Pual Ridge Research Articles

Tellurium Transport and Enrichment in Volcanogenic Massive Sulfide Deposits: Numerical Simulations of Vent Fluids and Comparison to Modern Sea-Floor Sulfides

Abstract Volcanogenic massive sulfide deposits may represent a significant future source of Te, which is a critical element important for the green energy transition. Tellurium is enriched in these settings by up to 10,000 times over its crustal abundance, indicating that fluids in sea-floor hydrothermal systems may transport and precipitate Te. The major element composition of these hydrothermal fluids is controlled by fluid-rock interaction and is well documented based on experimental, modeling, and natural studies; however, controls on Te mobility are still unknown. To better understand Te enrichment in this deposit type, numerical simulations of the mafic-hosted Vienna Woods and the felsic-hosted Fenway sea-floor vents in the Manus basin were performed to predict Te mobility in modern sea-floor hydrothermal vent fluids and Te deposition during sulfide formation. These simulations demonstrate that the mobility of Te in sea-floor hydrothermal systems is primarily controlled by fluid redox and temperature. Tellurium mobility is low in reduced hydrothermal fluids, whereas mobility of this metal is high at oxidized conditions at temperatures above 250°C. Numerical simulations of the reduced vent fluids of the mafic-hosted Vienna Woods site at the back-arc spreading center in the Manus basin yielded Te concentrations as low as 0.2 ppt. In contrast, the more oxidized model fluids of the felsic-hosted Fenway site located on Pual Ridge in the eastern Manus basin contain 50 ppt Te. The models suggest that Te enrichment in these systems reflects rock-buffer control on oxygen fugacity, rather than an enriched source of Te. In fact, the mafic volcanic rocks probably contain more Te than felsic volcanic rocks. The association of elevated Te contents in the felsic-hosted Fenway system likely reflects magmatic volatile input resulting in lower pH and higher Eh of the fluids. More generally, analysis of sulfide samples collected from modern sea-floor vent sites confirms that redox buffering by the host rocks is a first-order control on Te mobility in hydrothermal fluids. The Te content of sulfides from sea-floor hydrothermal vents hosted by basalt-dominated host rocks is generally lower than those of sulfides from vents located in felsic volcanic successions. Literature review suggests that this relationship also holds true for volcanogenic massive sulfides hosted in ancient volcanic successions. Results from reactive transport simulations further suggest that Te deposition during sulfide formation is primarily temperature controlled. Modeling shows that tellurium minerals are coprecipitated with other sulfides at high temperatures (275°–350°C), whereas Te deposition is distinctly lower at intermediate (150°–275°C) and low temperatures (100°–150°C). These predictions agree with geochemical analyses of sea-floor sulfides as Te broadly correlates positively with Cu and Au enrichment in felsic-hosted systems. The findings of this study provide an important baseline for future studies on the behavior of Te in hydrothermal systems and the processes controlling enrichment of this critical mineral in polymetallic sulfide ores.

Platinum solubility in silicate melts: The effects of sulfur (S2–), temperature, and melt composition

Platinum (Pt) solubility in silicate melt (SolPtSil) has been determined to understand its behavior in planetary magmas. However, the effect of sulfur (S) in the silicate melt on SolPtSil was paid little attention. Here we present experiments performed at 1–1.5 GPa, 1300–1700 °C, and oxygen fugacity (fO2) from IW−1.6 to IW+2.3 (IW denotes the iron–wüstite buffer) to determine SolPtSil for silicate melts with major element compositions corresponding to those of rhyolite, mid-ocean ridge basalt, high-FeO basalt, and Martian basalt. The results show that SolPtSil increases from ∼0.1 to 10 μg/g as the S content in silicate melt increases from nominally zero to ∼4000 μg/g, which indicates that Pt dissolves predominantly as Pt-sulfide species in S2–-bearing silicate melt. Our results also show that SolPtSil increases with increasing temperature and/or the silicate melt NBO/T (the ratio of non-bridging oxygens to tetrahedrally coordinated cations). Using our newly obtained SolPtSil and previous data, we developed an empirical model that describes SolPtSil as a multi-function of temperature, fO2, the silicate melt NBO/T, and the S2– content in silicate melt in the case at reducing conditions (fO2 < IW+3.5). The application of our empirical model to mantle partial melting suggests that Pt-rich alloys form only after sulfide liquid exhaustion because of the S-enhanced SolPtSil. We also applied our empirical model to predict the behavior of Pt during arc magmatic differentiation. The results agree with the observations that Pt-rich alloys formed before the saturation of sulfides in the oxidized Pual Ridge lavas. We suggest that the observed coexistence of Pt-bearing alloys and sulfide globules in oxidized arc magmas can be explained by an early formation of Pt-bearing alloys followed by a late formation of sulfide liquids. The strong effect of S2– on SolPtSil also implies that the oxidization of magmatic melts can lead to direct precipitation of Pt-bearing alloys due to the S2– to S6+ transformation, which can also be subsequently entrapped in the sulfide liquids formed later. Finally, our results show a strong effect of S2– in the silicate melt on lowering the metal–silicate melt partition coefficients of Pt, which should be considered when using Pt to trace planetary accretion and differentiation processes. Collectively, our study demonstrates an important role for S2– in controlling the behavior of Pt during planetary core–mantle–crust differentiation.

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.

Distribution and solubility limits of trace elements in hydrothermal black smoker sulfides: An in-situ LA-ICP-MS study

The key for understanding the trace metal inventory of currently explored VHMS deposits lies in the understanding of trace element distribution during the formation of these deposits on the seafloor. Recrystallization processes already occurring at the seafloor might liberate trace elements to later hydrothermal alteration and removement. To investigate the distribution and redistribution of trace elements we analyzed sulfide minerals from 27 black smoker samples derived from three different seafloor hydrothermal fields: the ultramafic-hosted Logatchev hydrothermal field on the Mid-Atlantic Ridge, the basaltic-hosted Turtle Pits field on the mid-atlantic ridge, and the felsic-hosted PACMANUS field in the Manus basin (Papua New Guinea). The sulfide samples were analyzed by mineral liberation analyser for the modal abundances of sulfide minerals, by electron microprobe for major elements and by laser ablation-inductively coupled plasma-mass spectrometry for As, Sb, Se, Te, and Au. The samples consist predominantly of chalcopyrite, sphalerite, pyrite, galena and minor isocubanite as well as inclusions of tetrahedrite–tennantite. Laser ablation spectra were used to evaluate the solubility limits of trace elements in different sulfide minerals at different textures. The solubility of As, Sb, and Au in pyrite decreases with increasing degree of recrystallization. When solubility limits are reached these elements occur as inclusions in the different sulfide phases or they are expelled from the mineral phase. Most ancient VHMS deposits represent felsic or bimodal felsic compositions. Samples from the felsic-hosted PACMANUS hydrothermal field at the Pual ridge (Papua New Guinea) show high concentrations of Pb, As, Sb, Bi, Hg, and Te, which is likely the result of an additional trace element contribution derived from magmatic volatiles. Co-precipitating pyrite and chalcopyrite are characterized by equal contents of Te, while chalcopyrite that replaced pyrite (presumably during black smoker growth) is enriched in Te relative to pyrite. These higher Te concentrations may be related to higher fluid temperature.

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.

Geologic setting of PACManus hydrothermal area — High resolution mapping and in situ observations

This study presents a systematic analysis and interpretation of autonomous underwater vehicle-based microbathymetry combined with remotely operated vehicle (ROV) video recordings, rock analyses and temperature measurements within the PACManus hydrothermal area located on Pual Ridge in the Bismarck Sea of eastern Manus Basin. The data obtained during research cruise Magellan-06 and So-216 provides a framework for understanding the relationship between the volcanism, tectonism and hydrothermal activity. PACManus is a submarine felsic vocanically-hosted hydrothermal area that hosts multiple vent fields located within several hundred meters of one another but with different fluid chemistries, vent temperatures and morphologies. The total area of hydrothermal activity is estimated to be 20,279 m2. The microbathymetry maps combined with the ROV video observations allow for precise high-resolution mapping estimates of the areal extents of hydrothermal activity. We find the distribution of hydrothermal fields in the PACManus area is primarily controlled by volcanic features that include lava domes, thick and massive blocky lava flows, breccias and feeder dykes. Spatial variation in the permeability of local volcanic facies appears to control the distribution of venting within a field. We define a three-stage chronological sequence for the volcanic evolution of the PACManus based on lava flow morphology, sediment cover and lava SiO2 concentration. In Stage-1, sparsely to moderately porphyritic dacite lavas (68–69.8 wt.% SiO2) erupted to form domes or cryptodomes. In Stage-2, aphyric lava with slightly lower SiO2 concentrations (67.2–67.9 wt.% SiO2) formed jumbled and pillowed lava flows. In the most recent phase Stage-3, massive blocky lavas with 69 to 72.5 wt.% SiO2 were erupted through multiple vents constructing a volcanic ridge identified as the PACManus neovolcanic zone. The transition between these stages may be gradual and related to progressive heating of a silicic magma following a recharge event of hot, mantle-derived melts.

Platinum-alloy and sulfur saturation in an arc-related basalt to rhyolite suite: Evidence from the Pual Ridge lavas, the Eastern Manus Basin

We have measured the platinum group element (PGE) and Re concentrations of arc-type lavas from the Pual Ridge and the surrounding area in the Eastern Manus Basin. These magmas followed an Fe-enrichment trend to produce a wide range of compositions with MgO varying between 8 and 0.1wt.%. We found distinct differences in the PGE geochemistry of the high (>3wt.% MgO) and the low-Mg lavas (<3wt.% MgO). During the early stages of fractionation, the concentrations of Pd, Cu, Au and Re increase whereas Ir, Ru, Rh and Pt decrease. Co-variations of Ir, Ru and Rh with Pt in the high-Mg lavas suggest that the depletion of these elements is due to Pt-rich alloy saturation. This is consistent with the high Pt contents in the high-Mg lavas, which is close to the solubility of Pt in the basaltic melt at similar conditions. In contrast, the concentrations of all PGE and Re drop rapidly in the low-Mg lavas (except for Ru and Ir), with the PGE concentrations falling at a rate that is appreciably faster than Cu, which we attribute to sulfide saturation. As a consequence, there is a marked decline in Pd/Cu in the low-Mg lavas and we suggest that this ratio is the best indicator of sulfide saturation in an evolving magmatic system.A feature of the data is that duplicate analyses of the same sample often do not agree within error. We attribute this scatter to the nugget effect, with nuggets of a Pt-rich alloy in the high-Mg lavas and sulfide blebs in the low-Mg lavas. The PGE concentrations of phenocryst-bearing high-Mg lavas are higher than in the associated glassy lavas, and scatter on MgO variation diagrams is significantly reduced if only glassy lavas are considered, which indicates that the micron scale Pt-rich alloy grains are intimately associated with the phenocrysts. Our results provide strong evidence that Pt-rich alloys can crystallize from a basaltic-andesitic magma, along with the silicate minerals, and fractionate Pt from Pd during magmatic differentiation. As a consequence, Pd/Pt increases during Pt alloy fractionation and this ratio can be used to identify Pt-rich metal saturation. The Pual Ridge alloys are Pt-rich because the primary magmas are Pt-enriched and Ir-depleted, which is typical of arc-related magmas.

The Magnetite Crisis in the Evolution of Arc-related Magmas and the Initial Concentration of Au, Ag and Cu

The association of large Au–Cu–Ag ore deposits with convergent margins is commonly attributed to the higher content of chalcophile elements in the parental magmas generated at subduction zones compared with mid-ocean ridges. We present new geochemical data for arc-like, relatively oxidized mantle-derived basalt to rhyolite magmas from the Pual Ridge and vicinity, Eastern Manus Basin, which show that the initial abundances of base and precious metals in the parental basalts are similar to those of mid-ocean ridge basalt (MORB). The contents of Au, Cu, and Ag are built up in the evolving Pual Ridge liquids to levels considerably in excess of those in MORB because, unlike MORB, they are not saturated in a sulfide phase, which is a consequence of their being more oxidized than MORB. The behaviour of S during the evolution of the Pual Ridge magmas is obscured by late-stage SO2 loss during eruption, but we show that it may be inferred by using Se as a proxy, because this element follows S closely during magmatic evolution except it is not lost during low-pressure (near sea-floor) degassing. The onset of magnetite fractionation at ~60 wt % SiO2 and an Mg-number of ~40 is accompanied by an abrupt decrease in the contents of Au, Cu and Ag, previously attributed to separation of Cu–Au-rich fluid, which is also shown by Se, implying that magnetite fractionation triggers sulfide saturation. Petrological modelling reveals that the amount of magnetite fractionation involved is sufficient to convert most of the S originally dissolved in the magma as sulfate (SO42–) to sulfide (S2–), triggering saturation in a Cu-rich sulfide phase, tentatively identified as bornite (Cu5FeS4). This sulfide phase sequesters Au and Ag, elements with the same valence as Cu in sulfides, but not other potentially chalcophile elements such as Ni, Re, and Pt, which suggests that the phase is crystalline rather than an immiscible sulfide melt. The relatively high contents of Cu and Au characteristic of evolved convergent margin magmas requires no enrichment from subducted material. Instead, the association of major Cu–Au deposits with convergent-margin magmatism results specifically from the process of magmatic evolution under oxidizing conditions. This same property also leads to early magnetite fractionation, triggering the abrupt saturation in the Cu-rich sulfide. Hence the easily recognizable trend of magmatic evolution under oxidizing conditions (i.e. the sharp drop in chalcophile element concentrations) may be an exploration guide to economic Au–Ag–Cu provinces, or a crucial pre-enrichment step in the formation of such deposits. The decrease in P2O5 and Sr at the same stage in the fractionation sequence (~60 wt % SiO2) indicates that saturation in apatite is concomitant with magnetite–sulfide saturation in the Eastern Manus Basin

Geochemistry of hydrothermal fluids from the PACMANUS, Northeast Pual and Vienna Woods hydrothermal fields, Manus Basin, Papua New Guinea

Processes controlling the composition of seafloor hydrothermal fluids in silicic back-arc or near-arc crustal settings remain poorly constrained despite growing evidence for extensive magmatic–hydrothermal activity in such environments. We conducted a survey of vent fluid compositions from two contrasting sites in the Manus back-arc basin, Papua New Guinea, to examine the influence of variations in host rock composition and magmatic inputs (both a function of arc proximity) on hydrothermal fluid chemistry. Fluid samples were collected from felsic-hosted hydrothermal vent fields located on Pual Ridge (PACMANUS and Northeast (NE) Pual) near the active New Britain Arc and a basalt-hosted vent field (Vienna Woods) located farther from the arc on the Manus Spreading Center. Vienna Woods fluids were characterized by relatively uniform endmember temperatures (273–285 °C) and major element compositions, low dissolved CO 2 concentrations (4.4 mmol/kg) and high measured pH (4.2–4.9 at 25 °C). Temperatures and compositions were highly variable at PACMANUS/NE Pual and a large, newly discovered vent area (Fenway) was observed to be vigorously venting boiling (358 °C) fluid. All PACMANUS fluids are characterized by negative δ D H 2 O values, in contrast to positive values at Vienna Woods, suggesting substantial magmatic water input to circulating fluids at Pual Ridge. Low measured pH (25 °C) values (∼2.6–2.7), high endmember CO 2 (up to 274 mmol/kg) and negative δ 34 S H 2 S values (down to −2.7‰) in some vent fluids are also consistent with degassing of acid-volatile species from evolved magma. Dissolved CO 2 at PACMANUS is more enriched in 13C (−4.1‰ to −2.3‰) than Vienna Woods (−5.2‰ to −5.7‰), suggesting a contribution of slab-derived carbon. The mobile elements (e.g. Li, K, Rb, Cs and B) are also greatly enriched in PACMANUS fluids reflecting increased abundances in the crust there relative to the Manus Spreading Center. Variations in alkali and dissolved gas abundances with Cl at PACMANUS and NE Pual suggest that phase separation has affected fluid chemistry despite the low temperatures of many vents. In further contrast to Vienna Woods, substantial modification of PACMANUS/NE Pual fluids has taken place as a result of seawater ingress into the upflow zone. Consistently high measured Mg concentrations as well as trends of increasingly non-conservative SO 4 behavior, decreasing endmember Ca/Cl and Sr/Cl ratios with increased Mg indicate extensive subsurface anhydrite deposition is occurring as a result of subsurface seawater entrainment. Decreased pH and endmember Fe/Mn ratios in higher Mg fluids indicate that the associated mixing/cooling gives rise to sulfide deposition and secondary acidity production. Several low temperature (⩽80 °C) fluids at PACMANUS/NE Pual also show evidence for anhydrite dissolution and water–rock interaction (fixation of B) subsequent to seawater entrainment. Hence, the evolution of fluid compositions at Pual Ridge reflects the cumulative effects of water/rock interaction, admixing and reaction of fluids exsolved from silicic magma, phase separation/segregation and seawater ingress into upflow zones.

Facies reconstruction from resistivity-at-the-bit images recorded in a submarine felsic volcanic succession

For the first time in history of the Ocean Drilling Program (ODP), Leg 193 drilled into an active hydrothermal vent field hosted in felsic submarine volcanic rocks using Logging-While-Drilling (LWD) techniques and specifically, the Resistivity-at-the-Bit (RAB) tool. In this manuscript, we present the results obtained from penetrating into the uppermost portion of hydrothermally altered dacite volcanic rocks hosting the subsurface of the PACMANUS active black smoker field along the Pual Ridge (Papua New Guinea). ODP Hole 1189C was drilled to a total depth of 166 m below seafloor (mbsf) and continuous electrical images of the borehole wall provide a full 360° coverage of the surrounding formation. For comparison, higher resolution borehole images were also recorded over an interval of approximately 50 m using a wireline Formation Micro Scanner (FMS). These data, together with geological observations obtained from drill cores provide the background for the interpretation of the electrical formation properties. The interpretation of the RAB images shows that coherent dacite facies and two types of breccias with distinctive clast size populations can be distinguished. These have been classified as breccia type I with predominance of coarse clasts containing maximum diameters of 95 cm and breccia type II with predominace of fine clasts containing maximum diameters of 55 cm. Coherent and breccia facies are intercalated in variable proportions with coherent facies becoming gradually dominant in the lower sections of the hole. The RAB data include the upper 10–30 m below surface where coring and traditional wireline logging data were not obtained. The interpretation of the RAB data confirms the principle observation from drill cores that the Roman Ruins site is located in a proximal to medial position relative to the eruptive vent and show that strongly altered fractured rocks extend to the seafloor enabling mixing of hot hydrothermal fluids and ocean bottom water. The continuous coverage of the RAB data is the only means of interpreting sub-bottom variations in the volcanic stratigraphy and the proportions of different volcanic facies.

Effects of hydrothermal alteration on Pb in the active PACMANUS hydrothermal field, ODP Leg 193, Manus Basin, Papua New Guinea: A LA-ICP-MS study

The conventional model of leaching volcanic rocks as a source of metals in a seafloor hydrothermal systems has been tested by examining the behavior of Pb and other trace elements during hydrothermal alteration. ODP Leg 193 drill sites 1188 (Snowcap) and 1189 (Roman Ruins) on Pual Ridge in the eastern Manus Basin offshore eastern Papua New Guinea provide a unique three-dimensional window into an active back-arc hydrothermal system. We investigate by means of a LA-ICP-MS microbeam technique the capacity of Pb to be leached from a host volcanic rock exposed to various types and intensities of alteration. Our results are in general agreement with previous studies that utilized bulk analytical techniques but provide a more detailed explanation of the processes. Fresh representative dacitic lavas from the Pual Ridge have an average whole rock Pb content of 5.2 ppm, an average interstitial glass Pb content of 5.6 ppm and an average plagioclase Pb content of 1.0 ppm. Altered matrix samples have highly variable Pb values ranging from 0 to 52.4 ppm. High Pb values in altered samples are associated with a low temperature chlorite and clay mineral assemblage, in some cases overprinted by a high temperature (up to 350 °C) silica-rich “bleaching” alteration. Only the most highly altered matrix samples have REE patterns that differ from the fresh Pual Ridge dacite. This may represent either different lava histories or alteration characteristics that have affected normally immobile REEs. Altered samples with the highest Pb values have similar REE patterns to those of the local unaltered lavas. They are compositionally similar to typical Pual Ridge dacites indicating a genetic relationship between the main regional volcanic suite and the subseafloor hydrothermally altered, Pb-enriched material. Relative loss/gain for Pb between the analyzed altered samples and a calculated precursor show a maximum relative gain of 901%. Samples with relative Pb gain from both drill sites are associated with lower temperature alteration mineral assemblages characterized by pervasive chloritization. The related lower temperature (220–250 °C) neutral to slightly acidic fluids have been ascribed by others to return circulation of hydrothermal fluids that did not interact with seawater. Because altered samples have a higher Pb content than the fresh precursor, leaching of fresh volcanic rocks cannot be the source of Pb in the hydrothermal systems.

Mineralogical, geochemical and isotopic characteristics of hydrothermal alteration processes in the active, submarine, felsic-hosted PACMANUS field, Manus Basin, Papua New Guinea

During ODP Leg 193, 4 sites were drilled in the active PACMANUS hydrothermal field on the crest of the felsic Pual Ridge to examine the vertical and lateral variations in mineralization and alteration patterns. We present new data on clay mineral assemblages, clay and whole rock chemistry and clay mineral strontium and oxygen isotopic compositions of altered rocks from a site of diffuse low-temperature venting (Snowcap, Site 1188) and a site of high-temperature venting (Roman Ruins, Site 1189) in order to investigate the water-rock reactions and associated elemental exchanges. The volcanic succession at Snowcap has been hydrothermally altered, producing five alteration zones: (1) chlorite ± illite-cristobalite-plagioclase alteration apparently overprinted locally by pyrophyllite bleaching at temperatures of 260–310°C; (2) chlorite ± mixed-layer clay alteration at temperatures of 230°C; (3) chlorite and illite alteration; (4) illite and chlorite ± illite mixed-layer alteration at temperatures of 250–260°C; and (5) illite ± chlorite alteration at 290–300°C. Felsic rocks recovered from two holes (1189A and 1189B) at Roman Ruins, although very close together, show differing alteration features. Hole 1189A is characterized by a uniform chlorite-illite alteration formed at ∼250°C, overprinted by quartz veining at 350°C. In contrast, four alteration zones occur in Hole 1189B: (1) illite ± chlorite alteration formed at ∼300°C; (2) chlorite ± illite alteration at 235°C; (3) chlorite ± illite and mixed layer clay alteration; and (4) chlorite ± illite alteration at 220°C. Mass balance calculations indicate that the chloritization, illitization and bleaching (silica-pyrophyllite assemblages) alteration stages are accompanied by different chemical changes relative to a calculated pristine precursor lava. The element Cr appears to have a general enrichment in the altered samples from PACMANUS. The clay concentrate data show that Cr and Cu are predominantly present in the pyrophyllites. Illite shows a significant enrichment for Cs and Cu relative to the bulk altered samples. Considerations of mineral stability allow us to place some constraints on fluid chemistry. Hydrothermal fluid pH for the chloritization and illitization was neutral to slightly acidic and relatively acidic for the pyrophyllite alteration. In general the fluids, especially from Roman Ruins and at intermediate depths below Snowcap, show only a small proportion of seawater mixing (<10%). Fluids in shallow and deep parts of the Snowcap holes, in contrast, show stronger seawater influence.

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.

Facies reconstruction of a hydrothermally altered dacite extrusive sequence: Evidence from geophysical downhole logging data (ODP Leg 193)

ODP Leg 193 drilled into the PACMANUS hydrothermal field (Papua New Guinea), which is an active hydrothermal vent field associated with felsic magmatism in a convergent geodynamic setting. The PACMANUS hydrothermal field is part of the eastern Manus Basin and is located near the crest of Pual Ridge, a 500 to 700 meters high felsic neo‐volcanic ridge. Two sites, Snowcap (Site 1188) and Roman Ruins (Site 1189), were drilled approximately 800 m apart. Although low core recovery limited the examination of the subsurface geology, geophysical downhole measurements provided continuous records of in‐situ physical properties with depth and were used to reconstruct lithostratigraphic profiles. Downhole logging operations included standard wire line logging and Logging While Drilling (LWD) measurements. Electrical resistivity images of the borehole wall were examined to find characteristic textures and three different volcanic facies were distinguished: coherent dacite, volcaniclastic dacite and brecciated dacite. In addition, intervals with sulfide mineralization were identified based on characteristic responses of the photoelectric factor log. A comparison of the reconstructed profiles from both sites shows considerable differences in the proportions of facies and the thickness of individual layers. Thick units of coherent facies are predominant at Snowcap and indicate a proximal position of the site within the volcanic system of Pual Ridge. At Roman Ruins, thin individual layers with rapid changes in volcanic facies and a higher proportion of volcaniclastic and brecciated dacite suggest a medial position. These differences in volcanic facies show that the volcanic architecture at Pual Ridge is characterized by small‐scale facies changes, emplacement of small‐volume individual lava flows and complex geometries of individual emplacement units. Geophysical logging data suggest that subseafloor hydrothermal activity is focused to the area around one hole at Roman Ruins. Downhole logs from this hole show characteristic cyclic patterns in the electrical resistivity and photoelectric factor logs indicating that hydrothermal sulfide mineralization is concentrated at the tops of individual lava flows representing paleo‐seafloor positions.

Contrasting evolution of hydrothermal fluids in the PACMANUS system, Manus Basin: The Sr and S isotope evidence

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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).