Magmatic Oxygen Fugacity Research Articles

Zircon U–Pb ages, Hf-O isotopes and trace elements of the multi-volcanism in the Ningwu ore district, eastern China: Implications for the magma evolution and fertility of iron oxide–apatite (IOA) deposits

The Ningwu ore district is the best-defined iron oxide–apatite (IOA) district in China and hosts >30 IOA deposits. At Ningwu, four volcanic cycles occurred over a short period of ∼ 10 Ma and yielded comparable lithogeochemical characteristics. However, only the sub-volcanic intrusive rocks from the Dawangshan Formation are associated with IOA-type mineralization. In this study, the mineralization related and unrelated formations have been distinguished based on zircon Hf-O isotope and trace element analyses to reveal the magma evolution and fertility of IOA deposits. Our results suggest that the Mesozoic volcanic rocks at Ningwu have three components of sources: enriched mantle, Archean crystalline basement, and Neoproterozoic crustal materials. The main sources for the magmas of the Longwangshan and Dawangshan formations include the Archean crust and enriched mantle, whereas those for the Gushan and Niangniangshan formations include Neoproterozoic crust and enriched mantle. According to the magmatic sources, the four volcanic formations in the Ningwu ore district can be divided into two groups: (1) the Longwangshan and Dawangshan formations and (2) the younger Gushan and Niangniangshan formations. The magmatic oxygen fugacities (fO2) for the Longwangshan and Dawangshan formations are ΔFMQ +0.6 to +2.2 and ΔFMQ +0.8 to +3.0, respectively. In comparison, the Gushan Formation has much lower magmatic fO2 values of ΔFMQ +0.2 to +0.6, whereas the Niangniangshan Formation yielded magmatic fO2 values peaking at ΔFMQ +0.6 to +2.4 and +3.2 to +4.2. Our new data, together with previous results, suggest that moderate magmatic fO2 (ΔFMQ +0.8 to +3), F-richness, high water contents, high proportions of enriched mantle materials, and a higher degree of magma evolution may affect the fertility of the magma for IOA deposit formation.

Magma mixing affected the Late Triassic porphyry mineralization in the Yidun arc in SW China

The Yidun arc was formed by the westwards subduction of the Garze-Litang Ocean in the Late Triassic. It trends from near north to south. Many intermediate-acid intrusions became widely distributed in the Yidun arc during this period. Large to super-large porphyry-skarn deposits developed only in the southern Yidun arc, with no corresponding deposit in the northern part. It has been found that some mafic microgranular enclaves (MMEs) are exposed, and there is certain evidence of magma mixing in the Yidun arc in recent years. To clarify the relationship between widely developed magma mixing and metallogenic differences in the Yidun arc, the zircon U-Pb dating and geochemistry of quartz monzonite porphyry (QMP) and its MMEs in the Pulang super-large porphyry Cu deposit in southern Yidun have been carried out. Then, the granodiorite (GD, ore-barren) and MMEs of the Daocheng batholith in the middle and the Cuojiaoma batholith (ore-barren) in the north of the Yidun arc were compared. The results show that the oxygen fugacities of the Pulang QMP and MMEs are higher, and the MMEs are slightly higher than those of the QMP overall. The oxygen fugacity of MME in the Daocheng GD is slightly higher than that of FMQ, but it is still lower than that of QMP and MME in Pulang. That is, the magmatic oxygen fugacity of the Daocheng GD is lower than that of the Pulang QMP and MME. There are more magmas fractionated in the south than in the north of Yidun due to the difference in geological structure between the north and south. Therefore, the degree of magma mixing is gradually reduced in the Yidun arc from south to north (Pulang, Daocheng to Cuojiaoma), and the metallogenic potential is also higher in the south and lower in the north.

Sulfur recycling in subduction zones and the oxygen fugacity of mafic arc magmas

Sulfur is important to a range of subduction zone processes, but the factors controlling the sulfur content of primitive arc magmas are poorly understood. In particular, uncertainties about the oxidation state of primary melts in the sub-arc mantle hinder efforts to understand the behavior of sulfur in arc magmas. Here, we use olivine-hosted melt inclusion data from 32 arc segments globally to characterize sulfur contents of magmas from a variety of subduction zones and identify the key processes that control the sulfur content of arc magmas. Average primary magma sulfur contents estimated from these data range from 466±220ppm (Mariana Forearc) to 4,264±819ppm (Ecuador). Primitive and more evolved magmas in both hot- and cold-slab subduction zones commonly have higher sulfur contents than mid-ocean ridge basalt (MORB) magmas. In most cases, these elevated sulfur contents require a subduction-modified mantle source with more sulfur than MORB-source mantle. Correlations between magma S and Cl concentrations and S/Dy and Th/Yb ratios in the global data set confirm that sulfur is partially sourced from the subducting slab. By comparing melt inclusion sulfur contents to sulfur solubility limits imposed by sulfide saturation, we find that 88% of arc magmas in our compilation require conditions more oxidizing than the quartz-fayalite-magnetite buffer (QFM). The relationship between S/Dy and calculated magmatic oxygen fugacity (fO2) suggests that slab-derived sulfate is responsible for the higher oxidation state of arc magmas compared to MORB. Model calculations of redox equilibrium between S and Fe species in basaltic melts when sulfate is added to the mantle wedge demonstrate the feasibility of this causal link, which can explain the common range of arc magma fO2 values observed for subduction zones globally. Correlations between magma sulfur contents, fO2, and the proportion of added slab-derived material inferred from trace elements can drive associations between high Sr/Y magmas and fertile porphyry ore deposits, as well as the MORB-like Cu contents of arc magmas.

Copper isotope fractionation in magmatic Ni–Cu mineralization systems associated with the variation of oxygen fugacity in silicate magmas

Significant variations of oxygen fugacity (fO2) in silicate magmas are widely suggested to play an important role in formation of magmatic Ni–Cu deposits in convergent margin settings. Copper isotopes have potential to track redox change of magmatic systems, but the linkage of silicate magma oxygen fugacity with Cu isotope fractionation in magmatic Ni–Cu mineralization systems is still uncertain. A combined study of Cu isotopes and estimated oxygen fugacities can provide novel and direct insights into this issue. In this study, four magmatic Ni–Cu deposits in Eastern Tianshan (NW China) were investigated in order to reveal the fO2 variations in silicate magmas as a function of redox-induced Cu isotope fractionation. The silicate magma fO2 of the four deposits widely varies from ∼QFM –2.2 to + 0.6, from ∼QFM –0.7 to 0.0, from ∼QFM –1.4 to –0.2, and from ∼QFM –1.2 to –0.6 for the Tulaergen, Huangshandong, Huangshannan, and Hulu deposits, respectively. Models using olivine compositions, Cu/Pd ratios, and estimated silicate magma fO2 suggest that the silicate magmas gradually became more oxidized as Ni–Cu mineralization proceeded. The closed-system R factor equation and the Rayleigh equation were used to model mineralization processes and related Cu isotope fractionation. Modelling using δ65Cu values and the estimated fO2 indicates that the evolved silicate magmas with high fO2 tend to incorporate heavier Cu isotopes. Our studies indicate that the fO2 variation of silicate magmas is the key factor governing Cu isotope fractionation in magmatic Ni–Cu mineralization systems in convergent tectonic settings, which is potentially applicable for other magmatic Ni–Cu deposits worldwide.

Redox evolution of crystallizing magmas with C-H-O-S volatiles and its implications for atmospheric oxygenation

The evolution of Earth’s atmospheric O2 levels on long timescales is modulated in part by the composition of magmatic gases, specifically their oxidation state and speciation. Here, we examine how magma differentiation affects the oxygen fugacity and composition of magmatic gases. Variations in temperature, pressure, composition, and redox conditions of evolving magmatic systems result in complexities, which have not been fully treated previously. Taking all these variables into account, we develop a multipronged approach to a comprehensive understanding on the role of magmatic gases on atmospheric oxygenation. This includes a compilation of petrological observations on the redox systematics of mid-ocean ridge basalts, a geochemical model for magma crystallization in oxygen-closed systems with variable initial redox conditions, and a thermodynamic framework for gas–melt systems with multi-component C-H-O-S volatiles.By considering magmatic differentiation at variable depths, we show that magma crystallization progressively changes the redox state of residual melts and coexisting gases due to differences in iron speciation between the melt and cumulate minerals. In thick crusts, high pressures of magma differentiation suppress magnetite saturation but promote garnet crystallization, increasing magmatic oxygen fugacity by ∼4–8 orders of magnitude and thereby resulting in oxidized gases. In thin crusts, the prevalence of magnetite prevents significant oxidation of the magma, yielding reduced gas emission. Hence, magmatic gases from thick crusts are much less effective O2 sinks, which does not appear to be affected by gas decompressional transport or polybaric degassing from the deep crustal magmatic zone to the atmosphere. Depending on how crustal thickness during active magmatism varies with time, magmatic gases may play an important role in modulating atmospheric O2 levels on million-year timescales. Rapid growth of continental crust in the late Archean appears to have been associated with the first evidence for significant crustal thickening during magmatism. If so, this would have led to a substantial decrease in O2 sink, possibly permitting atmospheric O2 to rise to a new state.

Geochronology, geochemistry, and Sr-Nd-Hf isotopic compositions of Permian granitoids from the South Altay, NW China: Implications for tungsten-copper skarn mineralization

The Bastielieke deposit is a W polymetallic skarn deposit discovered in the South Altay, Northwest China. W polymetallic mineralization mainly occurs within the contact zone between the Permian intrusions and the Kangbutiebao Formation (Fm.) metamorphic volcanic-sedimentary rocks. Permian granitoids associated with W (Cu) mineralization include several phases of granite: biotite granite (G1), monzogranite (G2) and two-mica monzogranite (G3) in L2 ore block, biotite monzogranite (G4) in L3 ore block. We obtained zircon U-Pb ages of G3, G4, and barren porphyritic monzogranite (G5) from Lamazhao granite of 275.0 Ma, 275.4 Ma, and 277.0 Ma, respectively. All samples from G3, G4, G5 show enrichment of SiO2, weakly peraluminous, enriched in light rare earth elements (LREE), and depleted in high field strength elements (HFSE). These Permian granitoids were mainly derived from mantle magma, and mixed with some lower crust components, although the Sr-Nb-Hf characteristics are slightly differences. G1, G2, and G4 were involvement of more mantle components than G3 and G5. G1 and G5 have experienced more obviously fractional crystallization than G2, G3, and G4, and G5 displays the highest fractionated degree. Moreover, G5 has the highest magmatic oxygen fugacity compared with G1, G2, G,3 and G4 according to variable △FMQ values (G1: −2.75 to 1.42, G2: −3.14 to 2.23, G3: −3.15 to 0.70, G4: −2.92 to 1.36, and G5: −1.63 to 2.82, respectively), as well as δEu values, δCe values, and Ce/Nd ratios. Thus, we proposed that tungsten skarn mineralization in the Bastielieke deposit area is mainly controlled by high fractional crystallization, and the contribution of magma source and oxidation state cannot be neglected. In addition, the copper could be offered by oxidized fluids exsolved from mafic magmas or Cu-bearing immiscible magmatic sulfides in copper-rich tungsten skarn system.

Porphyry Cu fertility of eastern Paleo-Tethyan arc magmas: Evidence from zircon and apatite compositions

The Paleo-Tethyan arc belt is host to a small number of porphyry Cu deposits the key controls on which remains poorly understood. It has been hypothesized that the Paleo-Tethyan arc magmas were reduced and therefore less fertile for porphyry Cu formation but few studies have been conducted to directly assess the magmatic oxygen fugacity. We investigate the magma fertility of eastern Paleo-Tethyan arc igneous rocks in the Sanjiang region, SW China, represented by the Maoding (268.3 ± 2.3 Ma), Suwalong (275.6 ± 1.4 Ma to 264.7 ± 1.4 Ma) and Baimaxueshan (255.7 ± 1.5 Ma to 249.3 ± 1.1 Ma) granitoids. They have geochemical features consistent with typical continental arc rocks, formed from partial melting of the mantle wedge, with increasing contamination of crust-derived melts over time. The presence of amphibole, high Sr/Y ratios (35 ± 20, n = 17), and lack of negative Eu anomalies (EuN/EuN⁎ = 1.04 ± 0.3, n = 17) are consistent with derivation from hydrous magmas. Zircons from the intrusive rocks have relatively low EuN/EuN⁎ (mostly <0.4) ratios and calculated ΔFMQ values (−2.1 ± 1.2, n = 86). Combined with apatite microphenocrysts with extremely low sulfur contents (< 0.12 wt% SO3), it suggests that the intrusions originated from water-rich but reduced arc magmas, that were less fertile for porphyry Cu formation. Our study highlights that magmatic fO2 is probably a critical control on the prospectivity of arc systems and confirms that zircon and apatite compositions can be used to evaluate porphyry Cu fertility.

Development of major element proxies for magmatic H2O content in oceanic basalts

Analysis of H2O concentrations in quenched glass has enabled significant improvements in our understanding of its role in mantle melting, magma differentiation, eruption dynamics, and the origin of mantle heterogeneity. Direct measurements of dissolved H2O in glass, however, are not always possible, and we lack robust methods of constraining magmatic H2O contents in aphyric, bulk rock samples that lack glass. Here, we present a major element hygrometer for mid-ocean ridge (MOR) and back-arc basin (BAB) basalt magmas based on the sensitivity of phenocryst phase assemblages to magmatic H2O contents, which translate into resolvable differences in liquid lines of descent (LLDs) as a function of magmatic H2O concentrations. Existing hygrometers lack sufficient resolution to be useful at the low H2O concentrations typical of MOR and BAB basalts (<1.0 wt%). We develop the major element proxy, Al2O3/FeO*(7.0) (fractionation-corrected to 7 wt% MgO), for determining magmatic H2O contents using cogenetic suites of oceanic basalts with well-defined LLDs and well-constrained H2O contents. H2O(7.0) positively correlates with Al2O3/FeO*(7.0) in the mid-ocean ridge basalt dataset, and this relationship is maintained in back-arc basin basalts with a broader range of water contents (up to 2.0 wt%). The main petrological control over this covariation is the role of H2O in suppressing plagioclase crystallization, while crystallization pressure and magmatic oxygen fugacity play lesser roles. Herein, we present an empirical model that uses Al2O3/FeO*(7.0) to estimate the magmatic water content in plagioclase-saturated oceanic basaltic magmas: H2O(7.0) = 1.109Al2O3/FeO∗(7.0) − 1.111. This model enables the estimation of magmatic H2O content using whole-rock major element data, which can be readily determined for aphyric or crystalline lavas that lack quenched glassy rinds, melt inclusions, or appropriate phenocryst assemblages.

Rapid transition to fertile magma and promotion of porphyry mineralization: A case study from the Don Javier deposit

The Don Javier porphyry Cu–Mo deposit formed contemporaneously with the Incaic I orogeny (∼60 Ma) in southern Peru. The causative dacite porphyry is hosted by the Yarabamba Superunit, which is the youngest batholithic unit in the Toquepala arc. In this study, we conducted elemental and isotopic analyses on samples from the dacite porphyry and Yarabamba Superunit in an effort to clarify the origins and formation mechanism of the deposit. Zircon U–Pb dating shows that the young part of the Yarabamba Superunit was emplaced at 65.4 ± 0.7 to 63.5 ± 0.8 Ma, and the causative dacite porphyry was emplaced at Don Javier between 59.9 ± 0.4 and 59.2 ± 1.1 Ma. The Yarabamba Superunit and dacite porphyry have similar εHf(t) and δ18O isotope values, ranging from –4.6 to +1.9 and 5.1‰ to 6.4‰, respectively. The Yarabamba Superunit has initial Sr and Nd isotope values of 0.70533–0.70579 and 0.51242–0.51244, respectively. Isotopic data suggest that the Yarabamba Superunit and dacite porphyry evolved from an isotopically homogeneous magma reservoir with minor crustal assimilation. However, the dacite porphyry has whole-rock and zircon Eu/Eu* values of 0.9 and 0.34, respectively, higher than those of the Yarabamba Superunit (0.6 and 0.13).Additionally, the dacite porphyry has significantly higher apatite S content (0.07 wt%) and XCl values (0.21) than those of the Yarabamba Superunit (apatite S: 0.01 wt%; XCl values: 0.15). The calculated magmatic oxidation state shows that the dacite porphyry has a significantly higher magmatic oxygen fugacity (ΔFMQ +1.1) than the Yarabamba Superunit (ΔFMQ –0.5).Together, the data suggest that the magmas formed the Yarabamba Superunit and causative dacite porphyry are characteristics of distinct tectonic regimes. The magma of the Yarabamba Superunit was generated during normal subduction and underwent low-pressure, H2O-poor, plagioclase-dominated fractionation while the magma of the dacite porphyry was generated during arc compression and underwent high-pressure, H2O-rich, amphibole-(garnet) dominated fractionation, which led to a high oxidation state and volatile-rich magma that promoted porphyry mineralization at Don Javier. This rapid elevation of the magmatic oxidation state and volatile content occurred synchronously with the Incaic I orogeny (∼60 Ma), which indicates that this orogeny had a significant impact on the formation of the very large–super-giant porphyry ore systems in southern Peru.

Volatile Characteristics of Apatite in Dacite from the Eastern Manus Basin and Their Geological Implications

As one of the youngest back-arc basins, the evolutionary behavior of magmatic volatiles in the Eastern Manus Basin has been poorly studied. Recently, apatite has received widespread attention for its powerful function in recording information on magmatic volatiles. In this paper, by determining the major element compositions and primary volatile abundances (F, Cl, SO3) of apatites in dacite, we analyze the compositions of volatiles before magma eruption in the Eastern Manus Basin, as well as their indications for magmatic oxygen fugacity and petrogenesis, so as to improve the study about the evolution of magmatic volatiles in this region. Experimental data indicate that apatite saturation temperatures range from 935 to 952 °C. All the apatites are magmatic apatites with F contents of 0.87−1.39 wt.%, Cl contents of 1.24−1.70 wt.%, and SO3 ≤ 0.06 wt.%. Analysis reveals that the apatites in this study crystallized from volatile-undersaturated melts, so their chemical compositions can be used as indicators of magmatic compositions. According to calculations, the minimum S concentrations of the host melts range from 2−65 ppm or 8−11 ppm. The crystallization and separation of magnetite caused the reduction state of melts, and the relatively low oxygen fugacity (ΔFMQ = −0.2 ± 0.9) caused low SO3 contents in apatites. In addition, F and Cl contents of the host melt were calculated to be 185−448 ppm and 1059−1588 ppm, corresponding to the H2O contents of 1.4−2.1% and 1.2−1.5% (error ± 30−40%), respectively. The high Cl/F ratio and H2O contents of samples indicate the addition of slab-derived fluids in the mantle source region of the Eastern Manus Basin. High F contents of the melts may be influenced by F-rich sediments, as well as the release of F from lawsonite and phengite decomposition. High Cl appears to originate from the dual influence of subduction-released fluids and Cl-rich seawater-derived components. Further, it is estimated that 14−21% of the total Cl concentrations in melts were added directly from subduction-released fluids, or higher.

Genesis and Metallogenic Characteristics of the Zhunsujihua Granitic Intrusions in Sonid Left Banner, Inner Mongolia, China

The Zhunsujihua porphyry Mo-Cu deposit is located in the Erenhot–East Ujimqin metallogenic belt in northeastern China. Granodioritic intrusions in the mining area are dominated by granodiorite and granodiorite porphyry, but the Mo mineralization is limited within the granodiorite. Zircon LA-ICP-MS U-Pb dating yields crystallization ages of 301.5 ± 3.0 Ma for granodiorite and 296.0 ± 3.0 Ma for granodiorite porphyry. These ages constrain the magmatic activity at the Zhunsujihua deposit that took place during the subduction of the Paleo-Asian oceanic plate. Whole-rock geochemical data suggest that the granodioritic intrusions belong to calc-alkaline and high-K calc-alkaline series, and are characterized by enrichment in K, Rb, U, Th, and Pb, and depletion in Nb, Ta, Ti, and P. The negative Eu, Ba, and Sr anomalies suggest that they have experienced extensive fractionation of plagioclase. Trace element compositions of zircons from the Zhunsujihua deposit provide constraints on the oxygen fugacity (ƒO2) of the magma, which is shown to high values with ∆FMQ = +0.5 to +5.6. The wide range of zircon εHf (t) (+1.3~+9.4) values, positive whole-rock εNd (t) (+2.5~+3.9) values, and relatively low initial (87Sr/86Sr)i (0.70367~0.70561) ratios indicate that the magmas mainly originated from a juvenile lower crust source derived from depleted mantle, but mixed with pre-existing crustal components. Moreover, the juvenile lower crust represents the main source of Mo for the Zhunsujihua deposit. A high magmatic oxygen fugacity and fractional crystallization played key roles in forming the Zhunsujihua deposit.

Slab-derived devolatilization fluids oxidized by subducted metasedimentary rocks

Metamorphic devolatilization of subducted slabs generates aqueous fluids that ascend into the mantle wedge, driving the partial melting that produces arc magmas. These magmas have oxygen fugacities some 10–1,000 times higher than magmas generated at mid-ocean ridges. Whether this oxidized magmatic character is imparted by slab fluids or is acquired during ascent and interaction with the surrounding mantle or crust is debated. Here we study the petrology of metasedimentary rocks from two Tertiary Aegean subduction complexes in combination with reactive transport modelling to investigate the oxidative potential of the sedimentary rocks that cover slabs. We find that the metasedimentary rocks preserve evidence for fluid-mediated redox reactions and could be highly oxidized. Furthermore, the modelling demonstrates that layers of these oxidized rocks less than about 200 m thick have the capacity to oxidize the ascending slab dehydration flux via redox reactions that remove H2, CH4 and/or H2S from the fluids. These fluids can then oxidize the overlying mantle wedge at rates comparable to arc magma generation rates, primarily via reactions involving sulfur species. Oxidized metasedimentary rocks need not generate large amounts of fluid themselves but could instead oxidize slab dehydration fluids ascending through them. Proposed Phanerozoic increases in arc magma oxygen fugacity may reflect the recycling of oxidative weathering products following Neoproterozoic–Palaeozoic marine and atmospheric oxygenation.

Geology and factors controlling the formation of the newly discovered Beimulang porphyry Cu deposit in the western Gangdese, southern Tibet

The porphyry Cu deposits in the Gangdese metallogenic belt were mainly formed in the eastern portion. The Beimulang is a newly discovered deposit formed in post-collisional setting in the western part of the belt and adjacent to the super-large Zhunuo porphyry Cu deposit. There are three epochs of magmatism at Beimulang, including pre-mineralization quartz porphyry (PQP; 49.7 ± 0.4 Ma), inter-mineralization monzogranite porphyry (IMGP; 14.8 ± 0.2 Ma), monzogranite (IMG; 14.1 ± 0.2 Ma), and late-mineralization granite porphyry (LGP; 11.7 ± 0.1 Ma). The various degrees of alteration and sulfide occurred at the IMG(P) and LGP indicate that there are at least two episodes of fluid exsolution events associated with different intrusions spanning ∼ 3 m.y. at Beimulang, which are unlike other Gangdese porphyry Cu deposits showing single fluid exsolution around only one porphyry at Miocene (e.g., Qulong, Zhunuo, Dabu). The PQP is characterized by high SiO2 (75.6–81.4 wt%) and K2O (2.1–4.3 wt%) concentrations, high Nb/Ta (8.4–11.3) ratios, and low Mg# (22–43), Ni (1.5–2.7 ppm) contents. They have high (87Sr/86Sr)i (0.705098–0.707767) ratios and low εNd(t) (-4.9 to −1.9) values. These geochemical characteristics indicate that they were probably derived from a juvenile basaltic lower crust during the ongoing Indian-Asian collision. The IMGP and IMG show similar petrography and geochemical characteristics, such as high SiO2 (65.0–69.8 wt%), K2O (3.6–4.3 wt%), Sr (550–809 ppm) contents, high Sr/Y (79–89) ratios, and low Mg# (43–48), Th/Yb, and Nb/Yb values, combined with high (87Sr/86Sr)i (0.707254–0.708017) and low εNd(t) (-7.9 to −6.0), implying a similar source probably originated in the subduction-modified lower crust metasomatized by Neo-Tethyan oceanic slab-derived fluids. The LGP displays trends produced by fractionation of magma with a composition similar to the IMG(P), but wall rock assimilation plays a significant role during magmatic evolution. Zircons from the IMG(P) show higher Eu/Eu* (>0.3), 10000*(Eu/Eu*)/Y (>4), and (Ce/Nd)/Y (>0.04) ratios than those of the PQP, indicating the fertile magmas were more oxidized and hydrous. The high magmatic oxidation state for the IMG(P) is also supported by using the zircon (ΔFMQ = +4.1 to + 6.1), and amphibole (ΔFMQ =+2.6 to + 3.4) oxybarometers. Their high magmatic water contents are supported by evaluations using amphibole (∼3.5 wt%) and plagioclase (∼6.7 wt%) compositions, respectively. However, the LGP has higher magmatic oxygen fugacity (Eu/Eu* >0.3, ΔFMQ = +4.2 to + 7.3) caused probably by auto-oxidation of the magmas, but lower magmatic water contents (Sr/Y = 31 ± 15; V/Sc = 6 ± 4) caused presumably by previous fluid exsolution and/or fractionation of hydrous silicate minerals during magma ascent than those of the IMG(P). Therefore, the highly hydrous and oxidized magmas with juvenile material are favorable to form porphyry Cu deposits in Tibetan orogen. On the contrary, magmatic evolution can probably reduce the water content of magma chamber, which is not beneficial to porphyry Cu mineralization.

Highly Oxidising Conditions in Volatile-Rich El Hierro Magmas: Implications for Ocean Island Magmatism

Abstract Recent studies investigating magmatic volatile contents indicate widespread enrichment of carbon, sulfur, and halogens in ocean island basalts (OIBs). At El Hierro in the Western Canary Islands, magmas with exceptionally high CO2 and S contents have been erupting throughout the Holocene. High S content of up to 5200 ppm requires an oxidised mantle source, but estimates of initial magmatic oxygen fugacity (fO2) are sparse. Here, we present estimates of fO2 and magmatic temperature for El Hierro together with a global mantle potential temperature dataset to evaluate redox and temperature conditions in the early stages of melt evolution for volatile-rich OIBs. Oxygen fugacities calculated using vanadium partitioning between melt inclusions (MIs) and their olivine hosts are &amp;gt;FMQ + 2.9 (2.9 log10 units above the fayalite-magnetite-quartz buffer), indicating that El Hierro magmas are highly oxidised. MI and matrix glass sulfur speciation data record fO2 between FMQ-1 to FMQ + 2; these values strongly depend on the position of the S2− to S6+ transition relative to the FMQ buffer. Nonetheless, glass sulfur speciation data record lower oxygen fugacity than V partitioning data, indicating MIs were able to maintain Fe3+/ΣFe and S6+/ΣS equilibrium with the surrounding melt during their evolution. The high fO2 of El Hierro magmas is coupled with an average mantle potential temperature estimate of 1443 ± 66°C (1σ, n = 17) for the broader Canary Islands, which is slightly higher than the average potential temperature estimated for adjacent mid-ocean ridge segments (1427 ± 33°C, 1σ, n = 474), albeit the two values are well within error. We find that ~98% of Canary Island rock compositions are not suitable for calculation of mantle potential temperatures using currently available methods. This is caused by the presence of substantial pyroxenite and volatile-enriched peridotite mantle domains under the Canary Islands. A wider compositional calibration of various petrological models is necessary to precisely determine mantle potential temperatures for volatile-rich alkali basalts. Our high oxygen fugacity estimates for El Hierro magmas reflect the fertile, fusible, and volatile-enriched nature of the mantle source beneath the Western Canary Islands.

Genesis and tectonic magmatic evolution of Carboniferous granites in the Hongliuhecao-Yuejinshan area of Beishan orogenic belt

北山造山带位于中亚造山带南缘，区内广泛发育古亚洲洋-陆演化的岩浆记录，是研究北山造山带构造-岩浆-成矿作用及其动力学背景的重要窗口。红柳河槽-跃进山地区是北山造山带中北部的重要组成部分，区内与成矿有关花岗岩的形成时代、成因及构造背景尚不明确，由此制约了对北山造山带内成岩成矿动力学背景的探讨。本次在系统、详细的野外地质调查、岩石（相）学观察的基础上，结合岩浆岩地球化学、锆石U-Pb年代学和Lu-Hf同位素等证据，深入剖析了北山造山带内红柳河槽-跃进山地区的花岗岩形成时代、类型、源区和动力学背景，取得了以下认识：（1）通过锆石LA-ICP-MS U-Pb定年测得与成矿有关的狼娃山二长花岗岩、红柳河槽正长花岗岩、跃进山北正长花岗岩、跃进山二长花岗岩形成时代分别为327.1±2.4Ma、326.7±2.4Ma、321.3±2.4Ma和309.7±2.2Ma，表明红柳河槽-跃进山地区铜、钨、钼、铅、锌等元素成矿与石炭纪（327.1~309.7Ma）岩浆活动有关；（2）通过花岗岩地球化学研究表明，红柳河槽-跃进山地区的花岗岩均属于钙碱性-高钾钙碱性、准铝质-弱过铝质系列，强烈富集LREE、LILE，亏损HREE、HSFE，具有典型弧岩浆的地球化学特征，花岗岩成因类型均为I型；（3）本次获得狼娃山二长花岗岩和红柳河槽正长花岗岩的ε_Hf（t）值分别为5.90~9.92（平均值7.93）和5.87~10.28（平均值8.11），t_DM2分别为0.703~0.959Ga和0.93~1.03Ga，且具有较低的Nb/Ta比值（5.1~14.2，平均值8.5），与典型新生下地壳来源岩浆（8.3）相似，表明岩浆物质可能源于新生下地壳岩石部分熔融；（4）根据锆石Ti温度计查明北山造山带红柳河槽-跃进山地区狼娃山二长花岗岩、红柳河槽正长花岗岩、跃进山北正长花岗岩、跃进山二长花岗岩的平均结晶温度分别为765℃、765℃、816.4℃、771.5℃，锆石Ce^（Ⅳ）/Ce^（Ⅲ）平均值分别为85.27、108.4、103.6、135.7，表明从石炭纪早期到晚期，岩浆的氧逸度呈逐渐升高的趋势。根据以上研究，结合前人研究成果，初步建立了北山造山带构造-岩浆演化模型如下：石炭纪北山造山带北部红石山洋向南侧公婆泉-明水-旱山复合构造单元（弧）之下发生俯冲，诱发新生下地壳部分熔融形成的壳源岩浆；其侵入到地壳浅部，并最终形成红柳河槽-跃进山花岗岩体。;Beishan orogenic belt is located in the southern margin of the Central Asian Orogenic Belt. Magmatic records of paleo-Asian ocean-continent evolution widely existed in this area, and as a result, it is an important window to study the tectonic magmatic mineralization and dynamic background of Beishan orogenic belt. Hongliuhecao-Yuejinshan area is an important component of the middle-northern part of Beishan orogenic belt, however, the formation age, genesis and tectonic background of granites related to mineralization in the area are not clear, which seriously restricts the discussion of diagenetic and metallogenic dynamic backgrounds in Beishan orogenic belt. Based on systematic and detailed field geological survey and petrographic (facies) observation, combined with the evidence of magmatic rock geochemistry, zircon U-Pb chronology and Lu-Hf isotope, we analyzes the formation age, type, source area and dynamic background of granite in Hongliuhecao-Yuejinshan area in Beishan orogenic belt, from which the following conclusions can be obtained:(1) According to zircon LA-ICP-MS U-Pb dating, the formation ages of Langwashan monzogranite, Hongliuhecao syenogranite, Yuejinshanbei syenogranite and Yuejinshan monzogranite related to mineralization are 327.1±2.4Ma, 326.7±2.4Ma, 321.3±2.4Ma and 309.7±2.2Ma, indicating that the mineralization of copper, tungsten, molybdenum, lead, zinc and other elements is related to the Carboniferous magmatism of 327.1~309.7Ma. (2) The geochemical study of granites shows that the granites in Hongliuhecao-Yuejinshan area are strongly enriched in LREE and LILE, while deficient in HREE and HSFE, which demonstrated that they have the geochemical characteristics of typical arc magma and the I-type granites, and belong to calc alkaline, high-K calc alkaline, quasi aluminous-weak peraluminous series. (3) The ε_Hf(t) values of Langwashan monzogranite and Hongliuhecao syenogranite are 5.90~9.92 (average 7.93) and 5.87~10.28 (average 8.11), respectively; and the t_DM2 values are 0.703~0.959Ga and 0.93~1.03Ga, respectively; furthermore, they have a low Nb/Ta ratio (5.1~14.2, average 8.5), which is similar to the typical magma from the newborn lower crust (8.3). All these charateristics indicate that the magma material may have originated from the partial melting of the newborn lower crust rocks. (4) According to the zircon Ti thermometer, the average crystallization temperatures of Langwashan monzogranite, Hongliuhecao syenogranite, Yuejinshanbei syenogranite and Yuejinshan monzogranite in the studying area are 765℃, 765℃, 816.4℃ and 771.5℃, respectively; while the zircon Ce^(Ⅳ)/Ce^(Ⅲ)average values are 85.27, 108.4, 103.6, 135.7, respectively, indicating that the oxygen fugacity of magma increased gradually from the early to the late Carboniferous. Based on the above analyses, together with previous research results, the tectonic magmatic evolution model of Beishan orogenic belt is preliminarily established:the Hongshishan ocean in the north of the Beishan orogenic belt subducted under the Gongpoquan-Mingshui-Hanshan composite tectonic unit (arc) in Carboniferous, which induced partial melting of the newborn lower crust and formed the magma; this crustal magam intruded into the shallow crust, and finally formed the Hongliuhecao-Yuejinshan granite.

Geochronology and petrogenesis of the Shuizi pyroxenite at Danba, western Yangtze Craton

扬子克拉通西缘在~260Ma发生短期内大规模峨眉山玄武岩溢流喷发。攀西地区发育的镁铁-超镁铁质岩被广泛认为是峨眉山大火成岩省的产物，但在北端松潘-甘孜岩区一直缺乏该类岩石的报道。本文首次报道扬子西缘丹巴水子乡单斜辉石岩的准确年龄，其锆石LA-ICP-MS U-Pb加权平均年龄为260.7±3.3Ma，表明其为峨眉山大火成岩省北端松潘-甘孜岩区镁铁-超镁铁质岩的组成部分。通过与攀枝花钒钛磁铁矿含矿岩体边缘相带苦橄岩和上部相带浅色辉长岩进行锆石微量元素对比显示，水子乡单斜辉石岩具有相近的高氧逸度，其ΔQFM为0~3，Ce_N/Ce_N^*平均为~30，该性质可能同样源自扬子西缘洋壳板片俯冲交代形成的较高氧逸度地幔源区。尽管如此，水子乡辉石岩体并未因高氧逸度而有明显的含钛磁铁矿饱和结晶，可能由其较低结晶分异程度造成。相比之下，攀枝花岩体经历了更高程度的含钛磁铁矿和斜长石分离结晶作用，伴随大规模的钒钛磁铁矿成矿。;Large-scale eruption of Emeishan flood basalt occurred in the western margin of the Yangtze Craton in a short period during~260Ma. Mafic-ultramafic rocks found in the Panxi area are widely considered to be products of the Emeishan Large Igneous Province (ELIP). However, there is a lack of reports on this kind of rocks in the Songpan-Ganzi area in the northern end of the ELIP. This paper first documents a precise age of the Shuizi pyroxenite in the Danba County. The zircon LA-ICP-MS U-Pb weighted mean age is 260.7±3.3Ma, indicating a representative mafic-ultramafic constituent of the northern end of the ELIP in the Songpan-Ganzi area. Comparing zircon trace elements of the Shuizi pyroxenite with those of picrite in the marginal zone and gabbro in the upper zone of the Panzhihua Fe-Ti-V oxide deposit were conducted. It is found that the Shuizi pyroxenite has similarly high magmatic oxygen fugacity, ΔQFM of 0~3 and average Ce_N/Ce_N^* of~30, to the Panzhihua mafic rocks. The high oxygen fugacity for both is considered to be derived from a mantle lithosphere source which had been metasomatized by subducted oceanic slab materials in the western margin of the Yangtze Craton. Although the Shuizi pyroxenite has high oxygen fugacity, there is less obvious Fe-Ti oxide saturation and precipitation here. This could be attributed to its relatively low degree of crystallization and differentiation. In contrast, the Panzhihua mafic-ultramafic intrusion experienced higher degree of titanomagnetite and plagioclase fractional crystallization, accompanied by a massive Fe-Ti-V oxide mineralization.

Experimental calibration of an Fe3+/Fe2+-in-amphibole oxybarometer and its application to shallow magmatic processes at Shiveluch Volcano, Kamchatka

AbstractOxygen fugacity is an important but difficult parameter to constrain for primitive arc magmas. In this study, the partitioning behavior of Fe3+/Fe2+ between amphibole and glass synthesized in piston-cylinder and cold-seal apparatus experiments is developed as an oxybarometer, applicable to magmas ranging from basaltic to dacitic composition. The partitioning of Fe2+ is strongly dependent on melt polymerization; the relative compatibility of Fe2+ in amphibole decreases with increasing polymerization. The Fe2+/Mg distribution coefficient between amphibole and melt is a relatively constant value across all compositions and is, on average, 0.27. The amphibole oxybarometer is applied to amphibole in mafic enclaves, cumulates, and basaltic tephra erupted from Shiveluch volcano in Kamchatka with measured Fe3+/FeTotal. An average Fe3+/Fe2+ amphibole-glass distribution coefficient for basalt is used to convert the Fe3+/FeTotal of amphibole in samples from Shiveluch to magmatic oxygen fugacity relative to NNO. The fO2 of primitive melts at the volcano is approximately NNO+2 and is faithfully recorded in amphibole from an amphibole-rich cumulate and the basaltic tephra. Apparently, higher fO2 recorded by amphibole in mafic enclaves likely results from partial dehydrogenation of amphibole during residence in a shallow andesite storage region. We identify three pulses of mafic magma recharge within two weeks of, a month before, and two to three months before the eruption and find that, at each of these times, the host andesite was recharged by at least two magmas at varying stages of differentiation. Application of the amphibole oxybarometer not only gives insight into magmatic fO2 but also potentially details of shallow magmatic processes.

Increasing sulfur and chlorine contents in ore-forming magmas: The key to Pulang porphyry Cu-Au formation, SW China

The giant Pulang porphyry Cu-Au district (446.8 million tonnes at 0.52% Cu and 0.18 g/t Au) is located in the southern Yidun terrane of the Sanjiang (Three-river) region, southwest China. It has six phases of porphyry intrusions: premineralized fine-grained quartz diorite (FQD) and coarse-grained quartz diorite porphyry (CQD), the main synmineralized quartz monzonite porphyry (QMP), and late synmineralized granodiorite porphyry (GP), diorite porphyry (DP), and aplite. New zircon U-Pb ages for the premineralized FQD and late synmineralized GP, DP, and aplite are 226.3 ± 2.6 Ma (MSWD = 1.9), 216.4 ± 1.0 Ma (MSWD = 2.0), 216.0 ± 1.1 Ma (MSWD = 1.7), and 216.0 ± 1.5 Ma (MSWD = 1.7), respectively. All Pulang porphyry rocks have high whole-rock Sr/Y ratios (37.8–184) with minor or no negative Eu anomalies (0.73–1.04), and are characterized by high large-ion-lithophile-element (LILE: Th, U, Ba, Rb) abundances and low high-field-strength-element (HFSE: Nb, Ta, Ti, Zr, Hf).Zircons from premineralized FQD and synmineralized GP, DP, and aplite rocks have consistently high EuN/EuN* ratios (mostly > 0.4). The magmatic oxygen fugacity calculated from zircon compositions, show that the FQD (ΔFMQ = 1.59 ± 0.43), GP (ΔFMQ = 1.52 ± 0.18), DP (ΔFMQ = 1.52 ± 0.30), and aplite (ΔFMQ = 1.75 ± 0.60) were all derived from moderately oxidized magmas. Magmatic water contents estimated from amphibole compositions and the extensive presence of amphibole phenocrysts suggest that H2O contents are >4 wt%. Together with the published results for premineralized CQD and main synmineralized QMP, it is concluded the causative magmas for the six porphyry phases at Pulang were all relatively oxidized and hydrous.The difference between premineralized FQD and CQD and synmineralized QMP, GP, and DP are reflected in magmatic chlorine and sulfur contents. To minimize the effect of hydrothermal alteration, the apatite microphenocrysts enclosed by least altered phenocrysts were selected for analyses. The Cl contents of magmatic apatite microphenocrysts from the synmineralized QMP (avg = 0.340 ± 0.192, n = 26), GP (avg = 0.317 ± 0.151, n = 21), and DP (avg = 0.179 ± 0.112, n = 18) are significantly higher than those from the premineralized FQD (avg = 0.046 ± 0.032, n = 9) and CQD (avg = 0.043 ± 0.034, n = 13). Similarly, the SO3 contents of igneous apatite microphenocrysts from the synmineralized QMP (avg = 0.259 ± 0.156, n = 25), GP (avg = 0.203 ± 0.163, n = 20), and DP (avg = 0.306 ± 0.173, n = 18) are higher than those from the premineralized FQD (avg = 0.181 ± 0.136, n = 10) and CQD (avg = 0.085 ± 0.068, n = 6). The S-Cl contents of synmineralized melts estimated from apatite S-Cl compositions are higher than those of premineralized magmas, suggesting that the elevated magmatic S-Cl contents in the synmineralized porphyries might have been important for Cu-Au mineralization. This was likely due to S-Cl rich mafic magma recharge, supported by the presence of abundant mafic magmatic enclaves in the synmineralized QMP.

Controls on the formation of porphyry Mo deposits: Insights from porphyry (-skarn) Mo deposits in northeastern China

AbstractPorphyry Mo deposits have traditionally been classified into two major classes, arc-related and Climax-type, based on the tectonic setting and chemistry of associated intrusions. Although there is a consensus that porphyry Mo systems were formed by the optimal coincidence of geological processes operating at different scales, it is unclear what key parameter(s) render systems productive and whether the two classes of porphyry Mo deposits are unique in their mode of formation, or if they share fundamentally similar geological processes. These questions are important as a clearer understanding of the optimum conditions for the formation of porphyry Mo deposits is a prerequisite for more efficient exploration.This contribution presents a detailed assessment of the factors affecting the formation of porphyry Mo deposits through the investigation of barren and mineralized intrusions from the arc-related Songbei-Yangjiazhangzi-Lanjiagou (SYL) ore zone and the Climax-type Hashitu deposit of northeastern China. Our results show that the syn-mineralization intrusions from the SYL ore zone are quite evolved (SiO2 ~ 75 wt%; Na2O+K2O ~ 8.7 wt%) and are characterized by apparent light rare earth element (LREE) enrichments (LaN/YbN = 2.7–33.1) and moderate negative Eu anomalies (Eu/Eu* = 0.4–0.7). They show enriched zircon Hf isotopic compositions [εHf(t) = –11.9 to –4.8], indicating their parental magmas were likely derived from an ancient crustal source. Melt inclusions from the SYL syn-mineralization intrusions contain negligible F and Cl. In contrast, Hashitu syn-mineralization intrusions are characterized by weak LREE enrichments (LaN/YbN = 2.2–6.9) and strong negative Eu anomalies (Eu/Eu* = 0.02–0.10), with SiO2 and Na2O+K2O contents similar to the SYL syn-mineralization intrusions. They show depleted zircon Hf isotopic compositions [εHf(t) = 3.1–5.0], indicating their parental magmas were likely derived from a juvenile crustal source. Melt inclusions from the Hashitu syn-mineralization intrusions contain up to 0.4 wt% F and 0.03 to 0.09 wt% Cl. However, in both cases, the syn-mineralization intrusions are Mo-poor (1–7 ppm Mo), oxidized (above the quartz-fayalite-magnetite buffer), water-saturated (4.4–7.8 wt% H2O), and were emplaced at palaeodepths of 3.3 to 8.3 km. These data imply that magma source composition is not a key factor in the formation of porphyry Mo deposits. In contrast, magma oxygen fugacity, water content, and emplacement depth appear to play fundamental roles in the formation of porphyry Mo deposits of both arc-related and Climax-type.Within individual deposits, no systematic differences between pre- and syn-mineralization intrusions are observed in terms of magma source, fractionation degree, oxygen fugacity, emplacement depth, and volatile and Mo contents. Instead, a crucial apparent difference lies in the geometry of the intrusions, i.e., pre-mineralization intrusions generally occur as flat, ponded bodies, whereas syn-mineralization intrusions commonly develop as small stocks or dikes. Our results, in combination with an examination of other porphyry Mo systems, suggest that the sudden depressurization of magma chambers and subsequent venting of voluminous fluids along focusing structures (such as small stocks or dikes) most likely plays a critical role in the formation of porphyry Mo deposits of both arc-related and Climax-type. The findings of this study indicate that fluid processes in the shallow crust are pivotal for the formation of porphyry Mo deposits and that settings with ideal magmatic-hydrothermal architectures are most likely to develop into productive porphyry Mo systems.

Cu-Mo infertile granite: Insights from the late cretaceous plutons in the Northern Yidun Terrane, eastern Tibetan Plateau

The Yidun Cu-Mo polymetallic metallogenic belt, located in the eastern Tibetan Plateau, is well known for numerous Mesozoic porphyry Cu–Mo deposits. The Cu-Mo deposits are only concentrated in the southern part of the belt while the granites situated in the north are generally barren in Cu-Mo mineralization. To understand the cause of Cu and Mo infertility, geochronology and petro-mineral geochemistry of three Cu- and Mo-barren granite plutons (Cilincuo, Rongyicuo, and Hagela) in the Northern Yidun Terrane were investigated. Petrogeochemical characteristics and zircon U-Pb dating (95–85 Ma) along with the tectonic evolution history confirmed that the three plutons formed in a Late Cretaceous intracontinental setting. The whole-rock geochemical compositions showed that these plutons can be classified as A-type granites and have undergone extensive magmatic differentiation. Based on the calculated magmatic oxygen fugacity using zircon composition and a set of geochemical criteria including whole-rock Sr/Y ratios, Eu/Eu* and Fe3+/Fe2+ values as well as apatite Cl-H2O contents, the parental magma of the Cilincuo pluton was identified as more oxidized and H2O-Cl-rich than those of the Rongyicuo and Hagela plutons. These features enable the Cilincuo pluton to have an elevated capacity of Cu mineralization compared to the other two plutons. However, the causative magmas of the three plutons are still not enough hydrous and oxidized for typical porphyry Cu ± Mo mineralization. The whole-rock Sr-Nd isotope values of these three plutons are consistent with Late Triassic Cu and Mo barren granites in the same region such as Dongcuo, Cuojiaoma and Sucuoma, suggesting that they were derived from a similar magma source which might lack the oxidized and hydrous Neoproterozoic arc root remnant.