Noble metals and associated trace elements in sulfides from the active TAG mound, ODP Site 158-957

Primary sulfides from cores of ODP Holes 158-957M, 158-957C, and 158-957H on the active TAG hydrothermal mound (Mid-Atlantic Ridge, 26°08'N) have been studied for concentrations of several chemical elements. Based on 262 microprobe analyses it has been found that the sulfides have extremely heterogeneous distribution of noble metals (Au, Ag, Pt, and Pd) and several associated elements (Hg, Co, and Se). Noble metals are arranged in the following order in terms of decreasing abundance, i.e. concentration level above detection limits (the number of analyses containing a specific element is given in parentheses): Au (65), Ag (46), Pt (21), and Pd (traces). The associated trace elements have the following series: Co (202), Hg (132), and Se (49). The main carriers of "invisible" portion of the noble metals are represented by pyrite (Au, Hg), marcasite and pyrite (Ag, Co), sphalerite and chalcopyrite (Pt, Pd), and chalcopyrite (Se). Noble metal distribution in sulfides reveals a lateral zonality: maximal concentrations and abundance of Au in chalcopyrite (or Pt and Ag in chalcopyrite and pyrite) increase from the periphery (Hole 957H) to the center (holes 957C and 957M) of the hydrothermal mound, while Au distribution in pyrite displays a reversed pattern. Co concentration increases with depth. Vertical zonality in distribution of the elements mentioned above and their response to evolution of ore genesis are under discussion in the paper.

Hydrocarbons in hydrothermal sulfides from the Rainbow Hydrothermal Field

The Rainbow Hydrothermal Field (36°N, Mid-Atlantic Ridge) is one of three presently known fields related to serpentinization of ultramafic rocks accompanied by formation of hydrogen- and methane rich solutions. Gas chromatographic and molecular gas chromatographic - mass spectrometric investigations of sulfide ores and sediments from this field confirmed predominantly biological nature of bitumoids related to high-temperature transformation of biomass of the hydrothermal biological community. At the same time ores of the Rainbow field contain significant amounts of compounds that are not directly related to biogenic synthesis. This fact suggests possibility of abiogenic synthesis of methane and even complex hydrocarbons during serpentinization of ultramafic rocks.

Chemistry of hydrothermal clasy and sulfides of ODP Site 106-649

Several meters of unconsolidated hydrothermal sediment were recovered from the Snake Pit hydrothermal field during ODP Leg 106. Polymetallic sulfides comprise most of the sediment with minor fragments of massive sulfide, organic debris, clay minerals, and fresh glass shards. Trace element and Sr-isotope contents of hydrothermal clays and sulfides from Holes 649B and 649G indicate that these minerals precipitated from a mixed hydrothermal fluid-seawater solution. Evaluation of the REE mineral data and the Snake Pit hydrothermal fluids shows that the REE distribution coefficients between the hydrothermal fluids and clay-sulfide mixes range from 100-500. This indicates that hydrothermal fluids originating in the root-zone of the Snake Pit hydrothermal system may be modified by the precipitation of hydrothermal minerals, either in the shallow subsurface or within chimney structures. Contrasting REE profiles of clay-sulfide aggregates and massive sulfides from Holes 649B and 649G may be accounted for by spatial and/or temporal variations in redox conditions in the plumbing system.

Lead in bottom sediments, nodules, and sulfides from the Black Sea

A study of lead distribution in recent, ancient Black Sea and Neweuxinian bottom sediment shows similar vertical distributions of the element in the oxygen and hydrogen sulfide zones of the sea; i.e. hydrogen sulfide contamination does not affect lead contents in bottom sediments of the sea. Lead distribution in sediment mass of the Black Sea reflects dependence of accumulation of the element on the hydrodynamic regime of the sea and forms of its migration. It is noted that absence of lead accumulation in Black Sea nodules results from specific nodule formation and from geochemical activity of the element. A large role of diagenetic sulfide formation in lead geochemistry is shown. Degree of lead accumulation in iron sulfides depends on conditions of sedimentation and on physical and chemical parameters in the sea.

Radionuclide concentrations, ratios, and derived ages of massive sulfides from ODP Leg 139 and Leg 169 sites

Massive sulfide samples from the Bent Hill area were analyzed for 230Th/234U and 231Pa/235U disequilibria. Apparent ages calculated from these ratios are between 8.2 and >300 ka. Concordant ages were found for only three samples that originate near the surface from the clastic sulfide zone and suggest "true" ages of between 8.5 and 16.0 ka (mean of 230Th and 231Pa ages). The uranium vs. depth distribution in the Bent Hill Massive Sulfide deposit suggests an open system for uranium for the deeper part of the deposit, which was probably caused by extensive recrystallization processes inhibiting true age determinations.