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Abstract: Seafloor hydrothermal systems play an important role in the metal budgets of the oceans via hydrothermal plumes, accumulation of seafloor massive sulfide deposits, and alteration of the oceanic crust. These processes have resulted in large-scale metal anomalies on the Pacific plate, most notably at the Nazca–Pacific plate boundary. This plate-scale variability in metal deposition has important implications for the fluxes of metals to subduction zones and possibly the metal endowment of arc-related mineral deposits. However, the relative contributions to the metal budget from black smokers, deep-sea sediments, Mn nodules and altered crust remain unclear. The Cu contents of more than 10,000 samples of seafloor massive sulfide deposits, subseafloor stockwork mineralization, nodules and sediments reveal that most of the Cu metal originally mobilized by high-temperature hydrothermal convection at the ridges is retained in the crust as subseafloor alteration and mineralization, never reaching the seafloor. This metal accounts for at least 80% of the labile Cu that may be released to subduction fluids driven off a down-going slab. Copper deposited in deep-sea sediments, which account for 17% of the total budget, is derived in part from plume fallout associated with ridge-crest hydrothermal activity but also from pelagic deposition of marine organic matter enriched in Cu metal. Massive sulfide deposits, nodules and manganiferous crusts account for only ∼3% of the Cu metal of the subducting slab. [Copyright &y& Elsevier]

Recent developments in the measurement of δ34S and δ33S in sulfide minerals provide a new framework to evaluate isotopic equilibrium among sulfide minerals and a new tool to investigate mineralizing processes or sources of sulfur in ore-forming systems. Isotopic equilibrium requires that values of δ34S and δ33S are highly correlated between mineral parrs and that deviations (quantified as Δ33S) from a reference mass-dependent fractionation are the same for both minerals within experimental error. In this contribution, we apply this framework for the first time to natural samples, and evaluate isotopic equilibrium among sulfide minerals from the Archean Kidd Creek volcanogenic massive sulfide deposit in Ontario, Canada. Two of eight sulfide mineral pairs showed clear isotopic disequilibrium, in agreement with interpreted paragenetic relationships; six other pairs have isotopic compositions that are consistent with equilibrium. We discuss the implications of these results for the interpretation of mineral paragenesis and sulfur isotope geothermometry at Kidd Creek. This technique offers new possibilities for evaluating sulfur isotope systematic in Archean (>2.45 Ga) ore deposits, in which the range of δ34S values is small but non-zero values of Δ33S are common. [ABSTRACT FROM AUTHOR]

• Review of the critical processes controlling ore formation in the New Ireland Basin. • Combining geological knowledge of the on- and offshore areas. • New constraints on the origin, timing, and location of pathways for metal-rich melts and fluids. • Significance of microplate tectonics for gold endowment. The Southwest Pacific region, and Papua New Guinea in particular, is spectacularly endowed with mineral resources, including some of the youngest and richest porphyry Cu-Mo-Au deposits in the world. Among them is the giant porphyry-epithermal Ladolam Au deposit on Lihir Island in the Tabar-Lihir-Tanga-Feni (TLTF) island chain, northeast of New Ireland. Its setting within a former forearc basin is very different from most Southwest Pacific porphyry and epithermal deposits. Our synthesis of published and previously unreleased data from ship-based multibeam and seismic studies, satellite gravimetry, geochemistry and geochronology reveals a far more complex crustal structure and composition than is presently understood from the geology of the islands alone. We show that the unique regional Au endowment results from the alignment of various preconditions that are prolific to ore formation: i) hydrous and metal-rich metasomatic veins in the mantle source, ii) second-stage, low volume partial melting due to incipient rifting, iii) high volatile contents and oxygen fugacities of the melts due to preferential melting of hydrous phases in the metasomatic veins, and iv) in the specific case of Lihir, unroofing of the volcanic edifice that led to boiling and rapid metal deposition. This study shows that the location of the Ladolam deposit on Lihir is controlled by large-scale structures that can be traced offshore and are the site of continuing submarine volcanism and epithermal-style Au mineralization. The observed structural framework is dominated by the emergence of trans -lithospheric faults that provided pathways for the melts to the seafloor, near-surface structural focusing of the ascending melts and fluids, and a regional tectonic stress regime that stabilized the conditions over a significant period of time and/or repeatedly. Marine seismic data confirms the complex structure of the TLTF island chain. Each island group sits on tilted blocks that form horst structures separated by half grabens developed due to regional NW-SE-directed extension. Regional compression perpendicular to the extension continues as a result of the transition from subduction to collision at the leading edge of the Ontong Java Plateau. The protracted, transtensional motion between distinct crustal blocks controls the location and timing of magmatism and mineralization. A kinematic link between volcanism at the location of Lihir and the splitting of New Ireland by NE-directed propagation of seafloor spreading in the Manus Basin is suspected. By combining onshore and offshore geology, we propose a new model of the evolution of the New Ireland Basin, magmatism along the TLTF island chain and ultimately ore deposit formation. This study demonstrates the importance of integrating offshore geology and geophysics into models that aim to explain the structural, magmatic, and sedimentary evolution of marginal basins that are host to economic mineral deposits. [ABSTRACT FROM AUTHOR]

Arc volcanism related to subduction is known to release large amounts of volatiles. Among them, S and Cl have the capability to transport metals from the Earth's interior to the upper crust where they may be concentrated in ore deposits. Metal fluxes, even in single volcanic systems, are high enough to form giant ore deposits within ~50 ka. The fact that such deposits are rare but concentrated in few, well-endowed regions must reflect unique magmatic and tectonic processes that control metal concentration and migration. Among the processes thought to be responsible for metal enrichment are crustal recycling, melting of metasomatized mantle, and microplate interactions that create pathways for mantle melts through the crust.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 large-scale 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. 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-to-Feni volcanic island chain that has emerged in the last 3.6 Ma from the New Ireland Basin, a much older sedimentary forearc basin relative to subduction along the stalled Manus-Kilinailau Trench. Here, we present a new interpretation of the regional geodynamic framework combining onshore and offshore data that extends the known geological framework of the area by a factor of 5-10 compared to the land cover alone. The distribution of onshore and offshore lithotectonic assemblages shows that magmatism in the Tabar-to-Feni island chain is not migrating along the chain, as previously suggested, but crosses the chain at a high angle to the Manus Kilinailau Trench, in accordance with a model of Pacific slab breakoff resulting from the stalled subduction. In the model, the general emergence of post-subduction high-K magmatism is closely related to slab breakoff, whereas the focusing of melts into distinct volcanic systems is controlled by regional crustal structure. Following post-Miocene adjustments of the North and South Bismarck microplates, the New Ireland Basin has been undergoing major lithospheric extension. Offshore seismic profiles image large normal faults between the individual island groups. These structures are genetically linked to rifting of New Ireland and extension in the eastern Manus Basin. The geometry of the crustal blocks in the basin indicate that Lihir is located at the point of maximum extension of the rift, thus explaining the anomalous magmatic and hydrothermal activity. [ABSTRACT FROM AUTHOR]

The extensional back-arc setting in the Aegean Sea, Greece, hosts polymetallic ore deposits that are temporally associated with the emplacement of Miocene granitoids and occur adjacent to the major detachment systems (e.g. North Cycladic and West Cycladic Detachment Systems). Various types of mineral deposits (e.g. carbonate replacement, skarn, vein-type Pb-Zn-Ag-(Au), and intermediate sulfidation epithermal deposits) formed during different stages of back-arc evolution and can be found in the metamorphic basement (Upper and Lower Cycladic Nappe), unmetamorphosed Pelagonian zone and Quaternary volcanic units along the active volcanic arc. Despite these host rock differences, a common hypogene mineralogical composition can be found across the Cycladic Islands, consisting of galena, sphalerite, pyrite, and minor chalcopyrite and sulfosalts. Galena (PbS) in particular is an abundant and geochemically robust mineral in these deposits. We present new galena Pb-isotope data collected via LA-Q-ICP-MS analysis of twelve deposits from Lavrion, the mainland in the north, across the islands of Tinos, Mykonos, Serifos, and Antiparos to Milos at the active volcanic arc in the south. Mineral separates have been analyzed from 31 samples with 20 spots on a minimum of 12 grains per sample. Galena shows fairly homogeneous Pb-isotopic compositions: 206Pb/204Pb: 18.68–18.91, 207Pb/204Pb: 15.67–15.75 and 208Pb/204Pb: 38.83–39.18, exhibiting a systematic geographic pattern. Whereas in the northern Cyclades predominantly vein-type deposits have low 206Pb/204Pb-isotope ratios (<18.85), the carbonate replacement, skarn and epithermal deposits in the western Cyclades have uniform and higher 206Pb/204Pb-isotope ratios (>18.85). The similarity of the 206Pb/204Pb-isotope ratios of galena from Lavrion and Milos suggests a common metal source underlies both regions, the Lower Cycladic Nappe, which contributed to the shallow crustal deposits in the western Cyclades and was accessible since the Late Miocene. In contrast, a mixture of more primitive metal sources, the Upper and Lower Cycladic Nappes, can explain the lower and heterogeneous 206Pb/204Pb-isotope ratios in the northern Cyclades. Based on Pb-isotope signatures the metal sources of the Cycladic Mineral District can be divided into two groups: a primitive and heterogeneous source in the northern Cyclades and a more evolved and homogenous source in the western Cyclades, and the two groups help define the fault trace of the proposed Trans Cycladic Thrust. [ABSTRACT FROM AUTHOR]

The LaRonde Penna Au-rich VMS deposit is the second largest known deposit of its class (58.8 Mt at 4.31 g/t, containing 8.1 Moz of Au). It is located in the Blake River Group of the Abitibi greenstone belt and it is part of the Bousquet district. The ore consists of massive to semi massive sulfide lenses (Au-Zn-Ag-Cu-Pb), stacked in the upper part of a steeply dipping, southward facing homoclinal submarine volcanic sequence composed of extensive tholeiitic basaltic flows (Hébécourt Formation) overlain by tholeiitic to transitional, mafic to intermediate effusive and volcaniclastic units (lower member of the Bousquet Formation) at the base and transitional to calc-alkaline (FII ± FI-type) intermediate to felsic effusive (domes and flow breccia) and intrusive rocks (cryptodomes and dikes) on top (upper member of the Bousquet Formation). This volcanic architecture is thought to be responsible for internal variations in ore and alteration styles: in the upper part of the mine, the 20 North lens comprises a transposed pyrite-chalcopyrite (Au-Cu) stockwork (20N Au zone) associated with a large footwall discordant to semiconformable quartz - biotite ± garnet assemblage (distal) which transitions laterally into a proximal quartz-garnet-biotite-muscovite zone (gains in MnO, Fe2O3 T, and MgO and losses of Na2O and K2O). The 20N Au zone is overlain by a pyrite-sphalerite-galena-chalcopyrite-pyrrhotite (Zn-Ag-Pb) massive sulphide lens (20N Zn zone). The 20N Zn zone tapers with depth in the mine (1900 m) and gives way to the 20N Au zone where it consists of transposed stringers and local semimassive sulphides (Au-rich pyrite and chalcopyrite) enclosed within a large aluminous alteration halo composed of a quartz-pyrite-kyanite-andalusite assemblage that is interpreted to be the metamorphic equivalent of an advanced argillic alteration that has many similarities to that of metamorphosed high-sulphidation systems where all oxides except SiO2 and Al2O3 have been leached. The variations in ore and alteration styles along the 20 North lens show that the hydrothermal system evolved in time and space from near-neutral seawater-dominated hydrothermal fluids, responsible for Au-Cu-Zn-Ag-Pb mineralization, to highly acidic fluids with possible direct magmatic contributions, responsible for Au ± Cu-rich ore and aluminous alteration in response to the evolving local volcanic setting that controlled the nature of the ore-forming fluids. The host sequence is interpreted to have been generated in an intermediate setting between back-arc basin and volcanic arc environments. This setting could be responsible, at least in part, for the Au enrichment of the VMS deposits of the Bousquet district and represent an exploration target in other volcanic belts. [ABSTRACT FROM AUTHOR]