Résumé
The drill cores of the approximately 3,5 Ga Dresser Formation at North Pole collected as part of the Pilbara Drilling Project consists of meta-komatiites overlain with barite-sulfide beds and silicified volcanogenic and bedded carbonate sediments that were deposited in a shallow water environment. This succession is connected to a network of barite- and silica-feeder veins intrusive in the underlying meta-komatiites, which are thought to represent the conduits for hydrothermal fluid circulation. The top of the veins gradually vanishes out into the bedded barite and sedimentary carbonate, which argues for periods of quiet water carbonate sedimentation on the seafloor, interrupted by episodes of hydrothermal alteration. In order to determine the environmental conditions (pressure and temperature gradients, pH, redox state, fluid mixing) pertaining to the development of this early ecosystem, we performed detailed bulk and in-situ major and trace element analyses of the different mineral paragenesis present in the different sedimentary (carbonate and volcanogenic) and hydrothermal (barite-sulfide) rock types. The bedded carbonates are mainly composed of fine-grained silica and Fe-carbonate (ankerite rhombs and fine-grained siderite linked to Fe-oxides) with smaller amounts of sulfides and carbonaceous material. Locally, the occurrence of calcite in association with hematite, sulphides and organic material in the core of the ankerite rhombs attest to some early oxidizing conditions in a generally reducing environment. This mineral association could be attributed to the activity of anoxygenic or oxygenic photosynthesizers thriving on the seafloor. The overall reducing environmental conditions during deposition of bedded carbonate on top, contrasts with the underlying lithologies, in which barite coexists with microscopic pyrite, sulfide laminates (ZnS and FeS (sub 2) ), muscovite and carbonate, which argues for strongly contrasting oxidizing-reducing conditions at a mineral scale. This contrast is also attested by whole rock Y/Ho ratio and EU (super *) values which evolve respectively from approximately 25 to approximately 45 (mean seawater value approximately 50) and approximately 1 to approximately 0,3 from bottom to top, indicating a progressive seawater influence towards the top of the sedimentary sequence. In contrast, Se/S and Ni/Co ratio obtained on individual sulfide grains show two major sources for hydrothermal fluids originating from basalt (Ni/Co (sub measured) : approximately 2,5) and komatiites (Ni/Co (sub measured) approximately 12), respectively, with no seawater influence (Se/S approximately 4-6.10 (super -4) ).