Subunit VIA is characterized by sulfide veins and minor sulfide disseminations. Ten samples from Subunit VIA were chosen for thin-section analysis: two from Hole 856H and eight from Hole 1035F. Two distinctive textures are evident within these samples: sulfide infilling of sedimentary features and subvertical sulfide veins (Fig. F4). Within the samples studied, pyrrhotite and chalcopyrite most commonly infill sedimentary pore structures. Euhedral pyrite crystals and minor (<5%) magnetite are common and generally present with chalcopyrite. Chalcopyrite is also present as a secondary mineral adjacent to pyrrhotite. In Hole 1035F, sulfide infilling of sedimentary pore spaces is similar; however, unlike the samples from Hole 856H, vein assemblages are more common than disseminated sulfide. Most of the disseminated sulfide in Hole 1035F is present as chalcopyrite impregnations (~0.5 mm). Magnetite (modal abundance of 5%-10%) is commonly found along vugs and near chalcopyrite in ~0.2-mm subhedral crystals. In interval 169-1035F-12R-1, 78-93 cm, disseminations are primarily pyrrhotite with minor pyrite alteration rims, as opposed to chalcopyrite disseminations previously described. In summary, the sulfide textures in Subunit VIA in both Holes 856H and 1035F are characterized by pyrrhotite and chalcopyrite infilling or replacement of the primary sedimentary texture.
Subvertical sulfide veins comprise the second dominant sulfide texture within Subunit VIA. Two types of veins are within this subunit. Veins in Subunit VIA of Hole 856H are characterized by Mode 1 extension fractures with one mineralization event, often evidenced by syntaxial textures. Secondary alteration of sulfide phases may occur subsequent to mineral infilling. Hole 1035F also has extension veins characterized by a single episode of mineral infilling, but multiple-event sulfide veins are also present. Multiple stages of fluid flow are evidenced by classic crack-seal textures and crosscutting networks of multiple narrow veins.
Veins in Hole 856H perpendicularly crosscut the bedding-parallel sulfide-impregnated layers. Typically, veins in Hole 856H are composed of pyrite with lesser amounts (<5%) of magnetite. Chalcopyrite commonly rims pyrite crystals. Pyrrhotite is also present at the rim of some pyrite veins. In Sample 169-856H-21R-1, 96-97 cm, the crosscutting vein is linked with a horizontal vein. The vertical and horizontal veins have a similar composition: pyrrhotite crystals (0.75-1.0 mm) with secondary Fe silicates along the cleavage traces and subhedral to euhedral sphalerite with pyrite.
In Core 169-1035F-11R, extension veins are lined with euhedral quartz crystals, whereas the interiors of the veins are infilled with sulfides (Fig. F5). The quartz-lined fractures are infilled with pyrrhotite (80%-90%), chalcopyrite (8%-15%), and magnetite (2%-5%). Many of these veins exhibit a distinctive Fe silicate alteration halo, and the sediments surrounding the veins are typically chloritized. These sections have a modal abundance of 20%-25% sulfide since sulfides are found not only within the veins but also as dissemination within the section.
Starting in Core 169-1035F-12R, the vein assemblages become more complex. Infilling of extension fractures remains the dominant vein formation mechanism; however, at least four sulfide phases exist within the veins, and anhydrite borders the vein (Fig. F6). Veins in this core are composed of pyrrhotite, sphalerite, pyrite, and chalcopyrite. Pyrrhotite is present as distinctive 0.1- to 0.25-mm crystals with Fe silicate alteration along the cleavage traces. Pyrite is present as subhedral to euhedral cubic crystals in larger veins (aperture of 1-2 mm), and in euhedral crystals with chalcopyrite and minor sphalerite in narrower (aperture <0.5 mm) and later veins. The other primary sulfide assemblage within this core is chalcopyrite, present as blebs and inclusions within the altered sediment (Fig. F7). The larger veins in Sample 169-1035F-12R-1, 39-42 cm, exhibit a greater degree of alteration, which is consistent with the presence of anhydrite vein selvages bordering the veins. The anhydrite vein selvages extend 0.5 to 1.0 mm from the vein and are subhedral to euhedral anhydrite blades. Throughout this core, anhydrite is bladed and radiating, suggesting renewed hydrothermal fluid flow since sulfate is not stable at the high temperatures consistent with the vein sulfides.
In summary, the sequence of sulfide deposition within the sulfide veined sediments in Holes 856H and 1035F are as follows:
Subunit VIB is composed of sediment with 2-10 vol% sulfide veins and/or impregnations (Fig. F8). The total sulfide content is less in this subunit than in Subunit VIA. Three samples were chosen from this unit: one from Hole 856H and two from Hole 1035H. The host sediments are typically altered to fibrous chlorite (needles <0.1 mm in length) that overgrows quartz grain boundaries. Quartz comprises ~40% of these sections.
Two types of sulfide deposition, similar to those found in Subunit VIA, also are within these samples. Chalcopyrite is present as both small (<0.2 mm) impregnations in sediment and as subvertical veins. This is in contrast to the veins in Subunit VIA, which are dominantly pyrite or pyrrhotite. The concentration of chalcopyrite disseminations in sediment decreases away from the subvertical veins. This is in contrast to the vein patterns evident in Subunit VIA, where there is no correlation between disseminated sulfide concentration and proximity to the vein.
The final sulfide feeder unit is the sulfide-banded and impregnated sediments where sulfides comprise 10-50 vol% of the core (Fig. F9). Eight thin sections were studied from Holes 856H and 1035H. Bands of chalcopyrite/isocubanite, which parallel sedimentary bedding, characterize these samples. Sulfide bands have either equant grain boundaries or rounded grain boundaries that result from alteration of the sulfide phases. Chalcopyrite is found with euhedral quartz and minor pyrrhotite and generally precipitates within the primary sedimentary pore spaces (Fig. F10).
In Hole 1035H, the sulfide bands are composed of predominantly cubic pyrite crystals 0.75-1.5 mm in size. The pyrite bands comprise ~50% of the opaque phases. Pyrrhotite stringers found throughout the pyrite have apertures of <0.1 mm and comprise ~15% of the opaque phases. Pyrrhotite is also found along the edges of the pyrite cubes (Fig. F11). Hematite commonly is found at the center of the veins and within large pyrite crystals. Hematite, as an alteration product, comprises ~30% of many samples. Narrow (<0.2 mm) veins of sphalerite and magnetite crosscut pyrite. Deeper within Hole 1035H (Core 21R), sulfide bands composed of chalcopyrite/isocubanite precipitated in the primary sedimentary pore structure.
In summary, the sulfide-banded unit is characterized by chalcopyrite/isocubanite banding that infills the primary sedimentary pore structure. Quartz and pyrrhotite are common, whereas sulfides contemporaneous with chalcopyrite are less abundant.
In contrast, pyrite is the dominant sulfide phase and pyrrhotite stringers crosscut and replace pyrite within Core 169-1035H-17R.