Apatite and Zircon Geochemistry in Yao’an Alkali-Rich Porphyry Gold Deposit, Southwest China: Implications for Petrogenesis and Mineralization

: The Yao’an gold deposit is located in the middle of the Jinshajiang-Ailaoshan alkali-rich metallogenic belt, and this belt hosts many porphyry-type Cu-Au-Mo deposits formed at 46–33 Ma. Yao’an porphyry gold-mineralization is intimately associated with biotite syenite porphyry, whereas the contemporaneous quartz syenite porphyry is barren. In this study, we compared the major and trace elements of apatite and zircon and isotopic compositions of zircon from the biotite syenite porphyry and quartz syenite porphyry, to explore their geochemical differences that may affect their mineralization potential. The results show that both porphyries were derived from the partial melting of the thickened lower crust, which has been modiﬁed by slab-derived ﬂuids, but has different mineral crystallization sequences, magma ﬂuid activities, and magma oxidation states, respectively. REE contents in apatite and zircon can be used to reveal the crystallization sequence of minerals. A rapid decrease of (La/Yb) N ratio in apatite from both porphyries may be caused by the crystallization of allanite. Large variation of Cl contents and negative correlation between F/Cl and (La/Yb) N in apatite from fertile porphyry indicate that it has experienced the exsolution of Cl-bearing hydrothermal ﬂuid. Higher Y/Ho and lower Zr/Hf in zircon from fertile porphyry indicate a stronger ﬂuid activity than barren porphyry. The high S, V, As contents, δ Eu, low δ Ce in apatite, as well as high Ce 4+ /Ce 3+ and log( f O 2 ) estimated from zircon geochemistry from fertile porphyry, indicate high a oxidation state of fertile porphyry, similar to other fertile porphyries in this metallogenic belt. High ﬂuid activity and ﬂuid exsolution are conducive to the migration and enrichment of metal elements, which are very important for mineralization. High oxygen fugacity inhibits the precipitation of metal in the form of sulﬁde, thereby enhancing the mineralization potential of rock. Therefore, the exsolution of Cl-bearing hydrothermal ﬂuid and high oxygen fugacity are the key factors promoting mineralization in Yao’an area.

In the Yao'an gold deposit, previous studies mainly focused on the geochronology and petrogenesis of syenite porphyry [31,34,38,40]. However, detailed studies on mineralogy are scarce in the Yao'an area, and it is especially unclear which factors control the difference in mineralization between fertile porphyry and barren porphyry. In this study, based on detailed mineralogical observations, the major and trace elements of apatite and zircon and Hf isotopes of zircon from the Yao'an syenite porphyry (including 'fertile' biotite syenite porphyry and 'barren' quartz syenite porphyry) were obtained. We also compared this data with those for other fertile and barren porphyries in this metallogenic belt. We discussed the magma evolution history, fluid activity, and oxygen fugacity of fertile and barren porphyry, for better understanding the differences in ore-forming potential.

Deposit Geology
The Yao'an gold deposit was discovered in 1987 with approximately 10 tons gold resources [37]. The faults and folds in this area are well developed, including the mainly NW-plunging Laojiezi composite anticline. The NE-trending fault controls the emplacement of syenite porphyry and mineralization. The syenite porphyry can be divided into biotite syenite porphyry and quartz syenite porphyry, which are mainly distributed in Baimaju and Xiaocaiyuan, respectively. Gold is hosted in the Baimaju biotite syenite porphyry, whereas the Xiaocaiyuan quartz syenite porphyry is barren ( Figure 2). Details of the petrographic characteristics are as follows.
The biotite syenite porphyry is grayish-white, and displays slight alteration with porphyritic textures (Figure 3c). Under the microscope, phenocrysts are dominated by orthoclase (10 vol%), biotite (5 vol%), and hornblende (<5 vol%). The orthoclase phenocrysts are characterized by idiomorphic or hypidiomorphic textures with abundant fractures. Dark brown biotites and hornblendes are common mafic minerals, form euhedral to subhedral prisms of variable size, and contain apatite and zircon as mineral inclusions (Figure 3f,g). The composition of the matrix is similar to that of the phenocrysts. Apatite, magnetite, and zircon are found as accessory minerals (Figure 3e  The quartz syenite porphyry is light gray, displays slight alteration, porphyritic textures, and block structures (Figure 3d). Under the microscope, phenocrysts are mainly orthoclase (10 vol%), quartz (5 vol%), with traces of biotite (<5 vol%), without hornblende. The orthoclase phenocrysts are typically subhedral to euhedral, have undergone clay alteration, and contain apatite and zircon as a mineral inclusion (Figure 3j,k). The quartz phenocrysts show harbor-like shapes or are finely granular. The groundmass composition is similar to that of the phenocrysts. The accessory minerals are mainly apatite, magnetite, zircon, and titanite. The content of hydrous minerals is lower than that in the biotite syenite porphyry (Figure 3i

Sampling
The biotite syenite porphyry (fertile porphyry) sample was collected from the Baimaju outcrop ( Figure 3a); the quartz syenite porphyry (barren porphyry) was collected from the Xiaocaiyuan outcrop ( Figure 3b). Zircon and apatite were separated by traditional density and magnetic methods and were then picked manually under a microscope. In order to investigate the internal characteristics of the zircon and apatite, polished sections of zircon and apatite were carbon coated for cathodoluminescence (CL) imaging. These work were completed at the Langfang Sincerity Geological Service Co., Ltd., Hebei, China.

EMPA and LA-ICP-MS of Apatite and Zircon
Major elements of the apatite were analyzed using a JEOL-JXA8230 electron microprobe analyzer (EMPA) in wavelength-dispersive spectrometry (WDS) mode in the MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing, China. The operating conditions include a 15 kV accelerating voltage, a 10 nA beam current, and a 5 µm beam diameter. To detect the characteristic X-ray patterns, twenty seconds (s) of counting time was chosen for most elements, whereas 20 s was chosen for F and 10 s was chosen for P and Ca. Natural minerals and synthetic materials were used as standards, and all of the standards were tested for homogeneity before their utilization for quantitative analyses. Matrix corrections were carried out using the ZAF correction program supplied by the instrument manufacturer.
Laser-ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) trace element analyses of apatite and zircon were performed at the National Research Center for Geoanalysis, Chinese Academy of Geological Sciences, Beijing, China, using a Finnigan MAT ELEMENT II high-solution inductively coupled plasma spectrometer (HR-ICP-MS) with a high-performance interface coupled with a New-Wave UP-193 nm Nd: YAG UV laser. Ablation was carried out by a He-Ar gas mixture (flow rate of 0.73 L He/min and 0.85 L Ar/min) and the resulting vapor was combined with argon before delivery into the ICP-MS. The analyses were conducted using a 35-µm spot diameter, a 10-Hz frequency, a 0.25-mJ/pulse power, during a 70-s analysis including 20 s measurement of gas blank, followed by ablation of the sample for approximately 40 s by raster. Each group of ten apatite analyses was bracketed by analysis of standard glass NIST612, NIST 610, and Kl2 to correct for mass bias drift during analytical session. The calibration were completed using internal standards and matrix normalization. The Ca content determined by EPMA was used as the internal standard for trace-element determinations by LA-ICP-MS.

Zircon Hafnium Isotopes
Hf isotope analyses were conducted using a LA-MC-ICP-MS. The detailed analytical procedures were described by [61]. A beam diameter of 40-60 µm was used, and helium carrier gas transported the ablated aerosol from the laser-ablation cell via a mixing chamber to the ICP-MS torch. The zircon standard GJ1 was analyzed to evaluate the accuracy of the laser-ablation results. Throughout the session, GJ1 yielded a mean value of 176 Hf/ 177 Hf = 0.282015 ± 0.000008 (2σ, n = 10), which was consistent with error with the published standard value [62]. To calculate the εHf values, we adopted the chondritic values of [63], that is ( 176 Hf/ 177 Hf) CHUR = 0.032 and ( 176 Hf/ 177 Hf) CHUR0 = 0.282772. We also used the decay constant λ = 1.865 × 10 year in the εHf calculations. To calculate depleted-mantle model ages (T DM1 ), we adopted the values of ( 176 Lu/ 177 Hf) DM = 0.0384 and ( 176 Hf/ 177 Hf) DM = 0.28325 [64]. Here, we calculated two-stage model ages (T DM2 ), which assumed that the parental magma was produced from average continental crust (i.e., 176 Lu/ 177 Hf = 0.015) that was derived from depleted mantle [65].

Results
In this article, we have obtained a total of 40 major elements and 40 trace elements in apatites. 50 trace elements and 28 Hf isotope were analyses for zircon. Apatite major and trace elements are listed in Table S1, and zircon trace elements and Hf isotopes are listed in Table S2.

Zircon Hf Isotopes
Zircon Hf analysis and calculated εHf and T DM2 from all samples are listed in Table S2.

Apatite and Zircon Geochemistry as Indicators of Magma Crystallization Sequence
Apatite and zircon trace element compositions are mainly controlled by magma composition, partition coefficient between apatite/zircon and melt, and crystallization of other minerals. As such, they can be used as good indicators to trace magma fractional crystallization [8,[68][69][70][71][72][73].
Th/U ratios of zircon from fertile and barren porphyry are negatively correlated with Yb/Gd ratios (Figure 7a), indicating that they have experienced the effects of fractional crystallization [72]. Ce/Sm and Yb/Gd ratios in zircon can be used to reveal the growth sequence of minerals [72], In Ce/Sm versus Yb/Gd diagram (Figure 7b), zircon from fertile and barren porphyry plots in similar field, reflect that their compositions were effected by crystallization of titanite or hornblende. Zircon from fertile porphyry has lower ∑MREE/∑HREE than those from barren porphyry, which indicates that zircon from fertile porphyry was mainly affected by hornblende, while zircon from barren porphyry mainly was affected by titanite, as hornblende is known to concentrate the MREE but titanite concentrated the HREE [74,75]. In addition, titanite is enriched in U and less in Th, and Yb has lower partition coefficient than Dy in hornblende [76], therefore zircon crystallizing late will have lower U/Th and higher (Yb/Dy) N ratio than crystallizing earlier. Lower U/Th and (Yb/Dy) N ratio in zircon from barren porphyry than those from fertile porphyry (Figure 8a), further indicating that zircon from barren porphyry was mainly affected by titanite, and zircon from fertile porphyry was mainly affected by hornblende. Moreover, most of the zircon grains were wrapped in feldspar, which indicates that zircon crystallizes earlier than feldspar. We used the apatite-saturation temperature calculation formula and Ti in zircon to estimate the crystallization temperature of apatite and zircon [77]. In this study, apatites having a higher crystallization temperature (876 to 987 • C) than zircons (792 to 825 • C), indicating that apatite crystallized earlier than zircon. In addition, the rapid decrease of (La/Yb) N value in apatite (Figure 8b) indicates the crystallization of allanite [74,75]. Therefore, the crystallization sequence of minerals in this article is as follows: allaniteapatite-hornblende-zircon-feldspar in fertile porphyry, and allanite-apatite-titanite-zirconfeldspar in barren porphyry. Furthermore, [7] we proposed that Sr/Y and δEu in apatite are good indicators for adakitic-like rocks, especially in altered or weathered rocks. As shown in Figure 9a, all apatites fall into the adakitic rock area.

Fluid Activity and Exsolution Is Favorable to Mineralization
Y/Ho and Zr/Hf ratios in zircon and Cl contents in apatite are important parameters to indicate magma fluid activity [11,78]. The Y/Ho ratio of chondrites is 27.7, and usually in the absence of fluid interaction, Y/Ho ratio in zircon maintained from 24-34. Strong magmatic fluid activity can promote the differentiation of Y and Ho; therefore, zircon crystallized from magma with strong fluid activity has a high Y/Ho ratio [78]. Zr and Hf has the same ionic radius and valence, strong fluid activity, and can promote more Hf to substitute Zr in zircon [78]. Cl, as an incompatible element, more readily enters the fluid phase than F during magma evolution [79][80][81][82][83][84][85][86]; therefore, high Cl contents and Cl/F ratios in apatite may be related to the high water content of magma.
Compared with zircon in the barren porphyry, zircon from fertile porphyry has higher Y/Ho and lower Zr/Hf ratios (Figure 9b), which indicates that the fluid activity in fertile porphyry is stronger. Apatite from fertile porphyry displays a higher Cl content relative to apatite in the barren porphyry, which indicates a higher water content of fertile porphyry. High fluid activity and water contents can cause the magma to readily reach water saturation and fluid exsolution during its evolution. As shown in Figure 4b, Cl contents in apatite from fertile porphyry decreased rapidly, F/Cl ratio in apatite from fertile porphyry show negative correlation with (La/Yb) N ratio (Figure 8b), indicating that fertile porphyry experienced exsolution of Cl-bearing hydrothermal, because Cl-bearing hydrothermal can carry more LREE relative to HREE and MREE. In addition, the crystallization of allanite can also lead to the decrease of (La/Yb) N value; as shown in Figure 8b, rapid decrease of (La/Yb) N value but constant F/Cl ratio indicates the crystallization of allanite, consistent with the observation of allanite under the microscope (Figure 3e). Furthermore, high Cl contents in apatite from fertile porphyry indicates that Cl is rich in magmatic fluid and since metal elements easily combine with Cl to form soluble complexes, this is conducive to the migration and enrichment of metal elements [87][88][89].
Both Eu and Ce have two valence states (Eu 2+ , Eu 3+ ; Ce 3+ , Ce 4+ ). Eu enters apatite in high valence, while Ce in low valence [12,91]. As shown in Figure 10a, δEu and δCe appear in an obvious negative correlation, indicate their contents in apatite are mainly controlled by oxygen state of magma. High δEu and low δCe in fertile porphyry indicate high oxidation state of the magma. S, V and As has variable valence states (−2, +4, +6), and S 6+ ; As 6+ ; V 6+ are favored by apatite. High S, V and As in apatite from fertile porphyry, indicate high oxidation state of fertile porphyry. In addition, all apatite from fertile porphyry and only four apatites from barren porphyry lie above the NNO buffer line (Figure 10b), which indicate higher oxidation state of fertile porphyry than those of the barren porphyry.
Ce has two valence states (+3 and +4), and Ce 4+ is favored by zircon because it can substitute Zr 4+ , which makes the zircon show positive Ce anomaly. In addition, Zircon can also be used to calculate f O 2 of its host magma [17][18][19]101]. The calculated Ce 4+ /Ce 3+ and log(f O 2 ) in zircon from fertile porphyry (54 to 429, average 134; −16 to −3, average −8) are higher than it from barren porphyry (6 to 25 average 156; −18 to −8, average −12), indicate that fertile porphyry has higher oxygen fugacity than barren porphyry (Figure 11a; Figure 12a,b). Previous studies have shown that the Ce/Nd and Ce/Sm ratio from zircon are positively correlated with oxygen fugacity [9]. Here, the higher Ce/Nd and Ce/Sm ratios in zircon from fertile porphyry are consistent with previous studies (Figure 11b). Compared with other fertile porphyry in the metallogenic belt, we can see that fertile porphyry in Beiya and Machangqing is characterized by high oxygen fugacity, whereas the Liuhe barren porphyry shows low oxygen fugacity (Figure 12a,b).  High Ce 4+ /Ce 3+ ratio signifies high oxygen fugacity. The Yao'an data are compared with those from the Beiya porphyry [6], Machangjing porphyry [32], and Liuhe porphyry [39], using the equation of [105].
Silicate melts with high oxygen fugacity can suppress sulfide saturation and therefore depletion of chalcophile elements (e.g., Cu) that are in forms of sulfide, which allows gathering of Cu in the Cl-S bearing fluid phase [106][107][108][109][110][111]. Therefore, a high oxygen fugacity is more likely to lead to the development of porphyry deposits.

Implications for the Magmatic Source
Previous studies have shown that Hf isotopic compositions of the chondrites and depleted mantle are different from those of the enriched lithospheric mantle and crust. Generally, 176 Hf/ 177 Hf ratios of chondrites and depleted mantle ( 176 Hf/ 177 Hf > 0.283335, εHf > 0) show a more radiogenic character than that of enriched lithospheric mantle and crustal components (εHf < 0) [64,112,113].
The Beiya and Machangqing alkali-rich porphyries, which are located near the Jinshajiang-Ailaoshan suture at the edge of the Yangtze plate, have high zircon εHf (Beiya: −4 to 4; Machangqing: 0.3 to 4.7) value and young two-stage Hf depleted mantle-model ages (Beiya: 0.85 to 1.37 Ga; Machangqing: 1.1 to 0.8 Ga) are considered to be derived from thickened juvenile lower crust mixed with lithospheric mantle [31,47]. In this paper, strong negative zircon εHf values (−15.24 to −7.75) and adakite-like rocks of the Yao'an fertile and barren porphyry, as well as the high f O 2 and apatite S-Cl contents, may indicate that the Yao'an intrusions are all derived from partial melting of the thickened lower crustal materials that have been modified by slab-derived fluids. Ancient two-stage Hf model ages (2.04 to 1.57 Ga) indicate that the crustal components maybe the Paleoproterozoic-Mesoproterozoic metamorphic basement.

Conclusions
According to the analysis of apatite and zircon from barren and fertile porphyry, the following conclusions can be drawn.
(1) REE contents in apatite and zircon can be used to reveal the crystallization sequence of minerals and are good indicators of adakitic rock.
(2) Y/Ho and Zr/Hf ratios in zircon and Cl contents variation in apatite from fertile porphyry indicate fertile porphyry has stronger fluid activity and experienced exsolution of Cl-bearing hydrothermal.
(3) The high S, V, As contents, δEu, low δCe in apatite, as well as high Ce 4+ /Ce 3+ and log(f O 2 ) estimated from zircon geochemistry from fertile porphyry, indicate high oxidation state of fertile porphyry, similar to other fertile porphyries in this metallogenic belt.
(4) In situ zircon Hf isotopic data show that Yao'an intrusions were derived from partial melting of the thickened lower crust that have been modified by slab-derived fluids.