Nina Ul'yanova

Mineralogy and geochemistry of sulfide ores in ocean-floor hydrothermal fields associated with serpentinite protrusions

The aim of this paper is to analyze and compare the mineralogy and geochemistry of copper–zinc sulfide ores from the Logachev-2 and Rainbow hydrothermal fields of the Mid-Atlantic Ridge (MAR), confined to serpentinite protrusions. It was found that the Zn(Fe) and Cu, Fe(Zn) sulfides had been deposited in black smokers pipes almost simultaneously from intermittently flowing, nonequilibrium H2S-low solutions of different temperatures. The Pb isotope composition confirmed that the source of lead had been the deep oceanic crust. The ores of the Rainbow Field contain 20-fold more Co than the ores restricted to the basalts and show a high ratio of Co/Ni = 46. The Rainbow ore is enriched in heavy S isotope (δSav = 10/oo) because of the constant flow of cold sea water into the subsurface zone of the hydrothermal system. The ore of the Logachev-2 field is 8 times higher in gold compared to the other MAR regions. The sulfide ores of the Rainbow and Logachev-2 fields have no analogues among the MAR ore occurrences in terms of the enrichment in useful components (Zn, Cd, Co, and Au).

Sulfide minerals in the Menez Gwen nonmetallic hydrothermal field (Mid-Atlantic Ridge)

Mineralogy and geochemistry of the sulfide-bearing rocks and ores discovered in the Menez Gwen field are studied. Samples were taken during the expedition of the Shirshov Institute of Oceanology (cruise 49, R/V Akademik Mstislav Keldysh. The mineral composition of rocks and ores were studied by the traditional methods of optical microscopy, scanning electron microscopy (CAMSCAN), and microprobe analysis (EPMA SX-50). Contents of trace elements were determined by the laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS). The Zn-Cu ore comprises zonal sulfide chimney intergrowths. Numerous Se-rich copper ore fragments occur in the volcanomictic layered gritstones and/or barite slabs. The mineral composition, zonality, and association of trace elements in ore are typical of “black smokers” formed at the basalt base near the Azores Triple Junction in the MAR. The results obtained make it possible to reconstruct formation history of the Menez Gwen hydrothermal field into the high-temperature (Cu-Se association in ore clasts), medium-temperature (Zn-Cu-As association in ore), and recent (Ba-SiO2 association) stages.

Sources of Minor and Rare-Earth Elements in Hydrothermal Edifices of Near-Continental Rifts with Sedimentary Cover: Evidence from the Guaymas Basin, Southern Trough

The mineralogy and geochemistry of a fragment of an active hydrothermal edifice from the Hydrothermal Hill of the Southern Trough valley of the Guaymas Basin in the Gulf of California were studied. The sample was collected from a depth of 1995 m by the Pisces manned submersible on cruise 12 of the R/V Akademik Mstislav Keldysh, Institute of Oceanology, Russian Academy of Sciences. The fragment and the edifice itself consists of two accrete pipes: ore (pyrrhotite) and barren (carbonate) combined in a single edifice by an outer barite–opal zone. The ore edifice is located in the rift zone of the Guaymas Basin with a thick sedimentary cover and is depleted in metals in comparison with ores from rift zones of the open ocean, which are not blocked by sedimentary deposits. This is explained by loss of metals at the boundary between hot sills and sedimentary rocks and by the processes of interaction of hydrothermal solutions with sedimentary deposits. The sedimentary series faciitates long-term preservation of endogenous heat and the ore formation process. Ore edifices of the Guaymas Basin are mostly composed of pyrrhotite, have a specific set of major elements, microelements and REEs, and contain naphthenic hydrocarbons. They may be search signs of hidden polymetallic deposits, considered to be the roots of ore occurrences localized under the surface of the bottom in young active rifts with high spreading and sedimentation rates, i.e., in near-continental areas of rifts of the humid zone with avalanche sedimentation.