Due to the complexity of 2D magnetic anomaly maps north of 18 degrees S and the sparsity of seismic data, the tectonic evolution of the northern Lau Basin has not yet been unraveled. We use a multi-method approach to reconstruct the formation of the basin at similar to 16 degrees S by compiling seismic, magnetic, gravimetric and geochemical data along a 185 km-long crustal transect. We identified a crustal zonation which preserves the level of subduction input at the time of the crust's formation. Paired with the seafloor magnetization, the crustal zonation enabled us to qualitatively approximate the dynamic spreading history of the region. Further assessment of the recent tectonic activity and the degree of tectonic overprinting visible in the crust both suggest a complex tectonic history including a dynamically moving spreading center and the reorganizing of the local magma supply. Comparing the compiled data sets has revealed substantial differences in the opening mechanisms of the two arms of the Overlapping Spreading Center (OSC) that is made up by the northernmost tip of the Fonualei Rift and Spreading Center in the east and the southernmost segment of the Mangatolu Triple Junction in the west. The observed transition from a predominantly tectonic opening mechanism at the eastern OSC arm to a magmatic opening mechanism at the western OSC arm coincides with an equally sharp transition from and strongly subduction influenced crust to a crust with virtually no subduction input. The degree of subduction input alters the geochemical composition, as well as the lithospheric stress response. Plain Language Summary The opening of back-arc basins is often described as analogy to mid-ocean ridge spreading, where the only difference is the force driving the extension. However, the northern Lau Basin is a prime example for the shortcomings of this analogy since its crust preserves an image of its complex tectonic history. The complexity results from the short-lived nature of zones of active rifting and spreading in the northern Lau Basin, which is very different from the temporally and spatially steady nature of spreading centers at mid-ocean ridges. The analysis of different methods (wide angle seismic data using ocean bottom seismometers, multi-channel seismic, magnetic, gravity, and geochemical data) has led us to conclude that the Lau Basin's crust at 15 degrees 30-17 degrees 20'S was formed by a dynamically changing, both in regard of magma composition and position, extensional system that consists of the Fonualei Rift and Spreading Center and the Mangatolu Triple Junction. Nevertheless, the crustal zonation, formed by the varying subduction influence during its formation, is still preserved and affects the stress response of the crust and thus the present-day tectonic behavior.

Epithermal-style and transitional shallow submarine hydrothermal systems were discovered at recently active volcanic centers along the Tonga-Kermadec Arc, SW-Pacific. Notable enrichments in As, Sb, Hg, Ag, and Au, low base metal contents, characteristic alteration patterns and sulfur isotope ratios resembling precipitates from volcanic subaerial hot springs suggest contributions from magmatic sources. The growing number of discoveries of epithermal-style mineralisation in shallow submarine hot springs have significant implications for land-based mineral exploration in ancient volcanic belts, which are traditionally targeted for volcanogenic massive sulfide deposits but are largely unexplored for epithermal-type mineralisation.

The northeastern Lau Basin is one of the fastest opening and magmatically most active back-arc regions on Earth. Although the current pattern of plate boundaries and motions in this complex mosaic of microplates is reasonably understood, the internal structure and evolution of the back-arc crust are not. We present new geophysical data from a 290 km long east-west oriented transect crossing the Niuafo'ou Microplate (back-arc), the Fonualei Rift and Spreading Center (FRSC) and the Tofua Volcanic Arc at 17 degrees 20S. Our P wave tomography model and density modeling suggest that past crustal accretion inside the southern FRSC was accommodated by a combination of arc crustal extension and magmatic activity. The absence of magnetic reversals inside the FRSC supports this and suggests that focused seafloor spreading has until now not contributed to crustal accretion. The back-arc crust constituting the southern Niuafo'ou Microplate reveals a heterogeneous structure comprising several crustal blocks. Some regions of the back-arc show a crustal structure similar to typical oceanic crust, suggesting they originate from seafloor spreading. Other crustal blocks resemble a structure that is similar to volcanic arc crust or a "hydrous" type of oceanic crust that has been created at a spreading center influenced by slab-derived water at distances <50 km to the arc. Throughout the back-arc region, we observe a high-velocity (Vp 7.2-7.5 km s(-1)) lower crust, which is an indication for magmatic underplating, which is likely sustained by elevated upper mantle temperatures in this region.

Abstract Back‐arc basins open in response to subduction processes, which cause extension in the upper plate, usually along trench‐parallel spreading axes. However, global seismic databases reveal that the majority of seismic events in the Lau Basin occur along transcurrent (strike‐slip) rather than extensional faults. To better characterize active deformation in this region, we compared centroid moment tensors (CMTs), calculated for large (Mw > 5), shallow (<30 km) seismic events, to the orientations of seafloor lineaments mapped throughout the Lau Basin. Ship‐based multibeam and satellite altimetry were combined with vertical gravity gradient data to create the lineament map. By comparing the possible focal planes of the CMTs to the orientations of the lineaments, the most likely fault plane solutions were selected, thus classifying the faults and establishing the nature of the highly variable stress regimes in the basin. We resolved the strike, dip, and dip direction of 308 faults and classified 258 additional structures by fault type. The analysis highlights a stress regime that is dominated by a combination of left‐lateral and right‐lateral strike‐slip faults, large‐scale transcurrent motion along rigid crustal‐scale fault zones, and nonrigid diffuse deformation along preexisting seafloor structures, with extension mainly limited to the tips of propagating rifts and spreading centers. By resolving many of the uncertain motions on the mapped lineaments of the Lau Basin, the CMT analysis addresses a number of questions concerning basin‐scale stress regimes and microplate development, complementing GPS measurements, and providing a more complete picture of the complexities of back‐arc basin development.

Giant ore deposits are rare but concentrated in few, wellendowed regions which must reflect unique magmatic and tectonic processes that control metal concentration and migration. The region of Papua New Guinea in the Western Pacific is particularly well-endowed in Cu and Au. Here, complex plate tectonic processes including subduction reversals, microplate formation and reorientations, and largescale lithospheric extension have led to the formation of a number of world-class Cu and Au deposits. The major Cu deposits are associated with continent collision in the Miocene. The Au deposits are Pliocene and younger and are more closely linked to recent microplate tectonics [1]. One of those is the giant Ladolam porphyry-epithermal Au deposit on the island of Lihir in easternmost Papua New Guinea. Lihir belongs to one of four island groups in the Tabar-toFeni chain that has emerged in the last 3.6 Ma from the New Ireland Basin, an older sedimentary forearc basin relative to the stalled Manus-Kilinailau Trench.

Marine minerals such as manganese nodules, Co-rich ferromanganese crusts, and seafloor massive sulfides are commonly seen as possible future resources that could potentially add to the global raw materials supply. At present, a proper assessment of these resources is not possible due to a severe lack of information regarding their size, distribution, and composition. It is clear, however, that manganese nodules and Co-rich ferromanganese crusts are a vast resource and mining them could have a profound impact on global metal markets, whereas the global resource potential of seafloor massive sulfides appears to be small. These deep-sea mineral commodities are formed by very different geological processes resulting in deposits with distinctly different characteristics. The geological boundary conditions also determine the size of any future mining operations and the area that will be affected by mining. Similarly, the sizes of the most favorable areas that need to be explored for a global resource assessment are also dependent on the geological environment. Size reaches 38 million km2 for manganese nodules, while those for Co-rich crusts (1.7 million km2) and massive sulfides (3.2 millionkm2) are much smaller. Moreover, different commodities are more abundant in some jurisdictions than in others. While only 19% of the favorable area for manganese nodules lies within the Exclusive Economic Zone of coastal states or is covered by proposals for the extension of the continental shelf, 42% of the favorable areas for massive sulfides and 54% for Co-rich crusts are located in EEZs.

Summary Deep-sea mining is seen as a potential way to provide future secure metal supply to global markets. The current rush to the seafloor in areas beyond national jurisdiction indicates that sound knowledge of the geological characteritics of the various commodities, a realistic resource assessment, and a social and political discussion about the cons and pros of their exploitation that is based on facts, not myths, is required. This contribution provides the most recent information on global deep-sea mineral resources and sets the stage for detailed talks in this session.

Rock and fluid samples were collected from three hydrothermal chimneys at the Endeavour Segment, Juan de Fuca Ridge to evaluate linkages among mineralogy, fluid chemistry, and microbial community composition within the chimneys. Mössbauer, midinfrared thermal emission, and visible-near infrared spectroscopies were utilized for the first time to characterize vent mineralogy, in addition to thin-section petrography, X-ray diffraction, and elemental analyses. A 282°C venting chimney from the Bastille edifice was composed primarily of sulfide minerals such as chalcopyrite, marcasite, and sphalerite. In contrast, samples from a 300°C venting chimney from the Dante edifice and a 321°C venting chimney from the Hot Harold edifice contained a high abundance of the sulfate mineral anhydrite. Geochemical modeling of mixed vent fluids suggested the oxic-anoxic transition zone was above 100°C at all three vents, and that the thermodynamic energy available for autotrophic microbial redox reactions favored aerobic sulfide and methane oxidation. As predicted, microbes within the Dante and Hot Harold chimneys were most closely related to mesophilic and thermophilic aerobes of the Betaproteobacteria and Gammaproteobacteria and sulfide-oxidizing autotrophic Epsilonproteobacteria. However, most of the microbes within the Bastille chimney were most closely related to mesophilic and thermophilic anaerobes of the Deltaproteobacteria, especially sulfate reducers, and anaerobic hyperthermophilic archaea. The predominance of anaerobes in the Bastille chimney indicated that other environmental factors promote anoxic conditions. Possibilities include the maturity or fluid flow characteristics of the chimney, abiotic Fe2+ and S2- oxidation in the vent fluids, or O2 depletion by aerobic respiration on the chimney outer wall.

Clark volcano of the Kermadec arc, northeast of New Zealand, is a large stratovolcano comprised of two coalescing volcanic cones; an apparently younger, more coherent, twin-peaked edifice to the northwest and a relatively older, more degraded and tectonized cone to the southeast. High-resolution water column surveys show an active hydrothermal system at the summit of the NW cone largely along a ridge spur connecting the two peaks, with activity also noted at the head of scarps related to sector collapse. Clark is the only known cone volcano along the Kermadec arc to host sulfide mineralization.Volcano-scale gravity and magnetic surveys over Clark show that it is highly magnetized, and that a strong gravity gradient exists between the two edifices. Modeling suggests that a crustal-scale fault lies between these two edifices, with thinner crust beneath the NW cone. Locations of regional earthquake epicenters show a southwest-northeast trend bisecting the two Clark cones, striking northeastward into Tangaroa volcano. Detailed mapping of magnetics above the NW cone summit shows a highly magnetized ring structure 350 m below the summit that is not apparent in the bathymetry; we believe this structure represents the top of a caldera. Oblate zones of low (weak) magnetization caused by hydrothermal fluid upflow, here termed burn holes, form a pattern in the regional magnetization resembling Swiss cheese. Presumably older burn holes occupy the inner margin of the ring structure and show no signs of hydrothermal activity, while younger burn holes are coincident with active venting on the summit.A combination of mineralogy, geochemistry, and seafloor mapping of the NW cone shows that hydrothermal activity today is largely manifest by widespread diffuse venting, with temperatures ranging between 56 degrees and 106 degrees C. Numerous, small (<= 30 cm high) chimneys populate the summit area, with one site host to the similar to 7-m-tall "Twin Towers" chimneys with maximum vent fluid temperatures of 221 degrees C (pH 4.9), consistent with delta S-34(anhydrite-pyrite) values indicating formation temperatures of similar to 228 degrees to 249 degrees C. Mineralization is dominated by pyrite-marcasite-barite-anhydrite. Radiometric dating using the Ra-228/Ra-226 and Ra-226/Ba methods shows active chimneys to be <20 with most <2 years old. However, the chimneys at Clark show evidence for mixing with, and remobilizing of, barite as old as 19,000 years. This is consistent with Nd and Sr isotope compositions of Clark chimney and sulfate crust samples that indicate mixing of similar to 40% seawater with a vent fluid derived from low K lavas. Similarly, REE data show the hydrothermal fluids have interacted with a plagioclase-rich source rock.A holistic approach to the study of the Clark hydrothermal system has revealed a two-stage process whereby a caldera-forming volcanic event preceded a later cone-building event. This ensured a protracted (at least 20 ka yrs) history of hydrothermal activity and associated mineral deposition. If we assume at least 200-m-high walls for the postulated (buried) caldera, then hydrothermal fluids would have exited the seafloor 20 ka years ago at least 550 m deeper than they do today, with fluid discharge temperatures potentially much hotter (similar to 350 degrees C). Subsequent to caldera infilling, relatively porous volcaniclastic and other units making up the cone acted as largescale filters, enabling ascending hydrothermal fluids to boil and mix with seawater subseafloor, effectively removing the metals (including remobilized Cu) in solution before they reached the seafloor. This has implications for estimates for the metal inventory of seafloor hydrothermal systems pertaining to arc hydrothermal systems.

Lalor is a recently discovered Au-Zn-rich volcanogenic massive sulphide (VMS) deposit. It is located in the Paleoproterozoic Snow Lake arc assemblage, host to numerous past producing Cu-Zn and Zn-Cu VMS deposits. Lalor is the largest deposit of the Snow Lake camp and also the richest in gold with reserves of 14.4 Mt grading 1.86 g/t Au, 24 g/t Ag, 0.6 wt.% Cu and 7 wt.% Zn and resources estimated at 12.6 Mt grading 3.85 g/t Au, 27.3 g/t Ag, 0.9 wt.% Cu and 2.3 wt.% Zn, for a total size of approximately 27 Mt and potentially containing 75 t Au. The deposit consists of distinct Zn-Cu-Pb±Au-Ag semi-massive to massive sulphide lenses and zones of disseminated Au-Ag-Pb-Cu sulphides. The ore zones are stratigraphically and/or structurally stacked in a complexly deformed and metamorphosed succession of intensely hydrothermally altered rocks of the Chisel mature arc sequence that hosts other Zn-rich VMS deposits. Preliminary mapping and lithogeochemistry results indicate that the stratigraphic footwall is composed of at least three distinct but highly altered mafic to felsic volcanic (and perhaps sedimentary) units. The alteration of the footwall is both extensive and intense. At least 11 distinct alteration assemblages have been defined based on the distribution and relative abundance of specific metamorphic minerals such as amphiboles, chlorite, cordierite, biotite, muscovite, pyrite, staurolite, garnet, kyanite, sillimanite, diopside and epidote. The various alteration assemblages may be in part due to varying protolith compositions, together with the superposition of several hydrothermal events. The hanging wall does not show any extensive alteration and may be in structural contact with the deposit. Five ore types can be defined. They include Zn±Cu-rich massive sulphide lenses and three distinct ore types that contain significant gold: (1) Cu-rich massive sulphides; (2) low sulphide calc-silicate zones with high Ag-Pb-Cu±As-Se-Te and; (3) anthophylliterich alteration zones with trace of finely disseminated pyrrhotite. The numerous alteration assemblages and the various ore styles result from a complex hydrothermal history and possible modifications during subsequent deformation and metamorphism. The gold endowment of the deposit, its size and its distinctive features compared to known anomalous and gold-rich VMS deposits make Lalor an ideal site to document and better understand gold enrichment processes in the VMS environment.

Reykjanes is a high-temperature subaerial geothermal system (275-340 degrees C) with fluid compositions comparable to those discharged from black smokers on the ocean floor. The reservoir concentrations below 1350 m depth of trace metals, prior to boiling, were determined from downhole samples; they include 154-2431 mu M Fe (9-140 mg/kg), 207-261 mu M Cu (14-17 mg/kg), 79-393 mu M Zn (5-27 mg/kg), 37-55 mu M Mn (2-3 mg/kg), and 0.61.4 mu M Pb (120-290 mu g/kg), with 6-31 nM Au (1-6 mu g/kg) and 250-960 nM Ag (28-107 mu g/kg). By constrast, the discharge liquid at the surface has orders of magnitude lower metal concentrations due to precipitation caused by boiling and vapor loss during depressurization. The metals in similar seawater-dominated high-temperature discharges from ocean-floor black smokers are typically lower in concentration than the pre-boiled (and unmixed) fluids from Reykjanes wells. Some seafloor fluids have boiled before reaching the seafloor; more importantly they may have entrained cold seawater just before discharge. These processes resulted in metal deposition prior to sampling at the seafloor and as evidenced from the formation of now-eroded volcanic-hosted massive sulfide deposits.

The Reykjanes geothermal system has a reservoir liquid that originated as seawater and has reacted with the surrounding basalt at temperatures up to 340 C. As the fluid ascends in wells the pressure decreases which leads to boiling and the precipitation of sulfides. In surface pipelines the scales consist mainly of sphalerite, chalcopyrite with minor bornite, pyrrhotite, and pyrite. The bulk composition of the scales downstream of the orifice plate at similar to 22 bar is 45 wt% Zn, 15 wt% Cu, 3 wt% Fe, 30 wt% S, 2 wt% SiO2, up to 950 mg/kg Au and up to 2.3 wt% Ag. The amount of scales formed in one year in three wells were up to 1.4 tonnes, consisting of 896 kg Zn, 284 kg Cu, 197 kg Fe, 22 kg Pb, similar to 5 kg Ag and 0.8 kg Au. The Reykjanes geothermal system is a subaerial analogue of high-temperature black smokers on the ocean floor, based on reservoir fluid compositions with high metal and trace element concentrations and the similar compositions of minerals that precipitate downhole and in surface pipelines.

Sulphate concentrations in the ocean prior to 2.4 Gyr ago were lower than today. The sulphur isotope systematics of 2.7-Gyr-old sulphide deposits suggests that these low concentrations were maintained by a balance between hydrothermal sources and microbial sulphate reduction.

The Seafloor Mineralization Working Group (SMWG) of the international non-profit organization InterRidge was formed is 2008 to address the issues surrounding the burgeoning interest in the mineral resources associated with hydrothermal vents found at the midocean ridge system. Following a successful workshop and colloquium on the issues surrounding deep sea mining at vent systems, the working group outlined some of the key science questions needed to better understand this complex system.