STRUCTURAL GEOLOGY

Two holes (Holes 1189A and 1189B) were cored at Site 1189, situated ~30 m apart. Hole 1189A was cored from the seafloor, whereas Hole 1189B was cased and coring started at 31 mbsf. The structures identified in the two holes were primary volcanic layering, brecciation of volcanic rocks, orientation of veins, and age relationship between veins. The structures were described on the structural visual core description forms, and the data were entered in the structural log (see "Site 1189 Visual Core Descriptions" and "Site 1189 Structural Logs").

Hole 1189A

Orientation of Volcanic Layering

Primary volcanic layering (So in the structural logs) was represented in the cores by trails of elongated vesicles and by millimeter-scale laminations, between 0.5 and 2 mm thick, which were interpreted to be flow banding. The 26 measurements show that the dip of primary layering varies from horizontal to vertical and that there is no obvious relationship between dip variation and depth in the hole (Fig. F99). The wide range of dips could be caused by the hole intersecting several detached and rotated lava blocks and/or different parts of lobate lava flows with differing primary dips within or between flows. Because of the very low recovery of core in the hole, it is impossible to offer any definite conclusion as to the meaning of these variations in dip. However, the intersection of thick sequences of breccia with rotated clasts suggests that block rotation is likely to be an important factor.

Breccias

Two different types of breccias were intersected in Hole 1189A. In lithologic Units 3 and 4 (i.e., interval 193-1189A-2R-1, 93-136 cm [10.63-11.06 mbsf]), 2- to 50-mm-long angular fragments of altered volcanic rocks are present in a matrix of coarse anhydrite, minor quartz, and scattered grains of pyrite (Fig. F100). The fragments vary in color from white to blue-green, presumably because of differences in clay mineral and chlorite content. These differences between the individual fragments show that they represent volcanic rocks that have gone through different degrees of alteration, but also that the alteration must have preceded the brecciation. The contacts between the fragments and the matrix are sharp, which show that little or no alteration has taken place during fragmentation and later cementation by the anhydrite matrix. A similar breccia is present in Unit 6 (i.e., interval 193-1189A-3R-1, 56-63 cm [19.96-20.03 mbsf]). Here, the anhydrite cement and flow-banded green clay-chlorite altered volcanic fragments are crosscut by quartz veins with minor pyrite (Fig. F101). Microscopic examination confirms that there are no alteration halos at the contacts between the fragments and the matrix.

Alteration both preceded and accompanied brecciation in Unit 9, the interval from 38.8 mbsf (i.e., Section 193-1189A-5R-1, 0 cm) to 49.22 mbsf (i.e., Section 193-1189A-6R-1, 62 cm). During the first stage, the volcanic rocks were completely altered to an assemblage of mainly white phyllosilicates (illite and pyrophyllite) and silica. During the second stage, the altered volcanic rocks were brecciated and the fragments altered to green chlorite and smectite (Fig. F102). In contrast to the breccias encountered in Units 3, 4, and 6 (Sections 193-1189A-2R-1 and 3R-1; discussed in the previous paragraph), the contacts between the gray-green matrix and the fragments are diffuse. The degree of alteration of the fragments varies from incipient to complete. Relict flow lamination was recognized in several of the fragments, and in many cases the clasts could be fitted together. Incipient brecciation was noted in several pieces of core in Unit 9 (i.e., interval 193-1189A-6R-1, 31-42 cm [48.91-49.02 mbsf]), where a 2- to 3-cm silica-pyrite-anhydrite vein contains several 2- to 10-mm unoriented fragments of flow-laminated volcanic rock and thin offshoots from the main vein intrude into the wall rock.

Similar types of breccias were also found in many of the pieces in Units 9, 10, 16, and 17 (i.e., Sections 193-1189A-6R-1, 7R-1, and 9R-1), alternating with pieces containing veins and vein-network structures, some showing minor brecciation.

Vein Mineralogy and Paragenesis

The veins in Hole 1189A consist of varying proportions of quartz, anhydrite, and pyrite. Minor chalcopyrite is associated with pyrite in some veins in Units 5, 16, 17, and 19 (i.e., Sections 193-1189A-3R-1, 9R-1, and 10R-1). Most of the veins are dominated by quartz with or without anhydrite and minor pyrite (Fig. F103). This is especially the case for Units 8, 9, 10, 15, and 19, where the veins form networks of <0.5- to 2-mm-thick veins and veinlets (Fig. F104). In Unit 19, some veins are dominated by quartz and some by anhydrite (Fig. F103).

In the brecciated volcanic rocks, especially in Unit 9 (i.e., Sections 193-1189A-5R-1 and 6R-1, as described above), quartz-pyrite-anhydrite veins cut across and replace both earlier infill and veins of anhydrite between the altered volcanic fragments. In a few cases, thin anhydrite veins cut across earlier anhydrite ± pyrite veins with alteration halos. Also, in some of the thin sections, hairline veins and fractures filled with quartz cut across veins of predominantly anhydrite or quartz.

Alteration halos around the veins in the brecciated units are typically gray to green, apparently consisting of clay and/or chlorite. In most other cases, the alteration halos consist of quartz, in one unit as poikiloblasts overgrowing the groundmass of the altered volcanic rocks (Fig. F105A, F105B). Rare second generation anhydrite and quartz veins (only seen in thin sections) do not seem to have any halos associated with them.

Microscopic examination shows that in many cases the veins exhibit a distinct zonation. Where anhydrite is present, it tends to occupy the center of the veins, whereas quartz forms the selvages (Fig. F105C, F105D). Pyrite and locally chalcopyrite are associated with the anhydrite. Pyrite in many cases rims the veins (Fig. F105A, F105C).

Vein Geometries

With respect to vein thickness, ~70% of the veins in Hole 1189A are <2 mm thick, whereas only 2% are thicker than 10 mm (Fig. F106A). Although the anhydrite-dominated veins are generally thinner than the quartz veins, the thickest veins consist of anhydrite with minor quartz and pyrite (the thickest is a vein >30 mm in width in Unit 7 [i.e., interval 193-1189A-3R-1, 89-94 cm]).

The dips of the veins range from horizontal to vertical. However, most of the veins have dips between 30° and 60° (Fig. F106B). There is no obvious systematic variation in the dips of the veins with depth in the hole (Fig. F106C). This is possibly because almost all the measurements were taken over the interval from 60 to 90 mbsf.

Hole 1189B

Orientation of Primary Volcanic Structures

Primary volcanic layering was identified in the cores of Hole 1189B by alternating millimeter-thick lamina, marked by differences in color from gray to green or white. This structure was interpreted to represent flow banding that had undergone different degrees of silica-clay alteration. Layering was also defined by trails of aligned, stretched, and flattened vesicles. Trails of plagioclase phenocrysts were also used to define the primary layering, although such indicators were rare. With three exceptions, the 15 measurements shown in Figure F107 are from lithologic Unit 19, between 128 and 139 mbsf, which is a highly silicified vesicular volcanic rock (see "Igneous Petrology"). The vesicles are variably stretched and flattened. The dips of the layering (So) and the plunges of stretched vesicles (Lo) range between 33° and 90° in this interval with most between 65° and 90° (Fig. F107). This localized steep layering and coeval stretching of vesicles over a thickness of >10 m could indicate that the hole intersected a major zone of upflow of lava, or even a dike.

Breccias and Vein Network Structures

Hole 1189B is characterized by brecciated rocks and vein network (stockwork) structures. Three intervals of breccias and veins were intersected, separated by highly altered, gray, siliceous volcanic rock crosscut by only a few veins. The breccia-vein network intervals are Unit 1 (i.e., Section 193-1189B-1R-1, 0 cm [31.00 mbsf]) to Unit 18 (i.e., Section 11R-1, 43 cm [128.03 mbsf]), Unit 20 (i.e., Section 13R-1, 20 cm [147.20 mbsf]) to Unit 26 (i.e., Section 16R-1, 74 cm [176.44 mbsf]), and Unit 32 (i.e., Section 17R-1, 104 cm [186.34 mbsf]) to Unit 36 (i.e., Section 18R-2, 115 cm [197.57 mbsf]), which is the bottom of the hole.

In the interval 31.00-128.03 mbsf, the brecciated rocks contain irregular fragments of highly altered volcanic rock, partly with flow banding or vesicles still preserved (Fig. F108). In some of the pieces, the breccias are polymict (i.e., contain fragments of different composition or texture). In most cases the breccias are matrix supported. The matrix or cement between the fragments above 49.7 mbsf consists predominantly of anhydrite with rare gypsum, whereas quartz dominates between 49.7 and 128 mbsf. Microscopic examination showed that gypsum, where present, replaces anhydrite. Pyrite is present throughout as part of the matrix, locally with minor amounts of chalcopyrite. The pieces of rock between the intervals of brecciated rock consist predominantly of highly altered volcanic rocks, crosscut by a fine network of quartz-pyrite veins.

In the interval between 147.20 and 176.44 mbsf, the brecciated rocks predominantly contain fragments of flow-banded volcanic rocks (Fig. F109). In contrast to the interval described in the previous paragraph, the brecciated pieces in this interval are commonly clast-supported (cf. Figs. F108, F109). The matrix here is mainly quartz with variable amounts of anhydrite and pyrite. In places, late anhydrite veins cut across both the matrix and fragments (Fig. F109).

Brecciation in this interval is caused by both volcanic and hydrothermal processes. In rare cases (Fig. F23), ductile flow facies are seen to cut brecciated rocks. Such examples are clear evidence of autoclastic brecciation.

Another example of brecciation and network veining is shown in Figure F110. In this case, the veins and open-space fill consist of quartz, anhydrite, sphalerite, pyrite, and trace chalcopyrite. Quartz forms the rim of the veins, and euhedral sphalerite and pyrite have been crystallized farther into the veins and into what must have formed as open spaces, whereas coarse anhydrite crystallized as the last phase in the middle of the veins and in the open spaces. The sphalerite has a light brown, Zn-rich core, whereas the rim is darker and Fe rich. In this case the brecciation is clearly secondary, affecting a primarily coherent volcanic rock. The different fragments all fit together and form a jigsaw-fit breccia structure. Movements between the fragments are evident, as traced by prominent flow bands (some of which are labeled A-G on Fig. F110B).

Another example of secondary brecciation and network veining is shown in Figure F111. Here, the veins consist of quartz, magnetite, and pyrite. Magnetite is located principally in the vein selvages, whereas pyrite occupies the center of the thicker veins and open spaces. The fluids affected the rocks by forming clay-silica alteration halos around the veins. Secondary hairline anhydrite veinlets cut across the breccia and vein network.

In general, vein-network formation obviously was later than the primary volcaniclastic brecciation of the rock. This is shown in Figure F109, where a dense network of mainly silica (according to XRD measurements, it is probably cristobalite; see "Hydrothermal Alteration") cuts across the fragments and the flow banding. In places, the fine and small veins are surrounded by halos of silica-clay alteration (Fig. F112).

The interval of brecciated rocks and vein networks from 186.34 mbsf to the bottom of the hole at 197.57 mbsf is very similar to the previous interval. However, veins and open-space fill in this interval contain fine-grained hematite together with quartz and anhydrite.

Vein Mineralogy and Paragenesis

The veins in Hole 1189B may contain quartz, anhydrite, pyrite, magnetite, and/or hematite with minor to trace sphalerite, chalcopyrite, and gypsum. More than 90% of the veins are dominated by quartz (Fig. F113). Veins dominated by anhydrite, partly replaced by gypsum, are found mainly in the networks in the upper part of the hole, especially in Units 1 to 3 (above 49.7 mbsf) and in veins intersecting the thick, rather monotonous, volcanic rock of Unit 19 (Fig. F114). Anhydrite also commonly forms late vein structures, crosscutting earlier quartz-dominated veins (see Figs. F109, F111, F115).

Hematite is present in the vein network and breccias in Unit 5 (interval 193-1189B-6R-1, 0-56 cm). Otherwise, hematite is found in the brecciated rocks and vein network in the lower part of the hole (186.34-197.57 mbsf), as described above. Magnetite is common in the network veins in Unit 26 (interval 193-1189B-16R-1, 7-88 cm) and in some of the veins in the upper part of Unit 32 (interval 193-1189B-17R-1, 104-145 cm), where it appears in selvages and halos around quartz veins with minor pyrite.

With respect to sulfides, pyrite is present as a minor component in most of the veins, whether they are single veins, or parts of a vein network. Sphalerite is a minor component in the vein networks in Unit 13 (i.e., Section 193-1189B-10R-1, Pieces 1 and 3) and is a trace component in the vein network in Units 32 through 34 (i.e., interval 193-1189B-18R-1, 0-142 cm). Chalcopyrite is a minor component in the breccias in the upper part of the hole in Units 1 through 3 (Sections 193-1189B-1R-1 and 2R-1) and is otherwise found in trace amounts, commonly associated with pyrite or sphalerite throughout the hole.

The veins in Hole 1189B can be divided into two generations. The second generation consists of late anhydrite veins, which cut across first-generation veins and vein networks consisting predominantly of quartz and pyrite ± iron oxides (Figs. F109, F111, F115).

Vein Geometries

Of the veins in Hole 1189B, 52% are 1 mm thick or less, ~40% are between 1 and 2 mm thick, and <1% are thicker than 10 mm (Fig. F116A). Most of the veins <1 mm thick belong to the fine vein network of quartz and pyrite in the brecciated intervals.

The dips of the veins range from horizontal to vertical. However, >50% of the veins have dips between 30° and 60° (Fig. F116B). In some cases, the veins tend to follow primary layering in the volcanic rocks (Fig. F115). There appears to be no systematic variation in dip of the veins with depth in the hole (Fig. F116C).

Summary

Holes 1189A and 1189B are ~30 m apart, and the vertical difference between their collars is ~6 m; therefore, the holes together provide an insight into the lateral variation of the Roman Ruins hydrothermal site.

There are several interesting similarities and differences between the two holes with respect to vein structures:

  1. The volcanic rocks in Hole 1189B are much more brecciated than the rocks in Hole 1189A.
  2. The vein intensity is higher and intervals of network veining are thicker in Hole 1189B.
  3. The brecciated rocks in the two holes are very similar, consisting of variably altered volcanic fragments and crosscut by vein networks of quartz with pyrite and minor anhydrite. However, in contrast to Hole 1189A, magnetite and hematite are present in the networks as minor components, and sphalerite and chalcopyrite are present as trace minerals in Hole 1189B.
  4. In both holes, late coarse-grained anhydrite veins crosscut vein networks and brecciated rocks.
  5. The distribution of dips of the veins in the two holes is very similar; in both, veins dipping between 30° and 60° are the most common.

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