Mineralogy and geochemistry of a hydrothermal silica-sulfide-sulfate spire in the caldera of Axial Seamount, Juan De Fuca Ridge

Three hyrothermal vent fields have been found in the 21 km (super 2) summit caldera of Axial Seamount, a large shield volcano on the central Juan de Fuca Ridge. In one, near the caldera's northern wall, are low-T (<35 degrees C) hydrothermal vents and large, chimney-like spires; from it an intact 160 kg spire has been recovered and has been studied. The spire is composed of amorphous silica, sphalerite, marcasite, baryte, wurtzite, chalcopyrite and galena, with trace amounts of pyrite, jordanite, Pb-As-Sb-Ag sulphosalts, tetrahedrite-tennantite, native S, and Fe and Mn oxides. Its bulk chemical composition is 28.5 SiO (sub 2) , 22.7 Zn, 8.7 Ba, 5.8 Fe, 0.43 Cu, 0.35 Pb, and 18.6% S with 570 ppm As, 350 Sb, 186 Ag, 4.9 Au, 522 Cd, 1470 Sr and 600 ppm Mn. Mineral paragenesis in the spite is related to changes in T, the degree of sea-water mixing, and pH-aS (sub 2) -aO (sub 2) conditions during its growth. Amorphous silica solubility in sampled vent fluids, fluid- inclusion microthermometry in wurtzite and determination of delta (super 18) O of baryte indicate that baryte and some sulphide formed early at <185 degrees C, followed with increasing T to approx 250 degrees C by a main stage of sulphide mineralization, then with T declining beneath 185 degrees C formation of some late baryte accompanied by trace sulphosalts, native S and Fe and Mn oxides; amorphous silica was formed throughout. The delta (super 34) S values for BaSO (sub 4) (+16.1 to +21.2permil), FeS (sub 2) (+1.1 to +3.9permil), and ZnS (+4.2 to +5.7permil) indicate isotopic disequilibrium among these phases and suggest local reduction of sulphate from sea-water at the site of deposition. The complex multistage history of the spire reflects growth during mixing between higher T fluids and sea-water, a process important for the accumulation of sulphides at many sea-floor vent sites and in their ancient analogues on land.