Magmatic-hydrothermal Ore Deposits Research Articles

Open-system behaviour of magmatic fluid phase and transport of copper in arc magmas at Krakatau and Batur volcanoes, Indonesia

The Sunda arc of Indonesia is an excellent example of how volcanic processes at convergent plate margins affect the distribution of metals and control the distribution of ore deposits. In this paper, we report microtextural observations and microanalytical data (SEM-EDS and LA-ICP-MS) of silicate and sulfide melt inclusions from fresh samples of volcanic rocks from the 2008 eruption of Mt. Krakatau and 1963 eruption of Mt. Batur, Sunda arc, Indonesia that bear implications on the concentration and transport of Cu and other chalcophile elements in mafic-intermediate magmas in arc settings. These multi-phase inclusions contain glass, amphibole and plagioclase, together with co-trapped apatite, magnetite, sulfides and lobed, drop-like Fe-oxide. We observed two stages of sulfide formation: 1) early-formed sulfide globules (pyrrhotite and intermediate solid solution), which derived from an immiscible sulfide melt and only occur as inclusions in phenocrysts; and 2) late-formed, irregular Cu-rich sulfides (intermediate solid solution to bornite), which were deposited in the presence of an aqueous fluid, and are contained as fluid phase precipitates in vapour bubbles of melt inclusions and in vesicles, as well as finely dispersed grains in the groundmass. Microtextural observations and X-ray elemental maps show that interaction between sulfide globules and aqueous fluid resulted in partial oxidation and transfer of Cu between the fluid and the sulfide phase. A compilation of whole-rock analyses from the Sunda arc indicates that Cu reaches ~250–300ppm in mafic samples (SiO2≤52wt.%), and then suddenly drops with progressive fractionation to <50ppm in intermediate-felsic samples. This behaviour can be explained by sulfide melt exsolution or degassing and scavenging of Cu occurring at various stages of magma fractionation (at MgO ~8–2.5wt.%). These trends can be effectively modelled by sulfide saturation during fractional crystallisation at oxygen fugacities varying from fO2=FMQ +0.8 to FMQ +1.4. In contrast, LA-ICP-MS analyses of whole multi-phase melt inclusions hosted in olivine, pyroxene and plagioclase indicate variable Cu and S contents (Cu up to ~6000ppm), which do not correlate with fractionation indicators (e.g. SiO2, MgO, Rb), consistent with co-trapping of CuS phases with silicate melt. The highest Cu concentrations, Cu/S and incompatible trace elements (e.g. Rb) were measured in plagioclase, which crystallised over a wider range of melt compositions in comparison with olivine and pyroxene, and preferentially contains late-formed Cu-rich sulfides. These results underline the importance of mafic-intermediate magmas as sources of Cu in magmatic-hydrothermal ore deposits, and suggest that S-rich fluid is of primary importance in the transport of Cu in arc settings.

Lithospheric Architecture of the Lhasa Terrane and Its Control on Ore Deposits in the Himalayan-Tibetan Orogen

Magmatic-hydrothermal ore deposits in collisional orogens are new targets for modern mineral exploration, yet it is unclear why they preferentially occur in some specific tectonic environments within these orogenic belts. We integrate geologic and geochemical data (especially zircon U-Pb dating and Lu-Hf isotope data) for Mesozoic-Cenozoic magmatic rocks and associated ore deposits in the Lhasa terrane, a highly endowed tectonic unit within the Himalayan-Tibetan orogen, and provide the first example in a continental collision terrane of the application of zircon Hf isotope data to image the lithospheric architecture and its relationship with ore deposits. Three crustal blocks are identified within the Lhasa terrane by the Hf isotope mapping method. They include a central long-lived Precambrian microcontinent with local reworking and two surrounding juvenile Phanerozoic crustal blocks with significant mantle contributions to constituent magmatic rocks. The three crustal blocks are bounded by two E-W–trending terrane-boundary faults, and each block is cut by two N-S–striking concealed faults. Isotopic signatures of zircons from the juvenile crustal blocks indicate that the Phanerozoic continental crust grew from several Mesozoic volcanic-plutonic arcs and by underplating of mantle-derived magmas generated during Mesozoic accretion and Cenozoic collision. Mesozoic subduction-related porphyry Cu-Au deposits and Cenozoic collision-related Cu-Mo deposits are exclusively located in regions with high e Hf(>5) juvenile crust. Cu enrichment during differentiation of high f o2 arc magmas is the key for the formation of Mesozoic subduction-related porphyry Cu-Au. By contrast, remelting of the lower crustal Cu sulfide-rich magmatic cumulates within the juvenile crust is interpreted to have played a key role in the formation of Cenozoic collision-related Cu-Mo deposits. Granite-related Pb-Zn deposits cluster in the oldest crustal regions or developed along the margin of the old crustal block bounded by lithospheric faults. The porphyry Mo deposits are localized along the reworked margins of the old crustal block. It is suggested that crustal reworking released Mo from the old crust to form porphyry Mo deposits, whereas leaching of Pb and Zn from the Paleozoic carbonate cover strata by felsic intrusion-driven fluids is critical to the formation of Pb-Zn ore deposits. Skarn Fe-Cu ore deposits are typically localized along a terrane boundary fault, i.e., lithospheric discontinuity, through which crust-derived felsic melt mixed with Cu-rich mantle-derived mafic magmas ascending upward. Associated granitoid rocks usually bear microgranular mafic enclaves and show a zircon Hf isotope array from negative to positive e Hf values (−7.3 to +6.7), supporting mixing of juvenile mantle and evolved crustal sources. The Hf isotope maps show temporal-spatial relationships between crustal structure and the location of ore deposits, demonstrating that the structure, nature, and composition of the crust controlled the localization of ore deposits and the migration of ore-forming metals in the terrane. This study shows that the lithospheric architecture of an orogenic terrane can be imaged by Hf isotope mapping to provide mappable units which can be used to explore for ore deposits at the terrane scale.

Metal Contents of Ore‐Forming Fluids in Magmatic‐Hydrothermal Ore Deposits

Acta Geologica Sinica - English EditionVolume 88, Issue s2 p. 1062-1063 Meeting Abstracts Metal Contents of Ore-Forming Fluids in Magmatic-Hydrothermal Ore Deposits Robert J. BODNAR, Corresponding Author Robert J. BODNAR Fluids Research Laboratory, Department of Geosciences, Virginia Tech, Blacksburg, VA 24061 USACorresponding author. E-mail: rjb@vt.eduSearch for more papers by this author Robert J. BODNAR, Corresponding Author Robert J. BODNAR Fluids Research Laboratory, Department of Geosciences, Virginia Tech, Blacksburg, VA 24061 USACorresponding author. E-mail: rjb@vt.eduSearch for more papers by this author First published: 29 December 2014 https://doi.org/10.1111/1755-6724.12379_1Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume88, Issues2Special Issue: Meeting Abstracts: The 14th Quadrennial International Association on the Genesis of Ore Deposits Symposium. August 19–22, 2014, Kunming, ChinaDecember 2014Pages 1062-1063 RelatedInformation

Tectonic, magmatic, and metallogenic evolution of the Tethyan orogen: From subduction to collision

This paper reviews the tectonic, magmatic, and metallogenic history of the Tethyan orogen from the Carpathians to Indochina. Focus is placed on the formation of porphyry Cu±Mo±Au deposits, as being the most characteristic mineral deposit type formed during both subduction and collisional processes in this region. Relatively little is known about the history of the Paleotethys ocean, which opened and closed between Gondwana and Eurasia in the Paleozoic, and few ore deposits are preserved from this period. The Neotethyan ocean opened in the Permian–Early Triassic as the Cimmerian continental fragments (the cores of Turkey, Iran, Tibet, and Indochina) rifted from the northern Gondwana margin and drifted northwards. These microcontinents docked with the Eurasian margin at various points in the Mesozoic and Cenozoic, and formed a complex archipelago involving several small back-arc basins and remnants of the Paleotethyan ocean. The main Neotethyan ocean and these smaller basins were largely eliminated by collision with India and Africa–Arabia in the early Eocene and early-mid Miocene, respectively, although Neotethyan subduction continues beneath the Hellenic arc and the Makran.The majority of porphyry-type deposits are found in association with Neotethyan subduction (mainly in the Mesozoic and Paleogene), and syn- to post-collisional events in the mid-Paleogene to Neogene. They are found throughout the orogen, but some sections are particularly well-endowed, including the Carpathians–Balkans–Rhodopes, eastern Turkey–Lesser Caucasus–NW Iran, SE Iran–SW Pakistan, southern Tibet, and SE Tibet–Indochina. Other sections that appear barren may reflect deeper levels of erosion, young sedimentary cover, or lack of exploration, although there may also be real reasons for low prospectivity in some areas, such as minimal subduction (e.g., the western Mediterranean region) or lithospheric underthrusting (as proposed in western Tibet).Over the last decade, improved geochronological constraints on the timing of ore formation and key tectonic events have revealed that many porphyry deposits that were previously assumed to be subduction-related are in fact broadly collision-related, some forming in back-arc settings in advance of collision, some during collision, and others during post-collisional processes such as orogenic collapse and/or delamination of subcontinental mantle lithosphere. While the formation of subduction-related porphyries is quite well understood, collisional metallogeny is more complex, and may involve a number of different processes or sources. These include melting of: orogenically thickened crust; previously subduction-modified lithosphere (including metasomatized mantle, underplated mafic rocks, or lower crustal arc plutons and cumulates); or upwelling asthenosphere (e.g., in response to delamination, slab breakoff, back-arc extension, or orogenic collapse).The most fertile sources for syn- and post-collisional porphyry deposits appear to be subduction-modified lithosphere, because these hydrated lithologies melt at relatively low temperatures during later tectonomagmatic events, and retain the oxidized and relatively metalliferous character of the original arc magmatism. Unusually metallically enriched lithospheric sources do not seem to be required, but the amount of residual sulfide phases in these rocks may control metal ratios (e.g., Cu:Au) in subsequent magmatic hydrothermal ore deposits. Relatively Au-rich deposits potentially form in these settings, as observed in the Carpathians (e.g., Roşia Montană), Turkey (Kisladag, Çöpler), and Iran (Sari Gunay, Dalli), although the majority of syn- and post-collisional porphyries are Cu–Mo-rich, and resemble normal subduction-related deposits (e.g., in the Gangdese belt of southern Tibet). This similarity extends to the associated igneous rocks, which, being derived from subduction-modified sources, largely retain the geochemical and isotopic character of those original arc magmas. While still retaining a broadly calc-alkaline character, these rocks may extend to mildly alkaline (shoshonitic) compositions, and may display adakite-like trace element signatures (high Sr/Y and La/Yb ratios) reflecting melting of deep crustal garnet amphibolitic sources. But they are otherwise hard to distinguish from normal subduction-related magmas.Small, post-collisional mafic, alkaline volcanic centers are common throughout the orogen, but for the most part appear to be barren. However, similar rocks in other post-subduction settings around the world are associated with important alkalic-type porphyry and epithermal Au±Cu deposits, and the potential for discovery of such deposits in the Tethyan orogen should not be overlooked.

The dynamic interplay between saline fluid flow and rock permeability in magmatic‐hydrothermal systems

AbstractMagmatic‐hydrothermal ore deposits document the interplay between saline fluid flow and rock permeability. Numerical simulations of multiphase flow of variably miscible, compressible H2O–NaCl fluids in concert with a dynamic permeability model can reproduce characteristics of porphyry copper and epithermal gold systems. This dynamic permeability model uses values between 10−22 and 10−13 m2, incorporating depth‐dependent permeability profiles characteristic for tectonically active crust as well as pressure‐ and temperature‐dependent relationships describing hydraulic fracturing and the transition from brittle to ductile rock behavior. In response to focused expulsion of magmatic fluids from a crystallizing upper crustal magma chamber, the hydrothermal system self‐organizes into a hydrological divide, separating an inner part dominated by ascending magmatic fluids under near‐lithostatic pressures from a surrounding outer part dominated by convection of colder meteoric fluids under near‐hydrostatic pressures. This hydrological divide also provides a mechanism to transport magmatic salt through the crust. With a volcano at the surface above the hydrothermal system, topography‐driven flow reverses the direction of the meteoric convection as compared to a flat surface, leading to discharge at distances of up to 7 km from the volcanic center. The same physical processes at similar permeability ranges, crustal depths, and flow rates are relevant for a number of active systems, including geothermal resources and excess degassing at volcanos. The simulations further suggest that the described mechanism can separate the base of free convection in high‐enthalpy geothermal systems from the magma chamber as a driving heat source by several kilometers in the vertical direction in tectonic settings with hydrous magmatism. These root zones of high‐enthalpy systems may serve as so‐called super‐critical geothermal resources. This hydrology would be in contrast to settings with anhydrous magmatism, where the base of the geothermal systems may be closer to the magma chamber.

Noble gas and halogen constraints on fluid sources in iron oxide-copper-gold mineralization: Mantoverde and La Candelaria, Northern Chile

The noble gas (Ar, Kr, Xe) and halogen (Cl, Br, I) composition of fluid inclusions in hydrothermal quartz and calcite related to the hypogene iron oxide-copper-gold (IOCG) mineralization at Mantoverde and Candelaria, Chile, have been investigated to provide new insights of fluid and salinity sources in Andean IOCG deposits. A combination of mechanical extraction by crushing and thermal decrepitation methods was applied and collectively indicate that fluid inclusions with salinities ranging from 3.4 up to 64 wt% NaCl equivalent have molar Br/Cl and I/Cl ratios of between 0.5 × 10−3 and 3.0 × 10−3 and I/Cl of between 8 × 10−6 and 25 × 10−6 in the majority of samples, with maximum values of 5.2 × 10−3 obtained for Br/Cl and 64 × 10−6 for I/Cl in fluid inclusions within individual samples. The fluid inclusions have age-corrected 40Ar/36Ar ratios ranging from the atmospheric value of 296 up to 490 ± 45, indicating the presence of crustal- or mantle-derived excess 40Ar in the fluid inclusions of most samples. The fluid inclusions have 84Kr/36Ar and 130Xe/36Ar ratios intermediate of air and air-saturated water. However, 40Ar/36Ar is not correlated with either 84Kr/36Ar or 130Xe/36Ar, and the fluid inclusion 36Ar concentrations of 0.2–3.5 × 10−10 mol/g (calculated from measured Cl/36Ar and thermometric salinity measurements) extend below the seawater value of 0.34 × 10−10 mol/g, suggesting that contamination with modern air is a minor artifact. The range of fluid inclusion Br/Cl and I/Cl ratios overlap those previously documented for the mantle and magmatic-hydrothermal ore deposits, and the fluids’ unusually low 36Ar concentration is consistent with the involvement of magmatic-hydrothermal fluids. Input of additional non-magmatic fluid components is suggested by the spread in Br/Cl and I/Cl to values characteristic of bittern brine sedimentary formation waters and near atmospheric 40Ar/36Ar. These data are compatible with mixing of magmatic-hydrothermal fluids and evaporated seawater, which was modified by interaction with back-arc basin sediments as the major control on ore formation. Low Br/Cl (<0.5 × 10−3) and I/Cl (<5.0 × 10−6) values that would provide evidence for evaporite dissolution as an important source of fluid salinity were not detected.

Combining in situ isotopic, trace element and textural analyses of quartz from four magmatic-hydrothermal ore deposits

This study couples in situ 16O, 17O and 18O isotope and in situ trace element analyses to investigate and characterize the geochemical and textural complexity of magmatic-hydrothermal quartz crystals. Euhedral quartz crystals contemporaneous with mineralization were obtained from four magmatic-hydrothermal ore deposits: El Indio Au–Ag–Cu deposit; Summitville Au–Ag–Cu deposit; North Parkes Cu–Au deposit and Kingsgate quartz-Mo–Bi–W deposit. The internal features of the crystals were imaged using cathodoluminescence and qualitative electron microprobe maps. Quantitative isotopic data were collected in situ using 157 nm laser ablation inductively coupled plasma mass spectrometry (for 40 trace elements in quartz) and sensitive high-resolution ion microprobe (for 3 isotopes in quartz). Imaging revealed fine oscillatory zoning, sector zoning, complex “macromosaic” textures and hidden xenocrystic cores. In situ oxygen isotope analyses revealed a δ18O range of up to 12.4 ± 0.3 ‰ in a single crystal—the largest isotopic range ever ascribed to oscillatory zonation in quartz. Some of these crystals contain a heavier δ18O signature than expected by existing models. While sector-zoned crystals exhibited strong trace element variations between faces, no evidence for anisotropic isotope fractionation was found. We found: (1) isotopic heterogeneity in hydrothermal quartz crystals is common and precludes provenance analysis (e.g., δD–δ18O) using bulk analytical techniques, (2) the trace element signature of quartz is not an effective pathfinder toward noble metal mineralization and (3) in three of the four samples, both textural and isotopic data indicate non-equilibrium deposition of quartz.

Selective copper diffusion into quartz-hosted vapor inclusions: Evidence from other host minerals, driving forces, and consequences for Cu–Au ore formation

Recent experimental studies have raised concerns that Cu concentrations in quartz-hosted fluid inclusions from magmatic–hydrothermal ore deposits do not represent pristine concentrations in the trapped fluids, but are modified by post-entrapment diffusional exchange through the host quartz. New microanalyses of fluid inclusions hosted in topaz show significantly lower Cu concentrations in vapor inclusions, compared to otherwise identical inclusions hosted by coexisting quartz, whereas coeval brine (hypersaline liquid) inclusions are very similar independent of host mineral in one sample. Sulfur is present as a major component in all vapor inclusions, as in most porphyry-related vapor inclusions, and Cu never exceeds S, but commonly matches the S content at a molar ratio of Cu:S⩽2 in vapor inclusions hosted by quartz.Univalent ions with a radius smaller than ∼1Å are known to diffuse rapidly through the channels of the quartz structure, parallel to its crystallographic c axis. Since only Cu concentrations differ between topaz- and quartz-hosted inclusions, we hypothesize that Cu+ and H+ re-equilibrate by diffusional ion exchange through these channels, while all other element concentrations remain essentially unchanged. A thermodynamic model considering charge-balanced Cu+H+ exchange and diffusive H2 re-equilibration of an initially Cu-poor but S-rich vapor inclusion with a typical rock-buffered fluid environment outside the host crystal demonstrates a strong chemical driving force for Cu+ to migrate from the surrounding rock into the fluid inclusion during cooling of the system. The driving force for Cu diffusion, against the gradient in total Cu concentration, is the abundant H+ liberated inside the inclusion by dissociation of HCl and particularly by the precipitation of CuFeS2 by reaction with the initially trapped H2S and/or SO2. Gold is not only a much larger ion, but is subject to an opposing driving force, suggesting that high concentrations of this larger ion analyzed in vapor inclusions probably represent true gold concentrations in magmatic–hydrothermal vapor.These findings imply that brine–vapor separation in porphyry deposits does not cause selective Cu transfer to the vapor, but is more likely to destabilize Cu complexes and promote copper ore deposition during decompression and unmixing of the two fluid phases. By contrast, Au may be selectively transferred into the vapor phase, allowing its transport through the deeper porphyry copper deposits to form epithermal gold deposits closer to the earth’s surface.

The partitioning of sulfur and chlorine between andesite melts and magmatic volatiles and the exchange coefficients of major cations

Andesite melts were equilibrated with an H–O–S-bearing volatile phase to determine the partition coefficients for S and Cl as a function of melt composition and oxygen fugacity. The experiments were conducted in rapid-quench MHC vessel assemblies at 200MPa and 1000°C, and over a range of imposed fO2 between NNO−1.2 and NNO+1.8. High fluid/melt mass ratios (∼15) were employed, allowing precise and accurate partition coefficients to be obtained by mass balance calculations. Chlorine exhibits Henrian behavior at ClO−0.5 activities typical for arc magmas, with DClvolatile/melt=1.36±0.06 (1σ) below 0.2wt.% Cl in the melt; at higher ClO−0.5 activities, DClvolatile/melt increases linearly to 2.11±0.02 at 1wt.% Cl in the melt. In the volatile phase: FeCl2∼NaCl>KCl∼HCl. The determination of cation exchange coefficients for major cations yielded: KK,Navolatile/melt=1.23±0.10 (1σ) and ∗KFe,Navolatile/melt=DFevolatile/melt/DNavolatile/melt=1.08±0.16 (1σ). Under these conditions, the concentration of HCl in the vapor is negatively correlated with the (Na+K)/(Al+Fe3+) ratio in the melt. Reduced sulfur (S2−) appears to obey Henry’s law in andesite melt–volatile system at fH2S below pyrrhotite saturation. The partition coefficient for S at fO2=NNO−0.5 correlates negatively with the FeO concentration in the melt, changing from 254±25 at 4.0wt.% FeO to 88±6 at 7.5wt.% FeO. Pyrrhotite saturation is reached when approximately 3.2mol% S is present in the volatile phase at fO2=NNO−0.5. At the sulfide/sulfate transition, the partition coefficient of S drops from 171±23 to 21±1 at a constant FeO content of ∼6wt.% in the melt. At fO2=NNO+1.8, anhydrite saturation is reached at ∼3.3mol% S present in the volatile phase. Aqueous volatiles exsolving from intermediate to mafic magmas can efficiently extract S, and effect its transfer to sites of magmatic-hydrothermal ore deposit formation.

The Trace Metal Chemistry of Deep Geothermal Water, Palinpinon Geothermal Field, Negros Island, Philippines: Implications for Precious Metal Deposition in Epithermal Gold Deposits

Palinpinon geothermal field, Negros Island, Philippines, is a high-temperature, liquid-dominated geothermal system in an active volcanic island-arc setting. Parallels have been drawn between the geologic setting and hydrothermal alteration styles recognized at Palinpinon with those that characterize several types of magmatichydrothermal ore deposits (e.g., porphyry, skarn, and high and intermediate sulfidation epithermal systems). The neutral chloride Palinpinon reservoir water chemistry is affected by boiling, mixing, and conductive cooling. The occurrence of base and precious metal scale deposits in several geothermal wells demonstrates that the deep reservoir fluid is capable of transporting and depositing base and precious metals. However, trace metal analysis of the deep reservoir fluids shows they are undersaturated with respect to gold (1–4 ig/kg). Chemical modeling of Palinpinon geothermal water predicts that boiling should be the most effective mechanism for base and precious metal deposition, producing sulfide assemblages similar to those present in well scale deposits. Mixing with acid-bearing sulfate waters can also produce these sulfide assemblages, but less efficiently and with gold and silver deposition occurring at lower temperatures (i.e., <110°C). Despite indications that modern hydrothermal fluids are capable of metal transportation, base and precious metal deposition is not strongly developed at Palinpinon. Base and precious metal concentrations in drill core and drill cuttings are one to two orders of magnitude below ore grade (<0.02 wt % Cu, <0.03 wt % Pb, <0.01 wt % Zn, <0.01 wt % Mo, <8 g/t Ag, and <0.05 g/t Au), and important features that characterize high-grade ore zones of porphyry (i.e., quartz stockwork veining) and epithermal deposits (i.e., residual vuggy quartz, massive crustiform veins, and/or hydrothermal breccia complexes) are apparently absent at Palinpinon. Such features intimate that high-grade ore deposition is promoted by large volumes of fluid and highly permeable wall rock. Their absence at Palinpinon indicates that permeability, fluid flux, and/or metal transport have been insufficient to form significant zones of base or precious metal enrichment.

The Sulfur Budget in Magmas: Evidence from Melt Inclusions, Submarine Glasses, and Volcanic Gas Emissions

Research Article| January 01, 2011 The Sulfur Budget in Magmas: Evidence from Melt Inclusions, Submarine Glasses, and Volcanic Gas Emissions Paul J. Wallace; Paul J. Wallace Deprtment of Geological Sciences, University of Oregon Eugene, Oregon 97403-1272, U.S.A., pwallace@uoregon.edu Search for other works by this author on: GSW Google Scholar Marie Edmonds Marie Edmonds Department of Earth Sciences University of Cambridge Cambridge, CB2 3EQ, United Kingdom, medm06@esc.cam.ac.uk Search for other works by this author on: GSW Google Scholar Author and Article Information Paul J. Wallace Deprtment of Geological Sciences, University of Oregon Eugene, Oregon 97403-1272, U.S.A., pwallace@uoregon.edu Marie Edmonds Department of Earth Sciences University of Cambridge Cambridge, CB2 3EQ, United Kingdom, medm06@esc.cam.ac.uk Publisher: Mineralogical Society of America First Online: 09 Mar 2017 © The Mineralogical Society Of America Reviews in Mineralogy and Geochemistry (2011) 73 (1): 215–246. https://doi.org/10.2138/rmg.2011.73.8 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Paul J. Wallace, Marie Edmonds; The Sulfur Budget in Magmas: Evidence from Melt Inclusions, Submarine Glasses, and Volcanic Gas Emissions. Reviews in Mineralogy and Geochemistry 2011;; 73 (1): 215–246. doi: https://doi.org/10.2138/rmg.2011.73.8 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyReviews in Mineralogy and Geochemistry Search Advanced Search The major magmatic volatile components—H2O, CO2, S, Cl, and F— play an important role in the formation, evolution, and eruption of magma. Knowledge of magmatic concentrations and fluxes of these volatiles is thus important for understanding explosive eruptive behavior of volcanoes, recycling of volatiles in subduction zones, formation of magmatic-hydrothermal ore deposits, fluxes of volcanic gases to Earth’s atmosphere, and potential climatic impacts of large volcanic eruptions. Over the past 30 years, new analytical techniques for measuring volatiles in melt inclusions and glasses from volcanic rocks and new developments in remote sensing technology used for quantifying... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Alkali metals control the release of gold from volatile-rich magmas

Magmatic-hydrothermal ore deposits are a major resource of gold. Volatiles exsolving from magmas rising through the Earth's crust are the key agent for enrichment of gold in these deposits. We report the first experimental determination of gold solubilities in the magmatic volatile phases at conditions typical of most magmas associated with gold deposits (T = 1000 °C, P = 150 MPa). We show that gold hydrosulfide complexes supersede gold chloride complexes, and more importantly, that the stability of gold hydrosulfide complexes is greatly increased by the presence of minute concentrations of KCl or NaCl (0.1–0.5 mol/kg H 2O). The amplifying effect of alkali chlorides on the solubility of gold in H 2S bearing volatiles may explain the preferential association of many giant hydrothermal gold deposits with high-potassium alkaline mafic to intermediate igneous rocks, which exsolve volatiles that simultaneously contain both H 2S and alkali chlorides in significant concentrations.

Numerical modeling of magmatic–hydrothermal systems constrained by U–Th–Pb–He time–temperature histories

This paper presents a 2D computational model which combines (U–Th)/He thermochronometry and U–Pb geochronology data with numerical modeling techniques to quantitatively constrain the thermal and exhumation history of igneous intrusions and associated magmatic–hydrothermal ore deposits. The model considers various parameters related to the cooling of intrusive bodies (conduction cooling, latent heat of crystallization and fusion, thermal convection within magma bodies, hydrothermal circulation induced by magma intrusion, exhumation and erosion) and quantifies the thermal and exhumation history of a given igneous intrusion directly from measured geo-/thermochronometry data. The complete thermal history of igneous bodies includes magmatic cooling and exhumation cooling. Numeric modeling indicates that the magmatic cooling of igneous bodies is complicated but can be further divided into two distinct stages. In the first stage the igneous body cools rapidly while the ambient country rock is simultaneously heated, and this stage is characterized by a very high cooling rate; during the second, longer duration cooling stage, both the igneous body and the ambient country rock cool slowly until they reach a final thermal equilibrium. The exhumation cooling stage is primarily controlled by erosion and exhumation processes. The cooling of igneous bodies is affected by many factors but intrusion size and emplacement depth are the principal controls on the cooling rate and the duration of the two magmatic cooling stages. Combined models for magmatic and exhumation cooling provide a complete digitized thermal history curve which can then be constrained using apatite (U–Th)/He, zircon (U–Th)/He, and zircon U–Pb age data. The digitized temperature–age curve defines the time and depth of emplacement, crystallization age intervals for mineralised zones, cooling rate, “cooled” and exposure ages, and exhumation/erosion rate for the ore deposit. The validity of the curve generated by the inverse thermal modeling routine is demonstrated using age data obtained from the Sar Cheshmeh porphyry copper deposit in Iran.

Sulfur Contents at Sulfide-Liquid or Anhydrite Saturation in Silicate Melts: Empirical Equations and Example Applications

We present a new empirical equation for estimating sulfur content in silicate melts at anhydrite saturation based on available experimental results and an updated empirical equation for estimating sulfur content in silicate melts at sulfide-liquid saturation by adding relevant experimental data that were not previously used and excluding controversial experimental data. The fits of our empirical equations to the data used for calibration are within 10 percent error. Only total pressure, temperature, and silicate melt composition, parameters that are often known or can be reasonably assumed for natural magmatic systems, are required for utilization of the equations. The empirical equations are useful for petrologic modeling of natural magmatic systems, including magmatic hydrothermal ore deposits. Several example applications including the effect of decompression on sulfur content at sulfide-liquid saturation in komatiite and high Mg basaltic magmas and the ratio of silicate minerals to immiscible sulfide liquids during fractional crystallization of these magmas are provided.

The role of sulfur in the formation of magmatic–hydrothermal copper–gold deposits

Essential resources of many rare metals including copper, zinc, molybdenum, silver and gold occur in natural sulfide mineral deposits. Understanding the origin of these metal resources has been limited by a lack of data about the geochemistry of sulfur, the most important and abundant element of ore deposits. We report the first directly measured sulfur concentrations in high-temperature fluids, together with their ore-metal contents, using a new method for sulfur quantification in fluid inclusions by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Co-genetic brine and vapor inclusions from magmatic–hydrothermal ore deposits and granitic intrusions show an excess of sulfur over ore metals, as required for efficient ore-mineral precipitation. The results demonstrate that S, Cu and Au are highly enriched in vapor-like magmatic fluids, implying that such low-salinity fluids are the key agent for the formation of porphyry copper and epithermal gold deposits.

Platinum solubility and partitioning in a felsic melt–vapor–brine assemblage

The solubility and partitioning of Pt in a S-free vapor – brine – rhyolite melt – Pt metal assemblage has been quantified at 800 °C, f O 2 = NNO and pressures of 100 and 140 MPa. Vapor and brine were sampled at run conditions by trapping these phases as glass-hosted fluid inclusions as the melt cooled through the glass transition temperature. The vapor and brine were in equilibrium with the melt at the time of trapping and, thus, represent fluids which were sampled at the termination of each experimental run. The microthermometrically determined salinities of vapor and brine are ∼2 and ∼63 wt.% NaCl eq. and ∼9 and ∼43 wt.% NaCl eq. at 100 and 140 MPa, respectively. Platinum solubilities in vapor, brine and glass (i.e., quenched melt) were quantified by using laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS). Equilibrium is discussed with reference to the major and trace element concentrations of glass-hosted fluid inclusions as well as the silicate melt over run times that varied from 110 to 377 h at 140 MPa and 159 to 564 h at 100 MPa. Platinum solubility values (±1σ) in H 2O-saturated felsic melt are 0.28 ± 0.13 μg/g and 0.38 ± 0.06 μg/g at 140 and 100 MPa, respectively. Platinum solubility values ( ± 1 σ x ¯ ) at 140 and 100 MPa, respectively, in aqueous vapor are 0.91 ± 0.29 μg/g and 0.37 ± 0.17 μg/g and in are brine 16 ± 10 μg/g and 3.3 ± 1.0 μg/g. The measured solubility data were used to calculate Nernst-type partition coefficients for Pt between vapor/melt, brine/melt and vapor/brine. The partition coefficient values ( ± 1 σ x ¯ ) for vapor/melt, brine/melt and vapor/brine at 140 MPa are 2.9 ± 1.0, 67 ± 27, and 0.13 ± 0.05 and at 100 MPa are 1.0 ± 0.2, 6.8 ± 2.4, and 0.15 ± 0.05. The partitioning data were used to model the Pt-scavenging capacity of vapor and brine during the crystallization-driven degassing (i.e., second boiling) of a felsic silicate melt over a depth range (i.e., 3–6 km) consistent with the evolution of magmatic-hydrothermal ore deposits. Model calculations suggest that aqueous vapor and brine can scavenge sufficient quantities of Pt, and by analogy other platinum group elements (PGE), to produce economically important PGE-rich magmatic-hydrothermal ore deposits in Earth’s upper continental crust.

Synthetic Fluid Inclusions. XVII. PVTX Properties of High Salinity H2O-NaCl Solutions (&gt;30 wt % NaCl): Application to Fluid Inclusions that Homogenize by Halite Disappearance from Porphyry Copper and Other Hydrothermal Ore Deposits

Fluid inclusions that homogenize by disappearance of the halite crystal after the vapor bubble has disappeared have been reported from a wide range of ore-forming systems. While this inclusion type is most commonly observed in porphyry copper and similar magmatichydrothermal ore deposits, they have also been reported from iron oxide copper-gold (IOCG), Mississippi Valley-type, orogenic (lode, greenstone) gold, massive sulfide, unconformity U, and mantotype deposits, as well as from non-ore associated granitoids and oceanic crust. The lack of experimental data to interpret microthermometric data from these inclusions, combined with poor petrographic characterization of inclusion origin, has led to a wide range of inferred PT trapping conditions for the inclusions. The relationship between liquid-vapor homogenization temperature (ThL-V), halite dissolution temperature (Tmhalite) and pressure has been determined using synthetic H2O-NaCl fluid inclusions that homogenize by halite disappearance. The experimental data cover the range ThL-V ≈ 150° to 500°C and Tmhalite ≈ 275° to 550°C. An empirical equation describing the relationship between pressure, ThL-V, and Tmhalite has been developed to estimate formation pressures from microthermometric data and is valid from pressures along the liquid + vapor + halite curve to 300 MPa. A detailed literature review reveals that the results of this study cannot be applied retroactively to estimate pressures of previously reported inclusions that homogenize by halite dissolution for several reasons. First, most workers have not collected and reported the data in a manner that allows one to determine if the inclusions have trapped a single, homogeneous phase and have not leaked or changed volume following trapping. Thus, it is not possible to determine if the inclusions show consistent microthermometric behavior within a group of coeval inclusions. Additionally, many published studies provide only summaries of results from numerous samples or present the data graphically, precluding application of our results to individual inclusions. However, based on our review, it appears that much of the published data for inclusions that homogenize by halite dissolution represent inclusions that have either trapped a halite crystal along with the liquid or have reequilibrated by necking and/or stretching. Results from this study have been used to estimate minimum formation pressures for inclusions from the Ditrau Alkaline Massif, Transylvania, Romania, the Musoshi stratiform copper deposit, Zaire, the Bismark skarn deposit, northern Mexico, the Naica chimneymanto deposit, Mexico, the Questa porphyry molybdenum deposit, New Mexico, and the Bingham Canyon porphyry Cu-Mo deposit, Utah. In each case, pressures estimated using results from the present study are in good agreement with previous pressure estimates. However, in some cases estimated pressures (both our estimates as well as those of other authors) appear to be unreasonably high, based on the geologic setting at the time of formation. These results highlight the currently limited understanding concerning pressures (depths) of ore formation.

Primary igneous anhydrite: Progress since its recognition in the 1982 El Chichón trachyandesite

Primary igneous anhydrite was first identified in 1982 El Chichón pumices. Analysis of the sulfur budget for the eruption provided compelling evidence that the pre-eruptive magma contained a significant gas phase at ∼ 7 km depth in order to account for the “excess gas release” of ∼ 5–9 million tons of SO 2 to the stratosphere by the eruption. Primary igneous anhydrite and a larger “excess gas release” of ∼ 20 million tons of SO 2 were noted for the significantly larger eruption of Mount Pinatubo in 1991, for which a separate gas phase at ∼ 7–9 km depth was also required by the sulfur budget. Pumices from both eruptions have mineral assemblages dominated by plagioclase and hornblende, with minor biotite, and show evidence for co-nucleation and mutual inclusions of anhydrite and apatite. Both magmas were also very water-rich and highly oxidized, with oxygen fugacities $1 log unit above the synthetic Ni–NiO buffer. Furthering the similarities between these two eruptions, ion-microprobe analyses of sulfur isotopic compositions of anhydrites in pumices from El Chichón and Mount Pinatubo both showed that individual crystals are isotopically homogeneous, but inter-crystalline variations in δ 34S are well beyond analytical error. This study reviews the history of thought regarding primary igneous anhydrite. Ten volcanic and plutonic analogues to the El Chichón and Mount Pinatubo magmas are discussed. All were similarly water-rich and highly oxidized magmas. Hornblende is the dominant mafic phenocryst in all but one plutonic example; in that case so much early anhydrite precipitated that no calcium remained to form hornblende. Biotite is very abundant in that exceptional plutonic case, and is present in six of the other examples with primary igneous anhydrite. Sphene is known as an indicator mineral of high magmatic oxygen fugacity; it was present in the 1982 El Chichón pumices, and in 6 of the other anhydrite-bearing samples. Hornblende and biotite phenocrysts are also characteristic of the intermediate-to-felsic plutonic rocks that typically host porphyry–Cu deposits. Such magmas are also known to be water-rich and highly oxidized. A close spatial and petrogenetic connection exists between magmas with primary igneous anhydrite and magmatic–hydrothermal ore deposits.

CHEMICAL SUBSTITUTIONS IN OXIDIZED TOURMALINE IN GRANITE-RELATED MINERALIZED HYDROTHERMAL SYSTEMS, WESTERN TURKEY

Tourmaline is the most common borosilicate mineral in granite-related magmatic-hydrothermal ore deposits in the Aegean region of western Turkey. Hydrothermal deposits there include the mesothermal argentiferous Pb–Zn–Cu deposit at Kadikalesi (Bodrum–Mugla) and the porphyry Au deposit at Kisladag (Esme–Usak), with related potassic igneous rocks and accompanying Tertiary strata of western Anatolia. At Kadikalesi, mesothermal vein-type mineralization is linked to monzodioritic intrusions in a subvolcanic suite, which is overlain by volcanic rocks ranging in composition from calc-alkaline to potassic. In the non-brecciated hydrothermal systems, tourmaline occurs as subhedral to anhedral crystals in monzodiorite or as an essential mineral in veins and their reaction zones. At Kisladag, magmatic-hydrothermal brecciation associated with multiphase intrusions of latite porphyry is related to the Kisladag caldera. Tourmaline is an abundant and widespread alteration-induced mineral throughout the Kisladag porphyry system. Electron-microprobe analyses of the tourmaline crystals collected from both hydrothermal deposits show significant chemical variation within the range of buergerite compositions. The main substitution mechanism involves the CaFe 2+ [Na −1 (Al,Fe 3+ ) −1 ] exchange vector. Compositions deviate from ideal schorl–dravite along a trend that closely approximates the uvite exchange-vector. Mossbauer spectroscopy shows the presence of significant Fe 3+ in all the tourmaline crystals, suggesting a high oxidation state for the ore-forming fluids, and for related hydrothermal systems associated with emplacement of subvolcanic intrusions.

Fluid-Fluid Interactions in Magmatic-Hydrothermal Ore Formation

Hydrothermal ore deposits have provided some of the first geological evidence for the interaction of multiple fluids in the Earth’s interior, including fluid mixing as well as fluid phase separation. Physical evidence comes from veins and breccias documenting the impact of fluids in breaking rocks and creating fracture permeability for ore formation, as a result of the fluid expansion that is commonly associated with phase separation between melts and multiple fluids. Chemical evidence of fluid–fluid interaction includes the precipitation of minerals in veins as a result of fluid boiling or mixing between chemically contrasting fluids. Fluid inclusions record interactions of multiple fluids, especially in magmatic-hydrothermal ore deposits and associated igneous rocks (Fig. 1⇓). In recent years, fluid inclusions have contributed to a more quantitative understanding of the role of fluid–fluid interactions, since new microanalytical techniques allow determination of element concentrations in all types of fluids, from low-density vapor through aqueous and hypersaline liquids to hydrous salt melts (Roedder 1984; Samson et al. 2003). Figure 1. Geological evidence for fluid phase separation in ore-forming magmatic-hydrothermal systems. (a) ‘Bonanza grade’ gold (∼1 wt%) that was precipitated together with fine-grained pyrite in a fluidized breccia resulting from instantaneous boiling and expansion of magmatic fluids ascending from an underlying andesite porphyry intrusion. The breccia consists of adularia–sericite altered clasts of sedimentary wall rock (s) and nodules of (originally colloidal?) pyrite with bright specks of gold (p+g), both cemented by roscoelite (r; ∼ vanadium-rich muscovite) and late calcite (c) (Zone VII ore body, Porgera, Papua New Guinea; Ronacher et al. 2004). (b) Bladed manganocalcite, the characteristic habit of calcite precipitated from aqueous epithermal fluids as a result of the pH change following loss of CO2 to a separating vapor phase (Madan, Bulgaria; Kostova et al. 2004). (c) Growth band in a euhedral quartz …