Pyrite Varieties at Pobeda Hydrothermal Fields, Mid-Atlantic Ridge 17°07′–17°08′ N: LA-ICP-MS Data Deciphering

Abstract

The massive sulfide ores of the Pobeda hydrothermal fields are grouped into five main mineral microfacies: (1) isocubanite-pyrite, (2) pyrite-wurtzite-isocubanite, (3) pyrite with minor isocubanite and wurtzite-sphalerite microinclusions, (4) pyrite-rich with framboidal pyrite, and (5) marcasite-pyrite. This sequence reflects the transition from feeder zone facies to seafloor diffuser facies. Spongy, framboidal, and fine-grained pyrite varieties replaced pyrrhotite, greigite, and mackinawite “precursors”. The later coarse and fine banding oscillatory-zoned pyrite and marcasite crystals are overgrown or replaced by unzoned subhedral and euhedral pyrite. In the microfacies range, the amount of isocubanite, wurtzite, unzoned euhedral pyrite decreases versus an increasing portion of framboidal, fine-grained, and spongy pyrite and also marcasite and its colloform and radial varieties. The trace element characteristics of massive sulfides of Pobeda seafloor massive sulfide (SMS) deposit are subdivided into four associations: (1) high temperature—Cu, Se, Te, Bi, Co, and Ni; (2) mid temperature—Zn, As, Sb, and Sn; (3) low temperature—Pb, Sb, Ag, Bi, Au, Tl, and Mn; and (4) seawater—U, V, Mo, and Ni. The high contents of Cu, Co, Se, Bi, Te, and values of Co/Ni ratios decrease in the range from unzoned euhedral pyrite to oscillatory-zoned and framboidal pyrite, as well as to colloform and crystalline marcasite. The trend of Co/Ni values indicates a change from hydrothermal to hydrothermal-diagenetic crystallization of the pyrite. The concentrations of Zn, As, Sb, Pb, Ag, and Tl, as commonly observed in pyrite formed from mid- and low-temperature fluids, decline with increasing crystal size of pyrite and marcasite. Coarse oscillatory-zoned pyrite crystals contain elevated Mn compared to unzoned euhedral varieties. Framboidal pyrite hosts maximum concentrations of Mo, U, and V probably derived from ocean water mixed with hydrothermal fluids. In the Pobeda SMS deposit, the position of microfacies changes from the black smoker feeder zone at the base of the ore body, to seafloor marcasite-pyrite from diffuser fragments in sulfide breccias. We suggest that the temperatures of mineralization decreased in the same direction and determined the zonal character of deposit.