Steven D. Scott

Comparative mineralogy and geochemistry of gold-bearing sulfide deposits on the mid-ocean ridges

A comparative study of the mineralogy and geochemistry of sulfide deposits on mid-ocean ridges in the Northeast Pacific and the Mid-Atlantic reveals common characteristics associated with primary gold enrichment. Average gold contents of 0.8 to 5 ppm Au occur in sulfides from Southern Explorer Ridge and Axial Seamount (Northeast Pacific) and from the TAG hydrothermal field and Snakepit vent field (Mid-Atlantic Ridge). The enrichment of gold in these deposits is consistently related to a phase of late-stage, low-temperature ( 1 ppm Au occur exclusively in pyritic assemblages and commonly with abundant Fe-poor sphalerite and a suite of complex Pb—Sb—As sulfosalts. Amorphous silica and, locally, barite or carbonate are important constituents of the gold-rich precipitates but do not contain gold themselves. High-temperature (350°C) black smoker assemblages, consisting dominantly of pyrite, chalcopyrite, pyrrhotite, isocubanite and abundant anhydrite are uniformly gold-poor (≤0.2 ppm Au). To the extent that individual sulfides can be mechanically separated, chemical analyses by neutron activation indicate that gold is most abundant in sphalerite (up to 5.7 ppm Au) but also occurs in pyrite and marcasite. Samples of sphalerite with abundant inclusions of fine-grained sulfosalts locally contain up to 18 ppm Au, suggesting that sulfosalts may be repositories for gold. No free gold has been observed at 4000 × magnification of polished specimens, indicating that the gold is present only as submicroscopic inclusions or as a chemical constituent within the sulfides. Samples from gold-rich deposits in the Northeast Pacific and Mid-Atlantic are compared with similar but relatively gold-poor sulfides from the Galapagos Rift and 13°N on the East Pacific Rise (EPR), and with barren sulfides from 11°N EPR, 21°N EPR, the Endeavour Ridge, and the Southern Juan de Fuca Ridge. Trace element analyses of more than 170 samples show that gold enrichment in almost all of the deposits is associated with high concentrations of Ag, As, Sb, Pb and Zn, and locally with high Cd, Hg, Tl, and Ga. In contrast, gold is typically depleted in samples with high Co, Se, and Mo. The close association of Au with Ag, As, Sb, and Pb may reflect the common behavior of these metals as aqueous sulfur complexes (e.g., [Au(HS)−2]) at low temperatures. Similar mineralogical and geochemical associations are observed in sulfide deposits from modern back-arc settings and in the ancient geologic record.

Characterization of Bacteriogenic Iron Oxide Deposits from Axial Volcano, Juan de Fuca Ridge, Northeast Pacific Ocean

Iron oxides from the caldera of Axial Volcano, a site of hydrothermal vent activity along the Juan de Fuca Ridge, are characterized by abundant bacterial structures that closely resemble the sheaths of Leptothrix ochracea , the stalks of Gallionella ferruginea , and the filaments of a novel iron oxidizing PV-1 strain. These bacteria are commonly associated with iron-oxide precipitates and are proposed to play two causal roles in the formation of iron oxides at Axial Volcano. First, by increasing the rate of Fe 2+ oxidation, and second, by lowering the concentration of Fe 3+ required for precipitation by providing a reactive surface for heterogeneous nucleation. Rapid rates of oxidation and precipitation caused by the bacteria likely contribute to the poorly ordered nature of the iron oxides, determined to be primarily 2-line ferrihydrite, and in one case, poorly ordered goethite. The iron-oxide precipitates consist dominantly of iron, silicon (mostly diatoms), and organic carbon with a suite of sorbed trace metals. The high metal uptake affinity of these poorly ordered iron oxides may be important in the global cycling of trace elements throughout the worlds oceans. Additionally, iron oxides precipitating on the surface of bacteria can preserve individual cells as microfossils making bacteriogenic iron oxides ideal proxies for paleoenvironmental and astrobiological studies.

Ultrastructure and potential sub-seafloor evidence of bacteriogenic iron oxides from Axial Volcano, Juan de Fuca Ridge, north-east Pacific Ocean

Iron oxides from the caldera of Axial Volcano, a site of hydrothermal vent activity along the Juan de Fuca Ridge, were found to consist predominantly of microbial structures in hydrated whole mounts examined using an environmental scanning electron microscope. Novel observations were made of the iron oxides revealing the spatial relationships of the bacteria within to be more consistent with microbial mats than mineral precipitates. The bacterial structures are attributed to the sheaths of Leptothrix ochracea, the stalks of Gallionella ferruginea, and the filaments of a novel iron oxidizing PV-1 strain, based on the distinctive morphological characteristics of these three bacteria. Energy dispersive X-ray spectroscopy revealed the presence and distribution of Fe, Si, and Cl on the bacterial sheaths, stalks and filaments. The iron oxides were identified by X-ray diffraction to be two-line ferrihydrite, a poorly ordered iron oxyhydroxide. Adsorption of Si in particular to two-line ferrihydrite likely contributes to its stability on the seafloor, and might also be a preservation mechanism creating microfossils of the bacterial structures encrusted with ferrihydrite. Presumptive evidence of the sub-seafloor presence of L. ochracea, G. ferruginea and PV-1 at Axial Volcano was obtained from the presence of these bacteria on a trap that had been placed within an active vent, and also in a vent fluid sample. If indeed these bacteria are present in the sub-seafloor, it may be an indication that the surface expression of iron oxide deposits at Axial Volcano is minimal in comparison to what exists beneath the seafloor.

Tectonics of rift propagation into a continental margin : Western Woodlark Basin, Papua New Guinea

Oblique convergence along the irregular boundary between the Pacific and Indo-Australian plates in the SW Pacific has resulted in a transpressional regime. The drag of the overriding Pacific plate has produced a breakup of the Solomon microplate from the Indo-Australian plate and has induced its rotation within the mega-shear zone between the two major plates. The microplate is being separated from the Indo-Australian plate by passive rifting and seafloor spreading in the Woodlark Basin which is propagating westward along the continental margin of Papua New Guinea. The present-day pole of the microplate rotation with respect to Indo-Australian plate is estimated to be near the Owen Stanley Fault Zone which represents the suture after Paleocene-Eocene arc-continent collision. The average rate of the seafloor spreading propagation is estimated to be 150 mm yr−1 over the last 3.4 Ma. The transition between seafloor spreading and continental rifting is characterized by a dramatic reduction in the production of new oceanic crust and by a change from localized deformation within oceanic lithosphere to distributed continental extension. Variations between the two structural styles are being balanced by an accommodation zone and a newly developing transform fault. The style of continental deformation progressively changes due to a gradual decrease in lithospheric thinning along the rift axis toward the pole of opening. Three accommodation zones are proposed to balance differential extension and slippage between individual series of tilted blocks.

14C ages of hydrothermal petroleum and carbonate in Guaymas Basin, Gulf of California: Implications for oil generation, expulsion, and migration

{sup 14}C dates have been obtained by accelerator mass spectrometry and by conventional techniques from hydrothermally derived petroleum, calcite, and vent water from the southern trough of Guaymas Basin, Gulf of California. The ages for petroleum range from 4240 to 5705 B.P., whereas those for calcite range from 7340 to 33,168 B.P. The CO{sub 2} in a sample of hydrothermal vent fluid (24% end member) yielded an apparent age of 14,820 B.P., which is within the calcite age range. These are not true ages, but rather they reflect the age of carbon within these materials. The data indicate that the conversion of sedimentary organic matter to petroleum occurs very rapidly and that the petroleum is mobilized from sedimentary organic matter in the upper 30 m of the sediment column ({le}500 m thick) underlying the sea-floor deposits. The large difference in ages between the petroleum and most of the calcites indicates that the carbon source in the calcites is more complex. The inorganic carbon pool probably originates from the dissolution of older detrital carbonates and autochthonous biogenic carbonates in the shallow sediments, and/or the decarboxylation of organic matter at greater depth, with a magmatic component of dead carbon.

Hydrothermal processes and contrasting styles of mineralization in the western Woodlark and eastern Manus basins of the western Pacific

Abstract The western Woodlark Basin (initial rifting of continental crust) and eastern Manus Basin (rifted arc crust) offshore eastern Papua New Guinea display contrasting styles of hydrothermal activity and mineralization. In the eastern Manus basin, en echelon felsic and mafic volcanic ridges have formed in a pull-apart basin of the rifted New Britain arc terrane. Here, the PACMANUS Cu-Zn-Pb-Ag-Au sulphide deposit is forming within an area of about 800 × 350 m on the flank of a dacite lava dome atop a prominent 20 km long and 250–350 m high volcanic ridge. The ridge is andesitic in its lower reaches, dacitic to rhyolitic on top and is adjacent to an extensive field of basalt. At Woodlark, submarine rhyolite domes are devoid of hydrothermal products, but extensive Fe-Si-Mn oxyhydroxide deposits are forming from low temperature fluids on Franklin Seamount, an axial basaltic andesite volcano near the tip of the oceanic propagator. Protruding through and perhaps underlying these oxyhydroxides are inactive, higher temperature, precious metal-rich (Ag to 545 ppm, Au to 21 ppm), barite-silica spires. The Franklin Seamount deposits are thought to cap a disseminated sulphide deposit within the volcano and represent a failed massive sulphide system. Both Franklin Seamount and PACMANUS provide models for ancient ores on land.

The Tuzo Wilson Volcanic Field, NE Pacific: Alkaline volcanism at a complex, diffuse, transform-trench-ridge triple junction

The Tuzo Wilson Volcanic Field (TWVF) consists of Quaternary alkaline volcanics erupted at the complex, diffuse triple junction between the Explorer, Pacific, and North American plates. It occurs in a region of distributed strain that lies between right-lateral, strike-slip faulting at the Queen Charlotte fault, seafloor spreading at the Explorer Ridge, and subduction at the Cascadia subduction zone. The TWVF is contained within a poorly defined graben structure and consists of two 500- to 700-m-high composite seamount volcanoes surrounded by numerous smaller vents, with a total edifice volume of about 12 km3. The erupted volcanics are large ion lithophile elements (LILE) and light rare earth element (LREE) enriched alkali basalts, hawaiites, mugearites, and benmoreiites. These volcanics also differ from mid-ocean ridge basalts by lacking Fe enrichment and Al depletion trends and by having low relative Fe, Sc, and Cr and high relative Al for a given MgO content, reflecting significant high-pressure pyroxene fractionation. They are petrologically similar to other alkaline volcanics found capping near-ridge seamounts in the east Pacific. Petrogenetic modeling combined with analysis of Sr, Nd, and Pb isotopic data shows that TWVF volcanism is ephemeral in nature and that the TWVF lavas were derived by small amounts of melting (≤3%) of an amphibole-bearing, LREE- and LILE-enriched heterogenous mantle similar to that underlying the adjacent Explorer spreading ridge. The new volcanological, petrological, and geochemical data presented constrain both the petrogenetic origin of TWVF lavas and the tectonic processes occurring in the triple junction region. Current wisdom states that the TWVF represents either a site of seafloor spreading or a hotspot/mantle plume. Our new data are incompatible with these models and instead are consistent with other geophysical data in suggesting that the TWVF represents minor volcanism associated with pull-apart structures developing between parallel strike-slip faults in a region of distributed strain. In short, the TWVF represents “leaky transform” volcanism in an oceanic setting.

Determination of selenium isotopic ratios by continuous-hydride-generation dynamic-reaction-cell inductively coupled plasma-mass spectrometry

The goal of this study was to develop an accessible method for the determination of selenium isotopes within selenium-rich geological samples and examine the influence of sample introduction, instrumental parameters, column separation and the application of standard-sample bracketing for mass bias and drift correction. Quantitative selenium separation and enrichment of samples was achieved by a column separation using 0.2 g thioglycollic impregnated cotton fiber prior to introduction of the sample to an ICP-DRC-MS. 0.6 ml min−1 premixed Ar (95%) + H2 (5%) was favored over CH4 and NH4 as a reaction cell gas and was used within the DRC coupled with optimized DRC rejection parameters RPa (0) and RPq (0.65) to effectively reduce the signal to background ratio of all measured selenium isotopes (m/z 76, 77, 78, 80 and 82). Ion signal intensity of all measured selenium isotopes were increased 100 fold over classic nebulization by mixing of 1% NaBH4 and acidified sample digestions in a membrane-less computer-controlled continuous hydride generator. Transient hydride ion signals were time-averaged for five readings and three replicates to produce an in-run precision (2σ) of ±0.45‰ δ82/76SeMerck (relative to a Merck titrosol ICP-MS standard) and ±0.85‰ δ82/76SeMerck over an 18 month period. In the absence of a selenium isotopic standard, the accuracy of the method was determined using four interlaboratory solutions and five geological standard reference materials covering 0 to −4.5‰ δ82/76SeMerck. Our results indicate excellent reproducibility within method precision. The minimum mass of Se required for isotopic ratio determination was 3 μg (>100 000 cps at m/z 82 and 76).

Chemosynthesis; an alternate hypothesis for Carboniferous biotas in bryozoan/microbial mounds, Newfoundland, Canada

Fossil tubes, stressed high-abundance low-diversity faunas, and abundant, low-temperature, hydrothermal mineralisation occur in Lower Carboniferous bryosoan/microbial carbonate mounds in southwestern Newfoundland, Canada. These occurrences, set in a rift-valley setting that was tectonically active during the Carboniferous, were previously regarded as marginal marine deposits laid down in subtidal, schisohaline environments adjacent to a rocky shoreline. We suggest that they may, instead, have formed by chemosynthetic processes at low temperature marine vents at a depth of 100 m, or greater

Experimental calibration of the sphalerite cosmobarometer

Abstract The pressure and temperature dependence of the composition of sphalerite in equilibrium with troilite + metallic iron has been determined experimentally at 2.5 and 5.0 kbar between 400° and 800°C using both the aqueous and anhydrous alkali halide flux recrystallization techniques. The measured pressure effect is larger than that calculated by us and by Schwarcz et al. (1975a), and is described by the equation ( T in Kelvins), P (kbar) = −3.576 + 0.0551 T −0.0296 T log mole % FeS . Assuming temperatures of final equilibration between sphalerite and troilite of 350°C for iron meteorites and 600°C for enstatite chondrites, published analyses of sphalerites provide estimates of pressures of formation and possible radii of parent objects of meteorites as follows: IA irons (Landes, Sardis, Gladstone, Bogou, Odessa, Toluca) 0.0 to 3.5 kbar, 0 to 442 km; E6 enstatite chondrites (Yilmia, Pillistfer) −0.2 to 0.7 kbar, 0 to 198 km.