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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.

Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) is a valuable tool for analyzing very small geological materials, like exposed melt inclusions, because it provides high mass and spatial resolution with little sample destruction. In this study, ToF-SIMS quantitative analysis of major and trace elements and isotope ratios were determined in a vapor bubble wall of a melt inclusion and in its host clinopyroxene. Two standard glass reference materials, BCR-1 and JB-2, of similar composition were used. Results indicate that quantification using ToF-SIMS is possible for many mineral-forming elements. High mass and spatial resolution elemental maps clearly show the preferential partitioning of S, Na, Cu, Au, Li, and K into the vapor bubble in the clinopyroxene-hosted melt inclusion. Moreover, the presence of C, H, and associated hydrocarbon fragments in the element maps suggest the heterogeneous entrapment of brine and hydrocarbon metal-bearing phases in the melt inclusion. Variability in isotope ratios found particularly in the standard reference materials suggests either or both heterogeneous distribution in the sample or analytical variability. In any case, more research is necessary in order to better constrain the multitude of variables.

The Menez Gwen (37°50′N) and Lucky Strike (37°17′N) seafloor hydrothermal systems on the Mid-Atlantic Ridge are influenced by the Azores plume, which results in a geochemically enriched character of the host basalts. Two groups of highly plagioclase phyric lavas can be identified at Menez Gwen. Group I lavas are highly vesicular and highly enriched in incompatible elements with high (La/Sm) N , Nb/Zr, Ba/Nb, Zr/Y and Nb/Y ratios. The enriched group I lavas resemble those at the Lucky Strike site. Group II porphyritic lavas are less vesicular, less enriched in incompatible elements with low (La/Sm) N , Nb/Zr, Ba/Nb, Zr/Y and Nb/Y ratios. These differences between group I and II lavas can be explained either by melting-induced mixing of a two-component mantle or mixing between two melts. Furthermore, the presence of identical assemblages of plagioclase phenocrysts in both groups is strong evidence for mantle heterogeneities. Silicate melt inclusions, hosted in plagioclase phenocrysts, exhibit small metallic precipitates of Fe + Ni + Cu ± Zn (± Co ± As) ± S ± Cl within the vapour bubbles. The heterogeneous trapping of distinct coexisting phases during plagioclase growth indicates that a high temperature ore-metal rich immiscible fluid must have been exsolved from the magma prior to eruption. On the other hand, lava vesicularities are remarkably different between the group I and group II lavas. They indicate ubiquitous degassing of magmatic volatiles, particularly in group I lavas that underwent significant pre-eruptive degassing. The presence of an ore-metal-rich fluid being exsolved from the magma chamber in conjunction with pre-eruptive degassing is a strong indicator of direct magmatic contribution of metals and volatiles to the seafloor hydrothermal systems. At the Menez Gwen site, timing of degassing and the enriched chemistry of lavas proved to be crucial factors contributing to the exsolution of a metal-rich fluid prior to magma eruption.

The Rainbow vent field is an ultramafic rock-hosted seafloor hydrothermal system located on the Mid-Atlantic ridge issuing high temperature, acidic, metal-rich fluids. Hydrothermal products include Cu–Zn–(Co)-rich massive sulfides with characteristics comparable to those found in mafic volcanic-hosted massive sulfide deposits. Petrography, mineralogy and geochemistry of nonmineralized and mineralized rocks sampled in the Rainbow vent field indicate that serpentinized peridotites host the hydrothermal vent system but serpentinization reactions occurred prior to and independently of the sulfide mineralization event. The onset of sulfide mineralization is reflected by extensive textural and chemical transformations in the serpentine-group minerals that show clear signs of hydrothermal corrosion. Element remobilization is a recurrent process in the Rainbow vent field rocks and, during simple peridotite serpentinization, Ni and Cr present in olivine and pyroxene are incorporated in the pseudomorphic serpentine mesh and bastite, respectively. Ni is later remobilized from pseudomorphic serpentine into the newly formed sulfides as a result of extensive hydrothermal alteration. Bulk-rock geochemistry and correlation coefficients discriminate the different processes: serpentinization, sulfide mineralization and superficial seafloor low-temperature processes related to the circulation of seawater (e.g. carbonatization, sulfide oxidation and B and U uptake).

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.