6.1. Robustness of apatite petrochronology in fingerprinting porphyry systems
Accurate chronological constraints are critical for establishing the timing of mineralization, deciphering mineralization processes, and developing mineral exploration models. Geochronological and geochemical fingerprints of mineralization processes can be preserved by apatite and zircon. Zircon has characteristics of high stability and sealing temperatures, and high U and low common Pb contents, and therefore is one of the most suitable minerals for U–Pb dating. However, zircon may be absent in some ore systems or may not directly represent mineralization. Therefore, in recent years, the development of LA–ICP–MS dating of other U-rich minerals that form in hydrothermal fluids or related intrusives has become a powerful method to determine the age of mineral deposits. Apatite, a common accessory mineral in magmatic rocks and hydrothermal deposits, stands out due to it already being widely employed in low temperature thermochronology research (Chew and Spikings, 2015; Prowatke and Klemme, 2006; Webster et al., 2009). Apatite can be U–Pb dated (Tc = 550–350 oC), fission track dated (Tc = 110–60 oC), and U–Th–Sm/He dated (Tc = 80–40 oC), forming a medium- to low-temperature continuous thermochronology that can comprehensively and continuously analyze tectono–thermochronological history (Carrapa et al., 2009; Glorie et al., 2019; Jepson et al., 2018). Furthermore, apatite can accommodate a variety of elements (e.g., S, Sr, U, Th, REE, etc.) and has high volatile contents, such as F, OH and Cl, making it an ideal mineral for both geological dating and tracing (Chu et al., 2009; Piccoli and Candela, 1994).
In this study, we selected apatite and zircon grains from the syenogranite and monzogranite in the Hutouya deposit to study thermochronology and in-situ trace element compositions, with the aim to test the consistency between apatite and zircon for petrochronology and fingerprinting of igneous processes in a porphyry–skarn system. Apatite grains in the two intrusives can be categorized into two types (FI-free Apatite I and FI-rich Apatite II; Fig. 8). The variations of textures and geochemical compositions in the two types of apatite are indicative of changing crystallization environments. The euhedral grains of Apatite I might be crystallized from a volatile-undersaturated magma. Conversely, Apatite II with lower Cl and higher F contents are only distributed in the more highly fractionated granite (Table S5, Fig. 8) and formed under a volatile-oversaturated stage demonstrated by the rich fluid inclusion contents (Andersson et al., 2019; Glorie et al., 2020; Mao et al., 2016; Pan et al., 2016; Qu et al., 2019b). Lower Cl and higher F contents of Apatite II could be attributed to the segregation of isolated fluid phase in the late aqueous magma (Chu et al., 2009; Doherty et al., 2014; Mathez and Webster, 2005; Sha and Chappell, 1999; Webster et al., 2009). Zircon grains in the two granites can be classified into Zircon I with transparent and bright zones, Zircon II with dark and metamict features, and Zircon III with mineral inclusions (Fig. 8) in the CL images, indicating that they were formed under different physicochemical conditions during the magmatic-hydrothermal evolution. Zircon I grains have a magmatic texture of well-developed bright oscillatory zones, and are most likely primary magmatic zircon that crystallized early in the evolution of granitic magma. The low Th and U contents and higher Zr/Hf ratios of Zircon I (Table S4) indicate they crystallized from volatileundersaturated anhydrous magma (Erdmann et al., 2013; Qu et al., 2019b; Zeng et al., 2016). Zircon II occurring as individual grains or overgrowth with the Zircon I might be of a successive later origin than the Zircon I (Fig. 8). Notably, high Th and U contents of Zircon II may be its crystallization from a volatile-enriched aqueous magma (Nasdala et al., 2001; Claiborne et al., 2006; Bacon et al., 2007; Geisler et al., 2007; Erdmann et al., 2013). Zircon III grains full of numerous hydrothermal mineral inclusions might be of the product of fluid interaction with previous Zircon II in a volatile-oversaturated environment, indicative of hydrothermal crystallization or hydrothermal alteration (Breiter and Skoda, 2012; Erdmann et al., 2013; Hoskin, 2005; Hoskin and Schaltegger, 2003). Collectively, apatite and zircon from the syenogranite and monzogranite in the Hutouya deposit experienced a prolonged crystallization process and were altered by late-stage exsolved fluids. The well-developed euhedral apatite and oscillatory primary magmatic zircon represent an early-crystallized phase from a least fractionated granite. Metamict zircon occurs as individual grains or overgrowth with the magmatic zircon formed under volatile–saturated aqueous magma during the magmatic-hydrothermal transition stage. Some formed zircon was altered by exsolved magmatic fluids in the most fractionated granite, indicating a volatile oversaturated environment. Meanwhile, apatite with abundant fluid inclusions and high F/Cl ratios from the most fractionated granite crystallized in this subsolidus stage.
Apatite I and Zircon I are interpreted to be of magmatic origins, and their ages therefore represent the time of magma emplacement. The apatite LA–ICP–MS U–Pb ages range from 235–220 Ma, which is consistent with the zircon U–Pb ages that range from 227–224 Ma. These ages are indistinguishable within error and indicate that the magmas were emplaced over a short time span, and cooled rapidly, given the closure temperature of apatite (~ 620 oC) and zircon (900 oC).
As hydrothermal activities and mineralization in porphyry–skarn systems are intimately tied to the emplacement of ore-forming intrusions (Razique et al. 2014), the associated hydrothermal and mineralization events at Hutouya probably have similar short durations of just a few million years or less (Zhong et al., 2018). At the regional scale, magmatism at Hutouya coincides with that in the other porphyry and skarn deposits in the QMB (Fig. 1C). For instance, Kaerqueka porphyry–skarn in the QMB formed circa 227 Ma (Feng et al., 2012), 224 Ma at Yazigou Cu–Mo deposits (Li et al., 2008), and 229.4 Ma at Kendekeke Fe deposits (Xiao et al., 2013). The granitoids associated with skarns in the area formed during the same period, including the ages at Hutouya (this study), 225 Ma at Galinge Fe skarn deposits (Zhao et al., 2013), and 227 Ma at Tawenchahan Fe skarn deposits (Feng et al., 2012).
It should be noted that geochemical compositions of apatite can be regarded as a tool to identify magmatic mineralization potentials. The magmatic oxygen fugacity is a key factor to fertile magmas of porphyry deposits (Lehmann, 1990; Sun et al., 2015, 2013; Wittenbrink et al., 2009; Zhang et al., 2017; Zhong et al., 2017). Oxidized magmas are more likely to form Cu porphyry deposits than reduced magmas (Imai, 2002; Li et al., 2017a; Liang et al., 2006; Lu et al., 2016), considering that under the high oxygen fugacity, sulfur in magmas mainly exists in the form of sulfate (SO42−) which has a much higher solubility in silicate melts than sulfide, that is, sulfide is difficult to reach saturation then precipitate during the magmatic stage thus facilitating metal accumulations in the late stage of magmatic evolutions (Ballard et al., 2002; Richards, 2003). Skarn and porphyry deposits have similar magmatic origins and evolution processes (Li et al., 2017b), so it is also applicable to Cu (–Pb–Zn) skarn deposits in controlling of the oxygen fugacity to fertil magmas of Cu porphyry deposits.
Zircon (Eu/Eu*)N and (Ce/Ce*)N values are effective indicators for evaluating the magmatic oxygen fugacity (Ballard et al., 2002; Gardiner et al., 2017; Trail et al., 2012). In the Hutouya Fe–Cu–Pb–Zn skarn deposit, the syenogranite and monzogranite have similar (Eu/Eu*)N and (Ce/Ce*)N values in zircon, (Table S4; Fig. 12). According to the Weibao Cu–Pb–Zn skarn deposit in the QMB, fertile intrusions have higher Ce4+/Ce3+ values than those of non-fertile intrusions (Zhong et al., 2018). We can draw a conclusion that parental magmas with the higher oxygen fugacity from fertile intrusions in Cu skarn deposits tend to form Cu mineralization. However, Pb and Zn are not easily controlled by oxygen fugacity and behave as incompatible elements. This means that magmas related to the Pb–Zn mineralization can be either high oxygen fugacity or low oxygen fugacity. Many studies have shown that both S-type granite (low oxygen fugacity magmas) and I-type granite (high oxygen fugacity magmas) can form Pb–Zn skarn deposits (Fu et al., 2017; Niu et al., 2017), which also supports that Pb-Zn mineralization is independent of magmatic oxygen fugacity conditions. In other words, the oxidation state is not a controlling factor for Pb–Zn mineralization within the Hutouya deposit.
Apatite Eu and Ce anomalies may be more easily affected by other factors unlike zircon Eu and Ce anomalies which are mainly controlled by oxygen fugacity conditions, therefore the relationship between apatite Eu and Ce anomalies and magmatic oxygen fugacity is not so similar (Piccoli and Candela, 1994). Nevertheless, if the physical conditions (specifically temperature and pressure) and concentrations of these elements in magma are relatively stable, apatites crystallizing from more oxidized magma will have higher Eu3+/Eu2+ but lower Ce4+/Ce3+ than reduced magma owing to ion substitution in the apatite structure, which results in apatites having strong negative Eu and positive Ce anomalies (Cao et al., 2012; Sha and Chappell, 1999). In this study, we further confirm that magmatism in the Hutouya deposit is similar to other skarn deposits in the QMB. Notwithstanding, this work shows that the fertile intrusions at Hutouya can be well defined by (Eu/Eu*)N and (Ce/Ce*)N parameters, that is strong negative Eu anomalies and weak positive Ce in apatite. Hence, apatite Ce anomalies including (Ce/Ce*)N, Ce4+/Ce3+, and Ce/Nd values are relatively more robust as proxies for magma oxidation state (Loader et al., 2017). The parameter (Eu/Eu*)N, although affected by many magmatic processes, can still reflect the magma redox state to some degree (Dilles et al., 2015). Moreover, previous studies found that Mn contents are significantly controlled by the oxygen fugacity with high apatite Mn contents in reduced magmas while low apatite Mn contents in oxidized magmas, which can be explained by the substitution of Ca2+ by Mn2+. Compared with Mn3+ and Mn4+, Mn2+ is more easily enriched in apatite, because the ionic radius of Mn2+is close to that of Ca2+ (Belousova et al., 2002). Mn mainly exists as Mn2+ at low oxygen fugacity with high Mn contents in apatite, while Mn mainly exists as Mn3+ and Mn4+ at the high oxygen fugacity with low Mn contents. In the Weibao deposit, apatite within two ore-forming intrusions exhibits much lower Mn concentrations than within the barren diorite porphyry. Apatite Mn contents of the two fertile intrusions in the Hutouya deposit are similar to the fertile intrusions in the Webao deposit (Zhong et al., 2018).
Notwithstanding, halogen contents of magmas (especially F and Cl contents) in apatite can also be regarded as an important indicator to evaluate productive magmas since halogens can effectively complex and transport metal elements (Coulson et al., 2001; Pan et al. 2016; Piccoli and Candela, 1994; Webster et al., 2004). A previous study showed that apatite, occurring as inclusions within biotite and hornblende, was one of the early crystal phases that appeared during crystallization (Tang et al., 2021), and thus the F and Cl partitioning between the apatite and the melt seems unlikely to be influenced by the crystallization of biotite and hornblende. Therefore, the contents of chlorine and fluorine in the melt predominantly affected by their magmatic sources can be evaluated from the concentrations of chlorine and fluorine in apatite. In this study, the fluorine contents of apatite is much higher than chlorine contents, because the partition coefficient of fluorine between apatite and melt is much higher than those of chlorine (Mathez and Webster, 2005). In addition, the chlorine contents of the two fertile intrusions in this study remain almost invariable, because the apatite/melt ratio is approximately constant at low Cl contents, but when the melt becomes saturated in Cl both partition coefficients increase rapidly as Cl content of the bulk system increases (Doherty et al., 2014). Furthermore, magmas formed by partial melting of lower crust materials usually show relatively stronger enrichment of F and depletion of Cl than those formed by dehydration melting in slab subduction environments (Ding et al., 2015; Jiang et al., 2018; Kendrick et al., 2011; Xu et al., 2022). Therefore, the volatile components of the parent magmas of syenogranite and monzogranite are mainly related to lower crustal melting.