Escanaba Trough Research Articles

Peculiarities of clay minerals formation in the Pleistocene sediments under specific tectonomagmatic and hydrothermal conditions of the Central Hill (Escanaba Trough, Gorda Ridge, Pacific Ocean). Communication 1. Hole ODP 1038B

Using a complex of analytical methods, clay minerals were studied in Pleistocene sediments from Hole ODP 1038B, 120.50 m deep, drilled on the northwestern edge of the Central Hill, located in the Escanaba Trough (Gorda Ridge) near a hydrothermal source with a temperature of 108°C, as well as in Pleistocene background terrigenous sediments from reference Hole ODP 1037B, drilled in the Escanaba Trough, 5 km south of Central Hill. The association of terrigenous clay minerals in sediments from Hole 1037B consists of mixed-layer smectite-illites, smectite, chlorite, illite, and kaolinite. In sediments from Hole 1038B in the interval from the bottom surface to a depth of 5–7 m, clay minerals are terrigenous. In the rest of the sedimentary section, clay minerals are represented by newly formed biotite, chlorite, and dioctahedral smectite. Their formation occurred under the conditions that arose during the intrusion of basaltic melt into the Escanaba trough with the formation of a laccolith and the subsequent rapid cooling of its flank; the intrusion was accompanied by the ascent of high-temperature hydrothermal fluid in the central discharge channel, interacting with the adjacent sediments. As a result, at the high-temperature stage of this interaction, finely dispersed biotite was formed in the sediments due to the original terrigenous clay minerals, K-feldspar and amphiboles. Then, at the rapid cooling of the hydrothermal fluid to a temperature presumably 270–330°C, partial replacement of biotite by chlorite. With further rapid cooling of the hydrothermal fluid to a temperature of 200°C and below and its mixing with sea water seeping into the sediments of the Central Hill, smectite was formed.

Peculiarities of Clay Mineral Formation in Pleistocene Sediments Under Specific Tectonomagmatic and Hydrothermal Conditions of the Central Hill (Escanaba Trough, Gorda Ridge, Pacific Ocean): Communication 2. Holes ODP 1038A and 1038H

Peculiarities of Clay Mineral Formation in Pleistocene Sediments Under Specific Tectonomagmatic and Hydrothermal Conditions of the Central Hill (Escanaba Trough, Gorda Ridge, Pacific Ocean): Communication 2. Holes ODP 1038A and 1038H

Seeing Through Metamorphism: Geochemical and Isotopic Studies of Amphibolite Facies Mafic-Siliciclastic (Besshi-Type) Massive Sulfide Deposits at the Elizabeth Copper Mine, Vermont, USA

Abstract The Elizabeth volcanogenic massive sulfide (VMS) deposit in Vermont is atypical among such deposits in containing both stratiform Fe-Cu-Zn-Mn mineralization and stratabound Al-Na-K-B enrichments and Ca-Mg depletions in wall rocks. This mafic-siliciclastic (Besshi-type) deposit is contained in a thick sequence of Lower Devonian pelitic schist, minor quartzite, and sparse amphibolite. Wall rocks to the sulfide ores are predominantly amphibole-bearing rocks lacking quartz. The deposit has been metamorphosed to middle amphibolite facies and is complexly deformed by two folding events and syndeformational thrust faults. Whole-rock analyses of altered metabasaltic wall rocks reveal locally high concentrations of SiO2 (up to 85.0 wt %), Al2O3 (up to 32.5 wt %), K2O (up to 4.02 wt %), or Na2O (up to 6.02 wt %). Data for relatively immobile trace elements (Sc, Cr, Ti, Zr, Th) and rare earth elements (REEs) indicate protoliths of low-Ti tholeiitic basalt broadly of normal mid-ocean ridge basalt (N-MORB) affinity. The altered wall rocks are mineralogically distinctive in containing, in many samples, abundant muscovite, phlogopite, albite, dolomite, tourmaline, and/or tremolite-actinolite. One very aluminous unit of coarse garnet-mica schist, also with a tholeiitic basalt protolith, has local domains composed of abundant staurolite, minor margarite and sillimanite, and rare corundum. All of these altered wall rocks are stratabound but generally discontinuous along strike, in contrast to Mn-rich stratiform lenses (coticules), for which mineralogical and geochemical data suggest deposition as VMS-related chemical sediments. Al-normalized calculations of whole-rock geochemical data for mineralogically different types of altered basalts, relative to an inferred least altered precursor in the mine sequence, show average major additions (>50%) of Mn, Na, and K, and major losses (>50%) of Mg and Ca for most types. Changes in Si, Ti, and Fe are generally negligible (±5%), except for siliceous, tourmaline-rich rocks that display on average a major addition of 254% Si. Whole-rock δ18O values of silica-poor samples with metabasaltic precursors range from 10.7 to 13.4‰. These values are uncorrelated with SiO2 contents and are mostly higher than that of the least altered metabasalt in the Elizabeth mine area (δ18O = 11.1‰), suggesting premetamorphic subseafloor alteration by relatively low-temperature (ca. 150°–250°C) VMS-related hydrothermal fluids. A lack of depletion of light REEs for most of the altered metabasalts, compared to the least altered precursor, is also consistent with a low-temperature alteration process. Sulfur isotope values for pyrrhotite, chalcopyrite, sphalerite, and pyrite in massive sulfide and disseminations in wall rocks range from 4.3 to 9.3‰, a typical range for sediment-hosted sea-floor hydrothermal systems, reflecting mixed sulfur sources derived mainly from footwall basalts and coeval seawater sulfate. Shale-normalized REE data for coticules show small negative Ce and Eu anomalies that suggest deposition of precursor Mn-rich sediments in mildly oxic waters. In contrast, most samples of massive sulfides lack Ce anomalies but have small to moderate positive Eu anomalies, reflecting mineralization under anoxic and reducing conditions at or above 250°C. The presence of negative Ce anomalies in the coticules and two samples of massive sulfide, but the lack of such anomalies in other massive sulfide samples, suggest deposition within a stratified water column in which the redoxcline fluctuated due to hydrothermal venting of reductants such as Fe2+, Mn2+, H2S, H2, and CH4, which episodically produced anoxic bottom waters during VMS mineralization. The length (≥3.4 km) and relatively narrow width (~500 m) of the Elizabeth sulfide deposit is attributed to formation in an elongate sea-floor graben that served as a locus of tholeiitic basaltic volcanism and hydrothermal mineralization. Morphological and geochemical data for highly altered metabasaltic wall rocks of the deposit provide evidence for pervasive subseafloor alteration, during and after exhalative chemical precipitation of sulfides and Mn-Fe sediments. Possible modern analogs are the predominantly sediment-hosted VMS deposits in Middle Valley and Escanaba trough in the northeast Pacific Ocean and Guaymas basin in the Gulf of California. The metalliferous sulfide deposits of the Atlantis II Deep in the Red Sea may be the best modern analog, based on their sheetlike morphology, sulfur isotope values, and associated Fe and Mn oxyhydroxide sediments that are potential protoliths of the coticule rocks in the Elizabeth mine sequence.

Authigenic biotite from hydrothermally altered terrigenous sediments of the Central Hill (Escanaba Trough, Gorda Ridge, Pacific Ocean, hole ODP 1038B)

For the first time, authigenic dispersed biotite was discovered in Pleistocene terrigenous sediments of the Central Hill, located in the Escanaba Trough in the southern part of the Gorda Ridge (northeastern sector of the Pacific Ocean), which accounts for almost the entire content of fine fractions 1 μm of some samples from ODP 1038B hole. The authigenic nature of biotite is associated with the metasomatic effect of hydrothermal solution on terrigenous clay minerals after intrusion of basaltic magma in the form of laccolith with a temperature of ~1200°C. The mineral composition of fine fractions of sediments was studied using complex analytical methods, including modeling of their diffraction patterns. It has been established that the dispersed micaceous mineral (biotite) is trioctahedral, high-iron, does not swell when saturated with glycol, but contracts after heating at 550°C. It is shown that in its structure there are no foreign layers, the height and composition of which differ from the micaceous layers. The decrease in the height of micaceous layers upon heating biotite to 550°C is mainly due to a decrease in the height of 2:1 octahedra due to the difference in the Fe2+–O and Fe3+–O bond lengths as a result of the oxidation of Fe2+ cations It has been established a limit value for the coefficient of variation CV, characterizing the absence of mixed-layering in a regular structure, which should not exceed a value of 0.10.

Fourier transform ion cyclotron resonance mass spectrometric analysis of NSO-compounds generated in hydrothermally altered sediments from the Escanaba Trough, northeastern Pacific Ocean

To date little is known about the functionalized and heteroatomic compounds present in hydrothermal petroleum and how these compounds are affected by high temperature vent fluids. To address this, the maltene fractions of three hydrothermal petroleum samples spanning the shallow depth of a sediment core collected in the Escanaba Trough of the Gorda Ridge in the northeast Pacific were analyzed by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) in both negative electrospray ionization (ESI) and negative atmospheric pressure photoionization (APPI) ionization modes. These polar compounds were structurally similar to asphaltenes, but did not exceed a nominal mass range of ∼800 Da. They were derived primarily from terrestrially sourced organic matter (OM) with common heteroatom arrangements of CaHbOc, CaHbOcNd, CaHbNd, CaHbOcSe, CaHbOcNdSe, CaHbNdSe, and CaHbSe. The samples included compounds containing up to 19 oxygen heteroatoms and ranged in size from 10 to 53 carbon atoms with double bond equivalent (DBE) values of 1–38. Molecular formulae with N and S heteroatoms were minor. Within this sulfidic environment the formation and preservation of sulfur heterocyclic compounds was very low, which could be due to sulfurization being out-competed by the high concentrations of metals in the vent fluids or as a result of the thermal instability of C–S bonds. The dominance of oxygenated, and not thiolated, compounds was primarily source-dependent with secondary alteration by aromatization in this system proceeding by H abstraction and reduction of sulfate to H2S. We identified differences in the distributions of heteroatomic compounds caused by the expulsion, thermochemical oxidation, and differentiation of petroleum products during the migration of fluids to the sediment–water interface.

Hydrothermal alteration of sediments in the high-temperature reaction zone at Central Hill of Escanaba Trough, Gorda Ridge

In sediments from Holes ODP 1038A and 1038H, drilled near the hot springs at Central Hill, Escanaba Trough (Gorda Ridge), redistribution of major elements occurs during the water-rock interaction. Contents of Si, Fe, Mg, Ca, Na and K have changed in altered sediments. In this process, an increase/decrease in contents of major elements in altered sediments shows a decrease/increase in their contents in fluids. The irregular distribution of major elements in altered sediments resembles a layer pie, which reflects the existence of lateral flows of hydrothermal fluids.

Coring disturbances in IODP piston cores with implications for offshore record of volcanic events and the Missoula megafloods

Piston cores collected from IODP drilling platforms (and its predecessors) provide the best long-term geological and climatic record of marine sediments worldwide. Coring disturbances affecting the original sediment texture have been recognized since the early days of coring, and include deformation resulting from shear of sediment against the core barrel, basal flow-in due to partial stroke, loss of stratigraphy, fall-in, sediment loss through core catchers, and structures formed during core recovery and on-deck transport. The most severe disturbances occur in non-cohesive (sandy) facies, which are particularly common in volcanogenic environments and submarine fans. Although all of these types of coring disturbances have been recognized previously, our contribution is novel because it provides an easily accessible summary of methods for their identification. This contribution gives two specific examples on the importance of these coring disturbances. We show how suck-in of sediments during coring artificially created very thick volcaniclastic sand layers in cores offshore Montserrat and Martinique (Lesser Antilles). We then analyze very thick, structureless sand layers from the Escanaba Trough inferred to be a record of the Missoula mega-floods. These sand layers tend to coincide with the base of core sections, and their facies suggest coring disturbance by basal flow-in, destroying the original structure and texture of the beds. We conclude by outlining and supporting IODP-led initiatives to further reduce and identify coring disturbances, and acknowledge their recent successes in drilling challenging sand-rich settings, such as during IODP Expedition 340.

The composition, origin and fate of complex mixtures in the maltene fractions of hydrothermal petroleum assessed by comprehensive two-dimensional gas chromatography

Sedimentary organic matter in hydrothermal systems can be altered by high temperature fluids to generate petroleum. The saturated and aromatic fractions of these hydrothermal oils are compositionally similar to conventional oil with the exception that they often contain higher concentrations of polycyclic aromatic hydrocarbons (PAH) as well as substantial mixtures of coeluting organic compounds that produce dramatically rising signal on the baseline of gas chromatograms termed unresolved complex mixtures (UCMs). Little is known about the compounds that compose UCMs and why or how they form. This is in part due to an inability to discriminate between in situ and migrated components that characterize the petroleum generated in hydrothermal systems. However, UCMs are also a product of the limitations imbedded in analytical separation techniques. With the advent of comprehensive two-dimensional gas chromatography (GC×GC), a revision of what should constitute molecular complexity needs to be considered. We address these problems by comparing the molecular compositions of the maltene fractions of three previously published hydrothermal petroleum samples using time of flight-mass spectrometry (GC×GC–ToF-MS) and 12 hydrothermal petroleum samples in cores from three locales using comprehensive two-dimensional gas chromatography with flame ionization detection (GC×GC–FID). The sediment cores were collected from Middle Valley, located off the axis of the Juan de Fuca Ridge, and the Escanaba Trough, along the Gorda Ridge, both in the NE Pacific Ocean, as well as from the Guaymas Basin in the Gulf of California. We define a UCM in GC×GC data to be a condition in which ⩾25% of the detected peaks within a chromatographic area coelute in either the first or second dimension. In turn, complex (CM) and simple mixtures (SM) are defined as having 5–24% and <5% coelution, respectively. All CM and UCMs were dominated by an array of configurational isomers, which becomes increasingly aromatic with higher molecular weight. We relate this to a multi-molecular complexity metric (MCM) by quantitatively comparing the difference in total peak variance and peak density for a GC×GC chromatogram. MCM values correlate with biomarker thermal maturity ratios for the Escanaba Trough and Guaymas Basin samples indicating that molecular complexity in these hydrothermal environments is in part a function of burial temperatures. Partial Least Squares (PLS) linear regression was applied to the total number of peak retention times as a proxy for the bulk molecular differences between each hydrothermal oil sample. Differences in the sample regressions correlate with the thermal maturity and the degree of PAH alkylation, indicating that this technique can be used to assess the degree of oxidative weathering due to dehydrogenation and hydrocarbon cracking. Subtracted chromatograms were then used to quantitatively track all of the individual molecular changes within the pyrolytic regime at Escanaba Trough. These subtracted chromatograms indicate that high molecular weight PAHs are highly mobile in hydrothermal fluids and may represent a phase partitioning that is occurring at greater depths. This phase condenses just below the seafloor to form an UCM in the near surface sediments. Saturated hydrocarbon biomarkers, such as hopanes, steranes and biphytanes are less mobile and more prone to being cracked and/or aromatized prior to migration toward the ocean floor. Together these techniques suggest that the molecular complexity of hydrothermal petroleum maximizes during the early stages of thermal maturation. The diversity of compounds forming these UCMs then decreases with increasing dehydrogenation, dealkylation and condensation reactions associated with elevated thermal stress and exposure to oxidants within the hydrothermal fluids.

Vacuolate-attached filaments: highly productive Ridgeia piscesae epibionts at the Juan de Fuca hydrothermal vents

Vacuolate sulfur bacteria with high morphological similarity to vacuolate-attached filaments previously described from shallow hydrothermal vents (White Point, CA) were found at deep-sea hydrothermal vents. These filamentous bacteria grow in dense mats that cover surfaces and potentially provide a significant source of organic carbon where they occur. Vacuolate-attached filaments were collected near vents at the Clam Bed site of the Endeavour Segment of the Juan de Fuca Ridge and from the sediment surface at Escanaba Trough on the Gorda Ridge. A phylogenetic analysis comparing their 16S rRNA gene sequences to those collected from the shallow White Point site showed that all vacuolate-attached filament sequences form a monophyletic group within the vacuolate sulfur-oxidizing bacteria clade in the gamma proteobacteria. Abundance of the attached filaments was quantified over the length of the exterior surface of the tubes of Ridgeia piscesae worms collected from the Clam Bed site at Juan de Fuca yielding a per worm average of 0.070 ± 0.018 cm3 (n = 4). In agreement with previous results for White Point filaments, anion measurements by ion chromatography showed no detectable internal nitrate concentrations above ambient seawater (n = 9). For one R. piscesae tube worm “bush” at the Easter Island vent site, potential gross epibiont productivity is estimated to be 15 to 45× the net productivity of the worms.

Modeling of gold scavenging by bismuth melts coexisting with hydrothermal fluids

The effect of gold scavenging by bismuth melts is investigated using equilibrium thermodynamic modeling of an aqueous solution–mineral–melt system. The calculations for the Au-Bi-Na-Cl-S-H-O system, performed at temperatures between 300 and 450 °C, demonstrate that Au concentrations in the melt are several orders of magnitude higher than in the coexisting fluid, indicating the possible formation of economic gold deposits from undersaturated aqueous fluids, in which mineralization would not be expected in the absence of a bismuth melt. The model applies to any deposit where a Bi melt is stable and coexists with a hydrothermal fluid; examples of such deposits are known from skarn, intrusion-related, orogenic, and volcanogenic massive sulfide (VMS) gold systems. In sulfur-poor systems the partitioning curves presented here can be used directly to correlate the gold concentration in the fluid and the Au grade in the ore (e.g., Escanaba Trough VMS deposit). These results also illustrate important principles generally applicable to understanding magmatic-hydrothermal and metamorphic deposits that may have contained significant volumes of more complex polymetallic melts.

Experimental deep-sea deployments reveal diverse Northeast Pacific wood-boring bivalves of Xylophagainae (Myoida: Pholadidae)

Only a single species of deep-sea wood boring bivalve has been known off the forested northwest coast of North America, although the Xylophagainae are so diverse in other areas that up to five species occur in a given length of wood. To determine whether additional species were present in the Northeast Pacific and if so, how they coexist, lengths of wood were experimentally deployed on heavily sedimented sites on the Cascadia Basin and Escanaba Trough, non-hydrothermally active basalt on Gorda Ridge and Axial Volcano, and an isolated sediment pond within the axis of Juan de Fuca Ridge, at depths of between 1,530 and 3,232 m. All locations were between 418and 488N, at least 240 km off the North American coast. Six previously unknown species, Xylophaga corona n. sp., X. zierenbergi n. sp., X. heterosiphon n. sp., X. oregona n. sp., X. microchira n. sp. and Xylopholas crooki n. sp., were collected and are described here. Dominant species differed in recoveries made after 10 and 24 months at Juan de Fuca Ridge and Cascadia Basin localities. Xylophaga microchira n. sp. dominated four of five deployments recovered within 12 months, but its abundance declined by 24 months, being largely replaced on Juan de Fuca Ridge by X. oregona n. sp., a species known only from this Ridge and from an incidental collection on the Nootka Fracture Zone. At Cascadia Basin sites, the abundances of X. heterosiphon n. sp. and of what are likely to be predatory polyclad flatworms increased as that of X. microchira n. sp. declined. Xylophaga microchira n. sp. is hypothesized to be specialized for early colonization, but to be competitively inferior to X. oregona n. sp., and more vulnerable to predators than X. heterosiphon n. sp. The dominance of competitively superior and more predator-resistant species in older deployments argues that biotic interactions contribute to coexistence of multiple species of wood-borers despite direct competition among them.

First description of deep-sea polyclad flatworms from the North Pacific: Anocellidus n. gen. profundus n. sp. (Anocellidae, n. fam.) and Oligocladus voightae n. sp. (Euryleptidae)

Two deep-sea polyclad species are described. Both species were found in association with wood-boring bivalves on oak and fir wood blocks that had been deployed on heavy sediment in the Cascadia Basin and Escanaba Trough in the North Pacific Ocean. Anocellidus profundus n. gen. n. sp. warrants erection of Anocellidae, n. fam. because of the unique position and orientation of the male copulatory apparatus. A. profundus lacks eyes but has long, nuchal tentacles and a ventral, arrowhead-shaped organ of putative sensory function. The male copulatory apparatus is located posterior to the male gonopore and is directed anteriorly. Highly muscularized spermiducal bulbs are present, a prostatic vesicle is lacking. A large Lang’s vesicle characterizes the female reproductive system. Oligocladus voightae n. sp. is defined by the presence of a mouth located anterior to the brain. Few and minute tentacular eyes are present. The seminal vesicle is connected to an auxiliary sperm storage vesicle, and a posterior anal pore is present in the main median branch of the intestine. All type material is deposited at the Field Museum of Natural History, Chicago, Illinois, USA.

Migration, Isolation, and Speciation of Hydrothermal Vent Limpets (Gastropoda; Lepetodrilidae) Across the Blanco Transform Fault

The Sovanco Fracture Zone and Blanco Transform Fault separate the Explorer, Juan de Fuca, and Gorda ridge systems of the northeastern Pacific Ocean. To test whether such offsets in the ridge axis create barriers to along-axis dispersal of the endemic hydrothermal vent animals, we examined the genetic structure of limpet populations previously identified as Lepetodrilus fucensis McLean, 1988 (Gastropoda, Lepetodrilidae). Mitochondrial DNA sequences and patterns of allozyme variation revealed no evidence that the 150-km-long Sovanco Fracture Zone impeded gene flow between the Explorer and Juan de Fuca populations. In contrast, the 450-km-long Blanco Transform Fault separates the limpets into highly divergent northern and southern lineages that we recognize as distinct species. We describe southern populations from the Gorda Ridge (Seacliff) and Escanaba Trough as Lepetodrilus gordensis new species and refer northern populations from the Explorer and Juan de Fuca ridge systems to L. fucensis sensu stricto. The species are similar morphologically, but L. gordensis lacks a sensory neck papilla and has a more tightly coiled teleconch. To assess the degree of isolation between these closely related species, we used the Isolation with Migration method to estimate the time of population splitting, effective sizes of the ancestral and derived populations, and rates of migration across the Blanco Transform Fault.

The Escanaba Trough, Gorda Ridge hydrothermal system: Temporal stability and subseafloor complexity

The composition and temperature of vent fluids sampled from the active hydrothermal system in Escanaba Trough, Gorda Ridge in 2000 and 2002 remain unchanged from the only time this field was previously sampled, in 1988. ODP Leg 169 drilled nine bore holes at this site in 1996, some within meters of the vents, yet this disturbance has not impacted the measured compositions or temperatures of the fluids exiting at the seafloor. The fluids have maximum measured temperatures of 218°C and contain ∼20% more chloride than local ambient seawater. Our interpretation is that the fluid compositions are generated by supercritical phase separation of seawater, with much of the water-rock reaction occurring within the ∼400m thick sedimentary section that overlies the basalt at this site. The ODP drilling results provide information on the mineralogy and composition of materials below the seafloor, as well as direct constraints not typically available on the physical conditions occurring below the seafloor hydrothermal system. Calculations utilizing geochemical modeling software suggest the fluids are close to saturation with a suite of minerals found subsurface, suggesting equilibrium between the fluids and substrate. These results provide an explanation for why the fluids have remained chemically stable for 14 yrs. The pore water data from drilling suggest that the hydrology and chemistry of the hydrothermal system are much more complex within the sediment cover than would be expected from the surface manifestations of the hydrothermal system. While the pore waters have chloride contents both greater and less than the local seawater, only fluids with higher chloride contents vent at the seafloor. Our calculations suggest that at the current conditions the “brines” (fluids with chlorinity greater than seawater) are actually less dense than the “vapors” (fluids with chlorinity less than seawater). These density relationships may provide an explanation for why the “brines” are now venting preferentially to the “vapors,” a situation opposite to what is usually observed or inferred.

Gold Enrichment and the Bi-Au Association in Pyrrhotite-Rich Massive Sulfide Deposits, Escanaba Trough, Southern Gorda Ridge

High gold contents (to 10.1 ppm, avg 1.4 ppm, n = 34) occur in pyrrhotite-rich massive sulfide samples from the sediment-covered floor of the Escanaba trough, the slow-spreading, southernmost segment of Gorda Ridge. These concentrations reflect the presence of primary gold, formed during high-temperature hydrothermal activity in mounds and chimneys, and secondary gold deposited during sea-floor weathering of massive sulfide. Primary gold occurs as fine-grained ( 2 μm) secondary gold grains have a porous, flaky morphology and occur in samples in which pyrrhotite is oxidized and replaced by Fe oxyhydroxides, Fe sulfate, and sulfur. Mounds and chimneys dominated by pyrrhotite and containing lesser amounts of isocubanite, chalcopyrite, and Fe-rich sphalerite were formed by high-temperature (estimated range 325°–275°C), reduced, low-sulfur vent fluids. The mineral and fluid compositions during this main stage of hydrothermal venting reflect subsurface interaction between circulating hydrothermal fluids and turbiditic sediment containing as much as 1.1 percent organic carbon. As the deposition of pyrrhotite, Cu-Fe sulfides, and sphalerite waned, a volumetrically minor suite of sulfarsenide, arsenide, Bi, and Au minerals was deposited from highly reduced, late main-stage fluids diffusing through mounds and chimneys. The low solubility of Au as a bisulfide complex and the absence of fluid mixing during this stage of hydrothermal activity apparently inhibited the precipitation of gold directly from solution. Instead, gold precipitation is thought to be linked to elevated concentrations of Bi in the late main-stage fluids. The textural relationships of Au and Bi minerals in pyrrhotite-rich samples, low melting point of native bismuth (271.4°C), and recent experimental results on Au and Bi in hydrothermal fluids contribute to the hypothesis that gold was effectively scavenged from the Escanaba trough vent fluids by coexisting droplets of liquid bismuth. Additional phase relationships of alloys in the Au-Bi system indicate that deposition of native bismuth and maldonite occurred at temperatures as low as 241°C. Bismuth droplets trapped in void space between main-stage mineral grains scavenged gold from ambient hydrothermal fluid to a greater extent than bismuth enclosed by late-forming pyrrhotite. The limited solid solution of Au in Bi can explain the apparent exsolution texture in which gold blebs are hosted by native bismuth. The electrum, native bismuth (with gold inclusions), and galena represent the last traces of gold mineralization from late main-stage fluids. During sea-floor weathering and the oxidation of pyrrhotite in the mounds and chimneys, secondary gold formed as aggregates of colloidal particles along pH gradients between acidic pore waters and ambient seawater. Gold was mobilized from earlier formed primary gold minerals and transported as aqueous chloride complexes. The reduction of Au(III) by residual Fe2+ in partly altered pyrrhotite and adsorption of colloids by Fe oxyhydroxides may have influenced the location of secondary gold grains within the alteration front. Solubility differences between gold and silver chloride complexes at low temperature account for the low Ag content of secondary gold grains. The high concentrations of Bi, and thus the association of Au and Bi minerals in pyrrhotite-rich massive sulfide, can be ascribed to the extensive interaction of hydrothermal fluids with sediment in the Escanaba trough. In contrast, the absence of the Au-Bi association in massive sulfides at other ridges, including other sediment-covered sites at Middle Valley and the Guaymas basin, reflects the predominantly basaltic source rocks.

Gold Enrichment and the Bi-Au Association in Pyrrhotite-Rich Massive Sulfide Deposits, Escanaba Trough, Southern Gorda Ridge

Gold Enrichment and the Bi-Au Association in Pyrrhotite-Rich Massive Sulfide Deposits, Escanaba Trough, Southern Gorda Ridge

Hydrothermal precipitates associated with bimodal volcanism in the Central Bransfield Strait, Antarctica

Polymetallic sulfide-sulfate mineralization enriched in Pb-Ag-As-Sb-Hg occurs in the Bransfield Strait, a late Tertiary-Quaternary marginal basin close to the Antarctic Peninsula. The mineralization is associated with bimodal volcanism and pelagic and volcaniclastic sediment in rifted continental crust. Hydrothermal precipitates have been recovered from two shallow (1,050–1,000 m water depth) submarine volcanoes (Hook Ridge and Three Sisters) in the Central Bransfield Strait. Mineralization at Hook Ridge consists of polymetallic sulfides, massive barite, and pyrite and marcasite crusts in semilithified pelagic and volcaniclastic sediment. Native sulfur commonly infills void space and cements the volcaniclastic sediment. The polymetallic sulfides are dominated by sphalerite with minor galena, enargite, tetrahedrite-tennantite, pyrite, chalcopyrite, and traces of orpiment cemented by barite and opal-A. The presence of enargite at Hook Ridge, the abundance of native sulfur, and the low Fe content of sphalerite indicate a high sulfur activity of the hydrothermal fluids responsible for mineralization. The sulfur isotopic composition of Hook Ridge precipitates documents the complexity of the sulfur sources in this hydrothermal system with variable influence of biological activity and possibly magmatic contributions. Homogenization temperatures and salinities of fluid inclusions in barite and opal-A suggest that boiling may have affected the hydrothermal fluids during their ascent. The discovery of massive barite-silica precipitates at another shallow marine volcano (Three Sisters volcano) attests to the potential for hydrothermal mineralization at other volcanic edifices in the area. The characteristics of the mineralization in the Bransfield Strait with rifting of continental crust, the presence of bimodal volcanism, including highly evolved felsic volcanic rocks, the association with sediments, and the Pb-Ag-As-Sb-Hg enrichment are similar to the setting of massive sulfide deposits in the Okinawa Trough, and distinct from those of sediment-dominated hydrothermal systems such as Escanaba Trough, Middle Valley, and Guaymas Basin. The geological setting of the Bransfield Strait is also broadly similar to that of some of the largest volcanogenic massive sulfide deposits in the ancient record, such as the Iberian Pyrite Belt.

04/00076 Improving flow properties of heavy crude petroleum: Khan, M. R. et al. PCT Int. Appl. WO 03 35,802 (Cl. C10G9/00), 1 May 2003, Appl. 2001/US50,791

Boron isotope and concentration data are presented for hydrothermal fluids from different tectonic settings that reflect derivation of boron from marine evaporites (Red Sea brines), non-marine evaporites (Salton Sea), clastic sediments (Escanaba Trough and Guaymas Basin), back-arc basalts (Mariana Trough) and mid-ocean ridge basalts (21° and 11–13°N East Pacific Rise; Juan de Fuca Ridge; and 23° and 26°N, Mid-Atlantic Ridge). Boron isotope systematics of these fluids are used to constrain the δ″B-values of tourmalines found in ancient massive sulfide deposits and tourmalinitcs that formed under conditions analogous to those of modern hydrothermal systems. The data also provide insights into the physical conditions required for the formation of tourmaline in certain geologic environments.

Got glass? Glass from sediment and foraminifera tests contribute clues to volcanic history

Volcanic glass fragments recovered from sediment collected in the Escanaba Trough and on traverses up the west walls at two locations on the Gorda Ridge greatly extend information on the petrologic diversity of the region obtained from outcrops. Compositions of glass grains found loose in sediments or agglutinated in the tests of benthic foraminifers provide information on compositions of unexposed flows that were covered by later eruptions and/or sediment. Many of the glass grains have compositions that closely match those of lava outcrops, indicating that they formed by spalling off pillow rims and quench granulation. Bubble-wall glass shards found only in the sediment give evidence of explosive eruptions that occurred despite great depth. Some of the glass fragments from the walls of the axial valley have unusually high K 2 O (to >0.7%) that is not represented by any sampled outcrop. Such enriched mid-oceanic-ridge basalt compositions have not previously been reported from the Gorda Ridge. Analyzing glass grains from the sediment in addition to rims of lava samples allows a more comprehensive characterization of the volcanic history of the region.

Hydrothermal petroleum in the sediments of the Andaman Backarc Basin, Indian Ocean

Recent sediments of the Andaman Backarc Basin, Indian Ocean, between the Andaman Nicobar islands and the Malay Peninsula have been analyzed for biomarker lipids. Three cores were selected: one each from the fault zone in a deep basin (a graben between two fault systems), another from a location adjacent to the fault, and the third from the topographic high in the rift valley. The molecular composition of the lipid classes ( n-alkanes, isoprenoids, alkylbenzenes, alkylcyclohexanes, hopanoids, polycyclic aromatic hydrocarbons, steranes, alcohols, sterols and fatty acids) was examined by gas chromatography (GC) and GC/mass spectrometry to understand the nature and source of the hydrocarbons present and the processes of maturation of organic matter. The data show that the hydrocarbons are of hydrothermal origin, derived from thermal alteration of sedimentary organic matter, consisting of a mixture predominantly of marine-derived components with some terrestrial inputs. Normal alcohols and fatty acids also corroborate the distribution of n-alkanes. The distribution profiles and various parameters computed from the concentration of the target compounds suggest that oxidative reactions and microbial degradation in this environment are insignificant. Triterpane and PAH compositions indicate that the thermal maturity of the bitumen in the samples is comparable to or lower than that found at other hydrothermal regions such as the Northern Juan de Fuca Ridge, Guaymas Basin and Escanaba Trough.