Ore-Fluid Evolution of the Sizhuang Orogenic Gold Deposit , Jiaodong Peninsula , China

The Sizhuang gold deposit with a proven gold resource of >120 t, located in northwest Jiaodong Peninsula in China, lies in the southern part of the Jiaojia gold belt. Gold mineralization can be divided into altered rock type, auriferous quartz vein type, and sulfide-quartz veinlet in K-feldspar altered granite. According to mineral paragenesis and mineral crosscutting relationships, three stages of metal mineralization can be identified: early stage, main stage, and late stage. Gold mainly occurs in the main stage. The petrography and microthermometry of fluid inclusion shows three types of inclusions (type 1 H2O–CO2 inclusions, type 2 aqueous inclusions, and type 3 CO2 inclusions). Early stage quartz-hosted inclusions have a trapped temperatures range 303–390 ◦C. The gold-rich main stage contains a fluid-inclusion cluster with both type 1 and 2 inclusions (trapped between 279 and 298 ◦C), and a wide range of homogenization temperatures of CO2 occurs to the vapor phase (17.6 to 30.5 ◦C). The late stage calcite only contains type 1 inclusions with homogenization temperatures between 195 and 289 ◦C. With evidences from the H–O isotope data and the study of water–rock interaction, the metamorphic water of the Jiaodong Group is considered to be the dominating source for the ore-forming fluid. The ore-fluid belonged to a CO2–H2O–NaCl system with medium-low temperature (160–360 ◦C), medium-low salinity (3.00–11.83 wt% NaCl eq.), and low density (1.51–1.02 g/cm3). Fluid immiscibility caused by pressure fluctuation is the key mechanism in inducing gold mineralization in the Sizhuang gold deposit.


Introduction
The giant Jiaodong gold deposit, with more than 4500 t proven gold reserves [1], is the most important gold producer of China [2,3].More than 150 deposits in Jiaodong area are generally divided into two styles, Jiaojia-style and Linglong-style [4].The mineralization of the former is the disseminated-stockwork type which occurs in the pyrite-sericite-quartz alteration belt and the latter's is the auriferous quartz vein type which is mostly controlled by subsidiary faults [5].However, both styles invariably occur in every single deposit in Jiaodong [6,7], with similar mineral paragenesis, alteration assemblages, and gold deposition ages [1,8].
Many previous inclusion studies show that three types of primary inclusions (H 2 O-CO 2 , aqueous, and CO 2 inclusions) have been founded in both the mineralization styles [6,9,10].They belong to the CO 2 -H 2 O-NaCl ± CH 4 fluid system with a medium-high temperature (158-393 • C) and a medium-low salinity (≤11.2 wt % NaCl eq.) [6][7][8][9][10].The fluid immiscible system was discussed and considered as a significant role in gold mineralization [6,10].The source of ore-forming fluids still remains a controversy: Magmatic water, mixing with meteoric water before and/or at mineralization, is considered the source based on a H-O isotope composition in many studies [11,12].Deng et al. (2015) [13] and Yang et al. (2017) [14] considered that the source of Jiaodong gold deposit may be from both the crust and mantle by carbon and sulfur isotope study; The δD values of hydrothermal sericite together with the O-S isotope indicate that the source of ore-forming metal is derived from the paleo-Pacific oceanic slab and its overlying sediments [6].
For the Sizhuang gold deposit, there are few studies including the geology, structures, and hydrothermal alteration [15], and only one published paper includes a fluid inclusion study.Wei et al (2015) [16] identified H 2 O-CO 2 and aqueous inclusions in quartz (hosted in auriferous quartz vein in early stage and main stage) and indicated a medium-low temperature (133-310 • C) and medium-low salinity (≤12 wt % NaCl eq.) H 2 O-CO 2 -NaCl fluid system.Just comparing the H-O isotope composition in the Sizhuang deposit with natural water, Wei et al (2015) [16] concluded that magmatic water was the dominating source of ore-fluid.
The origin of the Jiaodong gold province is still controversial.Only a simple comparison of isotopes cannot be convincing.In this study, three stages of quartz have been divided.Three mineralization styles have been identified and compared in the Sizhuang gold deposit which is well-known as a Jiaojia-style deposit.Three types of primary fluid inclusions were recognized by petrography and microthermometry.According to the H-O isotope, more comparisons with possibilities and the calculation of fluid compositions through a water-rock interaction will be done, the origin and evolution of ore-fluid in the Sizhuang gold deposit will be discussed.

Regional Geology
The Jiaodong Peninsula consists of two tectonic units, the Jiaobei Terrane of the North China Block and the Sulu Terrane of the Yangtze Block, which are bounded by Wulian-Qingdao-Yantai Fault [17] (Figure 1).The Jiaobei Terrane contains the Jiaobei Uplift in the north and Jiaolai Basin in the south.The Jiaobei Uplift includes more than 90% gold reserves and hosts the largest gold deposits of Jiaodong gold province [10,17] (Figure 1).
The major gold deposits in the Jiaobei Uplift are controlled by the NE-to NNE-trending fault system which cut through Linglong granite [4].The main faults from the west to east are Sanshadao, Jiaojia, Zhaoping, and Qixia Fault.All these faults are the subsidiary faults of the Tan-Lu Fault Zone (Figure 1).The Jiaojia Fault, which is the ore-controlling fault at the Sizhuang gold deposit (Figure 2), is one of the most significant faults in Jiaobei Uplift.A simplified geological map of the Jiaojia gold belt [17] showing the distribution of major faults and gold deposits: The Sizhuang gold deposit is located at the south segment of the Jiaojia Fault.

Deposit Geology
The Sizhuang gold deposit is situated in the footwall of the southern section of the Jiaojia Fault, about 28 km northeast to Laizhou City (Figure 1).It is a world-class gold deposit with a proven reserve of >120 t Au.Xenoliths of Jiaodong Group metamorphic rocks, mostly biotite amphibolite, only occurs in the hanging wall of Jiaojia Fault.The Linglong biotite granite is widely exposed in the mine and mainly in the footwall of the Jiaojia Fault (Figure 3).
The hydrothermal alteration zones, which develops along the Jiaojia Fault, can be divided into the pyrite-sericite-quartz alteration zone, sericite-quartz alteration zone, and K-feldspar alteration zone.The extent of sericite-quartz alteration is weaker and the volume of metallic sulfide (mostly pyrite) is fewer with the increasing distance to the Jiaojia Fault in the footwall.The content of K-feldspar in the alteration zone, especially in the K-feldspar alteration zone, is macroscopically higher than in Linglong biotite granite.
All gold mineralization occurs in the footwall of Jiaojia fault.The orebody is mainly controlled by the NE-to NNE-trending and SE-dipping Jiaojia fault system (Figure 2).The secondary faults, which are often filled with quartz veins and are surrounded by hydrothermal zones with a width from 0.5 m to more than ten meters, can be several hundreds to one thousand meters long and 0.1-1.5 m wide (Figure 3).Although the Sizhuang gold deposit is known as Jiaojia-style mineralization, three mineralization styles can be identified in the Sizhuang gold deposit.The first one is disseminated-stockwork type (Jiaojia-style) mineralization which is hosted in the first-order Jiaojia Fault (Figure 4a,d).There is less Jiaojia-style mineralization surrounding the subsidiary second-or third-order faults.The second style is the auriferous quartz vein style (Linglong-style) which almost occurs along the subsidiary secondor third-order faults (Figure 4b,e).The third type is sulfide-quartz veinlet in the K-feldspar altered granite (Figure 4c,f).These three mineralization styles have almost the same mineral assemblages and mineral paragenesis (Figure 4g-i).
The No. I and No. II orebody, characterized by disseminated-and stockwork-type ores, are respectively hosted in the pyrite-sericite-quartz alteration zone and sericite-quartz alteration zone which are located in the footwall of the Jiaojia Fault.The biggest No. III orebody (reserves 57.91% Au) occurs in the K-feldspar altered alteration zone.It is characterized by both Jiaojia-style and Linglong-style mineralization.The auriferous quartz veins are surrounded by Sericite-quartz alterations.As is pointed out above, there is no difference between different mineralization styles in mineral paragenesis in the Sizhuang gold deposit.The primary metallic mineral in ores is pyrite with lesser chalcopyrite, galena, sphalerite, pyrrhotite, and electrum.The gangue minerals mainly contain sericite, quartz, and calcite (Figure 6).With the study of cross-cutting relationships, textures, and mineral paragenesis, three mineralization stages have been divided which include the early stage, main stage, and late stage (Figure 7).The early stage is characterized by sericite-quartz-altered rocks with magmatic K-feldspar and white-milky quartz veins.There are less pyrite and few gold in the early stage.Few gold is deposited in this stage.
The main stage is characterized by the formation of quartz-pyrite-base-metal-sulfide stockworks and veins/veinlets, with a mass of gold and grey quartz in it.Gold occurs along the crack of pyrite.According to mineral assemblage, the main stage can be divided into two parts.In the earlier part, pyrite is the dominating sulfide.The later part is featured with the sulfide assemblage of pyrite, chalcopyrite, galena, and sphalerite.
The late stage is characterized by calcite veins/veinlets crosscutting the veins of main stage, with less pyrite and no visible gold in it.

Sampling and Analytical Procedures
Samples with different stages and mineralization styles were chosen for the fluid inclusion analyses on the basis of detailed fieldwork (Table 1).Twenty 0.2 mm-thick polished sections were used in the petrographic study.Quartz is the host mineral of fluid inclusion in early stage and main stage (Figure 8a,c).Calcite is the host mineral in late stage (Figure 8b,d).At length, eight sections were selected for microthermometric analysis, and 168 sets of data were obtained.

Fluid Inclusion Petrography
Quartz grains border upon or included by pyrite were chosen, and 27 inclusion clusters were identified, which are mostly interpreted to be primary in origin.The inclusions generally growing along trails, which may be secondary or pseudosecondary, are not used in following analysis.
By detailed petrography, following microthermometry and a comparison with a previous study in the Sizhuang gold deposit [16], three types of primary fluid inclusions were identified (Figure 9): H 2 O-CO 2 -NaCl inclusions (type 1), aqueous inclusions (type 2) and CO 2 inclusions (type 3).Most type 1 inclusions contain two or three phases (liquid H 2 O + CO 2 rich vapor or liquid H 2 O-liquid CO 2 -CO 2 rich vapor, respectively) at room temperature (Figure 9c,d).These inclusions (3-11 µm in diameter), the carbonic phases of which mainly account for 10-40%, commonly appear as groups or along growth zones and are widespread in any stages.Type 2 aqueous inclusions, which are commonly 2-13 µm in diameter, consist of H 2 O liquid and H 2 O vapor (Figure 9a,b).The degree of fill of H 2 O vapor mostly range 10-40%.Like most of the gold deposit in Jiaodong [6,[9][10][11], type 3 primary CO 2 inclusions are small (2-4 µm in diameter) and are infrequent in this study.They include CO 2 liquid and CO 2 vapor (Figure 9e).There is a phenomenon in main stage that both type 1 and type 2 fluid inclusions with similar total homogenization temperatures occur in the same inclusion cluster (Figure 9f), which suggests immiscibility during main stage.

Main Stage
In main stage, the three inclusion types mentioned above were all observed.For type 1 inclusions (Figure 10; Table 2), the carbonic phase melting temperatures (T mCO2 ) of H 2 O-CO 2 inclusions ranged from −60.5 to −56.5 • C (mean −57.6 • C, n = 55).
The integral clathrate-melting temperature (T mclath ) was between 5.2-7.9• C (mean 6.9 • C, n = 92).The homogenization of CO 2 (T hCO2 ) occurred in the vapor phase range 17.6 to 30.5 • C (mean 25.7 • C, n = 24) which was much wider than the range of the type 1's T hCO2 in early stage, which suggested the possibility of a bigger density change in main stage.The total homogenization (T hTOT ) into liquid occurred between 165 and 342 • C (mean 272 • C, n = 100).Some of the type 1 inclusions contained second phases at room temperature, which made them look like aqueous inclusions.However, when the temperature approximately cools down to 16 • C, they became 3-phases inclusions.
For CO 2 inclusions (Figure 10; Table 2), the carbonic phase melting temperatures (T mCO2 ) of CO 2 inclusions were −56.6 • C (n = 2).The homogenization of CO 2 (T hCO2 ) occurred in the vapor phase at 27.1 • C, nearly the same as T hCO2 of the mean of type 1.

Late Stage
Only type 1 H 2 O-CO 2 inclusions were identified in this stage.The carbonic phase melting temperatures (T mCO2 ) ranged between −58.
The range of densities of type 1 H 2 O-CO 2 inclusions was 0.516-1.017g/cm 3 (mean 0.886 g/cm 3 ).The range of type 2 aqueous inclusions was 0.506-0.937g/cm 3 .All in all, the density of fluid Sizhuang gold deposit ranged between 0.506 to 1.017 g/cm 3 .

Fluid Immiscibility and Pressure Fluctuation
As is mentioned above, the H 2 O-CO 2 and aqueous inclusions hosted in pyrite-related quartz in main stage can occur in a same inclusion cluster (Figure 9f), which can be considered as evidence of a fluid immiscibility.However, fluid mixing and necking down can result in this phenomenon as well [32,33].First and foremost, it should be eliminated that the fluid mixing and/or necking down may be the reasons for co-occurrences of the H 2 O-CO 2 inclusions and aqueous inclusions.
In a mixing fluid, the homogenization temperatures and salinities of H 2 O-CO 2 inclusions are different from those of aqueous inclusions [34].However, in this study, the ranges of salinities of H 2 O-CO 2 inclusions and aqueous inclusions are similar and the homogenization temperatures are discrepant, which indicates the fluid mixing is not the reason for type 1 and type 2 being in the same fluid cluster.Although necking down could generate this assemblage, the relatively constant degree of filling of vapor in each inclusion clusters and the low deformation level of quartz can even suggest that it should be eliminated.
Moreover, more evidences of immiscibility are shown in this study: (1) Gold mineralization related H 2 O-CO 2 inclusions and aqueous inclusions occupy in same assemblages (Figure 9f); (2) compared with H 2 O-CO 2 inclusions, aqueous inclusions have slightly lower total homogenization temperatures and higher salinities (Figure 11), which correspond with fluid immiscibility [35,36]; and (3) the similar homogenization temperatures of type 1 and type 3 suggest the enrichment of CO 2 may be caused by fluid immiscibility [37,38] (Figure 10; Table 2).Fluid immiscibility is vital in the Sizhuang deposit's gold mineralization.During fault movement, the fluid pressure decline can lead to the immiscibility of H 2 O-NaCl-CO 2 crustal fluid [39].Also, there is another argument that retrograde boiling can cause fluid immiscibility without a sharp decline in fluid pressure [40].In this study, there is a phenomenon in main stage that the T hTOT of type 2 aqueous inclusions is about 40 • C lower than that of the coexisting type 1 H 2 O-CO 2 inclusions (Figure 11).It can be caused by the partial CO 2 boiling in saturated aqueous fluid [41].Considering the Sizhuang gold deposit is controlled by a fault, CO 2 boiling may cause the fluid pressure fluctuation in the structural controlled Sizhuang gold deposit during faulting [41,42].
Fluid pressure can be reflected by the density of the fluid inclusions [43,44].In the Sizhuang gold deposit, the density ranges of the fluid inclusions in different stages and mineralization styles are wide (Figure 12), which show the evidence of a fluid pressure fluctuation during gold mineralization.which makes the result inaccurate.On the other hand, ore-fluid may have a complicated water-rock interaction during transportation and contacting with rocks [45].Therefore, more natural waters which may have a possibility of mineralization in the Sizhuang deposit (metamorphic water of the Jiaodong Group, magmatic water of the Linglong granite, magmatic water of the Guojialing granite) are compared with the H-O isotope composition in this study.
In general, similar to the majority of the gold deposits in Jiaodong, the spots of δ 18 O and δD are located between metamorphic water/magmatic water and meteoric water (Figure 13).In view of the H-O isotope geochemical background in the Jiaojia gold belt (Table 4) and the absence of Guojialing granite in the Sizhuang god deposit, all spots located in region A and B suggest that the metamorphic water of the Jiaodong Group and the magmatic water of Linglong granite may take part in the mineralization (Figure 13).There is a trend of spots approach to Mesozoic meteoric water, which indicates meteoric water may be a part of mineralization.Therefore, the mixing fluid of magmatic water of Linglong granite, metamorphic water of the Jiaodong Group and meteoric water is the source of ore-fluid.

Fluid Evolution and Water-Rock Interaction
As is noted above, ore-fluid may experience the water-rock interaction before mineralization.The water-rock interaction of ore-fluid and wall rocks follows the equation of meteoric-hydrothermal system [54]: Rock where δ i water and δ i Rock are the values of isotope compositions before the water-rock interaction; δ f water and δ f Rock are the final values after exchange; W and R are the numbers of atoms (oxygen or hydrogen in this study) in fluid and wall rock, respectively.When the water-rock exchange system is at equilibrium The H-O isotope composition of granite and metamorphic rocks in the giant Jiaodong gold deposit can be replaced by δD biotite and δ 18 O plagioclas (An 30 ) [50].∆ RW of δO 18 and δD were calculated and demonstrated in this system as follows [48,55]: The calculation of water-rock interaction uses the rate of quality of δD and δ 18 O in this study.In intermediate-acidic granite, the quality rates of oxygen and hydrogen have a stable relationship with the rates of atoms' numbers as follows [56]: Therefore, the final values of δO 18 and δD can be worked out with detailed calculation (Table S1). Figure 14a shows the H-O isotope composition and H-O isotope evolution after an exchange in the water-rock system in early stage (mean homogenization temperature 305 • C).Line 1 represents the evolution of exchange between the magmatic water of Linglong granite and Jiaodong Group metamorphic rocks.Line 2 represents the evolution of exchange between he metamorphic water of the Jiaodong Group and Linglong Granite.Line 3 represents the evolution of exchange between the meteoric water and Jiaodong Group metamorphic rocks.Line 4 represents the evolution of exchange between the meteoric water and Linglong granite.
Figure 14b shows the H-O isotope composition and H-O isotope evolution after exchange in the water-rock system in main stage (mean homogenization temperature 235 • C).Line 5 represents the evolution of exchange between the magmatic water of Linglong granite and Jiaodong Group metamorphic rocks.Line 6 represents the evolution of exchange between the metamorphic water of the Jiaodong Group and Linglong Granite.Lines 7-9 represents the evolution of exchange between the mixing fluid (metamorphic water of the Jiaodong Group and meteoric water mix with the rate 8:1, 4:1 and 2:1, respectively) and Linglong granite.Line 10 represents the evolution of exchange between the meteoric water and Linglong Granite.
In Figure 14a, the spots are under Line 1 distinctly, which suggests the magmatic water of Linglong granite may be not involved in fluid evolution in the early stage of the Sizhuang gold deposit.These spots are around Line 2, and one of them is on Line 3, indicating that the metamorphic water of the Jiaodong Group plays an important role in fluid evolution of the main stage.
In Figure 14b, the spots of the main stage mostly fall between Line 8 and Line 9, which suggest that the metamorphic water of the Jiaodong Group is the uppermost source of ore-fluid of the Sizhuang gold deposit.When the values of W/R are big (10-0.1),the mixing fluid of metamorphic water of the Jiaodong Group and meteoric water (with the mixing rate between 4:1 and 2:1) is propitious to gold mineralization.In conclusion, the source of the ore-fluid is the metamorphic water of the Jiaodong Group in early stage.In main stage, the metamorphic water of the Jiaodong Group and meteoric water mix with the rate between 4:1 and 2:1.The mixing fluid reacts with Linglong granite and extracts mineralization-related elements for gold deposition.

1.
Microthermometry showed less obvious differences between different mineralization styles, which suggested a similar mechanism of gold deposition.

Figure 2 .
Figure 2.A simplified geological map of the Jiaojia gold belt[17] showing the distribution of major faults and gold deposits: The Sizhuang gold deposit is located at the south segment of the Jiaojia Fault.

Figure 3 .
Figure 3.A sketch map of the Sizhuang gold deposit showing the orebodies and hydrothermal alteration: (a) The plan view of the Sizhuang deposit and (b) the geological cross sections along lines AB.The orebody was defined by the gold cut-off grade (1 g/t).

Figure 5 .
Figure 5. Field photographs showing the crosscutting relationships of the major geologic bodies from the Sizhuang gold deposit: (a,b) The K-feldspar altered granite crosscuts the biotite granite and (c,d) the quartz-sulfide vein crosscuts the K-feldspar altered granite, and the calcite vein crosscuts the quartz-sulfide vein.

Figure 6 .
Figure 6.Photomicrographs under transmitted light (a,b,h) and reflected light (c-g), showing important mineral assemblages in the Sizhuang gold deposit: (a) Sericite-quartz alteration consists of sericite and quartz, with few pyrite; (b) disseminated pyrite occurs in the matrix of sericite and quartz; (c-g) the primary sulfides in the main stage of the Sizhaung deposit; and (h) the calcite vein crosscuts the sericite and quartz.Abbreviations: Kfs = K-feldspar, Ccp = chalcopyrite, Qtz = quartz, Ser = sericite, Py = pyrite, Sp = sphalerite, Cc = calcite.

Figure 7 .
Figure 7.The paragenetic sequence of the hydrothermal alteration and mineralization at the Sizhuang gold deposit interpreted by crosscutting relationships and assemblages of minerals: The thickness means the abundance of minerals in the paragenetic sequence.

Figure 8 .
Figure 8. Photographs of the field view and ores shows the inclusion-hosted quartz (a,c) and calcite (b,d).Fluid-inclusion microthermometry was made on the Linkam THMSG 600 heating-cooling stage (−198 • C to 600 • C) on a Leitz microscope at the Fluid Inclusion Laboratories, China University of Geosciences, Beijing.Synthetic fluid inclusions were used to calibrate the stage to make sure for an accuracy of which the error is ±0.1 • C at temperatures <30 • C and ±1 • C at temperatures >30 • C, the heating rat of carbonic phase (T mCO2 ) and clathrate-melting temperatures (T mclath ) are 0.2 • C/min, and it is 1.0 • C/min for the carbonic phase (T hCO2 ).

Figure 9 .
Figure 9. Photomicrographs of the fluid inclusions in the Sizhuang gold deposits: (a,b) Type 2 aqueous inclusions contain two phases (liquid H 2 O and vapor H 2 O); (c,d) Type 1 H 2 O-CO 2 inclusions generally consist of three phases (liquid H 2 O, liquid CO 2 and CO 2 rich vapor); (e) Type 3 CO 2 inclusions are small and infrequent in this study; and (f) the inclusions with a total homogenization temperature of 298 • C is a type 2 aqueous inclusion.The other inclusions are Type 1 H 2 O-CO 2 inclusions.

Figure 10 .
Figure 10.Histograms of the microthermometric data showing the temperatures of T mCO2 , T mclath , T hCO2 and T hTOT and the salinity.

Figure 11 .
Figure 11.The temperatures ( • C) of the total homogenization temperatures (T hTOT ) versus salinity (wt% NaCl eq.):The solid and hollow signs represent the H 2 O-CO 2 inclusions and aqueous inclusions, respectively.

Figure 12 .
Figure 12.Histograms of the density of type 1 fluid inclusions: The range of inclusion density is wide in different stages or in different mineralization styles.

5. 2 .
Source and Evolution of Ore-Fluid 5.2.1.Source of Ore-Fluid H-O isotopes are commonly used for studying the source of fluid.Many previous studies compare the H-O isotopes compositions with those of magmatic water, metamorphic water and meteoric water in order to speculate the source of fluid.On the one hand, the range of natural H-O isotopes are wide,

Figure 13 .
Figure 13.The H-O composition of ore-fluids in early and main stages: Area A means metamorphic water of the Jiaodong Group mixed with meteoric water; Area B means magmatic water of the Linglong granite mixed with meteoric water; and Area C means magmatic water of the Guojialing granite mixed with meteoric water.The base map is modified after Guo et al. (2014) [53].

Figure 14 .
Figure 14.The evolution of the H-O composition during the water-rock interaction.

Table 1 .
The basic information of the samples in this study.

Table 2 .
The results of fluid inclusions microthermometry.

Table 4 .
The H-O isotope geochemical background in the Jiaojia gold belt.