Contribution Of Magmatic Fluids Research Articles

Magmatic degassing controlled the metal budget of the Axi epithermal gold deposit, China

Abstract From integrated textural and compositional studies of auriferous and barren pyrite/marcasite in the epithermal Axi gold deposit, China, we have identified a relationship between multiple gold mineralizing events, mafic magma recharge, and fluid-rock reactions. Three generations of pyrite (Py1–3) and four generations of marcasite (Mar1–4) record episodic gold mineralizing events, followed by silver-copper-lead-zinc-cadmium enrichment. The gold mineralizing events are recorded by high concentrations of subnanometer-sized gold in Py1, Py3, and Mar3 (max. = 147, 129, and 34 ppm, med. = 39, 34, and 12 ppm). Based on previous Re-Os age determinations of pyrite and U-Pb zircon ages of the andesitic wallrock, these gold events slightly postdate pulsed mafic magma recharge and represent the incursion of Au-As-S-rich magmatic volatiles into circulating meteoric water. Silver-Cu-Pb-Zn-Cd enrichment in Py2, Mar2, and Mar4 are consistent with quiescent degassing and gradual Ag-Cu-Pb-Zn-Cd enrichment in an evolved felsic magma. Barren Mar1 records the dominance of meteoric water and a limited magmatic fluid contribution. High-Co-Ni-V-Cr-Ti contents in porous cores of Py1 and Mar2 are attributed to wall rock alteration and dissolution-reprecipitation. The results provide convincing evidence that the metal budget (especially for Au, Ag, Cu, Pb, Zn, Sb) of the hydrothermal fluids and sulfides in epithermal systems are controlled by the influx of magmatic fluids and associated magma, whereas the enrichment of certain fluid-immobile elements, such as Co, Ni, V, Cr, and Ti, is caused in part by fluid-rock interaction.

Post-collisional magmatic-hydrothermal mineralization in the São Gabriel Terrane (southern Brazil): Insights from mineral chemistry, stable isotopes, and sulfide trace-elements of the Caçapava do Sul skarns

Prograde and retrograde magnesian skarns associated with the Caçapava do Sul Granitic Complex intrusion (central Dom Feliciano Belt, southern Brazil) host Cu–Fe (Mo–Au) mineralization. Vein and contact prograde skarns contain disseminated pyrite, pyrrhotite, molybdenite, and chalcopyrite formed from the interaction of mafic and felsic intrusions and dolomitic marbles at ca. 578 Ma. The prograde skarn mineralization is mainly localized in fold hinges and pinch-and-swell structures. Retrograde skarns host massive chalcopyrite and pyrite with chlorite and calcite associated with brittle structures at ca. 557 Ma. In this contribution, we provide mineral chemistry analyses of magmatic and skarn assemblages, calcite stable isotope analyses, and trace-element characterization of pyrite and chalcopyrite to assess ore-forming processes involved in the formation of the skarns. Amphibole-plagioclase geothermobarometry in granodiorite and monzonite apophyses of the Caçapava do Sul Granitic Complex yielded crystallization pressures between 4 and 5 kbar and temperatures from 680 to 750 °C. Biotite and amphibole composition of granodiorite, monzonite, and garnet-leucogranite apophyses suggest a trend of decreasing oxygen fugacity from early to late intrusions. Calcite-dolomite solvus thermometry in prograde skarns indicates temperatures between 580 and 630 °C whereas chlorite thermometry in retrograde skarn yields lower temperatures between 280 and 310 °C. Calcite from the retrograde skarns shows values of approximately −5.5‰ for δ13CVPDB and ∼9.0‰ for δ18OSMOW, indicating a major contribution of magmatic fluids during the retrograde stage whereas previous studies indicate a mixed igneous-sedimentary character of the calcite in prograde skarns. Our findings propose a multiphase magmatic-hydrothermal origin for the Cu–Fe (Mo–Au) skarn mineralization hosted by the Passo Feio Metamorphic Complex. The prograde and retrograde magnesian skarns of Caçapava do Sul were formed in the same age interval (578 - 557 Ma) as many other magmatic-hydrothermal base and precious metals mineralization in the São Gabriel Terrane. Hence, we suggest that the studied skarns belong to a multi-intrusive Cu-porphyry system in the São Gabriel Terrane formed during the post-collisional stage of the Dom Feliciano Belt.

Trace-element compositions of sulfides from inactive Tianzuo hydrothermal field, Southwest Indian Ridge: Implications for ultramafic rocks hosting mineralization

The recently explored inactive Tianzuo hydrothermal field, in the amagmatic segment of the ultraslow-spreading Southwest Indian Ridge (SWIR), is closely associated with detachment faults. In this site, sulfide minerals are hosted by serpentine-bearing ultramafic rocks and include high-temperature (isocubanite, sphalerite, and minor pyrrhotite) and low-temperature (pyrite I, marcasite, pyrite II, and covellite) phases. In this study, trace-element concentrations of isocubanite and pyrite II were used to elucidate mineralization processes in ultramafic rocks hosting sulfides. Results show that isocubanite is enriched in metals such as Cu, Co, Sn, Te, Zn, Se, Pb, Bi, Cd, Ag, In, and Mn, and pyrite II is enriched in Mo and Tl. The marked enrichment in Te, Cu, Co, and In in isocubanite (compared with Se, Zn, Ni, and Sn, respectively) is most likely due to the contribution of magmatic fluids from gabbroic intrusions beneath the hydrothermal field. The intrusion of gabbroic magmas would have enhanced serpentinization reactions and provided a relatively oxidizing environment through the dissolution of anhydrite precipitated previously in the reaction zone, within high temperature and low pH conditions. This might have facilitated the extraction of metals by initial hydrothermal fluids, leading to the general enrichment of most metals in isocubanite. Metals in pyrite II have compositions similar to those of isocubanite, except for strong depletion in magmatically derived Te, Cu, Co, and In. This means that serpentinization processes had a dominating role in pyrite II precipitation as well. The enrichment of pyrite II in Mo and Tl is also indicative of seawater contribution in its composition. The study concludes that serpentinization reactions contribute effectively both to high- and low-temperature sulfide mineralization at Tianzuo hydrothermal field, with gabbroic intrusions further promoting high-temperature sulfide mineralization, providing additional metals, fluids and heat. In contrast, low-temperature sulfide mineralization occurred during the cooling of gabbroic intrusions, with decreasing rates of serpentinization reactions and a significant influence of seawater.

Geochemical and isotopic evidence of volcanic plumbing system processes from fumarolic gases of Taal volcano, Philippines, prior to the January 2020 eruption

Significant changes in the chemical and isotopic composition of Taal fumarolic gas have been recorded observed before the January 2020 eruption, during the periods 2011 and 2016–2018. Increasing CO2/St, He/CO2, CO/CO2 and CO2/CH4 ratios were recorded during 2011 whereas increasing SO2/H2S, H2/CO2 ratios were recorded during the period 2017–2018. A decreasing on the CO2/CH4 and CO2/St ratios was observed for 2017–2018. These changes are attributed to an increased contribution of magmatic fluids to the hydrothermal system in both periods. Observed changes in H2 and CO contents suggest increases in temperature and pressure in the upper parts of the hydrothermal system of Taal volcano. The 3He/4He ratios corrected (Rc/Ra), and δ13C of fumarolic gases also increased during the periods 2010–2011 and 2017–2018 before the eruption onset. These variations were produced by two episodes of magmatic intrusion which favored degassing of a gas-rich magma at depth. During the 2010–2011 the magmatic intrusion of volatile-rich magma might have occurred from the mid-crustal storage region at shallower depths producing important he observed changes in pressure and temperature conditions, whereas a new injection of more degassed magma into the deepest zone of the hydrothermal system occurring in 2017–2018 might have favored the accumulation of gases in the subsurface, promoting conditions leading to a phreatic eruption.

Sulfur and copper isotopic composition of seafloor massive sulfides and fluid evolution in the 26°S hydrothermal field, Southern Mid-Atlantic Ridge

In-situ sulfur isotopic compositions of morphologically different pyrites from the SMAR (southern Mid-Atlantic Ridge) 26°S hydrothermal field, integrated with δ34S and δ65Cu values in associated chalcopyrite, reflect a recurring flux of magmatic-hydrothermal fluids and subsequent leaching by seawater. The δ34S values of sulfides (−1.27 to +5.25‰) indicate mixing of magmatic sulfur with seawater sulfate-derived sulfur. Compared to other Mid-Ocean Ridge (MOR) hydrothermal vent sites, a significant contribution of magmatic fluids is herein distinctive. Three concentric mineralogical zones around the conduit in an inactive black smoker sample include an inner chalcopyrite-dominated zone, an intermediate zone of idiomorphic and massive pyrite, and an outermost zone of porous and colloform pyrite with/without traces of chalcopyrite. The systematic increase in δ34S from the interior conduit (avg. −0.60‰, n = 8) through the intermediate zone (avg. +2.36‰, n = 14) to the exterior wall (avg. +3.02‰, n = 27), and contributions of seawater sulfate (0 to 10.7%, and to 18.6%, respectively) are most likely related to interaction between hydrothermal fluid and previously precipitated sulfides. A significant contribution of seawater sulfate-derived sulfur occurred likely via permeability or porosity upsurge across the chimney. Chalcopyrite with consistently positive δ65Cu values (+0.17 to +0.48‰), which are typical of mid-ocean ridge (MOR) hydrothermal vent fluids, is suggestive of reaction/mixing with hydrothermal fluids. Sulfur and Cu isotopic compositions of chalcopyrite further imitate the significant contributions of magmatic gases in the formation of metalliferous seafloor massive sulfides.

Mineralogy, geochemistry, and Sr–Pb and in situ S isotopic compositions of hydrothermal precipitates from the Tangyin hydrothermal field, southern Okinawa Trough: Evaluation of the contribution of magmatic fluids and sediments to hydrothermal systems

The Okinawa Trough (OT), is a back-arc basin where rifting is in its initial stages, and where hydrothermal fields (HTFs) have developed above terrigenous sediments and volcaniclastics layers of variable thickness. The source of ore-forming materials within the hydrothermal systems of the OT is complex, and may include seawater, magmatic rock, magmatic fluid, and sediments. The contribution of magmatic fluids and sediments to the hydrothermal systems of the OT is still uncertain. At least 10 modern seafloor HTFs have been reported from the OT, including the Jade, Hakurei, Iheya North Knoll, CLAM, Tangyin, Yonaguni Knoll IV, Hatoma, and Irabu. Here, we report results of mineralogical, geochemical, and whole-rock Sr–Pb and in situ S isotopic analyses of hydrothermal precipitates from the Tangyin HTF in the southern OT. This field was discovered during the HOBAB3 cruise in 2014 and contains four types of hydrothermal precipitates: Fe–Zn- and Zn–Pb-rich massive sulfides, silica-rich precipitates, and native sulfur-rich samples. Sulfates in Zn–Pb-, Fe–Zn-, and silica-rich samples have 87Sr/86Sr ratios of 0.71068–0.71750, which are higher than those of modern seawater and magmatic rocks, and indicate that the Sr is derived mainly from seawater and sediments. The Zn–Pb-, Fe–Zn-, and silica-rich samples have 206Pb/204Pb (18.5129–18.5307), 207Pb/204Pb (15.6374–15.6489), and 208Pb/204Pb (38.8759–38.9113) ratios that are intermediate between sediments and magmatic rocks, suggesting the Pb is a mixture of sediment- and magmatic-rock-derived Pb. The in situ S isotopic data indicate that the ore-forming fluids associated with all sample types had low δ34S values, with the Fe–Zn- (mean δ34S = 1.27‰ ± 1.38‰; SD, n = 12) and Zn–Pb-rich (mean δ34S = 0.93‰ ± 0.32‰; n = 36) massive sulfides, and silica-rich precipitates (mean δ34S = 2.85‰ ± 1.14‰; n = 58) having lower δ34S values than those of native sulfur-rich samples (mean δ34S = 4.23‰ ± 0.72‰; n = 5). The Zn–Pb- and Fe–Zn-rich massive sulfides, and silica-rich precipitates represent the medium–high temperature (>120 °C) early–middle stages of mineralization, formed under reducing conditions at low sulfur fugacity (fS2), based on the occurrence of low-fS2 mineral assemblages (e.g. high-Fe sphalerite ± isocubanite ± pyrrhotite). The low δ34S values of these three types of samples, and of the Hakurei and CLAM HTFs with low-fS2 mineral assemblages and low concentrations of Cu, Bi, and Au, most likely reflect the incorporation of biological sulfur previously formed by MSR within the sediments. The native sulfur-rich samples of the Tangyin HTF formed at low temperatures (<120 °C), and have slightly higher δ34S values than the Fe–Zn-, Zn–Pb-, and silica-rich samples, may reflect the incorporation of biological sulfur formed mainly by MSR during hydrothermal flow in a seawater recharge zone. Some sulfates in the Jade and Iheya North Knoll HTFs have δ34S values lower than that of modern seawater; this, in combination with high-fS2 oxidizing hydrothermal fluids, and high concentrations of Cu and Au in sulfide samples, indicates a magmatic fluid contribution to these HTFs.

Geochemistry of gas and water discharge from the magmatic-hydrothermal system of Guallatiri volcano, northern Chile

This work presents the first chemical and isotopic (δ13C-CO2, δ13C-CH4, 3He, 4He, 20Ne, 40Ar, 36Ar, δ18O, and δD) data for fluid discharges from Guallatiri volcano, a remote and massive stratovolcano, which is considered as the second most active volcano of the Central Volcanic Zone (CVZ) in northern Chile. Fumarolic gases had outlet temperatures of between 80.2 and 265 °C, and showed a significant magmatic fluid contribution marked by the occurrence of SO2, HCl, and HF that are partially scrubbed by a hydrothermal aquifer. The helium isotope ratios (< 3.2) were relatively low compared to those of other active volcanoes in CVZ, possibly due to contamination of the magmatic source by 4He-rich crust and/or crustal fluid addition to the hydrothermal reservoir. Geothermometry in the H2O-CO2-CO-H2-CH4 system suggests equilibrium temperatures of up to 320 °C attained in a vapor phase at redox conditions intermediate between those typical of hydrothermal and magmatic environments. Thermal springs located 12 km northwest of the volcano’s summit had outlet temperatures of up to 50.1 °C, neutral to slightly basic pH, and a sodium bicarbonate composition, typical of distal fluid discharges in volcanic systems. Cold springs at the base of the volcanic edifice, showing a calcium sulfate composition, were likely produced by interaction of shallow meteoric water with CO2- and H2S-rich gases. A geochemical conceptual model was constructed to graphically represent these results, which can be used as an indication for future geochemical monitoring and volcanic hazard assessment.

Paroxysmal eruptions tracked by variations of helium isotopes: inferences from Piton de la Fournaise (La R\xe9union island)

Helium (He) with its isotopes (3He, 4He) is a key tracer enabling the Earth’s mantle and dynamics to be characterized. Enrichment in primordial helium (3He) has been detected in volcanic gases of numerous magmatic systems in different geodynamic settings. Despite past use to monitor volcano-tectonic unrest, temporal 3He/4He variability in volcanic emissions is still poorly constrained. Here, we investigate noble gas chemistry of Piton de la Fournaise hotspot volcano, where temporal fluctuations of 3He/4He in response to the eruptive activity have never been studied. We compare the 3He/4He signature of volcanic gases and fluid inclusions and we highlight analogous evolution of the 3He/4He signature in both during the last decades of eruptive activity (1990–2017), even during the same eruption. We show that the maximum enrichment in 3He is found in magmatic fluids that fed the most voluminous eruptions which culminated in caldera collapse events. We argue that this enrichment in 3He mostly reflects a greater contribution of magmatic fluids from a primitive component of the mantle plume. These results emphasize that He isotopes may provide warnings of increases in deep magmatic contributions that potentially herald paroxysmal eruptions, as documented here at Piton de la Fournaise (2007) and also at Kilauea (2018).

Deciphering fluid flow at the magmatic-hydrothermal transition: A case study from the world-class Panasqueira W–Sn–(Cu) ore deposit (Portugal)

Deciphering the behavior of fluid flow at the magmatic-hydrothermal transition is crucial to understanding the physical processes leading to the formation of intrusion-related ore deposits and hence to predicting the locations of mineralized bodies. However, the hydrodynamics (direction and velocity) of the hydrothermal fluid flows related to this transitional stage remains poorly constrained. Here we present a coupled textural and chemical study performed on tourmaline growth bands to constrain fluid flow during the initiation of the hydrothermal system of the Panasqueira W–Sn–(Cu) deposit. This exceptional deposit consists of a dense network of flat wolframite and cassiterite-bearing quartz veins intensely developed above a well-known but hidden greisen cupola. The W–Sn mineralization is preceded by a tourmalinization stage occurring as metasomatic halos around veins and as selvages developed along the vein–wall-rocks contacts. Results emphasize the key role of the greisen cupola in fluid focusing and the role of fluid overpressure during vein opening. Velocity values highlight an efficient transport of metals in veins largely dominated by advective processes (10−4 to 10−3 m s−1), whereas fluid flow in the altered wall-rocks is slow and pervasive (10−6 to 10−5 m s−1), suggesting that element transport through the metasedimentary host rock was low and limited to the alteration haloes. LA-ICP-MS analyses of tourmaline growth bands reveal that fluids coming from the cupola are enriched in Na, K, Li, Sr and Sn, emphasizing the contribution of magmatic fluids during vein formation and the metasomatic alteration of the wall-rocks. More generally, this study demonstrates that the apical portions of granite bodies act as emanative centers of mineralizing fluids, and highlights the usefulness of mineral growth band analysis in the search for intrusion-related mineralization.

Formation of epithermal Sn-Ag-(Zn) vein-type mineralization at the Pirquitas deposit, NW Argentina: Fluid inclusion and noble gas isotopic constraints

The Pirquitas Sn-Ag-(Zn) deposit in northwestern Argentina is thought to be an analogue to the Miocene polymetallic epithermal Sn-Ag deposits of the southern Bolivian Tin Belt, but little is known in detail about the origin and evolution of ore-forming fluids at Pirquitas. This paper reports on a microthermometric study of fluid inclusions in quartz, sphalerite, Ag-Sn sulfides, and Ag-rich sulfosalts using transmitted near infrared and visible light, combined with noble gas isotope analyses of fluids released from mineral separates. The study focused on the vein-hosted mineralization, which formed during two major mineralization events, whereby the first event I comprises two stages (I-1 and I-2). All studied minerals exclusively contain aqueous two-phase inclusions, indicating that the ore-forming fluids did not undergo two-phase phase separation (boiling). Salinity of fluid inclusions in I-1 quartz that precipitated along with pyrite and pyrrhotite ranges between 0 and 7.5 wt% NaCl equiv. and homogenization temperatures (Th) are between 233 and 370 °C. Stage I-2 is characterized by abundant Sn-Ag-Pb-Zn-sulfides and a variety of Ag-rich sulfosalts. Fluid inclusions in stage I-2 Ag-Sn sulfides have salinities up to 10.6 wt% NaCl equiv. and Th between 213 and 274 °C. The deposition of stage I-2 ore is likely related to a new pulse of saline magmatic fluids to the hydrothermal system. The mineralization event II deposited the richest Ag ores at Pirquitas. Colloform sphalerite and pyrargyrite deposited during event II contain two-phase aqueous fluid inclusions with homogenization temperatures between 190 and 252 °C and salinities between 0.9 and 4.3 wt% NaCl equiv.Noble gas concentrations and isotopic compositions of ore-hosted fluid inclusions were determined from crushing hand-picked ore minerals from both mineralization events. With one exception, all samples yielded 3He/4He ratios between 1.9 and 4.1 Ra, which is within the range of published data from the volcanic arc and somewhat higher than typical values of meteoric water-derived hot-springs in the region. This demonstrates a significant contribution of magmatic fluids to the Pirquitas mineralization although no intrusive rocks are exposed in the mine region. Taking the noble gas evidence for a magmatic fluid source, we interpret the trends of decreasing Th and salinity values in fluid inclusions from events I and II to represent waning of the magmatic-hydrothermal system and/or increased admixing of meteoric water to the magmatic fluids.

A possible genetic relationship between orogenic gold mineralization and post-collisional magmatism in the eastern Kunlun Orogen, western China

Controversies in orogenic gold deposits remain, especially regarding the relative contribution of magmatic fluids to the mineralization. The Shuizhadonggou-Huanglonggou gold deposit in the Wulonggou gold field of the eastern Kunlun Orogen, western China consists mainly of sulfide disseminations in hydrothermally altered rocks with minor amounts of quartz-sulfide veins coeval to or overprinting the disseminated ores. Gold mineralization is hosted in diverse lithologies (i.e., Early Palaeozoic metamorphic rocks, Late Silurian syenogranite and Late Triassic porphyritic granodiorite and quartz diorite) and is structurally controlled by a second-order NW-trending sinistral-normal ductile shear zone and its subsidiary structures along the first-order Central Kunlun Fault. Gold-related hydrothermal alteration formed chlorite, sericite, quartz and calcite, with associated sulfide minerals dominated by pyrite, arsenopyrite, löllingite and pyrrhotite. Two types of hydrothermal sericite separates from a gold mineralized syenogranite yielded two 40Ar/39Ar ages of 237.0±2.0Ma and 230.8±1.7Ma, respectively, which postdated the Early Triassic regional metamorphism and ductile deformation by ~13m.y. Field relationships suggest that gold mineralization also overprinted ~220Ma porphyritic granodiorites and quartz diorites. These data indicate three gold mineralization events in the Late Triassic overlapped with post-collisional magmatism in the orogen. Gold-bearing sulfides have δ34S values mostly in the range of 2.0–5.5‰, consistent with a magmatic source of sulfur. Quartz from quartz-pyrite-sericite veins has calculated δ18OH2O values of 1.8–6.1‰ and δDH2O of −63 to −107‰. The stable isotopic and geochronological data suggest a magmatic origin for the ore-forming fluids. Therefore, the Shuizhadonggou-Huanglonggou and other gold deposits in the eastern Kunlun Orogen share many features of orogenic gold deposits (e.g., multiple-order ore-controlling structures, hydrothermal alteration, mineralization styles, close association between mineralization and magmatism, and H-O-S stable isotope compositions) and are genetically related to Late Triassic post-collisional magmatism.

Geochemistry of fluid discharges from Peteroa volcano (Argentina-Chile) in 2010–2015: Insights into compositional changes related to the fluid source region(s)

This study presents the first geochemical data of fluid discharges collected from February 2010 to March 2015 from the Planchón–Peteroa–Azufre Volcanic Complex (PPAVC), located in the Transitional Southern Volcanic Zone (TSVZ) at the border between Argentina and Chile. During the study period, from January 2010 to July 2011, Peteroa volcano experienced phreatic to phreatomagmatic eruption possibly related to the devastating Maule earthquake occurred on February 27, 2010. The compositional dataset includes low temperature (from 43.2 to 102°C) gas discharges from (i) the summit of Peteroa volcano and (ii) the SE flank of Azufre volcano, both marked by a significant magmatic fluid contribution, as well as bubbling gases located at the foothill of the Peteroa volcanic edifice, which showed a chemical signature typical of hydrothermal fluids. In 2012, strong compositional changes affected the Peteroa gases from the summit area: the acidic gas species, especially SO2, increased, suggesting an input of fluids from magma degassing. Nevertheless, the R/Ra and δ13C–CO2 values decreased, which would imply an enhanced contribution from a meteoric-hydrothermal source. In 2014–2015, the chemical and isotopic compositions of the 2010–2011 gases were partially restored. The anomalous decoupling between the chemical and the isotopic parameters was tentatively interpreted as produced by degassing activity from a small batch of dacitic magma that in 2012 masked the compositional signature of the magmatic fluids released from a basaltic magma that dominated the gas chemistry in 2010–2011. This explanation reliably justifies the observed geochemical data, although the mechanisms leading to the change in time of the dominating magmatic fluid source are not clear. At this regard, a geophysical survey able to provide information on the location of the two magma batches could be useful to clarify the possible relationships between the compositional changes that affected the Peteroa fluid discharges and the 2010–2011 eruptive activity.

Using the chemical analysis of magnetite to constrain various stages in the formation and genesis of the Kiruna-type chadormalu magnetite-apatite deposit, Bafq district, Central Iran

Textural and compositional data are presented for different types of magnetite in the Chadormalu iron deposit to discern the genesis of various styles of mineralization. Samples were chosen according to their paragenetic relations to apatite and their host setting: magnetite-apatite veins in the altered host rocks, disseminated magnetite-apatite assemblages in the marginal parts of the main ore body, and massive magnetite associated with irregular apatite veinlets from internal part of the main ore body. Scanning electron microscopy - back scatter electron (SEM-BSE) images reveal that there are three main generations of magnetite in each of the different magnetite-apatite assemblages. Primary magnetite (Mag1) features abundant porosity and a dark appearance. A second generation of magnetite (Mag2) replacing Mag1 shows a lighter appearance with both sharp and gradational contacts with the primary magnetite crystals. The two magnetite types are related to dissolution-precipitation processes due to changing physico-chemical parameters of the ore fluids. A third type of magnetite (Mag3) with a recrystallized appearance and foam-like triple junctions was mostly observed in magnetite-apatite veins in the main ore body and in veins hosted by altered rocks. Electron probe microanalyses (EPMA) were utilized to discriminate the various magnetite generations in the different magnetite-apatite assemblages. Applying published elemental discrimination diagrams shows that most primary magnetites fall into the hydrothermal- and Kiruna-type fields. Primary magnetite contains lower FeO (88.77–93.65 wt.%; average 91.5 wt.%), and higher SiO2 (0.21–2.26 wt.%; ave. 0.32 wt.%), Al2O3 (0.001–0.45 wt.%; ave. 0.053 wt.%), and CaO (0.002–0.48 wt.%; ave. 0.078 wt.%) contents, which might be related to magmatically derived fluids. Secondary magnetites have higher FeO (89.23–93.49 wt.%; ave. 92.11 wt.%), lower SiO2 (0.037–0.189 wt.%; ave. 0.072 wt.%), Al2O3 (0.004–0.072 wt.%; ave. 0.019 wt.%), and CaO (<0.034 wt.%; ave. 0.013 wt.%) possibly showing a lower contribution of magmatic fluids in the formation of Mag2. The magnetite Mag3 contains the highest FeO (91.25–93.8 wt.%; average 92.69 wt.%), low to moderate SiO2 (0.008–1.44 wt.%; ave. 0.13 wt.%), Al2O3 (<0.732 wt.%; ave. 0.059 wt.%), and CaO (<0.503 wt.%; ave. 0.072 wt.%), and appears to have formed by recrystallization of the previous two generations. The different major, minor, and trace element compositions of various magnetite generations might be due to an ore-forming fluid that was initially magmatic-hydrothermal and evolved to moderately brine-dominated meteoric fluids. The involvement of a basinal brine is supported by the occurrence of a late phase 34S-enriched pyrite in the Chadormalu deposit.

Geochemical peculiarities of ores from the largest Natalka gold deposit in Northeastern Russia

This study of the behavior of trace and rare earth elements in ores from the Natalka gold deposit allows us to draw several conclusions. It is suggested that ore formation is related to the regional metamorphism of the host terrigenous carbonaceous rocks, which could be the major source for trace and rare earth elements. Minor enrichment of the Natalka ores in W is evidence of the contribution of magmatic fluid, which could be superimposed on early quartz veins, in ore formation. Our results support the metamorphic–magmatic model of formation of economic gold–quartz deposits of the Yana–Kolyma Belt. The similarity of metasomatites of the Natalka deposit with disseminated gold–sulfide refractory ores from the Nezhdaninskoe and Bakyrchik deposits points to the possible presence of such ores in the Natalka deposit. Our data are important for forecasting regional metallogenic reconstructions, search, and evaluation of gold deposits.

REE in fluid inclusions of quartz from gold deposits of north-eastern Russia

Abstract Inductively coupled plasma-mass spectrometry (ICP-MS) has been used to determine rare earth element concentrations in aqueous solutions extracted from fluid inclusions. Quartz has been sampled from ores of three major types of polygenic gold hydrothermal systems of North-Eastern Russia: (1) gold-quartz-sulphide (Au-Q, Nezhdaninsk); (2) gold-antimony (Au-Sb, Sarylakh) and (3) intrusion-related gold-bismuth-siderite-polysulphide (Au-Bi-Sid, Arkachan) large deposits located in terrigenous rocks of the Verkhoyansk fold belt. The total concentration of REE in the fluid inclusions is not high (up to 52 ppm). The contribution of LREE dominates in REE balance (ΣLREE/ΣHREE=7.4–112.1). The chondrite-normalized REE patterns of inclusion fluids for the Au-Q and Au-Bi-Sid deposits are characterized by LREE enrichment with a positive or negative Eu anomaly. REE patterns for the regenerated quartz from Au-Sb deposits are characterized by pronounced differentiation between light and heavy lanthanides in fluid inclusions. Significant total REE concentration decreasing (on 1–2 order) from early to late stages of Nezhdaninsk and Arkachan deposits is revealed. The positive correlations of total REE concentrations with Rb, Cs, Li and B contents in fluid inclusions are shown. The REE distribution in fluid inclusions can be used as indicators of the contribution of magmatic fluid in the hydrothermal system.

Origin of Mineralizing Fluids of the Sediment-Hosted Navachab Gold Mine, Namibia: Constraints from Stable (O, H, C, S) Isotopes

The Navachab gold mine in the Damara belt of central Namibia is characterized by a polymetallic Au-Bi-As-Cu-Ag ore assemblage, including pyrrhotite, chalcopyrite, sphalerite, arsenopyrite, bismuth, gold, bismuthinite, and bismuth tellurides. Gold is hosted by quartz sulfide veins and semimassive sulfide lenses that are developed in a near-vertical sequence of shelf-type metasedimentary rocks, including marble, calcsilicate rock, and biotite schist. The sequence has been intruded by abundant syntectonic lamprophyre, aplite, and pegmatite dikes, documenting widespread igneous activity coeval with mineralization. The majority of quartz from the veins has δ 18O values of 14 to 15 per mil (V-SMOW). The total variations in δ 18O values of the biotite schist and calcsilicate rock are relatively small (12–14‰), whereas the marble records steep gradients in δ 18O values (17–21‰), the lowest values being recorded at the vein margins. Despite this, there is no correlation between δ 18O and δ 13C values and the carbonate content of the rocks, indicating that fluid-rock interaction alone cannot explain the isotopic gradients. In addition, the marble records increased δ 13C values at the contact to the veins, possibly related to a change in the physicochemical conditions during fluid-rock interaction. Gold is interpreted to have precipitated in equilibrium with metamorphic fluid ( δ 18O = 12–14‰ δ D = −40 to −60‰) at peak metamorphic conditions of ca. 550°C and 2 kbars, consistent with isotopic fractionations between coexisting calcite, garnet, and clinopyroxene in the alteration halos. The most likely source of the mineralizing fluid was a midcrustal fluid in equilibrium with the Damaran metapelites that underwent prograde metamorphism at amphibolite- to granulite-facies grades. Although there is no isotopic evidence for the contribution of magmatic fluids, they may have been important in contributing to the overall hydraulic regime and high apparent geothermal gradients (ca. 80°C/km−1) in the mine area.

Contribution of Magmatic Fluid to the Active Hydrothermal System in the JADE Field, Okinawa Trough: Evidence from Fluid Inclusions, Oxygen and Helium Isotopes

The Okinawa trough is an active, backarc-spreading basin in which hydrothermal fluids are venting from black and white smoker chimneys along a felsic volcanic ridge. The Okinawa seafloor hydrothermal precipitates are comparable to Kuroko-type massive sulfide deposits in many respects. Both systems occur in backarc settings and are hosted by a suite of bimodal volcanic rocks. They show mound-like features and contain predominantly Zn-Pb-Cu metal associations. Aqueous and CO2-rich hydrothermal fluids from the JADE field exhibit high trapping temperatures (up to 420°C), and moderate salinities (up to 8% NaCl equiv.). We provide evidence for the contribution of volatiles from a magmatic source to the actively forming submarine hydrothermal system. The following features in the JADE field support a magmatic component within this system: (1) the presence of a shallow-level felsic magma chamber 1-2 km beneath the seafloor; (2) anomalously high heat flow within the hydrothermal field; (3) a high content of CO2 and CH4 in the hydrothermal plume; (4) effervescence of volatile species (e.g., CO2, CH4, H2, N2,) in the hydrothermal fluid that forms independent discharges as CO2-rich fluid through CO2 hydrate pipes near black smoker chimneys; (5) helium isotope data demonstrating that these gases were derived from a mantle source; (6) high-temperatures and moderate to high fluid-inclusion salinities; (7) oxygen isotopic compositions of altered rocks and quartz indicating that mineralizing fluids have a δ18O composition consistent with a mixture of magmatic fluid and seawater.

Geochemical indicators of possible ongoing volcanic unrest at Nisyros Island (Greece)

Significant changes occurred in the chemistry of gases discharged from Nisyros fumaroles in the years, 1997–2001, following a strong seismic crisis. Increasing H2S/CO2 ratios and decreasing CH4/CO2 ratios are attributed to an increased contribution of magmatic fluids to the hydrothermal system. Some fumaroles showed concurrent increases in H2 and CO contents implying increases in temperature and pressure in the upper parts of the hydrothermal system. These changes reinforce the possibility of an increased hazard of hydrothermal eruptions at present with respect to the '90's. Moreover, both the present events and the historical hydrothermal eruptions may represent precursory signals of a new period of volcanic unrest at Nisyros.

The Candelaria-Punta del Cobre Iron Oxide Cu-Au(-Zn-Ag)Deposits, Chile

Several iron oxide-rich Cu-Au(-Zn-Ag) deposits define an approximately 5-km-wide and at least 20-km-long belt along the eastern margin of the coastal batholith near Copiapo, Chile. This belt includes the large Candelaria mine and a group of middle- and small-sized mines in the Punta del Cobre district, which is located about 3 km northeast of the Candelaria deposit. Estimated geologic resources of the belt are on the order of 700 to 800 million metric tons (Mt) at 1.0 percent Cu. The ore occurs in veins, breccia, and stringer bodies, and in replacement bodies that are roughly concordant to bedding. The orebodies are hosted mainly by volcanic and volcaniclastic rocks of the Punta del Cobre Formation and, in places, also occur in volcaniclastic intercalations in the lower part of the overlying Early Cretaceous Chanarcillo Group. Most of the larger orebodies in the belt are located where northwest-trending brittle faults intersect the contact between massive volcanic and volcaniclastic rocks. These northwest faults and a major northeast-trending ductile shear zone control portions of the ore of the Candelaria deposit. Chalcopyrite is the only hypogene Cu mineral. The Cu-Au ore is characterized by abundant magnetite and/or hematite and by locally elevated concentrations of Ag, Zn, Mo, and light rare earth elements. The ore is hosted mainly in zones with biotite-potassium feldspar ± calcic amphibole ± epidote alteration at Candelaria. In the Punta del Cobre district, ore in the deeper parts of the deposits is similarly associated, whereas at shallow levels it occurs in zones of biotite-potassium feldspar, or albite-chlorite ± calcite alteration. Mineralization at Candelaria-Punta del Cobre took place under relatively oxidized conditions manifested by the formation of specular hematite. In parts of the district, the pseudomorphic replacement of early specular hematite by magnetite during the main iron oxide formation marks a shift toward more reduced conditions or higher temperatures. The bulk of the magnetite probably formed at temperatures of about 500° to 600°C. The main sulfide stage followed with formation of pyrite and chalcopyrite at temperatures of >470° to 328°C. Subsequent martitization of the magnetite points to a temperature decrease. Cooling of the hydrothermal system is also indicated by the homogenization temperatures of <236°C of saline fluid inclusions in late-stage calcite. Oxygen isotope combined with microthermometric data suggest that magmatic fluids or nonmagmatic fluids equilibrated with magmatic silicates were dominant during the main copper mineralization. Relatively light oxygen isotope signatures of fluids in equilibrium with late calcite suggest mixing with a nonmagmatic fluid (e.g., basinal brines or meteoric waters) during the late stages of hydrothermal activity. Sulfur isotope ratios of chalcopyrite, pyrite, pyrrhotite, and sphalerite from the Bronce, Candelaria, Las Pintadas, Santos, and Socavon Rampa deposits range from δ 34SCDT values of –0.7 to +3.1 per mil. This narrow range of sulfur isotope ratios near 0 per mil is consistent with sulfur of magmatic origin. Anhydrite from the Candelaria mine paragenetically overlaps with chalcopyrite. Fluid inclusions in this anhydrite homogenize between 340° and 470°C and it has δ 34SCDT values between 14.5 and 17.5 per mil. A Δ sulfate-sulfide value of 13.4 per mil for a sample with coexisting anhydrite and chalcopyrite is consistent with sulfide-sulfate fractionation at temperatures on the order of 400°C. Ore lead isotope signatures are homogeneous and similar to those of least altered volcanic host rocks and nearby intrusive rocks. Radiometric ages, including new 40Ar/39Ar ages for hydrothermal alteration at Candelaria, point to a main Cu-Au mineralization event at Candelaria-Punta del Cobre at around 115 Ma. The ages indicate that ore formation was broadly coeval with batholithic granitoid intrusions and with regional uplift. They further imply that the Cu-Au(-Zn-Ag) deposits formed at shallow crustal levels (<3 km). The stable isotope data, the presence of previously reported hypersaline CO2-rich fluid inclusions in the main Cu ore stage and saline fluid inclusions in late-stage calcite, the oxidized character of the first ore-fluid pulses, and the mineralization age coeval with nearby intrusive activity are consistent with, but not unequivocally evidence of, magmatic fluid contribution into the hydrothermal system.

Volcanogenic-related origin of sulfide-rich quartz veins: evidence from O and S isotopes at the Géant Dormant gold mine, Abitibi belt, Canada

The Geant Dormant gold mine is a sulfide-rich quartz vein gold deposit hosted by a volcano-sedimentary sequence and an associated felsic endogenous dome and dikes. The auriferous quartz-sulfide veins were preceded by two synvolcanic gold-bearing mineralizing events: early sulfidic seafloor-related and later disseminated pyrite in the felsic dome. This deposit differs from classical Archean auriferous quartz vein deposits by the low carbonate and high sulfide contents of the veins and by their formation prior to ductile penetrative deformation. The δ18O values of quartz associated with seafloor-related auriferous sulfides average 11.9 ± 0.6‰ (n = 3). The seafloor hydrothermal fluids had a δ18O value of 3.2‰ calculated at 250 °C. The oxygen isotope composition of quartz and chlorite from veins average 12.5 ± 0.3‰ (n = 20) and 5.9 ± 1.1‰ (n = 4) respectively. Assuming oxygen isotope equilibrium between quartz and chlorite, the veins formed at a temperature of ∼275 °C, which is consistent with the calculated temperature of 269 ± 10 °C from chlorite chemistry. The gold-bearing fluids had a δ18O value of 4.7‰ calculated at 275 °C. The δ34S values of sulfides from the three gold events range from 0.6 to 2.8‰ (n = 32) and are close to magmatic values. Sulfur isotope geothermometry constrains the sulfide precipitation in the gold-bearing veins at a temperature of ∼350 °C. The similarity of the isotope data, the calculated δ18O of the mineralizing fluids and the likely seawater fluid source suggest that the three mineralizing events are genetically related to a volcanogenic hydrothermal system. The high value of the auriferous fluids (δ18O = 4.7‰) is attributed to a significant magmatic fluid contribution to the evolved seawater-dominated convective hydrothermal system. The two-stage filling of veins at increasing temperature from quartz-chlorite (275 °C) to sulfides (350 °C) may reflect the progressive maturation of volcanogenic hydrothermal systems. These results, together with field and geochemical data, suggest that formation of gold-rich volcanogenic systems require specific conditions that comprise a magmatic fluid contribution and gold from arc-related felsic rocks, coeval with the mineralizing events. This study shows that some auriferous quartz-vein orebodies in Archean terranes are formed in volcanogenic rather than mesothermal systems.