Randolph A. Koski

Mineralogy and chemistry of massive sulfide deposits from the Juan de Fuca Ridge

Six hydrothermal vent sites and associated benthic communities were located in the axial valley of the southern Juan de Fuca Ridge using transponder-navigated bottom photography. The hydrothermal deposits form ledges and shallow mounds within a central zone characterized by a linear bathymetric depression and numerous collapse features. The flat valley floor adjacent to the central zone consists of ferrobasalt lobate flows and sheet flows; sediment cover is minimal. Vent sites are characterized by concentrations of tube worms, clams, benthic siphonophores, and several unidentified fauna. Two types of massive sulfide were dredged from one of the vent sites. Type A samples are angular slabs of dark gray Zn-rich sulfide with interlayers and a thin, partly oxidized crust of Fe sulfide. These layered sulfide aggregates appear to be fragments of a sulfide wall enclosing an active hydrothermal vent. The outer sulfide wall is composed mainly of colloform Fe sulfide and Fe-poor sphalerite deposited under lower-temperature conditions whereby sea water and hydrothermal fluid mix above the discharge point. With continued sulfide deposition, the wall inhibits mixing of sea water and hydrothermal fluid. Inside the wall, the intensifying hydrothermal system deposits a higher-temperature assemblage of granular Fe-rich sphalerite, wurtzite, pyrite, and minor Cu-Fe sulfide. The zonation in wurtzite from Fe-rich cores to Fe-poor rims may result from a late-stage cooling of the hydrothermal fluid and(or) a change in fluid chemistry. The sulfide wall grows outward where a rupture in it permits the escape of high-temperature fluid and then deposition of a secondary shell over the breakthrough point. As temperature increases, earlier-formed minerals dissolve, and Zn, Fe, and Pb migrate toward the outer sulfide wall. Tube worms flourished in the outer wall of type A samples, and abandoned tube structures served as conduits for the late-stage hydrothermal fluids. Type B sulfide samples are subrounded, spongy-textured fragments composed almost entirely of dendritic aggregates of pale, Fe-poor colloform sphalerite and opaline silica. This type of sulfide is deposited in open space by moderate- to low-temperature fluid discharging at a slow but variable rate; the fluid becomes increasingly oxidizing, resulting in late-stage deposition of hematite, barite, and sulfur. Type B samples show little evidence of burrowing animals; this type of sulfide may be deposited in settings peripheral to sites of focused discharge.

Active hydrothermal vents and sulfide deposits on the southern Juan de Fuca Ridge

Massive-sulfide deposits rich in zinc and silver were recovered from the Juan de Fuca Ridge 500 km west of Oregon in September 1981. The samples recovered are composed largely of zinc sulfide, with lesser amounts of iron, lead, and copper sulfide. Most of the deposits occur at a series of hydrothermal vents within a relatively continuous depression in the center of a smooth 1-km-wide valley along the ridge axis. The depression appears to be formed by collapse of a lava lake possibly modified by extensional faulting. The axial valley floor outside the depression is underlain by fresh, glassy, ferrobasalt sheet and lobate flows. Two types of sulfide-mineral deposits were dredged from one of the hydrothermal vents: (1) angular slabs of dark-gray zinc sulfide; and (2) subrounded fragments of porous light-gray zinc sulfide. The samples contain fresh sphalerite, zoned wurtzite, pyrite, and minor amounts of marcasite, galena, and chalcopyrite-cubanite. The spreading rate of the Juan de Fuca Ridge and the composition of the sulfide samples are generally similar to the East Pacific Rise lat 21° N site; however, the texture and geologic setting of the sulfide deposits are significantly different.

Fossils of Hydrothermal Vent Worms from Cretaceous Sulfide Ores of the Samail Ophiolite, Oman

Fossil worm tubes of Cretaceous age preserved in the Bayda massive sulfide deposit of the Samail ophiolite, Oman, are apparently the first documented examples of fossils embedded in massive sulfide deposits from the geologic record. The geologic setting of the Bayda deposit and the distinctive mineralogic and textural features of the fossiliferous samples suggest that the Bayda sulfide deposit and fossil fauna are remnants of a Cretaceous sea-floor hydrothermal vent similar to modern hot springs on the East Pacific Rise and the Juan de Fuca Ridge.

Compositions, growth mechanisms, and temporal relations of hydrothermal sulfide-sulfate-silica chimneys at the northern Cleft segment, Juan de Fuca Ridge

Three active hydrothermal vents forming sulfide mounds and chimneys (Monolith, Fountain, and Pipe Organ) and more widely distributed inactive chimneys are spatially related to a system of discontinuous fissures and young sheet flow lavas at the northern Cleft segment, Juan de Fuca Ridge. The formation of zoned tubular Curich chimneys (type I) on the Monolith sulfide mound is related to focused flow of high-temperature (to 328°C) fluid. Bulbous chimneys (type II or “beehives”) at the Monolith and Fountain vents are products of diffuse high-temperature (to 315°C) discharge. A broader zone of vigorous mixing between the hydrothermal fluid and seawater results in quench crystallization of anhydrite-rich shells. Columnar Zn-sulfide-rich chimneys with narrow channelways (type III) are constructed where focused and relatively low-temperature (261°C) fluid vents directly from the basalt substrate. The bulk chemistry (low Cu; high Pb, Ag, and SiO2 contents), mineralogy (pyrite-marcasite-wurtzite-amorphous silica-anglesite), colloform and filamentous textures, and oxygen isotope characteristics of inactive (type IV) chimneys indicate a low-temperature (<250°C) origin involving diffuse and sluggish flow patterns and conductive cooling. Seafloor observations and 210Pb data indicate that (1) type IV chimneys are products of an earlier period of hydrothermal activity that ended no more than 60 years ago but prior to the sheet flow eruption, (2) the high-temperature Monolith and Fountain vents are manifestations of the same heating event (shallow emplacement of magma) that led to the sheet flow eruption and recent megaplumes, and (3) the Pipe Organ Vent is in a very youthful stage of development and chimney deposition postdates the sheet flow eruption.

Pb isotopes in sulfides from mid-ocean ridge hydrothermal sites

The authors report Pb isotope ratios of sulfides deposited at seven recently active mid-ocean ridge (MOR) hydrothermal vents. Sulfides from three sediment-starved sites on the Juan de Fuca Ridge contain Pb with isotope ratios identical to their local basaltic sources. Lead in two deposits from the sediment-covered Escanaba Trough, Gorda Ridge, is derived from the sediments and does not appear to contain any basaltic component. There is a range of isotope ratios in a Guaymas Basin deposit, consistent with a mixture of sediment and MOR basalt Pb. Lead in a Galapagos deposit differs slightly from known Galapagos basalt Pb isotope values. The faithful record of Pb isotope signatures of local sources in MOR sulfides indicates that isotope ratios from ancient analogues ca be used as accurate reflections of ancient oceanic crustal values in ophiolite-hosted deposits and continental crustal averages in sediment-hosted deposits. The preservation of primary ophiolitic or continental crustal Pb isotope signatures in ancient MOR sulfides provides a powerful tool for investigation of crustal evolution and for fingerprinting ancient terranes.

Bacterially mediated diagenetic origin for chert-hosted manganese deposits in the Franciscan Complex, California Coast Ranges

Numerous manganese deposits in the Franciscan Complex, California, occur as conformable lenses within bedded radiolarian chert-argillite sequences that are, in turn, intercalated within thicker sections of sandstone and shale. The field relations, composition, and petro-graphic and isotopic characteristics indicate that the manganese was concentrated by diagenetic reconstitution of siliceous and hemipelagic sediment during burial. The ore lenses are Mn-rich and Fe-poor assemblages consisting largely of rhodochrosite, manganese silicates, opal-CT (disordered cristobalite-tridymite), and quartz. Highly negative δ 13 C values for the carbonate carbon in rhodochrosite indicate that CO 2 likely originated from oxidation of methane; less negative values result from mixing of methanogenic carbon and CO 2 derived from bacterial degradation of organic matter. The δ 18 O values for the carbonate of rhodochrosite indicate temperatures of formation between 12 and 100 °C. The oxidation of methane prior to carbonate precipitation may have used the minor (0.4%–0.5%) Mn and Fe oxyhydroxides and oxides deposited with the sediment. The mobilization of manganese from biogenic and terrigenous sources in the sediment column into discrete horizons and the fractioriation of manganese from iron reflect the presence of oxidation-reduction boundaries and gradients in the sediment column. Fluids derived from compaction and silica-dehydration reactions in the transformation of opal-A (X-ray amorphous biogenic silica) to quartz were involved in transportation of principal components. Sedimentary and geochemical attributes suggest that the deposits formed in a deep-water environment in a zone of oceanic upwelling near a continental margin.

An estimate of hydrothermal fluid residence times and vent chimney growth rates based on210Pb/Pb ratios and mineralogic studies of sulfides dredged from the Juan de Fuca Ridge

Abstract The210PbPb ratios across two sulfide samples dredged from the Juan de Fuca Ridge are used to estimate the growth rate of the sulfide material and the residence time of the hydrothermal fluid within the oceanic crust from the onset of basalt alteration.210Pb is added to the hydrothermal fluid by two processes: (1) high-temperature alteration of basalt and (2) if the residence time of the fluid is on the order of the 22.3-year half-life of210Pb, by in-situ growth from222Rn ( Krishnaswami and Turekian, 1982 ). Stable lead is derived only from the alteration of basalt. The210Pb/Pb ratio across one sample was ∼ 0.5 dpm/10−6 g Pb, and across the other it was ∼ 0.4 dpm/10−6 g Pb. These values are quite close to the238UP/b ratios of basalts from the area, suggesting that the residence time of the hydrothermal fluid from the onset of basalt alteration is appreciably less than the mean life of210Pb, i.e., the time required for ingrowth from the radon. An apparent growth rate of 1.2 cm/yr is derived from the slope of the210Pb/Pb curve for one of the samples. This is consistent with its mineralogy and texture which suggest an accretionary pattern of development. There is no obvious sequential growth pattern, and virtually no gradient in210Pb/Pb across the second sample. This is consistent with alteration of the original210Pb/Pb distribution by extensive remobilization reactions which are inferred from the mineralogic and textural relationships of the sample.

Hydrothermal discharge zones beneath massive sulfide deposits mapped in the Oman ophiolite

The area in the Oman ophiolite containing the volcanic-hosted Bayda and Aarja massive sulfide deposits exposes a cross section of ocean crust and reveals to an unprecedented extent the fossil zones of hydrothermal upwelling that fed these sea-floor deposits. The fossil discharge zones are elongate areas of alteration and mineralization characterized by numerous small (metres to tens of metres in length), linear, discontinuous gossans. The gossans result from oxidation of hydrothermal pyrite replacing primary igneous phases and filling voids and fractures in the altered host rocks. The two deposits have separate discharge zones that appear to be sub-sea-floor extensions of their stockworks. The Bayda zone extends through the volcanic section into the upper sheeted dike complex and is interpreted as having formed on the ridge crest above an axial magma chamber; the Aarja zone terminates against a plagiogranite pluton that intrudes the lower volcanic section and is thought to have formed after Bayda in an off-axis environment. Structural, stratigraphic, and compositional characteristics of the Bayda and Aarja massive sulfide bodies are consistent with this interpretation. The geometry of the discharge zones suggests that in both cases upfiow occurred in broad zones (at least 400-600 m wide) that were elongated along strike (i.e., parallel to the spreading axis).

Metalliferous Sediment and a Silica-Hematite Deposit within the Blanco Fracture Zone, Northeast Pacific

A Tiburon ROV dive within the East Blanco Depression (EBD) increased the mapped extent of a known hydrothermal field by an order of magnitude. In addition, a unique opal-CT (cristobalite-tridymite)-hematite mound was discovered, and mineralized sediments and rock were collected and analyzed. Silica-hematite mounds have not previously been found on the deep ocean floor. The light-weight rock of the porous mound consists predominantly of opal-CT and hematite filaments, rods, and strands, and averages 77.8% SiO2 and 11.8% Fe2O3. The hematite and opal-CT precipitated from a low-temperature (≥ 115° C), strongly oxidized, silica- and iron-rich, sulfur-poor hydrothermal fluid; a bacterial mat provided the framework for precipitation. Samples collected from a volcaniclastic rock outcrop consist primarily of quartz with lesser plagioclase, smectite, pyroxene, and sulfides; SiO2 content averages 72.5%. Formation of these quartz-rich samples is best explained by cooling in an up-flow zone of silica-rich hydrothermal fluids within a low permeability system. Opal-A, opal-CT, and quartz mineralization found in different places within the EBD hydrothermal field likely reflects decreasing silica saturation and increasing temperature of the mineralizing fluid with increasing silica crystallinity. Six push cores recovered gravel, coarse sand, and mud mineralized variously by Fe or Mn oxides, silica, and sulfides. Total rare-earth element concentrations are low for both the rock and push core samples. Ce and Eu anomalies reflect high and low temperature hydrothermal components and detrital phases. A remarkable variety of types of mineralization occur within the EBD field, yet a consistent suite of elements is enriched (relative to basalt and unmineralized cores) in all samples analyzed: Ag, Au, S, Mo, Hg, As, Sb, Sr, and U; most samples are also enriched in Cu, Pb, Cd, and Zn. On the basis of these element enrichments, the EBD hydrothermal field might best be described as a base- and precious-metal-bearing, silica-Fe-oxide-barite deposit. Such deposits are commonly spatially and temporally associated with volcanogenic massive sulfide (VMS) ores. A plot of data for pathfinder elements shows a large hot spot at the northwestern margin of the field, which may mark a region where moderate to high temperature sulfide deposits are forming at depth; further exploration of the hydrothermal field to the northwest is warranted.

Massive sulfide metallogenesis at a late Mesozoic sediment-covered spreading axis: Evidence from the Franciscan complex and contemporary analogues

The Island Mountain deposit, an anomalous massive sulfide in the Central belt of the Franciscan subduction complex, northern California Coast Ranges, formed during hydrothermal activity in a sediment-dominated paleo-sea-floor environment. Although the base of the massive sulfide is juxtaposed against a 500-m-wide melange band, its gradational upper contact within a coherent sequence of sandstone, siltstone, and mudstone indicates that hydrothermal activity was concurrent with turbidite deposition. Accumulations of sulfide breccia and clastic sulfide were produced by mass wasting of the sulfide mound prior to burial by turbidites. The bulk composition of sulfide samples (pyrrhotite rich; high Cu, As, and Au contents; radiogenic Pb isotope ratios) is consistent with a hydrothermal system dominated by fluid-sediment interaction. On the basis of a comparison with possible contemporary tectonic analogues at the southern Gorda Ridge and the Chile margin triple junction, we propose that massive sulfide mineralization in the Central belt of the Franciscan complex resulted from hydrothermal activity at a late Mesozoic sediment-covered ridge axis prior to collision with the North American plate.