Genesis and Formation of the Tuwaishan Gold Deposit in Hainan Island, South China: Implications from H-O-S Isotopes

: The Tuwaishan gold deposit is located at the northeastern end of the Gezhen shear zone in the western part of Hainan Island, South China. It is one of a series of similar gold deposits hosted in the Mesoproterozoic basement rocks and structurally controlled by the Gezhen shear zone. The hydrothermal ore-forming period can be divided into quar tz -pyrite-arsenopyrite stage, quar tz -py- rite-base metal sul ﬁ des stage and quar tz -carbonate stage. Eleven gold-bearing quar tz vein samples yield δ D V-SMOW and δ 18 O V-SMOW values of − 75.9‰ to − 54.4‰ and +8.1‰ to +13.7‰, respectively, and the corresponding δ 18 O water values range from +3.1‰ to +8.7‰. In addition, the pyrite separates from 14 ore samples yield δ 34 S values of +4.5‰ to +7.9‰. The H-O-S isotopic data, along with ﬂ uid properties of the Tuwaishan and other gold deposits along the Gezhen shear zone, suggest that the ore-forming ﬂ uid and materials are of metamorphic rather than magmatic origin. Hence, we pro-pose that the Tuwaishan gold deposit is best classi ﬁ ed as orogenic gold deposit that resulted from regional metamorphism. Considering that the Mesoproterozoic basement rocks have experienced amphibolite facies metamorphism prior to the gold mineralization, the metamorphic devolatiliza-tion of the Ordovician-Silurian rocks at depth would provide a realistic source of ﬂ uid, gold and sulfur for the Tuwaishan and other gold deposits of the Gezhen gold belt.


Introduction
Hainan Island is located in the southern margin of the South China Block, and is adjacent to the northern part of the Indochina Block [1,2] (Figure 1a). More than 50 gold deposits and occurrences have been discovered on this island, making Hainan Island a prospective region for gold exploration in South China [3][4][5][6][7]. The Gezhen gold belt, which consists of a series of similar gold deposits along the NE-trending Gezhen shear zone in the western Hainan Island, has become the most prospective gold exploration target within the island [4,5,8]. Studies on ore deposit geology, ore-forming fluid characteristics and isotope compositions have been conducted to constrain the source and to further identify the genesis of the Gezhen gold deposits, but debates persist on whether those deposits are sedimentaryreworked, magmatic-hydrothermal or orogenic in genesis [5,8,[10][11][12][13]. The metamorphic rocks in the Mesoproterozoic Baoban Group have long been accepted as the source of Au for the Gezhen gold belt because (1) they are the host rocks of those gold deposits and (2) they possess a relatively high content of Au [5,11,12,[14][15][16][17]. However, the rocks in the Baoban Group were deposited at 1460-1430 Ma and have experienced amphibolite facies metamorphism at 1.3-1.0 Ga, whereas the Gezhen gold deposits were formed at ~243 Ma [8,[18][19][20][21][22]. On account of the metamorphic devolatilization model, Au and sulfur would be released into the fluid phase during the prograde metamorphic transition of greenschist to amphibolite facies [23][24][25][26][27][28]. Therefore, if the Gezhen gold deposits are orogenic in genesis, then reevaluation is required for whether the regional metamorphism of the Baoban Group rocks led to the formation of the Tuwaishan and other gold deposits along the Gezhen shear zone.
In this study, we present new H-O-S isotopic compositions of ore samples from the Tuwaishan gold deposit and summarize the previously published H-O-S isotopic data for the Gezhen gold deposits to identify the source characteristics of ore-forming fluid and materials of the Tuwaishan and other gold deposits along the Gezhen shear zone. In addition, we carried out integrated analyses of source characteristics, deposit geology, regional geology of the Gezhen gold belt, as well as tectonic settings of Hainan Island to further identify the potential source for gold mineralization along the Gezhen shear zone.

Regional Geology
Hainan Island is separated from the mainland of South China by the Qiongzhou Strait in the north and is regarded as the southernmost extension of the Cathaysia Block [1,2,24] (Figure 1a). It is tectonically located at the junction of the Eurasian plate, the Indian-Australian plate and the Pacific plate [29][30][31][32][33].
The Gezhen shear zone lies between the Changjiang-Qionghai and Jianfeng-Diaoluo E-W trending faults in the western part of the Hainan Island [9] (Figure 1b). It extends for more than 55 km with a width of 0.5-3 km [34][35][36]. The Gezhen shear zone trends 35-40°, dips to the northwest with dip angles gradually varying from 60-80° on the surface to 15-30° with increasing depth and is characterized by both sinistral and reverse movement [9,36,37] (Figure 2). It is a multi-phase superimposed brittle-ductile shear zone and is disrupted by later NW-and E-W-trending faults [5,38] (Figure 2). The youngest shear deformation age was 227.4 ± 0.2 Ma from 40 Ar/ 39 Ar dating of synkinematic muscovite along the shear zone [9].
The hanging wall rocks of the Gezhen shear zone include the Mesoproterozoic Baoban Group and contemporaneous granitic and mafic rocks while the footwall rocks comprise the Ordovician Nanbigou Formation and the lower Silurian Tuolie Formation [20,21,39,40] (Figure 2). The Baoban Group comprising the lower Gezhencun Formation and upper Ewenling Formation was deposited at 1460-1430 Ma and suffered amphibolite facies metamorphism at 1.3-0.9 Ga [18][19][20][21][22]. It represents the oldest crystalline basement of the Hainan Island [37,41]. The Gezhencun Formation rocks consist of gneiss and migmatitic gneiss and have experienced migmatization and upper amphibolite facies metamorphism, while the Ewenling Formation rocks consist of quartz-mica schist, mica-quartz schist, quartzite and thin graphite layers and have experienced lower amphibolite facies metamorphism [8,19,22,37,41]. The protoliths of the Gezhencun Formation rocks are inferred to be intermediate-felsic volcanic and volcaniclastic rocks based on their whole rock geological composition while the rhythmic layering as well as relict clastic texture suggest that the protoliths of the Ewenling Formation rocks were mainly siliciclastic rocks [19,22,37,41]. The Nanbigou Formation rocks are mainly composed of quartz-mica schist, quartz-sericite phyllite and metamorphic siltstone, interspersed with metamorphic basic volcanic rocks. The overall metamorphism of the Nanbigou Formation is greenschist facies while some areas have reached upper greenschist facies metamorphism [4,42]. The low-grade metamorphic successions in the Tuolie Formation consist of phyllite, slate and metamorphic sandstone, with limestone and tuff, and the overall metamorphism is lower greenschist facies [4,42].
Magmatic rocks outcropped along the Gezhen shear zone include the Mesoproterozoic granitic and mafic rocks, as well as Middle Permian Changjiang and Datian plutons [20,21,43]. The Mesoproterozoic granitoids intruded the Baoban Group at 1450-1430 Ma, and were generally subjected to metamorphism and deformation, resulting in the formation of granitic gneiss and granitic mylonite [18,21,39,40]. The Mesoproterozoic mafic rocks were formed at 1440-1420 Ma and were metamorphosed to plagioclase amphibolite, metabasic gabbro and metabasic diabase, which are mainly lenticular, dyke and stratified in the Baoban Group [20,44]. The Datian pluton intruded the Baoban Group, Nanbitou Formation and Tuolie Formation at 263 ± 1.2 Ma [43]. It is composed of biotite monzogranite and shows gneissic schistosity within the Gezhen shear zone [43]. The Changjiang pluton consists of biotite monzogranite with a formation age of 262 ± 2.2 Ma [43]. Along the Gezhen shear zone, from NE to SW, the Tuwaishan, Baoban, Erjia and Bumo gold deposits as well as other gold deposits and occurrences have been discovered, making up the Gezhen gold belt [5,8,[45][46][47]. Confirmed a total measured metal Au reserve of ~275 t, the Gezhen gold belt has become the most economically significant gold belt in the Hainan Island [4,5,8].

Deposit Geology
The Tuwaishan gold deposit is located at the northeastern end of the Gezhen shear zone, approximately 15 km southwest of the Changjiang City ( Figure 2). This gold deposit was discovered during the gold rush in the Hainan Island during 1988-1991 and has been mined underground since 1997 [48]. The combined proven gold reserves and historical gold production are about 22 t at 3.5 g/t [48].
Rocks in the deposit area include quartz-mica schist of the Ewenling Formation and mylonitized Mesoproterozoic granitoids on the hanging wall, as well as schist and phyllite of the Ordovician-Silurian metamorphic successions on the footwall of the Gezhen shear zone (Figures 2 and 3). The dominant structure in the deposit area is the Gezhen shear zone. It is approximately 1.5 km in width, trends 35-40°, dips to the northwest with dip angles ranging from 60° to 80° in the mining area ( Figure 3). Plagioclase amphibolites are locally observed as dykes or lenses in the Baoban Group rocks, and the dioritic porphyrites have also been found as dykes in the Mesoproterozoic metamorphic and magmatic rocks in the mining shafts [5,8,10,11]. More than 60 orebodies have been discovered in the granitic mylonites and are divided into 4 ore zones [48]. The orebodies are 150-300 m long, 5-25 m wide, strike 35-40° and dip NW from 75° on the surface to 55° with increasing depth [48]. All lenticular orebodies and gold-bearing veins are controlled by the Gezhen shear zone and its secondary fractures [5,8,47]. Two types of gold mineralization have been identified in the Tuwaishan gold deposit-disseminated ores hosted in the granitic mylonite and less common auriferous sulfide-bearing quartz veins (e.g., the smoky quartz veins; Figure 4). The granitic mylonites are the main wallrocks in the gold deposit while the mineralized quartz-mica schists and plagioclase amphibolites have also been observed. Silicification, sulfidization and sericitization have close relationships with gold mineralization while chloritization and carbonatization are also related to orebodies. Ore minerals include pyrite, arsenopyrite with lesser pyrrhotite, galena, sphalerite and chalcopyrite ( Figure 5). The occurrence of gold is predominantly native gold, and electrum grains occur adjacent to or in fractures of sulfide grains and gangue minerals as free gold ( Figure 5). Major gangue minerals include quartz, albite, sericite, chlorite and calcite (Figures 4 and 5).  Based on crosscutting relationships and mineral assemblages, the hydrothermal oreforming period of the Tuwaishan gold deposit can be divided into three stages. The early stage gray and milky quartz veins contain pyrite, arsenopyrite, gold and local pyrrhotite (Figures 4 and 5). The middle stage is characterized by smoky quartz veins crosscutting early stage quartz veins or superimposed on the early stage silicification, containing base metal sulfides and native gold, and is the main stage for gold mineralization (Figures 4  and 5). The late stage is represented by quartz-calcite veins commonly crosscutting the earlier two stages of quartz veins with trace sulfides and almost no gold, reflecting the end of this period of ore-forming hydrothermal activity ( Figure 4).

Sample Description and Analytical Methods
In this study, 11 and 14 ore samples have been collected at different sections from shaft 2, 3 and 4 of the Tuwaishan gold deposit to carry out H-O and S isotope analyses, respectively. Descriptions of the samples are presented in Table 1.
The H and O isotope analyses were completed at the Beijing Research Institute of Uranium Geology, China (BRIUG). Quartz grains were crushed into 40-60 mesh and fur-ther handpicked under a binocular microscope, followed by fine grinding and sieving before treatment with dehydrated ethanol to ensure a purity of better than 99% [49,50]. The H and O isotopic compositions of the purified quartz were measured using a MAT-253 gas isotope ratio mass spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). Hydrogen isotope was measured on water released from fluid inclusions of the quartz via thermal decrepitating. The purified quartz samples were first degassed through heating under a vacuum at 90 °C for 12 h. Then, the water was released from fluid inclusions by heating the samples to 400-500 °C [51]. The released water was trapped and reduced to H2 by zinc powder before analyses with gas isotope ratio mass spectrometer. Oxygen isotope analyses were based on the BrF5 extraction technique, with O being extracted by reacting with BrF5 [52]. The resulting O2 was reacted with graphite rods to produce CO2 before analyses with gas isotope ratio mass spectrometer. The isotopic results were standardized with Vienna-Standard Mean Ocean Water (V-SMOW) for H and O isotopes, while the analytical precisions were better than ±2% for δD values and ±0.2% for δ 18 O values. The isotopic fractionation of oxygen between quartz and water was calculated using the equation of 1000lnα = 3.38 × 10 6 /T 2 − 3.4 [52].

Results
The H and O isotopic compositions of gold ores from the Tuwaishan gold deposit, alone with those of the Baoban, Bumo, Erjia gold deposits as well as the wallrock granitic mylonites from previous studies are presented in Table 2 and plotted in Figure 6. The 11 ore samples of the Tuwaishan gold deposit yield δDV-SMOW and δ 18 OV-SMOW values from −75.9‰ to −54.4‰ and from +8.1‰ to 13.7‰, respectively. Given 360 °C as the trapping temperature for the main stage of gold mineralization of the Tuwaishan gold deposit [47], the corresponding δ 18 Owater values range from +3.1‰ to +8.7‰.
The S isotope data of pyrites from the Gezhen gold deposits as well wallrocks and dykes are listed in Table 3 and plotted in Figure 7. The 14 ore samples of the Tuwaishan gold deposit show δ 34 S values of +4.5‰ to +7.9‰ with a narrow variation of 3.4‰ and an average of +6.3‰.  [43]. Magmatic water, metamorphic water and meteoric water line are from [53]. Hydrogen and oxygen isotopic compositions of typical orogenic gold deposits are from [54].

Ore-Forming Fluid Source
Many workers have carried out H-O isotope analyses to constrain the fluid source of the Gezhen shear zone gold deposits, and it has been proposed that a mixture of magmatic water and metamorphic water with variable contents of meteoric water are most likely the source for the Gezhen gold deposits [5,[10][11][12]14]. The δDV-SMOW and calculated δ 18 Owater values are from −87.0‰ to −55.0‰ and from −0.1‰ to +8.1‰ for the Tuwaishan gold deposit [13,16,55,56], indicating a hybrid source for ore-forming fluid [53]. Some researchers proposed that the ore-forming fluid of the Tuwaishan gold deposit originated from a mixture of magmatic water and meteoric water [10,13], while some other researchers argued that a mixture of metamorphic and meteoric water with lesser contribution of magmatic water could be the source fluid for this deposit [17].
Our new H-O isotopic data for quartz grains separated from 11 ore samples of the Tuwaishan gold deposit predominantly fall into either the metamorphic water field or the magmatic water field or the overlap zone between them in the δDH2O vs. δ 18 OH2O plot (Figure 6), indicating a mixture of metamorphic and magmatic water for the ore-forming fluid source [53,[58][59][60]. Meanwhile, half of our new data fall into the typical orogenic gold fluid field in the δDH2O vs. δ 18 OH2O plot [54], suggesting the involvement of metamorphic water in the formation of the Tuwaishan gold deposit [24,54,61,62]. The ore-forming fluid of the Gezhen gold deposits is characterized as medium temperature (200-380 °C), low salinity (predominantly 3.0-7.0%NaCleqv), reducing (H2S exists in the gas phase composition) and CO2 rich [12,13,15,47,63]. It is compatible with metamorphic fluid but is significantly different from the magmatic fluid [64][65][66][67]. Thus, combined with ore-forming fluid properties and H-O isotopic compositions, it is proposed that the ore-forming fluid of the Tuwaishan gold deposit is most likely sourced from metamorphic water.

Sulfur Source
Although there are plenty of available sulfur isotopic data for the Gezhen gold deposits as well as the wallrocks, intrusions and dykes (Table 3, Figure 7), debates still exist for the sulfur source of the Gezhen gold deposits. This is partly because the similarity among δ 34 S values of the gold deposits, the Baoban Group metamorphic rocks, the granitic mylonites and the diorite porphyrite dykes (Table 3, Figure 7). Some researchers suggest that the sulfur was sourced from the magmatic fluid which has interacted with wallrocks of the Baoban Group metamorphic rocks or the granitic mylonites during its ascending, as the positive δ 34 S values of the Gezhen gold deposits are slightly higher than those of the magmatic fluid, and are close to those of the wallrocks [10,16] (Table 3; Figure 7). However, some other workers believe that the sulfur was originated from the metamorphism of the Baoban Group rocks [12,17,68]. The indistinctive positive δ 34 S values are not fully consistent with either magmatic fluid or sedimentary fluid [12]. In addition, the narrow variation of δ 34 S values for the Gezhen gold deposits indicates a sulfur isotope homogenization during regional metamorphism [10,17,[69][70][71]. Considering the high gold content of the Baoban Group rocks as well as their close spatial relationship with the Gezhen gold deposits, many researchers believe that the metamorphism of the Baoban Group rocks played a key role in the formation of those gold deposits [12,16,17,68].
Field and microscopic observations suggest that ore minerals of the Gezhen gold deposits are predominantly sulfides including pyrite, arsenopyrite, sphalerite, galena and chalcopyrite whereas sulfates are absent [16,47]. Thus, the sulfur isotope compositions of sulfides should be approximately the same as those of the ore-forming fluid [69,72]. Our new δ 34 S values of pyrite separates from 14 ore samples from the Tuwaishan gold deposit show positive values (from +4.5‰ to +7.9‰) with a narrow variation (3.4‰), which strongly imply a sulfur isotope homogenization caused by regional metamorphism during the formation of the gold deposit [27,54,70]. Furthermore, the same δ 34 S values among the Tuwaishan, Baoban, Erjia and Bumo gold deposits request a common source for all gold deposits along the Gezhen shear zone (Table 3; Figure 7). Thus, the plagioclase amphibolites as well as the diorite porphyrite dykes are not likely the sources of sulfur for the Gezhen gold deposits due to their scattered distribution and limited scales [5,16]. It is concluded that the sulfur of the Tuwaishan gold deposit, as well as other gold deposits along the Gezhen shear zone, is originated from regional metamorphism of sources rocks.

Genesis and Formation of the Tuwaishan Gold Deposit
The Gezhen gold deposits have been traditionally classified as sedimentary-reworked [10][11][12] or magmatic-hydrothermal origin [13]. Xu et al. [5] summarized previous studies on deposit geology, ore-forming fluid characteristics and isotope compositions of the Gezhen gold deposits and concluded that these gold deposits were best classified as orogenic in genesis. Our new data on H-O-S isotopic compositions of the Tuwaishan gold deposit further support the orogenic gold classification not only because the H-O-S isotopic compositions of the Tuwaishan gold deposit are consistent with those of typical orogenic gold deposits worldwide [6,54,73,74] (Figures 6 and 7) but also because the comprehensive interpretations of our new H-O-S isotopic data with ore-forming fluid properties favor a metamorphic rather than a magmatic origin.
As discussed above, the Tuwaishan gold deposit belongs to orogenic gold deposit and is most likely sourced from metamorphism. Therefore, the host rocks of the Baoban Group metamorphic rocks are seemingly the potential sources of both ore-forming fluid and materials for the Tuwaishan gold deposit as well as other gold deposits along the Gezhen shear zone. In fact, it has been widely accepted by early researchers on Hainan gold deposits that the Baoban Group rocks are the source rocks for the Gezhen gold deposits [5,11,12,[14][15][16][17]. Accordingly, it has been proposed that the metamorphic fluid released from regional metamorphism of the Baoban Group rocks mixed with magmatic fluid or the magmatic fluid that extracted Au from the Baoban Group rocks during its ascending, which deposited Au and sulfides in appropriate spaces in the Gezhen shear zone, is the major mechanism of gold mineralization along the shear zone [11,16,17].
However, despite the fact that the Gezhen gold deposits were sourced from metamorphism, and the Baoban Group metamorphic rocks are the host rocks, those Mesoproterozoic basement rocks are not necessarily the source for the Gezhen gold deposits [8]. Based on the metamorphic devolatilization model for orogenic gold deposits, most of sulfur and Au would have been released from the source rocks into the metamorphic fluid during the prograde metamorphic transition from the greenschist to the amphibolite facies [23,24,[75][76][77]. This is when the source rocks were first heated through a temperaturepressure window that broke chlorite, pyrite, organic matter and various other minerals and contributed to the fluid phase [24]. The Baoban Group rocks together with the Mesoproterozoic mafic rocks have experienced amphibolite facies regional metamorphism at 1.3-0.9 Ga [18,19,22], during which time those Mesoproterozoic basement rocks have lost most of their Au and sulfur [8,24,54,77]. Therefore, the Baoban Group rocks could not be a source for the fluid and gold for the gold mineralization at ca. 243 Ma along the Gezhen shear zone [8]. Instead, The Ordovician Nanbigou Formation rocks and the Silurian Tuolie Formation rocks on the footwall of the Gezhen shear zone have experienced greenschist facies and lower greenschist facies regional metamorphism, respectively [4,42]. Even through the timing and duration for this metamorphism have not been well constrained, the continuous amalgamation process from back-arc consumption (272-252 Ma) to orogenic assembly (251-243 Ma) between the South China and Indochina Blocks represents an important tectono-thermal event that would promote regional metamorphism in Hainan Island [2,8,78]. Considering the fact that the Ordovician-Silurian rocks are on the footwall of the reverse Gezhen shear zone (Figure 2), and the dip angle gradually decreases with increasing depth [9,36], metamorphic devolatilization of these rocks at depth would provide a realistic source of fluid, gold and sulfur for the gold mineralization along the shear zone. Therefore, we propose that the Ordovician-Silurian rocks on the footwall of the Gezhen shear zone are most likely the sources of ore-forming fluid and materials for the Tuwaishan gold deposit and other gold deposits along the Gezhen shear zone.

Conclusions
New H-O-S isotopic data of the Tuwaishan gold deposit in Hainan Island are reported here. In combination with H-O-S isotopic compositions and ore-forming fluid properties of the Gezhen gold deposits from previous works, this study identified the oreforming fluid and material source characteristics of the Tuwaishan and other gold deposits along the Gezhen shear zone. Furthermore, with integrated studies on source characteristics, deposit geology and regional geology of the Gezhen gold deposits, as well as tectonic settings of Hainan Island, a realistic source has been proposed for the Tuwaishan and other gold deposits along the Gezhen shear zone. The main conclusions are as follows: (1) The δD and δ 18 O values for ore-forming fluid of the Tuwaishan gold deposit are from −75.9‰ to −54.4‰ and from +3.1‰ to +8.7‰, indicating a source of metamorphic fluid.
(2) The δ 34 S values for the ore-related pyrites of the Tuwaishan gold deposit are from +4.5‰ to +7.9‰, reflecting a source of metamorphism for the ore-forming materials. (3) The Tuwaishan and other gold deposits along the Gezhen shear zone are orogenic gold deposit that were formed as a result of regional metamorphism. (4) The Mesoproterozoic basement rocks could not be the source of the Gezhen gold deposits. Instead, the Ordovician-Silurian rocks on the footwall of the Gezhen shear zone are most likely the source for the Tuwaishan and other gold deposits along the shear zone, even though more work is required to confirm such a source. Data Availability Statement: All data generated or used during the current study are available in full within this article.