Raymond Binns

Hydrous metasomatism of oceanic sub-arc mantle, Lihir, Papua New Guinea : petrology and geochemistry of fluid-metasomatised mantle wedge xenoliths

Abstract Ultramafic, mafic and sedimentary xenoliths have been recovered from a recently erupted, shoshonitic submarine cinder cone (Tubaf and Edison volcanoes) from the Tabar–Lihir–Tanga–Feni island arc, located in the New Ireland basin of Papua New Guinea. These samples represent a proxy drill hole that can be re-assembled into an ‘ophiolite-type’ model of oceanic lithosphere. Petrographic and geochemical examination of the gabbroic and depleted mantle xenoliths indicates that the New Ireland fore-arc lithosphere is a fragment of ancient Pacific Plate generated at a mid-ocean ridge spreading centre and transported to the Pacific–Australian Plate margin. Convergent margin processes subjected the harzburgitic mantle wedge to hydrofracturing and hydration metasomatism at T=790–1030°C as a consequence of dewatering of a subducted slab. Advection of a high-density, H2O-rich fluid containing a substantial dissolved component (alkali aluminosilicate melt and aqueous carbon and sulphur species) through these mantle fractures caused a net transfer of soluble elements from the lower to upper mantle wedge and created a network of oxidised (ΔFMQ≈1.8–2.0) metasomatised peridotite enriched in orthopyroxene, clinopyroxene, phlogopite, amphibole, magnetite, and Fe–Ni sulphides. The vein mineral assemblage magnetite+sulphide indicates precipitation from a hydrous fluid with high SO2/H2S, consistent with the hydrous fluid being derived from dehydration of subducted, altered oceanic crust. Preferential partial melting of these metasomatically enriched mantle wedge regions could account for the highly oxidised, sulphur- and alkali-rich nature of the high-K calc-alkaline volcanoes of the Tabar–Lihir–Tanga–Feni island chain.

Parental basaltic melts and fluids in eastern Manus backarc Basin: implications for hydrothermal mineralisation

The eastern Manus Basin is an actively forming backarc extensional zone behind the New Britain Island are, which hosts a number of submarine volcanic edifices and hydrothermal fields. Isotopic and trace element geochemical characteristics of the edifices are comparable with those of the adjacent subaerial New Britain are, and differ significantly from those of MORE-like lavas on and near the Manus Spreading Ridge in the central part of the basin. Fractional crystallisation dominates magma evolution from primitive basalts to andesites, dacites and rhyodacites in the eastern Manus Basin, but several lineages with differing trace element enrichment have been delineated. Melt inclusions within olivine phenocrysts (Fo(82-92)) Of two representative east Manus basalts, respectively, with modest (0.2 wt%) and high (0.8 wt%) potassium contents, host ubiquitous CO2-bearing vapour bubbles, denoting presence of an immiscible fluid phase at early stages of crystallisation. Bubbles often carry precipitate phases whose abundance is broadly proportional to the bubble size reaching a maximum in fluid bubbles with little or no melt. Among the precipitates, detected by laser Raman spectroscopy and EDS-scanning electron microscopy, carbonates are common and include magnesite, calcite, ankerite, rhodochrosite and nahcolite (NaHCO3). Gypsum, anhydrite, barite, anglesite, pyrite, and chalcopyrite have also been found. Some amorphous precipitates recrystallise after bubbles are opened to Na-Ca carbonates, halite and Na-K-Ca alumine-silicates. Copper abundances decrease from basalt to dacite across the eastern Manus fractionation spectrum, whereas Pb behaves as an incompatible element, increasing to highest values in the dacites. Zinc abundance reaches maximum concentrations in andesite, and decreases during further fractionation. Loss of Cu especially from the fractionating magmas, in the absence of immiscible sulphide liquid, strongly implies metal partitioning into CO2-H2O fluid, which is degassed significantly during magma fractionation. Hydrothermal fluids in the PACMANUS system may carry a direct contribution of the magmatic metal-bearing fluid, exsolved from the crystallising are-like magmas at this immature backarc basin, and are able to transport and concentrate major amounts of ore metals, particularly Cu.

Evidence for rhenium enrichment in the mantle wedge from submarine arc–like volcanic glasses (Papua New Guinea)

The low Re abundance in arc-type volcanic rocks characterized by high 1 8 7 Os/ 1 8 8 Os ratios is an unsolved puzzle of the 1 8 7 Re- 1 8 7 Os isotope system, leaving a significant gap in our understanding of the evolution of the upper mantle-continental crust system. Here we report new observations of high Re concentrations in fresh, submarine-erupted-i.e., relatively undegassed-island arc-like volcanic glasses dredged from the eastern Manus Basin, offshore Papua New Guinea. These observations, together with previously published reports of high Re concentrations in arc-type melt inclusions, indicate that undegassed arc-type volcanic rocks and the mantle wedge are enriched in Re. Consequently, the Re concentration in the continental crust is likely to be as high as ∼2 ppb, much higher than previously estimated. The low Re concentrations in subaerial arc-type volcanic rocks are probably due to Re loss during magma degassing.

Tectonics of rift propagation into a continental margin : Western Woodlark Basin, Papua New Guinea

Oblique convergence along the irregular boundary between the Pacific and Indo-Australian plates in the SW Pacific has resulted in a transpressional regime. The drag of the overriding Pacific plate has produced a breakup of the Solomon microplate from the Indo-Australian plate and has induced its rotation within the mega-shear zone between the two major plates. The microplate is being separated from the Indo-Australian plate by passive rifting and seafloor spreading in the Woodlark Basin which is propagating westward along the continental margin of Papua New Guinea. The present-day pole of the microplate rotation with respect to Indo-Australian plate is estimated to be near the Owen Stanley Fault Zone which represents the suture after Paleocene-Eocene arc-continent collision. The average rate of the seafloor spreading propagation is estimated to be 150 mm yr−1 over the last 3.4 Ma. The transition between seafloor spreading and continental rifting is characterized by a dramatic reduction in the production of new oceanic crust and by a change from localized deformation within oceanic lithosphere to distributed continental extension. Variations between the two structural styles are being balanced by an accommodation zone and a newly developing transform fault. The style of continental deformation progressively changes due to a gradual decrease in lithospheric thinning along the rift axis toward the pole of opening. Three accommodation zones are proposed to balance differential extension and slippage between individual series of tilted blocks.

Hydrothermal processes and contrasting styles of mineralization in the western Woodlark and eastern Manus basins of the western Pacific

Abstract The western Woodlark Basin (initial rifting of continental crust) and eastern Manus Basin (rifted arc crust) offshore eastern Papua New Guinea display contrasting styles of hydrothermal activity and mineralization. In the eastern Manus basin, en echelon felsic and mafic volcanic ridges have formed in a pull-apart basin of the rifted New Britain arc terrane. Here, the PACMANUS Cu-Zn-Pb-Ag-Au sulphide deposit is forming within an area of about 800 × 350 m on the flank of a dacite lava dome atop a prominent 20 km long and 250–350 m high volcanic ridge. The ridge is andesitic in its lower reaches, dacitic to rhyolitic on top and is adjacent to an extensive field of basalt. At Woodlark, submarine rhyolite domes are devoid of hydrothermal products, but extensive Fe-Si-Mn oxyhydroxide deposits are forming from low temperature fluids on Franklin Seamount, an axial basaltic andesite volcano near the tip of the oceanic propagator. Protruding through and perhaps underlying these oxyhydroxides are inactive, higher temperature, precious metal-rich (Ag to 545 ppm, Au to 21 ppm), barite-silica spires. The Franklin Seamount deposits are thought to cap a disseminated sulphide deposit within the volcano and represent a failed massive sulphide system. Both Franklin Seamount and PACMANUS provide models for ancient ores on land.

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

Ocean Drilling Program (ODP) Leg 193 investigated two sites of hydrothermal activity along the crest of the Pual Ridge in the eastern Manus Basin. A site of low-temperature diffuse venting, Snowcap (Site 1188), and a high-temperature black smoker site, Roman Ruins (Site 1189), were drilled to depths of 386 and 206 m below seafloor (mbsf), respectively. Although the two sites are <1000 m apart, the 87Sr/86Sr and ?34S signatures of anhydrite recovered at both sites are very different. The data suggest a complex interplay among hydrothermal fluid, magmatic fluid, and seawater during alteration and mineralization of the PACMANUS (Papua New Guinea–Australia–Canada–Manus) system. These new results significantly expand the subsurface data on seafloor hydrothermal systems and may begin to explain the earliest processes of multistage mineralization and alteration history that typify ancient massive sulfide systems.

Submarine volcanism and hydrothermal venting studied in Papua, New Guinea

From March 11 to April 5,1994, the German research vessel Sonne mapped the largely uncharted offshore areas of the Tabarto-Feni island chain in the New Ireland Basin of Papua, New Guinea. The Epithermal Deposits Southwestern Pacific Ocean (EDISON) cruise was organized as part of a multidisciplinary program to study the regional tectonic setting of the Tabar-to-Feni chain, to document recent submarine volcanism, and to investigate seafloor hydrothermal activity on the submerged flanks of the volcanos. The New Ireland Basin occupies a forearcposition with respect to the formerly active Manus-Kilinailau arc-trench system and hosts a series of Pliocene to recent alkaline volcanos that are built on rifted Miocene sedimentary basement. Several of the volcanos have large, high-level porphyry stocks, and several have active geothermal systems, including gold-depositing hot springs and the giant Ladolam gold deposit on the island of Lihir.

New hydrothermal activity and alkalic volcanism in the backarc Coriolis Troughs, Vanuatu

The Vanuatu Australia Vents Expedition (VAVE) to the Coriolis Troughs in southern Vanuatu during September 2001 aboard the RV Franklin discovered a new hydrothermall vent field-herein informally named Nifonea-and recent alkallic volcanic activity. The Nifonea field in the central Vate Trough was located by coincident light transmission and CH4 anomalies in a hydrothermal plume of ∼60 km2 extent, best developed between 1600 and 1750 m depth at ∼150 m above the seafloor. Extensive hydrothermal fauna and yellow-brown crusts and mounds cover an area of ∼1 km2. Very fresh, glassy, variably vesicular, sparsely phyric and aphyric basalt, trachybasalt, and basaltic trachyandesite (with ∼5-6 wt% combined alkalies at ∼ 51%-53% SiO2 and enriched light rare earth elements, Nb, and Zr) samples were dredged from youthful curtain, tube, and sheet flows, plus iron oxyhydroxide deposits. The alkalic composition of lavas in this tectonic setting is unique and attributed to thin ocean crust being developed in an incipient rifting phase involving a relatively low percentage of source-mantle melting. The Coriolis Troughs are among Earths most youthful backarc basins and thus provide valuable insights to incipient rifting and hydrothermal processes.