Geology topics

Wayne C. Shanks

Publications and source records attributed to Wayne C. Shanks.

62 records · Page 4Linked to original sources

USGS1990

Deep-water hydrocarbon seeps in Guaymas Basin, Gulf of California

Acoustically discovered hydrocarbon seeps along a transform fault zone on the Sonoran margin of Guaymas Basin and in the Southern Trough of Guaymas Basin were examined and sampled during dives of DSV Alvin . Seepage of methane and heavier hydrocarbons occurs through shallow pockmarks along the eroding crest of a steep anticline, 1600 m below sea level. Extensive ledges of aragonite crop out around the rims of the pockmarks; isotopic analysis indicates that carbonate precipitation is a result of methane oxidation. Seepage zones within the pockmarks support dense communities of Calyptogena , half buried in the mud; tubeworms ( Lamellibracheae sp.) have colonized many of the aragonite outcrops. Though the margin site is only a few kilometers from a high-temperature sulfide-precipitating and petroleum-discharging vent system at a nearby center, its structural setting is more akin to pockmarks described from continental shelves, and its chemosynthetic fauna is more like that around low temperature seeps on other continental slopes and margins (e.g. Oregon, U.S.A., and Japan). The seep in the Southern Trough is a condensate-type petroleum (C 1 -C 40 ) and rises as a plume from a young vent-mound system about 2000 m below sea level.

Gulf of California↗

USGS1988

Isotopic studies of epigenetic features in metalliferous sediment, Atlantis II Deep, Red Sea

The unique depositional environment of the Atlantis II Deep brine pool in the Red Sea produces a stratiform metalliferous deposit of greater areal extent than deposits formed by buoyant-plume systems typical of the midocean ridges because of much more efficient metal entrapment. Isotopic analyses of strontium, sulfur, carbon, and oxygen from the metalliferous sediments indicate that three major sources contribute dissolved components to the hydrothermal system: seawater, Miocene evaporites, and rift-zone basalt. An areally restricted magnetite-hematite-pyroxene assemblage formed at high temperatures, possibly in response to hydrothermal convection initiated by intrusion of basalt into the metalliferous sediment. A correlation between smectite Fe/(Fe+Mg) ratios and oxygen isotope temperatures suggests that smectite is a potentially important chemical geothermometer, and confirms geochemical calculations indicating that Mg-rich smectite is more stable than Fe-rich smectite at elevated temperatures.

Canadian Mineralogist↗

USGS1988

Composition of massive sulfide deposits from the sediment-covered floor of Escanaba Trough, Gorda Ridge: implications for depositional processes

Massive sulfide deposits with two distinct compositions are spatially related to volcanic edifices that penetrate up to 500 m of turbiditic sediment in Escanaba Trough (ET), southern Gorda Ridge. The mineralogy, metal content, sulfur isotope composition, and hydrocarbon content of massive sulfides from ET reflect the extensive interaction between underlying turbiditic sediment and hydrothermal fluid. Pyrrhotite-rich sulfide mounds are inferred to have formed under gentle temperature and chemical gradients related to diffuse, low-velocity flow through disrupted sediment near the sediment-capped hills.

Canadian Mineralogist↗

USGS1987

Sulfur and lead isotope studies of stratiform Zn-Pb-Ag deposits, Anvil Range, Yukon: Basinal brine exhalation and anoxic bottom-water mixing (Canada)

Five stratiform Zn-Pb-Ag deposits are known in Early Cambrian metapelitic rocks along a curvilinear trend in the Anvil Range, central Yukon. The Anvil Range deposits occur along the southwestern boundary of the Selwyn basin in the stratigraphic transition zone between metapelites of the Mt. Mye unit and calcareous phyllites of the overlying Vangorda unit. The massive sulfides are associated closely with anomalously thick graphitic phyllites, apparently related to a second-order basin. A typical Anvil cycle of mineralization begins with a ribbon-banded graphitic-quartzitic-pyritic unit. This grades upward into sulfide-bearing quartzite, quartzitic massive sulfide, massive sulfide, and finally a baritic massive sulfide horizon. Sericitic alteration envelopes irregularly encompass each deposit and locally are developed best in footwall rocks. Detailed sulfur isotope studies have been carried out on the DY and Grum deposits and on one representative drill hole from the Faro deposit. The delta 34 S values of sulfide minerals generally range from 10 to 22 per mil and are similar in all three deposits. The delta 34 S values of pyrite in unmineralized samples from the district exhibit a wider range, from 6 to 34 per mil, and show distinct upward stratigraphic increase due to a stagnation cycle in the basin. The delta 34 S values of barite samples are strongly dependent on bottom-water conditions and mode of mixing during brine exhalation. They range from 22 to 26 per mil in the Faro deposit to 36 to 42 per mil in the DY deposit. These variations are due to mixing of isotopically light sulfate (18-20ppm) in Ba-bearing ore fluid and isotopically heavy residual sulfate (30-60ppm) in anoxic seawater. Thirty-eight samples of galena from the DY, Grum, Faro, SB, and Swim deposits have been analyzed for lead isotope ratios. In general, the lead isotope ratio data indicate an upper crustal lead source, with the Proterozoic Grit unit which is inferred to underlie the district being the most likely source rock. A small component of mantle leads from mafic igneous rocks or due to source rock inhomogeneity is also indicated. Graphitic host lithologies, lack of stratiform iron oxides, delta 34 S values of sedimentary pyrite, and sulfide sulfur-organic carbon variations in unmineralized cores indicate formation of the Anvil deposits in strongly reduced bottom water related to a previously unknown Early Cambrian anoxic event.

Economic Geology↗

USGS1987

Preliminary report, cruises L1-86-NC and L2-86-NC, Escanaba Trough, Gorda Ridge

Eight large (up to 200 m across) and many small massive sulfide deposits were photographed and sampled at the sediment-covered Escanaba Trough, southern Gorda Ridge, during S.P. Lee cruises L1-86-NC and L2-86-NC in 1986. The deposits are associated with two volcanic edifices within the sedimentary fill along the axis of Escanaba Trough. The sulfide and other hydrothermal minerals are deposited on the seafloor as well as within the sedimentary section. High concentrations of Pb and As in some of the sulfide samples indicates significant interaction between hydrothermal fluids and sediment at depth.

Open-File Report↗

USGS1986

Isotopic constraints on the origin of the Atlantis II, Suakin and Valdivia brines, Red Sea

The origin of three Red Sea submarine brine pools was investigated by analysis of the S and O isotope ratios of dissolved sulfate and Sr isotope ratios of dissolved Sr in the brines. Sulfur and O isotope ratios of sulfate and Sr isotope ratios of evaporitic source rocks for the brines were measured for comparison. The S, O and Sr isotope ratios of evaporites recovered from DSDP site 227 are consistent with an upper Miocene evaporites age. The Valdivia Deep brine formed by karstic dissolution of Miocene evaporites by overlying seawater and shows no signs of hydrothermal input. The Suakin Deep brines are derived from, or have isotopically exchanged with Miocene or older evaporites. There has been only minor dilution of the brine by overlying seawater. Strontium isotope ratios of Suakin brine may indicate addition of a minor (15%) amount of volcanic Sr to the brine, but there is no evidence of high temperature brine-rock interaction. The sulfate in the Atlantis II brine was apparently derived from seawater. The O isotope ratio of sulfate in the present Atlantis II brine could reflect isotopic exchange between seawater sulfate and the brine at approximately 255°C. Approximately 30% of the Sr in the Atlantis II brine is derived from the underlying basalt, probably by hydrothermal leaching. Atlantis II brine is the only known example from the Red Sea which has a significant high-temperature hydrothermal history.

Geochimica et Cosmochimica Acta↗

USGS1981

Seawater sulfate reduction and sulfur isotope fractionation in basaltic systems: interaction of seawater with fayalite and magnetite at 200–350°C

Sulfate reduction during seawater reaction with fayalite and with magnetite was rapid at 350°C, producing equilibrium assemblages of talc-pyrite-hematite-magnetite at low water/rock ratios and talc-pyrite-hematite-anhydrite at higher water/rock ratios. At 250°C, seawater reacting with fayalite produced detectable amounts of dissolved H 2 S, but extent of reaction of solid phases was minor after 150 days. At 200°C, dissolved H 2 S was not detected, even after 219 days, but mass balance calculations suggest a small amount of pyrite may have formed. Reaction stoichiometry indicates that sulfate reduction requires large amounts of H + , which, in subseafloor hydrothermal systems is provided by Mg metasomatism. Seawater contains sufficient Mg to supply all the H + necessary for quantitative reduction of seawater sulfate. Systematics of sulfur isotopes in the 250 and 350°C experiments indicate that isotopic equilibrium is reached, and can be modeled as a Rayleigh distillation process. Isotopic composition of hydrothermally produced H 2 S in natural systems is strongly dependent upon the seawater/basalt ratio in the geothermal system, which controls the relative sulfide contributions from the two important sulfur sources, seawater sulfate and sulfide phases in basalt. Anhydrite precipitation during geothermal heating severely limits sulfate ingress into high temperature interaction zones. Quantitative sulfate reduction can thus be accomplished without producing strongly oxidized rocks and resultant sulfide sulfur isotope values represent a mixture of seawater and basaltic sulfur.

Geochimica et Cosmochimica Acta↗

USGS1977

Ore transport and deposition in the Red Sea geothermal system: a geochemical model

Thermodynamic calculation of distribution of dissolved aqueous species in the Red Sea geothermal brine provides a model of ore transport and deposition in good agreement with observed accumulations of base metal sulfides, anhydrite, and barite. The Red Sea brine is recirculated seawater that acquires high salinity by low-temperature interaction with Miocene evaporites and is subsequently heated to temperatures in excess of 200°C by interaction with recent rift zone intrusive rocks. At temperatures up to 250°C, NaSO − 4 and MgSO 0 4 are the dominant sulfur-bearing species. H 2 S forms by inorganic sulfate reduction at the higher temperatures but is maintained at a uniform concentration of about 2 ppm by the strength of the sulfate complexes. Chloride complexes solubilize metals at the higher temperatures, and thus sulfide and metals are carried together into the Atlantis II Deep. Below 150°C, the brine becomes supersaturated with respect to chalcopyrite, sphalerite, galena, and iron monosulfide due to chloride-complex dissociation. Sulfide precipitation rates, based on the rate of brine influx, are in good agreement with measured sedimentation rates. Anhydrite precipitates as crystalline fissure infillings from high-temperature inflowing brine. Barite forms from partial oxidation of sulfides at the interface between the lower hot brine and the transitional brine layer.

Geochimica et Cosmochimica Acta↗