Search

The textures and compositions of hydrothermal apatite grains have been reported in many iron oxide-apatite (IOA), porphyry, and rare-earth elements (REE) polymetallic deposits. However, such information for apatite in hydrothermal gold deposit is not available. In this study, we present integrated textural, elemental, and in situ oxygen isotopic data of hydrothermal apatite grains from the Hatu gold deposit (56t Au, average grade: 5 g/t) in the West Junggar, Northwest China, to investigate the detailed mineralization processes and nature of ore-forming fluids. Hydrothermal apatite is present within the quartz-sulfide ores at the Hatu gold deposit. Apatite grains of the Hatu gold ores contain abundant fluid inclusions and fine sulfide crystals, indicating that they are closely related to gold mineralization. Three types of hydrothermal fluorapatite (Ap1, Ap2, and Ap3) were revealed by Cathodoluminescence (CL) imaging. The Ap1 and Ap2 formed in the auriferous ore-forming stage, whereas the Ap3 formed in the post-ore stage. The three types of apatite have distinct total REE concentrations (216-751 ppm for Ap1, 14-120 ppm for Ap2, and 914-1422 ppm for Ap3) as well as chondrite-normalized REE distribution patterns. The textures and compositions of different apatites are inconsistent with dissolution-re-precipitation processes but instead point to three episodes of apatite formation in the Hatu hydrothermal system. The Ap3 has different REE and lower Eu/Eu* values (0.97-1.72) than Ap1 (1.34-4.50) and Ap2 (1.69-4.95), indicating a change of fluid compositions and a decrease of oxygen fugacity from the auriferous ore-forming stage to post-ore stage. All three types of apatite (Ap1, Ap2, and Ap3) show relatively high and consistent oxygen isotope compositions ranging from 14.2 ± 0.3â° to 15.2 ± 0.2â°, from 14.5 ± 0.2â° to 15.0 ± 0.3â°, and from 13.8 ± 0.3â° to 15.4 ± 0.2â°, respectively. Their calculated [delta].sup.18O.sub.fluid values are higher than magmatic-hydrothermal fluids but consistent with those from metamorphic fluids derived from meta-basalts. The near-zero [delta].sup.34S values of hydrothermal pyrite grains in the gold ores are much higher than those of pyrite framboid grains in the ore-hosted sedimentary rocks (- 41.1 to - 24.3â°), suggesting the sulfur may be derived from meta-basalts rather than sedimentary rocks in the mining area. Taken together, we suggest that Hatu is an orogenic gold deposit formed by fluids and metals derived from metamorphic de-volatilization of meta-basalts at depth. Our study highlights that a combination of CL imaging, in situ trace elements, and oxygen isotope compositions of hydrothermal apatite can be a novel and powerful tool to trace the nature of ore-forming fluids in the hydrothermal gold deposits., Author(s): Jiahao Zheng [sup.1] [sup.2] [sup.3], Ping Shen [sup.3], Wanyi Feng [sup.1] Author Affiliations: (1) grid.411604.6, 0000 0001 0130 6528, Zijin School of Geology and Mining, Fuzhou University, , 350108, [...]

The Wulong gold deposit (> 80 t Au) is located at the northeastern margin of the North China Craton (NCC). Gold in the most economically important quartz veins (Stages 2 and 3) is associated with a varied assemblage of Bi and Te minerals. Stage 2 is characterized by, in a temporal order, native gold-native bismuth-maldonite-hedleyite, bismuthinite-Bi-sulfotellurides, electrum-hessite-Bi-Pb-Ag sulfosalts, Bi-Pb sulfosalts, and Bi-Pb-sulfotellurides. All of these minerals except maldonite, electrum, hessite, and Bi-sulfosalts are present in Stage 3 veins, which also contain jonassonite and unnamed minerals with stoichiometry of Bi8Te3 and Bi5Te3. These complex Bi mineral assemblages resulted from changes in Pb, Ag, Te and Bi concentrations in reduced hydrothermal fluids both locally and temporally, from Stage 2 to 3. Texturally, approximately 85% of the gold grains are associated with almost all Bi minerals in each main-stage mineralization. Quartz-vein ores from Stages 2 and 3 have high Bi concentrations (up to 7332 ppm) and show a positive correlation between Bi and Au, with Bi/Au ratios > 10. These textural and geochemical associations, together with the occurrence of numerous droplet-like blebs and small inclusion trails of native bismuth and Bi-tellurides with native gold, maldonite, or jonassonite, suggest that Bi-rich melts may have formed from the hydrothermal fluids of Stages 2 and 3, and acted as an Au scavenger in a complex Bi-Au-Pb-Ag-Te system. Hence, the intimate association of Bi and Au at Wulong is the result of Au enrichment via the liquid bismuth collector mechanism. Unlike Wulong, however, other deposits in the NCC, such as Yangzhaiyu (Xiaoqinling district), Guilaizhuang (Jiaodong district) and Dongping (Jidong district), are characterized by low Bi concentrations, and are much richer in Te than Bi, so these systems could not generate low-melting point Bi-rich polymetallic melts. The low-sulfur Bi-Au-Pb-Ag-Te association at Wulong is consistent with that observed in reduced intrusion-related gold systems elsewhere and suggests that hydrothermal fluids may have been derived from the co-located Early Cretaceous diorite-granite porphyry dyke swarm. [ABSTRACT FROM AUTHOR]

[Display omitted] • Chlorite, epidote, and apatite in five stages show different geochemical features. • Magmatic hydrothermal fluids played a remarkably crucial role during the overprinting period. • The mechanism of copper precipitation is distinct in different stages. Porphyry copper deposits can be deformed by subsequent magmatic-tectonic-metamorphic evolution, resulting in the remobilization of pre-existing ores and the consequent overprinting mineralization. This process might potentially increase the ore grade and/or resource. We focus on the Duobaoshan porphyry Cu (Mo) deposit, two periods were identified in this study, i.e., porphyry period (including Stage 1: potassic alteration; 2: propylitic alteration, and 3: chlorite-sericite alteration and porphyry mineralization) and overprinting period (including Stage 4–1: ductile–brittle deformation; Stage 4–2 and Stage 4–3: overprinting mineralization). Metal sulfide veins occur along the schist in Stage 4–2 and Stage 4–3, features that differ from the typical porphyry mineralization of Stage 3 (veinlets, stockworks, and dissemination). Here, we present in situ Sr isotope data for apatite and compositions of chlorite, epidote, and apatite to reveal the source and nature of fluids and mechanism of precipitation during overprinting mineralization. The in situ 87Sr/86Sr ratios of apatite from Stages 3, which range from 0.7030 to 0.7046, are slightly higher than those observed in the Ordovician granodiorite. Meanwhile, the Sr isotope ratios for apatite from Stages 4–2 and 4–3 are 07033–0.7040 and 0.7013–0.7045 respectively. These ratios broadly consistent with those of the surrounding Ordovician granodiorite and basaltic andesite, suggesting that the ore-forming fluids for all these stages originated from magmatic sources. Chlorite in Stage 4–2 has a higher Mn, Co, Ni, Zn content compared to chlorite in Stage 3, 4–3, whereas epidote and apatite in Stage 4–2 have higher Ca, Sr, Pb content compared to those in Stage 3, 4–3, implying that the fluids in Stage 4–2 underwent strongly water–rock reactions and extracted metal elements from surrounding rocks. Copper was only detected in Stage 4–3 chlorite (15.13–500.20 ppm) and epidote (7.91–19.29 ppm), indicating a high concentration of copper in the fluid, and due to the pre-existing magnetite dissolution, chlorite, epidote, and apatite in Stage 4–3 exhibit high V and Fe relative to other stages. Fluid mixing likely occurred during Stage 3, leading to the precipitation of copper. Strong water-reaction in Stage 4–2 produce changes in fluid natures that facilitate sulfide deposition during this stage. While in Stage 4–3, copper precipitation is controlled by a combination of rising pH and changing sulfur content and valence. Our study highlights that in the Duobaoshan deposit, late magmatic-hydrothermal fluids remobilized pre-existing ore bodies, and the copper precipitation mechanisms differed during the main copper mineralization events. [ABSTRACT FROM AUTHOR]

The Tuwu deposit is one of the largest porphyry copper deposits in the Eastern Tianshan, Northwest China. Ore bodies of the deposit are associated with the tonalite porphyry, which intruded the Early Carboniferous Qi´eshan Group. The hydrothermal ore‐forming processes include the porphyry mineralization episode with potassic, chlorite‐sericite/albite, phyllic, and propylitic alteration and the overprinting mineralization episode. These hydrothermal alteration episodes were studied on the basis of mass transfer and element mobility. The mass balance data and isocon diagrams reveal that Cu is highly associated with the chlorite‐sericite (albite) stage and followed by phyllic and potassic stages. On the basis of electron probe analysis, the SO3 contents of the igneous apatite from the tonalite porphyries range from 0.016 to 0.288 wt.%, which are higher than those from the diorite porphyries (SO3 = 0.088–0.179 wt.%). Similarly, the apatite saturation temperature (861–926°C) and magmatic S content (mean = 52.16 ppm) from the tonalite porphyries are also higher that from the diorite porphyries (T = 813–877°C; S = 28.25 ppm). These results suggest that the S content of the apatite is related to the temperature and S content of the melt. In the hydrothermal alteration zones, the F favours partitioning into apatite under lower temperature, the F contents of hydrothermal apatite increase from the potassic zones to the propylitic zones accompanying with the decrease of temperature. Cl element is much more sensitive to the pH, pressure, and fluid composition than F element. The Cl contents of the apatite increase from the potassic zones (0.18–0.46 wt.%) to the chorite‐sericite (albite) zones (0.28–0.58 wt.%), resulting from the decrease of fluid pH and pressure. The lower Cl contents of apatite (0.07–0.48 wt.%) in the phyllic zones were due to the lower Cl content of hydrothermal fluids. The highest Cl contents (0.09–0.69 wt.%) of apatite in the propylitic zones were caused by the decrease of pressure. The δ18Owater values of quartz in chlorite‐sericite (albite) zones and phyllic zones range from −0.8‰ to −0.4‰ and from −5.3‰ to 4.8‰, and the δD values vary from −57‰ to −47‰ and from −52‰ to −44‰, respectively. Gaseous compositions of the fluid inclusions in quartz are dominated by H2O with minor CO2, and the liquid compositions mainly consist of Na+ and Cl−. These data imply that the ore‐forming fluids belong to the H2O‐NaCl system and evolved from magmatic water to meteoric water in origin. [ABSTRACT FROM AUTHOR]

Abstract The North Balkhash–West Junggar Metallogenic Belt is one of the most important areas within the Central Asian Metallogenic Domain. The belt comprises the North Balkhash and West Junggar regions, which have significantly different Cu reserves. We present mineral geochemistry, whole-rock major and trace element chemistry, zircon Hf and O isotope compositions, and zircon trace element chemistry, on the basis of which we conclude that porphyry copper deposits within the belt formed by different processes. We interpret ore-related magmas in the West Junggar region to have formed in an immature arc setting by the partial melting of lower crust (30–40 km), with some mantle input and sediments contamination. In contrast, ore-related magmas within the North Balkhash region are interpreted to have formed in a mature arc setting by the partial melting of lower crust (~40 km), with mantle input and possible upper-crustal contamination. The oxygen fugacities of ore-related intrusions within the North Balkhash region are typically higher than those within the West Junggar region. A high magmatic water content is considered to be important, but not sufficient, for the formation of large porphyry copper deposits. Thicker crust and high oxygen fugacity are the two main reasons for the higher Cu reserves in the North Balkhash region compared with the West Junggar region. Graphical abstract Unlabelled Image Highlights • Sources of PCDs in the West Junggar region are principally juvenile lower crust and form in the normal crust. • PCDs in the West Junggar region have different formation processes. • Thicker crust and higher oxygen fugacity are crucial factors resulting in significantly reserves difference. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Ore-forming tonalite porphyry has high oxygen fugacity and sulfur contents. • Three hydrothermal alteration stages are recognized. Decreasing of t emperature and pressure result in copper precipitation. • Copper transported by brine at high to low temperature, but also by vapor at high temperature. Abstract The Aktogai Group (more than 12.5Mt of Cu and 80t of Au) is located in the western part of the Central Asian Orogenic Belt (CAOB). The mineralization of the Aktogai deposit is associated with the Early Carboniferous tonalite porphyry which intruded into the Koldar complex. The ore-forming tonalite porphyry has higher oxygen fugacity and sulfur contents than those of wall rocks based on results of mineralogical chemistry analyses. Hydrothermal fluid activities result in the potassic, sericite(-chlorite) and propylitic alteration, and are associated with disseminated and/or vein mineralization. The alteration and mineralization include three stages. Potassic stage is characterized by hydrothermal K-feldspar, biotite, magnetite, "A" veins and disseminated mineralization. Sericite(-chlorite) stage that partly overprinted on potassic stage is characterized by sericite, chlorite, "C" veins and disseminated mineralization. Propylitic stage is characterized by chlorite, epidote and "C" veins, and develops in the tonalite porphyry and wall rocks. Five types of fluid inclusions have been recognized in the Aktogai deposit, including liquid-rich (L-type), vapor-rich (V-type), daughter mineral-bearing (S-type), CO 2 -bearing (C-type) and minor SO 2 -bearing fluid inclusions. In the potassic stage, ore-forming fluid belongs to H 2 O-NaCl-CO 2 system with four types fluid inclusions, homogeneous temperatures and salinities of fluid inclusions are 320 °C to 448 °C and 0.35 wt% to 51.08 wt%, respectively. In the sericite(-chlorite) stage, the fluid belongs to H 2 O-NaCl-CO 2 system with L-type, V-type and C-type fluid inclusions, homogeneous temperatures and salinities of fluid inclusions are 243 °C to 315 °C and 1.22 wt% to 13.27 wt%, respectively. In the propylitic stage, the fluid belongs to H 2 O-NaCl system with L-type and V-type fluid inclusions, homogeneous temperatures and salinities of fluid inclusions are 211 °C to 328 °C and 0.35 wt% to 11.46 wt%, respectively. A relatively negative δ34S‰ values (−4.61‰ to −3.18‰) indicate that the sulfur derived from a high oxygen fugacity magma. Decreases of the temperature, pressure and oxygen fugacity of the ore-forming fluid result in metal precipitation in the Aktogai deposit. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Ore-forming fluids are moderate-high temperature and salinity, and relatively oxidized, and they evolved from magmatic to meteoric in origin. • Ore-forming materials were mainly derived from magma. • Temperature, redox, pH, and fluid-rock interaction are important controls on the transportation and precipitation of sulfides. Abstract The Tuwu porphyry copper deposit is located in the Eastern Tianshan Mountains, Xinjiang, China. Lentiform-like ore bodies are hosted in the Early Carboniferous Qi'eshan group, and Cu mineralization shows a close association with tonalite porphyry. Porphyry and country rocks have undergone extensive potassic, chlorite-sericite, phyllic and propylitic alteration. The mineralized veins in the Tuwu deposit can be divided into: earlier quartz–biotite ± magnetite ± chalcopyrite veins (stage I), quartz–chalcopyrite ± bornite ± pyrite veins (stage II) and quartz ± molybdenite ± chalcopyrite ± pyrite ± calcite veins (stage III), and later calcite ± quartz veins (stage IV). The Stage II and III are the main Cu mineralization stages. At the Tuwu deposit, three types of fluid inclusions have been distinguished: liquid-rich (L-type), vapor-rich (V-type) and halite-bearing (S-type), respectively. Laser Raman spectra identified magnetite, hematite, chalcopyrite, and pyrite at S-type inclusions. The quartz veins in stage I and II contain all inclusion types. L-type and V-type inclusions are recognized in stage III quartz minerals, but only L-type inclusions are present in stage IV quartz minerals. Fluid inclusions in quartz of stage I, II, III and IV are characterized by homogenization temperature vary of 275–492 °C, 232–477 °C, 199–287 °C and 133–227 °C, with corresponding NaCl equiv. salinities of 2.2–58.7 wt% NaCl equiv. , 1.4–58.3 wt% NaCl equiv. , 1.1–6.0 wt% NaCl equiv. , and 1.1–4.2 wt% NaCl equiv. , respectively. Initial ore-forming fluids of the Tuwu deposit belong to relatively oxidized H 2 O-NaCl system with moderate-high temperature and salinity. Sulfur isotope data from chalcopyrite and pyrite suggest a magmatic source. Two types of chlorite (type 1 and type 2) are recognized based on the relationship with metal sulfides. Electron microprobe analysis shows that chlorites are plotted in ripidolite and pycnochlorite. Decreasing Fe/(Fe + Mg) ratios of chlorites indicate that the meteoric water was involved in the magmatic water, combined with the positive correlation with AlIV, implying that the chlorite formed in a relatively reduced and acidic environment. The hydrothermal fluids in type 1 chlorite bring large quantities of reduced metallic minerals compared to the barren fluids in type 2 chlorite. The formation temperatures of chlorite are roughly coincident with the results that obtained from fluid inclusion studies, which show a decrease trend. Changes in temperature, redox, pH, and fluid-rock interaction likely play an important role in the transportation and precipitation of metal sulfides of the Tuwu deposit. [ABSTRACT FROM AUTHOR]

The Eastern Tianshan orogen contains many late Paleozoic porphyry Cu and magmatic Cu-Ni deposits. Recent studies demonstrate that several early Paleozoic volcanogenic massive sulfide (VMS) Cu-polymetallic and porphyry Cu deposits were discovered in the northern part of Eastern Tianshan. This study presents zircon U-Pb, whole-rock geochemical, and Sr-Nd isotopic data for granites and diorites from the Huangtupo VMS Cu-Zn deposit, northern part of the Eastern Tianshan. Our results can provide constraints on the genesis of intermediate and felsic intrusions as well as early Paleozoic geodynamic setting of the northern part of Eastern Tianshan. LA-ICP-MS zircon U-Pb analyses suggest that the granites and diorites were formed at 435 ± 2 Ma and 440 ± 2 Ma, respectively. Geochemical characteristics suggest that the Huangtupo granites and diorites are metaluminous rocks, exhibiting typical subduction-related features such as enrichment in LILE and LREE and depletion in HFSE. The diorites have moderate Mg#, positive ε Nd (t) values (+6.4 to +7.3), and young Nd model ages, indicative of a depleted mantle origin. The granites exhibit mineral assemblages and geochemical characteristics of I-type granites, and they have positive ε Nd (t) values (+6.7 to +10.2) and young Nd model ages, suggesting a juvenile crust origin. The early Paleozoic VMS Cu-polymetallic and porphyry Cu deposits in the northern part of Eastern Tianshan were genetically related. The formation of the early Paleozoic magmatic rocks as well as VMS and porphyry Cu deposits in the northern part of Eastern Tianshan was due to a southward subduction of the Junggar oceanic plate. [ABSTRACT FROM AUTHOR]

Newly-discovered Suyunhe Mo deposit (0.57 Mt Mo metal), located in the Barluk Mountains of the West Junggar terrain, Xinjiang, NW China, is the largest porphyry Mo deposit discovered in Xinjiang so far. The Suyunhe Mo deposit is associated with the Suyunhe intrusive complex which was emplaced into the volcano-sedimentary sequences of the Middle Devonian Barluk Group. The Suyunhe complex consists of the deep granite intrusion and the shallow granite porphyry, granodiorite porphyry, monzonitic granite porphyry, and tonalite porphyry stocks which are cut by the felsic dikes. Dominant Mo mineralization is hosted in the Barluk Group volcano-sedimentary rocks but is minor in the granite, granite porphyry, granodiorite porphyry, and monzonitic granite porphyry of the Suyunhe complex. We report in situ zircon U–Pb ages, whole rock geochemistry, Sr–Nd isotopic and zircon Lu–Hf isotopic data for the ore-bearing intrusive rocks from the Suyunhe deposit. Secondary ion mass spectrometry (SIMS) zircon U–Pb dating indicates that the deep granite emplaced at 298.4 ± 1.9 Ma–295.3 ± 3 Ma and the shallow granodiorite porphyry intruded during 294.7 ± 2.1 Ma–293.7 ± 2.3 Ma. The Suyunhe granite intrusion, including fine- and medium-grained granite, belongs to the high-K calc-alkaline series. Most of them have low Zr + Nb + Y + Ce (< 350 ppm) and 10,000 Ga/Al (< 2.7) values, showing a geochemical affinity to I-type granite. These granites are enriched in light rare earth elements (LREE) and depleted in heavy rare earth elements (HREE) and Eu, Ba, Sr, Nb, P and Ti, indicating that they have experienced advanced fractional crystallization. They have low initial 87 Sr/ 86 Sr ratios (0.70318–0.70380), high whole-rock εNd(t) values (+ 4.4 to + 6.0), high zircon εHf(t) values (+ 10.8 to + 14.6), and young whole-rock Nd and zircon Hf model ages. They also show low Mg# (< 0.35). These features demonstrate that the ore-bearing granite formed by partial melting of the juvenile lower crust and experienced advanced fractional crystallization. The Suyunhe granite porphyry and monzonitic granite porphyry are compositionally similar to the granite, indicating that they share a common magma source and process with the granite. The Suyunhe granodiorite porphyries belong to slightly high-K calc-alkaline series and have LREE enrichment and HREE, Nb and Ti depletion. They have low ( 87 Sr/ 86 Sr) i ratios (0.70421–0.70444), high εNd(t) values (+ 4.9 to + 6.2), high zircon εHf(t) values (+ 9.6 to + 15.2), and young whole-rock Nd and zircon Hf model ages. They also show low Mg# (< 0.39) with one reception of 0.48. These features indicate that the granodiorite porphyries have a relatively deeper magma source region of the lower crust and have not experienced as much fractional crystallization. The Suyunhe barren tonalite porphyries show enrichment of LREE and depletion of HREE, Nb and Ti depletion, and have chemical composition (e.g., Mg# = 0.41–0.48) different from the Suyunhe granodiorite porphyries, though isotopically they are indistinguishable. This suggests that the parental magma of the barren tonalite porphyries originated from melting of a lithospheric mantle. Therefore, the Suyunhe ore-bearing magmas derived from the juvenile lower crust which has a high potential to bear porphyry Mo deposit and result in the formation of the Suyunhe large porphyry Mo deposit. [ABSTRACT FROM AUTHOR]

[Display omitted] • Mg, Fe, Al, and Ti are most different between chlorite PCDs and chlorite Barren. • Three diagrams are proposed to distinguish chlorite PCDs from chlorite Barren. • Redox states and formation temperatures result in the differences. In the porphyry Cu ± Au ± Mo deposits (PCDs), chlorite and its mineral assemblages are indicators for exploration. However, chlorite found in PCDs may have different origins because it is a common mineral in different geological environments. Therefore, it is necessary to identify the differences in the composition of PCDs-related chlorites (chlorite PCDs) and chlorite formed in other geological environments (chlorite Barren). Here, based on occurrences, mineral assemblages, and compositions of the chlorites from thirty PCDs and barren areas (including geothermal systems, chlorite GSs ; low-grade metamorphic rocks, chlorite LGMRs ; sedimentary rocks, chlorite SRs), we used principal component analysis (PCA) and random forest (RF) methods to find discrimination approaches. Results show that chlorite PCDs and chlorite Barren differ in the Fe, Mg, Al, Ti, and possibly Mn, K, and Ca elements. Based on these differences, wt.% ratios of (MgO + 100*TiO 2)/(FeO + 100*CaO), TiO 2 /(CaO + K 2 O + Na 2 O), Al 2 O 3 /(100*K 2 O*CaO), MgO/(100*CaO*Na 2 O), SiO 2 /100(CaO + K 2 O), and FeO/100TiO 2 and diagrams of Fe/(Fe + Mg) (a.p.f.u. ratio) vs TiO 2 /(Na 2 O + K 2 O + CaO) (wt.% ratio), Fe/(Fe + Mg) (a.p.f.u. ratio) vs MgO/(100*Na 2 O*CaO) (wt.% ratio), and Fe/(Fe + Mg) (a.p.f.u. ratio) vs SiO 2 /Al 2 O 3 (wt.% ratio) are proposed to distinguish chlorite PCDs from chlorite Barren. These discrimination approaches have better results in distinguishing chlorite PCDs from chlorite LGMRs and chlorite SRs but are less effective for chlorite GSs. By comparing the physicochemical conditions, the highly oxidized conditions and high temperature of the ore-related intrusions and fluid properties result in the compositional differences between chlorite PCDs and chlorite Barren. Although further researches remain to be done, this study provides potential approaches for identifying chlorite PCDs based on major elements. [ABSTRACT FROM AUTHOR]

Most porphyry copper deposits worldwide contain magnetite, hematite, and anhydrite in equilibrium with hypogene copper-iron sulfides (chalcopyrite, bornite) and have fluid inclusions with CO >> CH that are indicative of high fO. In contrast, the Baogutu porphyry Cu deposit in the West Junggar terrain (Xinjiang, China) lacks hematite and anhydrite, contains abundant pyrrhotite and ilmenite in equilibrium with copper-iron sulfides (chalcopyrite), and has fluid inclusions with CH >> CO that are indicative of low fO. The mineralized intrusive phases at Baogutu include the main-stage diorite stock and minor late-stage diorite porphyry dikes. The main-stage stock underwent fractional crystallization and country-rock assimilation-contamination, and consists of dominant diorite and minor gabbro and tonalite porphyry. The country rocks contain organic carbons (0.21-0.79 wt.%). The δCv values of the whole rocks (−23.1 to −25.8 ‰) in the wall rocks suggest a sedimentary organic carbon source. The δCv values of CH (−28.2 to −36.0 ‰) and CO (−6.8 to −20.0 ‰) in fluid inclusions require an organic source of external carbon and equilibration of their ΔC values (8.2-25.0 ‰) at elevated temperatures (294-830 °C) suggesting a significant contribution of thermogenic CH. Mineral composition data on the main-stage intrusions, such as clinopyroxene, hornblende, biotite, magnetite, ilmenite, sphene, apatite, and pyrrhotite, suggest that the primary magma at Baogutu was oxidized and became reduced after emplacement by contamination with country rocks. Mineral compositions and fluid inclusion gas compositions suggest that the redox state of the system evolved from log fO > FMQ + 1 in the magma stage, to log fO < FMQ as a consequence of country rocks assimilation-contamination, to log fO > FMQ in the hydrothermal stage. Though oxidized magma was emplaced initially, assimilation-contamination of carbonaceous country rocks decreased its fO such that exsolved fluids contained abundant CH and deposited a reduced assemblage of minerals. [ABSTRACT FROM AUTHOR]

Highlights: [•] Yandong porphyry Cu deposit is hosted by a volcanic complex and a felsic intrusion. [•] Diorite and plagiogranite porphyries yield SIMS zircon U–Pb ages of 340Ma and 332Ma, respectively. [•] The volcanic complex and plagiogranite porphyry formed in an Early Carboniferous island arc. [•] Plagiogranite porphyry is productive porphyry and caused alteration and mineralization. [Copyright &y& Elsevier]

Abstract: We report SIMS U–Pb zircon, elemental and Nd–Sr–Pb isotopic data for the volcanic rocks and their subvolcanic units from the Xiemisitai Mountains in the West Junggar Region (Xinjiang, China), aiming to determine their ages, source regions and tectonic setting. Zircon U–Pb dating results of the volcanic rocks (422.5±1.9Ma–411.2±2.9Ma) indicate that the volcanic strata occurred in the Xiemisitai Mountains at Late Silurian–Early Devonian time rather than Middle Devonian as suggested previously. They are probably the eastern extension of the early Paleozoic (Cambrian–Silurian) Boshchekul–Chingiz volcanic arc of East Kazakhstan in China. The Xiemisitai volcanic rocks include andesite, rhyolite, and their pyroclastic equivalents. The subvolcanic units are felsite and granite porphyry. Andesite is moderately LREE-enriched, with a marked negative Nb anomaly and Th/Yb-enrichment. Rhyolite, felsite and granite porphyry are enriched in LREE and Th and depleted Nb. The initial Sr isotope values of the Xiemisitai volcanic rocks and their subvolcanic units range from 0.702294 to 0.704456, εNd(420Ma) from +0.19 to +1.88 with Nd model ages ranging from 600 to 803Ma. Initial Pb isotope ratios (206Pb/204Pb) are 17.77–17.96, (207Pb/204Pb) are 15.49–15.54 and (208Pb/204Pb) are 37.56–37.80. These features suggest that the Xiemisitai volcanic rocks and their subvolcanic units derived from a mantle wedge significant mixed by subducted material (EMI) in a subduction zone. They also indicate significant involvement of Neoproterozoic primitive crust in the formation of the Xiemisitai magmatic arc. These rocks are associated with a Late Silurian–Early Devonian southward subduction of the oceanic lithosphere in the northern West Junggar Region. This gives rise to an EW-trending Boshchekul–Chingiz volcanic arc and associated metallogenic belt in the northern West Junggar Region and adjacent Kazakhstan. [Copyright &y& Elsevier]

Abstract: The Baogutu porphyry copper belt lies in the Darbut transitional island arc of the western Junggar, in the western section of the Central Asian Orogenic Belt in NW China. Our new petrographic results for the ore-bearing porphyry stocks in the Baogutu porphyry copper belt recognize them as diorite porphyry stocks rather than the granodiorite porphyry stocks as previously identified. The copper mineralization is hosted in the diorite, diorite porphyries and related breccias of the diorite porphyry stocks. Geochemical data indicate that the ore-bearing porphyries have a predominantly intermediate composition with a transitional character from tholeiite to calc-alkaline, and are enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE) with a clear negative Nb anomaly. REE patterns show distinct enrichments in LREE relative to HREE. The rocks also exhibit high initial εNd(t) (+2.7 to +6.3) ratios and low initial 87Sr/86Sr values (0.70359–0.70397). Many samples are chemically similar to adakites (e. g. Yb<1.9 ppm, Y<18 ppm, Sr/Yb>20, 87Sr/86Sr<0.7045). These data are consistent with a transitional island arc from immature arc to mature arc and suggest that the ore-bearing porphyry system was derived from the partial melting of multiple sources including oceanic crust and a subduction-modified mantle wedge, with melts undergoing significant crystal fractionation during convergence between the paleo-Junggar ocean and the Darbut arc. [Copyright &y& Elsevier]

• The primary magma is still oxidized in the reduced Suyunhe porphyry Mo deposit. • The highly differentiated and fluorite-rich magma is crucial for the formation of the Suyunhe deposit. • The addition of reduced sequence or gases (e.g. CH 4) improves the efficiency of metal precipitation. Some porphyry Mo deposits with relatively reduced ore assemblages, characterized by high reduced gases (e.g. CH 4) and a lack of magnetite and hematite, are generally considered as the reduced porphyry Mo deposits. Nevertheless, the role of oxygen fugacity (f O 2) in controlling Mo enrichment and precipitation of molybdenite in such reduced porphyry Mo deposits is poorly understood. The Suyunhe reduced porphyry Mo deposit with a Mo metal reserve of 0.57Mt at an average grade of 0.05–0.09%, located in the West Junggar terrane in Xinjiang, is associated with Early Permian granitic rocks, which emplaced into the volcano-sedimentary sequences of the Middle Devonian Barluk Formation. In this study, we report that the variation of magmatic f O 2 , the possible mechanism of Mo enrichment, and the contributions of the reduced gases or matter to molybdenite precipitation. Results indicate that the primitive magma is still oxidized with f O 2 > NNO (Ni–NiO oxygen buffer), but it is then reduced due to contamination by the reduced sequences. Furthermore, the decreasing magmatic f O 2 does not significantly prevent the enrichment of Mo metals during magmatic evolution. By contrast, the evolved parental magma, characterized by highly differentiated and fluorite-rich, is responsible for improving Mo concentration of melts. Most of reduced gases are derived from decomposition of organic matter occurring in the reduced volcanic sedimentary rocks of Barluk Formation. More importantly, these reduced gases (e.g. CH 4) play a role in molybdenite precipitation, which may effectively improve the efficiency of metal precipitation. [ABSTRACT FROM AUTHOR]

The article discusses research on oxidation conditions of granitic magmas that are associated with Porphyry Copper Deposits in the Central Asian Orogenic Belts. Topics discussed include evaluation of the relationship between the oxidation condition and metal-fertility of magmas, selection of grains with no inclusions and no sector zoning for trace element analysis with a laser ablation inductively coupled plasma mass spectrometry and analysis of impact of oxidation conditions of magmas.

Graphical abstract Highlights • The primary magma of Baogutu reduced porphyry Cu deposit is oxidized and its reduced features are secondary. • δ56Fe of less altered plutonic rocks plot into the field of arc magma, well consistent with the high oxygen fugacity of primary magma. • Plutonic rocks and wall rocks turn to be heavy Fe isotopes enrichment during the hydrothermal alteration. • This is the first time to report the Fe isotopic composition and fractionation of reduced porphyry Cu deposit. • Fe isotopes may be a potential tool to constrain the redox state of primary magma and reveal the origin of reduced porphyry Cu deposit. Abstract Reduced porphyry Cu deposits are scarce and their origins remain enigmatic. The Late Carboniferous Baogutu reduced porphyry Cu deposit in the western Junggar Terrane, NW China, provides an opportunity to address this issue. In this study, we conducted systematic analyses in terms of petrography, mineral chemistry, major elements, trace elements and Fe isotopes to investigate the redox state of its primary magma and further constrain the origin of Baogutu copper deposit. Based on the petrological observation and mineral chemistry, two magmatic stages have been recognized during the formation of ore-forming diorite. The early stage is characterized by the crystallization of magnetite and high Mg# [Mg/(Mg + Fe2+)] hornblende under oxidized condition (ΔNNO > 2.36, calculated by the equation given by Scaillet and Evans (1999). The late stage is characterized by the crystallization of ilmenite and low Mg# biotite under reduced condition (ΔNNO <−0.6). The large decrease of oxygen fugacity (log f O 2 , more than 3 units) between the two stages may result from the contamination of organic carbon during the emplacement of magma. In addition, major and trace element compositions of the Baogutu ore-forming diorites show the affinity of arc magmas which are like those that host oxidized porphyry Cu deposits. In this study, we first report the Fe isotopic compositions of reduced porphyry copper deposit and discuss its application for the origin of ore deposit. The δ56Fe values of less altered diorites and granodiorites (+0.083‰ and +0.054‰) fall into the field of arc magmas, consistent with their major and trace elements results. The δ56Fe values of sulfides (chalcopyrite and pyrite) show a large span (−0.228 to +0.614‰) and the early formed disseminated sulfides are enriched in light Fe isotopes relative to the late formed vein-hosted sulfides, which indicates that hydrothermal fluid evolves toward heavy Fe isotope enrichment. The average δ56Fe of all hydrothermal sulfides (chalcopyrite and pyrite) is much higher than that of Baogutu silicate rocks, which demonstrates that the hydrothermal fluid are Fe isotopically heavy and it well explains the positive correlations between δ56Fe and indices of hydrothermal alteration for altered plutonic rocks and wall rocks. In summary, major elements, trace elements and Fe isotopes of the Baogutu porphyries show the affinity of arc magmas, same as the typical oxidized porphyry Cu deposits. Both petrological observations and Fe isotope features of the Baogutu porphyry Cu deposit indicate that the primary magma of Baogutu porphyries is oxidized and its reduced features are secondary. [ABSTRACT FROM AUTHOR]

The Suyunhe large porphyry Mo deposit (∼0.57 Mt molybdenum), located in the West Junggar, NW China, is the largest known porphyry Mo deposit in Xinjiang. Granitoids in this deposit are mainly characterized by three closely spaced intrusive centers (known as stocks I, II and III respectively). The stocks I and III mainly consist of barren granodiorite porphyry and tonalite porphyry, whereas the stock II is mainly composed of fertile monzonitic granite porphyry and granite porphyry. Based on detailed major and trace element, and Sr–Nd isotopic analyses, two distinct compositional groups can be identified. The first group of high-silica end-members (HSE) is characterized by high SiO 2 (mostly >75 wt%), low MgO (0.07–0.69 wt%) and Mg# (0.19–0.36), significant Eu depletion in the chondrite-normalized diagram, and low Sr/Y and La/Yb, as well as noticeably negative anomalies of Ba, Sr, P and Ti in the primitive mantle-normalized diagram. The second group of low-silica end-members (LSE), however, displays adakite-like features with lower SiO 2 (<75 wt%), higher MgO (0.52–1.32 wt%) and Mg# (0.32–0.52; mostly >0.4), and higher Sr/Y (mostly >20) and La/Yb (>8). The depleted Sr–Nd isotopic characteristics (ε Nd (T) = 3.5–6.4 and I sr = 0.7026–0.7055) and young two-stage model ages of HSE and LSE indicate that they were both derived from partial melting of juvenile lower crust that might be triggered by asthenosphere upwelling subsequent to a slab rollback event. However, the depths of initial melting might be different. The current evidence demonstrates that HSE in the Suyunhe deposit formed by partial melting of juvenile crust at depths of less than ∼33 km with a plagioclase residue, whereas that for LSE occurred at depths of >40 km where a garnet residue existed and the crust was thickened. The lower source depth, as well as subsequently strong plagioclase fractionation, results in the absence of adakite-like characteristics in HSE. The Ce 4+ /Ce 3+ and Eu N /Eu N ∗ ratios in zircons of HSE are much lower than ore-forming intrusions from porphyry Cu deposits in the Central Asian Orogenic Belt, but noticeably higher than barren intrusions from the Lachlan fold belt and ore-bearing intrusions from small-intermediate porphyry Mo deposits from the East Qinling–Dabie and the Nanling metallogenic belts, China, indicating that neither too high nor too low oxygen fugacities are favorable for large porphyry Mo deposits. Based on previous studies of adakitic rocks in the world, adakite-like LSE in the Suyunhe deposit are believed to have higher oxygen fugacities, and thus be less fertile than HSE. We finally suggest that adakites and adakite-like rocks are unproductive for porphyry Mo deposits. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The CAMD is one of the most important porphyry-type deposit domains on earth. • CAMD porphyry deposits are associated with the evolution of three Paleo-oceans. • MASH and slab melting models can explain the genesis of some PCDs in the CAMD. • A new model involves 'enriched lower crust and relictic oceanic ridge subduction' • Melting of juvenile lower crust was essential for Mo mineralization. Abstract A series of porphyry-type Cu-(Mo)-(Au) deposits including some giant world-class deposits such as Kounrad, Aktogai, Kal'makyr, Oyu Tolgoi and Chalukou occur in the Central Asian metallogenic domain (CAMD). The outline of the CAMD almost overlaps with that of the Central Asian orogenic belt (CAOB), which is located between the East Europe, Siberia, Tarim and North China cratons. The CAMD comprises sparsely exposed tectonic slices of Precambrian high-grade metamorphic basement rocks that are sandwiched between Phanerozoic accretionary complexes and arc rocks. According to geological, metallogenic, geochronological and tectonic setting characteristics, the CAMD comprises three metallogenic provinces: (1) the Kazakhstan Cu-Au-Mo, (2) the Mongolia Cu-Au and (3) the Northeast China Mo-Cu metallogenic provinces. Early Paleozoic (∼490 to 440 Ma) and Late Paleozoic (∼330 to 295 Ma) large scale porphyry-type mineralization in the Kazakhstan metallogenic province is intimately associated with the evolution of the Central Tianshan–Yili–Northern Tianshan–Kokchetav–Chingiz Paleozoic arc system which was terminated by the late Permian closure of the western segment of the Paleo-Asian Ocean. In contrast, the ∼370 Ma and ∼240 Ma porphyry-type mineralization of the Mongolia metallogenic province is related to the Tuva–Mongolia arc collage system that was formed during the evolution of the Paleo-Asian Ocean in the Paleozoic and the Mongol–Okhotsk Ocean in the early Mesozoic. Furthermore, the Northeast China metallogenic province was affected by three distinct geodynamic processes that were associated with the Paleo-Asian, Mongol–Okhotsk and Paleo-Pacific Oceans during the Paleozoic (>250 Ma), the early Mesozoic (>200 Ma) and the late Mesozoic (200–90 Ma), respectively. Except for the Early Paleozoic Cu mineralization at Duobaoshan (∼480 Ma) and Bainaimiao (440 Ma), the porphyry-type mineralization of this metallogenic province is predominantly characterized by widespread Mesozoic Mo mineralization. In general, porphyry-type deposits of the CAMD have geological, structural, metallogenic, alteration and fluid characteristics, which are consistent either with those related to the oceanic subductional arc settings of the Circum-Pacific metallogenic domain or with those of the continental collisional environment in the Tethyan metallogenic domain. The involvement of highly oxidized intermediate to felsic magmas is fundamental for the formation of porphyry deposits. Even though the prevalent MASH and slab melting models can be employed to interpret the genesis of some porphyry Cu deposits in the CAMD, a here newly suggested model, which is based on 'pre-enriched mafic lower crust and the subduction of a relictic mid-oceanic ridge', is thought to explain the large scale copper mineralization in the Balkhash–West Junggar metallogenic belt of the Kazakhstan metallogenic province. Nevertheless, most of the porphyritic suites associated with the Mo deposits in the Northeast China metallogenic province have relatively depleted mantle-like neodymium and hafnium isotopic compositions, suggesting that the ore-forming granitic magma may have predominantly been derived from juvenile lower crust. Upwelling of asthenosphere may have triggered the melting of the lower crust during post-collisional extension subsequent to the closure of the Paleo-Asian Ocean in the Triassic, the subduction of the Paleo-Pacific ocean in the Jurassic and the thinning of the lithosphere caused by rollback of the Paleo-Pacific plate in the Cretaceous. Ancient crustal components were only locally found to have been involved in the magma source of several medium-large Mo deposits. [ABSTRACT FROM AUTHOR]