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Sulfide droplets from fresh Mid-Ocean-Ridge Basalt (MORB) glasses show different textures. Some are fine-grained droplets consist of Monosulfide Solid Solution (Mss) and Intermediate Solid Solution (Iss) micrometric intergrowths with pentlandite at the Mss-Iss interface and disseminated Fe-oxide grains; other droplets display a characteristic "zoned" texture consisting of segregated massive grains of Mss and Iss, with euhedral Fe-oxides and pentlandite occuring as equant grains and as flame-shaped domains in the Mss formed by exsolutions. The difference in the textures implies a difference in the crystallization history of the sulfide droplets. These different textures are observed in droplets that are only millimeters apart in the same sample, and thus had an identical cooling history. Therefore, some other factors controlled the textural development. There is relationship between the size and the texture of the droplets. The larger sulfide droplets tend to have zoned textures and the smaller ones fine-grained textures. We propose that the latter have experienced greater undercooling before crystallization. The reason for the delay in crystallization could be that, in the small sulfide droplets, large stable grains with low surface to volume ratio cannot form, which results in higher effective solubility of the Mss. Due to the high degree of undercooling in the small droplets, there were numerous nucleation sites and the diffusion rates of the crystal components in the liquid were lower, leading to fine-grained Mss-Iss intergrowths. In contrast, larger droplets with lower effective solubility of Mss began to crystallize at higher temperature, and thus had fewer nucleation sites, higher diffusion rates, and more time for sulfide differentiation.

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10Volcanogenic Massive Sulphide (VMS) deposits are commonly enriched in Cu, Zn and 11Pb and can also be variably enriched in Au, As, Sb, Se and Te. The behaviour of these elements 12 during hydrothermal alteration of the oceanic crust is not well known. Ocean Drilling Program 13 (ODP) Hole 1256D penetrates a complete in-situ section of the upper oceanic crust providing a 14 unique sample suite to investigate the behaviour of metals during hydrothermal alteration. A 15 representative suite of samples was analysed for Au, As, Sb, Se and Te using low detection limit 16 methods, and a mass balance of metal mobility has been carried out through comparison with a 17 fresh Mid-Oceanic Ridge Basalt (MORB) glass database. The mass balance shows that Au, As, 18Se, Sb, S, Cu, Zn and Pb are depleted in the sheeted dyke and plutonic complexes with mobilities 19 of -46±12 %, -27±5 %, -2.5±0.5 %, -27±6 %, -8.4±0.7 %, -9.6±1.6 %, -7.9±0.5 % and -44±6 % 20 respectively. Arsenic and Sb are enriched in the volcanic section due to seawater-derived fluid 21Manuscript Click here to download Manuscript: MIDE-D-14-00144-1-Patten-revised.docx 2 circulation. Calculations suggest that large quantities of metal are mobilised from the oceanic 22 crust but only a small proportion is eventually trapped as VMS mineralisation. The quantity of 23 Au mobilised and the ratio Au to base metals are similar to that of mafic VMS and a ten times 24 enrichment of Au would be needed to form a Au-rich VMS. The Cu-rich affinity of mafic VMS 25 deposits could be explained by base metal fractionation both in the upper sheeted dykes and 26 during VMS deposit formation. 27Keywords: VMS deposit, Au-rich VMS, ODP Hole 1256D, hydrothermal alteration in the 28 oceanic crust 29

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10Fluxes of metals during the hydrothermal alteration of the oceanic crust have far reaching effects 11 including buffering of the compositions of the ocean and lithosphere, supporting microbial life and the 12 formation of sulphide ore deposits. The mechanisms responsible for metal mobilisation during the 13 evolution of the oceanic crust are complex and are neither fully constrained nor quantified. Investigations 14 into the mineral reactions that release metals, such as sulphide leaching, would generate better 15 understanding of the controls on metal mobility in the oceanic crust. We investigate the sulphide and 16 oxide mineral paragenesis and the extent to which these minerals control the metal budget in samples from Ni, Cu, Zn, As, Ag, Sb, Se, Te, Au, Hg and Pb. In the volcanic section, low temperature fluid circulation 34 (<150 °C) leads to low temperature sulphide precipitation in the form of pyrite fronts that have high As 35 concentrations due to uptake from the circulating fluids. Deep late low temperature circulation in the 36 sheeted dyke and the plutonic complexes results in local precipitation of patchy sulphides and local metal 37 remobilisation. Control of sulphides over Au, Se and Cu throughout fast-spreading mid-oceanic crust 38 history implies that the generation of hydrothermal fluids enriched in these metals, which can eventually 39 form VMS deposits, is strongly controlled by sulphide leaching. 40

Volcanogenic massive sulphide deposits are enriched in metals that are either derived from the hydrothermal alteration of the basement rocks or supplied by exsolution of metal-rich volatiles during magmatic differentiation. The extent to which each process contributes to metal enrichment in these deposits varies between different tectonic settings. Ocean Drilling Program Hole 786B recovered >800 m of upper oceanic crust from a supra-subduction zone setting and includes a 30 m-thick mineralised zone. In-situ S isotopic compositions of pyrite decrease from 5.89±2.87 ‰ δ 34 S in the upper mineralised zone down to -3.34±2.09 2 ‰ δ 34 S in the extensively altered central mineralisation zone, potentially indicating strong magmatic fluid input in this area. Whole rock data and in-situ trace element analyses in sulphide minerals show enrichment of Ag, As, Au, Bi, Mo, S, Se, Sb and Te in the mineralised zone. Evaluation of metal behaviour during magmatic differentiation and primary metal fertility of basement rocks suggests that degassing melt is the main source for the high Au, Se and S enrichment observed in the mineralised zone. Magmatic volatile exsolution occurred late during the magmatic differentiation (~2 wt.% MgO), concomitant with oxide crystallisation and metal depletion in the melt. Comparison of Ocean Drilling Program Hole 786B with volcanogenic massive sulphide deposits hosted by boninitic volcanic successions, such as in the Semail ophiolite, the Newfoundland Appalachians and the Flin Flon Belt, suggests that magmatic fluid exsolution could be a common mechanism for Au enrichment in bi-modal mafic volcanogenic massive sulphide deposits.The upper alteration zone (799-815 mbsf) is characterised by alteration of primary LCBon to corrensite, smectite and albite, with common veins of quartz and carbonate (Alt et al. 1998). Sulphides occur dominantly in quartz and carbonate veins although disseminated pyrite and chalcopyrite are also present in the matrix. Vein-hosted sulphides are pyrite with accessory sphalerite, marcasite, chalcopyrite and trace of

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