Magmatic Oxidation Research Articles

Amphibole fractionation controls the redox state of magmas in porphyry Cu ore systems: Constraints from amphibole-rich rocks in eastern China

Porphyry Cu ore deposits host most of the Cu, Au, and Mo reserves worldwide and generally form under highly oxidizing conditions. However, it is unclear what causes the high redox state (i.e., the source or subsequent magmatic differentiation). Amphibole fractionation is a key step in the formation of porphyry Cu ore deposits. In this study, amphibole-rich lithologies and derivative intermediate-silicic porphyries from two porphyry Cu systems (Laiyuan and Tongling) in eastern China were investigated to constrain the magma redox state during amphibole fractionation. At Laiyuan, the amphibole-rich rocks are hornblende-bearing gabbroic fragments or lenses hosted in granodiorite, consisting of two crystal populations: group I—brown, anhedral hornblende cores, with clinopyroxene ± olivine relics; and group II—green hornblende rims and matrix hornblende. At Tongling, the amphibole-rich lithologies are clinopyroxene-hornblende cumulates or hornblende-bearing gabbro xenoliths hosted by intermediate-silicic porphyries. Based on the clinopyroxene-liquid thermobarometer and amphibole-liquid thermobarometer, the brown amphibole and clinopyroxene from two systems crystallized under 4.6−5.9 kbar and 990−1000 °C, but their green amphiboles crystallized under lower pressure-temperature conditions (∼630−850 °C and 0.5−1.4 kbar). The results of amphibole and zircon-based oxybarometers show that their parent melts have similar moderate oxygen fugacities of ΔFMQ +1.0 ± 0.5, comparable to those of primitive arc magmas, but after amphibole fractionation their residual intermediate-silicic melts had obviously increased oxygen fugacities of ΔFMQ +2.5 ± 1.0. Modeling results show that the increase in oxygen fugacity is probably caused by the preferred partitioning of ferrous Fe into amphibole and clinopyroxene during amphibole fractionation. Amphibole differentiation is a common process in porphyry Cu systems; such processes may facilitate the oxidation of causative magmas and, therefore, the occurrence of porphyry Cu deposits.

Optimal magmatic oxidation conditions for giant porphyry copper deposits.

It is widely accepted that moderately to highly oxidized magmas are needed to form porphyry copper deposits (PCDs). However, only a few studies have attempted to investigate the effects of variable magmatic fO2 values on the formation of PCDs. Based on previously published studies, the magma oxygen fugacity of giant PCDs is mainly concentrated between ΔFMQ -0.5 and ΔFMQ +2.5, with the first quartile, median, and third quartile being +0.8, +1.3, and +1.7, respectively. In this work, we have carried out numerical modeling, which shows that the S2- used to precipitate Cu accounts for approximately 90% of total sulfur at ΔFMQ +1 but drops to 33%-38% and 15%-25% at ΔFMQ +2 and ΔFMQ +3, respectively. This leads to a decrease of 88%-96% in Cu precipitation efficiencies from ΔFMQ +1 to ΔFMQ +3. Our petrological model further shows that the maximum Cu precipitation efficiency (and, therefore, Cu endowments) occurs at around ΔFMQ +1. We, therefore, highlight that moderately oxidized magma (ΔFMQ=∼1) rather than highly oxidized ones (ΔFMQ > ∼2) is optimal for the generation of giant PCDs.

Sulfur species and gold transport in arc magmatic fluids.

The sulfur species present in magmatic fluids affect the global redox cycle, the Earth's climate and the formation of some of the largest and most economic ore deposits of critical metals. However, the speciation of sulfur under conditions that are relevant for upper crustal magma reservoirs is unclear. Here we combine a prototype pressure vessel apparatus and Raman spectroscopy to determine sulfur speciation in arc magmatic fluid analogues in situ over a range of geologically relevant pressure-temperature-redox conditions. We find that HS-, H2S and SO2 are the main sulfur species in the experimental fluids, while the concentrations of S6+ species and S2 - and S3 - sulfur radical ions are negligible, in contrast to previous experimental work. The measured gold solubilities in the experimental fluids are highest when sulfur is dominantly present as HS- and H2S species and greatly exceed thermodynamic predictions. Our results imply that HS-, rather than sulfur radicals, accounts for the high solubilities of gold in magmatic-hydrothermal fluids. We also find that magmatic sulfur degassing is a redox process under oxidizing conditions and may lead to additional magma oxidation beyond that imparted by slab-derived fluxes and crystallization.

The Weideshan Batholith and Felsic Dykes in the Eastern Jiaodong Peninsula: Is There Any Possible Relation to Gold Mineralization?

The genetic link between Weideshan-type rocks and gold mineralization in the Jiaodong Peninsula remains unclear. In this study, we examined the geochemical characteristics, water content, and oxidation states of Weideshan-type rocks and associated felsic dykes to assess the potential of a Weideshan-type batholith in directly contributing the fluids responsible for extensive gold mineralization. The findings reveal that the emplacement timing of Weideshan-type magmatic rocks in the Jiaodong Peninsula is slightly different from the timing of gold mineralization. Additionally, high zircon Eu/Eu* values indicate a relatively high water content within the Weideshan batholith. However, due to limited data on crystallization pressures, it remains equivocal whether water was saturated during the emplacement of the Weideshan batholith. Even if fluid saturation did occur, the magmatic oxidation states of Weideshan-type rocks are notably high (>FMQ + 1.5), which is incompatible with the reduced mineral assemblages typical of Jiaodong gold deposits. Therefore, our study suggests that the genetic link between gold mineralization and Weideshan-period granitic magmatism may be weak.

Processes controlling magma fertility at Buenavista del Cobre porphyry copper deposit (Cananea, México): A new petrogenetic model based on zircon U-Pb dating and apatite geochemistry

The Buenavista del Cobre is a world-class porphyry Cu-Mo deposit located in the Cananea Mining District, northern Sonora, México. Using zircon U-Pb dating, we show that the Proterozoic Cananea Granite, unconformably overlain regionally by Paleozoic limestones hosting skarn deposits underlies the present-day pit. We also present new crystallization ages for host rocks of copper mineralization in the District, dating for the first time a volcanic rock of the Henrietta Formation at 186.8 ± 1.1/3.0 Ma and the El Torre Syenite at 176.3 ± 1.1/2.9 Ma. Zircon U-Pb dating of the different porphyritic bodies reveals that the magmatic activity at Buenavista del Cobre lasted at least 4 Myr, from 59.7 ± 0.5/1.1 Ma to 56.1 ± 0.2/0.9 Ma. The deposit is composed by several porphyry intrusions referred to as “ore-rich” and “ore-poor” based on their individual metal contributions, which provides the opportunity to study the origin and processes enhancing magma fertility in an individual deposit. Combining our new geochronological dataset with geochemistry of apatite from the different porphyries allows us to propose a new petrogenetic model for the Buenavista del Cobre deposit. As the apatite Eu and Ce anomalies overlap with no clear difference between the ore-rich and ore-poor intrusions, we propose that the magmatic oxidation states of the magmas were similar. However, differences in apatite REE signatures, as well as variations in apatite Sr compositions between the two groups suggest that fractional crystallization processes in the parental magma influence the fertility of the porphyries. Additionally, apatite Cl contents of ore-rich porphyry intrusions are higher (>0.4 wt%) than the ore-poor intrusions (<0.2 wt%), suggesting an important role of the initial Cl content of the magmas in the mineralization process. These observations give new insights on the petrogenetic processes at origin of porphyry magma fertility. We propose that the evolution of the parental melt by fractional crystallization of hydrous minerals (hornblende) at upper crustal levels induced low H2O content of the residual magma, resulting in the formation of ore-poor porphyries. In contrast, we suggest that fractionation of anhydrous minerals (plagioclase) increased the H2O content in the residual melt, leading to the formation of ore-rich porphyries. Our new data allow us to propose an original genetic model for the Buenavista del Cobre deposit, which involves two cycles of supply, cooling and partial crystallization. This contribution shows that petrogenetic processes controlling porphyry copper magmas fertility are recorded in the composition of apatite at the deposit scale and highlights the importance of considering apatite geochemistry as an exploration tool.

Crustal magma oxidation state and endowments in porphyry copper deposits

Crustal magma oxidation state and endowments in porphyry copper deposits

Amphibole fractionation as a key driver for oxidation of magmas in convergent margins

During the process of differentiation, the magmas in convergent margins undergo an increase of oxidized nature, accompanied by a decreased Fe content and concentration of heavy Fe isotopes. Garnet and amphibole are both Fe-rich minerals, which can be responsible for this phenomenon through fractional crystallization. One prevailing hypothesis suggests that Fe2+-rich garnet cumulates in the arc root as a "crustal redox filter." However, the stability of garnets is highly dependent on pressure conditions. In contrast, amphibole can crystallize under a broader range of temperature and pressure conditions and is a more common mineral phase in magmas. As such, the contribution of amphibole might have been underappreciated. Here, we conducted elemental composition, zircon trace element, and high-precision Fe isotope analyses on Miocene magmatic rocks from the Gangdese arc to trace the evolution of magmatic oxidation. The results indicate that the enrichment of heavy Fe isotopes in these magmas is primarily controlled by amphibole-dominated fractional crystallization rather than garnet. This also implies that amphibole fractional crystallization may play a role in enhancing the oxygen fugacity of the magmas. Taking a global perspective, we found a pervasive correlation between amphibole fractional crystallization and Fe isotope fractionation in magmatism at convergent plate margins, indicating its widely applicable influence on oxidation. The influence of garnet cannot be entirely neglected in some specific scenarios, such as within thickened continental arcs, but its impact is generally limited. Continuous amphibole fractional crystallization increases oxidation, facilitating the mobilization and concentration of Cu within the magma, thereby enhancing the potential for porphyry deposit formation. This impact is especially notable in spatiotemporally related magmatic events and could be decisive in determining the magmatic mineralization potential.

Magmatic evolution and metallogenic diversity of the late Cretaceous granites in the Yidun terrane: Constraints from zircon and apatite geochemistry

Porphyry-hydrothermal type Cu-Mo(−W) deposits in the southern Yidun terrane (SYT) and hydrothermal-skarn type Sn-Pb-Zn-Ag deposits in the northern Yidun terrane (NYT) are closely related to late Cretaceous granitic intrusions in time and space. However, the genetic mechanisms and controlling factors responsible for the metallogenic diversity remain unclear. We investigated zircon and apatite compositions of the Sn-Pb-Zn-Ag-associated granites in northern Yidun terrane and the Mo-W-Cu and Mo-Cu-associated granites in southern Yidun terrane. The Cretaceous granites are distributed along a N–S-trending belt having ages ranging from 106 to 73 Ma, with a transition from predominant plagioclase crystallization to amphibole-dominated crystallization. Our data show that the granites from the NYT have lower zircon Eu/Eu* (0.06 ± 0.04), Ce4+/Ce3+, and ΔFMQ (FMQ is fayalite-magnetite-quartz) values compared to the granites in the SYT, indicating that the granites from the NYT are relatively reduced (ΔFMQ = -1.03 ± 1.02) in contrast to the oxidized granites in the SYT (ΔFMQ = 1.10 ± 1.05). Furthermore, apatite can record the volatile compositions of magma before fluid exsolution. Apatites in granites from the NYT with low XF/XCl ratios (≤120) are interpreted to have crystallized from volatile-undersaturated magma. These apatites are characterized by low Cl contents ranging from 0.04 to 0.12 wt% (0.07 ± 0.02 wt%; n = 21) and low SO3 contents of 0.01–0.06 wt%. In contrast, apatites in granite from the SYT with low XF/XCl ratios (≤80) have high Cl contents of 0.05 to 0.34 wt% (0.16 ± 0.07 wt%; n = 52) and SO3 contents of 0.01–0.46 wt%. The chemical compositions of zircon and apatite indicate that the Mo-W-Cu and Mo-Cu-associated granites in the SYT have higher fO2 and melt S contents before fluid exsolution than the Sn-Pb-Zn-Ag-associated granites in the NYT. Therefore, the contrasting magmatic oxidation states and S contents are critical in controlling the metallogenic diversity of the Cretaceous granites in the Yidun terrane. The Xingguolongba and Cuopu granites, first reported in this article, have U-Pb ages consistent with the Rongyicuo and Cuomolong granites. Furthermore, they exhibit similar geochemical properties, such as oxygen fugacity and volatiles, suggesting the possibility of forming Sn-Pb-Zn-Ag deposits.

Case study of the large-scale Mo-W mineralization in Eastern Qinling, China: Geology and genesis of the Yechangping porphyry-skarn system

Eastern Qinling Orogen is the largest Mo province in the world, hosting nine giant Mo deposits. Four giant Mo deposits contain economic W mineralization, while the mechanism of W mineralization remains unclear. The Yechangping deposit is a representative porphyry-skarn Mo-W deposit, with four paragenetic stages of prograde skarn, retrograde skarn, polymetallic sulfides and carbonate stage. Magmatism at Yechangping includes early barren monzogranite porphyry at 159.0 ± 1.5 Ma and late ore-related granite porphyry at ca. 145 Ma, and the latter porphyry shows a higher magmatic fractionation than the early barren monzogranite porphyry. Both barren and ore-related porphyries have similar high magma oxidation states, which may efficiently extract Mo during partial melting and hinder subsequent molybdenite crystallization until the porphyry-skarn Mo mineralization. Three types of scheelites are distinguished by their texture, paragenetic relationship and geochemistry, including Sch-1a in the retrograde skarn stage, and Sch-1b and Sch-2 in the polymetallic sulfides stage. Scheelite displays decreasing MoO3 contents from 7.01–51.59 % in Sch-1a, through 3.15–17.51 % in Sch-1b, to 0–1.51 % in Sch-2. The decreasing MoO3 contents in scheelite are attributed to the transformation of dominant Mo-bearing phases, i.e., from retrograde skarn stage powellite to polymetallic sulfides stage molybdenite. We suggest that the Mo-W mineralization is triggered by the decreasing oxygen fugacity, pH, and temperature, supported by thermodynamic modeling. In addition, the precipitation of fluorite and destabilization of fluorine complexes may have also promoted the Mo-W mineralization. Zircon Hf isotopes yielded εHf(t) values of −14.9 to −7.5 and TDM2 ages of 1671–2147 Ma, indicating that Yechangping porphyries were primarily sourced from the ancient basement of the Huaxiong Block, with additional contribution from the North Qinling Accretionary Belt. Skarn W mineralization in the southern part of Huaxiong Block may be controlled by both magmatic fertility and carbonate wall-rocks.

Oxidation of magmas during gain and loss of H2O recorded by trace elements in zircon

Hydroxylation-oxidation reactions of the type H2O + Fe→2+Fe3+ + OH− + 1/2 H2 oxidise Fe2+ in olivine and pyroxenes in oceanic lithosphere prior to subduction and in the supra-subduction-zone mantle wedge and oxidise Fe2+ in silicate melts as dissolved H2O accumulates during magmatic differentiation. Collectively, the coupled hydroxylation-oxidation of melts and of minerals in melt sources tends to produce positive correlations of hydration state and oxidation state of silicate melts, about which there has been much dispute and discussion in literature. We introduce a melt “hygro-barometer” which provides evidence from trace elements in zircon that melt oxidation state rose as dissolved H2O accumulated in fluid-undersaturated residual melts during mafic-to-felsic differentiation at Moho-vicinity depths. Zircon-based measures are consistent with experimental evidence of melt oxidation during gain of dissolved H2O. Examples from many arc igneous complexes demonstrate further oxidation of residual melts as hydrothermal fluid exsolved and segregated during trans-crustal ascent of the magmas. Zircons in those suites show that much of the contrast between redox states of dry mid-ocean-ridge and hydrous arc magmas (especially porphyry copper magmas) develops as H2O is lost during trans-crustal ascent of arc magmas because decompression-induced exsolution and open-system fractional segregation of hydrothermal fluid selectively extracts reduced members of the principal redox couples (H2–H2O, S4+–S6+, Fe2+–Fe3+), thereby raising the oxidation state of residual melt.

Zircon fertility indicators compromised by mineral inclusion contamination: A case study from the Taoxikeng W deposit, South China

Zircon trace element compositions have been widely used as a tool to assess the ore-forming potential of magmatic-hydrothermal systems. However, many zircons contain accessory mineral inclusions which, if accidentally analyzed, could compromise reported zircon trace element signatures, especially when the mineral inclusions are sub-micrometer and thus hard to monitor using conventional microscopes. However, how mineral inclusion contamination influences the robustness of zircon fertility indicators (e.g., Ce and Eu anomalies) is still not well illustrated. In this study, we report U-Pb ages and trace element compositions of zircons from ore-forming muscovite granite plutons of the Taoxikeng W deposit, South China. The studied zircons are characterized by abundant small apatite inclusions. Although these had little influence on zircon Eu/Eu* (and thus probably the interpreted level of magmatic fractionation and melt water concentration), they could have resulted in a misestimation of the magmatic oxidation state either via the zircon Ce/Ce* or Loucks et al. (2020) methods. This is especially important for granite-related W-Sn systems since their magmatic zircons generally contain abundant mineral inclusions, which may be accidentally sampled during zircon trace element analysis. In practice, we can use La as a monitor of REE-enriched mineral (i.e., apatite, titanite) inclusions, although the criterion for identifying pure zircon trace element data is still controversial and awaits further work.

Contrasting magmatic controls on the genesis of Fe-Ti-V oxide deposits in the Emeishan large igneous province using apatite Sr-Nd isotopes and apatite-zircon trace elements

We use Sr-Nd isotopes of apatite and trace element compositions of apatite-zircon pairs from a major ore layer in each of three selected magmatic Fe-Ti-V oxide ore deposits (Hongge, Panzhihua, and Taihe) in the Emeishan large igneous province (LIP) in southwestern China to contrast the magmatic controls on ore formation. The average contents of REEs in apatite from a representative ore zone/layer in each of these deposits are higher than the predicted values of apatite crystallizing from a melt with REE contents assumed to be the same as the average values of high-Ti basalts in the Emeishan LIP, confirming that the ore-forming magmas all experienced higher degrees of fractional crystallization by major silicate minerals than the average basalt. The apatites Sm/Yb and Sr/Y indicate that the Panzhihua magma was generated at a shallower depth and experienced higher degree of plagioclase fractional crystallization than Taihe and Hongge. The apatite Sr-Nd isotopes can be reproduced by ~ 8 wt% contamination with Precambrian gneiss-schist in the parental magma for Hongge and by ~ 10 wt% and ~ 25 wt% contamination with marbles in the parental magmas for Panzhihua and Taihe, respectively. A recycled, high-T altered oceanic gabbroic component in the mantle source is a viable alternative to the marble contamination model for Taihe. Coexisting zircon trace element compositions reveal that the parental magma for Taihe (ΔFMQ+4.0) is more oxidized than those for Hongge (ΔFMQ−0.4) and Panzhihua (ΔFMQ−0.7), questioning that magma oxidation played a critical role in the genesis of Fe-Ti-V oxide ore deposits in the Emeishan LIP.

Geochronology, petrogenesis, and magmatic oxidation state of the Mangling intrusive complex, Northern Qinling Belt, Central China: Implications for magma fertility and tectonic setting

The Mangling intrusive complex has different dioritic to granitic phases and is spatially and temporally related to molybdenum deposits in the Qinling Orogen. Zircon U-Pb dating of the Mangling intrusive complex indicates that dioritic rocks (biotite diorite and biotite diorite enclave; ca. 150−147 Ma) formed earlier than granitic rocks (medium- to fine-grained and fine-grained monzogranite and K-feldspar granite; ca. 145−141 Ma). The Mangling dioritic rocks exhibit large ion lithosphere element (e.g., Rb) and light rare earth element enrichment and high field strength element (e.g., Nb, Ta, and Ti) depletion. They have low to moderate SiO2 (51.33−58.16 wt%), high MgO (3.10−4.75 wt%) and Mg# (48−60), and negative zircon εHf(t) values (−11.6 to −6.8), suggesting origination from the continental lithospheric mantle that may have been metasomatized by previous sediment-derived melts and slab-derived fluids constrained by high Nb/Y, Th/Yb, and Rb/Y ratios. The Mangling granitic rocks are I-type granites and have high SiO2 (67.90−81.88 wt%) and low MgO (0.16−0.74 wt%). They have low and negative zircon εHf(t) values (−18.7 to −1.9) and old zircon Hf two-stage model ages (2334−1287 Ma), as well as similar mineral fractionation (e.g., hornblende, biotite, sphene, and apatite) with the Mangling dioritic rocks, indicating that they were derived from the remelting of old crustal rocks (e.g., Xiong’er and Kuanping groups) by the evolved underplated mafic magma. Compared with the Taoguanping mineralized monzogranite in the Northern Qinling Belt, zircon geochemistry (e.g., Ce4+/Ce3+, Eu/Eu*, and ΔFMQ [relative fayalite-magnetite-quartz buffer]) indicates that magma of the Mangling intrusive complex (except the biotite diorite) has high oxygen fugacity and small fractionated granitic intrusions, which are coeval with the biotite diorite enclave or younger than the Mangling granitic rocks, may have potential for generating porphyry molybdenum mineralization. The combination of this study and previous studies corroborates that the Qinling Orogen underwent an intracontinental orogenic evolution in a post-collisional compression to extension transitional setting during the Late Jurassic to Early Cretaceous, affected by far-field Paleo-Pacific slab subduction.

Apatite geochemistry as a proxy for porphyry-skarn Cu genesis: a case study from the Sanjiang region of SW China

In view of the importance of the magmatic oxidation state and volatile composition (H2O-S-Cl) in porphyry Cu mineralization, identification of magmatic information has become the basis for the evaluation of the Cu-mineralization potential of porphyries. Apatite, as a common accessory mineral in porphyry, is recognized as an effective petrogenetic and metallogenic indicator. In this study, we analyze the in situ major and trace elements and Nd isotope content of apatite from four granitic plutons in the Yidun Terrane, SW China, in order to provide new insights into the identification of the magmatic features of Cu-mineralized plutons. The plutons investigated in this study are a Cu-mineralized pluton (Hongshan) and three non-mineralized plutons (Cilincuo, Rongyicuo, and Hagela). Chemical composition and textural analysis indicate that the apatites are all of magmatic origin and have not been altered. Our results show that apatite δCe values rather than δEu and Ga values are more valid indicators of the magma oxidation state of the four plutons. Based on apatite δCe values and zircon compositions, the parental magma of the Hongshan pluton is identified as having been more oxidized than those of the other three plutons. Moreover, the higher Cl content (0.06–0.23 wt%) in the Hongshan apatite compared to that in apatite from the other three plutons (0.01–0.09 wt%), indicates that the parental magma of the former pluton was more enriched in Cl. Apatite from the Hongshan pluton contains more SO3 (average 0.05 wt%) than that from the other three plutons (SO3 content mostly lower than the detection limit of electron probe micro-analysis), which may be related to higher magmatic oxygen fugacity of the Hongshan pluton. In addition, the Nd isotopic composition of apatite [εNd(t): −13.6−5.9] demonstrates that the Cu-mineralized and non-mineralized plutons were derived from the melting of ancient lower crust. Furthermore, based on the geochemical signature of adakite revealed by apatite Sr/Y and δEu values, the Hongshan pluton can be considered to have been produced as the result of partial melting of the thickened lower crust. Finally, an index system consisting of apatite δCe, Sr/Y, and F/Cl values is established to distinguish between the Cu-mineralized and non-mineralized plutons.

The Genetic Link between Iron-Oxide–Apatite and Porphyry Cu–Au Mineralization: Insight from the Biotite–Pyroxene–Zircon Study of the Nihe Fe Deposit and the Shaxi Cu–Au Deposit in the Lower Yangtze Valley, SE China

Different ore deposit types may evolve from a common magmatic-hydrothermal system. Establishing a genetic link between different deposit types in an ore cluster can not only deepen the understanding of the magmatic-hydrothermal mineralization process but can also guide exploration. Both the Nihe iron-oxide–apatite (IOA) deposit and the Shaxi porphyry Cu–Au deposit in the Lower Yangtze Valley, Anhui, Southeast China, formed in the Luzong Cretaceous volcanic basin at ~130 Ma. We examined a temporal–spatial and potential genetic link between these deposits based on stratigraphic lithofacies sections, biotite and clinopyroxene mineralogical chemistry, zircon chronology, Hf isotopes, and trace elements. Stratigraphy, petrology, mineralogical chemistry, and available fluid inclusion results support that the emplacement depth of the Nihe ore-related porphyry is shallower than that of the Shaxi porphyry. The magmatic zircon and hydrothermal zircon from Nihe provided U–Pb ages of 130.6 ± 0.7 Ma and 130.7 ± 0.7 Ma, respectively. The magmatic zircon U–Pb age (130.0 ± 0.8 Ma) of Shaxi overlaps with its molybdenite Re–Os age (130.0 ± 1.0 Ma). The agreement between the mineralization and porphyry emplacement ages of Nihe and Shaxi indicates a temporal coincidence and supports a possible genetic link between the two deposits, considering their close spatial relationship (in the same ore district, 15 km). The zircon Hf isotopes and trace elements support the evolution of both deposits from an enriched lithospheric mantle, although the Shaxi deposit may have experienced contamination of the Jiangnan-type basement. Both deposits lie above the fayalite-magnetite-quartz buffer, but the Nihe magmatic zircons are of lower temperature and less oxidized than that of Shaxi. The much higher Eu/Eu* and Yb/Dy values of zircons from Shaxi are likely caused by the suppression of early plagioclase crystallization and the prevalence of amphibole fractionation, thus indicating more hydrous content of the Shaxi ore-related magma. Additionally, the Shaxi ore-related porphyry has higher zircon Hf concentrations, suggesting that the porphyry Cu–Au deposit has experienced a greater degree of magma fractionation. Our study highlights that the Nihe IOA deposit and the Shaxi porphyry Cu–Au deposit have a common magma source, while different extent of crust contamination, magma oxidation state, hydrous content, and degree of magma fractionation collectively result in the two distinct ore deposits. This possible genetic link suggests a great potential of porphyry Cu–Au-PGE mineralization in the Middle–Lower Yangtze River metallogenetic belt, especially in the deep part of the IOA district in the Luzong Cretaceous volcanic basin.

Two stages of porphyry Cu mineralization at Jiru in the Tibetan collisional orogen: Insights from zircon, apatite, and magmatic sulfides

Continental collision−related porphyry copper (Cu) deposits provide significant global copper resources, but their genesis remains controversial because it is not clear whether remelting of remnant arc-derived lower-crustal Cu-rich cumulates is critical to their formation. We investigated zircon and apatite compositions of the Jiru porphyry Cu deposit in the Gangdese belt of southern Tibet, which is characterized by weaker early Eocene mineralization and more pronounced Miocene mineralization. Our data demonstrate that apatite hosted in zircon can record the volatile compositions of magma before fluid exsolution. Zircon-hosted apatites from the early Eocene granite that have low XF/XCl ratios (≤3) and are interpreted to have crystallized from volatile-undersaturated magma have Cl contents of 0.96−2.14 wt% (1.86 ± 0.38 wt%; n = 15) and SO3 contents of 0.01−0.14 wt% (0.08 ± 0.04 wt%; n = 15). In contrast, zircon-hosted apatites from the Miocene porphyry that have low XF/XCl ratios (≤10) and are interpreted to have crystallized from volatile-undersaturated magma have Cl contents of 0.40−0.56 wt% (0.47 ± 0.06 wt%; n = 11) and SO3 contents of 0.20−0.92 wt% (0.59 ± 0.29 wt%; n = 11). In addition, the early Eocene magma was relatively reduced (ΔFMQ = 0.86 ± 0.55, where FMQ is fayalite-magnetite-quartz) compared with the oxidized Miocene magma (ΔFMQ = 2.04 ± 0.43). The contrasting magmatic oxidation states and apatite/melt S contents were likely critical in controlling the different scales of Cu mineralization during the early Eocene and Miocene, as more highly oxidized magma can dissolve much larger amounts of sulfur and metals (e.g., Cu). Melt Cl content may not play a critical role in magma fertility, since the Jiru early Eocene granite had higher melt Cl contents than the Miocene porphyry. Magmatic sulfides (pyrrhotite, chalcopyrite, and pyrite) were not recognized in the Miocene zircons but did occur in the early Eocene zircons, and these sulfides were demonstrated to have crystallized before fluid exsolution in the shallow magma reservoir. The Jiru early Eocene magmatic sulfide saturation might not have enhanced the amount of copper available to hydrothermal fluids during the early Eocene. Late-stage sulfide saturation in the previous arc magmas might reduce the potential for collision-related porphyry Cu mineralization that is associated with remelting of the previous arc lower crust.

Possible source of Au in the Jiaodong area from lower crustal sulfide cumulates: Evidence from oxygen states and chalcophile elements contents of Mesozoic magmatic suites

The genesis of giant Jiaodong gold province remains perplexing although many ore-forming models have been proposed in the past decades. The key issues are what caused the burst of gold around 120 Ma and possible sources of gold. Here we report gold and platinum group elements (PGE) data and magmatic oxidation states of the pre-mineralization Mesozoic plutonic rocks and some mafic dykes to discuss their potential effects on gold mineralization. Gold concentrations of the Late Jurassic Linglong granite are 0.015–0.278 ppb. The Early Cretaceous plutonic rocks (129–127 Ma) have the same levels of Au concentrations (Au = 0.031–0.047 ppb). Both of the Jurassic and Cretaceous magmatic rocks are depleted in Au compared to normal granitic rocks worldwide (∼1 ppb). Palladium and platinum concentrations in the Linglong granite are 0.160–0.385 ppb and 0.046–0.116 ppb, respectively. Meanwhile, Pd and Pt contents of Cretaceous magmatic rocks are 0.069–0.363 ppb and 0.063–0.465 ppb, respectively.Oxygen fugacities of the Late Jurassic Linglong granite calculated by biotite oxybarometer are FMQ + 0.5 to FMQ + 1.2 (where FMQ refers to fayalite-magnetite-quartz buffer). Oxidation states of the Early Cretaceous magmatic rocks estimated via amphibole oxybarometer are from FMQ + 0.3 to FMQ + 1.1 for Guojialing granodiorite. Zircon trace element data yield overall higher fO2 values than those estimated with biotite and amphibole compositions. Oxygen fugacities are from FMQ – 0.6 to FMQ + 1.9 (sample CJ-3) and FMQ – 0.7 to FMQ + 1.1 (sample LL-1) for the two Linglong granite samples. Zircons from the Early Cretaceous Guojialing pluton yield oxidation states between FMQ – 0.8 and FMQ + 1.9 (DCW-1), which are about one log unit higher than those of the Beijie pluton (BJ-1: FMQ – 1.0 to FMQ + 0.7). However, oxidation states from nearly syn-mineralization mafic dyke reach FMQ + 2.0 to + 3.0 when estimated via zircon oxybarometer. Oxidation states given by whole rock Fe3+/Fe2+ ratios also support the results calculated by mineral compositions.There is no fractionation crystallization of redox-sensitive minerals, like magnetite or garnet, as indicated by geochemical data, which implies that the oxidation states of magmas may reflect their source regions. However, the depleted nature of Au and PGE must imply that these elements are missing. We propose that sulfide saturation at the deep crust can be invoked to explain the extremely low gold and PGE contents of these magmatic rocks. The sulfide cumulates act as sinks for sulfide-loving elements (e.g., Au), which are pre-enriched source for later large-scale gold mineralization in Jiaodong area. The underplated water-rich mafic magma from enriched mantle source ponded at the lower crust, continuously crystallized anhydrous minerals (e.g., olivine and clinopyroxene) and reached volatile saturation at the very moment. This volatile, probably carrying gold, resorbed the sulfide cumulate and escaped to shallow levels to form the giant Jiaodong gold deposits.

Using apatite to differentiate metallogenic potential and environment of granitic rocks: A case study from the Tongshanling W-Sn-Cu-Pb-Zn ore field, Nanling Range (South China)

The Tongshanling ore field is one of the most representative W–Sn–Cu–Pb–Zn polymetallic deposits in the central-western Nanling Ore Belt, which developed multiple mineralization types, including Cu–Pb–Zn (Tongshanling), W–Sn (Weijia), Pb–Zn (Jiangyong), and Mo (Yulong) deposits. Here, we analyzed apatite from the Tongshanling granodiorite and the Weijia granite porphyry, integrating major and trace elements, U–Pb dating, and Nd isotopic analyses to compare the difference in geochronology, magma source, redox environment, and evolution between the Tongshanling and Weijia deposit. The U–Pb dating of the Tongshanling and Weijia apatite samples yield a lower intercept age of 166.2 ± 11.3 Ma (1σ, MSWD = 0.22) and 160.87 ± 8.92 Ma (1σ, MSWD = 0.44) on Tera-Wasserburg diagrams, respectively. The 147Sm/144Nd and 143Nd/144Nd ratios of the Tongshanling apatite samples are 0.136769–0.182878 and 0.512159–0.512246, respectively, with corresponding εNd (t = 166 Ma) ranging from −8.32 to −6.60, suggesting that the Tongshanling granodiorite was produced by partial melting of metaigneous rocks in the deep crust. The Tongshanling apatite shows a right-inclined REE pattern, while the Weijia apatite displays a “V” type REE pattern. The Tongshanling apatite shows higher Eu/Eu* values, and lower Mn and Ce/Ce* values than the Weijia apatite, indicating that the former formed in higher magmatic oxidation state while the latter formed in more reduced conditions. The granodiorite in the Tongshanling deposit is characterized by a lower degree of fractionation than the granite porphyry in the Weijia deposit, of which the Weijia apatite has lower Sr, Mg contents, and higher Y contents. Furthermore, apatite from Cu-bearing granites exhibits lower F/Cl ratios compared to those from W-bearing granites in the South China Block. In the Tongshanling area, apatite from the Tongshanling granodiorite has higher Cl contents than the Weijia apatite, indicating that the crustal basement of the Tongshanling area was affected by the high Cl fluid derived from the Proterozoic oceanic subduction and remelted to form granite in the Mesozoic, while the Weijia granite porphyry formed from remelting of a sedimentary source. The Tongshanling granodiorite and Weijia granite porphyry with different degrees of fractionation, magma source, and halogens contents can be effectively distinguished by the geochemistry of apatite. The difference in trace elements (Sr, Y, Mg, and REE) and isotopes of apatite can be applied to the identification of different types of ore-bearing granites in the Nanling Range.

Titanite geochemistry and its indicators for Cu-mineralized magmas: Insight from three dioritic intrusions in the Tongling area of the Lower Yangtze River metallogenic belt

Titanite has been regarded as a potential petrogenetic and metallogenic indicator owing to its stability and enrichment of trace elements. To verify the availability of geochemical titanite proxies in the Cu-mineralized magma system and to reveal magmatic constraints on Cu mineralization related to magmatic hydrothermal fluid, in situ major and trace element abundances and Nd isotopes of igneous titanite were determined for three Mesozoic diorite intrusions in the Tongling ore district of eastern China. The investigated intrusions were the Tongguanshan, Fenghuangshan, and Hucun intrusions, all of which host porphyry-skarn Cu deposits. Our study shows that the titanite δEu value and Fe/Al ratio, rather than the δCe and Ga content, are more effective indicators of the magma oxidation state in all three intrusions. Based on the titanite δEu value and Fe/Al ratio, the causative magma of the Fenghuangshan intrusion (titanite δEu = 0.50 ± 0.08; Fe/Al = 0.21 ± 0.07) was identified as being less oxidized than those of the Tongguanshan (titanite δEu = 0.64 ± 0.09; Fe/Al = 0.93 ± 0.15) and Hucun (titanite δEu = 0.80 ± 0.03; Fe/Al = 0.91 ± 0.04) intrusions. This result was consistent with the zircon Ce4+/Ce3+ ratio. Moreover, owing to the limited effect of fluid exsolution and fractional crystallization on the F content of the investigated titanite, the higher titanite F content of the Fenghuangshan intrusion (titanite F = 0.40 ± 0.09 wt%) implies that the parental magma of this intrusion is more F-enriched than those of the Hucun (titanite F = 0.17 ± 0.09 wt%) and Tongguanshan (titanite F = 0.10 ± 0.07 wt%) intrusions; however, titanites from all three Cu-mineralized intrusions had similar or lower F content than those from intrusions without Cu mineralization. The above findings indicate that magmas with high oxygen fugacity have relatively high potential for porphyry-skarn Cu mineralization, while the high enrichment of F is not necessary for Cu mineralization. Additionally, we found that titanite εNd(t) values can inherit whole-rock εNd(t) values and thus can be used to investigate petrogenesis. This study confirms that titanite geochemistry is an efficient petrogenetic and metallogenic indicator for Cu-mineralized magma systems when the prerequisites are met.

Geochronology and Geochemistry of the Mamupu Cu‐Au Polymetallic Deposit, Eastern Tibet: Implications for Eocene Cu Metallogenesis in the Yulong Porphyry Copper Belt

AbstractThe Mamupu skarn‐type Cu‐Au polymetallic deposit represents the first discovery of a medium deposit in the southern Yulong porphyry copper belt (YPCB), eastern Tibet. The Cu‐Au mineralization mainly occurs as chalcopyrite in breccia, within the plate‐like carbonate interlayer, being closely related to chloritization (e.g., chlorite, magnetite and epidote) and skarnization (e.g., diopside, tremolite and garnet). The ore‐related quartz syenite porphyry (QSP) and granodiorite porphyry (GP) were emplaced at 40.1 ± 0.2 Ma and 39.9 ± 0.3 Ma, respectively. The QSP of Mamupu is an alkaline‐rich intrusion, relatively enriched in LREE, LILE, depleted in HFSE, with no significant negative Eu and Ce anomalies, slightly high (87Sr/86Sr)i, low ∊Nd(t), uniform (206Pb/204Pb)i and ∊Hf(t) values, which indicates that the porphyry magma may be caused by both the mixing of metasomatized EM II enriched mantle and thickened juvenile lower crust. The QSP in the Mamupu deposit shares a similar genesis of petrology to other ore‐related porphyries within the YPCB. High oxygen fugacity and water content of the magmas are essential for the formation of porphyry and skarn Cu deposits. The QSP has similar high magmatic oxidation states and water content to the Yulong deposit, which indicates that the Mamupu has a high prospecting potential. Differences in the geological characteristics and scale of mineralization between the Mamupu and other YPCB deposits may be due to the different emplacement depths of ore‐related intrusions, as well as differences in the surrounding rocks.