Sulfur In Hydrothermal Fluids Research Articles

Molybdenum mineralization genetically linked with magmatism at the Shipingchuan deposit, SE China

Most molybdenum (Mo) deposits are considered to be associated with magmatic systems; however, their genetic link is not clearly resolved when using, for example, bulk sulfur (S) and lead (Pb) isotopes of sulfides dominated by pyrite separates. Here, we present microtextures and in situ trace-element results of pyrite, in situ S isotopic compositions of molybdenite and pyrite, and Pb isotopes of pyrite and K-feldspar determined by laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) and multicollector (MC) LA-ICP-MS. Combined with geochronologic data for both the magmatism and Mo mineralization, these in situ methods allowed the genetic link between Mo mineralization and magmatism to be unraveled at Shipingchuan, SE China. The Shipingchuan deposit is characterized by molybdenite-quartz veins hosted by faults or fractures around a biotite K-feldspar granite. Zircon LA-ICP-MS U-Pb and molybdenite Re-Os dating results confirm that the biotite K-feldspar granite and mineralization were coeval and formed during the end of the Early Cretaceous (ca. 105 Ma), whereas postmineralization monzogranite porphyry dikes formed at 93.7 Ma. Pyrite shows a close mutual relationship with molybdenite and is characterized by Co-Ni-As-Te-Cu-Pb-Ag-Bi−rich cores (PyI) revealed by LA-ICP-MS data. Co/Ni and calculated (Se/S)fluid ratios for both cores (PyI) and rims (PyII) indicate a magmatic-hydrothermal origin. The in situ S isotopic compositions (δ34S) of molybdenite are 2.1‰ to 6.8‰, which are identical to those of molybdenite separates. PyI has δ34S values of −6.9‰ to 0.8‰, whereas PyII exhibits slightly more negative values of −7.5‰ to −0.2‰. These analyses indicate reliable results for in situ S isotopes in molybdenite, which could be more appropriate to constrain the origin of sulfur in hydrothermal fluids than results from pyrite analysis. The in situ S isotopic compositions of sulfides confirm a magmatic source for sulfur. The in situ Pb isotopic compositions of pyrite are consistent with those of K-feldspar from the biotite K-feldspar granite, indicating a common source. The heterogeneous As/Ni, As/Sb, and As/Bi values, as well as the variations of δ34S values of both PyI and PyII, indicate progressive oxidization of the hydrothermal fluids. These results illustrate that the metal-rich fluids were released from the granite and migrated along faults and fractures around the granite in an extensional setting. The precipitation of molybdenite-quartz veins at Shipingchuan was triggered by changes in temperatures and redox conditions, which were caused by mixing of magmatic fluids and meteoric water. Results in this study provide an example of tracing the link between Mo mineralization and magmatism using in situ S isotopic compositions of molybdenite and Pb isotopes of pyrite and K-feldspar.

Protracted and polyphased gold mineralisation in the Agnew District (Yilgarn Craton, Western Australia)

The Archean orogenic gold deposits exhibits common characteristics including coeval emplacement age often proposed to be associated with a single late tectonic gold mineralisation event. Although valid for a large majority of deposits in the Yilgarn Craton, this model does not account for certain atypical gold deposits. In the world-class Agnew District (Yilgarn Craton, Western Australia) structural and paragenetic relationships, combined with a compilation of geochronology and multiple sulfur isotopes, indicate that mineralisation developed during a two-stage process involving different fluid sources. The initial event dated at c. 2660 Ma is related to the onset of folding during Kalgoorlie Orogeny and presents characteristics compatible with magmatic intrusion related mineralisation. A second mineralisation event dated at c. 2630 Ma presents a mineral and metal inventory typical to Archean orogenic-like gold mineralisation, and developed at a later stage of the orogenesis. Measurements of δ34S and Δ33S from spatially constrained samples from the two temporally distinct mineralising events further ascertain contrasting fluid compositions and/or reservoirs. Combined with geochronological data obtained on the poly-magmatic Lawlers granitic complex, we argue for a protracted ore forming process. The first stage of mineralisation has a magmatic affinity and is followed by a second stage of mineralisation whereby Au-bearing hydrothermal solution incorporated an Archean sedimentary sulfur reservoir. We propose that the change in source reservoir for sulfur in hydrothermal fluids may reflect a regional change in tectonic regime from convergent to transcurrent motion, allowing fluids to be first sourced from mantle-derived magmas followed later by metamorphic fluids.

Extreme variation of sulfur isotopic compositions in pyrite from the Qiuling sediment-hosted gold deposit, West Qinling orogen, central China: an in situ SIMS study with implications for the source of sulfur

High spatial resolution textural (scanning electron microscope (SEM)), chemical (electron microprobe (EMP)) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS)), and sulfur isotopic (secondary ion mass spectrometry (SIMS)) analyses of pyrite from the Qiuling sediment-hosted gold deposit (232 ± 4 Ma) in the West Qinling orogen, central China were conducted to distinguish pyrite types and gain insights into the source and evolution of sulfur in hydrothermal fluids. The results reveal an enormous variation (−27.1 to +69.6 ‰) in sulfur isotopic composition of pyrite deposited during three paragenetic stages. Pre-ore framboidal pyrite, which is characterized by low concentrations of As, Au, Cu, Co, and Ni, has negative δ34S values of −27.1 to −7.6 ‰ that are interpreted in terms of bacterial reduction of marine sulfate during sedimentation and diagenesis of the Paleozoic carbonate and clastic sequences, the predominant lithologies in the deposit area, and the most important hosts of many sediment-hosted gold deposits throughout the West Qinling orogen. The ore-stage hydrothermal pyrite contains high concentrations of Au, As, Cu, Sb, Tl, and Bi and has a relatively narrow range of positive δ34S values ranging from +8.1 to +15.2 ‰. The sulfur isotope data are comparable to those of ore pyrite from many Triassic orogenic gold deposits and Paleozoic sedimentary exhalative (SEDEX) Pb-Zn deposits in the West Qinling orogen, both being hosted mainly in the Devonian sequence. This similarity indicates that sulfur, responsible for the auriferous pyrite at Qiuling, was largely derived from the metamorphic devolatization of Paleozoic marine sedimentary rocks. Post-ore-stage pyrite, which is significantly enriched in Co and Ni but depleted in Au and As, has unusually high δ34S values ranging from +37.4 to +69.6 ‰, that are interpreted to result from thermochemical reduction of evaporite sulfates in underlying Cambrian sedimentary rocks with very high δ34S values. The variations in Au content and sulfur isotopic compositions across a single ore-stage pyrite grain may reflect displacement of indigenous groundwater with low δ34S values by auriferous metamorphic fluids with high δ34S values. The very low-grade metamorphism of the host rocks and the metamorphic derivation of sulfur for the ore pyrite indicate that the Qiuling sediment-hosted gold deposit is an epizonal manifestation of an orogenic gold system in the West Qinling orogen.

Sources and thermo-chemical sulfate reduction for reduced sulfur in the hydrothermal fluids, southeastern SYG Pb-Zn Metallogenic Province, SW China

Located on the western Yangtze Block, the Sichuan (四川)-Yunnan(云南)-Guizhou(贵州) (SYG) Pb-Zn metallogenic province has been a major source of base metals for China. In the southeastern SYG province, structures are well developed and strictly control about 100 Pb-Zn deposits. Almost all the deposits are hosted in Devonian to Permian carbonate rocks. Lead-zinc ores occur either as veinlets or disseminations in dolomitic rocks with massive and disseminated textures. Ore minerals are composed of pyrite, sphalerite and galena, and gangue minerals are calcite and dolomite. Sulfide minerals from four typical Pb-Zn deposits are analyzed for sulfur isotope compositions to trace the origin and evolution of hydrothermal fluids. The results show that δ34S values of sulfide minerals range from +3.50‰ to +20.26‰, with a broad peak in +10‰ to +16‰, unlike mantle-derived sulfur (0±‰). However, the mean δ34Ssulfide and δ34SΣS-fluids values are similar to that of sulfate-bearing evaporites in the host rocks (gypsum: ⊃+15‰ and barite: +22‰ to +28‰) and Cambrian to Permian seawater sulfate (+15‰ to +35‰). This suggests that reduced sulfur in hydrothermal fluids was likely derived from evaporates in the host rocks by thermo-chemical sulfate reduction (TSR). Calculated δ34SΣS-fluids values of the Shanshulin(杉树林), Qingshan(青山), Shaojiwan(筲箕湾) and Tianqiao(天桥) Pb-Zn deposits are +21.59‰, +18.33‰, +11.43‰ and +10.62‰, respectively, indicating sulfur-bearing hydrothermal fluids may be evolved from the Shanshulin to Qingshan and then the Shaojiwan to Tianqiao deposition sites along the Yadu(垭都)-Ziyun(紫云) lithospheric fracture in the southeastern SYG province.

The oxidation state of sulfur in magmatic fluids

Sulfur compounds in volcanic gases are responsible for the global cooling after explosive eruptions and they probably controlled the early evolution of the Earth's atmosphere. We have therefore studied the oxidation state of sulfur in aqueous fluids under the pressure and temperature conditions and oxygen fugacities typical for magma chambers (0.5–3kbar, 650–950°C, Ni–NiO to Re–ReO2 buffer conditions). Sulfur speciation was determined by Raman spectroscopy of quenched fluids trapped as inclusions in quartz. Our results show that sulfur in hydrothermal fluids and volcanic gases is much more oxidized than previously thought and in particular, some explosive eruptions may release a significant fraction of sulfur as SO3 or its hydrated forms. In the pressure range from 500 to 2000bar, the equilibrium constant K1 of the reaction 2H2S+3O2=2SO2+2H2O in aqueous fluids can be described by lnK1=−(57.1±7.1)+(173,480±7592)T−1, where T is temperature in Kelvin. The equilibrium constant K2 for the reaction SO2+½O2=SO3 in aqueous fluids, where SO3 may include hydrated forms, such as H2SO4, was found to be strongly pressure dependent, with lnK2=−(5.2±5.7)+(19,243±5993)T−1 at 1500bar; lnK2=−(11.1±1.3)+(25,383±1371)T−1 at 2000bar and lnK2=−(22.1±2.2)+(37,082±2248)T−1 at 2500bar. Our data imply that volcanoes may directly inject hexavalent sulfur in the form of H2SO4 into the atmosphere, not only on Earth, but possibly also on Venus and on Mars, when it was still tectonically active. Remote measurements from satellites may have underestimated the sulfur yield of some recent eruptions. Moreover, the mechanisms of the interaction of volcanic gases with the stratosphere need to be reconsidered.

ARSENOPYRITE MELTING DURING METAMORPHISM OF SULFIDE ORE DEPOSITS

Arsenopyrite is present as a minor phase in many different types of ore deposits. Here we investigate a number of ore deposits metamorphosed to mid-amphibolite facies and above to show that in some environments, arsenopyrite is likely to melt during metamorphism, but in others it will persist until it is converted to lollingite + pyrrhotite. The fate of arsenopyrite is governed by the sulfur fugacity imposed by the surrounding mineralogy during metamorphism. At the Hemlo gold deposit, Canada, which contains a range of disseminated sulfi des, the breakdown of barite promoted conditions of high f(S2) and melting of arsenopyrite during prograde metamorphism. On the other hand, at several massive sulfi de deposits including Osborne Lake, Montauban and Geco in Canada, high f(S2) conditions were instead generated through pyrite breakdown on the pyrite–pyrrhotite buffer, also causing arsenopyrite to melt in favorable parts of the deposits. In contrast, metamorphic processes that inhibit high f(S2) through consumption of sulfur promote the solid-state conversion of arsenopyrite to lollingite and pyrrhotite rather than melting. In most mineral deposits, the strongest infl uence on sulfur fugacity is the pyrite-to-pyrrhotite reaction, which buffers f(S2) to increasingly elevated values as temperature increases. Once pyrite is consumed, however, f(S2) no longer is maintained at elevated values. If rocks hosting arsenopyrite are able to conserve pyrite to middle-amphibolite-facies conditions (beyond 491°C at 1 bar, or ~560°C at 5 kbar), arsenopyrite melting will occur. If not, arsenopyrite melting is unlikely, though still possible. Of the mechanisms that promote pyrite decomposition at metamorphic conditions below arsenopyrite melting, sequestration of sulfur by iron silicates or oxides (or both) to form pyrrhotite may be the most effective in many types of deposit. At the Calumet deposit and in some parts of the Geco deposit, this process was found to be effective in converting pyrite to pyrrhotite in magnetite-rich rocks. Pyrite consumption and low-f(S2) conditions are also promoted to a small extent by incorporation of sulfur in hydrothermal fl uids, such as an introduced fl uid or those generated by dehydration reactions, this effect becoming more signifi cant as temperature rises. Deposits where arsenopyrite is likely to melt during metamorphism include pyrite-rich massive sulfi de deposits as well as disseminated deposits lacking abundant iron silicates and oxides. The As-rich melts that result are highly effective in incorporating and mobilizing other metals, particularly gold and silver, as demonstrated at the Challenger deposit (Australia) and at Hemlo, Ontario.

Seafloor hydrothermal clay alteration at Jade in the back-arc Okinawa Trough: mineralogy, geochemistry and isotope characteristics

Seafloor hydrothermal activity at Jade has resulted in extensive alteration of the host epiclastic sediments and pumiceous tuffs, forming mica, kaolins (kaolinite and halloysite), Mg-rich chlorite, talc, montmorillonite, and a mixed-layer mineral of dioctahedral chlorite and montmorillonite (Chl/Mont). Clay mineral assemblages show a vertical variation, which reflects variable amounts of cold seawater incorporated into hot hydrothermal fluids in subsurface sediments and tuff. However, mixing alone cannot explain the occurrence of abundant kaolin minerals at Jade. The formation of kaolin minerals requires much more acidic fluid than expected from simple mixing of hydrothermal fluids and cold seawater. Low pH values are likely attained by oxidation of H 2S either dissolved in the hydrothermal fluid or released from the fluid during decompression. The fluid reaching the seafloor is discharged into cold seawater, which caused precipitation of sulfides close to vents and native sulfur and barite at the margins of the vent areas. Halloysite, barite and anhydrite show Sr isotope compositions similar to marine Sr, indicating the derivation of marine Sr directly from seawater or by the dissolution of calcareous nannoplanktons. The isotopic compositions of kaolinite (δ 18O = +7.4‰, δD = −23‰), Chl/Mont (δ 18O = +7.0‰, δD = −32‰), and mica (δ 18O = +5.4 to +9.9‰, δD = −30 to −26‰) suggest fluids of a heated seawater origin. The O isotopic data yielded formation temperatures of 170°C for kaolinite, 61 to 110°C for halloysite, and 145 to 238°C for mica. Barite δ 34S values (+21.0 to +22.5‰) are very similar to the marine sulfate value, confirming that the barite formation took place due to mixing of Ba-bearing hydrothermal fluids and sulfate-rich seawater. Native sulfur shows a large variation in δ 34S in one hand specimen probably because of rapid disequilibrium precipitation of S during fluid exhalation on the seafloor. Sulfur in hydrothermal fluids is usually consumed to form metal sulfides. Therefore, abundant native sulfur at Jade suggests high H 2S/metals ratios of the hydrothermal fluids. The alteration assemblages and isotopic data of hydrothermal minerals from Jade are very similar to those of Kuroko-type barite deposits of middle Miocene age, which formed from fluids of high S/metals ratios at less than 200°C. At Jade, there is only one black smoker actively discharging high temperature (∼320°C) fluid, but there are many fossil sulfide chimneys and mounds in the area. The mineralogy and high Au and Cu in these precipitates suggest highly metalliferous hydrothermal activity in the past. These activities likely resulted in discharge of hydrothermal plumes and fall-outs of sulfides and sulfates on the seafloor. These fall-outs were incorporated in sediments far from the vent areas. They are now recorded as high metal contents in sediments with no petrographic and mineralogical evidence of in-situ hydrothermal activity. Some are high as 8,100 ppm for Cu, 12,500 ppm for Zn, 1,000 ppm for As, 100 ppm for Ag and 21,000 ppm for Pb. Detrital grains of montmorillonite in such sediments are coated with Fe-oxyhydroxides during the suspension in seawater before settling on the seafloor. The depths of such metal anomalies in sediments suggest high levels of metalliferous hydrothermal activities from 1,800 to 300 ybp.

Geochemistry and genesis of mesothermal gold deposits in Korea: Base metal-rich gold mineralization of the Byungjibang mine, Hwoingsung area.

Electrum (79.67-88.68 atom.%Ag)-sulfide mineralization of the Byungjibang mine in the Hwoingsung area represents the first recognized example of mesothermal gold mineralization of late Cretaceous age in South Korea, and was deposited in three stages of mineralogically simple, base metal-rich quartz±calcite veins that filled the fractures in Cretaceous biotite granite and quartz porphyry. The K-Ar dating of hydrothermal vein muscovite indicates that mineralization took place during the late Cretaceous (81.9±1.8 Ma). Fluid inclusion data show that main ore mineralization (stage I) formed from high temperature (225°-370°C), H2O-NaCl(-CO2) fluids with low salinity (1.4-6.7 wt. % eq. NaCl). The deposition of abundant base-metal sulfides (sphalerite, chalcopyrite, galena, etc.) forming distinct sulfide bands was mainly a result of primary boiling and associated CO2 effervescence of the fluid with temperatures between about 320° and 370°C under considerable crustal depths (at least 1.3 to 2.2 km deep). Gold-silver deposition occurred paragenetically later than base-metal sulfide deposition, and was the result of cooling (down to 200°-250°C) and decreasing sulfur activity caused by mixing with meteoric groundwaters. Calculated sulfur isotope composition of ore fluids (δ34SΣS=3.4 to 5.6‰) indicates an igneous source of sulfur in hydrothermal fluids, probably the nearby Cretaceous quartz porphyry dikes. Measured and calculated, oxygen and hydrogen isotope compositions of stage I ore fluids (δ18Owater=3.1 to 7.9‰, δDwater=−72 to −87‰) indicate that the auriferous hydrothermal fluids at Byungjibang formed originally from either a magmatic water or an isotopically highly exchanged meteoric water under low water-rock ratio condition, and evolved through progressive mixing with an isotopically evolved meteoric groundwater (which is represented by stage III fluids; temperature=down to 130°C, δ18Owater=−6.4 to −7.2‰, δDwater=−86‰).