Importance of Magmatic Water Content and Oxidation State for Porphyry-Style Au Mineralization: An Example from the Giant Beiya Au Deposit, SW China

: The Beiya Au deposit is the largest Cenozoic Au deposit in the Jinshajiang-Ailaoshan porphyry metallogenic belt. Numerous studies document that high water content and f O 2 are vital factors for the generation of Au mineralization. In this belt, only the Wandongshan and Hongnitang districts are considered to be of economic importance, while the other districts, such as Bailiancun, are barren. So in order to reveal the importance of water content and oxidation state for Beiya porphyry-style Au mineralization, the amphiboles and zircons compositions are used to evaluate the physicochemical conditions (e.g., pressure, temperature, f O 2 , and water content) of the Wandongshan ore-fertile porphyries and Bailiancun ore-barren porphyries observed in the Beiya Au deposit. The results show that the water content of the Wandongshan parent magma ( ≤ 4.11 ± 0.4 wt %) are slightly higher than those of the parent magma at Bailiancun ( ≤ 3.91 ± 0.4 wt %), while the emplacement pressure of the Wandongshan parent magma (31.5–68.6 MPa) is much lower than that of the parent magma at Bailiancun (142.3–192.8 MPa), indicating that the Wandongshan magma reached water saturation earlier. In addition, the Wandongshan porphyries crystallized from more oxidized magma (average of ∆ FMQ = +3.5) with an average temperature of 778 ◦ C compared to the Bailiancun porphyries (average of ∆ FMQ = +1.5) with a mean magmatic temperature of 770 ◦ C. The Ce 4+ /Ce 3+ ratio of zircon in the Wandongshan ore-related intrusions (average Ce 4+ /Ce 3+ of 62.00) is much higher than that of the Bailiancun barren porphyries (average Ce 4+ /Ce 3+ of 23.15), which further conﬁrmed Wandongshan ore-related magma is more oxidized than the Bailiancun barren magma. Therefore, melts that are more enriched in water and with a high oxidation state will be more fertile to form an economic porphyry-style Au system.


Introduction
Porphyry systems supply three-quarters of the world's Cu and one-fifth of the world's Au resources [1]. It's widely accepted that high water content (>4 wt %) [2,3] and a high oxidation state (>FMQ + 1.3-2) [4,5] of the magma are critical factors for the generation of Cu-Au mineralization. The solubility of Cu and Au in hydrous silicate melts was proven to be affected by the amount and proportion of dissolved sulfate and sulfide in S-bearing magma, whereas f O 2 controls the speciation of sulfur [5][6][7]. Under high f O 2 conditions, the majority of sulfur exists as SO 2 or SO 4 2− with very low

Geological Setting
The Sanjiang (Nujiang-Lancangjiang-Jinshajiang) region is an important part of the Tethys tectonic domain, located in Southwestern China [25,26]. The Jinshajiang-Ailaoshan suture zone is located in the eastern Himalaya-Tibetan plateau of the Tethyan domain and is bounded by the western Yangtze Craton to the southeast ( Figure 1). The regional tectonic evolution includes subduction of the Paleo-Tethyan oceanic slab, and magmatic arc formation followed by arc-continent collision during the Paleozoic-Mesozoic era, and then Indo-Asian continental collision during the Cenozoic [27,28]. The collision between the Indo and Asian continents have caused more than 300 km of strike-slip movement along the Ailaoshan-Red River shear zone since the Paleocene [29,30]. As a consequence of this strike-slip, lithospheric-scale extension has occurred, causing a set of secondary NNW-trending strike-slip faults and folds, and the emplacement of numerous alkali-rich intrusions along the Jinshajiang suture zone [31][32][33]. These intrusions form a 2000-km-long and 50-to 80-km-wide alkaline magmatic belt, and are associated with several important porphyry and skarn polymetallic (Cu, Au, Mo, Zn, Pb, and Ag) deposits [34,35] in the Jinshajiang-Ailaoshan metallogenic belt (Figure 1b) [7], from north to south: the Yulong Cu, the Beiya Au, the Machangqing Cu-Mo-Au, the Yao'an Au, the Habo Cu-Mo-Au, and the Tongchang Cu-Mo deposits.

The Wandongshan Monzogranite Porphyry
The Wandongshan monzogranite porphyry intrusion has a gray-white color and porphyritic textures. Phenocrysts form about 50 vol % of the rock, and include K-feldspar, plagioclase, quartz, biotite, and minor amphiboles ( Table 1). The K-feldspar phenocrysts make up about 5-15 vol % and usually have euhedral to subhedral shapes that range in size from 0.2 to 0.5 mm, with occasional Carlsbad twinning (Figure 3a). The plagioclase phenocrysts (1-5 mm) are mostly euhedral in shape, showing tabular biotite inclusions ( Figure  3c). The quartz phenocrysts are irregular, with a circular shape. The amphibole phenocrysts are mostly

The Wandongshan Monzogranite Porphyry
The Wandongshan monzogranite porphyry intrusion has a gray-white color and porphyritic textures. Phenocrysts form about 50 vol % of the rock, and include K-feldspar, plagioclase, quartz, biotite, and minor amphiboles ( Table 1) shape. The amphibole phenocrysts are mostly euhedral, usually measuring 0.4 mm in size (Figure 3b). The groundmass comprises fine-grained feldspar and quartz. The accessory minerals are apatite, magnetite, titanite, and zircon. The mineralization is dominated by pyrite and chalcopyrite within quartz veins, as well as disseminated in the monzogranite. euhedral, usually measuring 0.4 mm in size (Figure 3b). The groundmass comprises fine-grained feldspar and quartz. The accessory minerals are apatite, magnetite, titanite, and zircon. The mineralization is dominated by pyrite and chalcopyrite within quartz veins, as well as disseminated in the monzogranite.

The Bailiancun Monzogranite Porphyry
The monzogranite porphyry intrusion at the Bailiancun deposit is characterized by porphyritic texture. Phenocrysts consist of K-feldspar, quartz, plagioclase, and amphibole. The groundmass includes K-feldspar, quartz, and amphiboles ( Table 1). The accessory minerals are dominated by apatite and zircon. The K-feldspar phenocrysts make up about 30 vol % with a euhedral to subhedral habitus, and measuring 0.4 mm in size (Figure 3e). The quartz phenocrysts have subrounded circular shapes. The plagioclase phenocrysts account for 25 vol % and are mostly euhedral. The amphibole phenocrysts are small (0.3 mm), with euhedral shapes (Figure 3f).

The Bailiancun Monzogranite Porphyry
The monzogranite porphyry intrusion at the Bailiancun deposit is characterized by porphyritic texture. Phenocrysts consist of K-feldspar, quartz, plagioclase, and amphibole. The groundmass includes K-feldspar, quartz, and amphiboles ( Table 1). The accessory minerals are dominated by apatite and zircon. The K-feldspar phenocrysts make up about 30 vol % with a euhedral to subhedral

Analytical Methods
Electron microprobe analysis was conducted on amphiboles using a JEOL JXA-8800 Superprobe at the Institute of Mineral Resources, Chinese Academy of Geological Sciences. The microprobe was operated at an accelerating voltage of 15 kV, a beam current of 20 nA, and a beam size of 5 µm. Matrix corrections were performed using the ZAF correction program supplied by the instrument manufacturer. Chemical compositions of amphiboles can be used to evaluate the physicochemical conditions (e.g., pressure, temperature, f O 2 , and water content) of the parental magma from which the amphibole crystallized [25,36,37].
Zircon trace elements were determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at the Key Laboratory of Crust-Mantle Materials and Environments, Chinese Academy of Sciences (CAS), Hefei. The LA-ICP-MS analyses used a Finnigan Neptune MC-IPS-MS with a high-performance interface coupled with a New-Wave UP-213nm Nd: YAG UV laser. The diameter of the laser ablation pit was 45 µm and the mean power output was 4 W. Signal and background measuring durations were about 70 and 40 s, respectively. Further analytical details are given in Liu et al. [38]. Zircon is a widespread accessory mineral that retains its primary chemical and isotope compositions from the time of crystallization [39], which means that the magma oxygen state during crystallization can be recorded by the zircon composition. Zircon solubility is mainly controlled by the temperature and major oxide concentrations of the magma [40]. Titanium exists as Zr 4+ and Si 4+ isotopes, so the Ti content are controlled by the temperature of the zircon crystallization [41][42][43]. This study uses the Ti content of the zircon crystals in zircon-saturated melts to estimate the temperature of the melt (log(Ti) = 6.01 ± 0.03 − (5080 ± 30)/T(K)) [42].

Amphibole Compositions
Major element compositions and chemistry of amphibole from the monzogranite porphyry at Wandongshan and Bailiancun districts are reported in Supplementary Materials, Table S1.
The emplacement pressure of the parental magma at Wandongshan and Bailiancun can be estimated using the Al-in-amphibole geobarometer [25,36]. These results show that the emplacement pressures of Wandongshan porphyries range from 31.5 MPa to 68.6 MPa, which corresponds to a paleodepth of 1.2-2.4 km (average 1.8 km) under lithostatic pressure (Table 2) [36]. In contrast, the emplacement pressures of Bailiancun porphyries are significantly higher (142. , which corresponds to a deeper paleodepth of 5.4-7.3 km (average 6.7 km). Furthermore, water content are directly measure by the component of Wandongshan amphiboles (H 2 O melt = 5.215 [6] Al* + 12.28) [25]. These results show that the water content of the Wandongshan parent magma exhibit a wider range of 3.05 ± 0.4 wt % to 4.11 ± 0.4 wt %, whereas those of the Bailiancun parent magma have a tight range of 3.46 ± 0.52 wt % to 3.91 ± 0.4 wt % (Table 2, Figure 4b). Moreover, the amphibole composition is affected by the oxidation state of its parental melt, thus they allow the direct analysis of the oxygen state of their parental magmas [25]. The oxidation state deduced from amphiboles of the Wandongshan porphyry range from ∆NNO + 1.00 to ∆NNO + 1.89 (NNO is the nickel-nickel oxide oxygen buffer), slightly higher than that of the Bailiancun porphyries (∆NNO + 0.39 to ∆NNO + 0.67, Table S1).  Table S2).

Discussion
As mentioned above, the Wandongshan Au-bearing porphyries and Bailiancun barren porphyries have different physicochemical conditions (e.g., pressure, temperature, fO2, and water content). It is widely accepted that high oxidation state (high fO2) and high H2O content are two key factors responsible for Au mineralization [4,12,13].

The Water Content of the Parental Magmas
A high magmatic fluid flux ensures the exsolution of an aqueous volatile phase, which is considered as the sine qua non for magmatic hydrothermal ore-forming systems [2]. Both the Wandongshan and Bailiancun monzogranite porphyries contain amphibole phenocrysts. The amphibole phenocrysts at Wandongshan and Bailiancun are mostly euhedral in shape, and are characterized by embayed textures indicative of resorption (Figure 3b,f), suggesting that these amphiboles crystallized early. The presence of amphibole during the early stages of crystallization demonstrates that the parental magma at Wandongshan and Bailiancun districts are hydrous melts (>4 wt % H2O) at elevated pressures [45,46]. Furthermore, the water content of the Wandongshan parent magma are characterized by a wider range of 3.05 ± 0.4 wt % to 4.11 ± 0.4 wt %, whereas those of the Bailiancun parent magma have a tight range of 3.46 ± 0.52 wt % to 3.91 ± 0.4 wt % (Table 2), which further confirmed the parental magma at Wandongshan and Bailiancun districts are relatively water-rich.
The emplacement pressure of the parental magma at Wandongshan and Bailiancun can be estimated using the Al-in-amphibole geobarometer [25,36]. These results show that the emplacement pressures of Wandongshan porphyries range from 31.5 MPa to 68.6 MPa, but those of Bailiancun porphyries are

Discussion
As mentioned above, the Wandongshan Au-bearing porphyries and Bailiancun barren porphyries have different physicochemical conditions (e.g., pressure, temperature, f O 2 , and water content). It is widely accepted that high oxidation state (high f O 2) and high H 2 O content are two key factors responsible for Au mineralization [4,12,13].

The Water Content of the Parental Magmas
A high magmatic fluid flux ensures the exsolution of an aqueous volatile phase, which is considered as the sine qua non for magmatic hydrothermal ore-forming systems [2]. Both the Wandongshan and Bailiancun monzogranite porphyries contain amphibole phenocrysts. The amphibole phenocrysts at Wandongshan and Bailiancun are mostly euhedral in shape, and are characterized by embayed textures indicative of resorption (Figure 3b,f), suggesting that these amphiboles crystallized early. The presence of amphibole during the early stages of crystallization demonstrates that the parental magma at Wandongshan and Bailiancun districts are hydrous melts (>4 wt % H 2 O) at elevated pressures [45,46]. Furthermore, the water content of the Wandongshan parent magma are characterized by a wider range of 3.05 ± 0.4 wt % to 4.11 ± 0.4 wt %, whereas those of the Bailiancun parent magma have a tight range of 3.46 ± 0.52 wt % to 3.91 ± 0.4 wt % (Table 2), which further confirmed the parental magma at Wandongshan and Bailiancun districts are relatively water-rich.
The emplacement pressure of the parental magma at Wandongshan and Bailiancun can be estimated using the Al-in-amphibole geobarometer [25,36]. These results show that the emplacement pressures of Wandongshan porphyries range from 31.5 MPa to 68.6 MPa, but those of Bailiancun porphyries are significantly higher, ranging from 142.3 MPa to 192.8 MPa ( Table 2). The solubility of H 2 O in silicate magmas is mainly controlled by pressure and to a lesser extent by temperature [45]. The Wandongshan parent magma could reach water saturation at 31.5-70.9 MPa if they contain~2.5% H 2 O (Figure 4b). By contrast, the Bailiancun parental magma would need more water when they reach water saturation at 142. 3-192.8 MPa. Therefore, it is expected that the Wandongshan parental magma will achieve water saturation easier, and the fluid exsolution would take place at 1.2-2.4 km, which consistent with the estimated trapping depth for Stage I S-type inclusions (~2 km depth under lithostatic conditions) [17].

Magmatic Oxidation State
The lg(f O 2 ) vs. T and Ce anomaly vs. 1/T diagrams are divided into several oxygen fugacity fields by curves, which can be used to estimate the oxidation state of the magma. The data points of the Wandongshan monzogranite porphyry are mainly plotted within the field between magnetite-hematite buffer curve (MH) and fayalite-magnetite-quartz buffer curve (FMQ), while the f O 2 of the intrusions from the Bailiancun area are lower than those of the Wandongshan porphyries (Figure 5b,c). This result confirms that the porphyries from the Wandongshan district are more oxidized than those the porphyries from the Bailiancun district.
Furthermore, direct measurement of f O 2 also implies that the Wandongshan porphyries are more oxidized, the f O 2 deduced from amphibole chemistry of the Wandongshan porphyry range from ∆NNO + 1.00 to ∆NNO + 1.89 (NNO is the nickel-nickel oxide oxygen buffer), slightly higher than that of the Bailiancun porphyries (∆NNO + 0.39 to ∆NNO + 0.67, Figure 6a). This result is consistent with the values calculated by zircon chemistry. The estimated f O 2 using zircon chemistry for the Wandongshan porphyry is ∆FMQ + 3.5, slightly higher than that of the Bailiancun monzogranite porphyry (∆FMQ + 1.5; Figure 5a). Moreover, the Wandongshan monzogranite porphyries have an average zircon Ce 4+ /Ce 3+ ratio of 62.00 (Table 2), and an average zircon Ce anomaly of 76.11 (Figure 5c), much higher than those of the Bailiancun monzogranite porphyry (23.15, 30.98, respectively). Those differences above indicate the formation of Au-bearing porphyries requires elevated f O 2 .  Table 2). The solubility of H2O in silicate magmas is mainly controlled by pressure and to a lesser extent by temperature [45]. The Wandongshan parent magma could reach water saturation at 31.5-70.9 MPa if they contain ~2.5% H2O (Figure 4b). By contrast, the Bailiancun parental magma would need more water when they reach water saturation at 142. 3-192.8 MPa. Therefore, it is expected that the Wandongshan parental magma will achieve water saturation easier, and the fluid exsolution would take place at 1.2-2.4 km, which consistent with the estimated trapping depth for Stage I S-type inclusions (~2 km depth under lithostatic conditions) [17].

Magmatic Oxidation State
The lg(fO2) vs. T and Ce anomaly vs. 1/T diagrams are divided into several oxygen fugacity fields by curves, which can be used to estimate the oxidation state of the magma. The data points of the Wandongshan monzogranite porphyry are mainly plotted within the field between magnetite-hematite buffer curve (MH) and fayalite-magnetite-quartz buffer curve (FMQ), while the fO2 of the intrusions from the Bailiancun area are lower than those of the Wandongshan porphyries (Figure 5b,c). This result confirms that the porphyries from the Wandongshan district are more oxidized than those the porphyries from the Bailiancun district.
Furthermore, direct measurement of fO2 also implies that the Wandongshan porphyries are more oxidized, the fO2 deduced from amphibole chemistry of the Wandongshan porphyry range from ΔNNO + 1.00 to ΔNNO + 1.89 (NNO is the nickel-nickel oxide oxygen buffer), slightly higher than that of the Bailiancun porphyries (ΔNNO + 0.39 to ΔNNO + 0.67, Figure 6a). This result is consistent with the values calculated by zircon chemistry. The estimated fO2 using zircon chemistry for the Wandongshan porphyry is ΔFMQ + 3.5, slightly higher than that of the Bailiancun monzogranite porphyry (ΔFMQ + 1.5; Figure 5a). Moreover, the Wandongshan monzogranite porphyries have an average zircon Ce 4+ /Ce 3+ ratio of 62.00 (Table 2), and an average zircon Ce anomaly of 76.11 (Figure 5c), much higher than those of the Bailiancun monzogranite porphyry (23.15, 30.98, respectively). Those differences above indicate the formation of Au-bearing porphyries requires elevated fO2.

Implications for Au Mineralization
Beiya is one of the largest Au deposits in China, with by-products Cu, Fe, Ag, Pb, and Zn [23][24][25]. Au and other ore metals were derived from the alkaline melt as documented by Yang et al. [47], and the mineralized fluids at Beiya were mainly derived from magmatic water [16,17]. Those indicate that the Beiya alkaline melt provides ore-forming fluids and metallic elements for the formation of porphyry Au mineralization [16].

Implications for Au Mineralization
Beiya is one of the largest Au deposits in China, with by-products Cu, Fe, Ag, Pb, and Zn [23][24][25]. Au and other ore metals were derived from the alkaline melt as documented by Yang et al. [47], and the mineralized fluids at Beiya were mainly derived from magmatic water [16,17]. Those indicate that the Beiya alkaline melt provides ore-forming fluids and metallic elements for the formation of porphyry Au mineralization [16].
In contrast to Bailiancun porphyry, the Wandongshan porphyries are characterized by higher water content and higher f O 2 , but lower emplacement pressure (Table 2, Figure 7). Au solubility was investigated as a function of f O 2 in sulfur-saturated silicate melts [12], indicating Au solubility in sulfur-free silicate melt increases with increasing of f O 2 [48]. In addition, high f O 2 favors sulfur as sulfate dissolved in the evolved magmas and suppresses magmatic sulfide precipitation [4,7], which can cause sequestration of metals before they partition into the aqueous phase [10]. On the other hand, a high H 2 O content and a low pressure results in the Wandongshan parental magma reaching water saturation easier, because water solubility increases with increasing pressure [44], hence ensuring exsolution of aqueous volatile phases and the partitioning of Au between sulfides and silicate melt efficiently [12,49,50]. Therefore, melts that are more enriched in water and with a high oxidation state will be more fertile to generate Au mineralization. In contrast to Bailiancun porphyry, the Wandongshan porphyries are characterized by higher water content and higher fO2, but lower emplacement pressure (Table 2, Figure 7). Au solubility was investigated as a function of fO2 in sulfur-saturated silicate melts [12], indicating Au solubility in sulfur-free silicate melt increases with increasing of fO2 [48]. In addition, high fO2 favors sulfur as sulfate dissolved in the evolved magmas and suppresses magmatic sulfide precipitation [4,7], which can cause sequestration of metals before they partition into the aqueous phase [10]. On the other hand, a high H2O content and a low pressure results in the Wandongshan parental magma reaching water saturation easier, because water solubility increases with increasing pressure [44], hence ensuring exsolution of aqueous volatile phases and the partitioning of Au between sulfides and silicate melt efficiently [12,49,50]. Therefore, melts that are more enriched in water and with a high oxidation state will be more fertile to generate Au mineralization.

Figure 7.
Cartoons illustrating the different water content, magmatic oxidation state, and emplacement depths of the parental magmas at Wandongshan (a) and Bailiancun (b), respectively. Previous studies show that the parent magma at Wandongshan (a) and Bailiancun (b) districts are derived from the same magma source [22]. This study reveals that the magma of Wandongshan experienced fluid saturation at about 1.19-2.68 km, while the magma of Wandongshan experienced fluid saturation at about 6.48-7.28 km. The water content and magmatic oxidation state of Wandongshan parental magmas are both higher than those of the Bailiancun parental magmas.

Conclusions
Chemical compositions of zircons and amphiboles are used to evaluate the physicochemical conditions (e.g., pressure, temperature, fO2, and water content) of the parental magma. We found the water content of the Wandongshan parent magma (≤4.11 ± 0.4 wt %) are slightly higher than those of the parent magma at Bailiancun (≤3.91 ± 0.4 wt %). Furthermore, the fO2 deduced from amphibole chemistry of the Wandongshan porphyry range from ΔNNO + 1.00 to ΔNNO + 1.89, slightly higher than that of the Bailiancun porphyries (ΔNNO + 0.39 to ΔNNO + 0.67). This result is consistent with the values calculated by zircon chemistry. The Wandongshan porphyry has higher zircon Ce 4+ /Ce 3+ ratio (average of 62) and high fO2 value (average of ΔFMQ = +3.5) than those of Bailiancun porphyry (average of Ce 4+ /Ce 3+ = 23.15; ΔFMQ = +1.5). Obviously, the Wandongshan ore-related porphyry has higher water content and more oxidized than the Bailiancun barren porphyry. Therefore melts that are more enriched in water and with a high oxidation state will be more fertile to form an economic hydrothermal system especially when emplaced at shallow levels. Cartoons illustrating the different water content, magmatic oxidation state, and emplacement depths of the parental magmas at Wandongshan (a) and Bailiancun (b), respectively. Previous studies show that the parent magma at Wandongshan (a) and Bailiancun (b) districts are derived from the same magma source [22]. This study reveals that the magma of Wandongshan experienced fluid saturation at about 1.19-2.68 km, while the magma of Wandongshan experienced fluid saturation at about 6.48-7.28 km. The water content and magmatic oxidation state of Wandongshan parental magmas are both higher than those of the Bailiancun parental magmas.

Conclusions
Chemical compositions of zircons and amphiboles are used to evaluate the physicochemical conditions (e.g., pressure, temperature, f O 2 , and water content) of the parental magma. We found the water content of the Wandongshan parent magma (≤4.11 ± 0.4 wt %) are slightly higher than those of the parent magma at Bailiancun (≤3.91 ± 0.4 wt %). Furthermore, the f O 2 deduced from amphibole chemistry of the Wandongshan porphyry range from ∆NNO + 1.00 to ∆NNO + 1.89, slightly higher than that of the Bailiancun porphyries (∆NNO + 0.39 to ∆NNO + 0.67). This result is consistent with the values calculated by zircon chemistry. The Wandongshan porphyry has higher zircon Ce 4+ /Ce 3+ ratio (average of 62) and high f O 2 value (average of ∆FMQ = +3.5) than those of Bailiancun porphyry (average of Ce 4+ /Ce 3+ = 23.15; ∆FMQ = +1.5). Obviously, the Wandongshan ore-related porphyry has higher water content and more oxidized than the Bailiancun barren porphyry. Therefore melts that are more enriched in water and with a high oxidation state will be more fertile to form an economic hydrothermal system especially when emplaced at shallow levels.