Eastern Manus Back-arc Basin Research Articles

Origin of Cu in the PACMANUS hydrothermal field from the eastern Manus back-arc basin: evidence from mass balance modeling

Hydrothermal precipitates associated with active vents in the eastern Manus Basin, an actively opening back-arc basin in the Bismarck Sea, Papua New Guinea, are among the most Cu-rich on the modern seafloor. The volcanic rocks associated with this mineralization may be insufficiently enriched in Cu to account for the Cu content of the sulfides by simple leaching. The PACMANUS hydrothermal field lies in the eastern portion of the eastern Manus Basin. Mass balance modeling of the PACMANUS hydrothermal system indicates that simple leaching of a stationary reaction zone (0.144 km3) by hydrothermal fluids cannot yield the Cu found in associated sulfide deposits because unacceptably high leaching, transportation and precipitation efficiencies are required to derive the Cu in sulfides by leaching processes. With 100% leaching, transport and precipitating efficiency, 0.166 km3 of volcanic rocks would need to be leached to account for the Cu budget of hydrothermal sulfide deposits. The key requirement for forming metal-rich magmatic fluids is a large amount of metals available to enter the exsolved vapor phase. Magmas generated in the eastern Manus Basin inherently have high fO2 because of metasomatism of the mantle source by oxidized materials from the subducted slab, leading to copper enrichment in the magma chamber. Moreover, the presence of Cu in gas-rich melt inclusi on bubbles in Pual Ridge andesite is evidence that degassing and partitioning of Cu into the magmatic volatile phase has occurred in the eastern Manus Basin. Numerical mass balance modeling indicates that approximately 0.236 Mt Cu was potentially transferred to the hydrothermal system per cubic kilometer magma. Magmatic degassing seems to play a more significant role than leaching.

Magma plumbing and hybrid magma formation at an active back-arc basin volcano: North Su, eastern Manus basin

North Su is the central volcanic edifice of the SuSu Knolls hydrothermal district, located in an extensional transform zone of the eastern Manus back-arc basin. The submarine volcano hosts a vigorous hydrothermal system with black and white smokers, diffuse fluid discharge, and accumulation of native sulfur. Among its dacitic to andesitic lavas are remarkable hybrid rocks, containing highly magnesian olivine (Fo81–94) phenocrysts with thin orthopyroxene reaction rims in a dacitic matrix. We investigated these rocks in order to understand their formation, to constrain temperature and depth of magma storage, and to identify links between magmatic processes and hydrothermal activity. The compositions, zonations and reaction rims of phenocrysts suggest that the hybrid lavas formed by mixing when dacitic to rhyodacitic magma was invaded by Mg-rich andesite to basaltic andesite with boninite affinities. The resulting melts show strong enrichment of large ion lithophile elements, moderate enrichment of light rare earth elements and depletion of Nb, Ta and Ti relative to N-MORB, characterizing them as back-arc basin magmas. Compositional zonations at the outermost rims of olivine phenocrysts suggest a period of no more than a few days between the inferred mixing event and eruption. Mineral-melt thermometers indicate a temperature of ∼1010 °C for the hybrid magma. Mineral-melt barometers give a wide range of pressures with a mean of 19 ± 217 MPa, reflecting equilibration at shallow depths as well as limited barometer performance at low pressures. In addition, many clinopyroxene phenocrysts lack equilibrium between their strongly zoned rims and the host melt. The H2O (0.9–5.3 wt%) and CO2 (70–4730 ppm) contents of melt inclusions in clinopyroxene and plagioclase phenocrysts provide better constraints on the pressure range for pre-eruptive magma storage. The data suggest that dacite phenocryst crystallization and magma mixing occurred at 40–140 MPa pressure, about 1–5 km beneath the summit of North Su. The sulfur–H2O relationship in melt inclusions reflect a degassing trend that is consistent with formation of an aqueous-sulfurous magmatic fluid, inferred to produce highly acidic vent fluids at North Su (Seewald et al., 2015). The melt inclusion data also indicate significant fractionation between F, Cl and H2O after magma degassing, possibly reflecting formation of a high-salinity brine at depth and fluorine fixation in minerals formed in the subsurface. Our results suggest that the dynamic magmatic system, with magma mixing occurring shortly before ascent and eruption, causes rapid changes in composition and quantity of exsolved fluids. Due to the short distance to the seafloor, such fluid fluctuations are rapidly transferred to the hydrothermal systems. This may be the main cause for the observed temporal, spatial and compositional variations in hydrothermal activity at North Su, and possibly at other vent fields of the Manus basin.

Arsenic and Antimony in Hydrothermal Plumes from the Eastern Manus Basin, Papua New Guinea

Studies on the concentrations of arsenic (As) and antimony (Sb) in seawater columns are very important for tracing hydrothermal plumes and understanding fluid characteristics of seafloor hydrothermal systems. The total As, Sb, Mn, and Cl− concentrations of three hydrothermal plume seawater column samples have been studied at Stations 18G, 18K, and 18B in the eastern Manus back-arc basin, Bismarck Sea, Papua New Guinea. At Stations 18G and 18K, the plumes above North Su and near the Suzette site in the SuSu Knolls hydrothermal field are both enriched in As, Sb, and Mn and depleted in Cl, as a result of contribution of As-Sb-Mn-enriched and Cl-depleted vent fluid outputs to the hydrothermal plume, which is most likely generated in the subseafloor by fluid-rock interaction, magma degassing, or phase separation (boiling of hydrothermal fluid). The plume at Station 18B is enriched in As, Sb, Mn, and Cl, suggesting that As-Sb-Mn-Cl-enriched fluid discharges from vents, which have been generated by fluid-rock interaction. The concentrations of As and Sb anomalous layers, like manganese (Mn), are higher than those of the other layers in the three hydrothermal plume seawater columns. As and Sb with Mn showed a positive correlation (R2>0.8, p<0.05), and the distributions of As and Sb within the hydrothermal plume are not controlled by particle adsorption or biogeochemical cycles, suggesting that As and Sb, like Mn, can be used to detect and describe the characteristics of hydrothermal plumes in seawater environment. In addition, anomalous layer with As/Sb ratio lower than those of ambient seawater at the same temperature is found in the eastern Manus basin, suggesting that the As/Sb ratio may also act as an effective tracer reflecting the effect of hydrothermal activity on As and Sb in the seawater column.

The competing effects of sulfide saturation versus degassing on the behavior of the chalcophile elements during the differentiation of hydrous melts

AbstractThere is a lack of consensus regarding the roles of sulfide saturation versus volatile degassing on the partitioning of Cu and Ag during differentiation and eruption of convergent margin magmas. Because of their oxidized character, volatile‐rich magmas from the Eastern Manus Back‐arc Basin (EMBB) only reach sulfide saturation following magnetite‐driven reduction of the melt: the so‐called “magnetite crisis.” If sulfide saturation typically precedes volatile saturation, the magnetite crisis will limit the proportion of Cu and Ag that can partition from the melt into an exsolving volatile‐rich phase, which may contribute to the sporadic occurrence of magmatic‐hydrothermal ore deposits at convergent margins. However, it is unclear whether the magnetite crisis is a common or rare event during differentiation of volatile‐rich magmas. We report major and trace element data for submarine volcanic glasses from the Tonga arc‐proximal Valu Fa Ridge (VFR; SW Pacific). Cu‐Se‐Ag systematics of samples erupting at the southern VFR suggest magnetite fractionation‐triggered sulfide saturation. The similarity in chalcophile element systematics of the southern VFR and EMBB samples is unlikely to be coincidental, and may indicate that the magnetite crisis is a common event during differentiation of hydrous melts. However, unlike many convergent margin magmas, it is unlikely that the evolving VFR and EMBB were saturated in a S‐bearing volatile phase prior to magnetite fractionation. Hence, the metal‐depleting magnetite crisis may be restricted to back‐arc basin magmas that do not degas volatiles prior to magnetite fractionation and potentially convergent margin magmas fractionating at high pressures in the continental crust.

Episodic Subseafloor Hydrothermal Activity Within the Eastern Manus Back-Arc Basin Determined by Uranium-Series Disequilibrium in Barite

The geochronology of seafloor hydrothermal activity from several mineralized, active and inactive vent fields at Suzette and Pual Ridge, eastern Manus back-arc basin, Bismarck Sea, Papua New Guinea, has been established using the 228Th/228Ra, 228Ra/226Ra, and 226Ra/Ba uranium-series disequilibrium methods for barite-rich samples. These hydrothermal systems underwent episodes of hydrothermal fluid circulation that produced barite over the last ~6,000 years. Precipitation of hydrothermal barite, disseminated within the host rock and lining fractures, vugs, and conduits, was controlled by decreasing temperatures and mixing of hydrothermal fluids with oxidized seawater. Repeated fluctuations in hydrothermal fluid temperature and chemistry resulted in complex sequences of barite and sulfide minerals lining vugs and conduits, whereas rapid cooling following fracture events led to precipitation of only barite and some Fe oxide coatings on barite crystals. A wide range in ages of barite within drill cores (i.e., 380–5,990 yrs before present (BP)) reflect repeated pulses of hydrothermal fluids through the altered volcanic and mineralized rocks that make up the hydrothermal mound structures, with the younger ages (i.e., 34–220 yrs BP), representing more recent barite-precipitating fluid circulation accessing fractures, veins, and vugs. Barite from chimney samples is mostly young (i.e., 23–1,680 yrs BP), with the majority precipitated between 23 to 41 yrs BP; reflecting the structural stability (standing life) of the chimneys and possible influences from varying hydrothermal fluid chemistry or reactivation of vent fields. Three generalized age groupings, representing major episodes of barite-precipitating hydrothermal fluid circulation, are resolved from the Suzette area (19–45, 810–1,900, and 3,740–4,710 yrs BP, including 1σ errors); with the most recent ages recorded from both the Suzette and Pual Ridge areas. Fast-spreading rates within the Manus basin, an abundance of faults and variable magmatic inputs are major factors controlling the frequency and longevity of these episodes of hydrothermal activity. Mineral resource models and comparisons indicate that the Cu-Zn-Ag-Au–rich Suzette deposit is small; however the measured age range is comparable to ages given for modeled (typical) seafloor massive sulfide deposits from back-arc and arc environments. We suggest that the scale of the Suzette deposit may have been limited during formation by a low hydrothermal fluid flux and/or inappropriate physical and chemical conditions for sufficient metal precipitation from hydrothermal fluids.

Metalliferous Sediments Associated with Presently Forming Volcanogenic Massive Sulfides: The SuSu Knolls Hydrothermal Field, Eastern Manus Basin, Papua New Guinea

Short sediment cores were examined from the active SuSu Knolls hydrothermal field in the eastern Manus back-arc basin, Papua New Guinea, in order to explain the origin of the currently accumulating metalliferous sediments. The mineralogy and geochemistry of the sediments were studied by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray fluorescence, and instrumental neutron activation analysis. Sediments of the Suzette site originated from the deposition of volcaniclastic material with the addition of a hydrothermal component from the mass wasting of old oxidized chimneys. A strong mass wasting event was recognized at the base of the studied sedimentary succession from the occurrence of chalcopyrite, pyrite, and barite aggregates, similar to those found in chimneys, as well as the presence of atacamite. The metalliferous component is characterized by high concentrations of Cu (up to 2.3%) and Au (up to 3.5 ppm), elevated concentrations of As, Ba, Zn, and Fe, as well as a positive Eu anomaly. The material derived from eroded chimneys was deposited together with abundant glass fragments eroded from the volcanic edifice. The mass wasting was succeeded by deposition of volcaniclastic sediment containing dacite fragments, Ca plagioclase, pyroxene, glass shards, cristobalite, Si-rich amorphous material, alunite, pyrite, barite, and magnetite. The volcaniclastic sediment may have originated from hydrothermal eruptions at North Su and South Su. The present-day plume at Suzette is a likely source of rare barite aggregates, amorphous Cu-containing aggregates, and Cu-Fe sulfides creating geochemical anomalies of Ba and Cu in the uppermost sediments at Suzette. Interbedded volcaniclastic and hemipelagic sediments were retrieved east and west of SuSu Knolls. The enrichment of Mn in these sediments is considered to be a result of settling of Mn oxides from a hydrothermal plume.

Textural and mineralogical changes associated with the incipient hydrothermal alteration of glassy dacite at the submarine PACMANUS hydrothermal system, eastern Manus Basin

Variably altered dacite from the PACMANUS vent field in the eastern Manus back-arc basin, Papua New Guinea, was studied to determine the textural and mineralogical characteristics of hydrothermal alteration taking place in the immediate subsurface of this modern seafloor hydrothermal system. Detailed textural investigations show that fluid flow through the glassy dacite has been strongly controlled by the primary volcanic textures. Quench fractures and networks of interconnected perlitic cracks linking vesicles provided pathways for hydrothermal fluids flowing up to the seafloor. Hydrothermal alteration along these pathways resulted in the formation of pseudoclastic textures. Textural evidence suggests that alteration of the glassy dacite has not been sustained. The samples have been affected by incipient hydrothermal alteration that is typically not preserved in ancient volcanic-rock-hosted massive sulfide deposits. Interaction of the glassy dacite with hydrothermal fluids primarily resulted in the conversion of volcanic glass to dioctahedral smectite. Only minor amounts of trioctahedral smectite were formed. Destruction of the volcanic glass and the formation of smectite caused pronounced changes in the chemistry of the dacite samples, in particular a decrease in the SiO 2 whole-rock content and the Na 2O/K 2O ratio. The two alkali elements behaved differently during hydrothermal alteration due to preferential incorporation of K into the interlayer position of the newly formed dioctahedral smectite. Smectite formation occurred under rock-dominated conditions although the addition of Mg was required to form trioctahedral smectite from the silicic volcanic glass. Primary plagioclase was resistant to hydrothermal alteration highlighting the fact that the destruction of volcanic glass and feldspar are not necessarily contemporaneous in massive sulfide forming hydrothermal systems. Incipient alteration of the glassy dacite close to the seafloor occurred at temperatures below 150 °C in an environment that allowed the development of steep temperature gradients. Comparison of the new data to the findings of deep drilling during ODP Leg 193 suggests that the smectite-rich alteration in the immediate subsurface of the PACMANUS hydrothermal vent field represents the low-temperature equivalent of illite- and chlorite-rich alteration associations forming in the upflow zones of the hydrothermal fluids in the deeper portion of the volcanic sequence.

Vigorous Exsolution of Volatiles in the Magma Chamber beneath a Hydrothermal System on the Modern Sea Floor of the Eastern Manus Back-Arc Basin, Western Pacific: Evidence from Melt Inclusions

Vigorous Exsolution of Volatiles in the Magma Chamber beneath a Hydrothermal System on the Modern Sea Floor of the Eastern Manus Back-Arc Basin, Western Pacific: Evidence from Melt Inclusions

Vigorous Exsolution of Volatiles in the Magma Chamber beneath a Hydrothermal System on the Modern Sea Floor of the Eastern Manus Back-Arc Basin, Western Pacific: Evidence from Melt Inclusions

Image analyses and ion probe analyses were carried out on melt inclusions in olivine, pyroxene, and plagioclase phenocrysts of vesicular volcanic rocks that host the actively forming massive sulfides at the PACMANUS hydrothermal field in the eastern Manus back-arc basin, western Pacific. The melt inclusions, ranging in size from 150 μm, contain one or more bubbles. Most of them have bubble/melt inclusion volume ratios (Vb/Vm) of >5.5 vol percent, higher than those for the melt inclusions with their bubbles generated from shrinkage (Vb/Vm, 3.6 vol %) or postentrapment exsolution of volatiles (Vb/Vm, 3.7 vol %). This, together with the presence of vapor bubbles with little melt (Vb/Vm, 80 to >95 vol %) in pyroxene and olivine phenocrysts, suggests that a vapor phase was formed from vigorous exsolution of volatiles or the boiling of a magma in the crystallizing magma chamber. The Vb/Vm ratios show distinct distribution patterns in the phenocryst minerals, indicating that there was more than one vesiculation event in the magma chamber. Melt inclusions with no visible bubbles have the highest contents of H2O (2.1–2.5 wt %). The H2O contents of the glass in the melt inclusions are inversely related to the size of the coeval bubbles. The significant variation of H2O contents (1.0–2.5%) in the melt inclusions suggests that the H2O was degassed from the crystallizing magma. The positive correlations of S, Cl, and possibly F with H2O indicate that all these volatiles were exsolved from the melt into the vapor phase. The negative correlation of H2O with alkali contents in the glass of melt inclusions suggests that the magma was crystallizing in the presence of a fluid phase. It is estimated that the magma might have lost up to 65 percent of its volatiles while crystallizing. Significant amounts of magmatic fluid can be degassed from a magma during its crystallization in a shallow magma chamber. An individual vesiculation event may last at least 5 to 48 years, as estimated from the growth of the phenocrysts. This estimate is a minimum because not all the phenocrysts crystallize at the same time. The estimated life of a black smoker chimney is 2 to 10 years, which is within the duration of an individual vesiculation event. This implies that the magmatic fluid could be a major source of ore metals during the growth of individual sulfide chimneys. Through multiple vesiculation events in a crystallizing magma chamber, the magma could provide large quantities of ore metals and volatiles to a hydrothermal system over a long period of time to form a large massive sulfide deposit on the sea floor.

S, Sr, and Pb isotopic systematics of hydrothermal chimney precipitates from the Eastern Manus Basin, western Pacific: Evaluation of magmatic contribution to hydrothermal system

Sulfur, strontium, and lead isotopic analyses were performed on chimneys from the hydrothermal vent fields (the PACMANUS and Susu knolls) in the eastern Manus back arc basin. Lead isotope composition varies in a narrow range (206Pb/204Pb = 18.75–18.78, 207Pb/204Pb = 15.51–15.54, and 208Pb/204Pb = 38.31–38.40). Strontium isotopic ratios (87Sr/86Sr) of sulfate minerals vary from 0.7051 to 0.7077, falling between those of seawater (0.7090) and the hydrothermal fluid end‐member (0.7050). The δ34S values of the chimneys are −8.0 to −3.9‰ for the Susu knolls sulfide, +1.0 to +4.3‰ for the PACMANUS sulfides, and +7.4 to +20.6‰ for sulfate. The Susu knolls sulfides have the lowest values so far reported for volcanic‐hosted massive sulfides from modern seafloor. The low δ34S values of sulfates and sulfides cannot be explained by a simple seawater circulation model without an additional source of light sulfur. This is also supported by the presence of sulfate in hydrothermal end‐member fluid indicated by sulfur and strontium isotopic ratios in chimney sulfates. Biogenic sulfur and/or oxidation of H2S after boiling are excluded from the possible sources of light sulfur because of the lack of sediment cover and nonequilibrium in isotopic fractionation by boiling at seafloor. Disproportionation of magmatic SO2, introduced into hydrothermal system through preeruptive degassing, is the most plausible mechanism for the observed low δ34S values. The lighter δ34S values and more radiogenic lead isotope compositions of the Susu knolls samples indicate that magmatic degassing was more vigorous at the Susu knolls than PACMANUS.

Fluid inclusion evidence for subsurface phase separation and variable fluid mixing regimes beneath the deep‐sea PACMANUS hydrothermal field, Manus Basin back arc rift, Papua New Guinea

Altered volcanic rocks were cored from over 350 m below the seafloor at the Papua New Guinea‐Australia‐Canada Manus Basin Hydrothermal Field (PACMANUS) deep‐sea hydrothermal field, in the eastern Manus back arc basin. Fluid inclusions in anhydrite veins reveal phase separation and fluid mixing beneath the seafloor. The anhydrite precipitated from high‐temperature fluids (150–385°C). At Roman Ruins, a site of active high‐temperature venting (220–276°C, measured by submersible), the fluid inclusion thermal depth profile is uniform and high temperature (242–368°C). At Snowcap, a site of warm water effusion (6–65°C), the fluid inclusions indicate high temperatures at depth (270–385°C) but both low and high temperatures in the shallower section. This indicates a flow regime dominated by vertical advection and shallow entrainment and mixing with cool seawater. Inclusions at Snowcap exhibit extreme salinity variations due to phase separation at temperatures above 350°C. Fluids contain Na, Cl, Fe, Zn, Mg, and Ba and a minor gas component such as CO2 or CH4. Most inclusions at Roman Ruins exhibit salinities that fall within the range of those observed at modern active vent sites along the mid‐ocean ridge system. Fluid inclusion temperatures support a hypothesis, developed previously from Sr‐isotopic analysis, that the subseafloor at Snowcap is characterized by mixing between deep‐sourced hot hydrothermal fluids and cold seawater‐like fluid. Both heating of seawater and cooling of upwelling hydrothermal fluids can be recognized by combining isotopic and fluid inclusion data. In contrast to Snowcap, the regime at Roman Ruins is less varied, with uniformly high‐temperature upwelling fluids that have hydrothermally dominated Sr‐isotopic ratios.

Drill core-based facies reconstruction of a deep-marine felsic volcano hosting an active hydrothermal system (Pual Ridge, Papau New Guinea, ODP Leg 193)

Pual Ridge is a deep-marine, felsic volcanic edifice in the eastern Manus back-arc basin (Papua New Guinea) with an estimated volume of ∼6 to 9 km 3. It is 1–1.5 km wide, 20 km long and rises 500–600 m above the surrounding ocean floor. The active PACMANUS hydrothermal field on the crest of Pual Ridge at 1640–1690 m below sea level was the target of Ocean Drilling Program Leg 193. Variably altered dacite lavas have been recovered from the subsurface of a low-T discharge site (Snowcap) and a high-T black smoker site (Roman Ruins) reaching a maximum depth of 380 m below seafloor (mbsf). Volcanic facies interpretation of these cores is difficult due to incomplete recovery and widespread pseudoclastic textures generated by fracturing and multi-phase, incomplete fluid–dacite interaction. However, distinction of genuine volcaniclastic facies and facies with alteration-related clastic appearance is important in order to define paleo-seafloor positions within the volcanic stratigraphy, that may be prospective for massive sulfide mineralization. This has been accomplished using remnant primary characteristics indicative of transportation such as polymictic composition, grading or textural evidence for differential movement of individual clasts. Three phases of volcanic activity can be distinguished and a proximal facies association dominated by coherent facies of dacite lavas exists below Snowcap. At Roman Ruins, a medial facies association consists of lava flows with about equal proportions of coherent and volcaniclastic facies. Endogenous growth was an important process during lava flow emplacement and groundmass textures such as perlite, flow banding and spherulites indicate that cooling rates were variable, locally allowing for high-temperature devitrification. A tube pumice breccia unit is interpreted as the resedimented facies of a quench fragmented, highly vesicular dacite lava carapace. Sulfide accumulations in the subsurface are restricted to Roman Ruins suggesting that the abundance of volcaniclastic facies favored ascent of metal-bearing fluids due to the inherent high permeability. Evidence for mineralization at paleo-seafloor positions at 110 and 195 mbsf indicate a complex interrelationship between volcanic and hydrothermal activity. A facies model has been developed which may help to locate subaqueous, felsic lava-dominated volcanic centers and associated sulfide deposits in ancient volcano–sedimentary successions.

Magmatic Degassing of Volatiles and Ore Metals into a Hydrothermal System on the Modern Sea Floor of the Eastern Manus Back-Arc Basin, Western Pacific

Magmatic Degassing of Volatiles and Ore Metals into a Hydrothermal System on the Modern Sea Floor of the Eastern Manus Back-Arc Basin, Western Pacific

Magmatic Degassing of Volatiles and Ore Metals into a Hydrothermal System on the Modern Sea Floor of the Eastern Manus Back-Arc Basin, Western Pacific

Magmatic fluids were degassed before and during the eruption of vesicular volcanic rocks that host the actively forming massive sulfides at the PACMANUS hydrothermal field in the eastern Manus back-arc basin, western Pacific. The dredged samples of fresh lavas, ranging in composition from basalt to rhyolite, define a calc-alkalic trend that is thought to have resulted from fractionation in a common magma chamber. These rocks have variable vesicularity and vesicle size distributions that record the degassing history of the magma. The highly vesicular basalt and basaltic andesite are the least fractionated melt and experienced preeruptive, syneruptive, and posteruptive degassing. The weakly vesicular felsic rocks formed from an evolved magma that was largely degassed before its eruption. Vesicularity tends to decrease with Si, K, Ba, and Zr and to increase with Ca, Mg, Fe, and Sc in bulk samples, suggesting that the degassing of volatiles was linked to crystal fractionation of the magma. A volatile-rich magma is indicated by high concentrations of H 2 O (0.9–2.5%) and Cl (to 0.45%) in mafic melt inclusions in phenocrysts of the basaltic andesite. A fluid phase in the melt inclusions indicates that the magma was saturated with volatiles in the magma chamber. Volatiles exsolve as an immiscible fluid with increasing crystal fractionation, and the composition of the degassed magmatic fluid changes with the evolving magma. The fluid is CO 2 -dominated during the degassing of weakly fractionated mafic magma and becomes a mixture of CO 2 and H 2 O as H 2 O is increasingly exsolved from the highly fractionated felsic magma. The ore metals in the degassed fluid, as inferred from the compositions of the metallic precipitates found in the vesicles of melt inclusions and matrix glass, progressively change from Ni + Cu + Zn + Fe in basalt and basaltic andesite, to Cu + Zn + Fe in andesite, to Cu + Fe in dacite, to Fe in rhyodacite, and to Fe + Zn (+ Pb?) in rhyolite. This trend suggests that magmatic fluids, released from a fractionating magma, could be a source of metal for various types of ore deposits and mineral occurrences. Significant amounts of magmatic fluid can be degassed during the fractionation of a shallow magma chamber. At least 1.7 percent H 2 O is estimated to have exsolved from the magma at PACMANUS. If a fluid is concentrated in a shallow magma chamber and is discharged along a favorable structural zone that extends to the sea floor, it will contribute large quantities of volatiles and ore metals directly to a sea-floor hydrothermal system, a process demonstrated for the geothermal systems of subareal volcanoes. The focused discharge of a magmatic fluid as a result of preeruptive degassing, particularly in a fractionated felsic magma, could be responsible for the Fe, Cu, Zn, and Pb metals in the sulfide chimneys at PACMANUS. By analogy, a magmatic fluid can provide a major source of ore metals for large or super-large volcanogenic massive sulfide deposits in the geologic record.

Gold Content of Eastern Manus Basin Volcanic Rocks:Implications for Enrichment in Associated Hydrothermal Precipitates

Hydrothermal precipitates associated with active vents in the eastern Manus back-arc basin, Papua New Guinea, are among the most gold rich yet discovered on the modern sea floor. The volcanic rocks associated with this mineralization were investigated to determine if they are sufficiently enriched in gold to account for the gold content of the sulfides by simple leaching and to determine whether or not any evidence for a magmatic fluid exists. The gold content of unaltered volcanic glass and glassy volcanic rocks from the eastern Manus basin ranges from <1 to 15 ppb and averages 6 ± 3 (1 σ ) ppb. These concentrations are similar to volcanic rocks from the Lau, Japan, and Yamato back-arc basins but are significantly higher than those from midocean ridges and submarine-arc volcanic rocks. Modeling of the PACMANUS hydrothermal system indicates that for a stationary reaction zone unacceptably high leaching and transport and precipitation efficiencies are required to derive gold in the sulfides by leaching processes. Downward migration of the high-temperature reaction zone, as the magma that is driving the circulation cools, will result in exposure of the hydrothermal fluids to much more rock than in the static scenario. Consequently sufficient gold may be leached to account for the gold in the sulfides. Primitive mantle-normalized metal contents of PACMANUS sulfides are similar to those of associated volcanic rocks, rather than the basement volcanic rocks that are the more likely source of leached metals. This similarity implies a close genetic relationship between the metal content of the host volcanic rocks and the sulfides, such as derivation from the same magma. A significant positive correlation of gold and copper contents of the volcanic rocks indicates a similar behavior of these two metals during magmatic evolution. During fractional crystallization both metals are initially enriched in the melt. Peak concentrations of copper and gold in the melt are reached at an SiO2 content of 57 percent, after which the concentration decreases rapidly. Such a rapid decrease is believed to be due to a combination of magnetite fractionation and pre-eruptive degassing of the andesitic magma. A dual source of gold is proposed for the PACMANUS deposit, whereby gold is leached from the subsea-floor rocks but is also added to the hydrothermal system by a direct contribution from an exsolved magmatic fluid. Such a dual source may be an important factor in generating the gold-rich precipitates found in western Pacific submarine arcs and back arcs.

Facies architecture of the felsic lava‐dominated host sequence to the Thalanga massive sulfide deposit, Lower Ordovician, northern Queensland

The Thalanga volcanic‐hosted massive sulfide deposit occurs in the Cambro‐Ordovician Mt Windsor Subprovince in northern Queensland. The orebody comprises steeply dipping, stratiform, sheet‐like, polymetallic massive sulfide lenses. Overall, the volcanic facies architecture at Thalanga is dominated by quartz‐ and/or feldspar‐phyric lavas and synvolcanic intrusions that comprise coherent facies and in situ and resedimented autoclastic facies. Systematic phenocryst logging (mineralogy, abundance, size) has been used to discriminate separate emplacement units of rhyolite in the footwall and dacite in the hangingwall. Some of the petrographically different rhyolite and dacite types can also be distinguished using immobile‐element geochemistry. Rhyolitic lavas and intrusions in the footwall are weakly to strongly altered. Apparent clastic textures resulting from hydrothermal alteration and metamorphism are widely developed in the coherent facies. Genuine clastic textures are characterised by clasts with randomly oriented internal laminar or banded fabric (e.g. rotated, flow‐laminated clasts), marked and consistent differences in quartz phenocryst abundance and/or size range between clasts and matrix, and normal grading. Mass‐flow‐emplaced, rhyolitic breccia units delineate palaeo‐sea‐floor positions in the footwall that are potentially prospective for exhalative massive sulfide mineralisation. A comparison of the distribution of clastic and coherent facies with the geometry of strongly altered zones in the footwall indicates that intense hydrothermal fluid flow was independent of the facies arrangement. The massive sulfide lenses conformably overly altered footwall rhyolite and occur in a distinctive facies association which includes coarse quartz‐phenocryst‐rich rhyolitic sills with peperitic contacts and crystal‐rich polymictic breccia. The hangingwall to the orebody consists of largely unaltered dacitic lavas and synvolcanic intrusions and minor dacitic pumice breccia, dacitic breccia and polymictic volcanic breccia. The facies architecture shows that the Thalanga massive sulfide deposit formed in a below‐storm‐wave‐base depositional environment on top of an elevated, lava‐dominated, rhyolitic volcanic centre. A modern analogue for the setting of the Thalanga massive sulfide is the PACMANUS hydrothermal field on the crest of the dacite lava‐dominated Pual Ridge in the eastern Manus backarc basin (Papua New Guinea).

Acidic and sulfate-rich hydrothermal fluids from the Manus back-arc basin, Papua New Guinea

Hot (≥ 88-120 °C) and acidic (pH ≤ 2.1) hydrothermal fluids rich in sulfate were discovered venting in the DESMOS caldera (depth = 1926 m), eastern Manus back-arc basin, Bismarck Sea, surrounded by Papua New Guinea. The abundant sulfate (≥ 32.8 m M , higher than the seawater value of 28 m M ) with elemental sulfur deposition around the vents, and remarkably low δD(H 2 O) and δ 34 S(H 2 S) values (−8.1‰ and −5.6‰, respectively), are suggestive of the incorporation of a magmatic fluid and the disproportionation of the exsolved SO 2 from a magma body. The DESMOS fluid may be similar in origin to the acidic sulfate-chloride hot springs associated with subaerial volcanic activity. In contrast to the typical hydrothermal end member Mg concentration of 0, the DESMOS fluids are rich in Mg (46–52 m M ), probably because of Mg dissolution by acid attack upon magnesium silicate minerals.

Possible contribution of a metal-rich magmatic fluid to a sea-floor hydrothermal system

THE source of the hydrothermal fluids vented in active volcanic areas on the sea floor1–3 has been a matter of some debate4–7; they may arise purely from the interaction of circulating sea water with the hot rocks through which it passes1,3,8, or there may be an admixture of a fluid escaping from magma at depth, as is seen in subaerial geothermal systems9. The answer to this question also bears on the origin of the sulphide ores deposited by sea-floor hydrothermal systems, and their ancient analogues8,10,11 preserved on land. Here we present direct evidence for the presence of magmatic fluid in the lavas that host an actively forming massive sulphide deposit in the eastern Manus back-arc basin. We find high concentrations of chlorides and sulphides of ore-forming metals such as copper, zinc and iron in CO2-rich gaseous bubbles found both in melt inclusions trapped in the phenocrysts of the volcanic rocks, and in the matrix glass. We conclude that a metal-rich fluid was present in the magma before eruption, and probably exsolved as the pressure decreased. This finding suggests the possibility for the contribution of large quantities of ore-forming metals to a sea-floor hydrothermal system.

Actively forming polymetallic sulfide deposits associated with felsic volcanic rocks in the eastern Manus back-arc basin, Papua New Guinea

Actively forming polymetallic sulfide deposits associated with felsic volcanic rocks in the eastern Manus back-arc basin, Papua New Guinea

Volcanism in Australasia. A Collection of Papers in Honour of the Late G.A.M. Taylor, G.C.

North Su is the central volcanic edifice of the SuSu Knolls hydrothermal district, located in an extensional transform zone of the eastern Manus back-arc basin. The submarine volcano hosts a vigorous hydrothermal system with black and white smokers, diffuse fluid discharge, and accumulation of native sulfur. Among its dacitic to andesitic lavas are remarkable hybrid rocks, containing highly magnesian olivine (Fo81–94) phenocrysts with thin orthopyroxene reaction rims in a dacitic matrix. We investigated these rocks in order to understand their formation, to constrain temperature and depth of magma storage, and to identify links between magmatic processes and hydrothermal activity. The compositions, zonations and reaction rims of phenocrysts suggest that the hybrid lavas formed by mixing when dacitic to rhyodacitic magma was invaded by Mg-rich andesite to basaltic andesite with boninite affinities. The resulting melts show strong enrichment of large ion lithophile elements, moderate enrichment of light rare earth elements and depletion of Nb, Ta and Ti relative to N-MORB, characterizing them as back-arc basin magmas. Compositional zonations at the outermost rims of olivine phenocrysts suggest a period of no more than a few days between the inferred mixing event and eruption. Mineral-melt thermometers indicate a temperature of ∼1010 °C for the hybrid magma. Mineral-melt barometers give a wide range of pressures with a mean of 19 ± 217 MPa, reflecting equilibration at shallow depths as well as limited barometer performance at low pressures. In addition, many clinopyroxene phenocrysts lack equilibrium between their strongly zoned rims and the host melt. The H2O (0.9–5.3 wt%) and CO2 (70–4730 ppm) contents of melt inclusions in clinopyroxene and plagioclase phenocrysts provide better constraints on the pressure range for pre-eruptive magma storage. The data suggest that dacite phenocryst crystallization and magma mixing occurred at 40–140 MPa pressure, about 1–5 km beneath the summit of North Su. The sulfur–H2O relationship in melt inclusions reflect a degassing trend that is consistent with formation of an aqueous-sulfurous magmatic fluid, inferred to produce highly acidic vent fluids at North Su (Seewald et al., 2015). The melt inclusion data also indicate significant fractionation between F, Cl and H2O after magma degassing, possibly reflecting formation of a high-salinity brine at depth and fluorine fixation in minerals formed in the subsurface. Our results suggest that the dynamic magmatic system, with magma mixing occurring shortly before ascent and eruption, causes rapid changes in composition and quantity of exsolved fluids. Due to the short distance to the seafloor, such fluid fluctuations are rapidly transferred to the hydrothermal systems. This may be the main cause for the observed temporal, spatial and compositional variations in hydrothermal activity at North Su, and possibly at other vent fields of the Manus basin.