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Fluid circulation at basement ⁄ cover unconformities is of first importance for metal transfer and especially the formation of Pb-Zn, F, Ba and U-deposits. This is typically the case for world-class Proterozoic U deposits (Canada, Australia, Gabon) in basins, which show many similarities with younger Pb-Zn-F-Ba systems (Irish Paleozoic Pb-Zn deposits, F-Pb-Zn-Ba deposits related to extensional tectonics from Spain, western France and Silesia and fluid movements related to continental rifting in the Rhine graben). As fluid mixing near the basement ⁄ cover unconformity is one of the key factors for ore formation, a series of parameters have been considered for both systems: the time gap between basin formation and metal deposit, the origin and nature of the ore fluids, the temperature of fluid end members and the style of migration. Results show great similarities in all fluid systems: (i) a wide range of fluid salinity indicating the lack of homogeneity of fluid chemistry at the scale of the reservoirs, (ii) the deep penetration of brines through faults and dense networks of microfractures within the basement below the unconformity, (iii) local fluid-rock interaction leading to porosity increase and significant fluid changes in fluid chemistry, (iv) a pulsatory fluid regime during fluid trapping, (v) anisothermal fluid mixing revealed by a systematic temperature gap between brines and recharge fluids, (vi) stages of fluid movements facilitated by discontinuous opening related to later tectonic ⁄ telogenetic stages linked to major geodynamic events, typically without related sedimentation and burial (exception in a few cases characterized by the synchronous production and penetration of surface brines and ore genesis). By analogy with younger systems, the conditions of burial and penetration of brines in the Archean basement suggest that thermal convection drove the brine movements, and was possibly linked to extensional tectonics in a part of the giant mid-Proterozoic U-deposits.

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Fluid inclusions trapped in quartz veins hosted by a leucogneiss from the southern part of the Naxos Metamorphic Core Complex (Attic‐Cycladic‐Massif, Greece) were studied to determine the evolution of the fluid record of metamorphic rocks during their exhumation across the ductile/brittle transition. Three sets of quartz veins (V‐M2, V‐BD & V‐B) are distinguished. The V‐M2 and V‐BD are totally or, respectively, partially transposed into the foliation of the leucogneiss. They formed by hydrofracturing alternating with ductile deformation accommodated by crystal‐plastic deformation. The V‐B is discordant to the foliation and formed by fracturing during exhumation without subsequent ductile transposition. Fluids trapped during crystal–plastic deformation comprise two very distinct fluid types, namely a CO2‐rich fluid and a high‐salinity brine, that are interpreted to represent immiscible fluids generated from metamorphic reactions and the crystallization of magmas respectively. They were initially trapped at ∼625 °C and 400 MPa and then remobilized during subsequent ductile deformation resulting in various degrees of mixing of the two end‐members with later trapping conditions of ∼350 °C and 140 MPa. In contrast, brittle microcracks contain aqueous fluids trapped at 250 °C and 80 MPa. All veins display a similar δ13C pointing to carbon that was trapped at depth and then preserved in the fluid inclusions throughout the exhumation history. In contrast, the δD signature is marked by a drastic difference between (i) V‐M2 and V‐BD veins that are dominated by carbonic, aqueous‐carbonic and high‐salinity fluids of metamorphic and magmatic origin characterized by δD between −56‰ and −66‰, and (ii) V‐B veins that are dominated by aqueous fluids of meteoric origin characterized by δD between −40‰ and −46‰. The retrograde P–T pathway implies that the brittle/ductile transition separates two structurally, chemically and thermally distinct fluid reservoirs, namely (i) the ductile crust into which fluids originating from crystallizing magmas and fluids in equilibrium with metamorphic rocks circulate through a geothermal gradient of 30 °C km−1 at lithostatic pressure, and (ii) the brittle upper crust through which meteoric fluids percolate through a high geothermal gradient of 55 °C km−1 at hydrostatic pressure.

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