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doi:10.2204/iodp.proc.331.106.2011

Petrology

Two holes were attempted at Site C0016, the NBC hydrothermal sulfide mound at Iheya North Knoll.

Hole C0016A was spudded without a guide base into the crest of the NBC Mound. The hole failed spectacularly (see “Operations”), with a broken drill pipe and the bit left in formation. No core was recovered. We did observe flashing due to phase separation in video images from the ROV, both in vents at the top of the mound within 2 m of the hole prior to drilling and also during drilling within the hole itself. These observations make it extremely unlikely that the fluid forming the NBC Mound has undergone phase separation within the underlying substrate.

Hole C0016B was drilled with a guide base immediately at the foot of the NBC mound, 20 m west of Hole C0016A. It was cored over an interval of 0 to 45 mbsf, with a recovery of only 2.1 m (4.7%) of core. As a result of this poor recovery, it is not possible to provide a holistic description of the alteration and mineralization within the sequence drilled. Instead, we detail the material collected in each core and provide a hypothesis on the relationships among the samples.

Core 331-C0016B-1L (0–9 mbsf)

We recovered 79 cm of competent hard rock as Core 331-C0016B-1L. During drilling, the bit penetrated very rapidly to ~6 mbsf and then slowed dramatically. It is therefore likely that the upper portion of the hole consists of soft and/or unconsolidated material and that the material recovered is from 6–9 mbsf.

Massive and semimassive sulfide

Intervals 331-C0016-1L-1, 0–31 cm, and 1L-1, 34–64 cm, comprise hard black clastic-textured massive sulfide (Fig. F2). The two intervals are similar, with the upper described in drill core as comprising rounded 1–5 mm fragments of ~10% soft-clay altered volcanic rock and ~10% hard siliceous volcanic rock, cemented in a hard matrix of ~60% sphalerite, 15% pyrite, and quartz. Coarser grained (2–3 mm) sphalerite, pyrite, galena, and possible chalcopyrite are associated with a late anhydrite vein down one side of the section.

The second interval of massive sulfide, interval 331-C0016-1L-1, 34–64 cm, is similar to the upper interval, with less abundant altered clasts (5%) and scattered irregular 2–4 cm patches of pale clay with sugary anhydrite (interpreted as possible late void fill). Sulfide is more abundant in the lower interval and is visually estimated as ~85% of the total volume, with half of this being pyrite.

A 3 cm interval separates the massive sulfide units and consists of hard gray, strongly silicified material with its clastic texture almost obliterated by silicification and only 15% sphalerite and 10% pyrite (visually estimated) (Fig. F2). This interval is a discrete fragment within the core, so its relationship to the surrounding massive sulfide intervals, the true depth interval it represents, and to what degree it is representative of that interval cannot be determined with any degree of certainty.

A polished thin section was cut of the upper massive sulfide interval. The section is estimated to contain 40% sphalerite, 10% pyrite, 4% galena, 1% chalcopyrite, 20% muscovite/illite, and 15% quartz, a mineralogy which is broadly in agreement with both the visual description of the core and the interpretation of X-ray diffraction (XRD) results for the massive sulfide (Table T2). In the section, illite/muscovite-pyrite altered clasts of volcanic rock, typically ~5 mm in size, and fragments of quartz, typically 1–2 mm in size, are cemented in a crystalline matrix of quartz-muscovite-sulfide (pyrite-sphalerite-chalcopyrite-galena) and late coarsely crystalline anhydrite (Fig. F3A, F3B).

Consistent paragenesis is present for the sulfide and sulfate phases in the section. Early iron-poor sphalerite appears to be detrital in origin and typically occurs as 0.5 mm subhedral, slightly rounded crystals, which commonly exhibit chalcopyrite disease (a dusting of submicrometer-sized “blebs” of chalcopyrite in sphalerite). This first generation of sulfide is overgrown by the other sulfide phases present in the sample. Pyrite occurs as euhedral to subhedral grains, both as single crystals and as overgrowths, with galena, on early sphalerite and is itself replaced by and overgrown by chalcopyrite (Fig. F3C). A second generation of iron-poor sphalerite overgrows early sphalerite (Fig. F3D) and other sulfide phases and also forms colloform- and atoll-textured aggregates (Fig. F3E), implying crystallization into open space. The final phase of precipitation is represented by coarse 2–3 mm anhydrite crystals, which overgrow all other phases in the sample (Fig. F3F).

Silicifed volcanic rock

The bottommost 15 cm of Core 331-C0016-1L consists of hard gray strongly silicified volcanic rock with ~5% coarsely disseminated 1 mm pyrite and sphalerite throughout the body of the rock. Coarser 2–3 mm sulfide (pyrite-sphalerite-minor galena) is associated with a quartz-anhydrite-10% sulfide vein that runs from the top of the section to ~10 cm (Fig. F4). Fine 1–2 mm wide irregular anastomosing veinlets of quartz with 10% sulfide also cut the rock. Like the semimassive sulfide farther up the core, the silicified volcanic rock is a discrete piece of core, so its spatial relationship to the overlying sulfide interval cannot be determined.

A polished thin section of silicified volcanic rock contained an estimated 60% quartz, 25% muscovite/illite, 5% anhydrite, 5% sphalerite, 5% pyrite, and trace quantities of galena and chalcopyrite, a mineralogy which is broadly in agreement with both the visual description of the core and the interpretation of XRD results for the massive sulfide (Table T2). The section shows intergrown quartz-muscovite/illite-pyrite and sphalerite (Fig. F5A, F5B), with late coarse anhydrite veining and open space fill (Fig. F5C). There is no visible evidence of volcanic fabric in the thin section.

The most abundant sulfides are 1 mm subhedral sphalerite and 0.1 mm euhedral pyrite. The disseminated and vein sulfides display similar paragenesis to that seen in the overlying massive sulfide (see above). Early sphalerite is locally overgrown by a second generation of sphalerite or pyrite and/or minor galena. Pyrite shows replacement by and overgrowths of chalcopyrite (Fig. F5D, F5E).

Core 331-C0016B-2L (9–27 mbsf)

A total recovery of 31 cm was recorded for Core 331-C0016B-2L, with three distinct lithologies represented by three discrete pieces of core (Fig. F6).

Silicified volcanic rock

The uppermost 7 cm of the core comprises hard gray strongly silicified volcanic rock with a faint remnant clastic texture. About 5% pyrite and trace sphalerite are disseminated as very fine crystals and scattered coarser 0.1 mm euhedra throughout the rock. This unit is similar to the bottommost 15 cm recovered in Core 331-C0016B-1L and has a similar mineralogy, with quartz, pyrite, sphalerite, and minor muscovite detected by XRD (Table T2).

Coarse anhydrite

Interval 331-C0016B-2L-CC, 7–19 cm, is composed of a snow white coarsely crystalline anhydrite aggregate composed of 1–2 cm tightly intergrown acicular crystals (Fig. F6). The crystals show rough alignment in part of the sample, but their orientation relative to the core is meaningless because of the uncertainty of the true orientation of the sample. Most of the material is sulfide-free, except for a dark vein of 1–2 mm euhedral sphalerite-minor pyrite that lies along one edge of the interval.

Quartz-clay altered volcaniclastic breccia

Interval 331-C0016B-2L-CC, 20–31 cm (19–20 cm is void space) is volcanic breccia containing subangular 2 mm to 3 cm (dominantly 5–10 mm) soft gray clay–altered and slightly harder white sugary silica-clay-anhydrite-altered volcanic clasts hosted in a dark hard siliceous matrix (Fig. F6). Fine-grained pyrite is disseminated throughout the matrix, averaging ~3%, although the distribution is uneven. The clasts are also unevenly pyritic, containing up to 5% fine-grained pyrite as disseminations and occasional vesicle fill. The edge of an anhydrite-sphalerite-pyrite-trace galena vein runs along the top of the fragment. The vein is clearly truncated by drilling and is up to 1 cm wide, as exposed.

This breccia is similar in texture and structure to the material recovered from below 26 mbsf at Site C0013, although unlike that material, Mg chlorite was not detected via XRD at Site C0016. The mineralogy of the rock by XRD is quartz, pyrite, anhydrite, and muscovite/illite.

Core 331-C0016B-3L (27–45 mbsf)

Quartz-chlorite altered volcanic rock

A total of 99.5 cm of material was recovered as Core 331-C0016B-3L. The entire core consists of dark green to gray quartz-chlorite-altered volcanic rock with abundant stockwork veining (see below). The groundmass of the rock exhibits faintly defined fragments as large as ~1 cm in diameter, which may be clasts or which may have been generated by hydraulic fracture and veining of the rock. Very fine grained pyrite is disseminated throughout the rock at trace levels. XRD analysis identified an assemblage of quartz, chlorite, and pyrite.

Two generations of veining are present in the rock (Fig. F7):