HYDROTHERMAL ALTERATION

Hydrothermally altered volcanic rocks were recovered at Site 1189 beneath the Roman Ruins hydrothermal site, a site of active, high-temperature (>240C) (Douville et al., 1999) fluid outflow and sulfide chimney development at the PACMANUS hydrothermal field (Tables T2, T4; Figs. F2, F17, F46, F47). At Site 1189, two RCB holes, Hole 1189A and Hole 1189B, were cored over intervals of 0.0-125.8 and 31.0-206.0 mbsf, respectively. Although the two holes are located only 30 m apart within the chimney field, the exceptionally poor recovery in the upper cored portion of Hole 1189B makes it extremely difficult to effectively compare them (total recovered material in Cores 193-1189B-1R to 10R between 31.0 and 127.6 mbsf is 1.49 m, at an average recovery of 1.54%). Nevertheless, it is apparent that Hole 1189B intersected a much more strongly mineralized sequence than the equivalent interval in Hole 1189A, indicating rapid lateral variation in alteration style within the volcanic sequence below Roman Ruins hydrothermal site. This is not surprising, given the focused nature of hydrothermal outflow onto the seafloor at the site, although both holes were located near chimneys.

General Overview

The following detailed description of hydrothermal alteration in coherent volcanic rocks and breccias from beneath the Roman Ruins chimney site is based primarily on visual descriptions of cores from Hole 1189A and Hole 1189B and is supplemented by thin-section petrography (Fig. F48) and XRD analyses of bulk samples (Figs. F46, F47). The portable infrared mineral analyzer (PIMA) analyses were also conducted on a large number of core samples from Hole 1189A. However, the spectra were not processed on board, and it was not possible to utilize PIMA results for this report.

Alteration was classified in the core primarily by the visual detection under binocular microscope of the following key minerals: soft green to bluish-green clay; soft white clay, and hard silica polymorphs. Sulfide and oxide mineralization are readily distinguished by the presence of abundant pyrite and other sulfides (principally chalcopyrite and sphalerite) and rare jasperoidal silica-iron oxide, respectively. These rocks are discussed more fully in "Sulfide and Oxide Petrology". Sulfides, predominantly pyrite, are also present as minor to trace phases within altered rocks throughout the drilled section.

Alteration Types and Relative Timing of Alteration Events

Whereas the lithology summaries (Tables T2, T4) reflect the dominant alteration type in any particular unit, it is common that multiple overprinting alteration styles can be distinguished in individual core pieces from Site 1189. Similar to Site 1188 (see "Hydrothermal Alteration" in the "Site 1188" chapter), alteration appears to be multistage, with a general evolution from early green silica-clay (GSC) alteration, followed by pale gray to white silica-white clay-anhydrite bleaching, and finally hard silicification. Late anhydrite quartz pyrite veins are present in cores from Site 1189. However, they were much less abundantly recovered than similar veins seen at Site 1188 and do not exhibit the bleached, banded alteration halos that are commonly developed around these veins at Snowcap hydrothermal site.

Variation of Alteration with Depth

Alteration at Site 1189 shows a similar variation with depth to that seen at Site 1188. However, a steeper temperature gradient is suggested, as equivalent transitions are developed at shallower depths. In Hole 1189A, only four pieces of fresh dacite, with alteration restricted to films of opaline silica and clay on fracture surfaces, were recovered before penetrating a sequence of bleached volcanic rocks with cristobalite, mixed-layer chlorite-smectite phases, and anhydrite as the principal secondary minerals (Fig. F46; Tables T2, T5). These variably bleached rocks extend to a curated depth of ~10 mbsf and are underlain by pervasively and completely altered, flow-banded, brecciated, GSC altered rocks, intercalated with more coherent, bleached, vesicular volcanic units. The mineralogy of this interval, which extends to a depth of 68 mbsf, is dominated by illite, chlorite, and either cristobalite (to 25 mbsf) or quartz (below 25 mbsf). In Hole 1189A, the transition between cristobalite and quartz is sharp and shallow, when compared to the similar transition at Site 1188, where it is gradual between 100 and 120 mbsf. Within the intercalated GSC altered and bleached rocks at Site 1189, patchy silicification is present and becomes more pronounced downhole, as indicated by the increasing quartz content and hardness of the rocks. A zone of poikiloblastic silicification is present between 68 and 78 mbsf in Hole 1189A, in which illite is rare and smectite and chlorite are the dominant phyllosilicates. Despite the common development of poikiloblastic quartz in this lithologic unit, the extent of alteration is not as high as in the overlying rocks, and the unit contains abundant fresh plagioclase microlites and phenocrysts. Similar units with poikiloblastic quartz and relict fresh plagioclase are also present in the deeper part of Hole 1189B (>120 mbsf). Potassium feldspar is also present and is considered to be an alteration product, although this could not be confirmed unequivocally by thin-section petrography. Pervasive silicification (overprinting remnant GSC and bleached domains) is the dominant style of alteration in the remainder of Hole 1188A (78-117 mbsf). The alteration assemblage is dominated by quartz, illite, and chlorite with remnant igneous plagioclase commonly preserved.

In Hole 1189B, the upper 30 m was not cored. However, rapid fluctuations in the ROP suggest that a sequence of intercalated hard rocks (most likely fresh volcanic rock) and very soft rocks (highly altered or mineralized material, rich in sulfide, sulfate, and/or clay) occupy this cased interval. A sand sample recovered from the hammer bit used to drill this interval consists of pyrite, anhydrite, and gypsum. From 30 to ~120 mbsf, Hole 1189B intersected a sequence of strongly brecciated, stockwork veined, GSC altered rocks with patchy silicification, which increases in intensity downhole, together with narrow intervals of both sulfide mineralization (pyrite, with subsidiary chalcopyrite and sphalerite) and patches of jasperoidal quartz with pyrite (Fig. F47). The mineralized samples are discussed in more detail in "Sulfide and Oxide Petrology". Although clearly more sulfide rich than the equivalent interval in Hole 1189A, the dominant alteration assemblage of quartz, illite, and chlorite in Hole 1189B is similar, except for the common occurrence of potassium feldspar below 70 mbsf (Table T6). The extremely poor recovery in the upper part of Hole 1189B makes further correlation impractical between the holes.

The lower portion of Hole 1189B, between ~120 and 200 mbsf, comprises a sequence of highly silicified, massive vesicular lavas, which show little evidence of sulfide mineralization, and flow-banded, brecciated units with highly to completely developed silicification overprinting GSC alteration and with widespread development of quartz-pyrite anhydrite sphalerite hematite veining. This sequence shows varying mineralogy, with less strongly altered units dominated by cristobalite (primarily developed by devitrification of volcanic glass), whereas the completely altered rocks contain quartz. A range of phyllosilicate phases (illite, chlorite, smectite, chlorite-smectite mixed layer, and illite-bearing mixed layer) are found in both cristobalite- and quartz-bearing lithologies, as is potassium feldspar.

Detailed Description of Alteration

A detailed description of the significant features of hydrothermal alteration at Site 1189 is presented in the following pages. As discussed previously, attempts to correlate between the upper parts of the two holes cored at the site are hampered by the poor recovery in the upper portion of Hole 1189B, compounded by the more sulfidic nature of material intersected in it. Consequently, the alteration in Holes 1189A and 1189B will be described separately, before being discussed jointly at the conclusion of this section of the "Site 1189" chapter.

Hand specimen descriptions of the alteration of individual core pieces (see the "Site 1189 Alteration Logs") and descriptions of individual thin sections are presented in Figure F48 and in the "Site 1189 Thin Sections". Additionally, Tables T2 and T4 present a summary of the hydrothermal alteration of the lithologic units in Holes 1189A and 1189B, whereas a summary of mineralogy as determined by XRD analysis is presented in Tables T5 and T6.

Hole 1189A

Films on Fresh Aphyric Dacite

Fresh dacite (<2% alteration) was only recovered in the first core in Hole 1189A (Unit 1: interval 193-1189A-1R-1, 0-17 cm) (Table T2). Alteration is limited to the development of patchy coatings of grayish silica-clay sulfate and occasional films of Fe oxyhydroxide along cracks and on vesicle walls. Based on an XRD analysis of interval 193-1189A-1R-1, 0-4 cm, opaline silica is the dominant component of these films, with quartz as a minor phase.

Green Silica-Clay Alteration

The GSC alteration is the earliest style of pervasive hydrothermal alteration observed at Site 1189 and may be overprinted by patchy bleaching or, more commonly, silicification. As at Site 1188, GSC alteration may be pervasive, giving rocks an overall pale green to blue green color, or it may form remnant patches of greenish coloration within otherwise pale, silicified rocks. Pervasive GSC alteration is best developed in zones of strong hydrothermal brecciation in Hole 1189A (Unit 4: interval 193-1189A-2R-1, 101-137 cm; Unit 6: interval 193-1189A-3R-1, 56-63 cm; Unit 9: Cores 193-1189A-5R through 6R; Unit 11: interval 193-1189A-7R-1, 59-72 cm; Unit 13: interval 193-1189A-7R-1, 92-99 cm; Unit 16: Core 193-1189A-9R; and Unit 19: interval 193-1189A-10R-1, 4-135 cm), where fragments of GSC altered volcanic rocks are cut and/or enclosed by hairline to massive silica pyrite and, with increasing depth, quartz-(pyrite) vein networks. More coherent GSC altered units (Unit 8: Core 193-1189A-4R-1 and Unit 10: interval 193-1189A-7R-1, 0-59 cm) are intercalated with the breccias. GSC altered clasts are also present in a polymict volcaniclastic breccia near the base of the hole (Unit 23: interval 193-1189A-13R-1, 51-59 cm).

GSC alteration typically pseudomorphs primary igneous features, preserving perlitic textures and flow banding, even at the microscopic scale (Figs. F49, F50). Originally microlitic GSC altered rocks commonly contain domains in which remnant igneous plagioclase microlites are surrounded by microcrystalline silica-clay alteration. Very fine grained disseminated primary magnetite is also preserved in some samples, although these crystals commonly exhibit evidence of incipient alteration and breakdown to leucoxene.

In brecciated units, angular flow-laminated clasts hosted in a silica matrix commonly show evidence of rotation relative to each other (Fig. F49). It is often unclear whether this rotation is a result of hydrothermal brecciation or whether it reflects an initial volcaniclastic texture in the rock. Flow-top breccias commonly contain angular clasts, so the lack of rounding of fragments in the breccias is not a clear indication of a strictly hydrothermal origin (McPhie et al., 1993). Nevertheless, more coherent intervals of GSC alteration typically exhibit incipient veining and fracturing, and thin-section petrography records the presence of interlinked vein networks parallel to or cutting across flow lamination. These fractures could develop into broad veins, such as those seen in more strongly brecciated core intervals, allowing transportation and rotation of volcanic fragments. Consequently, on the weight of currently available evidence, a hydrothermal origin for most breccias is preferred.

In Hole 1189A, GSC altered breccia clasts are hosted by anhydrite-cristobalite-(pyrite) stockworks at shallow depths (Unit 4: 10.71-11.07 mbsf). However, with increasing depth, quartz is the dominant silica polymorph, instead of cristobalite, and anhydrite generally decreases in abundance. Clasts from Unit 6 (19.96-20.03 mbsf) are hosted by anhydrite-quartz-(pyrite) veins (Fig. F51), whereas in Units 9 (38.80-49.29 mbsf), 11 (58.89-59.02 mbsf), 13 (59.22-59.59 mbsf), 16 (77.70-78.56 mbsf), and 19 (87.41-88.72 mbsf) anhydrite is present only as late vug fill and quartz-(pyrite) veins form the stockwork (Fig. F50). These changes are similar to those seen at Site 1188 and are supported by thin-section descriptions and XRD analyses of GSC altered units from Hole 1189A (Table T5). Illite is present in all examples of GSC alteration chosen for XRD analysis. A disordered chlorite-smectite mixed-layer phase was detected only in Unit 4, whereas all other samples contain chlorite, which generally increases in abundance downhole.

Pyrite is present as the dominant sulfide phase in all intervals of GSC alteration. Traces of chalcopyrite, generally as a late phase in quartz veins and vug fill, are also present in most polished thin sections examined (Fig. F48). The petrography and temporal relationships between these sulfide phases are discussed in "Sulfide and Oxide Petrology".

Bleaching

Pervasive bleaching is most common in the upper parts of Hole 1189A (Unit 2: intervals 193-1189A-1R-1, 17-34 cm, and 2R-1, 0-93 cm; Unit 5: interval 193-1189A-3R-1, 0-56 cm; and Unit 7: interval 193-1189A-3R-1, 63-96 cm). Intervals of bleaching are also present in the lower parts of the hole (Unit 17: intervals 193-1189A-9R-1, 86-89 cm, and 10R-1, 0-7 cm; Unit 22: intervals 193-1189A-12R-1, 128-138 cm, and 13R-1, 0-51 cm; Unit 23: interval 193-1189A-13R-1, 51-59 cm; and Unit 24: interval 193-1189A-13R-1, 59-72 cm), but these units are all silicified to some degree.

Bleached units are typically pale gray to white in appearance and may be distinguished from pervasively silicified rocks by their relative softness and from GSC altered rocks by color (GSC altered rocks are pale gray-green to blue-green). With the exception of vesicles (coated or filled with anhydrite, silica, and pyrite) and rare remnant perlitic textures, macroscopic igneous features are generally absent in these units. However, it appears that most of the pervasively bleached units represent massive, sparsely to moderately vesicular lavas, and thin sections of these units commonly display remnant domains of microlitic plagioclase (Figure F52).

XRD analysis (Table T5) and thin-section observations of bleached units indicate that silica polymorphs, clays, and anhydrite are the main alteration minerals. Cristobalite is found in Units 2 and 5 (0.17-10.63 and 19.40-19.96 mbsf, respectively). As with the GSC altered rocks, quartz dominates in the remaining, deeper units. Chlorite and illite are also present in bleached units throughout the depth of the hole, but rarely comprise more than a few volume percent of the rock. Remnant igneous plagioclase and probable secondary potassium feldspar (Unit 7 only) may also be present. Silica-(pyrite) anhydrite veins with silica-clay altered halos are the most abundant vein type in the bleached rocks. Coarse-grained anhydrite is the final phase to precipitate in these veins (Fig. F53) and is also present as vuggy cavity fill. Fine-grained pyrite and traces of chalcopyrite are disseminated in the rocks and concentrated in vugs. The petrography and relationships between these sulfide phases are discussed in "Sulfide and Oxide Petrology".

Silicification

As at Snowcap hydrothermal site (Site 1188), silicification is the dominant style of alteration for deeper lithologic units in Hole 1189A at Roman Ruins hydrothermal site. All rocks, irrespective of the dominant alteration style, host silica-(pyrite) bearing veins, and all lithologic units below Unit 7 (Cores 193-1189A-4R to 13R; 29.10-116.82 mbsf) are patchily to pervasively silicified (Table T2). In all cases, silicification is the final stage of pervasive alteration and may overprint either bleaching or GSC alteration. However, even later anhydrite-(silica-pyrite) veins locally overprint silicification.

In general, the degree of silicification increases with depth in Hole 1189A, as demonstrated by an increasing hardness of rocks in hand specimen. Granular microcrystalline to subhedral crystalline quartz replaces the groundmass and fills voids and vesicles of rocks, often preserving igneous textures. In many cases, volcanic features such as perlitic fractures or flow banding provide early fluid pathways, as indicated by the alteration halos commonly developed along these features. Anhydrite is not a major component of the silicified units in Hole 1189A but is found as late vesicle and vein fill in some instances.

Petrographic work confirms that quartz is the main alteration phase introduced to the rock during silicification (Figs. F48, F54). Typically, all vesicles in the rock are filled with crystalline quartz, and the groundmass is pervasively replaced by microcrystalline silica (identified as quartz by XRD analysis) intergrown with fine clay (illite and chlorite, again from XRD analyses) (Table T5). Quartz-pyrite veining is widespread and some veins contain late anhydrite infill. Remnant microlitic igneous plagioclase and very fine grained magnetite (partly replaced by leucoxene) are preserved in some sections. As well as pyrite, traces of chalcopyrite are present in all thin sections of silicified units from Hole 1189A. The petrography and relationships between these sulfide phases are discussed in "Sulfide and Oxide Petrology".

Poikiloblastic Silicification

Unit 15 (Core 193-1189A-8-R; 68.00-69.28 mbsf) is a patchily silicified, sparsely vesicular, plagioclase-phyric volcanic rock with weakly to strongly developed quartz-pyrite ( anhydrite) veining. However, the poikiloblastic style of alteration observed in this rock is unique to Site 1189 and has not been intersected at any other site during Leg 193. Consequently, the unit is discussed separately here.

The core from Unit 15 is cream colored in hand specimen, preserving traces of an original vesicular texture and cut by a fine (mostly <0.5 mm wide) anastomosing to crosscutting network of pyrite veinlets with well-developed siliceous alteration halos (Fig. F55). Rare examples develop into very fine grained quartz-pyrite veins as wide as 5 mm. Pyrite-silica veining exhibits an overall increase in intensity toward the lower part of the unit. Rare rounded xenolithic fragments of a coarse grained dark gray-green silicified plagioclase-chlorite-bearing rock are also found within the unit.

In thin section, Unit 15 contains 1%-2% fresh plagioclase phenocrysts, as long as 1 mm, and rare quartz phenocrysts hosted in a moderately clay-silica altered groundmass, with abundant weakly oriented acicular plagioclase microlites. Vesicles are filled with coarse quartz crystals, which in some cases host euhedral pyrite or chalcopyrite crystals in their centers. Narrow granular quartz veins typically host pyrite and minor chalcopyrite. Silicification is present as coarse-grained poikiloblastic quartz crystals (Fig. F56), which grow in halos along quartz-pyrite veins, quartz phenocrysts, quartz amygdules, and, where best developed (Sample 193-1189A-8R-1, 42-44 cm), within the groundmass of the rock. The poikiloblasts replace the formerly glassy groundmass of the rock, enclosing plagioclase microlites. When they overgrow quartz-bearing veins, or surround amygdules or quartz phenocrysts, the poikiloblasts are commonly optically continuous with the adjacent quartz in these structures.

The XRD spectra of Unit 15 are dominated by plagioclase, along with quartz in the lower, more strongly veined and silicified portion of the unit (Table T5). Quartz and pyrite are present throughout the unit, as is chlorite. Illite, smectite, and K-feldspar are also present as minor to trace phases in some samples.

Hole 1189B

The core from Hole 1189B is divided into an upper sequence (Units 1-14; 31-119 mbsf) roughly corresponding to the interval cored in Hole 1189A, but which is dominated by sulfide-rich, partly stockwork breccias, and a lower sequence (Units 15-36; 119-198 mbsf) comprising intercalated moderately to highly altered coherent units, highly to completely altered monomict breccias, and polymict volcaniclastic breccias and sandstones. The curated depth of the boundary (top of Unit 15) is at 118.53 mbsf. However, given the poor core recovery and the procedure for assigning sample depths (see "Shipboard Scientific Procedures" in "Introduction" in the "Explanatory Notes" chapter), it likely corresponds to a downhole logging discontinuity at 123 mbsf.

Upper Sequence (31-119 mbsf)

Rocks recovered from the uppermost 119 m of Hole 1189B include fragments of massive to semimassive sulfides (described in "Sulfide and Oxide Petrology"), completely GSC altered clasts of volcanic rocks embedded in vein networks variably rich in anhydrite, quartz, pyrite, and hematite, and rare GSC altered vesicular units. Core recovery across this interval is extremely poor, making it impossible to comment on the relationship between the different lithologies. However, the generally small size (<5 cm) of the individual core pieces and the widespread presence of veins within the recovered material implies a strongly veined and fractured sequence of rocks. Additionally, a very high ROP means that the missing intervals are exceptionally soft, suggesting sulfide, sulfate, and/or clay-rich lithologies.

Vesicular volcanic rocks in the upper sequence are restricted to Unit 2 (interval 193-1189B-1R-1, 29-39 cm; 31.29-31.39 mbsf), Unit 7 (interval 193-1189B-6R-1, 67-78 cm; 79.67-79.78 mbsf), Unit 9 (interval 193-1189B-8R-1, 0-24 cm; 98.40-98.64 mbsf), Unit 11 (interval 193-1189B-8R-1, 33-73 cm; 98.73-99.13 mbsf), and Unit 13 (interval 193-1189B-10R-1, 0-33 cm; 117.90-118.13 mbsf). Rocks from these units are completely and pervasively altered, predominantly to green clay (GSC alteration), but show an increasing extent of patchy silicification down the hole. The sample from Unit 2 is very soft and contains muscovite, chlorite, and pyrite, with only traces of quartz; Unit 7 exhibits millimeter-scale quartz-rich spots in the groundmass; Unit 9 is patchily silicified with silicified halos along pyrite veins; and Units 11 and 13 are pervasively silicified with quartz-pyrite-filled amygdules and variably developed quartz-pyrite veins (Figs. F57, F58). This progression in the extent of silicification is similar to that observed in Hole 1189A. With the exception of Unit 2, thin-section petrography and XRD analyses (Table T6) indicate that the major components of these units are quartz, illite, chlorite, and pyrite, along with potassium feldspar in some cases.

The remaining units of the upper sequence of Hole 1189B are breccias (partly stockwork breccias) with variable sulfide contents and gangue mineralogy. Unit 3 (Sections 193-1189B-2R-1 through 3R-1, 84 cm; 40.1-50.5 mbsf) consists of soft GSC altered rock fragments cemented in and cut by an anhydrite-pyrite stockwork. The XRD studies and thin-section observations indicate that gypsum is present in these rocks, replacing anhydrite, which, in turn appears to have replaced altered rock fragments (Fig. F59). Visual inspection of hand specimens from this interval suggests that individual samples contain up to 65% anhydrite, whereas others contain up to 60% pyrite and may also show significant concentrations of chalcopyrite. Unit 4 (Section 193-1189B-5R-1; 69.30-69.85 mbsf) is also a stockwork breccia with completely GSC altered perlitic clasts that, in thin section, show incipient silicification, indicated by poikiloblastic growth of quartz similar to that seen in Unit 15 of Hole 1189A. In contrast to Unit 3, the stockwork in Unit 4 is quartz-pyrite and the slight silicification of the clasts of Unit 4 may be related to the more siliceous breccia cement. Unit 5 (interval 193-1189B-6R-1, 0-56 cm; 79.00-79.56 mbsf) is characterized by its distinct red color, which is related to an abundance of hematite, accompanied by quartz and minor pyrite, in the quartz grains of the stockwork (Fig. F60). The volcanic clasts in Unit 5 are distinctly perlitic and contain K-feldspar, chlorite, illite, quartz, and disseminated pyrite. Two pieces of a breccia with anhydrite-quartz-pyrite stockwork and flow-banded, amygdaloidal clasts (Fig. F61) make up Unit 8 (interval 193-1189B-7R-1; 88.70-88.92 mbsf), whereas Unit 10 (interval 193-1189B-8R-1, 24-33 cm; 98.64-99.73 mbsf) and Unit 12 (interval 193-1189B-9R-1; 108.10-108.19 cm) consist of greenish gray, soft clasts embedded in a quartz-pyrite stockwork.

Lower Sequence (119-198 mbsf) - Coherent Units

The coherent units of the lower sequence of Hole 1189B are Unit 15 (interval 193-1189B-10R-1, 63-77 cm; 118.53-118.67 mbsf), Unit 19 (interval 193-1189B-11R-1, 43 cm, through 13R-1, 20 cm; 128.03-147.14 mbsf), and Units 26-28 (interval 193-1189B-15R-2, 37 cm, through 17R-1, 28 cm; 167.96-185.57 mbsf). These units are variably altered (25%-100% secondary phases in thin sections), vesicular to amygdaloidal volcanic rocks with rare quartz-pyrite magnetite or anhydrite veining. Alteration of these units is pervasive, but rarely complete. A marked feature of the coherent rocks is the development of large round to ovoid vesicles that are often filled or lined with quartz (Fig. F62). Moreover, they characteristically lack the flow banding and spherulitic textures that are characteristic of the brecciated units (see below).

With the exception of Unit 26, coherent units from the lower sequence of Hole 1189B rarely contain significant (>1%) sulfide. In Unit 26, a network of fine quartz-pyrite-magnetite veinlets with a magnetite-bearing alteration halo is developed (Fig. F63). These veins are moderately sulfide-rich (5%-20% contained pyrite) and in some cases coalesce to form brecciated zones with quartz-pyrite infill (refer to "Structural Geology").

The mineralogy, revealed by XRD analysis (Table T6) and thin-section studies, of coherent volcanic rocks from the lower sequence of Hole 1189B may be grouped into two assemblages. In Units 15, 19, and 26, the dominant silica polymorph is cristobalite (occurring as groundmass replacement and vesicle fill and interpreted to primarily be a product of devitrification of volcanic glass) with minor quartz (as vesicle fill, or more rarely in veins) and potassium feldspar, illite, and chlorite/illite-bearing mixed-layer phases. In contrast, Units 27 and 28 contain quartz in the groundmass and as vesicle fill, with potassium feldspar and chlorite or chlorite-smectite mixed-layer phases. Quartz also commonly forms irregular poikiloblasts, overgrowing clay-altered groundmass. Remnant microlites and less abundant phenocrysts of igneous plagioclase are present in both cristobalite- and quartz-bearing units, although thin-section studies (Fig. F48) indicate that the abundance of plagioclase in the latter (<20%) is generally lower than in the former (20%-40%). These observations indicate that quartz is the dominant silica polymorph in more extensively altered units, whereas cristobalite predominates in less altered rocks. This interpretation is consistent with the more complex phyllosilicate mineralogy of the cristobalite-bearing lithologies, which may be interpreted as disequilibrium assemblages that result from less intense fluid-rock interaction.

Lower Sequence (119-198 mbsf) - Monomict Breccias

A large proportion of the core recovered from the lower sequence of Hole 1189B is composed of monomict breccias (Units 16, 17, 21-25, 29, 30, 32-34, and 36) (see Table T4 for intervals). These brecciated rocks are composed of spherulitic, microlitic, and perlitic flow-banded volcanic fragments (refer to "Igneous Petrology") hosted in quartz-pyrite and anhydrite-pyrite cements (Figs. F64, F65, F66). The degree of brecciation is highly variable, and individual core pieces may be coherent, with a pseudobrecciated texture provided by alteration to light greenish gray material along fracture networks, overprinting flow banding (Fig. F67). Both brecciated and coherent intervals commonly exhibit zoned alteration patterns, with less altered kernels (Figs. F64, F67). Despite the development of pseudoclastic textures caused by alteration, the widespread recognition of clast rotation, scattered examples of flow-banded domains intruding breccias (refer to "Igneous Petrology"), and intercalated polymict units (see below) indicate that many of these brecciated units are at least partially volcaniclastic in origin.

As for the coherent intervals, two distinct alteration assemblages are present in monomict breccias from the lower sequence of Hole 1189B (Table T6). The majority of units (Units 16, 17, 21-24, 30, and 32) are quartz-dominated, with clay (generally chlorite, chlorite/smectite mixed layer phases, and/or illite) and potassium feldspar (in most units). Anhydrite is abundant in the cement of some breccias, which are scattered throughout the sequence (refer to the "Site 1189 Alteration Log"). Minor remnant igneous plagioclase is present in some intervals. In contrast, Units 25, 33, and 34 contain cristobalite and remnant plagioclase as major phases, potassium feldspar, and a very complex clay assemblage (illite, chlorite, smectite, chlorite/smectite mixed-layer phases, and illite-bearing mixed-layer phases). Rare quartz veins are also present in some intervals. Unit 34 also may contain minor talc and actinolite. However, no thin section of the unit was cut and the XRD spectra are somewhat ambiguous, so this remains to be confirmed. It is apparent that, like the coherent units, quartz-dominated assemblages are developed in more strongly altered brecciated rocks, whereas cristobalite, most likely produced by the devitrification of volcanic glass, is confined to the less altered intervals.

Lower Sequence (119-198 mbsf) - Polymict Volcaniclastic Breccias and Sandstones

Three intervals of polymict volcaniclastic breccia (Unit 18: 127.77-128.03 mbsf; Unit 20: 147.20-147.49 mbsf; and Unit 35: 196.78-197.16 mbsf) and a volcaniclastic sandstone (Unit 31: 186.26-186.34 mbsf) exhibiting graded bedding are present within the lower sequence of Hole 1189B. These rocks show a similar style of alteration to the monomict breccias. They contain a range of perlitic, flow-banded, and vesicular to vuggy clasts, which show moderate to complete GSC alteration and/or silicification and are hosted in a quartz-sulfide (dominantly pyrite) magnetite matrix (Fig. F68). In the breccias, some clasts contain narrow sulfide veinlets that are terminated by the matrix, implying that hydrothermal activity occurred prior to erosion and deposition of the units. Similarly, a glassy volcanic clast in the volcaniclastic sandstone contains chalcopyrite and sphalerite (Fig. F69), which are not found in the matrix of the rock, again implying preerosional hydrothermal activity. A thin section of a sample from Unit 35 (Sample 193-1189B-18R-2, 65-67 cm) contains minor amounts of acicular actinolite and possible epidote as inclusions in quartz. Actinolite was also provisionally identified by XRD analysis in one sample from Unit 34 and may be a common minor phase in rocks from the lowermost part of Hole 1189B.

Quartz is the only silica polymorph observed in the polymict volcaniclastic rocks of the lower sequence of Hole 1189B. Some or all of the clasts in all thin section (Fig. F48) and XRD samples (Table T6) also contain minor remnant plagioclase microlites, within a clay- (illite-chlorite) and quartz-rich groundmass. The clasts are hosted in a fine (for the sandstone) to coarsely crystalline quartz matrix, with 1%-3% pyrite. Minor anhydrite, which is partly replaced and overgrown by barite (Fig. F70), is also present in the matrix of Unit 20.

Discussion

Holes 1189A and 1189B were targeted in the midst of a zone of high-temperature focused fluid flow at the Roman Ruins sulfide chimney field. Despite this, the nature of alteration in Hole 1189A (cored interval 0-125.8 mbsf) is similar to that observed at Site 1188, a low-temperature diffuse vent field at Snowcap hydrothermal site (see "Hydrothermal Alteration" in the "Site 1188" chapter). The sequence once again shows a systematic change from cristobalite to quartz as the silica polymorph within alteration (Figs. F46, F53; Table T2), which broadly corresponds to a brecciated zone of GSC alteration, increasingly overprinted by silicification at depth. However, the transition from cristobalite to quartz, which takes place at 25 mbsf in Hole 1189A, is sharp and shallow when compared to the gradual transition over 100-120 mbsf at Site 1188. This is interpreted to reflect a higher temperature gradient at Site 1189.

The upper portion (31-~120 mbsf) of Hole 1189B, which is sited 30 m from Hole 1189A, is also represented by a sequence of dominantly brecciated, GSC altered volcanic rocks, with generally increasing silicification downhole. However, the rocks within Hole 1189B are much more sulfide-rich than those from Hole 1189A and contain intervals of polymetallic (pyrite-chalcopyrite-sphalerite) stockwork and semimassive to massive sulfide mineralization. As a result of the extremely poor recovery in this portion of Hole 1189B, it is not possible to determine the relationship of these units to other, less mineralized intervals within the sequence. However, it seems likely that this part of Hole 1189B successfully drilled a focused fluid discharge network (i.e., a mineralized stockwork). In contrast, Hole 1189A appears to have intersected a slightly peripheral alteration sequence that can be attributed to more diffusional fluid flow around the main conduits, similar to the system drilled at Site 1188. This rapid lateral variation in alteration style and intensity is not unexpected in a fracture-controlled fluid flow regime.

The lower sequence of altered volcanic rock recovered from Hole 1189B (~120-198 mbsf) is distinctly different in character to the upper sequence. It consists of highly silicified, massive vesicular lavas that show little evidence of sulfide mineralization, intercalated with flow-banded, brecciated units with highly to completely developed silicification that overprints GSC alteration, and widespread development of quartz-pyrite anhydrite sphalerite hematite veining. This sequence shows varying mineralogy. Less strongly altered units are dominated by cristobalite (primarily developed by devitrification of volcanic glass). In contrast, very highly to completely altered rocks contain quartz. The flow-banded, brecciated units are mostly quartz-bearing, whereas the vesicular lavas mostly contain cristobalite, although there are exceptions to this rule. This relationship suggests the possibility of lithologically controlled alteration zones at depth beneath Roman Ruins hydrothermal site. However, the lower sequence of Hole 1189B, where this relationship is developed, is almost entirely deeper than the bottom of Hole 1189A. Consequently, it is not possible to test this hypothesis by attempting to correlate units between the two holes at Site 1189.

Significance of the Cristobalite-Quartz Transition

Although cristobalite is a high-temperature silica polymorph, metastable low-temperature forms of the mineral are commonly found (Deer et al., 1992). These forms are widespread in devitrified acidic volcanic rocks (Williams et al., 1982) and shallow volcanic-hosted hydrothermal systems. In their review of southwestern Pacific Rim gold-copper deposits, Corbett and Leach (1998) find that cristobalite is a typical alteration mineral within low-temperature (<150C), high crustal-level epithermal-style mineralization, whereas quartz becomes the dominant polymorph with increasing temperature and depth. Consequently, the change from cristobalite to quartz at Sites 1189 and 1188 is interpreted to be caused by an increase in temperature with depth. This hypothesis is supported by the generally decreased abundance of anhydrite in quartz-dominant alteration. Anhydrite becomes insoluble in seawater at temperatures >150C (Bischoff and Seyfried, 1978), requiring that downwelling seawater would precipitate anhydrite at or around the 150C isotherm. At deeper (and hotter) levels within the hydrothermal system, this would effectively confine anhydrite to open fractures that may allow sudden influxes of relatively cool water. The fluid entering these fractures would be conductively heated, precipitating anhydrite as fracture fill, as observed.

In contrast to Site 1188, where fresh volcanic rocks were recovered to a depth of 34 mbsf, moderately altered bleached volcanic rocks were encountered only in the first 9 m core of Hole 1189A. Other significant mineralogical differences, such as the change from cristobalite to quartz as principal silica polymorph and the decrease in the importance of anhydrite as an alteration mineral, also take place at much shallower depths at Site 1189. These differences imply a higher temperature gradient during subseafloor alteration for Roman Ruins hydrothermal site than for Snowcap hydrothermal site, which is presumably related to the higher temperature venting observed at the former site.

The contrasting temperature gradients at Snowcap hydrothermal site and Roman Ruins hydrothermal site may reflect differences between the hydrological properties of the subseafloor at the two hydrothermal sites. For instance, the local development of less permeable layers at Snowcap hydrothermal site may promote pooling and mixing of fluids in the subsurface, followed by a slow upward flow through a poorly connected fracture network, resulting in diffuse surface hydrothermal activity. In contrast, at Roman Ruins hydrothermal site, the permeability structure appears to allow more vigorous hydrothermal circulation and the development of efficient fluid conduits that permit focused venting of high-temperature hydrothermal fluids at the seafloor. However, at this early stage of investigation, it is not possible to provide a detailed discussion of the specific factors that influence fluid flow at the two sites.

Significance of the Sulfates

As outlined above, the presence of anhydrite in many samples indicates that the interacting fluids had temperatures >150C. However, occasional replacement of anhydrite by gypsum and barite implies that anhydrite is no longer stable in these rocks and that conditions have changed after the anhydrite was deposited. The partial replacement of anhydrite by gypsum in Unit 3 (between 40 and 50 mbsf) suggests that the present-day temperature at this depth is below ~40C, the temperature below which anhydrite transforms to gypsum (MacDonald, 1953). Moreover, the replacement of anhydrite by barite in Unit 20 (~150 mbsf) indicates that anhydrite was unstable. The presence of apparently unmodifed anhydrite in other sections of the cores from Site 1189, however, suggests that anhydrite is generally not unstable throughout the drilled sequence. This complexity in sulfate distribution may be attributed to temporal variations in fluid flow and fluid-rock interaction along a variably connected fracture network.

Evidence for Prior Hydrothermal Activity

The presence of apparent presedimentation sulfide veining in clasts from polymict volcaniclastic breccias and sulfide mineralization in a clast within the volcaniclastic sandstone at a depth approaching 200 mbsf implies that the hydrothermal system at Roman Ruins hydrothermal site has been active for an extended period of time. Similarly, the recognition of textures that indicate exhalative deposition of sulfides, which are now located subsurface (refer to "Sulfide and Oxide Petrology"), also indicates that active seafloor venting is not unique to the present time. Rather, this evidence suggests that hydrothermal activity and felsic volcanism have been ongoing and intimately related over a period of time at the PACMANUS hydrothermal field.

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