpengxi granodiorite pluton and the Xiekeng diorite-granodiorite pluton, Luo et al. [39] proposed that such magmatism resulted from break-off of the subducted oceanic slab after collision in the early stages of Indosinian orogeny.Guo et al. [18] argued that they are formed as a continental margin arc in northeastern Tibet due to northward subduction during consumption of the paleo-Tethys ocean.Huang et al. [47] reported zircon U-Pb ages of 234.1 Ma from the Shehaliji quartz monzonite, and suggested the Early Indosinian granitoids in the West Qinling were formed in an active continental margin.Wei [48] reported the Xiahe granitoids were emplaced between 244 to 248 Ma, and proposed that the Xiahe granitoids were formed in a continental margin arc setting.The Yeliguan quartz diorite and Xiahe quartz diorite porphyry show adakitic geochemical features, and have been interpreted to have formed in the active plate margin [38].The petrology, geochronology and geochemistry of quartz diorite, quartz diorite porphyry and dioritic MME indicate that the granitoids in the Dewulu skarn copper deposit were products of arc magmatism in an active continental margin setting [12,50].We therefore propose that the ore-hosting diorite from the Zaorendao deposit in this study and the coeval magmatism throughout the Tongren-Xiahe-Hezuo polymetallic district, formed along an active continental margin, and define a regionally similar geodynamic process associated with the northward subduction of the paleo-Tethyan ocean (Figure 8).[12].These granitoids turned out to be generated by melting of a lower crustal component with additional input of a mafic component derived from the mantle, indicating a regional magma mixing process [12].Furthermore, the high Mg# values (58.25 and 60.00 [51]) of the diorite from the Zaorendao deposit are significantly higher than the melts produced by melting of metabasalts and eclogites (usually, Mg# < 45 [68]), indicating that mixing of a basic magma is required for the origin.Thus, taking into account the regional occurrence of coeval igneous rocks (250-235 Ma), we propose that the Zaorendao diorite was derived mainly from the partial melting of ancient Paleoto Meso-Proterozoic crustal material and has been mixed with a lesser number of mantle-derived components.

Geodynamic Implications
The widespread early to middle Triassic magmatism provide a window to better understand the geodynamic evolution of the Tongren-Xiahe-Hezuo polymetallic district.Li et al. [37] proposed that the Tongren granodiorite shows geochemical features of arc-related granitoids, and formed in a subduction-related regime in response to slab roll-back of the northward-subducting A'nimaque-Mianlue oceanic lithosphere.Combining zircon U-Pb dating with geochemical study of Shuangpengxi granodiorite pluton and the Xiekeng diorite-granodiorite pluton, Luo et al. [39] proposed that such magmatism resulted from break-off of the subducted oceanic slab after collision in the early stages of Indosinian orogeny.Guo et al. [18] argued that they are formed as a continental margin arc in northeastern Tibet due to northward subduction during consumption of the paleo-Tethys ocean.Huang et al. [47] reported zircon U-Pb ages of 234.1 Ma from the Shehaliji quartz monzonite, and suggested the Early Indosinian granitoids in the West Qinling were formed in an active continental margin.Wei [48] reported the Xiahe granitoids were emplaced between 244 to 248 Ma, and proposed that the Xiahe granitoids were formed in a continental margin arc setting.The Yeliguan quartz diorite and Xiahe quartz diorite porphyry show adakitic geochemical features, and have been interpreted to have formed in the active plate margin [38].The petrology, geochronology and geochemistry of quartz diorite, quartz diorite porphyry and dioritic MME indicate that the granitoids in the Dewulu skarn copper deposit were products of arc magmatism in an active continental margin setting [12,50].We therefore propose that the ore-hosting diorite from the Zaorendao deposit in this study and the coeval magmatism throughout the Tongren-Xiahe-Hezuo polymetallic district, formed along an active continental margin, and define a regionally similar geodynamic process associated with the northward subduction of the paleo-Tethyan ocean (Figure 8).  .Schematic cartoon illustrating the petrogenesis of the ore-hosting diorite at Zaorendao and regional Triassic magmatism in the Tongren-Xiahe-Hezuo polymetallic district (modified after [12]).SCLM = sub-continental lithospheric mantle.

Figure 1 .
Figure 1.(A) Major tectonic domains of China and the location of the Qinling Orogenic Belt.Insetshows the location of the study area (modified after[12]).(B) Simplified geological map of the Tongren-Xiahe-Hezuo district of the West Qinling orogen, showing distributions and ages of Early Triassic granitoids and ore deposits (modified after[12] and[21]).

Figure 1 .
Figure 1.(A) Major tectonic domains of China and the location of the Qinling Orogenic Belt.Insetshows the location of the study area (modified after[12]).(B) Simplified geological map of the Tongren-Xiahe-Hezuo district of the West Qinling orogen, showing distributions and ages of Early Triassic granitoids and ore deposits (modified after[12] and[21]).

Figure 2 .
Figure 2. (A) Sketch geologic map of the Zaorendao gold deposit and (B) selected exploration cross section showing the occurrence of gold ores (after [51]).

Figure 2 .
Figure 2. (A) Sketch geologic map of the Zaorendao gold deposit and (B) selected exploration cross section showing the occurrence of gold ores (after [51]).

Figure 4 .
Figure 4. Photographs and photomicrographs of the diorite in the Zaorendao deposit showing its geologic relationship with Triassic slate (A) and texture and mineral association (B-D).Blue box lines in Figure B show locations of doubly polished thin sections.Mineral abbreviations: Bt = biotite, Hbl = hornblende, Pl = plagioclase, Qz = quartz, Ser = sericite.

Figure 4 .
Figure 4. Photographs and photomicrographs of the diorite in the Zaorendao deposit showing its geologic relationship with Triassic slate (A) and texture and mineral association (B-D).Blue box lines in Figure (B) show locations of doubly polished thin sections.Mineral abbreviations: Bt = biotite, Hbl = hornblende, Pl = plagioclase, Qz = quartz, Ser = sericite.

Figure 5 .
Figure 5. Representative CL images of zircons separated from diorite sample 17ZRD01 with identified analytical spot, U-Pb age (Ma) and εHf(t) value.Red circle: U-Pb beam.Yellow dash circle: Lu-Hf beam.

Figure 5 .
Figure 5. Representative CL images of zircons separated from diorite sample 17ZRD01 with identified analytical spot, U-Pb age (Ma) and ε Hf (t) value.Red circle: U-Pb beam.Yellow dash circle: Lu-Hf beam.

Figure 7 .
Figure 7. Plots of εHf(t) values vs. U-Pb ages for zircons from the diorite in the Zaorendao deposit (A, B). (C) Histogram, and frequency curves of εHf(t) values.(D) Histogram, and frequency curves of corresponding two-stage model age (TDM2).Date from the igneous rocks of Tongren-Xiahe-Hezuo district are shown (Date are from Reference [12] and references therein.

Figure 7 .
Figure 7. Plots of ε Hf (t) values vs. U-Pb ages for zircons from the diorite in the Zaorendao deposit (A, B). (C) Histogram, and frequency curves of ε Hf (t) values.(D) Histogram, and frequency curves of corresponding two-stage model age (T DM2 ).Date from the igneous rocks of Tongren-Xiahe-Hezuo district are shown (Date are from Reference [12] and references therein.The 35 analyses for an estimated age at ca. 247 Ma yield 176 Lu/ 177 Hf and 176 Hf/ 177 Hf ratios of 0.000093 to 0.001847 and 0.282278 to 0.282575.They display ε Hf (t) values varying from −12.0 to −1.8, with an average of −6.7, and the two-stage model age (T DM2 ) range from 1.4 Ga to 2.0 Ga with an average of 1.7 Ga.It may indicate the contributions of both juvenile and ancient crustal materials.The inherited core gives 176 Lu/ 177 Hf and 176 Hf/ 177 Hf ratios of 0.001655 and 0.281608, with ε Hf (t) values of T DM2 of 2.6 Ga.

Table 1 .
Compilation of previously published zircon U-Pb ages of the Triassic magmatism in the Tongren-Xiahe-Hezuo polymetallic district.

Table 2 .
Zircon LA-ICP-MS U-Pb data of the ore-hosting diorite in the Zaorendao deposit.

Table 3 .
Lu-Hf isotopic composition of zircons from the ore-hosting diorite in the Zaorendao deposit.