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We document the discovery of an active, shallow, seafloor hydrothermal system (known as the Seven Sisters Vent Field) hosted in mafic volcaniclasts at a mid-ocean ridge setting. The vent field is located at the southern part of the Arctic mid-ocean ridge where it lies on top of a flat-topped volcano at ~130 m depth. Up to 200 &deg, C phase-separating fluids vent from summit depressions in the volcano, and from pinnacle-like edifices on top of large hydrothermal mounds. The hydrothermal mineralization at Seven Sisters manifests as a replacement of mafic volcaniclasts, as direct intraclast precipitation from the hydrothermal fluid, and as elemental sulfur deposition within orifices. Barite is ubiquitous, and is sequentially replaced by pyrite, which is the first sulfide to form, followed by Zn-Cu-Pb-Ag bearing sulfides, sulfosalts, and silica. The mineralized rocks at Seven Sisters contain highly anomalous concentrations of &lsquo, epithermal suite&rsquo, elements such as Tl, As, Sb and Hg, with secondary alteration assemblages including silica and dickite. Vent fluids have a pH of ~5 and are Ba and metal depleted. Relatively high dissolved Si (~7.6 mmol/L Si) combined with low (0.2&ndash, 0.4) Fe/Mn suggest high-temperature reactions at ~150 bar. A 13C value of &minus, 5.4&permil, in CO2 dominated fluids denotes magmatic degassing from a relatively undegassed reservoir. Furthermore, low CH4 and H2 (&lt, 0.026 mmol/kg and &lt, 0.009 mmol/kg, respectively) and 3He/4He of ~8.3 R/Racorr support a MORB-like, sediment-free fluid signature from an upper mantle source. Sulfide and secondary alteration mineralogy, fluid and gas chemistry, as well as 34S and 87Sr/86Sr values in barite and pyrite indicate that mineralization at Seven Sisters is sustained by the input of magmatic fluids with minimal seawater contribution. 226Ra/Ba radiometric dating of the barite suggests that this hydrothermal system has been active for at least 4670 &plusmn, 60 yr.

The Iberian Pyrite Belt (IPB) is a major metallogenic province with numerous volcanic-hosted massive sulfide (VMS) deposits. Albernoa, Serra Branca and Neves-Corvo represent three well-known felsic volcanic centers in the Portuguese portion of the IPB. Albernoa has no known VMS mineralization, whereas gossans are well known in Serra Branca. Neves-Corvo is a world-class VMS deposit with high grade and tonnage for massive Cu and Zn sulfide ores as well as for Sn (massive and stringer cassiterite ores; now exhausted). Magmatic-hydrothermal fluids may contribute to the metal budget in some VMS-forming systems. In the IPB, direct evidence for this has been lacking and the magmatic link was not yet fully established. This work presents the first quartz-hosted melt inclusion data of phyric felsic volcanic rocks from the Albernoa, Serra Branca and Neves-Corvo areas in the IPB. Melt inclusions of intermediate composition found in Albernoa felsic volcanics show evidence for early stages of crustal melt assimilation supporting previous petrogenetic models for the IPB. High Sn contents (up to 46 ppm) found in the melt inclusions from all volcanic centers are a primary magmatic feature on these IPB volcanic rocks and are above the background value for the IPB felsic volcanic rocks. Estimated temperature of formation of magmatic quartz, in excess of 800 °C, suggests that all three volcanic centers gathered favorable conditions for sustaining long-lived hydrothermal cells. Highest estimated temperatures (>892 °C) are attributed to one sample from Albernoa and one sample from Neves-Corvo. Their similar, pre-eruptive, Cu and Sn distribution supports the model for the Neves-Corvo deposit indicating that Sn and (at least part of the) Cu decoupled from a common magmatic source in the early stages of the ore-forming process. The occurrence of barren volcanic sequences originating from Sn-rich melts is evidence that a set of concurrent factors are additionally required to generate a high-grade VMS-deposit. Melt inclusion data can be used as a tool to assist future exploration of favorable volcanic centers in the IPB.

The most recent submarine eruption observed offshore the Azores archipelago occurred between 1998 and 2001 along the submarine Serreta ridge (SSR), ~ 4–5 nautical miles WNW of Terceira Island. This submarine eruption delivered abundant basaltic lava balloons floating at the sea surface and significantly changed the bathymetry around the eruption area. Our work combines bathymetry, volcanic facies cartography, petrography, rock magnetism and geochemistry in order to (1) track the possible vent source at seabed, (2) better constrain the Azores magma source(s) sampled through the Serreta submarine volcanic event, and (3) interpret the data within the small-scale mantle source heterogeneity framework that has been demonstrated for the Azores archipelago. Lava balloons sampled at sea surface display a radiogenic signature, which is also correlated with relatively primitive (low) 4He/3He isotopic ratios. Conversely, SSR lavas are characterized by significantly lower radiogenic 87Sr/86Sr, 206Pb/204Pb and 208Pb/204Pb ratios than the lava balloons and the onshore lavas from the Terceira Island. SSR lavas are primitive, but incompatible trace-enriched. Apparent decoupling between the enriched incompatible trace element abundances and depleted radiogenic isotope ratios is best explained by binary mixing of a depleted MORB source and a HIMU­type component into magma batches that evolved by similar shallower processes in their travel to the surface. The collected data suggest that the freshest samples collected in the SSR may correspond to volcanic products of an unnoticed and more recent eruption than the 1998–2001 episode.

Petrological, geochemical, and Nd isotopic analyses have been carried out on rock samples from the Rainbow vent field to assess the evolution of the hydrothermal system. The Rainbow vent field is an ultramafic-hosted hydrothermal system located on the Mid-Atlantic Ridge characterized by vigorous high-temperature venting (similar to 365 degrees C) and unique chemical composition of fluids: high chlorinity, low pH and very high Fe, and rare earth element (REE) contents (Douville et al., Chemical Geology 184:37-48, 2002). Serpentinization has occurred under a low-temperature (< 270 degrees C) retrograde regime, later overprinted by a higher temperature sulfide mineralization event. Retrograde serpentinization reactions alone cannot reproduce the reported heat and specific chemical features of Rainbow hydrothermal fluids. The following units were identified within the deposit: (1) nonmineralized serpentinite, (2) mineralized serpentinite-stockwork, (3) steatite, (4) semimassive sulfides, and (5) massive sulfides, which include Cu-rich massive sulfides (up to 28wt% Cu) and Zn-rich massive sulfide chimneys (up to 5wt% Zn). Sulfide mineralization has produced significant changes in the sulfide-bearing rocks including enrichment in transition metals (Cu, Zn, Fe, and Co) and light REE, increase in the Co/Ni ratios comparable to those of mafic Cu-rich volcanic-hosted massive sulfide deposits and different Nd-143/Nd-144 isotope ratios. Vent fluid chemistry data are indicative of acidic, reducing, and high temperature conditions at the subseafloor reaction zone where fluids undergo phase separation most likely under subcritical conditions (boiling). An explanation for the high chlorinity is not straightforward unless mixing with high salinity brine or direct contribution from a magmatic Cl-rich aqueous fluid is considered. This study adds new data, which, combined with the current knowledge of the Rainbow vent field, brings compelling evidence for the presence, at depth, of a magmatic body, most likely gabbroic, which provides heat and metals to the system. Co/Ni ratios proved to be good tools used to discriminate between rock units, degree of sulfide mineralization, and positioning within the hydrothermal system. Deeper units have Co/Ni < 1 and subsurface and surface units have Co/Ni > 1.