Magmatic Volatile Phase Research Articles

Using tourmaline to trace Li mineralization in the Mufushan granitic batholith, South China

Granites and rare metal pegmatites of the Mufushan granitic batholith form a continuous magmatic sequence linked by fractional crystallization. Tourmaline is present in muscovite leucogranites and all types of pegmatites, including highly evolved Li-rich pegmatites. We utilized major element, trace element and in-situ B isotope analyses of tourmaline to investigate the effects of magmatic fractional crystallization and magmatic volatile phase (MVP) exsolution on Li migration and exceptional Li enrichment. Eight types of tourmaline are identified across three rock units: (i) Isolated (Tur Ia) and nodular (Tur Ib) tourmaline within muscovite leucogranites; (ii) black tourmaline in veins and/or clusters (Tur IIa), as isolated crystals (Tur IIb) and in tourmaline-quartz segregations (Tur IIc) within Li-poor pegmatites; and (iii) tourmaline as isolated pink crystals with zoning patterns (Tur IIIa), as isolated pink crystals and/or radiating clusters (Tur IIIb), and as isolated crystals enclosed in quartz block (Tur IIIc) within Li-rich pegmatites. Tourmaline in Mufushan muscovite leucogranites and Li-poor pegmatites belongs to the alkali-group and schorl series with Mg/(Mg + Fe) ratios of 0.10–0.31 and 0.12–0.48, respectively, containing almost no Li* and F. In contrast, tourmaline in Li-rich pegmatites exhibits schorl-elbaite and elbaite-rossmanite compositions with low Mg/(Mg + Fe) ratio (avg. = 0.01), and evolved Li* (0.01–0.90 apfu, avg. = 0.41 apfu) and F (0.00–0.91 apfu, avg. = 0.36 apfu) contents. A pronounced increase in YAl, (Li* + Mn) contents, and Y[Al/(Al + Fe)] ratio is observed across the transition from Li-poor to Li-rich pegmatites, consistent with the anticipated pattern of fractional crystallization. The concentration of Li exhibits a sharp increase in Li-rich pegmatites (avg. Li = 6786 ppm) compared to Li-poor pegmatites (avg. Li = 114 ppm) and leucogranites (avg. Li = 469 ppm). Lithium contents increase and reach a peak during the crystallization of Tur IIIb (6686–11,667 ppm), and have lower peak contents during the precipitation of Tur IIIc (8261–9160 ppm), indicating that the incorporation of Li is influenced by MVP accumulation and exsolution. MVP exsolution significantly reduces the solubility of Nb, Ta, and Be in the residual melt, promoting the precipitation of beryl and columbite group minerals and facilitating the migration of fluid-mobile elements such as Li, Rb, Cs, and Ga to form lepidolite. The B isotope compositions of tourmaline range from −14.8 ‰ ~ −12.6 ‰ in Li-poor pegmatites to −17.1 ‰ ~ −14.0 ‰ in Li-rich pegmatites. Rayleigh fractionation modeling reveals that MVP saturation occurs after approximately 60 % B was removed from the pegmatite melt. The compositional variation of tourmaline demonstrates that Li enrichment is not only governed by continuous fractional crystallization, but also by MVP-related accumulation and exsolution mechanism.

Fluid inclusions in magmatic ilmenite record degassing in basic magmas

Magmatic volatile phases play a major role in igneous systems, but indirect sampling of the magmatic fluid, especially for basic magmas, remains challenging to document. Here, we report compelling evidence of primary fluid inclusions trapped within magmatic ilmenite from two different basic intrusive settings: Armorican Massif (France) and Central Iberian Zone (Spain). Fluid inclusions have a solute chemistry dominated by sodium, calcium, chlorine, sulfur and iron, with detectable contents of metals and metalloids and thus likely record the onset of late-stage magmatic volatile saturation in these basic magmas. Hence, we argue the presence of fluid inclusions in ilmenite may be a good indicator for magma degassing in these settings and importantly records the magmatic-hydrothermal transition. Additionally, this study suggests that the trapping of the magmatic volatile phase during ilmenite (and other opaque minerals) crystallization may be more common but at present an underappreciated phenomenon in basic igneous magmatic systems.

Migration and focusing of porphyry deposit-forming fluids through aplitic mush of the Saginaw Hill cupola, Arizona, United States

Porphyry-type Cu ± Au ± Mo deposits form in the upper (ca. 2–5 km deep) parts of large, long-lived magmatic-hydrothermal systems in which mineralising fluids are thought to be derived from mid-to shallow-crustal magma chambers. Increasingly, however, magmatic systems are viewed as consisting of mush with minor and transient lenses of magma, with mush being a variably packed framework of crystals with interstitial melt and magmatic volatile phase (MVP). In this context, questions remain as to the source (mainly depth) and mechanisms of transport and focussing of the vast volumes of fluids required for shallow level porphyry-type mineralisation. Even more problematic is a paucity of first-order textural evidence for the presence of mush in magmatic-hydrothermal systems, including those which host porphyry-type deposits. To address this, we have studied the aplitic porphyry cupola of the Saginaw Hill magmatic system, Tuscon, Arizona, United States, where magmatic-hydrothermal features are exceptionally well exposed, including a massive silica cap, quartz unidirectional solidification textures (USTs), stockworks of multiple generations of variably mineralised quartz veins and mineralised miarolitic cavities. From field-to micro-scale textural and geochemical studies, particularly observations of vermiform quartz between earlier generations of magmatic quartz and feldspar, we evidence the development of fluid pathways through mush at the magmatic-hydrothermal transition. These are shown to connect and provide fluids and ore constituents to the mineralised miarolitic cavities and early quartz vein stockworks. We suggest that this process should be considered in all new genetic, exploration and numerical models for porphyry and similar types of magmatic-hydrothermal ore-deposits.

Mineralogical constraints on pegmatite genesis and rare metal mineralization in the Mufushan batholith, South China

Pegmatites are commonly associated with or strongly influenced by the crystallization of magmatic volatile phases (MVP) as ascending magmas undergo differentiation. The mechanism of fractional crystallization, liquid immiscibility, and water–rock interactions that influence the rare metals concentration in pegmatites are still unclear. We analyzed the mineralogy (incl. monazite, garnet, zircon, and muscovite) from pegmatites to unravel the timing and magmatic-hydrothermal processes for the giant Renli-Chuanziyuan pegmatite Li-Nb-Ta mineralization in the Mufushan region (South China). Five types of pegmatites have been identified at Mufushan, i.e., those containing mainly orthoclase (O-pegmatite), microcline (M-pegmatite), albite-orthoclase (AO-pegmatite), microcline-albite (MA-pegmatite), and spodumene-albite (S-pegmatite).Monazites from the Renli high-Th Li-mica pegmatite exhibit magmatic affinity. The ∼140 Ma age of monazite U-Pb suggests a temporal relationship between the Li-rich pegmatite and its muscovite granite host. Garnet from these pegmatites is magmatic and belongs to the almandine-spessartite series, and contains feldspar and apatite inclusions. Continuous fractional crystallization likely controlled the pegmatite development, as the garnet spessartite content increases with distance from the Mufushan batholith. Garnet in the spodumene-albite pegmatites has very low REE content, and it represents a late stage of crystallization related to magmatic-hydrothermal fluids. Compositional variation in muscovite from all of the pegmatites is negligible. The K/Rb ratio (a proxy for the degree of fractionation) increases gradually from the O- to S-pegmatite, whilst the muscovite F content correlates positively with the degree of magma fractionation. The transformation of muscovite to lepidolite in pegmatites can occur through magmatic-hydrothermal crystallization or metasomatism. Zircon in pegmatites generally occurs as metasomatic remnants, with uneven zoning shown in cathodoluminescence (CL) images. Hydrothermal zircon U-Pb ages are concentrated in the 140–134 Ma range, consistent with the garnet leucogranite ages. This suggests that pegamtites and the garnet leucogranite were formed in the same magmatic-hydrothermal activity. The accumulated MVP and incompatible elements may have migrated from deeper, more crystal-rich zones under medium degree of fractionation. The MVP may contain abundant dissolved melt components as single-phase hydrous silicate liquids, forming typical quartz-/feldspar-dominated pegmatite and the diversity of rare-metal deposits.

Lithium isotopic systematics and numerical simulation for highly-fractionated granite-pegmatite system: Implications for the pegmatite-type rare-metal mineralization

Lithium (Li) isotope behavior during fractionation of highly-evolved granitic pegmatites and associated rare metal mineralization remains unclear, especially concerning the relative importance of magmatic versus metasomatic processes. In this study, we investigated the geochemistry and Li isotopes of the giant Renli-Chuanziyuan lithium-niobium-tantalum (Li-Nb-Ta) deposit in the Mufushan batholith (South China). We found that muscovite granite in the Mufushan complex can be divided into two groups (high-Li and low-Li). δ7Li value of the high-Li and low-Li group increases and decreases, respectively, with increasing Li content, which is likely resulted from fractional crystallization and/or magmatic volatile phase (MVP) exsolution. The MVP exsolution began with the solidification of muscovite granite, as evidenced by the zoned micas. The Li isotope fractionation, modeled by Rayleigh distillation during MVP exsolution, is consistent with the δ7Li value of Li-poor pegmatite, indicating that MVP exsolution is related to the pegmatite formation.The Mufushan Li-poor and Li-rich pegmatites have very high and low δ7Li value, respectively. Based on the Δ7Li(bulk-mica) value and numerical simulation, we suggest that the pegmatite formation at Mufushan could be linked to extensive MVP accumulation, while the different Li elemental and isotopic compositions of the two types of pegmatite are probably caused by MVP migration. In fact, published granite-pegmatite data worldwide also show negative Li vs. δ7Li correlation, indicating that such MVP exsolution, accumulation and migration are probably a common phenomenon for evolution of granitic pegmatite. Therefore, the different Li isotopes and Li-Rb-Cs proportion of Li-poor and Li-rich pegmatites indicate that MVP exsolution, migration and accumulation are key to rare metals enrichment and distinguish various types of rare-metal mineralization (Li and Nb-Ta).

Variations in water saturation states and their impact on eruption size and frequency at the Aso supervolcano, Japan

The exsolution of a magmatic volatile phase in the plumbing systems of volcanoes plays a key role in controlling growth dynamics of subvolcanic reservoirs and eruptive styles. By using common petrological proxies found in volcanic deposits, such as melt inclusions and apatite crystals, the presence of such an exsolved magmatic volatile phase can be traced, specifically the exsolution of water from silicate melt. To monitor variations in the water saturation state of magmas from the Aso volcanic complex (Kyushu, Japan) prior to and during the catastrophic Aso-4 caldera-forming event (at ∼86 ka BP), we investigate a set of pre-Aso-4 and Aso-4 deposits, combining volatile budgets (F, Cl, OH, S) of melt inclusions, matrix glasses, and apatite crystals, with sulfur isotope signatures in apatite. F-Cl-OH partitioning in apatite along with melt inclusion data from pre-Aso-4 units indicate water-undersaturated conditions during magma evolution until around 10 ka prior to the Aso-4 event. In contrast, eruptions occurring within the last ∼10 ka prior to the Aso-4 event indicate the presence of a water-rich exsolved magmatic volatile phase in the eruptible portions of the subvolcanic reservoir. Likewise, sulfur systematics in apatite and melt inclusions from the Aso-4 event suggest the exsolution of a water-rich magmatic volatile phase at some point prior to the caldera-forming event. Recharge of mafic magmas shortly before the Aso-4 eruption induced chemical hybridization in the resident upper crustal mush, bringing the reservoir back to less evolved compositions and water-undersaturated conditions. This hybridization event is recorded by volatile contents of both apatite and matrix glasses from late-erupted, crystal-rich products of the Aso-4 event, all yielding water-undersaturated signatures. During this hybridization event, chemical dilution and partial redissolution of the exsolved volatile phase reduced the magma compressibility significantly, so that additional magma influx from depth might have allowed a sharply increasing overpressurization in the subvolcanic reservoir and served as a potential trigger for the cataclysmic Aso-4 eruption. Drawing from our observations made on Aso, we propose that recharging of large silicic upper-crustal reservoirs with increased volumes of drier and more mafic melts can influence their water saturation states and associated physical properties. Such changes could contribute to the triggering of large-scale caldera-forming events.

Apatite evidence for a fluid-saturated, crystal-rich magma reservoir forming the Quellaveco porphyry copper deposit (Southern Peru)

Large volume, intermediate-felsic magma reservoirs are the source of melt and mineralising fluids which generate porphyry copper deposits. Cooling and crystallisation of hydrous magmas drives the exsolution and expulsion of a magmatic volatile phase—a process which remains challenging to constrain in porphyry Cu systems where the record of magma volatile compositions is rarely preserved. Here, we use the halogen compositions of apatite inclusions shielded as inclusions within zircon to constrain volatile evolution in magma reservoirs which pre-date and are synchronous with porphyry Cu mineralisation at Quellaveco, Southern Peru. Geochemical and textural data confirm that the zircon-included apatites escaped re-equilibration with hydrothermal fluids, unlike apatites found in the groundmass of the same rocks. We, therefore, recommend that future studies attempting to reconcile magmatic volatile budgets using apatite in porphyry Cu systems should focus on apatite inclusions in zircon. By combining the apatite inclusion data with numerical modelling, we find evidence that the magma reservoir sourcing porphyry Cu mineralisation remained fluid-saturated for the entire period recorded by apatite crystallisation. By contrast, the pre-mineralisation batholith shows more variable, potentially fluid-undersaturated behaviour. Our modelling suggests that in order to attain the porphyry melt volatile compositions inferred from apatite, the magma reservoir must have exsolved a large proportion of its volatile budget, consistent with having been held at high crystallinity (40–60% crystals). This crystallisation interval coincides with peak chlorine and copper extraction from intermediate-felsic magmas, and would have permitted efficient fluid migration and accumulation at the roof of the system. We suggest that the storage of large-volume, long-lived, crystal-rich magma reservoirs in magmatic arcs may be a critical step in generating world-class porphyry copper deposits.

Multistage in situ fractional crystallization of magma produced a unique rare metal enriched quartz-zinnwaldite-topaz rock

Highly fractionated granites and pegmatites have special petrogenesis and potential economic significance due to the enrichment of incompatible elements (Li, Rb, Cs, etc.) and flux elements (F, B, Li, etc.) in the magmatic volatile phase (MVP). Here we decipher the origin of the giant Zhengchong Rb-dominated deposit associated with highly fractionated A-type granite in South China. An extreme fractional crystallization (up to 98.3%) was required to generate super enrichment of incompatible elements and flux elements by the model of single-step in situ crystallization. An alternative model incorporates episodic remobilization and multistage in situ crystallization of a long-lived mush. The MVP and incompatible elements migrate via reactive flow at intermediate degree of crystallization (70–80%) in each intrusion fits with the observed geochronological and geochemistry data. This geochemical model can help us understand the evolution of magma reservoirs and the concept of magmatic distillation columns.

Tracing Volatiles, Halogens, and Chalcophile Metals during Melt Evolution at the Tolbachik Monogenetic Field, Kamchatka

AbstractMelt storage and supply beneath arc volcanoes may be distributed between a central stratovolcano and wider fields of monogenetic cones, indicating complex shallow plumbing systems. However, the impact of such spatially variable magma storage conditions on volatile degassing and trace element geochemistry is unclear. This study explores magma generation and storage processes beneath the Tolbachik volcanic field, Kamchatka, Russia, in order to investigate the evolution of the magmatic volatile phase and, specifically, the strong enrichment of chalcophile metals (in particular, Cu) in this system. We present new geochemical data for a large suite of olivine- and clinopyroxene-hosted melt inclusions (and host phenocrysts) from five separate monogenetic cones within the Tolbachik volcanic field. These high-Al composition magmas likely reflect the homogenised fractionation products of primitive intermediate-Mg melt compositions, stored at shallow depths after significant fractional crystallisation. Boron isotope compositions and incompatible trace element ratios of the melt inclusions suggest a deeper plumbing system that is dominated by extensive fractional crystallisation and fed by melts derived from an isotopically homogeneous parental magma composition. Volatile components (H2O, CO2, S, Cl, F) show that magmas feeding different monogenetic cones had variable initial volatile contents and subsequently experienced different fluid-saturated storage conditions and degassing histories. We also show that melts supplying the Tolbachik volcanic field are strongly enriched in Cu compared with almost all other Kamchatka rocks, including samples from the Tolbachik central stratocones, and other volcanoes situated in close proximity in the Central Kamchatka Depression. The melt inclusions record Cu concentrations ≥450 μg/g at ca. 4–5 wt.% MgO, which can only be explained by bulk incompatible partitioning behaviour of Cu, i.e. evolution under sulphide-undersaturated conditions. We suggest that initial mantle melting in this region exhausted mantle sulphides, leading to sulphide undersaturated primitive melts. This sulphide-free model for the high-Al cone melts is further supported by S/Se and Cu/Ag values that overlap those of the primitive mantle and MORB array, with bulk rock Cu/Ag ratios also overlapping other with other global arc datasets for magma evolution prior to fractionation of a monosulfide solid solution. We therefore demonstrate that the combination of novel chalcophile metal analyses with trace element, isotopic, and volatile data is a powerful tool for deciphering complex magmatic evolution conditions across the entire volcanic field.

Biotite as a recorder of an exsolved Li-rich volatile phase in upper-crustal silicic magma reservoirs

Abstract The magmatic-hydrothermal transition is key in controlling the fate of many economically important elements due to the change in partitioning when melt and magmatic fluid coexist. Despite its increasing economic importance, the behavior of lithium (Li) in such environments remains poorly known. We illustrate how compositionally unusual biotites from the rhyolitic Bishop Tuff (California, USA) and Kos Plateau Tuff (Greece) may contain a magmatic volatile phase trapped between layers of biotite crystals. Despite originating in pristine deposits and showing the expected X-ray diffraction spectra, these biotites return low (<95 wt%) analytical totals via electron microprobe (EMP) consistent with the presence of considerable amounts of light elements (non-measurable by EMP). Lithium contents and isotope ratios in these biotites are remarkable, with abundances reaching >2300 ppm, exceptionally light Li isotopic compositions (δ7Li as low as –27.6‰), and large isotopic fractionation between biotite and corresponding bulk samples (Δ7Libt–bulk as low as –36.5‰). Other mineral phases, groundmass glass, and melt inclusions from the same units do not support an extremely Li-rich melt prior to eruption. Biotites from phonolitic systems (Tenerife [Canary Islands] and Campi Flegrei [Italy]) do not show such extreme compositional differences, with biotite and melt showing roughly equivalent Li contents, underscored by significantly reduced Δ7Libt–bulk to a maximum of –10.9‰. We ascribe the difference in behavior to the near-liquidus appearance of biotite in alkaline magmatic suites, before widespread exsolution of a magmatic volatile phase in the magma reservoir, while in rhyolitic suites, biotite crystallizes at low temperature, trapping the coexisting exsolved fluid phase in the reservoir.

Mineral-scale variation in the trace metal and sulfur isotope composition of pyrite: implications for metal and sulfur sources in mafic VMS deposits

The link between metal enrichment and the addition of a magmatic volatile phase in volcanogenic massive sulfide deposits and actively forming seafloor massive sulfide deposits remains poorly characterized. This is especially true when considering how metal, sulfur and fluid flux change with time. In this study, we combine in situ sulfur isotope (δ34S; n = 31) measurements with trace metal chemistry of pyrite (n = 143) from the Mala VMS deposit, Troodos, Cyprus. The aim of our study is to assess the links between volatile influx and metal enrichment and establish how, or indeed if, this is preserved at the scale of individual mineral grains. We classify pyrite based on texture into colloform, granular, disseminated and massive varieties. The trace metal content of different pyrite textures is highly variable and relates to fluid temperature and secondary reworking that are influenced by the location of the sample within the mound. The sulfur isotope composition of pyrite at Mala ranges from − 17.1 to 7.5‰ (n = 31), with a range of − 10.9 to 2.5‰ within a single pyrite crystal. This variation is attributed to changes in the relative proportion of sulfur sourced from (i) SO2 disproportionation, (ii) thermochemical sulfate reduction, (iii) the leaching of igneous sulfur/sulfide and (iv) bacterial sulfate reduction. Our data shows that there is no correlation between δ34S values and the concentration of volatile elements (Te, Se) and Au in pyrite at Mala indicating that remobilization of trace metals occurred within the mound.

The role of magmatic fluids in the ~3.48 Ga Dresser Caldera, Pilbara Craton: New insights from the geochemical investigation of hydrothermal alteration

Hydrothermal fluids played a key role in establishing the environmental conditions in which ancient stromatolites grew within the North Pole Chert of the ~3.48 Ga Dresser Formation (Pilbara Craton, Western Australia). However, there has been uncertainty as to the physicochemical conditions of the hydrothermal system in relation to (i) the distribution of hydrothermal alteration, (ii) the relative contribution of seawater and/or magmatic volatiles to the hydrothermal fluids, and (iii) the origin of some of the major elements mobilized in the hydrothermal fluids.This study examines the hydrothermal alteration of the underlying North Star Basalt in order to better understand the nature of the circulating fluids and to determine the processes responsible for the transport and accumulation of metals and metalloids to the near surface environment. Detailed geological mapping reveals a complex distribution of alteration mineral assemblages that is controlled at all stratigraphic depths by the distance to the major fluid pathways that are now represented by hydrothermal silica veins. With increasing distance from the vein margins, alteration assemblages change from argillic (kaolinite–quartz) to phyllic (illite–quartz), and then to propylitic (chlorite–albite–epidote) and actinolitic (actinolite–albite–chlorite–epidote) at more distal positions. Illite Ar–Ar dating of argillic–altered basalt proximal to major hydrothermal veins immediately below the North Pole Chert confirms a syn–depositional hydrothermal origin of alteration, and demonstrates that the mineralogical and chemical features developed through the circulation of hydrothermal fluids were largely preserved after subsequent thermal overprints at 3.25, 3.06, and 2.29 Ga.The spatial distribution of the alteration mineral assemblages indicates that the fluids circulating in the hydrothermal system were highly acidic (pH < 3) for at least some time during the evolution of the Dresser Caldera. Such highly acidic fluid conditions were likely promoted by the input of magmatic volatile phases, such as HCl, SO2, H2S, and F. Bulk geochemical analyses of altered basalts reveal that large amount of metals, including Fe, Mg, Ni, and Zn, were leached from the North Star Basalt during hydrothermal alteration and delivered to the surface. Furthermore, our data indicate that K and Ba were introduced into the hydrothermal system from external reservoir(s). Although the contribution of K–rich seawater cannot be completely discounted, we argue that the bulk of K and Ba was sourced from an underlying magma chamber undergoing fractional crystallization of a melt with TTG–like composition.

The physical and chemical evolution of magmatic fluids in near-solidus silicic magma reservoirs: Implications for the formation of pegmatites

AbstractAs ascending magmas undergo cooling and crystallization, water and fluid-mobile elements (e.g., Li, B, C, F, S, Cl) become increasingly enriched in the residual melt until fluid saturation is reached. The consequential exsolution of a fluid phase dominated by H2O (magmatic volatile phase or MVP) is predicted to occur early in the evolution of long-lived crystal-rich “mushy” magma reservoirs and can be simulated by tracking the chemical and physical evolution of these reservoirs in thermomechanical numerical models. Pegmatites are commonly interpreted as the products of crystallization of late-stage volatile-rich liquids sourced from granitic igneous bodies. However, little is known about the timing and mechanism of extraction of pegmatitic liquids from their source. In this study, we review findings from thermomechanical models on the physical and chemical evolution of melt and MVP in near-solidus magma reservoirs and apply these to textural and chemical observations from pegmatites. As an example, we use a three-phase compaction model of a section of a mushy reservoir and couple this to fluid-melt and mineral-melt partition coefficients of volatile trace elements (Li, Cl, S, F, B). We track various physical parameters of melt, crystals, and MVP, such as volume fractions, densities, velocities, as well as the content in the volatile trace elements mentioned above. The results suggest that typical pegmatite-like compositions (i.e., enriched in incompatible elements) require high crystallinities (&amp;gt;70–75 vol% crystals) in the magma reservoir, at which MVP is efficiently trapped in the crystal network. Fluid-mobile trace elements can become enriched beyond contents expected from closed-system equilibrium crystallization by transport of MVP from more-evolved mush domains. From a thermomechanical perspective, these observations indicate that, rather than from melt, pegmatites may more likely be generated from pressurized, solute-rich MVP with high concentrations of dissolved silicate melt and fluid-mobile elements. Hydraulic fracturing provides a mechanism for the extraction and emplacement of such pegmatite-generating liquids in and around the main parental near-solidus mush as pockets, dikes, and small intrusive bodies. This thermomechanical framework for the extraction of MVP from mushes and associated formation of pegmatites integrates both igneous and hydrothermal realms into the concept of transcrustal magmatic distillation columns.

A missing link between ancient and active mafic-hosted seafloor hydrothermal systems – Magmatic volatile influx in the exceptionally preserved Mala VMS deposit, Troodos, Cyprus

Reconciling observations between ancient volcanogenic massive sulfide (VMS) and actively forming seafloor massive sulfide (SMS) deposits is critical for understanding the sources and processes that govern metal enrichment in marine hydrothermal systems. For a mafic VMS deposit, the Mala VMS mound located within the Troodos ophiolite, Cyprus, is unusual as pyrite is enriched in magmatic volatile elements (Au, Cu, Te and Se), sulfide δ34S values average − 3.8‰ ± 1.9‰ (1σ, n = 28), and gypsum averages +14.5‰ ± 2.0‰ (1σ, n = 26) - in stark contrast to the bulk of Troodos VMS pyrite, which averages +4.6‰ ± 2.8‰. To date, this combination of features has only been observed in actively forming SMS deposits in immature, subduction-influenced environments and rarely in ancient VMS deposits hosted in felsic environments. Traditionally, the leaching of igneous rocks is considered as the primary source of metals in mafic VMS deposits. However, at Mala, and perhaps other active SMS deposits in mafic environments, we suggest that Au, Cu, Te and Se were initially sourced from the direct contribution of a magmatic volatile phase where SO2 underwent disproportionation, a signature that is later overprinted by reacted seawater during deposit maturation and is therefore not usually preserved in ancient analogues. Thus, the exceptional preservation of Mala provides evidence of a magmatic volatile contribution in the early stages of mafic VMS deposit formation.

U-series histories of magmatic volatile phase and enclave development at Soufrière Hills Volcano, Montserrat

Injection of volatile-rich mafic magma prior to an eruption may trigger episodes of volcanism and can act to transfer metals from depth. However, petrologic knowledge of the timescales from mafic injection to eruption have thus far been focussed on mineral-scale studies of chemical zoning patterns. The study of mafic enclaves dispersed within eruption products can provide insights into the interaction between deep and shallow reservoirs. We combine 238U-230Th-226Ra-210Pb isotope data with trace element concentrations across the interface of two contrasting mafic enclaves in contact with their host andesite from the 2010 eruption at Soufrière Hills Volcano (SHV), Montserrat to investigate the history of mass exchange between the mafic enclave and the andesite host. The application of these time-sensitive isotopes highlights complexities in the transfer of volatiles and metal elements between magmas and the enclaves' potential as eruption triggers. The enclaves exhibit (210Pb/226Ra)0 ratios >1 consistent with volatile input to the subsurface plumbing system a few decades prior to eruption. Samples of the andesitic host, however, which make up the bulk of the eruptive products, have (210Pb/226Ra)0 ≤ 1 suggesting no net volatile gain in the decades leading up to eruption, or that melt-volatile interaction is on a timescale unresolvable by 210Pb226Ra systematics (i.e. <2 years). Variations in trace elements such as Cu, Pb and Ba show loss of a magmatic volatile phase and transport of metals within the deeper part of the plumbing system during differentiation of magmas feeding SHV. Our results do not support that volatile transfer into the andesite via enclaves is a direct trigger of explosive eruptions although the enclaves are likely syn-eruptively formed. 238U-230Th-226Ra-210Pb and trace element systematics at SHV support a role for fresh magma influx during periods of unrest, but long-term accumulation of the andesite.

Timing of multiple magma events and duration of the hydrothermal system at the Yu’erya gold deposit, eastern Hebei Province, China: Constraints from U–Pb and Ar–Ar dating

The Yu’erya deposit is a large granite–hosted gold deposit in the eastern Hebei Province classified as an intrusion–related gold system. The Yu’erya complex contains the causative intrusion which was the source of hydrothermal metal-bearing fluids. This composite granite complex contains miarolitic pegmatite(–like) cavities and pegmatite veins. A combination of detailed field observations and U–Pb dating by SHRIMP and LA–ICP–MS is used to document multiple magmatic events which occurred at 210–220, 175–173, 171–170, 167–166 and ~161 Ma formed the Yu’erya granite complex. The initiation of the hydrothermal system, as determined via Ar–Ar dating, is closely related to the exsolution of a magmatic volatile phase from the G1 granite at ca.176 Ma. The hydrothermal system operated for ~16 my (176–160 Ma) and produced multiple mineralising events (~176, 172–168 and 162–160 Ma) linked to emplacement of repeated magma pulses from 175 Ma to 161 Ma. Our results suggest that multiple magmatic–hydrothermal events over a time period of several million years may be a fundamental factor responsible for the enrichment process during formation of intrusion-related gold deposits and thus may be a requirement for forming economic orebodies.

Linking Mineralogy to Lithogeochemistry in the Highland Valley Copper District: Implications for Porphyry Copper Footprints

Abstract The Highland Valley Copper porphyry deposits, hosted in the Late Triassic Guichon Creek batholith in the Canadian Cordillera, are unusual in that some of them formed at depths of at least 4 to 5 km in cogenetic host rocks. Enrichments in ore and pathfinder elements are generally limited to a few hundred meters beyond the pit areas, and the peripheral alteration is restricted to narrow (1–3 cm) halos around a low density of prehnite and/or epidote veinlets. It is, therefore, challenging to recognize the alteration footprint peripheral to the porphyry Cu systems. Here, we document a workflow to maximize the use of lithogeochemical data in measuring changes in mineralogy and material transfer related to porphyry formation by linking whole-rock analyses to observed alteration mineralogy at the hand specimen and deposit scale. Alteration facies and domains were determined from mapping, feldspar staining, and shortwave infrared imaging and include (1) K-feldspar halos (potassic alteration), (2) epidote veins with K-feldspar–destructive albite halos (sodic-calcic alteration), (3) quartz and coarse-grained muscovite veins and halos and fine-grained white-mica–chlorite veins and halos (white-mica–chlorite alteration), and two subfacies of propylitic alteration comprising (4) prehnite veinlets with white-mica–chlorite-prehnite halos, and (5) veins of epidote ± prehnite with halos of chlorite and patchy K-feldspar. Well-developed, feldspar-destructive, white-mica alteration is indicated by (2[Ca-C] + N + K)/Al values &amp;lt;0.85, depletion in CaO and Na2O, enrichment in K2O, and localized SiO2 addition and is spatially limited to within ~200 m of porphyry Cu mineralization. Localized K2O, Fe2O3, and depletion in Cu, and some enrichment in Na2O and CaO, occurs in sodic-calcic domains that form a large (~34 km2) nonconcentric footprint outboard of well-mineralized and proximal zones enriched in K. Water and magmatic CO2-rich propylitic and sodic-calcic–altered rocks form the largest lithogeochemical footprint to the mineralization in the Highland Valley Copper district (~60 km2). Calcite in the footprint is interpreted to have formed via phase separation of CO2 from a late-stage magmatic volatile phase. Several observations from this study are transferable to other porphyry systems and have implications for porphyry Cu exploration. Feldspar staining and shortwave infrared imaging highlight weak and cryptic alteration that did not cause sufficient material transfer to be confidently distinguished from protolith lithogeochemical compositions. Prehnite can be a key mineral phase in propylitic alteration related to porphyry genesis, and its presence can be predicted based on host-rock composition. Sodic-calcic alteration depletes the protolith in Fe (and magnetite) and, therefore, will impact petrophysical and geophysical characteristics of the system. Whole-rock loss on ignition and C and S analyses can be used to map enrichment in water and CO2 in altered rocks, and together these form a large porphyry footprint that extends beyond domains of enrichment in ore and pathfinder elements and of pronounced alkali metasomatism.

Vapor Transport and Deposition of Cu-Sn-Co-Ag Alloys in Vesicles in Mafic Volcanic Rocks

AbstractMetallic sublimates coated by sulfides and chlorides line the vesicle walls of mafic volcanic lava and bombs from Kīlauea, Vesuvius, Etna, and Stromboli. The metallic sublimates were morphologically and compositionally similar among the volcanoes. The highest concentrations of S and Cl occurred on the surface of the sublimates, while internally they had less than 1 wt % S and Cl in most cases, leading us to classify them as alloys. The major components of the alloys were Cu, Sn, Co, and Ag based on electron microprobe analyses and environmental scanning electron microscope element maps. Alloy element maps showed a covariance of Cu-Sn, while Co and Ag concentrations varied independently. Laser ablation-inductively coupled plasma-mass spectrometry analysis of matrix glass and melt inclusions in bombs from Stromboli showed appreciable amounts of Cu, Co, and Sn. We propose a model for the origin of the metallic grains, which involves syneruptive and posteruptive magma degassing and subsequent cooling of the basalt vesicles. During syneruptive vapor phase exsolution, volatile metals (Cu, Co, and Sn) partition into the vapor along with their ligands, S and Cl. The apparent oxygen fugacity (fO2) in these vapor bubbles is low because of the relative enrichment of the exsolved gas phase in H2 relative to H2O in silicate melts, due to the much higher diffusivity of the former in silicate melts. The high fH2 and low fO2 induces the precipitation of metal alloys from the vapor phase. Subsequently, the reducing environment in the vesicle dissipates as the cooling vapor oxidizes and as H2 diffuses away. Then, metal-rich sulfides (and chlorides) condense onto the outer surfaces of the metal alloy grains either due to a decrease in temperature or an increase in fO2. These alloys provide important insights into the partitioning of metals into a magmatic volatile phase at low pressure and high temperature.

Mechanisms and patterns of magmatic fluid transport in cooling hydrous intrusions

Volatile outgassing from hydrous magma intrusions emplaced and cooling in the Earth's upper crust is key to a number of geologic processes including volcanic eruptions and ore deposition, yet the physical interactions between production, storage and transport of a magmatic volatile phase within magmatic intrusions and their large-scale thermal evolution have remained elusive. We performed numerical simulations of aqueous volatile transport in generic magma chambers as they crystallize in response to conductive and convective cooling and thereby transition from a crystal - volatile - melt suspension to a mush and eventually to a rigid rock. We used simplified but realistic material properties based on published phase equilibrium experiments, published grain-scale modeling results and general geological constraints.We found that the intrusion depth and water content exert decisive control on the rates and mechanisms of intrusion outgassing. Above a critical volatile content, highly permeable capillary tubes develop in crystal mush regions with intermediate crystal volume fractions (∼ 0.5 to 0.7), and such grain-scale tubes provide rapid intrusion-scale degassing paths. These tubes are located in a ring-like mush region inside the intrusion, allowing strongly lateral flow and focusing towards the top of the intrusion. If the water content of the magma is close to saturation at the time of emplacement (5 to 6 wt% H2O) and the depth of the magma chamber roof is at least 4 km deep, initially distributed breakthrough points may coalesce to a single area of fluid release. Large-scale intrusion geometry determines this location of focused fluid release, and cooling rate and size of the intrusion determine overall fluid flux. Feedbacks between local hydrofracturing and heat advection break up the focused fluid expulsion into many short-lived pulses. These conditions for large-scale fluid focusing favor the formation of porphyry copper deposits, but might also trigger caldera-forming ignimbrite eruptions.

Origin and Evolution of Magmas in the Porphyry Au-mineralized Javorie Volcano (Central Slovakia): Evidence from Thermobarometry, Melt Inclusions and Sulfide Inclusions

Abstract The effect of magmatic sulfide precipitation on the potential of magmatic systems to produce porphyry-type ore deposits is still a matter of debate. In particular, we need to know whether magmatic sulfide precipitation has an impact on the Cu and Au content of the exsolving magmatic volatile phases and, by this way, on the Cu/Au ratio of porphyry deposits. The Javorie volcano is a perfect place to explore these questions. First, it hosts several Au-only porphyry-type mineralized occurrences which have among the lowest Cu/Au ratios reported in the literature. Secondly, the geology of the Javorie volcano and the timing of porphyry Au mineralization are well established. The evolution of the Javorie magmatic system was reconstructed by detailed petrographic studies and laser ablation inductively coupled plasma mass spectrometry analysis of minerals, melt inclusions and sulfide inclusions. The Javorie volcano was formed during the post-subduction magmatic activity affecting the Western Carpathians. It is a typical stratovolcano, composed dominantly of basaltic andesites and andesites which were intruded by several small stocks of dacitic to dioritic composition. According to our thermobarometric data, the volcano was fed by a transcrustal magmatic system in which two levels of magma chambers could be identified. Part of the magma evolved in the lower crust as suggested by the occurrence of magmatic garnet antecrysts in some of the studied rocks. The occurrence of magmatic sulfide inclusions in garnet indicates that sulfide saturation was reached in this lower crustal magma chamber. Most of the rocks crystallized in an upper crustal magma chamber (∼2 ± 1 kbar) that was fed by a basaltic to basaltic andesite magmas. A large variation in temperatures, ranging between 820°C and 1025°C, recorded by the extrusive and intrusive rocks suggest either that the upper crustal magma chamber was thermally zoned, or that the temperature of the whole magma chamber varied dramatically during its lifetime. Magmatic sulfide inclusions are present in all minerals and rocks of the upper crustal magma chamber, independent of their timing relative to porphyry Au mineralization (pre-, syn-, post-ore). These observations suggest that the magmatic system was sulfide saturated during its entire evolution. With very few exceptions, the precipitating sulfides were composed of monosulfide solid solution containing 0·2–9·2 wt % Cu and 0·05–11 ppm Au. The presence of these magmatic sulfides, together with results of a numerical model, suggest that the primitive magma feeding the upper crustal magma chamber contained less than 2·75 wt % H2O and that only a minor part of the magmatic sulfides was fractionated out of the system. Finally, the Cu/Au ratios measured in the magmatic sulfide inclusions and the ones predicted for the exsolved aqueous fluids are 10 to 100 times higher than the Cu/Au ratios of the porphyry deposits. Therefore, the extremely low Cu/Au ratios of the porphyry deposits must have been acquired during the hydrothermal stage.