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doi:10.2204/iodp.proc.331.103.2011

Petrology

As was noted in “Lithostratigraphy,” pervasive hydrothermal alteration and sulfide mineralization are ubiquitous in material recovered from Site C0013, which was targeted at a high–heat flow site ~0.1 km east of the main high-temperature chimneys of the Iheya North hydrothermal field (Takai et al., 2010). Following the failure of Hole C0013A, Holes C0013B (cored interval 0.0–9.5 mbsf), C0013C (cored interval 3.0–12.7 mbsf), and C0013D (cored interval 3.0–35.8 mbsf) were drilled within 5 m of each other along an approximate north–south line (Fig. F2). Hole C0013E, which utilized the drilling guide base, was spudded ~10 m west and 2 m upslope from the other three holes and cored from 0.0 to 54.5 mbsf. On returning to the site, Holes C0013F (cored interval 0.0–7.5 mbsf), C0013G (cored interval 7.5–9.3 mbsf), and C0013H (cored interval 9.3–9.9 mbsf) were drilled between Hole C0013E and the other holes. Given the close proximity of the holes, they can effectively be considered to represent a single drilled section for the purposes of petrological interpretation. Holes C0013G and C0013H recovered only drill-washed material and therefore provide no useful petrological information.

The following description of hydrothermal alteration and mineralization from Site C0013 is based primarily on visual descriptions of cores from Holes C0013B–C0013F and is supplemented by XRD analyses of representative samples, scanning electron microscope (SEM) imaging, and energy dispersive spectrometry (EDS) characterization of selected samples, along with a small number of polished thin section descriptions of more competent lithologies. Most of the rocks drilled at the site are composed of very fine grained material. With the exception of anhydrite, quartz, sulfide minerals, and native sulfur, most mineralogical determinations were made or confirmed using the results of XRD analysis. This is particularly true of Mg chlorite, talc, kaolinite, and muscovite within the altered sequence.

Difficulties encountered in drilling at the site (melted core liners and variable ground conditions leading to badly disturbed core, broken core, and patchy recovery), when coupled with the large number of whole-round samples removed from the core prior to lithological description, make it difficult to interpret the temporal and spatial relationships between alteration and mineralization styles observed in the core, except at the grossest level.

Overview of hydrothermal alteration and mineralization at Site C0013

Hydrothermal alteration and mineralization at Site C0013 may be spatially divided into five assemblages, which exhibit a broadly consistent vertical distribution from the seafloor (Fig. F14):

1. From 0 to ~4 mbsf, generally moderately hydrothermally altered, variably sulfidic sediment is characterized by detrital sulfide (sphalerite-pyrite-covellite)–sulfate (anhydrite-barite) mineralization and local development of native sulfur, as well as the presence of kaolinite and muscovite.

2. Pale gray “hydrothermal clay” that features kaolinite and muscovite as well as anhydrite is best developed in Holes C0013C and C0013D, where it extends from ~4 to 6.5 mbsf. In Hole C0013E, it extends from 4 to 5 mbsf, and in Hole C0013F, from 4 to 5.5 mbsf.

3. A zone of pale bluish-gray to white mottled “hydrothermal clay,” featuring Mg chlorite as well as coarse (typically 4–5 cm) rounded nodules and brecciated, drilling-disturbed veins of white opaque anhydrite ± dolomite ± talc ± calcite ± quartz ± sphalerite ± pyrite underlies the kaolinite-muscovite-bearing alteration and extends to at least 23 mbsf in Holes C0013D and C0013E. Below this depth, recovery becomes too poor to confirm the presence of the assemblage. Fine-grained disseminated pyrite and sphalerite are present in low abundances (typically <<1%) throughout the zone, and rare narrow irregular anhydrite-sulfide and quartz-sulfide veins are also observed.

4. Recovery from below ~26 mbsf in Holes C0013C and C0013D, the two deepest holes at the site, consists of hard quartz and Mg chlorite–altered volcanic breccia with scattered quartz-sulfide (sphalerite-pyrite ± covellite ± native copper) veining and trace fine disseminated pyrite within clasts. This material is interpreted to be volcanic basement. The base of this unit is not well defined, as recovery from the lower portions of both Holes C0013C and C0013D is poor, although material recovered from Core 331-C0013E-9X was curated at a depth of 45 mbsf.

5. An interval featuring abundant euhedral anhydrite crystals, 1–2 cm in length, interpreted to be veins dismembered by drilling, overlaps the kaolinite and Mg chlorite–bearing alteration zones between ~4 and 5 mbsf and 9 and 10 mbsf. These veins are interpreted to overprint the other alteration at the site.

Near-surface sulfidic sediment

Variably sulfidic sediment was intersected in the upper portion of all five successful holes drilled at Site C0013 and is best represented in Sections 331-C0013E-1H-1 through 1H-5 (0.0–4.6 mbsf). Even with the limited drilling available from the seafloor at the site (only Holes C0013B, C0013E, and C0013F attempted to core the interval from 0 to 3 mbsf), it is apparent that this material shows rapid lateral and vertical variation in composition and grain size, suggesting that the sediment originated locally.

Sand- to grit-sized anhydrite crystal fragments and less abundant barite are present throughout the interval, as are 0.5–3 mm euhedral and subhedral fragments of sphalerite, pyrite, and, at significantly lower abundance, covellite. Galena and sulfosalts, including tetrahedrite-tennantite, are also detected by SEM as intergrowths with sphalerite. The sulfide-sulfate assemblage is reminiscent of the mineralogy of sulfide chimneys and mounds at Iheya North Knoll (Glasby and Notsu, 2003) and is interpreted to be sediment derived from the breakdown of nearby hydrothermal mounds. In Hole C0013E, the section from 0.16 to 4.1 mbsf averages >10% sulfide (visually estimated), with some layers as thick as 11 cm exceeding 50% total sulfide. The near-surface sequence is less heavily mineralized in the other holes drilled, although sulfide content locally exceeds 10% (visually estimated) over some intervals.

A distinctive yellow native sulfur–bearing horizon was also intersected in the uppermost 7.5 cm of Hole C0013E (Fig. F15A). This material contains fragments of quartz-altered volcanic rock, along with minor sphalerite and barite cemented in native sulfur. It is interpreted to represent sediment that has been infiltrated by liquid native sulfur on or close to the seafloor. Native sulfur melts at temperatures between 112° and 119°C at 1 atm (Haynes, 2010), implying that a similar temperature existed near the seafloor at the time this horizon formed. Native sulfur is also abundant in the sulfidic sediment at the top of Hole C0013F, occurring as a cement, similar to that seen in Hole C0013E, both at the top of the hole and as crystalline 1–3 mm linings on what must have been gaseous or fluid-filled voids in silty mud (Fig. F15B). Native sulfur is present to 0.7 mbsf, implying that it precipitated from H₂S-rich fluid as it passed through the sediment.

XRD analysis of samples from within the sulfidic sediment detected minor quantities of kaolinite and muscovite, in addition to the phases noted above (Table T5).

SEM imaging of anhydrite from the sulfidic sediment shows evidence of incipient dissolution and replacement by gypsum (Fig. F16), implying that current temperatures in parts of the material are below ~40°C, the temperature below which anhydrite transforms to gypsum (MacDonald, 1953). Framboidal pyrite (Fig. F17), partially devitrified volcanic glass (Fig. F18), and opaline silica (Fig. F19) are also observed by SEM, providing additional evidence that relatively low temperature conditions are locally present. However, the widespread presence of anhydrite crystal fragments within the sequence implies that temperatures of close to 150°C were prevalent (see “Coarsely crystalline anhydrite veining,” below).

Kaolinite-muscovite alteration

Kaolinite-muscovite-anhydrite–bearing “hydrothermal clay” is typically pale gray to white. In drill core, the assemblage is massive in character, except when overprinted by coarsely crystalline euhedral anhydrite veining. In addition to kaolinite and muscovite, vermiculite may also be present (suggested by XRD). It is also considered likely that cryptocrystalline clays are present in the samples, although this will need to be confirmed through postexpedition research. Fine-grained disseminated pyrite is present throughout the alteration zone in low abundances (typically <<1%).

In detail, kaolinite-muscovite–bearing alteration is interlayered with more sulfidic horizons. This is not surprising, as ongoing alteration of footwall material would be expected in an actively discharging hydrothermal system during successive pulses of strong hydrothermal activity.

Mottled Mg chlorite and nodular anhydrite alteration

Mg chlorite + anhydrite alteration is observed from ~6 to 12.5 mbsf in Hole C0013C (bottom of hole), from ~6 to 23 mbsf in Hole C0013D, from ~5 to 23 mbsf in Hole C0013E, and from ~5.5 to 9 mbsf in Hole C0013F (bottom of hole). The assemblage is distinguished visually by a distinct bluish-gray color and the presence of coarse (typically 4–5 cm), white, opaque, hard, rounded nodules and brecciated, drilling-disturbed intervals (possible veins) of anhydrite (Fig. F20). In many cases, anhydrite nodules also contain lesser amounts of one or more of dolomite, calcite, quartz, or talc and 1%–2% fine-grained sphalerite and pyrite. Fine-grained disseminated pyrite and sphalerite are present in low abundances (typically <<1%) throughout the zone, and rare narrow irregular anhydrite-sulfide and quartz-sulfide veins are also observed.

Many anhydrite nodules within the mottled alteration zone show pitted surfaces and truncation of internal zonation—clear evidence of erosion following precipitation (Fig. F20). This evidence indicates that the nodules have been either transported to their present location or precipitated and then chemically eroded during the evolution of the hydrothermal system at Site C0013. It is not possible to distinguish between these two possibilities with the information available from the site to date, but either possibility implies that the nodular anhydrite is the earliest alteration event detected at Site C0013.

Quartz + Mg chlorite–altered volcanic basement

Material recovered from below ~26 mbsf at Site C0013 consists of moderately to highly quartz + Mg chlorite–altered volcaniclastic breccia and conglomerates, interpreted to be volcanic basement. These rocks are best represented in Core 331-C0013E-7L but were also recovered in Cores 331-C0013E-6X and 331-C0013E-9X, as well as Core 331-C0013D-4X.

The variation in volcanic texture of these rocks is discussed in “Lithostratigraphy.” The rocks are distinguished by their high hardness and distinct quartz stockwork veining (Fig. F21). Thin section examination reveals that ~40% of the rock remains as volcanic glass, with the remainder devitrified and replaced by quartz, chlorite, and minor biotite (Fig. F22).

Sulfide mineralization is scarce in the altered volcanic rocks, mostly limited to very fine grained disseminated pyrite euhedra scattered in trace quantities throughout. Most quartz veins are barren of mineralization. However, a handful of mineralized quartz-sulfide veins were noted in the intervals intersected and were observed to contain sphalerite, pyrite, covellite, and, in one interval, somewhat unexpectedly, fine intergrowths of native copper and organic carbon (Fig. F23).

Coarsely crystalline anhydrite veining

Coarsely crystalline euhedral anhydrite was intersected in Hole C0013C from ~5.5 to 9.0 mbsf, in Hole C0013D from ~4.0 to 12.0 mbsf, in Hole C0013E from ~4.5 to 11.5 mbsf, and in Hole C0013F from ~5.5 to 8.8 mbsf (bottom of hole) (Fig. F14). Although logged in drill core as “hydrothermal gravel” with a clay matrix, it is probable that the anhydrite crystals, which typically are between 1 and 2 cm in length, represent a vein network that has been heavily fragmented by drilling.

Anhydrite becomes insoluble in seawater at temperatures >150°C (Bischoff and Seyfried, 1978), requiring that downwelling seawater would precipitate anhydrite at or around the 150°C isotherm. The concentration of coarsely crystalline anhydrite across a relatively narrow depth interval at Site C0013 implies precipitation by this means. The coarsely crystalline anhydrite zone overlaps the transition, confirmed by XRD, from muscovite-kaolinite to Mg chlorite–dominated “clay” alteration at Site C0013, which occurs at ~6 mbsf in Holes C0013C, C0013D, and C0013F and at ~5 mbsf in C0013E. The presence of anhydrite veins across both alteration zones implies that the veins are unrelated to these alteration assemblages and represent a later overprinting event.

Significance of the transition in “clay” mineralogy

The transition from kaolinite-muscovite–rich to chlorite-rich rocks with increasing depth at Site C0013 is similar to the gradation from paragonitized to chloritized rocks that is documented for the basement underneath the Trans-Atlantic Geotraverse hydrothermal mound at the Mid-Atlantic Ridge (26°N) (Humphris et al., 1995). The difference in mineralogy between Iheya North Knoll and the Mid-Atlantic Ridge is partially explained by the lack of iron and the abundance of potassium available within the sequence at the site when compared with the mafic volcanic rocks of the mid-ocean-ridge setting.

The change in alteration mineralogy is consistent with the generally accepted compositional evolution of upwelling hydrothermal fluids in seawater-dominated submarine systems. Along the fluid flow path in a hydrothermal convection cell, it can be anticipated that Mg fixation in chlorite leads to a decrease in fluid pH, leading in turn to an increase in fluid K content, which should stabilize potassic phases such as muscovite and kaolinite in the shallower parts of a felsic-hosted system (Seyfried et al., 1999).

In their study of hydrothermal clay alteration associated with high-temperature discharge and sulfide mineralization at the Jade hydrothermal field, Izena Caldera, also in the Okinawa Trough, Marumo and Hattori (1999) report a similar vertical transition from Mg chlorite at depth to near-surface kaolinite and native sulfur. They interpret this variation to reflect variable amounts of cold seawater incorporated into hot hydrothermal fluids in subseafloor sediments and tuff. They note, however, that mixing alone cannot generate sufficiently low fluid pH to explain the abundance of kaolinite minerals at the Jade hydrothermal field. They infer that low pH values are attained through oxidation of H₂S dissolved in the hydrothermal fluid or released from the fluid during decompression. We propose a similar model for Site C0013, supported by the high H₂S values recorded for many of the cores recovered at the site. In this model, the Mg chlorite–dominated alteration observed from 5 to 6 mbsf at Site C0013 and the less intense kaolinite-muscovite alteration appear to be part of a single alteration event, produced by the chemical evolution of upwelling high-temperature hydrothermal fluid. Therefore, at the time at which this alteration formed, Site C0013 was likely a site of hydrothermal discharge.

Accurate temperature estimation based on phyllosilicate and clay mineralogy in active hydrothermal systems is notoriously difficult (see for example, the discussion in Binns et al., 2007), as mineral stability is strongly influenced by fluid chemistry (which may vary rapidly, both temporally and spatially), rock composition, and confining pressure, and in many cases samples are in disequilibrium, limiting the use of many experimental studies. Although kaolinite may be stable at temperatures of up to ~290°C (Hurst and Kunkle, 1985) its stability field broadens with decreasing temperature, and it is most commonly found associated with lower temperature (~150°C) hydrothermal activity (Marumo, 1989). The predominance of Mg chlorite below ~6 mbsf at Site C0013 suggests alteration temperatures in the range of ~220°–300°C through this interval (Browne, 1978; Árkai, 2002).

Relative timing of alteration and mineralization at Site C0013

Based on the limited drilling at Site C0013, the following sequence is inferred for the evolution of hydrothermal activity, summarized schematically in Figure F24:

1. The earliest alteration event preserved is represented by the white anhydrite ± dolomite ± talc ± calcite ± quartz ± sphalerite ± pyrite nodules and veins. As was noted above, this material shows evidence of either mechanical or chemical erosion, implying that it predates the pervasive alteration that affects the drilled sequence at the site. It is possible that this early anhydrite was initially formed by a very similar mechanism to that proposed for the coarsely crystalline anhydrite veins that are also seen (see below).

2. As discussed above, quartz + Mg chlorite alteration of volcanic basement, pervasive Mg chlorite alteration of sediment, and near-surface kaolinite-muscovite alteration are interpreted to be part of a single alteration event in a discharging hydrothermal system. This event is the second phase of alteration recorded at Site C0013. Variably sulfidic sediments at the top of the sequence are interpreted to represent the eroded remnants of sulfide chimneys that once stood at the site.

3. As was discussed previously, it is likely that the coarse-grained crystalline anhydrite veining was formed by heating of downwelling seawater to a temperature >150°C. Given the disturbed nature of much of the core recovered at the site, clear textural relationships are not observed. However, it is likely that this alteration event postdates the other hydrothermal alteration and records a recent influx of seawater into the system after discharge at the site ceased.

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