 The geochemical variability of massive sulfides from mid-ocean ridges is explored with multivariate statistical analysis. The observed variability is mostly related to the temperature of deposition, the ridge spreading rate and zone refining. 2 The nature of the substrate (mafic vs. ultramafic rocks) seems to play a secondary role in controlling metal associations.

The Cogne magnetite deposit (Western Alps, Italy) is the largest in a series of apatite and sulphide-free magnetite orebodies that are hosted in serpentinites belonging to western Alpine ophiolitic units. The nearly endmember composition of magnetite, which is unusual for an ultramafic setting, and the relatively high tonnage of the deposit (18*10^6 tonnes at 45–50 wt% Fe) make Cogne an intriguing case study to explore magnetite-forming processes in ophiolites. The Cogne magnetite shows variable textures, including nodular ores, veins and fine-grained disseminations in serpentinites after mantle peridotites and totally serpentinized melt-impregnated peridotites (troctolites). An increase in Co/Ni ratio from magnetite-poor serpentinized peridotites (0.05) to nodular magnetite ores (>1) is observed. Trace element\ud analyses of magnetite from different sites and lithologies by laser-ablation inductively-coupled mass spectrometry indicate that magnetites have typical hydrothermal compositions, characterized by high Mg and Mn (median values up to 24,100 and 5000 ppm, respectively), and low Cr, Ti and V (median values up to 30, 570 and 60 ppm, respectively). Moreover, the variations in trace element compositions distinguish magnetite that is hydrothermal fluid-controlled [highest (Mg, Mn, Co, Zn)/Ni ratios] from magnetite whose composition is affected by host-rock chemistry (highest Ni ± Ti ± V). U-Th-Pb dating of magnetite-associated uraninite constrains the formation of the deposit to the Late Jurassic (ca. 150 Ma), during an advanced stage of the opening of the Alpine Tethys. Thermodynamic modelling of fluid-rock interactions indicates that fluids produced by seawater–peridotite or seawater–Fe-gabbro are not sufficiently Fe-rich to account for the formation of the Cogne deposit. This suggests that fractionation processes such as phase separation were critical to generate hydrothermal fluids capable to precipitate large amounts of magnetite in various types of ultramafic host-rocks. The oceanic setting and geochemical and mineralogical similarities with some modern ultramafic-hosted volcanogenic massive\ud sulphide deposits on mid-ocean ridges suggest that the exposed mineralized section at Cogne may represent the deep segment of a seafloor, high-temperature (300–400 C) hydrothermal system. The occurrence of similar magnetite enrichments in present-day oceanic settings could contribute to explain the presence of significant magnetic anomalies centred on active and inactive ultramafic-hosted hydrothermal fields

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A subduction complex of the northwestern Alps consists of serpentinites, eclogitic metagabbros, flysch-like metasediments, meta-ophicarbonates, and gneissic slices. Unlike other subduction complexes, it contains unusual hybridized rocks described here for the first time in the northwestern Alps. They are preserved as patches interstitial in the metagabbro and as layers within metagabbros and serpentinites. The hybridized rocks are made of high modal zoisite/clinozoisite + white mica pseudomorphs of lawsonite, garnet, and amphibole associated with an Alpine eclogite-facies fabric. While these eclogitic metagabbros are chemically comparable to oceanic oxide gabbros from the ultraslow Southwest Indian Ridge, the layers are extremely enriched in Al 2 O 3 and CaO and depleted in TiO 2 , MgO, and SiO 2 relative to metagabbros. Patches have a geochemical signature that is intermediate between that of layers and metagabbros. Trace element compositions of hybridized rocks suggest a contribution from a fluid derived from a mixed source made of sediments and serpentinites. Except for Ba, Rb, and K, layers are comparable to the global subducting sediments, indicating a sedimentary contribution, whereas the enrichment in Cr indicates a serpentinite contribution. Metasediment dehydration and chemical exchange of Ca and Sr have resulted in significant lawsonite crystallization in the subduction zone, as reflected by the ubiquitous presence of lawsonite pseudomorphs. In light of the unique textures and geochemical signature of the lawsonite pseudomorph-bearing hybridized rocks, an origin by fluid-rock interaction and Ca-metasomatism in the subduction environment is inferred and considered in the Western Alps context.

The aim of this paper is the investigation of the role of diagenesis in the transformation of clastic sulfide sediments such as sulfide breccias from the Semenov-3 hydrothermal field (Mid-Atlantic Ridge). The breccias are composed of marcasite–pyrite clasts enclosed in a barite–sulfide–quartz matrix. Primary hydrothermal sulfides occur as colloform, fine-crystalline, porous and radial marcasite–pyrite clasts with inclusions or individual clasts of chalcopyrite, sphalerite, pyrrhotite, bornite, barite and rock-forming minerals. Diagenetic processes are responsible for the formation of more diverse authigenic mineralization including framboidal, ovoidal and nodular pyrite, coarse-crystalline pyrite and marcasite, anhedral and reniform chalcopyrite, inclusions of HgS phase and pyrrhotite–sphalerite–chalcopyrite aggregates in coarse-crystalline pyrite, zoned bornite–chalcopyrite grains, specular and globular hematite, tabular barite and quartz. The early diagenetic ovoid pyrite is enriched in most trace elements in contrast to late diagenetic varieties. Authigenic lower-temperature chalcopyrite is depleted in trace elements relative to high-temperature hydrothermal ones. Trace elements have different modes of occurrence: Se is hosted in pyrite and chalcopyrite; Tl is related to sphalerite and galena nanoinclusions; Au is associated with galena; As in pyrite is lattice-bound, whereas in chalcopyrite it is related to tetrahedrite–tennantite nanoinclusions; Cd in pyrite is hosted in sphalerite inclusions; Cd in chalcopyrite forms its own mineral; Co and Ni are hosted in chalcopyrite.