Genesis of the Daping Gold Deposit in the Middle Xuefeng Mountain Area, Southern China: Constraints from Geochemistry, Fluid Inclusion, and H-O-S Isotope

Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring (Central South University), Ministry of Education, Changsha 410083, China Department of Geology, Northwest University, Xi’an 710069, China No.407 Geological Team, Hunan Bureau of Geology and Mineral Exploration and Development, Huaihua 418000, China School of Mines, Inner Mongolia University of Technology, Hohhot 010051, China Key Laboratory of Geoscience Spatial Information of Land and Resources, Chengdu 610059, China Geomathematics Key Laboratory of Sichuan Province, Chengdu 610059, China


Introduction
As one of the crucial gold producers of south China, the Jiangnan Orogen Belt (JOB) has a total gold reserve of >970 tons [1] and thus attracted more and more attention from the metallogeny geologists. The middle Xuefeng Mountain which belongs to the western section of the Jiangnan Orogen is located in the transitional region between the Cathaysia plate and the Yangtze plate (Figure 1(a)) [1][2][3][4][5][6][7][8]. At present, 21 gold deposits (points) have been discovered, and among them, the Chanziping gold deposit [9][10][11][12][13] and the Daping gold deposit [14] have a scale of large size and medium size, respectively. The exploration of the Daping gold deposit began in 1987 [15] and has proved gold reserves of more than 10 tons. Previous studies on the geological characteristics and metallogenic chronology indicate that the Daping gold deposit belongs to the shear zone type [16,17] with ore-forming age of 204.8 Ma which belongs to Indosinian [18]. However, the ore-forming fluids, geochemical characteristics of the main and trace elements, rare earth elements, and isotopes of the deposit have not been systematically studied, and its metallogenic mechanism and process are still unknown.
This paper attempts to reveal the source of metallogenic materials, metallogenic mechanism, and deposit type of the Daping gold deposit by petrogeochemistry, H-O-S isotopes, and ore-forming fluids and provide more metallogenic information for further exploration and prediction of the Daping gold deposit and other similar gold deposits with the same metallogenic characteristics. 3.1. Deposit Features. The stratum of the Daping gold deposit is composed of the Gaojian Group of the Qingbaikou System and Changan Formation of the Nanhua System, and the gold-bearing faults are NE-trending, NW-trending [20,21], and NNE-trending. Crossing through the middle part of the study area (Figure 2), the NE-trending ductilebrittle shear cleavage zone may serve as the ore-forming fluid passageways as well as auriferous host structures.

The Orebody
Characteristics. At present, 23 auriferous veins were found out with a length of 120-2100 m and a width of 1.30-70 m. The auriferous veins which occurred in the ductile shear zone or adjacent fault segments are NW-trending of I 6 , I 7 , I 9 , I 12 , and I 19 or NE-NNE-trending of I 17 , I 20 , I 21 , and I 26 . The NW-trending veins intersect the NE-trending vein at a large angle, and both the NEtrending and NW-trending auriferous veins have inclination angles of above 70° (Figure 3). At present, 38 ore bodies have been found out between the elevation of -40 m and 340 m, and among them, 7 main ore bodies have lengths of 170-470 m with an average thickness of 1.36-4.85 m and an average grade of 1.63-25.80 ppm. The alteration types of the Daping gold ores include silicification, sericitization, chlori-tization, carbonization, and clayization (Figures 4(a), 4(c), and 4(h)). The intensity of silicification, pyritization, and sericitization has positive relations with the intensity of gold mineralization.
3.3. Ore Characteristics. According to the differences of goldbearing structures, the Daping gold ores can be divided into quartz vein type (gold mineralization mainly occurred in quartz veins and nearby metal sulfides), altered rock type (gold mineralization mainly occurred in alteration rocks), and tectonic breccia type (gold mineralization mainly occurred in tectonic breccia rocks) ( Figure 5). The metal minerals of ores are composed of pyrite, arsenopyrite, chalcopyrite, galena, sphalerite, and stibnite, and among them,         8 Geofluids   10 Geofluids pyrite and arsenopyrite are the main gold-bearing minerals. The gangue minerals are mainly including quartz, carbonate minerals, albite, sericite, and muscovite. According to the mineral assemblages, the metallogenic process of the Daping gold deposit can be divided into two periods (metallogenic period and supergene period) and four stages (Table 1, Figure 4). The first stage has mineral assemblages of quartz+pyrite+arsenopyrite±carbonate minerals and the second stage has mineral assemblages of quartz+polymetallic sulfide minerals (pyrite, arsenopyrite, chalcocite, galena, chalcopyrite, tetrahedrite)±chlorite±carbonate minerals which are the main gold mineralization stages. The third stage has mineral assemblages of quartz and carbonate minerals. The supergene stage is characterized by the mineral of limonite±patina which is formed by the oxidation of metal sulfides.  4.2. Analytical Methods. The major elements and trace element of whole rock were tested in samples H3, H5, H6, H7, H8, H26, H28, H34, H40, H43, H46, H51, H52, H62, and H84. Hydrogen and oxygen isotopes of mineral quartz were tested in samples H28, H34, H22, H10, H16, H26, H9, and H46. Sulfur isotope of arsenopyrite and pyrite was tested in samples H34, H22, H16, H26, H7, and H46. The homogenization temperature, freezing temperature, and laser Raman spectra of the ore-forming fluid inclusions were tested in samples H7, H10, H21, H28, and H43.

Sampling and Analytical Methods
The major elements, trace element, and isotopes of sulfur, hydrogen, and oxygen of the Daping gold ore samples were measured in the Australian Real Analysis Test (Guangzhou) Co., Ltd. The major elements were tested by the X-ray fluorescence instrument of ME-XRF26d with precision and accuracy better than ±5%. The trace elements and the rare earth elements are measured by instruments of M61-MS81, and the relative error is less than 10%. Sulfur isotope was measured by an instrument of S-ISTP01L with accuracy of better than 0.02%. Hydrogen isotope was tested by the instrument of H-ISTP01 with accuracy of better than 0.3%. The oxygen isotope was measured by the instrument of O-ISTP01 with accuracy of better than 0.03%. δ 18 12 Geofluids

Major Elements.
The test results of the major elements illustrate that the Daping gold ores have a high content of SiO 2 and S and low content of Al 2 O 3 , TiO 2 , CaO, Na 2 O, MnO, and K 2 O (Table 3). With the increasing of gold mineralization, the intensity of silicification alteration and sulfide mineralization increased significantly. The content values of U, Rb, and Co in the ores of Daping gold deposit are smaller than Th, Sr, and Ni. The content value of Zr is greater than Hf. The original mantle-standardized spider map shows that the elements of Li, Cr, Ni, Zn, Rb, Th, and Sr enriched obviously, and on the contrary, the elements of Sc, Co, Cu, Ga, Cs, Tl, and Zr were depleted ( Figure 6).

Rare Earth Element.
Rare earth elements are important indications for the analysis of the ore-forming material sources due to their stable chemical properties and the specificity of distribution [24]. REE testing results (Table 5)

Hydrogen and Oxygen
Isotope. The hydrogen and oxygen isotope testing results (Table 6) of quartz mineral samples show that the value of δD H 2 O is from -51‰ to 62‰ (-58.6‰ in average), and the value of δ 18 O H 2 O is from -1.44‰ to 5.42‰ (1.92‰ in average).

Sulfur
Isotope. The sulfur isotope is important for studying the source of ore-forming materials [26,27]. Six sulfur isotope samples of metal sulfides (pyrite or arsenopyrite) were tested, and the testing results (Table 7) show that the value of δ 34 S is from -1.00‰ to 1.98‰ (-0.13‰ in average), which are close to the average S isotope of the magmatic hydrothermal deposit of 1.68‰ [28,29].
5.6. Ore-Forming Fluid Inclusion Testing. According to the microscope observation, the ore-forming fluid inclusions are in the shape of circle, ellipse, or irregular. The metallogenic period of the Daping gold deposit can be divided into three stages. The inclusions of stage one are two-phase inclusions and liquid phase inclusions. The inclusions of stage  H34-S1 Arsenopyrite -0.94 H22-S1 Arsenopyrite -1 H16-S1 Arsenopyrite -0.76 H26-S1 Arsenopyrite -0.67 H7-S1 Pyrite 1.98 H46-S1 Pyrite 0.23 13 Geofluids two are two-phase inclusions, liquid phase inclusions, and gas inclusions. The inclusions of stage three are two-phase inclusions and gas phase inclusions. Based on the laser Raman spectra testing, the composition of the liquid phase inclusions is H 2 O, and the composition of the gas phase inclusions is H 2 O and CO 2 . For detailed characteristics of the fluid inclusions, see Table 1 [30][31][32]. The gold content of the base rocks is 6.5-14 times than the average bulk continental crust of 3.0 ppb [33] and thus can provide an abundant initial gold source for the formation of the gold deposits in the middle Xuefeng Mountain area.
Trace element studies show that the Daping gold ores enrich in Li, Cr, Ni, Zn, large ion lithophile elements of Rb and Sr, and high field strength element of Th and deplete in elements of Sc, Co, Cu, Ga, Cs, Tl, and Zr ( Figure 6). The chondrite standard distributions of REE curves of gold ores have similar patterns of right-dip indicating that they probably have the same material sources and origins [25]. The rare earth elements of the gold ores are characterized by the strong enrichment of LREE ( Figure 10). The δEu values range from 0.54 to 0.75 which show negative Eu anomaly, indicating that Daping gold deposit may form in the reducing environment [24,34].
On the diagram of δ 18 O H 2 O vs. δD H 2 O (Figure 11), auriferous quartz of the Daping gold ores is mainly located between the magmatic water and meteoric water and partly located at magmatic water, which indicates that the metallogenic fluids may be the mixing fluids of magmatic waters and meteoric water. Considering that the Zhonghuashan granite and Huangmaoyuan granite (belonging to Baimashan complex granites) (Figure 1) are only a few kilometers away from Daping gold deposit and have δ 18 O value of 9.74-11.2‰ [35] which overlap partially with δ 18 O value of 10.1-15.7‰ of the Daping gold-bearing quart, this indicates that the metallogenic fluids may partly come from the deep concealed granite with mixing of the meteoric water, which is similar to the Chanziping deposit ( Figure 12). The δ 34 S‰ values of sulfide isotope from metal 14 Geofluids sulfides of the Daping gold deposit range from -1 to 1.98, which are consistent with the granite; Gaojian Group ( Figure 12) implied that both the granite and Gaojian Group may contribute to the sulfur sources.
Most granites in middle Xuefeng Mountain and adjacent regions belong to S-type granites [35][36][37] and have low gold contents (e.g., the Baimashan complex granites have average gold content of 1.25 ppb [38]). Considering that the magma source of the granites have gold contents of 19.46-41.86 ppb which are much higher than the 1.25 ppb of granites, thus, the gold element may aggregate in the magmatic hydrothermal fluids and may partly contribute to the formation of Mesozoic regional gold deposits.

Metallogenic
Mechanism. Based on Rb-Sr dating of quartz, the Daping gold deposit and Chanziping gold deposit occurred in 204.8 Ma and 205.6 Ma, respectively [18]. Large-scale regional thrusting nappe structures and associated acid magma intrusion activities occurred at 225-201 Ma of the Indosinian period [18,36,37]. Thus, the   15 Geofluids metallogenic epoch of regional gold mineralization is a little younger than the intrusion time of granites.
The tectonic schistose foliation developed in auriferous veins (for example, I 17 and I 21 ) and ductile deformation of auriferous quartz veins (Figures 5(b), 5(d), and 5(e)) provides important evidence for the existence of brittle and ductile shear zones. The development of tectonic schistose foliation structure provides a migration channel for oreforming fluids and serves as the main place for the precipitation and enrichment of ore-forming materials [39]. Driven by the thermal gradient of concealed granite, the gold metallogenic fluids migrate along the brittle and ductile shear zones, and in this process, the gold element of the adjacent stratum also adds to the fluids. When the ore-forming fluids reach the shallow stratum, and under the environment of depressurizing and fluid immiscibility, the thermodynamic equilibrium of CO 2 and oxygen fugacity were destroyed, and thus, the gold element precipitated in the quartz and metal sulfides to form the gold ores [16].
6.3. Ore Genetic Type. As an important type of gold deposits in the world, the orogenic gold deposits provide at least 30% of global gold reserves [40], and 17 giant gold deposits (>500 t Au) around the world belong to the orogenic gold type. Since the evolutionary history of the orogenic belt can be recorded in the formation of the orogenic gold  16 Geofluids deposits [41], the study of the ore-forming process and genesis of the orogenic gold deposits can provide valuable information of metamorphism and uplift-erosion process of the orogenic belt and thus attracted more and more geologists to conduct research on such type of gold deposits. Before the jargon of orogenic gold deposits was proposed, the classification of the gold deposits which occurred in the orogenic belts or greenstone belts was in chaos. For instance, based on the differences of the surrounding rocks, the gold deposits were classified into green belt type, turbidite type, and BIF (banded iron formation) type [42]; based on the differences of mineralization characteristics, the gold deposits were classified into quartz vein type, altered rock type, and breccia type [43]; based on the differences of ore-controlling factors, the gold deposits were classified as shear zone type [44]. Groves et al. [45] proposed that the gold deposit formed in the tectonic environment of the squeezing or compression in the convergence region of the plates and has close genetic relationship with the orogenic process which can be classified as orogenic gold deposit, and thus, the gold types mentioned above should be classified as one type of orogenic gold deposit. The middle Xuefeng Mountain area is located in the transition zone between Cathaysian plate and Yangtze plate and had undergone many periods of crustal tectonic movement and forms a large number of faults and folds as well as a series of ductile-brittle shear zone structures and multiple metallogenic episodes including Paleozoic (e.g., Zixi gold deposit of 425 Ma [46]) and Late Triassic (e.g., Chanziping and Daping gold deposit). In the Indosinian period, due to the strong NW-SE compression tectonic activities and the intrusion activities of regional acidic magmas, the magmatic hydrothermal fluids upwelled along the shear zone, and the gold elements in the stratum were activated, migrated, and gradually precipitated and enriched in the proper weak tectonic structures (for instance, ductile-brittle shear structures).
Similar Mesozoic gold deposits were also reported in the Jiangnan Orogen, e.g., Yanlinsi, Hengjiangchong, Wangu, Huangjindong, Jinjing, Mali, Fenshuiao, and Dayan [1,4,[47][48][49][50][51][52]. And generally speaking, its gold mineralization has close spatial relations with regional granitic intrusions [1]. H-O-C-S-Pb isotopic data of ore-forming fluids indicate that its ore-forming materials mainly source from granitic magma and minor from basement metamorphic stratum ( [1] and references therein). The ore-controlling structure of the shear zone of Daping gold deposit is similar to many world typical orogenic gold deposits [53][54][55][56][57][58]. In addition, the Daping gold deposit has many similar characteristics to the typical orogenic gold deposits (Table 9), for instance, the tectonic background of orogen, the ore-bearing rocks of semideep sea or deep sea sediments, the ore-type of quartz    Most of the Archaean gold deposits occur in greenstone belts, and the ore-hosting rocks are mainly tholeiitic volcanic rocks. Phanerozoic gold deposits are mainly hosted in semideep sea or deep sea turbidite. The host rocks generally underwent shallow-medium metamorphism of greenschist facies and amphibolite facies The wall rocks of the Daping gold ores are deep sea and semideep sea tuffaceous flysch formations of the Qingbaikou System with lithology assemblages mainly including slate, sandy slate, and sericite slate Ore-controlling structure The first-level fault is a large tectonic belt which cuts through the crust with a length of more than 100 km; the second-level fault has length of 1-10 km, and gold ore bodies are often located in the secondary level tectonic zones. The gold mineralization often occurred in the shear fracture and tension fracture The Daping gold deposit is located in ca. 10

Data Availability
The data used to support the findings of this study are included within the article.