Log fO2 Research Articles

Factors controlling the formation of large porphyry Cu deposits: A case study from the Jiurui ore district of Middle-Lower Yangtze River Metallogenic Belt using in situ zircon and apatite chemistry from syn-mineralization intrusions

The geological processes leading to the formation of porphyry Cu deposits have been well studied and documented. However, the major factors controlling the scale of mineralization resulting in giant ore deposits, remain poorly understood. In this study, we evaluate the compositions of zircon and apatite from syn-mineralization intrusions associated with two porphyry Cu deposits (Chengmenshan and Xiangutai) in the Jiurui ore district of the Middle-Lower Yangtze River Metallogenic Belt, in an attempt to reveal the possible role of mantle-contribution to ore-forming materials in controlling metal endowment in porphyry systems. The Chengmenshan and Xiangutai deposits share similar characteristics including age of formation and type of host-rocks but with different Cu endowment. Chengmenshan (2.6 Mt at grade 0.8%) is classified as well-endowed whereas Xiangutai (280,000 t at grade 0.5%) is poorly-endowed. New laser ablation ICP-MS U-Pb ages of zircon from the Chengmenshan intrusions are between 144.8 ± 1.4 Ma and 147.3 ± 1.2 Ma. These ages are similar to the U-Pb ages for the Xiangutai intrusions (146.1 ± 1.3 Ma to 149.3 ± 1.1 Ma). Both Chengmenshan (average 699 °C) and Xiangutai (average 692 °C) intrusions have similar calculated Ti-in-zircon temperatures for magma crystallization and both are inferred to have crystallized from highly oxidized magmas on the basis of calculated values of Ce/Ce*, Eu/Eu* and Ce(IV)/Ce(III) in zircon and Mn in apatite. Water contents of magma are both high and similar based on zircon trace element ratios. However, the average εHf(t) value (−3.2) of zircon from the Chengmenshan intrusions is much higher than the Xiangutai (−13.2). Furthermore, the Chengmenshan apatites show lower 87Sr/86Sr ratios (0.70780 to 0.70909, average 0.70842) than those from the Xiangutai intrusion (0.70964 to 0.71061, average 0.71017). Intrusions associated with both deposits share a similar age and physio-chemical characteristics (e.g. log (fO2), T, water content) but have different magmatic sources. This suggests that the contribution of mantle material is a controlling factor in the formation of large porphyry Cu deposits in the Jiurui ore district and even in the whole metallogenic belt.

Deserpentinization in Subduction Zones as a Source of Oxidation in Arcs: a Reality Check

AbstractPrevious studies have concluded that dehydration of serpentinites in subduction zones produces oxidizing fluids that are the cause of oxidized arc magmas. Here, observations of natural samples and settings are combined with thermodynamic models to explore some of the factors that complicate interpretation of the observations that form the basis of this conclusion. These factors include the variability of serpentinite protoliths, the roles of carbon and sulfur in serpentinite evolution, variability in serpentinization in different tectonic settings, changes in the bulk compositions of ultramafic rocks during serpentinization, fundamental differences between serpentinization and deserpentinization, and the absence of precise geothermobarometers for ultramafic rocks. The capacity of serpentinite-derived fluids to oxidize sub-arc magma is also examined. These fluids can transport redox budget as carbon-, sulfur-, and iron-bearing species. Iron- and carbon-bearing species might be present in sufficient concentrations to transport redox budget deep within subduction zones, but are not viable transporters of redox budget at the temperatures of antigorite breakdown, which produces the largest proportion of fluid released by serpentinite dehydration. Sulfur-bearing species can carry significant redox budget, and calculations using the Deep Earth Water (DEW) model show that these species might be stable during antigorite breakdown. However, oxygen fugacities of ∼ΔFMQ + 3 (where FMQ refers to the fayalite–magnetite–quartz buffer, and ΔFMQ is log fO2 – log fO2, FMQ), which is close to, or above, the hematite–magnetite buffer at the conditions of interest, are required to stabilize oxidized sulfur-bearing species. Pseudosection calculations indicate that these conditions might be attained at the conditions of antigorite breakdown if the starting serpentinites are sufficiently oxidized, but further work is required to assess the variability of serpentinite protoliths, metamorphic pressures and temperatures, and to confirm the relative positions of the mineral buffers with relation to changes in fluid speciation.

Geochemistry of Fe-Ti oxide and sulfide minerals in gabbroic rocks and magnetitite of the Archean Mayurbhanj mafic complex (eastern India): Magma fractionation, thermometry and oxygen fugacity of re-equilibration, and implications for Ni-Cu mineralization

The Archean Mayurbhanj mafic complex in the Singhbhum Craton (eastern India) contains a magmatic Fe-Ti-(V) oxide ore deposit within a gabbro-norite-anorthosite suite of rocks. Immediate country rocks belong to the early Archean Gorumahisani greenstone belt and 3.1 Ga granitic complex. The gabbro-noritic rocks of the Mayurbhanj complex consist of plagioclase (An26−69), clinopyroxene (En42−54Fs4−36Wo22−42), and orthopyroxene (En67−81Fs15−28Wo4−5) as cumulus phases with Fe-Ti-oxides (magnetite and ilmenite) and sulfides (pyrite, pyrrhotite, chalcopyrite) as disseminated phases. Two-pyroxene thermometry yields temperatures of 1199 °C − 956 °C which are close to the primary liquidus temperature of pyroxene from a fractionated basaltic magma. The co-existing magnetite and ilmenite pairs in gabbro and magnetitite yield equilibrium temperatures of 498–797 °C and oxygen fugacities of 11.38–27.1 (log fO2) reflecting various stages of cooling and subsolidus re-equilibration of the mineral phases. Disseminated sulfides are mainly present along the grain boundaries of accessory Fe-Ti oxide grains, within interstitial spaces of silicate minerals, as inclusions within the silicate minerals and along altered fracture zones of the silicate host whereas the magnetitite bodies are devoid of such sulfide mineralization. In-situ trace element analysis of oxide and sulfide minerals from the gabbro-noritic rocks and magnetitite ores reveals a higher concentration of Cr (22301–67433 ppm) and Ni (896–3880 ppm) in the magnetite of gabbro than in the magnetitite bodies. This is due to early fractionation of the accessory magnetite in the gabbro from a Fe-rich basaltic magma. The lower concentration of V (30211–34608 ppm) in the accessory magnetite of the gabbro-noritic rocks than in the magnetitite (48534–64855 ppm) suggests partitioning of vanadium into clinopyroxene within the gabbroic rocks. Higher chalcophile element (Pb ≈ 2–11 ppm, Zn ≈ 6773–8139 ppm, Mo ≈ 25–68 ppm) concentrations in magnetite of the gabbro-noritic rocks indicate equilibration with co-magmatic sulfide mineral assemblages. The partitioning behavior of HFSE (Zr, Hf, Ta, Nb) in the magnetite and ilmenite is mainly dependent on the Ti4+ content of the minerals. The rim of iron-oxide minerals around sulfide grains and negative Ni anomaly in the trace element pattern of pyrite suggest that these pyrite grains have formed after pyrrhotite which is magmatic in origin. Enrichment of Pb (≈ 27–3120 ppm), Ag (≈ 8–15 ppm), Bi (≈ 0.2–9 ppm), As (≈ 48–6561 ppm), and Sb (≈ 0.4–18 ppm) in a few pyrites suggests hydrothermal alteration of primary magmatic pyrite grains.

An experimental investigation of F, Cl and H2O mineral-melt partitioning in a reduced, model lunar system

It is believed that the Moon formed following collision of a large planetesimal with the early Earth. Over the ∼4 Gyr since this event the Moon has been considerably less processed by geological activity than the Earth, and may provide a better record of processes and conditions in the early Earth-Moon system. There have been many studies of magmatic volatiles such as H, F, Cl, S and C in lunar materials. However, our ability to interpret variable volatile contents in the lunar sample suite is dependent on our understanding of volatile behaviour in lunar systems. This is currently constrained by limited experimental data. Here, we present the first experimental mineral-melt partitioning coefficients for F, Cl and H2O in a model lunar system under appropriately reduced conditions (log fO2 to IW-2.1, i.e. oxygen fugacity down to 2.1 log units below the Fe-FeO buffer). Data are consistent with structural incorporation of F, Cl and OH− in silicate melt, olivine and pyroxene under conditions of the lunar mantle. Oxygen fugacity has a limited effect on H2O speciation, and partitioning of H2O, F and Cl is instead largely dependent on mineral chemistry and melt structure. Partition coefficients are broadly consistent with a mantle source region for lunar volcanic products that is significantly depleted in F, Cl and H2O, and depleted in Cl relative to F and H2O, compared to the terrestrial mantle. Partitioning data are also used to model volatile redistribution during lunar magma ocean (LMO) crystallisation. The volatile content of lunar mantle cumulates is dependent upon proportion of trapped liquid during LMO solidification. However, differences in mineral-melt partitioning during LMO solidification can result in significant enrichment on F relative to Cl, and F relative to H2O, in cumulate phases relative to original LMO composition. As such, Cl depletion in lunar volcanic products may in part be a result of LMO solidification.

Mineral chemistry of hydrothermal alteration assemblage in hanging wall Shahapur granite associated with vein-type Gogi uranium deposit, Bhima Basin, Eastern Dharwar Craton, India: Implications for physico-chemical conditions of ore formation

Eastern Dharwar craton (EDC) is a major uranium province in India hosting number of unconformity related, vein-type and carbonate hosted strata-bound uranium deposits. Gogi uranium deposit (GUD) is a structurally controlled hydrothermal ore deposit (~1.35 Ga), hosted in a reverse fault zone between the foot-wall Mesoproterozoic Bhima Group carbonates and the hanging wall basement granites (~2.5 Ga) of the EDC. The hanging wall granites, locally known as Shahapur granite show extensive chloritisation and epidotisation, and constitute the propylitic outer zone of the main hydrothermal system of the GUD. We conducted systematic mineralogical and geochemical studies on hydrothermally altered assemblages to understand physico-chemical conditions of hydrothermal fluid. Quartz, epidote, chlorite, titanite, fluorite and hematite are the main hydrothermal mineral phases. Chlorites occur in two different forms, as pseudomorphic replacement of hornblende and as vein/cavity fillings. Epidotes occur as coarse prismatic grains in the main veins, or as replacement of albite in the wall rock. Mineralogical, textural data and geochemical mass balance calculation indicate that about 17.9% of total albites (Modal abundance = ~48%) and all the hornblendes (modal abundance = ~7.5%) of the parent rock were broken down and reacted to form quartz, chlorite, epidote, secondary titanite and hematite. The hydrothermal reaction resulted in creation of 13 vol% secondary porosity in the host rock and acidification of the evolved fluid. Thermodynamic calculation on chlorite compositions indicates ~ 250 °C temperature of formation and oxidising nature of ambient fluid, with log fO2 ranging between −37 and −31 (avg. −35). Available fluid inclusion data, in conjunction with chlorite geothermometric results, show fluid entrapment pressure between 625 bar and 792 bar, which is consistent with emplacement of hydrothermal fluid in low-pressure brittle deformation zone. We surmise that U6+ was liberated from basement granite by reaction with oxidised acidic fluid, transported as chloride, carbonate complexes and precipitated in high permeability fault-zone, possibly because of mixing with reduced- basin derived fluids.

Insights into salty metamorphic fluid evolution from scapolite in the Trans-North China Orogen: Implication for ore genesis

Scapolite is abundant in medium to high grade metamorphic rocks with evaporitic calc-silicate bulk compositions, and because it can be Cl− and SO42− rich, is able to provide a unique view into the evolution of deep brines during orogenic cycles. In the Zhongtiao Mountains, part of the Trans-North China Orogen (TNCO), scapolite occurs in two distinct mineralogical associations; scapolite-I in carbonate-bearing biotite schist and scapolite-II in amphibolites. Initially, metamorphic Cl-rich scapolite-I formed in the schist at low fluid-rock ratio and peak pressure (meionite content = 32–37%, Cl = 2.34–2.75 wt.%). This observation, together with high molar Cl/Br ratios (Cl/Br = 973–2236), implies a halite-bearing evaporitic protolith. During a second stage of scapolite formation in the calc-silicates, rims overgrew scapolite-I with distinct compositions (Me% = 31–32, Cl = 2.64–3.0 wt.%, Cl/Br = 408–892). The lower molar Cl/Br ratios in scapolite-I rims imply that the later fluid was distinctly different to the earlier metamorphic fluid, and may have evolved through a stage of halite saturation induced by phase separation. Halite saturation would drive Br enrichment in the remaining fluid because Cl is preferentially sequestered in halite. Also at this time, amphibolites were subjected to intense open system fluid-rock interactions resulting in formation of scapolite-II (Me% = 30–32, Cl = 2.74–2.97 wt.%, Cl/Br = 291–1426), similar to that of the scapolite-I. S6+/∑S ratios in scapolite obtained via synchrotron XANES indicate that equilibration between fluid and scapolite would buffer the fluid to oxidized conditions, with oxygen fugacity (log fO2) beyond −15. The scapolitic rocks in this region were thus able to play a key role in controlling the salinity and oxidation state of the deep metamorphic fluid, thereby creating enhanced potential for scavenging of a range of metals within the metamorphic terrane and promoting syn-orogenic hydrothermal ore formation.

Genesis of Fanshan lithocap, Luzong volcanic basin, Anhui Province, China – Indications from alunite and pyrite isotopes

Previous research in the Luzong volcanic basin in the middle-lower Yangtze River area, east China, mostly focused on iron oxide-apatite (IOA) deposits, but the area also hosts an extensive lithocap of previously unknown affinities. This study proposes a genetic model for the Fanshan lithocap and clarifies the relationship between the lithocap and the IOA deposits, based on stable isotope analyses.There are four types of alunite in the Fanshan lithocap: granular alunite with APS cores, bladed alunite in vugs, needle alunite co-existing with pyrite, and very fine-grained powdery alunite co-existing with kaolinite and jarosite. We determined the S isotopic compositions of the different types of alunite: bladed and needle alunite produced results (20.30–22.29‰ and 18.30–22.98‰, respectively) indicating a magmatic-hydrothermal origin, whereas powdery alunite produced lower isotopic compositions (−4.38 to −4.48‰), consistent with the associated pyrite (−8.06‰), and indicating a supergene origin.The δ34S values of the needle alunite and of co-existing pyrite in the quartz–alunite–pyrite alteration zone reflect isotopic equilibrium between sulfate and sulfide at 180–220 °C. The pH increased from <2 to about 5, and the log fO2 decreased from −33 to −46 from proximal to distal alteration zones (quartz–alunite–pyrite to quartz–kaolinite–dickite to quartz–illite+/−smectite).The O and H isotopes of the magmatic-hydrothermal alunite (δ18O 14.2–19.9‰ and δD −74.4 to −67.0‰) indicate that the hydrothermal fluid during the wall rock alteration was dominantly magmatic in origin (δ18O 3.34–9.04‰ and δD −73.28 to −80.68‰). The H2S/SO4 of the bulk hydrothermal fluid during acid alteration was ~3 due to the deposition of alunite, similar to that of high-sulfidation epithermal deposits worldwide.The δ34SΣS of the hydrothermal fluid that produced the Fanshan lithocap was 0.12‰, suggesting the sulfur was probably derived from the mantle; that is significantly different from results obtained from previous work on the IOA deposits in the Luzong basin, which indicated that the sulfur was produced by mixing of a magmatic-hydrothermal fluid with sulfur from Triassic marine sedimentary rocks.Based on our isotopic data we conclude that the Fanshan lithocap is not related to the IOA deposits and its fluids were not derived from the diorite porphyrites that are related to IOA deposits in the Luzong basin. We infer that the intrusion that supplied magmatic-hydrothermal fluids to the lithocap is probably at depth below the lithocap, and may have been similar to intrusions that in many parts of the world are responsible for developing copper–gold porphyry deposits below lithocaps similar to the Luzong lithocap.

Implications of aplite dykes for mineralization in the Late Cretaceous vein-type Xiuwacu W-Mo deposit in the southern Yidun Terrane, SE Tibetan Plateau

Abstract The Xiuwacu W-Mo deposit with 24,000 t of Mo and 23,000 t of WO3, is located in the Southern Yidun terrane (SYT), SW China; being the only quartz-vein type W-Mo deposit in the region. In the deposit, aplite dykes are spatiotemporally associated with W-Mo orebodies; however, their genetic relationship remains unclear. Here we present the result of a study of the petrography, zircon U-Pb dating, whole-rock geochemistry, Sr-Nd-Pb isotopes, zircon trace elements and Hf isotopes of the aplite dykes. The Xiuwacu aplites are emplaced at 78.7 ± 1.9 Ma and have high-silica, alkali-rich and metaluminous -peraluminous compositions. They are enriched in light rare elements (LREEs), large-ion lithophile elements (LILEs; U, Rb, Th), and depleted in heavy rare earth elements (HREEs), high-field strength elements (HFSEs; Nb, P, Ti), and with strong negative Eu anomalies (δEu = 0.09–0.58, average = 0.36). The aplites have an average estimated crystallization temperature of 700 °C, and relatively low magmatic oxygen fugacity ([log(fo2)] = −33.2 to −16.1 average = −26.1) which is substantially lower than the Late Triassic biotite granite but similar to the Late Cretaceous porphyritic monzogranite in the deposit. All the aplite samples have varying (87Sr/86Sr)i ratios (0.701240–0.719258), negative eNd(t) values (−10.33 to −3.22), negative eHf(t) values (−5.3 to −3.3), and ancient two-stage Hf depleted-mantle model ages (TDM2(Hf) = 1886–2070 Ma). These geochemical characteristics demonstrate that the aplite dykes were formed by the partial melting of the ancient lower crust under an extensional setting, which was triggered by the NW-SE ward subduction of the neo-Tethys ocean slab during the Late Cretaceous. The ancient lower crust is interpreted to have provided the main source of W and Mo for the Xiuwacu deposit. The enrichment of W and Mo by fractional crystallization of the aplite magmas played an important role in concentrating W-Mo during the formation of the deposit. Aplite dykes have also been found in several Late Cretaceous Mo and/or Cu deposits in the SYT. Considering their common occurrence within the orebodies, the aplite dykes could be an effective prospecting indicator.

Can volcanism build hydrogen-rich early atmospheres?

Hydrogen in rocky planet atmospheres has been invoked in arguments for extending the habitable zone via N2-H2 and CO2-H2 greenhouse warming, and providing atmospheric conditions suitable for efficient production of prebiotic molecules. On Earth and Super-Earth-sized bodies, where hydrogen-rich primordial envelopes are quickly lost to space, volcanic outgassing can act as a hydrogen source, provided it balances the hydrogen loss rate from the top of the atmosphere. Here, we show that both Earth-like and Mars-like planets can sustain atmospheric H2 fractions of several percent across relevant magmatic fO2 ranges. In general this requires hydrogen escape to operate somewhat less efficiently than the diffusion limit. We use a thermodynamical model of magma degassing to determine which combinations of magma oxidation, volcanic flux and hydrogen escape efficiency can build up appreciable levels of hydrogen in a planet's secondary atmosphere. On a planet similar to the Archean Earth and with a similar magmatic fO2, we suggest that the mixing ratio of atmospheric H2 could have been in the range 0.2-3%, from a parameter sweep over a variety of plausible surface pressures, volcanic fluxes, and H2 escape rates. A planet erupting magmas around the Iron-Wüstite (IW) buffer (i.e., ∼3 log fO2 units lower than the inferred Archean mantle fO2), but with otherwise similar volcanic fluxes and H2 loss rates to early Earth, could sustain an atmosphere with approximately 10-20% H2. For an early Mars-like planet with magmas around IW, but a lower range of surface pressures and volcanic fluxes compared to Earth, an atmospheric H2 mixing ratio of ∼2-8% is possible. On early Mars, this H2 mixing ratio could be sufficient to deglaciate the planet. However, the sensitivity of these results to primary magmatic water contents and volcanic fluxes show the need for improved constraints on the crustal recycling efficiency and mantle water contents of early Mars.

Petrogenesis and geodynamic setting of granitoids at Machangqing Cu–Mo (Au) deposit, western Yangtze craton, southwestern China: constraints from zircon U–Pb and molybdenite Re–Os geochronology, Lu–Hf isotopes, and geochemistry

The Machangqing Cu–Mo (Au) deposit is located in the central part of the Jinshajiang – Red River belt in the Sanjiang orogen, which lies across the Qiangtang terrane and western Yangtze craton, southwestern China. Zircon U–Pb dating constrains that the granite porphyry and porphyritic granite emplacements occurred at 35.92 ± 0.31 Ma and 34.92 ± 0.31 Ma, respectively. The Re–Os model ages of molybdenite are 34.94 ± 0.38 Ma. The new ages presented here, along with previously published data in the region, define a short duration of potassic magmatism and mineralization from 37 Ma to 34 Ma in the Jinshajiang – Red River belt. Zircon Ce4+/Ce3+ values of the porphyritic granite and granite porphyry vary from 50.32 to 1579.20 (averaging 481.01) and 33.18 to 1511.80 (averaging 452.98), respectively, and the log(fo2) values vary from –6.66 to −23.86 and −9.88 to −25.18, respectively, which plot within the range of the fayalite–magnetite–quartz buffer curve to the magnetite–hematite buffer curve, indicating an oxidized magma source, which may have facilitated the Cu–Au enrichment. Zircons from granitoids show εHf(t) values ranging from −0.75 to +2.33 and crustal model ages between 0.9 and 1.1 Ga. The features of Lu–Hf isotopes and wide range of Mg#, Cr, and Ni contents imply that the magmas of the Machangqing granitoids were probably derived from partial melting of juvenile lower crust and mixed with some mantle melts. Combined with the features of the Machangqing granitoids, the following evolution process are concluded. During the Cenozoic, the India–Asia continental collision triggered upwelling of hot asthenosphere and underplating of the thickened juvenile lower crust, which caused the formation of mafic and felsic magmas. Those magmas ascended, mixed, crystallized, and formed Machangqing ore-bearing granitoids in an intracontinental extension setting.

Petrogenesis of the late Paleoproterozoic Luyashan igneous charnockite-enderbite suite, North China Craton and its comparison with metamorphic counterparts

Orthopyroxene-bearing granitoids of both magmatic and metamorphic origin occur in various terranes of the world, although their petrogenetic distinction remains equivocal. Here we investigate an example of unmetamorphosed magmatic charnockites (enderbite-charnockite) of the Late Paleoproterozoic Luyashan pluton in the North China Craton, where both charnockitic (granitic) and enderbitic (granodioritic) granitoids occurs in the pluton. In a thin section scale, charnockitic domains (~1–2 cm) are present within the outcrop scale enderbitic granitoid, that are composed of orthopyroxene, biotite, plagioclase, K-feldspar, quartz, ilmenite, magnetite, and pyrrhotite. Whereas the enderbite domain shows amphibole, clinopyroxene and apatite additionally. Mineral textures and orthopyroxene inclusions in amphibole and biotite preserved early stage orthopyroxene from both domains, where they carry pure endmember ilmenite and magnetite as the major oxide phases with similar compositions. Log (aH2O) - Temperature pseudosection modeling at log fO2 = −15 and pressure 800 MPa shows that orthopyroxene might be crystallized at ~900 °C. Subsequently, the mixed system achieved a higher water activity due to CO2 escape during cooling. During cooling, both the enderbitic and charnockitic domain are stable at a water content of >1 wt% or water activity (aH2O) of 0.4 or above. Our results suggest that mixing of immiscible granodioritic and granitic magma caused the formation of the enderbitic and charnockitic domains at similar water activity. Orthopyroxene inclusions in amphibole and biotite, lack of dehydration and oxidation textures, and absence of pyrite and apatite with monazite precipitates in the Luyashan igneous charnockites suggest that these rocks are significantly different from their metamorphic counterparts. Our study suggests that orthopyroxene crystallization in igneous charnockites is mainly dictated by the temperature and pressure at which magma starts crystallizing, rather than water activity. However, in the case of metamorphic charnockites, in the absence of partial melting, metasomatic orthopyroxene formation depends on low water activity of fluids and their dissolution capacity.

Fluid inclusion and isotope (O, H, C, Sr) constraints on the orogenic gold mineralization at the Enche Concha and Tunel prospects, Gurupi Belt, Brazil

The gold mineralization at the Enche Concha and Tunel prospects, northwestern portion of the Gurupi Belt, is hosted in dacites and phyllites that belong to the ca. 2160 Ma-old metavolcano-sedimentary Chega Tudo Formation. These host rocks are variably deformed, from a spaced cleavage to schistose and mylonitic varieties. The hosting structures have previously been defined as produced in the Paleoproterozoic. The hydrothermal alteration processes include carbonatization, sericitization, sulfidation, and quartz veining. These processes are both pervasive and fissure-filling in style. Pyrite is largely predominant over chalcopyrite and sphalerite and gold occurs as a refractory phase in pyrite. Fluid inclusion, stable (O, H, C) and Sr isotopes studies indicate that the hydrothermal alteration and gold mineralization occurred between 260 and 370°C (mostly ~328°C), and 0.85 to 2.8 kbar, from a low-salinity (~5% NaCl), aqueous-carbonic (XCO2 = 0.03-0.13), relatively reduced (log fO2 = -32.1), and near neutral (pH = 5.0-6.2) metamorphic fluid (d18O = +9.4 to +11.3‰, dD = -25 to -28‰). Contributions from deep-seated (lower crust and mantle) sources are suggested by a d13CCO2 value of -7.6‰, and 87Sr/86Sr ratios between 0.702699 and 0.705141, which are lower than those found in the country rocks. Gold was transported by the Au(HS)2- complex and precipitation occurred in response to phase separation, identified by fluid inclusions, and fluid-rock interactions, given by the lowering of the d13CCO2 values. The geological and genetic characteristics are equivalent to those of the orogenic gold system previously defined for the Gurupi Belt.

Primary magmatic biotite in granitoid intrusions associated with Late Mesozoic Jinchanggouliang-Erdaogou lode gold deposits, North China Craton: Linkage to gold mineralization

The Late Mesozoic Jinchanggouliang and Erdaogou lode gold deposits in the central segment of the northern North China Craton are mainly hosted in Late Archaean metamorphic rocks of the Jianping Formation and Jurassic volcanic-sedimentary rocks. These deposits are spatially and temporally associated with a series of I-type granitoid intrusions: i.e., the 128–136 Ma Duimiangou granodiorite complex with a massive margin and a porphyritic center at Jinchanggouliang, the 161.4 ± 1.1 Ma Loushang quartz diorite intrusion and numerous syenite porphyry dykes (160.9 ± 1.0 Ma) at Erdaogou, and the ~ 227–217 Ma Xitaizi monzogranite batholith. Compositions of primary magmatic biotite in these granitoid intrusions have been used to investigate the nature of granitoid magmas and the evolution of magmatic processes and to identify possible linkages between gold mineralization and granitoid magmatism. All primary magmatic biotite grains are Mg-rich in composition (XMg = Mg/(Mg + Fe) = 0.41–0.69). The Cl contents of biotite (0.09–0.42 wt%) in the Loushang quartz diorite and the Duimiangou massive granodiorite correlate negatively with XMg and are consistent with the “Mg-Cl avoidance” rule. The Cl contents of biotite (0.01–0.14 wt%) in the Duimiangou porphyritic granodiorite, syenite porphyry and Xitaizi monzogranite, on the other hand, remain relatively constant and may be affected by multiple variables such as Cl-rich fluids and subtle changes in temperature. Halogen fugacity of the parental magma calculated from biotite shows that the Duimiangou granodiorite complex at Jinchanggouliang and syenite porphyry dykes at Erdaogou have the log(fHF/fHCl) ranges of −0.71 to −0.27 and −2.16 to −1.08, respectively, and have similar log(fH2O)/(fHCl) values from 3.53 to 4.23. Oxygen fugacities (logfO2) calculated from biotite equilibria for the Duimiangou granodiorite complex and syenite porphyry dykes are −13.7 to −7.2, straddling on the nickel-nickel oxide and hematite-magnetite buffers. The Xitaizi monzogranite and the Loushang quartz diorite have lower log(fO2) values from −14.9 to −12.4, between the nickel-nickel oxide and hematite-magnetite buffers. The Duimiangou granodiorite complex and syenite porphyry dykes with high oxygen fugacities are similar to oxidized magmas associated with porphyry Cu-Mo-Au deposits. These results suggest that multiple episodes of I-type granitoid magmas in the study area were rich in Cl and high in oxygen fugacity, especially those associated with the Duimiangou granodiorite complex and syenite porphyry dykes. The I-type melts represented by the Duimiangou granodiorite complex and syenite porphyry dykes as well as their related Cl-rich, oxidizing fluids were likely efficient in dissolving, transporting and precipitating gold for the formation of the Jinchanggouliang and Erdaogou lode gold deposits.

Molybdenite Re–Os dating, petrology, and geochemistry of granitoids in the Bondar Hanza porphyry Cu deposit (Urumieh‐Dokhtar magmatic arc), Iran: Insight into petrogenesis, mineralization, and tectonic setting

The Bondar Hanza porphyry Cu deposit, hosted by a granitoid stock, is located 120 km south of Kerman city in the elongated NW–SE trending Urumieh‐Dokhtar magmatic arc (UDMA), Iran. The granitoid stock is a multiphase intrusive body, 2 km2 in surface area, which comprises microdiorite–microquartz diorite and granite–granodiorite. Petrological and geochemical analyses show that granitoids are peraluminous, magnesian, calcic to alkali‐calcic, and non‐adakitic intrusions characterized by negative Eu* anomalies. Thermobarometric and oxygen fugacity calculations indicate that magma emplacement occurred at 719–784°C, &lt;100 MPa, and at a log fO2 of NNO +1.6 to +3.2. Sulphur isotopic composition of sulphide minerals (δ34S = 5.0–6.8 ‰) are similar to other porphyry Cu deposits worldwide. Molybdenite separates yield Re–Os ages of 28.22–28.03 Ma, indicating that the Cu mineralization and associated magmatism occurred during the Oligocene. The Bondar Hanza is one of the several granitoids from small porphyry Cu deposits in the UDMA that is related to island arc sub‐productive granitoids. It seems that the largest ore‐hosting porphyry systems within the UDMA are generally restricted to Miocene intrusions with adakitic affinity (e.g., Sarcheshmeh granitoids) and that the temporally discrete non‐adakitic magmatic systems are sub‐productive to barren. These data are interpreted to show that magmatism in the Bondar Hanza region was likely associated with partial melting of juvenile lower crust induced by north‐eastward subduction of the Neo‐Tethys oceanic lithosphere.

The speciation of carbon, nitrogen, and water in magma oceans and its effect on volatile partitioning between major reservoirs of the Solar System rocky bodies

The composition of atmospheres and the resulting potential for planetary habitability in the rocky bodies of our Solar System and beyond is strongly controlled by the volatile exchange between their silicate reservoirs and exospheres. The initial budget and speciation of major volatiles, like carbon (C), nitrogen (N) and water (H2O), in the silicate reservoirs and atmospheres was set during the formation stages of rocky bodies. However, the speciation of these major volatiles in reduced silicate melts prevalent during the differentiation stages of rocky bodies and its effect on the partitioning of volatiles between major rocky body reservoirs is poorly known. Here we present SIMS and vibrational spectroscopy (FTIR and Raman) data, determining C solubility, H content, and speciation of mixed C-O-N-H volatiles in graphite saturated silicate glasses from high P (1–7 GPa)-T (1500–2200 °C) experiments reported in Grewal et al. (2019b, 2019a). The experiments recorded oxygen fugacity (log fO2) between IW–4.3 and IW–0.8. C-O-N-H speciation varied systematically as function of fO2 at any given P-T. We find out that C-N−, CO32-, N2, and OH− are the dominant species in the oxidized range (>IW–1.5), along with some contributions from C-H, N-H, and C-O bearing species. Between IW–3.0 and IW–1.5, C is bonded as C-O either in the form of isolated C-O molecules or Fe-carbonyl complexes, or as C-H in hydrocarbons, or as combination of both in esters, while almost all of the H is bonded with the dominant N species, i.e., NH2− or NH2-. At the most reduced conditions (<IW–3.0), C is present mostly in the form of C-H bearing species, while anhydrous N3− followed by N-H bearing molecules are the dominant N bearing species. Magma oceans (MOs) in highly reduced bodies like Mercury would contain most of their C as graphite if MO is carbon saturated and the dissolved C and N would be chemically bonded with the silicate network either in the form of anhydrous C4− and N3−, or hydrogenated C-H and N-H bearing species depending on H content of the silicate melts. MOs relevant for Mars, the Moon, Vesta, and angrite parent body would contain C and N mostly in the form of C-O and N-H bearing species, respectively. If the composition of Earth’s accreting material evolved from reduced to oxidized, then initially a significant amount of the C and N budget would be locked in the silicate reservoirs, which would subsequently be released to the proto-atmosphere(s) at later stages. The retention of proto-atmosphere(s) formed by MO degassing on Earth could have provided important precursors for prebiotic chemistry which possibly led to the eventual habitability of our planet. Additionally, based on the dominant speciation of N versus C in silicate melt as a function of fO2, we also predict that DNalloy/silicate is unaffected by fH2 under highly reduced conditions (<IW–3), while DCalloy/silicate is affected. Therefore, caution must be taken during the application of experimentally determined DNalloy/silicate and DCalloy/silicate to nominally anhydrous MOs.

Diffusion and partition coefficients of minor and trace elements in magnetite as a function of oxygen fugacity at 1150\xa0\xbaC

Lattice diffusion coefficients and partition coefficients have been determined for Li, Mg, Al, Sc, Ti, Cr, V, Mn, Co, Ni, Cu, Zn, Ga, Y, Zr, Nb, Mo, In, Lu, Hf, Ta and U in single crystals of natural magnetite as a function of oxygen fugacity (fO2) at 1150 °C and 1 bar by equilibration with a synthetic silicate melt reservoir. Most experiments were run for twelve hours, which was sufficient to generate diffusion profiles from 25 to > 1000 µm in length. The results were checked at one condition with two additional experiments at 66.9 and 161 h. The profiles were analysed using scanning laser-ablation inductively-coupled-plasma mass-spectrometry. Diffusion coefficients (D) were calculated by fitting data from individual element diffusion profiles to the conventional diffusion equation for one-dimensional diffusion into a semi−infinite slab with constant composition maintained in the melt at the interface. Equilibrium magnetite/melt partition coefficients are given by the ratio of the interface concentrations to those in the melt. Plots of log D as a function of log fO2 produce V-shaped trends for all the investigated elements, representing two different mechanisms of diffusion that depend on (fO2)−2/3 and (fO2)2/3. Diffusion coefficients at a given fO2 generally increase in the order: Cr < Mo ≈ Ta < V < Ti < Al < Hf ≈ Nb < Sc ≈ Zr ≈ Ga < In < Lu ≈ Y < Ni < U ≈ Zn < Mn ≈ Mg < Co < Li < Cu. Thus, Cu contents of magnetites are most susceptible to diffusive reequilibration, whereas the original content of Cr should be best preserved.

Contribution of magma mixing to the formation of porphyry-skarn mineralization in a post-collisional setting: The Machangqing Cu-Mo-(Au) deposit, Sanjiang tectonic belt, SW China

The Machangqing Cu-Mo-(Au) deposit in the central of Jinshajiang–Red River fold belt is a Cenozoic porphyry-skarn deposit in the western Yangtze Craton, southwestern China. Igneous rocks at Machangqing mainly consist of mafic and felsic rocks, including mafic dykes and barren syenite porphyry, as well as mineralized monzonite and granite porphyries with mafic microgranular enclaves (MMEs) in the ore-bearing porphyries. In this study, we compiled new and published geochemical and isotopic data for these intrusions, their MMEs and ore sulfides from the Machangqing deposit. Both mineralized and barren felsic porphyries, MMEs and mafic dyke were formed during the late Eocene (33–37 Ma), and have shoshonitic affinities (K2O + Na2O > 6 wt%, K2O/Na2O > 1 and Sr/Y > 40) ratios. They are enriched in LILE and LREE, but depleted in HFSE. Both mineralized and barren felsic porphyries are characterized by high SiO2 (64.67–72.81 wt%) and Al2O3 (13.43–16.10 wt%), but low Cr (average 37.63 ppm) and Ni (average 24.64 ppm) contents and low Mg# values (11–66). They have low whole–rock εNd(t) (–6.5 to –3.3) and relatively high (87Sr/86Sr)i (0.7061–0.7076), which fall inside the range of coeval mafic rocks in western Yunnan. The geochronological, geochemical and isotopic evidence suggest that the felsic magmas were formed by partial melting of the juvenile lower crust that had been injected by shoshonitic mafic magma as documented by the MMEs. The mafic dykes have high Mg# (71–76), Cr (average 492 ppm), Ni (average 195 ppm) contents, and were likely derived from partial melting of upper mantle sources metasomatized by paleo-subduction processes. The MMEs at Machangqing were produced by the mingling of metasomatized mantle-derived mafic melts with the juvenile Neoproterozoic thickened crust-derived felsic melts. The MMEs have spheroidal shapes and contain acicular apatite, indicating that the MMEs represent globules of coeval mafic melt injected into and partly assimilated by the felsic magmas. The ore sulfides have similar trace elemental and Pb isotopic features (206Pb/204Pb = 18.562–18.659; 207Pb/204Pb = 15.628–15.664; 208Pb/204Pb = 38.894–38.986) to the coeval mafic dykes, and their S-isotopic compositions (-2.24 to + 3.5‰) resemble typical mantle-source sulfides, suggesting a metallogenetic link with the coeval mafic magmas/melts and MMEs. Magmas of both the ore-bearing and barren porphyries are relatively oxidized in the FMQ to MH range (zircon Ce4+/Ce3+ = 87–950, log(fO2) = –22 to −5), although the ore-bearing ones are more oxidized. Petrological modelling suggests that the mineralized porphyries are derived from magma mixing dominated by partial melts of metasomatically-enriched mafic mantle, while the barren porphyries are products of magma mixing dominated by a juvenile lower crust component. We propose that magma mixing between lower crustal material with injected mafic mantle-derived magma has provided sufficient metals, sulfur and water to form the Machangqing Cu-Mo-(Au) deposit. The hydrous mafic magmas may have also remobilized the residual sulfides remaining in the continental lithospheric mantle (CLM), thus adding to the metal budget of these fertile magmas.

Formation of primitive achondrites by partial melting of alkali-undepleted planetesimals in the inner solar system

Acapulcoites-lodranites, ureilites, brachinites, brachinite-like achondrites and winonaites are the main groups of primitive achondrites. They are variably depleted in incompatible lithophile elements (Al, Na, K and rare earth elements) and siderophile/chalcophile elements relative to chondrites and are interpreted as the residual mantle of planetesimals from which silicate melts and sulfide/metal melts were extracted. We use a series of melting experiments conducted with various chondritic compositions (CV, CM, CI, H and LL) to constrain the oxygen fugacity (fO2), the temperature, extent of melting and the initial bulk composition of the parent bodies of primitive achondrites. They melted at different and variable fO2: ΔIW −0.5/−1.0 for brachinites, ΔIW −1.3/−2.5 for ureilites, ΔIW −1.6/−2.7 for acapulcoites/lodranites and ΔIW −2.5/−3.0 for winonaites (with ΔIW = log fO2 – (log fO2)IW; IW being the iron-wustite buffer). Those main groups of primitive achondrites, which have nucleosynthetic anomalies characteristic of the “non-carbonaceous” reservoir and the inner solar system, were not initially depleted in Na2O and K2O relative to the sun’s photosphere. This suggests, in accordance with the enrichment in the heavy isotopes of Zn, Rb and K in eucrites, that the depletion of moderately volatile elements in planetesimals that melted to a larger extent (e.g. Vesta, the angrite parent body) resulted from evaporative losses during partial melting. The depletion of moderately volatile elements in terrestrial planets is likely inherited from partial melting and differentiation of small planetary bodies rather than from the incomplete condensation of the solar nebula.

Widespread production of silica- and alkali-rich melts at the onset of planetesimal melting

We present the results of melting experiments on a suite of carbonaceous and ordinary chondritic compositions (CV, CM, CI, H and LL) performed at low pressure (0.1–13.1 MPa) and over a range of oxygen fugacity (log fO2 − (log fO2)IW = −2.5 to −1 and +0.8, IW being the iron-wustite buffer). These experiments constrain the composition of partial melts (F = 5–25 wt.%) of chondritic planetesimals. Most experiments (IW −2.5 to −1) were conducted in Molybdenum-Hafnium Carbide pressure vessels, which prevented the loss of alkali elements from the melt. The results show that all planetesimals not significantly depleted in moderately volatile elements relative to the sun’s photosphere (e.g. CI, H and LL compositions) produced low-degree melts (<15 wt.%) rich in SiO2, Al2O3 and alkali elements, regardless of the fO2. Despite their high apparent viscosities (104-5 Pa.s), such low-density partial melts (2400–2500 kg/m3) were mobilized and, upon crystallizing, formed rocks containing up to 80 vol.% of plagioclase An10-30 (i.e. oligoclase) such as the trachyandesite achondrites Graves Nunataks 06128/9, Northwest Africa 6698 and 11575, the Almahata Sitta clast ALM-A, as well as smaller “albitic clasts” in polymict ureilites and “alkali-silica-rich” inclusions in non-magmatic iron meteorites. We suggest that silica- and alkali- rich melts were widespread in small bodies of the early solar system but that much evidence was erased by subsequent stages of melting and planetary accretion and differentiation.

New Magmatic Oxybarometer Using Trace Elements in Zircon

Abstract We derive a novel method for determining the oxidation state of a magma as zircon crystallized, with a standard error of ±0·6 log unit ƒO2, using ratios of Ce, U, and Ti in zircon, without explicit determination of the ionic charge of any of them, and without independent determination of crystallization temperature or pressure or parental melt composition. It yields results in good agreement with oxybarometry on Fe–Ti oxide phenocrysts and hornblende phenocrysts quenched in eruptive I- and A-type dacites and rhyolites, but our zircon oxybarometer is also applicable to slowly cooled plutonic rocks and applicable to detrital and xenocrystic zircons. Zircon/melt partition coefficients of Ce and U vary oppositely with ƒO2 variation in the silicate melt. The Ce/U ratio in zircon also varies with the Ce/U element ratio in the silicate melt. During mafic-to-felsic magmatic differentiation, Ce and U are incorporated mainly in calcium-dominated lattice sites of clinopyroxene, hornblende, apatite, and occasionally titanite and/or allanite, all of which have a similar degree of preference for Ce over U. We employ the U/Ti ratio in zircon and in silicate melt as a magmatic differentiation index. Convergent- and divergent-plate-margin differentiation series consistently follow the relation log (Ce/U) ≈ –0·5 log (U/Ti) + C' in silicate melts of basaltic to rhyolitic composition. That correlation permits thermodynamic derivation of the oxybarometry relation among those elements in zircon: log fO2(sample)−log fO2(FMQ)≈42n+1log[Ce/(Ui×Ti)z]+C, wherein Ui denotes age-corrected initial U content, FMQ represents the reference buffer fayalite + magnetite + quartz, superscript z denotes zircon, and n varies with the average valence of uranium in the zircon’s parental silicate melt. We empirically calibrate this relation, using 1042 analysed zircons in 85 natural populations having independently constrained log ƒO2 in the range FMQ – 4·9 to FMQ + 2·9, to obtain the equation log fO2(sample)−log fO2(FMQ)=3·998(±0·124) log[Ce/(Ui×Ti)z]+2·284(±0·101) with a correlation coefficient R = 0·963 and standard error of 0·6 log unit ƒO2 in calc-alkalic, tholeiitic, adakitic, and shoshonitic, metaluminous to mildly peraluminous and mildly peralkaline melts in the composition range from kimberlite to rhyolite. Thermodynamic assessment and empirical tests indicate that our formulation is insensitive to varying crystallization temperature and pressure at lithospheric conditions. We present a revised equation for Ti-in-zircon thermometry that accounts appropriately for pressure as well as reduced activity of TiO2 and SiO2 in rutile- and quartz-undersaturated melts. It can be used to retrieve absolute values of ƒO2 from values of ΔFMQ obtained from a zircon analysis.