Magmatic-hydrothermal Source Research Articles

Genesis of the Xiaobeigou gold deposit in the Jiapigou gold district, Northeast China: Constraints from fluid inclusions, quartz H–O isotopes, and pyrite in situ sulfur isotope and trace element compositions

The Xiaobeigou gold deposit (>20 t @ 17.36 g/t) is located in the Jiapigou gold district (JGD), Northeast China, representing a large-sized quartz vein-type gold deposit hosted by Neoarchean basement gneiss and is related to Mesozoic intrusions in space and time. Four hydrothermal stages were identified from early to late, namely, (1) barren quartz stage, (2) quartz–pyrite stage, (3) gold–quartz–polymetallic sulfide stage, and (4) quartz–carbonate stage. Three main types of fluid inclusions (FIs) have been identified: water-rich (H2O–NaCl; W-type), mixed aqueous-carbonic (CO2–H2O–NaCl; C-type) and pure carbonic (CO2; PC-type) FIs. The early-stage quartz contains W- and C-type FIs that yield moderate homogenization temperatures (270–379 °C) and moderate-low salinities (4.82–17.87 wt% NaCl equivalent). The main-stage quartz grains contain all three types of FIs that yield medium–low temperature (179–263 °C) and moderate-low salinity (2.82–9.86 wt% NaCl equivalent). Only W-type FIs are recognized in late-stage quartz grains, which have low temperatures (133–183 °C) and low salinities (1.91–7.73 wt% NaCl equivalent). The initial ore-forming fluid of the Xiaobeigou gold deposit was a moderate homogenization temperature and moderate-low salinity CO2–H2O–NaCl ± CH4 ± N2 hydrothermal fluid system. Fluid immiscibility is the main mechanism leading to the precipitation of gold and other metal sulfides. The H–O isotopic values (δ18OH2O = −6.87 to 5.90‰; δD = −102 to −78‰) and composition of the quartz suggest that the initial ore-forming fluids consisted mainly of magmatic water mixed with meteoric water during the late stage of the mineralization process. The S isotope data (δ34S = 3.2 to 4.4‰, avg. 3.6‰) indicate that the ore-forming materials are magmatic sources. EPMA and LA–ICP–MS data show that a large amount of gold mineralization mainly occurs in the main ore-forming stage. The high Co/Ni ratio indicates that the pyrite may be a magmatic–hydrothermal source. Taking into account all available data and regional geological history, we suggest that a magmatic–hydrothermal source for the Xiaobeigou gold deposit in the JGD and the ore formation is related to the subduction of the Paleo-Pacific Plate beneath Eurasia.

Geochemical Characteristics of the Mineral Assemblages from the Niukutou Pb-Zn Skarn Deposit, East Kunlun Mountains, and Their Metallogenic Implications

The Niukutou Pb-Zn deposit is typical of skarn deposits in the Qimantagh metallogenic belt (QMB) in the East Kunlun Mountains. In this study, based on detailed petrographical observations, electron microprobe analyses (EMPAs), and laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) analyses, we report the major and trace element compositions of the typical skarn mineral assemblages (garnet, pyroxene, ilvaite, epidote, and chlorite) in this deposit. Three hydrothermal mineralization stages with different mineral assemblages of the prograde metamorphic phase were determined, which were distributed from the inside to the outside of the ore-forming rock mass. Grt1+Px1 (Stage 1), Grt2+Px2 (Stage 2), and Px3 (Stage 3) were distinguished in the Niukutou deposit. Furthermore, the ilvaites in the retrograde metamorphic phase can be divided into three stages, namely Ilv1, Ilv2, and Ilv3. The ore-forming fluid in Stage 1 exhibited high ∑REE, U, and Nd concentrations and δEu, δCe, and LREE/HREE values, which were likely derived from a magmatic–hydrothermal source and formed at high temperatures, high fO2 values, and mildly acidic pH conditions, and probably experienced diffusive metasomatism in a closed system with low water/rock ratios. In Stages 2 and 3, the ore-forming exhibited lower ∑REE, U, and Nd concentrations and δEu, δCe, and LREE/HREE values, with high Mn content that had likely experienced infiltrative metasomatism in an open system with high water/rock ratios. From Ilv1 to Ilv3, the δEu and U contents decreased, whereas the Mn content increased, indicating that the oxygen fugacity of mineralization was in decline. The ore-forming fluid evolution of the Niukutou deposit can be characterized as follows: from Stage 1 to Stage 3, the hydrothermal fluid migrated from the deep plutons to the shallow skarn and marble; the environment altered from the high fO2 and temperature conditions to low fO2 and temperature values, and the pH and Mn contents increased. The fluids contained considerable metal ore-forming materials that were favorable for the enrichment and precipitation of the Fe content. In the retrograde metamorphic phase, with the decrease in oxygen fugacity (from Ilv1 to Ilv3), the temperature and oxygen fugacity of the theore-forming fluid environment decreased, ultimately becoming conducive to the dissolution and precipitation of Pb and Zn elements.

Metallogenic model of the Fengyan stratabound Zn-Pb deposit in Fujian of southeastern China: Constraints from fluid inclusions, C[sbnd]O[sbnd]S-Pb isotopes, and pyrite chemistry

The Fengyan stratabound Zn-Pb deposit (total resources: 24.7 Mt of ore @ 4.09 % Zn and 1.92 % Pb) is hosted by Neoproterozoic marble and metavolcanic rocks in the Meixian ore field of southeastern China. Hitherto, the genesis of this deposit has been a matter of debate, including a Neoproterozoic volcanogenic massive sulfide genesis (VMS) or an Early Cretaceous magmatic-hydrothermal origin. Based on fluid inclusion data from various hydrothermal stages, stable isotope data of hydrothermal minerals, and in-situ trace element analyses of poly-stage pyrite (Py1-3), the deposit can be classified as a distal skarn-type. A complex paragenesis reflects four hydrothermal stages: stage I (pyroxene + garnet + Py1), stage II (epidote + hematite + magnetite + Py2), stage III (quartz + sphalerite + galena + Py3a + Py3b), and stage IV (calcite). The prograde skarn comprises andradite-rich garnet (on average Ad57Gr34Py9) and pyroxene (on average Hd58Di10Jo32) with high Mn contents. Fluid inclusion data reveal salinity and temperature ranges for the prograde and retrograde skarn fluids as 11.5 to 14.6 wt% NaCl equiv. at 278 to 338℃ and 7.5 to 11.2 wt% NaCl equiv. at 235 to 275℃, respectively. The entrapment pressure is calculated as 130 bars. The sulfide ore minerals of stage III were precipitated at lower temperatures (186 to 259℃) from aqueous fluids with lower salinities (<6.6 wt% NaCl equiv.). Late-stage calcite was deposited from a more dilute (<2.2 wt% NaCl equiv.) and cooler (<200℃) fluid with extremely depleted δ18Ofluid values of −10.1 to −2.0 ‰ (V-SMOW), consistent with a meteoric origin. A mean δ34SV-CDT of 1.7 ± 0.9 ‰ of sulfide minerals points to a magmatic sulfur source. Lead isotopic compositions of sphalerite separates (206Pb/204Pb = 18.295–18.381, 207Pb/204Pb = 15.644–15.662, and 208Pb/204Pb = 38.494–38.724) are similar to those of the Yanshanian granitic bodies in the region but different from those of the ore-hosting marble and metavolcanic rocks. Calcite in the late stage has δ13CV-PDB values of −5.4 to −1.5 ‰, consistent with a predominant magmatic origin. Variations of Co, Ni, Se, and As in pyrite of different generations are interpreted to reflect changes in temperature during hydrothermal evolution. The Co/Ni ratios in all generations of pyrite, mainly between 1 and 10, indicate a magmatic-hydrothermal source. Overall, our new data indicate progressive mixing of a granite-derived magmatic fluid with meteoric waters, as is expected in a distal skarn deposit.

Genesis of the Xinfang magmatic-hydrothermal gold deposit, Liaodong Peninsula, China: Constraints from pyrite Re-Os isotopes, C, O, S, Pb, Si, He and Ar isotopes

The Xinfang gold deposit (>20 t@ 2.85 g/t) is situated in the southwest of the Liaodong Peninsula. Ore bodies are hosted by the Archean gneiss in the footwall and Neoproterozoic metasandstone in the hangingwall of the Xinfang Metamorphic Core Complex. Here, we conduct pyrite Re-Os dating, C, O, Si, He-Ar, S, and Pb isotopes in order to constrain the ore-forming age and genesis of the deposit. Field observations show that three ore-forming processes are recognized: (1) quartz- coarse-grained pyrite stage (stage Ⅰ), (2) quartz-polymetallic sulfide-gold stage (stage Ⅱ), and (3) quartz – calcite ± pyrite stage (stage III). Pyrite from the quartz-polymetallic sulfide-gold vein yielded Re-Os isochron age of 121.1 ± 1.2 Ma (Initial 187Os/188Os = 1.49 ± 0.01, MSWD = 1.2), which is interpreted to be the ore-formation age.Theδ13CV-PDB and 18Ov-SMO W values of calcite from stage III range from −5.1‰ to −4.9‰ and from 9.7‰ to 10.1‰, and the δ30Si values of quartz from stage Ⅰto stage III range from −0.5‰ to −0.4‰, −0.1‰ to −0.3‰, and −0.2‰ to −0.1‰, respectively, indicating that ore-forming fluids were derived from a magmatic-hydrothermal source. Sulfur isotopic compositions of sulfides between −1.9 ‰ and 4.4 ‰ (mean = 2.4 ‰), are consistent with that of magmatic sulfur. The galena yielded a wide range of lead isotopic composition (206Pb/204Pb = 16.995–17.252, 207Pb/204Pb = 15.335–16.343, 208Pb/204Pb = 37.461–37.580) suggesting an external lead source. The He and Ar isotope compositions of pyrite and galena (3He/4He = 0.14Ra-0.62Ra), indicate a crustal source with minor mantle contributions. Combining all available data results, we propose that a magmatic -hydrothermal source for the Xinfang gold deposit and the ore formation was related to the subduction of the Paleo-Pacific Plate beneath Eurasia.

Geochronology and S–Pb–O–H isotopic constraints on the generation of the Luoyang Fe deposit in southwest Fujian Province, SE China

AbstractThe Luoyang Fe polymetallic deposit is a well‐known Makeng‐type ore deposit in a late Paleozoic basin in southwest Fujian, southeast China. To investigate the generation of Makeng‐type Fe deposits, we conducted an integrated study of geochronology and S–Pb–O–H isotope compositions of the Luoyang Fe deposit. The LA–ICP–MS zircon U–Pb ages of the granite and Re–Os ages of the molybdenite suggest that the emplacement of the granite was coeval with the mineralization of the Luoyang deposit at 133–131 Ma. The H–O and S–Pb isotope compositions indicate that the metallogenic material of the Luoyang deposit had a magmatic hydrothermal source, and was derived mainly from the upper crust with a low degree of contamination of mantle material. The Pb isotope analyses of the ore minerals show that the Luoyang Fe deposit formed in an orogenic setting.

Geology and genesis of the Early Paleozoic Zhengguang intermediate-sulfidation epithermal Au-Zn deposit, northeast China

Zhengguang is a partly eroded (about ≥500 m being eroded), structurally controlled intermediate sulfidation (IS) epithermal gold-zinc deposit, characterized by wide distribution of base metal sulfides, Fe-poor sphalerite and Mn-calcite, hosted by the Late Cambrian-Early Ordovician basaltic-andesitic volcanic and volcano-sedimentary rocks in northeast China. The deposit is located 4 and 8 km from the Early Ordovician Tongshan (including several IS veins about ~500 m away in the east called East Tongshan) and Duobaoshan porphyry copper deposits, respectively. A 484.8 ± 8.7 Ma (MSWD = 198) of Re-Os isochron age on pyrite indicates that it is the most ancient intermediate sulfidation deposit that has been dated.Four stages of veins have been discerned at Zhengguang. Early magnetite ± chalcopyrite ± epidote ± chlorite and K-feldspar ± pyrite ± chalcopyrite veins (stage I) were associated with restricted K-feldspar ± epidote (locally with chlorite) alteration and propylitic (epidote ± chlorite ± calcite) assemblages. Two primary epithermal gold stages developed at Zhengguang: pyrite-quartz ± Fe-poor sphalerite ± chalcopyrite ± galena ± electrum ± petzite ± hessite ± tetrahedrite/tennantite ± adularia veins (stage II) and (Mn-)calcite-quartz-pyrite ± Fe-poor sphalerite ± chalcopyrite ± galena ± hessite ± native gold (stage III), plus a late barren stage characterized by little sulfides (calcite ± chlorite ± hematite ± quartz, stage IV). Gold occurs chiefly as micro-inclusions of electrum and petzite in pyrite and sphalerite of stages II and III.Stable isotope analyses and fluid inclusion results indicate a magmatic-hydrothermal component in the ore fluids. Fluid inclusion data from stage I yielded the highest salinity (∼6.9 wt% NaCl) and homogenization temperature (average 264 °C). Calculated oxygen isotope composition of fluid (δ18Owater up to 4.9‰) equilibrated with quartz indicates a magmatic-hydrothermal source during stage I. A progressive addition of meteoric water component is suggested from stage I to IV of Zhengguang based on the average δ18Owater values equilibrated with quartz and homogenization temperatures (stage I: δ18Owater = 3.3‰, Th = 264 °C; stage II: δ18Owater = −1.4‰, Th = 182 °C; stage III: δ18Owater = −3.4‰, Th = 154 °C; stage IV: δ18Owater = −6.3‰, Th = 121 °C).Our new pyrite Re-Os dating result shows that Zhengguang formed at the same time with Tongshan and Duobaoshan within errors. Comparable sulfur isotopic compositions of pyrite from Zhengguang and Tongshan, analogous low-Fe content in sphalerite and similar mineral assemblage and alteration pattern between Zhengguang and East Tongshan IS veins collectively indicate a genetic connection between Zhengguang and Tongshan, which may derive from fluids exsolved from the same magma chamber. High-temperature assemblages (e.g., K-feldspr ± pyrite ± chalcopyrite) at Zhengguang indicate that there is potential for the discovery of porphyry copper–gold mineralization below the current level of diamond drilling. The confirmation of the Zhengguang as an Early Paleozoic epithermal deposit implies a good preservation and exploration potential of ancient epithermal deposits in eastern Central Asian Orogenic Belt.

Ore genesis of skarn mineralization in continental collision orogens: A case study from the Pusangguo Co-bearing Cu–Pb–Zn deposit in Tibet

The Gangdese belt in southern Tibet, is renowned for hosting numerous collision-related porphyry Cu-Mo deposits. The recent discovery of Pusangguo Co-bearing Cu–Pb–Zn skarn deposits is the first reported Co-bearing deposit in Gangdese belt as well as in Tibet. Researches on the geological and metallogenic characteristics of Pusanguo indicate that this Co-bearing Cu-Pb-Zn deposit was formed in a collisional setting. We document mineralogy, fluid inclusion studies, and H–O–S–Pb isotope systematics of the skarns and ores of the Pusangguo deposit. Intense endoskarn alteration, dominated by green and pink epidote and quartz, is widely disseminated throughout the host granodiorite as well as in veinlets. Exoskarn alteration is characterized by the presence of prograde grandite (andradite32–100 and grossular0–68) with minor pyroxene (diopside0–46, hedenbergite38–74, and johannsenite1–62). Retrograde epidotes from both the endoskarn and exoskarn show a narrow range of Fe3+/(Fe3+ + Al) ratios (0.25–0.34). Ore minerals include chalcopyrite, sphalerite and galena, with lesser pyrite, pyrrhotite, bornite, Co minerals and falkmanite.Four paragenetic stages of skarn formation and ore deposition are recognized in the Pusangguo Cu–Pb–Zn deposit: prograde (stage I), retrograde (stage II), early sulfide (stage III), and late sulfide (stage IV). Fluid inclusions in all stages are dominated by two-phase liquid-rich (type L) inclusions. During the major phase of mineralization (stage III), fluid inclusions formed at low temperatures (180–240 °C) and with low salinities (0.5–8.6 wt% NaCl equiv.). This is in contrast to other skarn deposits within the Gangdese porphyry copper belt (e.g., the Zhibula, Jiama, and Bangpu deposits) that formed from highly saline fluids at high temperatures. Hydrogen-Oxygen isotopic values obtained from garnet, epidote, quartz and calcite are δDH2O = −179‰ to − 213‰ and δ18OH2O = −9.1‰ to 9.3‰, whereas S-Pb isotopic values for sphalerite, chalcopyrite, pyrite and galena are δ34S = −1.0‰ to 1.2‰, estimated δ34Sfluid = −0.05‰. Such data are indicative of ore-forming fluids and metals that originated from a magmatic–hydrothermal source related to granodiorite emplacement during the Miocene post-continental collision. Extensive mixing between the magmatic-hydrothermal and meteoric fluids led to the co-precipitation of Cu, Pb and Zn minerals from low-salinity fluids at relatively low temperatures. The Pusangguo Co-bearing Cu-Pb-Zn skarn deposit shows consistent features with other (porphyry-related) skarn deposits in the Gangdese belt, except for relatively low temperature and salinity. The research is of great significance for understanding cobalt as an associated mineral in the Gangdese sulfide skarn deposit in Tibet.

Geochemical Characteristics of Mineral Assemblages from the Yamansu Iron Deposit, NW China, and Their Metallogenic Implications

The Yamansu deposit, which is hosted in the volcanic-sedimentary sequence of the Carboniferous Yamansu Formation in Eastern Tianshan, NW China, contains many skarns, and the orebodies occur in the ore district in stratoidal, banded or lenticular forms. Four alteration stages, namely, albite–tourmaline–apatite–Grt1 (Stage I), K-feldspar–Grt2 (Stage II), magnetite–chlorite–epidote (Stage III), and quartz–calcite–axinite–Grt3 (Stage IV), are distinguished in the Yamansu deposit. The mineral geochemistry associated with each different stage is presented to provide a better understanding of the corresponding metallogenic processes. The ore-forming fluid in Stage I was derived from a magmatic–hydrothermal source and formed at high temperatures with many volatiles. This ore-forming fluid, which contained considerable metallogenic materials during the early stage, likely experienced diffusive metasomatism in a closed system with low water/rock (W/R) ratios. Mineral geochemical analyses show that the Fe content gradually increases from Stage I to Stage II, indicating that accumulated ore-forming materials were available during changes in the physicochemical conditions from a reducing environment with neutral pH to oxidizing conditions with mildly acidic pH. During the main metallogenic stage (Stage III), mineral assemblages reflect moderate- to high-temperature conditions, and the ore-forming fluid was created and destroyed periodically; the magnetite ores were deposited in a fluctuating fluid system. The multilayered orebodies, multigenerational garnets, and minerals with oscillatory zoning indicate that the ore-forming fluid may have developed periodic fluctuations, and this special multistage fluctuation of the hydrothermal fluid in the Yamansu deposit was the key factor controlling the multiple extraction, enrichment and precipitation of metallogenic materials.

Geology, fluid inclusions and isotope geochemistry of the Herenping gold deposit in the southern margin of the Yangtze Craton, China: A sediment-hosted reduced intrusion-related gold deposit?

The Herenping gold deposit is a typical gold deposit in the Xuefengshan arcuate tectonic zone in the southern margin of the Yangtze Craton, and the major feature of the gold mineralisation in the zone is the varying degrees of tungsten and antimony mineralisation in the ore deposits. The Herenping gold deposit occurs in the reddish-purple sandy slate in the middle section of the Madiyi Formation of the Neoproterozic Banxi Group and is controlled by the fault-fold zone associated with the Liulincha anticlinorium and F8 fault zone. However, its sources of mineralisation and relationships between hydrothermal fluids and magmatic rocks are unclear, which similar to those of many gold deposits in metasedimentary terranes in the worldwide.The mineralisation can be divided into four stages: an albite-quartz stage (I), an albite-quartz-pyrite-gold stage (II), a quartz-gold-polymetallic sulphide stage (III), and a quartz-carbonate stage (IV). The gold mineralisation occurred in albite-quartz veins in stage II and in sulphide-rich quartz veins in stage III in the deposit. A study of the fluid inclusions reveals that the homogenisation temperature and salinity were slightly higher in stage II (Th., 186–348 °C; W., 5.2–17.43 wt% NaCl equiv.) than in stage III (Th., 145–249 °C; W., 2.9–15.57 wt% NaCl equiv.), indicating that the temperature and salinity of the ore-forming fluid are gradually decreasing. The H-O (δD = −62.3 to −53.0‰; δ18O = 4.2–11.8‰) isotope characteristics reveals that the ore-forming fluid is a magmatic hydrothermal source. The sulphur (δ34S, −4.8–2.0‰) isotope characteristics and the alteration mineral assemblage (sericite-chlorite-pyrite) indicates that the ore-forming fluids was reduced. The lead (206Pb/204Pb = 17.419–17.436, 207Pb/204Pb = 15.519–15.535, and 208Pb/204Pb = 37.86–37.90) isotope characteristics reflect the ore-forming materials were likely derived from a deep magmatic source. In addition, the geochemical characteristics of albitite veins show that they are silicon rich (64.02–66.26 wt%) and significantly reduced. Notably, the Herenping gold deposit is located in the western portion of the tungsten/tin metallogenic provinces in South China, is host in epimetamorphic detrital rocks and was formed at least two stages of mineralisation; typical gold-bearing albite-quartz veins developed during the primary stage. These characteristics are consistent with those of reduced intrusion-related deposits such as the Muruntau, Kumtor and Telfer deposits. Thus, the Herenping gold deposit may be a sediment-hosted intrusion-related gold deposit. Based on the phase transition pattern from sulphide-rich quartz veins to albite-quartz veins to albitite veins with increasing depth in the deposit, we propose that reduced intrusions deep in the Herenping gold deposit could be discovered in the future, which implies the possible existence of a reduced intrusion-related gold system in the Xuefengshan arcuate tectonic zone.

Isotopic geochemistry of the Jinwozi gold deposit in the eastern Tianshan orogen, NW China: implications for the ore genesis

The Jinwozi gold deposit in the eastern Tianshan orogenic belt in northwest China has been mined for more than 100 years and previous studies have mostly classified it as an intrusion‐related gold deposit. In this paper, the age of the Jinwozi intrusion and the isotopic geochemistry characteristics of the deposit were studied to investigate the ore genesis and ore genetic type. Results showed that: (1) The Jinwozi pluton, which is the host rock of the orebodies, was emplaced during 395–390 Ma, thus substantially predating gold mineralization (243–228 Ma); (2) δ18O values of fluid in the main mineralized stage (stage II) vary from −0.9 to 5.0‰, with an average of 2.0‰. Those of late quartz–calcite stage (stage III) fluids are between −13.3 and 0.4‰, with an average of −7.6‰. δD values of fluids in stage II vary from −97 to −84‰, with an average of −89‰, slightly higher than those of stage III fluids (−85 to −120‰, average −108‰). δ13C values of the ore‐forming fluid in stage II vary from −0.3 to 3.2‰, with an average of 1.3‰. δ34S values of pyrite from both stage II and III are generally high, ranging from 3.9 to 7.7‰, with an average of 6.0‰. Lead isotopic compositions of sulphides from stage II and III are similar, 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios have ranges from 17.7780 to 18.4750, 15.4881 to 15.6965, 38.1560 to 38.7370, respectively. Two Pb isotope model ages of 450 and 268 Ma were yielded from these data. The oxygen, hydrogen, carbon, sulphur and lead isotope compositions all indicate that the ore‐forming fluids at Jinwozi may dominantly derive from a mixture of metamorphic and meteoric water, but is unlikely from a magmatic–hydrothermal source. Based on the geological and geochemical evidence, we infer that the Jinwozi gold mineralization may have occurred during the collision between the Kazakhstan–Junggar Plate and the Tarim Plate, accompanied by metamorphism which generated the ore‐forming fluids through the dehydration and decarbonation of country rocks. Such processes, combining with other geological and geochemical characteristics, indicate that the Jinwozi gold deposit can be classified as orogenic type. Copyright © 2014 John Wiley &amp; Sons, Ltd.

The role of granites in volcanic-hosted massive sulphide ore-forming systems: an assessment of magmatic–hydrothermal contributions

Assessment of geological, geochemical and isotopic data indicates that a significant subgroup of volcanic-hosted massive sulphide (VHMS) deposits has a major or dominant magmatic–hydrothermal source of ore fluids and metals. This group, which is typically characterised by high Cu and Au grades, includes deposits such as those in the Neoarchean Doyon-Bousquet-LaRonde and Cambrian Mount Lyell districts. These deposits are distinguished by aluminous advanced argillic alteration assemblages or metamorphosed equivalents intimately associated with ore zones. In many of these deposits, δ34Ssulphide is low, with a major population below −3‰; δ34Ssulphate differs from coexisting seawater and Δ34Ssulphate–sulphide ∼ 20–30‰. These characteristics are interpreted as the consequence of disproportionation of magmatic SO2 as magmatic–hydrothermal fluids ascended and cooled and as a definitive evidence for a significant magmatic–hydrothermal contribution. Other characteristics that we consider diagnostic of significant magmatic–hydrothermal input into VHMS ore fluids include uniformly high (>3 times modern seawater values) salinities or very 18O-enriched (δ18O > 5‰) ore fluids. We do not consider other criteria [e.g. variable salinity, moderately high δ18Ofluid (2–5‰), δ34Ssulphide near 0‰, metal assemblages or a spatial association with porphyry Cu or other clearly magmatic-hydrothermal deposits] that have been used previously to advocate significant magmatic–hydrothermal contributions to be diagnostic as they can be produced by non-magmatic processes known to occur in VHMS mineral systems. However, in general, a small magmatic–hydrothermal contribution cannot be excluded in most VHMS systems considered. Conclusive data that imply minimal magmatic–hydrothermal contributions are only available in the Paleoarchean Panorama district where coeval seawater-dominated and magmatic–hydrothermal systems appear to have been physically separated. This district, which is characterised by chloritic and sericitic alteration assemblages and lacks aluminous advanced argillic alteration assemblages, is typical of many VHMS deposits around the world, suggesting that for “garden variety” VHMS deposits, a significant magmatic–hydrothermal contribution is not required. Other than deposits associated with advanced argillic alteration assemblages, the only deposit for which we ascribe a major magmatic–hydrothermal contribution is the Devonian Neves Corvo deposit. This deposit differs from other deposits in the Iberian Pyrite Belt and around the world in being extremely Sn-rich, with the Sn closely associated with Cu and in having formed from high 18O-rich fluids (δ18Ofluid ∼8.5‰). We consider these characteristics, particularly the last, as diagnostic of a significant magmatic hydrothermal contribution. Our analysis indicates that two subgroups of VHMS deposits have a major magmatic–hydrothermal contribution: Cu/Au-rich deposits with aluminous alteration assemblages and reduced, very Sn-rich deposits in which Sn was introduced in a high-temperature ore assemblage. Comparison with “normal” VHMS deposits suggests that these subgroups of VHMS deposits may form in specialised tectonic environments. The Cu/Au-rich deposits appear to form adjacent to magmatic arcs, an environment conducive to the generation of hydrous, oxidised melts by melting metasomatised mantle in the wedge above the subducting slab. This contrasts with the back-arc setting of “normal” VHMS deposits in which relatively dry granites (In this contribution, we use the term granite sensu latto) formed by decompression melting drive seawater-dominated hydrothermal circulation. The tectonic setting of highly Sn-rich VHMS deposits such as Neves Corvo is less clear; however, thick continental crust below the ore-hosting basin may be critical, as it is in other Sn deposits.

Focused and diffuse effluxes of CO 2 from mud volcanoes and mofettes south of Mt. Etna (Italy)

Several sites with anomalous emissions of carbon dioxide were investigated in the region south of Mt. Etna volcano in order to assess the types of emission (focused and/or diffuse), their surface extension and the total output of CO 2. Most of the studied emissions are located on the southwest boundary of Mt. Etna, near the town of Paternò. They consist of three mud volcanoes (known as Salinelle), one spring with bubbling gas (Acqua Grassa) and one area of diffuse degassing (Peschería). Another site (Naftía Lake) with remarkable gas emissions (bubbling gas into a lake as well as adjacent areas of diffuse soil degassing) is located further southwest of Mt. Etna in an area of extinct Quaternary volcanism on the northwest margin of Hyblean Mts. In all of these areas the origin of the highest CO 2 emissions is clearly magmatic, and degassing to the atmosphere occurs mostly through tectonic structures, probably at a regional scale. The magmatic source that feeds anomalous degassing in the above areas is likely to be the same that feeds volcanic activity at Mt. Etna. Focused degassing was measured at each emission vent using devices that measure the air speed, whereas diffuse soil degassing was measured using the accumulation chamber method. In total, 712 measurements were carried out (146 in focused degassing vents, 566 on diffuse degassing areas). Single CO 2 output values ranged from 1.8 10 − 5 to 1.68 kg s − 1 . In the case of diffuse degassing areas, statistical analyses allowed to discriminate between biogenic CO 2 and CO 2 deriving from a magmatic-hydrothermal source. Only the efflux values from the latter source were considered in the output estimates. The total estimated output thus obtained was about 2.61 kg s − 1 , relevant to a total surface of about 146,500 m 2 (which includes only the magmatic CO 2 emissions). This value is comparable with that of most non-volcanic emissions from geothermal and/or faulted areas of central-southern Italy, as well with the CO 2 output from some of the volcanic areas of Italy.

Mineralogical and stable isotope studies of gold–arsenic mineralisation in the Sams Creek peralkaline porphyritic granite, South Island, New Zealand

At Sams Creek, a gold-bearing, peralkaline granite porphyry dyke, which has a 7 km strike length and is up to 60 m in thickness, intrudes camptonite lamprophyre dykes and lower greenschist facies metapelites and quartzites of the Late Ordovician Wangapeka formation. The lamprophyre dykes occur as thin (< 3 m) slivers along the contacts of the granite dyke. δ18Omagma values (+5 to +8‰, VSMOW) of the A-type granite suggest derivation from a primitive source, with an insignificant mature crustal contribution. Hydrothermal gold–sulphide mineralisation is confined to the granite and adjacent lamprophyre; metapelite country rocks have only weak hydrothermal alteration. Three stages of hydrothermal alteration have been identified in the granite: Stage I alteration (high fO2) consisting of magnetite–siderite±biotite; Stage II consisting of thin quartz–pyrite veinlets; and Stage III (low fO2) consisting of sulphides, quartz and siderite veins, and pervasive silicification. The lamprophyre is altered to an ankerite–chlorite–sericite assemblage. Stage III sulphide veins are composed of arsenopyrite + pyrite ± galena ± sphalerite ± gold ± chalcopyrite ± pyrrhotite ± rutile ± graphite. Three phases of deformation have affected the area, and the mineralised veins and the granite and lamprophyre dykes have been deformed by two phases of folding, the youngest of which is Early Cretaceous. Locally preserved early-formed fluid inclusions are either carbonic, showing two- or three-phases at room temperature (liquid CO2-CH4 + liquid H2O ± CO2 vapour) or two-phase liquid-rich aqueous inclusions, some of which contain clathrates. Salinities of the aqueous inclusions are in the range of 1.4 to 7.6 wt% NaCl equiv. Final homogenisation temperatures (Th) of the carbonic inclusions indicate minimum trapping temperatures of 320 to 355°C, which are not too different from vein formation temperatures of 340–380°C estimated from quartz–albite stable isotope thermometry. δ18O values of Stage II and III vein quartz range from +12 and +17‰ and have a bimodal distribution (+14.5 and +16‰) with Stage II vein quartz accounting for the lower values. Siderite in Stage III veins have δ18O (+12 to +16‰) and δ13C values (−5‰, relative to VPDB), unlike those from Wangapeka Formation metasediments (δ13Cbulk carbon values of −24 to −19‰) and underlying Arthur Marble marine carbonates (δ18O = +25‰ and δ13C = 0‰). Calculated δ18Owater (+8 to +11‰, at 340°C) and \(\delta^{13}{\text{C}}_{{\rm CO}_{2}}\)(−5‰) values from vein quartz and siderite are consistent with a magmatic hydrothermal source, but a metamorphic hydrothermal origin cannot be excluded. δ34S values of sulphides range from +5 to +10‰ (relative to CDT) and also have a bimodal distribution (modes at +6 and +9‰, correlated with Stage II and Stage III mineralisation, respectively). The δ34S values of pyrite from the Arthur Marble marine carbonates (range from +3 to +13‰) and Wangapeka Formation (range from −4 to +9.5‰) indicate that they are potential sources of sulphur for sulphides in the Sams Creek veins. Another possible source of the sulphur is the lithospheric mantle which has positive values up to +14‰. Ages of the granite, lamprophyre, alteration/mineralisation, and deformation in the region are not well constrained, which makes it difficult to identify sources of mineralisation with respect to timing. Our mineralogical and stable isotope data does not exclude a metamorphic source, but we consider that the source of the mineralisation can best be explained by a magmatic hydrothermal source. Assuming that the hydrothermal fluids were sourced from crystallisation of the Sams Creek granite or an underlying magma chamber, then the Sams Creek gold deposit appears to be a hybrid between those described as reduced granite Au–Bi deposits and alkaline intrusive-hosted Au–Mo–Cu deposits.