Site 1188 targeted the Snowcap hydrothermal site, a zone of low-temperature diffuse venting within the PACMANUS hydrothermal field. Six attempts to drill were made at this site, resulting in three holes from which core and/or logging data were obtained. Hole 1188A, the initial hole at the site, intersected vertically extensive pervasive hydrothermal alteration, which often obscured primary textural features to the extent that it was not possible to identify individual units on the basis of igneous lithology alone. Poor hole conditions forced the abandonment of this hole at a depth of 211.6 mbsf. Hole 1188B was drilled with the LWD tool to 72.0 mbsf; an attempt to core deeper with the ADCB recovered only two wash samples. Following the abandonment of this operation, a concerted effort was made to drill a hole into the deeper part of the hydrothermal system at Snowcap. After three unsuccessful attempts, Hole 1188F provided cores from a deeper sequence of hydrothermally altered volcanic rocks from 218.0 to 386.7 mbsf.

General Overview

The following detailed description of hydrothermal alteration in coherent volcanic rocks and breccias from beneath Snowcap hydrothermal site is based primarily on visual descriptions of cores from Holes 1188A and 1188F and is supplemented by thin-section petrography and XRD analyses of bulk samples. Portable infrared mineral analyzer (PIMA) analyses were also conducted on a large number of core samples from both holes. The spectra will be further processed on shore, but for some intervals, particularly in Hole 1188F, initial results provide additional mineralogical information that complements the XRD data. These are discussed where appropriate in the text.

Alteration was classified in drill core primarily by the visual detection under binocular microscope of the following key minerals: soft green to bluish green clay, soft white clay, hard silica polymorphs, and magnetite. Anhydrite and pyrite are present throughout the drilled section, generally as trace to minor phases. An overview of the principal alteration types is given in Table T7.

Six main types of hydrothermal alteration were delineated in cores from Site 1188:

  1. Patchy silica-clay and Fe oxide films developed on fracture surfaces and as vesicle linings in otherwise fresh vesicular porphyritic rhyodacite.
  2. Pervasive opaline silica alteration of a single perlitic volcanic unit;
  3. Pervasive blue-green to green-gray silica-clay (GSC) alteration that generally pseudomorphs textural features such as perlite and flow banding. Silica occurs as cristobalite and/or quartz.
  4. Pervasive and vein-associated bleaching (whitish gray silica-clay ± anhydrite alteration) that results in the destruction of primary igneous textures other than vesicles.
  5. Patchy to pervasive silicification, forming a dominant quartz-illite assemblage that shows a general trend toward quartz-illite-chlorite at depths >300 mbsf.
  6. Impregnation with Fe oxides (dominantly magnetite, with minor hematite), in rare cases accompanied by hercynite, forming dark gray to black alteration patches and vein halos.

The characteristics, distribution, and relationships between each of these types of alteration are discussed in detail below. The diagnostic features of the three major alteration styles are also summarized in Table T7. In broad terms, the spatial distribution of alteration below Snowcap hydrothermal site is relatively straightforward (Fig. F35). A 34-m-thick cap of fresh rhyodacite to dacite, possibly intercalated with altered units that may not have been recovered, is underlain by a sequence of pervasively bleached vesicular volcanic rocks to a depth of ~50 mbsf. A zone of strongly anhydrite-silica (cristobalite or quartz)-pyrite veined and brecciated GSC altered rocks, intercalated with more massive bleached units and cut by late anhydrite veins with bleached halos, extends from ~50 to 105 mbsf. An additional sequence of bleached rocks with variably developed cristobalite-pyrite and quartz-pyrite stockwork veins underlies this sequence to a depth of ~125 mbsf. Between 125 and 375 mbsf (the bottom of Hole 1188F), silicification is the dominant style of alteration at Site 1188. As noted above, this style of alteration is typified by a quartz-illite assemblage, which tends toward quartz-illite-chlorite at depths >300 mbsf. Magnetite-enriched intervals are sporadic within the silicified sequence, between 135 and 185 mbsf in Hole 1188A and from 320 to 375 mbsf in Hole 1188F. The sequence of pervasively silicified rocks is crosscut by late anhydrite veins with banded gray to white bleached alteration halos. These veins are most abundant between 220 and 280 mbsf in Hole 1188F but are present in all cores below 125 mbsf.

The only two samples recovered as wash from Hole 1188B at a depth of 72.0 m are cristobalite-bearing, pervasively bleached, variably vesicular volcanic rocks, very similar to lithologies cored between 50 and 105 mbsf in Hole 1188A. The recovery of these two samples suggests the alteration profile in Hole 1188B is similar to that in the upper part of Hole 1188A.

Tables T2 and T4 provide a summary of the distribution, lithology, and alteration features of the 26 units identified in Hole 1188A and the 46 units in Hole 1188F, respectively (refer also to Figs. F4, F20). Lithologic subdivisions were made based on remnant volcanic characteristics (e.g., the recognition of perlitic texture or flow banding and the presence or absence of phenocrysts or vesicles) and/or the dominant style of alteration within a given interval. A detailed description of the alteration of individual core pieces is presented (see the "Site 1188 Alteration Log").

Downhole Distribution of Secondary Minerals

Thin-section and XRD investigations (Tables T8, T9, T10) reveal systematic changes in secondary mineral assemblages with depth at Site 1188, which broadly mimic the changes in alteration type outlined above (Fig. F35). The most pronounced changes are in the mineralogy of the prevailing silica polymorphs, the abundance of anhydrite, and the appearance and disappearance of particular phyllosilicate phases and magnetite.

Above 34 mbsf, opaline silica and smectitic clay are the principal secondary phases. Between 34 and 50 mbsf, cristobalite, chlorite-smectite mixed-layer phases, and anhydrite are the most abundant alteration minerals. Below 50 mbsf, pyrophyllite, occasional chlorite, illite, and barite (in trace amounts) join cristobalite and anhydrite. Barite has not been detected below 105 mbsf. With a handful of exceptions (see "Detailed Description of Hydrothermal Alteration in Lithologic Units from Site 1188"), cristobalite is not a major alteration mineral below 120 mbsf and anhydrite becomes less abundant, whereas quartz and illite increase. Magnetite and chlorite are present in most rocks from between 140 and 185 mbsf in Hole 1188A, where quartz and illite dominate the secondary mineral assemblage, whereas anhydrite is restricted to vein and vesicle fill. In addition, corrensite is locally abundant from 165 to 185 mbsf. A quartz-illite-dominated alteration assemblage with late anhydrite veining is continued in the uppermost section of Hole 1188F (220-280 mbsf), although chlorite and magnetite are rare and corrensite is absent. PIMA analyses and thin-section observations from this interval indicate that the white clay present in the banded, bleached halos associated with these veins is pyrophyllite, although this phase was not identified in XRD spectra. Chlorite becomes abundant below 280 mbsf and is joined by sporadic magnetite below 320 mbsf. Two magnetite-bearing samples from ~350 mbsf have small amounts of brucite, alunite (in one case), and hercynite spinel.

Thin-section observations and whole-rock geochemistry (mainly Al2O3, K2O, and MgO concentrations) indicate that phyllosilicate phases commonly comprise >30 vol% of the rock in all styles of alteration and are, therefore, much more abundant than the relatively small phyllosilicate peaks in the XRD spectra might suggest. However, this phenomenon is not unusual in unoriented polymineralic powder diffraction samples and is not considered to be a significant discrepancy.

The transitions from opaline silica to cristobalite to quartz below 105 mbsf indicate a general increase in alteration temperature with depth.

Alteration Styles and Relative Timing of Alteration Events

The alteration history of Site 1188 is multistage and complex. There are several episodes of fracturing and fluid flow, as manifested by multiple veining and alteration events.

Only small fragments of fresh to incipiently altered dacites could be recovered from the uppermost 34 m of basement at Snowcap. Below 34 mbsf, the extent of alteration is high to complete and the dominant style of alteration is pervasive replacement, although in some places alteration along veins and in patches overprints earlier pervasive alteration.

As discussed above, the most abundant secondary minerals in rocks recovered from Site 1188 are silica polymorphs (quartz or cristobalite), followed by phyllosilicates (abundant illite, with less common smectitic clay, pyrophyllite, chlorite-smectite mixed-layer phases, and chlorite), then anhydrite, pyrite, and magnetite. These phases either replace igneous material or occur as vein, vesicle, and vug fill. The common occurrence of anhydrite is remarkable. Anhydrite is most abundant as vein and vesicle fill or in breccia cement, but is also observed in thin sections replacing igneous groundmass or plagioclase, particularly in bleached units between 34 and 125 mbsf in Hole 1188A.

It is generally very difficult to associate specific alteration styles, other than bleaching, with discrete vein sets. The GSC alteration and silicification appear to be chiefly pervasive, with hydrothermal fluids percolating along microcracks, perlitic cracks, void space in breccias, and taking advantage of the generally high porosity/permeability of the fractured vesicular lavas to produce highly to completely altered rocks. In contrast, bleaching is rarely truly pervasive. Late, localized bleaching is clearly associated with anhydrite-pyrite veining in Hole 1188F, and concentric zonation of alteration halos adjacent to anhydrite-coated fractures is still recognizable even in the heavily bleached intervals between 35 and 105 mbsf in Hole 1188A.

The earliest recognizable stage of hydrothermal alteration at Site 1188 is pervasive GSC alteration, which is intercalated with and persists as remnants within bleached units in the upper part of Hole 1188A (~35-125 mbsf). The development of bleached alteration halos along anhydrite-pyrite veins, and of patches of light gray alteration within GSC altered rocks, suggests that pervasive GSC alteration preceded bleaching. Both pervasively GSC altered rocks and bleached rocks often exhibit localized silicification, either in vein halos or in and around vesicles, implying that there is a phase of silicification that postdates both GSC alteration and the early pervasive bleaching event in this part of the hydrothermal system.

Pervasive silicification is the dominant alteration type in the remainder of the cored interval (125-375 mbsf) at Site 1188. We use the active term "silicification" because the abundance of quartz as replacements and vesicle fill in most thin sections from this interval suggests that SiO2 has been added. Also, preliminary shipboard geochemistry indicates SiO2 enrichment in the silicified sequences relative to their precursors (Fig. F36; see "Geochemistry"). However, these conclusions are tentative and may be modified as a result of postcruise research.

Remnant domains of GSC altered rock are preserved within pervasively silicified rocks, particularly between 150 and 185 mbsf in Hole 1188A and between 260 and 280 mbsf in Hole 1188F. We consider that an early GSClike alteration, subsequently obscured by pervasive silicification except in remnant domains, was initially developed throughout the entire drilled sequence and that patchy silicification higher in the sequence, where it shows a similar temporal relationship to the GSC alteration and also postdates pervasive bleaching, is likely to be related to the pervasive silicification at depth. This is the principal basis for assigning pervasive silicification to the final major stage of alteration, although postcruise research may reveal a more complex evolutionary sequence.

Additional evidence for GSClike alteration prior to pervasive silicification in the deeper portions of the hydrothermal system is provided by the fact that, in pervasively silicified rocks, quartz typically overgrows other secondary phases such as magnetite, hematite, and, most significantly, clays pseudomorphing plagioclase microlites. Pyrite is part of all alteration assemblages, but silicified rocks are generally relatively enriched in pyrite, and quartz and pyrite are commonly intergrown.

The latest identifiable stage of hydrothermal alteration at Site 1188 is more localized and is represented by bleached and banded quartz-pyrophyllite halos associated with anhydrite (± quartz ± pyrite) veins. These veins, which always cut preexisting alteration, are developed most strikingly in the upper part of Hole 1188F (~220-280 mbsf), but are present throughout the interval from 105 to 375 mbsf. The mineralogy of these veins and their associated alteration is similar to that of pervasively bleached units at shallower depths. However, these veins clearly postdate all other alteration styles at Site 1188 and are observed to cut quartz veins that are developed in pervasively bleached rocks between 105 and 125 mbsf (see "Structural Geology"), clearly demonstrating that the anhydrite veins represent a later hydrothermal event. Late anhydrite is also present throughout the silicified sequence in the centers of vesicles and vugs that are commonly lined with quartz and pyrite. Additionally, multiple late stages of anhydrite formation are inferred on the basis of silicified or bleached halos along anhydrite veins that are cut by later anhydrite veins and the presence of cyclically zoned alteration halos on many veins, implying multiple fluid pulses (see "Structural Geology").

Detailed Description of Hydrothermal Alteration in Lithologic Units from Site 1188

Films on Fresh Porphyritic Dacite

Fresh porphyritic rhyodacite was the only lithology sampled from the upper portion of Hole 1188A (0-33.95 mbsf) (Table T2), although the very low core recovery (average <3%) in this interval means that the presence of other types of material cannot be confidently discounted. Fresh rhyodacite pebbles were also sporadically recovered farther down the hole, but this material was limited to the top of individual core intervals and was interpreted to have caved in from the upper levels.

Alteration of fresh rhyodacites is limited to the development of patchy films of grayish silica-clay and rust-colored iron oxide or oxyhydroxide on fracture surfaces and vesicle walls. Very fine grained euhedral pyrite is commonly present in trace quantities on these films. The XRD analyses (Table T8) indicate that opaline silica and smectitic clay are the dominant components of the films.

Opaline Silica

Pervasive alteration to opaline silica (confirmed by XRD analysis) is confined to Unit 2, which lies immediately beneath unaltered rhyodacite in Hole 1188A (33.95-34.08 mbsf). As it is not possible to distinguish between opaline silica and cristobalite in hand specimen, this type of alteration is included with pervasive silicification in the alteration description for the hole.

Rocks from Unit 2 have a remnant perlitic texture, cut by fine silica veinlets, along which alteration is particularly intense, resulting in a pseudoclastic texture (Fig. F37). Very fine grained pyrite is disseminated throughout the rock in trace amounts. Thin-section description and XRD analysis (Table T8) indicate that opaline silica is the dominant phase present in the unit, with minor smectite, illite, pyrite, and remnant igneous microlites of plagioclase. Silica(+clay) alteration is controlled by perlitic fractures and decreases in intensity toward the centers of perlitic pseudoclasts (Fig. F38).

Unit 2 is underlain by pervasively bleached vesicular volcanic rocks and marks a sharp increase in alteration intensity for Hole 1188A.

Green Silica-Clay Alteration

The GSC style of alteration is commonly overprinted by bleaching or silicification and is consequently interpreted to be the earliest style of pervasive alteration at Site 1188. It is most readily identified in core samples by its distinct blue-green to gray-green color and its softness when compared to silicified rocks. Although sporadically developed in most units below 49 mbsf, this style of alteration is best developed in Units 5, 6, 8, and 10, which lie in a 50-m zone of predominantly brecciated rocks between 49 and 99 mbsf (Table T2). More massive bleached units (Units 7 and 9; discussed below) are intercalated with the breccias.

Typically, GSC alteration leaves pseudomorphs of primary igneous features, preserving perlitic textures and flow banding (Figs. F39, F40). Even where best developed, GSC altered rocks have been extensively veined (Figs. F7, F40, F41). They are cut by anhydrite-(pyrite) vein networks with rare silica-(pyrite) veins in the shallower portions of the hydrothermal system (Units 5, 6, and 8) (Fig. F42) and are cut predominantly by quartz(-pyrite) veins with associated patchy to pervasive silicification at greater depths (Unit 10) (Fig. F43). Remnant domains of soft GSC alteration are also sporadic in silicified units below 125 mbsf (Fig. F35; see the "Site 1188 Alteration Log"), where they appear to exhibit a similar mineralogy to the main GSC altered interval between 50 and 100 mbsf, with superimposed quartz alteration.

Patchy to pervasive bleaching, associated with late anastomosing anhydrite-(pyrite) and quartz-(pyrite) vein networks, overprints the GSC altered breccias (Fig. F41). This style of bleaching is best developed in Unit 8 (Table T2), which lies between two massive bleached intervals (Units 7 and 9; see below) that are interpreted to represent the same type of alteration.

Unit 10 differs from the overlying breccias in that it is cut by a siliceous vein network, rather than the anhydrite-dominated veins that are abundant farther up the section. It exhibits well-developed silicification of altered volcanic fragments, particularly in vein halos. Scattered later anhydrite veins with bleached alteration halos cut the siliceous breccia.

Thin sections from GSC altered rocks commonly exhibit pervasive alteration of original volcanic glass to finely intergrown cristobalite and tan-brown clay. Where this type of alteration is developed, plagioclase microlites seem to be largely unaltered. It appears that the altering fluids used perlitic fractures and cooling cracks to invade the rock. However, original perlitic and spherulitic textures are often preserved. Spheroidal domains, where present, are typically altered to clay and surrounded by an intensely silicified matrix (Fig. F44). Vugs and vesicles are generally lined by cristobalite with blocky anhydrite in the center. Anhydrite is abundant in thin sections from Units 5, 6, and 8, but mostly as late-stage vein and vug fill. It does not seem to be a major component of the more pervasive GSC alteration style. Pyrite is present in silica ± anhydrite veins and is abundant in patches where silicification is intense. Quartz was clearly identified in thin sections from Units 8 and 10 in the form of veins and vesicle fill together with pyrite (Fig. F45. The sections cut from Unit 10 contain quartz-pyrite-magnetite veins with chlorite-hematite halos, replacing former igneous groundmass (Fig. F46). Veins of this type were not observed in other GSC altered units.

Powder XRD investigation of GSC altered breccias indicates that silica polymorphs and anhydrite (as breccia matrix, postdating GSC alteration) are prominent alteration minerals (Table T8). The most abundant phyllosilicate phases are chlorite and illite, but pyrophyllite is also present, particularly in bluish green sections of the core. Traces of barite were also detected in most of the samples analyzed.

The silica polymorphs delineated by XRD show a consistent change with depth. Cristobalite is the dominant polymorph in Units 5 and 6, whereas quartz and cristobalite are both present in significant quantities in Units 8 and 10. This change from cristobalite to quartz with increasing depth is similar to the pattern recorded from the Trans-Atlantic Geotransverse (TAG) hydrothermal field (Hopkinson et al., 1999) and is consistently seen through all styles of alteration at Site 1188 (see below).


Pervasive bleaching is well developed in the upper part of Hole 1188A, where bleached units alternate with GSC altered units (Table T2). Unit 3 (moderately to highly altered), and Units 4, 7, 9, and 11-14 (completely altered) all exhibit bleaching, which is dominated by silica and clay with generally minor anhydrite. In contrast to the gray-green to blue-green GSC altered rocks, bleached units are typically white to light gray in appearance and may be distinguished from pervasively silicified rocks by their relative softness. Pervasively bleached units fall into three broad categories (Table T2): generally featureless to weakly vesicular units (termed massive) from shallower depths (Units 3, 4, 7, and 9; 39-97 mbsf; intercalated with GSC altered units), bleached volcaniclastic units (Units 12 and 14), and strongly veined and brecciated units with silica stockworks (Units 11 and 13; 106-117 mbsf; intercalated with the volcaniclastic units).

The lower boundary of pervasive bleaching is marked by Unit 15 (125.70-125.94 mbsf), a medium- to coarse-grained crustiform to granular anhydrite-pyrite vein set, with small fragments of bleached volcanic rock. However, bleached alteration halos associated with anhydrite ± pyrite ± quartz veins are present throughout all of the core recovered from Site 1188. These veins clearly postdate pervasive GSC alteration and silicification and are, therefore, interpreted to represent a later alteration event than the pervasive bleaching developed between 40 and 125 mbsf.

Massive Bleached Units

Massive bleached units exhibit pervasive replacement of volcanic rock by cristobalite, anhydrite, and clay. All igneous textures, except for vesicles, are generally destroyed. Vesicles are partially or fully filled by crustiform to bladed anhydrite and very fine grained drusy pyrite (Units 3, 4, and the upper portion of Unit 7) or by cristobalite/quartz ± pyrite (the lower part of Unit 7 and throughout Unit 9). Late irregular anhydrite-pyrite veins are abundant in Unit 4. The pieces of core that comprise Units 5 and 9 commonly exhibit concentric zonation from white rims to gray cores (Fig. F47), representing alteration halos around fluid pathways—fractures or veins—that were not recovered.

Thin-section petrography and XRD analyses of bleached volcanic rocks indicate that anhydrite, silica polymorphs, and clays are the main alteration minerals (Table T8). Remnant microlitic igneous plagioclase is also rarely present, and barite is a minor alteration phase in some samples. Silica species show a similar zonation to that seen for the GSC altered rocks. Cristobalite is the only polymorph detected in Unit 4 and the upper part of Unit 7. The lower portion of Unit 7, as well as Unit 9, contains significant quantities of both cristobalite and quartz, with the latter mostly occurring as late silica-pyrite vesicle fill. Chlorite and smectite are the main clay phases in Unit 4, whereas illite and pyrophyllite are more abundant in the deeper bleached units.

Bleached Volcaniclastic Units

Bleached volcaniclastic lithologies (Units 12 and 14) contain soft, rounded, granule- to pebble-sized sulfate-clay altered clasts, which are cemented by quartz (Fig. F48). Variations in the color of individual clasts from white to beige and gray are interpreted to reflect compositional differences between the precursors of the clasts, meaning that the unit is polymictic. Larger clasts occasionally exhibit hard siliceous rims, implying that silicification occurred after the bleaching event. Pyrite is finely disseminated throughout the siliceous cement.

Thin-section petrography and XRD analyses (Table T8) of bleached volcaniclastic units indicate that quartz is the dominant phase, replacing the groundmass of the volcanic fragments as very fine grained intergrowths along with birefringent clay (illite, from XRD analysis) and occurring in veins with pyrite (Fig. F49). Anhydrite is present as late vug fill in Unit 14.

Brecciated Bleached Units

Units 11 and 13 comprise completely bleached, sparsely vesicular volcanic rocks that are crosscut by a strongly developed silica-anhydrite-(pyrite) stockwork vein system (Fig. F50). In Unit 11, these stockwork veins are cut by irregular, anastomosing, vuggy anhydrite veins that contain as much as 5% pyrite, mostly as drusy cavity fill. Unit 13 also displays late anhydrite veins, which are less vuggy than those in Unit 11 and have distinct narrow siliceous alteration halos. Pyrite occurs as fine disseminations throughout Units 11 and 13. The silica stockwork in Units 11 and 13 is interpreted to be equivalent to the silica cement of the intercalated volcaniclastic units and shows asimilar later relationship to pervasive bleaching.

Thin-section petrography and XRD analyses indicate that silica polymorphs are the dominant alteration minerals (Table T8), occurring as granular veins and intergrown with fine clays as pervasive groundm ass replacement. Cristobalite is the dominant polymorph of silica in Unit 11, whereas quartz predominates in Unit 13. Traces of talc were also detected by XRD in rocks from Unit 13, whereas remnant igneous plagioclase and disordered chlorite-smectite mixed-layer phases are present in Unit 11. Fine-grained euhedral pyrite is developed throughout, showing increased abundance toward veins. Anhydrite is present as late vein and vesicle fill in Unit 13. Relict igneous Ti magnetite shows partial breakdown to leucoxene and is often partially overgrown by pyrite.

Anhydrite Vein

Unit 15 consists of five pieces (as large as 4-5 cm across) of almost pure anhydrite, probably representing centimeter-thick anhydrite veins. These pieces are rubble from the uppermost part of Core 193-1188A-15R, and they probably represent an in situ lithologic unit (125.70 to 125.92 mbsf, curated depth), although they may also have fallen down the hole. XRD analysis (Table T8) confirms that this vein comprises near pure anhydrite. A few percent drusy quartz and pyrite accompany the anhydrite as accessory minerals.


Pervasive silicification (quartz-illite-chlorite ± magnetite) is the dominant style of alteration in the deeper part of Hole 1188A (Units 16-25; 125.94-184.60 mbsf) and throughout Hole 1188F (Units 27-72; 218.00-374.91 mbsf). Silicified units vary in color from light gray to almost black, and chloritic examples may be green. However, they are easily distinguished from GSC alteration and bleaching in drilled core by their relatively high hardness.

Silicified units commonly retain remnant primary textures, including vesicles (Units 16, 17, 19, 22, 27-29, 45-50, 57, 59-66, and 68-72), perlitic texture (Units 19 and 21), flow banding (Units 17, 44, and 59) and lamination (Unit 18). Thin-section observations and XRD analyses (Table T8) record the presence of preserved plagioclase phenocrysts and/or microlitic plagioclase (discussed below), implying that pervasive silicification is not strongly plagioclase destructive.

Within the silicified sequence, cristobalite is only found in Unit 21 (along with quartz) and in the less intensely altered Units 22, 31, and 40, where it is the sole silica polymorph detected. All other XRD analyses conducted on silicified units indicate that quartz is the dominant alteration mineral (Tables T8, T9). Although mostly indistinguishable in hand specimen, variability of the clay components in the secondary mineral assemblage over the 250-m-wide interval of silicification is indicated by thin-section and XRD studies. The following description of alteration styles is broken down into sections according to these changes.

Silicification in Hole 1188A (126-185 mbsf)

Two very distinctive silicified units were intersected within the Hole 1188A silicified sequence: a finely laminated unit (Unit 18; 136.91-137.79 mbsf) and a relatively weakly altered vesicular volcanic unit (Unit 22; 164.95-165.23 mbsf). As will be outlined below, these units mark significant changes in the secondary mineralogy profile. Silicified units within the hole are accordingly divided at these horizons into an upper silicified succession (Units 16 and 17; 125.94-136.91 mbsf), a middle silicified succession (Units 19-21; 137.79-164.95 mbsf), and a lower silicified succession (Units 23-25; 165.23-184.60 mbsf).

Upper Silicified Succession (126-137 mbsf). The uppermost unit of the silicified succession within Hole 1188A (Unit 16) contains abundant anhydrite veins, as much as 1 cm in thickness, present in the rubble that makes up the core from the upper portion of the unit. These anhydrite veins in Unit 16 may be genetically related to Unit 15, which consists almost entirely of anhydrite vein material (see above). Unit 17 and the lower part of Unit 16 comprise hard, pervasively silicified vesicular volcanic rocks, which are cut by quartz-pyrite veinlets much less than 1 mm in width. These latter veins have 1-mm-wide anhydrite selvages, surrounded by well defined 2- to 5-mm quartz-rich alteration halos. Thin sections of rocks from Unit 17 show complete replacement of volcanic glass by very fine grained phyllosilicates and quartz, with patchy preservation of microlitic igneous plagioclase. Pyrite is present throughout the rock as fine-grained vesicle fill, overgrowing quartz and, on occasion, is overgrown by anhydrite. XRD analyses of Units 16 and 17 indicate that illite is the principal phyllosilicate phase, with subsidiary chlorite in Unit 17 (Table T8).

Laminated Unit (137-138 mbsf). Unit 18 is an intensely silicified, finely laminated gray-green rock (Fig. F51) that was initially thought to be a sedimentary rock. However, examination of two thin sections (Samples 193-1188A-16R-2, 12-15 cm, and 40-43 cm) reveals the unit to be a highly altered flow-banded volcanic flow. The rocks are heavily silicified, but retain aligned plagioclase microlites and scattered phenocrysts (Fig. F52). Former volcanic glass has been altered to very fine grained phyllosilicates (mostly chlorite by XRD analysis) and quartz. Pyrite is disseminated in the altered groundmass.

Middle Silicified Succession (138-165 mbsf). Units 19 to 21 exhibit pervasive silicification and patchy development of bleaching and chloritic (green) alteration. All three units are cut by fine quartz-(pyrite) veinlets. Minor (trace to 3%) very fine grained euhedral magnetite is present as patchy disseminations and on fracture surfaces in Unit 19 and in the upper part of Unit 20 (Table T2). Irregular, anastomosing to simple anhydrite-minor magnetite-trace pyrite veins, which have distinct bleached to siliceous alteration halos, are also present in Unit 18. These appear to be contiguous with the quartz-pyrite veinlets.

Two thin sections were made of rocks belonging to Unit 19 (Samples 193-1188A-17R-1, 90-93 cm, and 17R-2, 33-37 cm). Both samples are completely altered, and the igneous groundmass is replaced by quartz and very fine grained phyllosilicates (chlorite and illite, from XRD analysis) with patchy development of anhydrite. Subhedral magnetite (absent in overlying units), occurring both within anhydrite veins and in the groundmass of the rock, shows evidence of breakdown to "leucoxene" and is commonly replaced or overgrown by pyrite (Fig. F53). Although its presence in veins clearly indicates that magnetite is a product of hydrothermal deposition at Site 1188, within Unit 19 the mineral always shows evidence of breakdown and is, therefore, clearly not stable as alteration progresses. Ultimately, magnetite would be expected to break down fully. This is considered to be a possible factor contributing to the patchy distribution of magnetite throughout Hole 1188F, where magnetite is most abundant in apparently less pervasively altered rocks with remnant igneous plagioclase.

As noted above, the dominant silica polymorph present in Unit 21 is cristobalite (Table T8). This implies that metasomatism within this unit (and the underlying Unit 22; see below) occurred at lower temperatures than those reached in the surrounding quartz-bearing lithologies. The interval probably represents a pocket of rock that avoided the passage of high-temperature fluids. As no thin section was cut from Unit 21, the textural and temporal relationships between cristobalite and quartz, which are also present in the unit (Table T8), are not known. However, it is possible that the cristobalite represents an earlier, lower temperature alteration event, which has been incompletely overprinted by quartz-dominated silicification.

Weakly Altered Volcanic Unit (165 mbsf). Unit 22 comprises fine-grained vesicular volcanic rocks that are weakly to moderately silicified and have plagioclase and magnetite microcrysts in the groundmass. Abundant silica- and anhydrite-filled microvesicles also contain traces of very fine grained euhedral pyrite. The unit is cut by rare irregular microcrystalline silica-anhydrite veins with silicified halos.

Petrography and XRD analysis of Unit 22 indicates that the main style of alteration is fine-grained cristobalite and clay (chlorite, from XRD analysis) replacement of the formerly glassy groundmass. Microlitic and rare phenocrystic plagioclase is unaltered. Fine-grained disseminated magnetite shows evidence of incipient alteration to leucoxene. Cristobalite and later-stage anhydrite fill vesicles.

Lower Silicified Succession (165-185 mbsf). The lower silicified succession in Hole 1188A (Units 23 to 25) comprises green to black, blotchy units with remnant soft, green, clay-rich patches and intervals. The latter, interpreted to be remnant GSC alteration, are surrounded by dark-colored fine-grained quartz-(pyrite) flooding (Fig. F54), which is associated with an anastomosing network of hairline fractures. Vuggy cavities in the matrix are lined by very fine grained crystalline quartz. Very fine grained euhedral pyrite is noticeably more abundant in silicified zones, particularly in the vuggy cavities. Magnetite appears to be replaced by pyrite in the silicified zones, requiring that it predates the silica-pyrite flooding. Very rare, poorly defined quartz-anhydrite-(pyrite) veins also appear to be overprinted by the silicification. Vuggy anhydrite vesicle fill is present in Unit 24.

Thin sections from Units 23 and 25 reveal that the rocks are fine grained and heavily silicified (Fig. F55) with remnant partially altered plagioclase in the groundmass. Some samples show relict perlitic structures. Pyrite forms large subhedral crystals with inclusions of magnetite. Titanomagnetite microphenocrysts are altered to dark opaque patches with trellislike magnetite lamellae, some of which are overgrown by a lighter gray oxide (probably maghemite).

The lower silicified succession of Hole 1188A is the only interval encountered at Site 1188 where XRD analyses (Table T8) reveal corrensite as the dominant clay component. The occurrence of corrensite in this interval (165-185 mbsf) is interesting because it has been demonstrated that corrensite is indicative of a formation temperature of ~250°-270°C (Schiffman and Fridleifsson, 1991; Beaufort and Meunier, 1994; Lackschewitz et al., 2000).

Unit 26 (193-202 mbsf). The final unit described in Hole 1188A (Unit 26; 192.70-202.29 mbsf) consists of vuggy massive sulfate-silica rubble with traces of disseminated pyrite and iron oxide spotting. It is considered likely that this unit, which most closely resembles Unit 15, is material that has fallen down the hole. It is therefore likely that an uncored vertical interval of ~30 m (192.70-218.0 mbsf) lies between the deepest in situ cores of Hole 1188A and the highest of Hole 1188F.

Silicification in Hole 1188F (218-375 mbsf)

The silicified units from Hole 1188F are divided on the basis of mineralogy into upper and lower sequences.

The upper sequence includes Units 27 through 45 (Cores 193-1188F-1Z to 19Z; 218-269 mbsf). It is characterized by the general absence of phyllosilicate phases other than illite in XRD spectra (Table T9) and by an almost complete absence of remnant igneous plagioclase. Two apparently spherulitic volcanic units (Units 31 and 40) show a distinctively different style of alteration to the other lithologies within the interval and are discussed separately.

The lower sequence extends from Unit 46 to Unit 72 at the bottom of the hole (Cores 193-1188F-22Z to 44Z; 282-375 mbsf). It is distinguished from the upper sequence by the widespread identification of phyllosilicate phases other than illite (most notably chlorite) in XRD spectra, the presence of remnant igneous plagioclase, and the sporadic occurrence of macroscopically visible magnetite below 320 mbsf. The magnetite-enriched lithologies are distinctive from the remainder of the lower sequence from Hole 1188 and are consequently discussed separately.

Upper Silicified Sequence (218-269 mbsf). Units 27 to 45 collectively comprise a sequence of dominantly pale to medium gray, sparsely vesicular, aphyric to weakly plagioclase-phyric volcanic rocks with occasional remnant spherulitic and flow-banded textures. Small (<1 mm) and, in some cases, large vesicles are typically filled with crystalline quartz ± pyrite, producing a distinctive spotted texture of small ovoid quartz amygdules in the more vesicular intervals. Some intervals are dark greenish gray, reflecting the presence of green chloritic clay.

The rocks of the upper sequence of Hole 1188F are cut by a network of anhydrite ± pyrite ± quartz veins, which have widths ranging from <0.1 to 10 mm, but are mostly between 1 and 3 mm wide. These veins usually exhibit cyclical alteration halos of dark and light bands, which are typically an order of magnitude wider than the associated vein (Fig. F56). Hard, dark bands contain more quartz, whereas the paler bands are softer and richer in clay. The vein network and its associated alteration appear to overprint the pervasive silicification, which is the dominant style of alteration in Units 27 to 45 (see below).

Thin-section petrography of samples from the upper sequence of Hole 1188F shows a consistent mineralogy dominated by quartz (4%-65%) and clays (25%-60%), with lesser anhydrite (<1%-17%) and pyrite (<1%-5%). Two principal clay species are present—illite and an apparently amorphous clay, which is brown when viewed in transmitted light and has a white, waxy appearance in reflected light, owing to internal reflections (Fig. F57). The distribution of this latter clay corresponds to the softer pale bands observed around anhydrite veins in hand specimen. Initial PIMA results indicate the presence of pyrophyllite, even in intervals where it could not be identified by XRD (e.g., in the upper sequence of Hole 1188F). It is considered likely that the amorphous-looking clay observed in thin section is pyrophyllite. Chlorite is also present in minor quantities in rocks with a greenish color.

The rocks of the upper sequence of Hole 1188F typically contain pseudomorphed igneous plagioclase microcrysts that are completely replaced by phyllosilicates. In weakly porphyritic intervals, palimpsest plagioclase phenocrysts are represented by fine-grained, often concentrically zoned intergrowths of illite, illite + possibly halloysite, or illite + pyrophyllite (Fig. F58). In two thin sections (Samples 193-1188F-13Z-1, 30-36 cm, and 14Z-1, 62-64 cm) remnant unaltered plagioclase was observed. Anhydrite mainly occurs as medium- to coarse-grained bladed crystals in narrow veins, which generally contain pyrite and may also contain subhedral quartz. The veins have distinct banded alteration halos, corresponding to quartz-rich and pyrophyllite-rich bands. Quartz is present throughout the rock as medium-grained crystalline aggregates filling amygdules, and as fine-grained granular groundmass intergrown with illite. Pyrite and anhydrite also are present as vesicle fill, both with and without quartz.

The gross textural relationships between minerals in rocks from the upper sequence of Hole 1188F suggest at least two stages of alteration. Early pervasive quartz-illite replacement of the primary volcanic mineralogy is overprinted by locally developed, banded quartz-pyrophyllite alteration associated with anhydrite-pyrite veining. Thin sections of core pieces that do not exhibit anhydrite veins and associated zoned alteration typically contain only subordinate pyrophyllite (Fig. F59).

In an apparently volcaniclastic interval within Unit 27 (interval 193-1188F-1Z-2, 0-9 cm), 1-cm rounded clasts are composed of finely crystalline quartz and illite, with very little pyrophyllite and almost no pyrite. They contrast sharply with the volcanic matrix of the rock, which is very porous, contains 2%-3% plagioclase pseudomorphed by clay, and is predominantly composed of pyrophyllite and illite with very little quartz (Fig. F60). The "clasts" are, therefore, likely to be xenoliths of altered volcanic rock that were incorporated into a lava during eruption. Similar relationships are observed in surface rocks in the Snowcap hydrothermal site (Yeats et al., 2000), where altered xenoliths are hosted by fresh aphyric dacitic glass. In this case, the silica-illite altered fragments are hosted by a rock that has subsequently experienced pyrophyllite alteration.

The XRD analyses of Units 41 to 44 (Table T9) indicate the possible presence of brittle mica as a major clay phase. However, the mineral was not identified in thin section, and the samples that contain it were otherwise similar to the remainder of the upper sequence of Hole 1188F.

Spherulitic Units (234 and 242 mbsf). Units 31 and 40 are distinctly out of character with respect to the remainder of the upper sequence of Hole 1188F. Each of these intervals (193-1188F-6Z-1, 45-47 cm, and 13Z-1, 80-83 cm) is represented by single small, flat, rounded pebblelike pieces <5 cm in diameter. They are identical, so it is possible that they actually represent a single lithologic interval, one or both having fallen from higher in the hole, although neither was at the end of a core. The units have an apparent spherulitic texture with white spheroidal domains hosted in a dark green siliceous matrix.

In thin section, these rocks consist of altered, brown, isolated and coalesced subcircular domains of randomly oriented, partially devitrified volcanic glass and microlitic plagioclase, generally 0.1-1 mm in diameter, which rarely have central radiating crystal aggregates. Brown clay rims the margins and forms concentric rings in some of the domains, and many are overgrown by radiating cristobalite (Fig. F61). Although the texture is not entirely typical of spherulitic volcanic rocks, it is considered likely that it has a similar origin (see "Igneous Petrology"). Similarly, the presence of volcanic glass, fresh plagioclase, and cristobalite (most likely produced by devitrification) indicates that these rocks are not strongly affected by the quartz-illite alteration that is widespread elsewhere in the upper sequence of Hole 1188F. The only clay species detected by XRD analysis is chlorite.

Because Hole 1188F was cased to well below the depth of the cristobalite-quartz transition seen at ~100-110 mbsf in Hole 1188A, Units 31 and 40 must derive from an interval or intervals that escaped the otherwise pervasive quartz-illite-dominated alteration of the surrounding rocks.

Lower Silicified Sequence (282-375 mbsf). The lower sequence of Hole 1188F (Units 46 to 72; Cores 193-1188F-22Z to 44Z) immediately follows an interval from 272.9 to 282.1 mbsf of no recovery and is distinguished from the upper sequence by the common occurrence of phyllosilicates other than illite, the appearance of relict fresh plagioclase, and the sporadic occurrence of magnetite-enriched rocks. Rocks from Units 46 to 49 (interval 193-1188F-22Z-1, 0 cm, to 26Z-1, 26 cm) have <5% plagioclase in thin section, and XRD analysis only detected significant amounts of plagioclase in one of nine samples (Table T9). Minor amounts of chlorite, chlorite-smectite mixed-layer phases, and smectite were identified by XRD in this interval (Table T9).

From Units 50 to 72 (interval 193-1188F-26Z-1, 26 cm, to the end of Core 44Z), plagioclase was detected by XRD in 27 of 32 samples (Table T9), and thin-section observations suggest that up to one-third of the rock is composed of remnant fresh to slightly clay-altered igneous plagioclase. Even the most plagioclase-rich samples show complete alteration of the interstitial groundmass and incipient replacement of plagioclase by clay (Fig. F62). Fresh plagioclase is rarely identifiable in hand specimen. Consequently, Units 50 to 72 have commonly been described as completely altered in visual core descriptions (VCD) (Table T4). Thin-section examination reveals, however, that approximately half of these rocks are completely altered (>95% alteration), with the remainder being very highly to highly altered.

Illite continues as an alteration mineral in rocks of the lower sequence of Hole 1188F, as does brown clay, which, although not detected in XRD analyses, is identified as pyrophyllite, based on initial PIMA results and its optical properties. Illite abundance diminishes with increasing depth in Hole 1188F, whereas that of chlorite increases. PIMA results suggest that, overall, pyrophyllite is less abundant in rocks from the lower silicified sequence than in the upper silicified sequence and that it is restricted to rocks that have lower abundances of chlorite. Chlorite is distinguished in most XRD spectra from the lower sequence and is identified in thin section below a depth of 337 mbsf (Sample 193-1188F-34Z-1, 45-47 cm), although it is not always possible to optically distinguish chlorite from other phyllosilicates. Chlorite typically replaces groundmass and may also line or fill vugs. Where chlorite is clearly identified, it forms green, pleochroic mats with anomalous Berlin blue birefringence colors. As was observed in the upper sequence, pyrophyllite alteration is patchy or confined to banded vein halos, once again suggesting it postdates the early quartz-illite-chlorite alteration.

Similar to the upper sequence in the hole, rocks representing the lower sequence of Hole 1188F often have spotted textures, which can usually be attributed to amygdules filled with quartz ± pyrite, less common anhydrite and chlorite, and very rare magnetite. The vein mineralogy continues to be dominated by anhydrite-pyrite ± quartz, with rare quartz-clay veins. However, anhydrite-pyrite veins with centimeter-wide zoned halos—a distinct characteristic of many rocks from Units 27 to 45—are much less common in the lower sequence.

Anhydrite is less abundant in the lower sequence of Hole 1188F than in Hole 1188A and the upper sequence of Hole 1188F. It is generally restricted to veins and late vesicle fill, and its abundance is estimated to be <2% in all thin sections representing the lower sequence. In contrast, more than half of the thin sections from the upper sequence are estimated to contain >3% anhydrite.

Magnetite-Enriched Lithologies (323-375 mbsf). Six magnetite-enriched units were encountered in Hole 1188F (intervals 193-1188F-31Z-1, 0-10 cm, in Unit 52; 34Z-1, 24-71 cm, in Unit 55; 37Z-1, 29-35 cm, in Unit 58; 39Z-1, 55-117 cm, in Unit 63; 40Z-1, 0-16 cm, in Unit 65; 41Z-1, 0-17 cm, in Unit 68; and 43Z-1, 56-114 cm, and Core 44Z in Unit 72). These units vary in thickness from 6 to 149 cm curated length and are often limited to single pieces intercalated with a sequence of more typical quartz-illite-chlorite altered rocks.

In hand specimen, magnetite appears to be finely disseminated and intergrown with clay and quartz, giving the rock a dark gray to black color. Magnetite also is present as vesicle fill (e.g., in Unit 65), where it is intergrown with clay and overgrown by pyrite filling the centers of amygdules (Fig. F63). In Unit 55, a slightly brecciated volcanic rock, magnetite contents are highest in the quartz-rich cement that hosts silicified angular clasts. Yet different modes of magnetite occurrence are revealed in Unit 58 (magnetite-rich halos around quartz-anhydrite-pyrite-rich domains) and Unit 72 (magnetite-rich halo along a diffuse pyrophyllite-quartz vein with a discontinuous anhydrite-pyrite core) (Fig. F64).

Although magnetite modes of up to 15% were estimated while preparing the VCDs, thin-section examination suggests there is <5% magnetite in the rocks. However, other oxides and spinels (hematite, maghemite, and hercynite) are in some places associated with magnetite. Thin sections of Units 55 and 58 (intervals 193-1188F-34Z-1, 45-47 cm, and 37Z-2, 31-33 cm, respectively) contain as much as 3% of a green to brown spinel, tentatively identified as hercynite (Fig. F65). This spinel is often rimmed by magnetite, and both phases are most abundant as inclusions in matrix quartz. Hematite is commonly found as inclusions in quartz (Fig. F66A), where it is associated with granular magnetite, but it may also form clusters of bladed crystals set in a mat of chlorite and a colorless, high-birefringence phase (Fig. F66B), which may be alunite or brucite (both phases were identified by XRD in the sample).

Relatively coarse (0.1-0.5 mm) opaque aggregates, with four- and six-sided shapes, were observed in polished thin sections of magnetite-enriched rocks. These aggregates represent the dark magnetic spots observed in hand-specimen samples and comprise fine (generally <10 mm) magnetite laths enclosed in a dark opaque matrix with rare pale internal reflections in reflected light. They are most abundant in domains that contain fresh igneous plagioclase. This and their shapes suggest they may be remnant altered igneous titanomagnetite. Subhedral pyrite overgrows and replaces some of the aggregates (Fig. F67). Similar magnetite-bearing aggregates were observed in the lower units of Hole 1188A, where they are also partly replaced by pyrite.

Summary of Spatial Variation of Alteration at Site 1188

The style and mineralogy of hydrothermal alteration at Site 1188 can be summarized as follows:

  1. Fresh rhyodacite, with patchily developed silica-clay and iron oxide films, extends from 0 to 34 mbsf.
  2. A pervasively silicified (opaline silica, with minor illite and smectite) perlitic volcanic unit (only 15 cm recovered) underlies the fresh rhyodacites and marks the upper limit of pervasive alteration at Site 1188.
  3. A zone of massive bleached volcanic rocks (cristobalite-clay-anhydrite-pyrite alteration) underlies the opaline unit and extends to a depth of 49 mbsf.
  4. A zone of intensely veined and fractured breccias and pseudobreccias with veins of anhydrite-silica(cristobalite grading to quartz with increasing depth)-pyrite intercalated with more massive bleached units (cristobalite-illite-anhydrite alteration grading to quartz-illite-anhydrite alteration, with minor pyrophyllite and patchy chlorite throughout) extends from 49 to 99 mbsf. Bleaching and silicification overprint earlier GSC alteration, and quartz veining becomes more predominant toward the base of the interval.
  5. A zone of hydrothermally brecciated, silica-veined, bleached units with intercalated coarse-grained volcaniclastic sediment extends from 106 to 126 mbsf. Silica veining and silica cementation of the sedimentary units postdates pervasive bleaching, but is cut by late anhydrite veins with bleached alteration halos. Cristobalite is the dominant silica polymorph at the top of the interval, but quartz dominates in the lower portion. Except for narrow anomalous units of partially altered volcanic rock, the deepest recorded occurrence of cristobalite in the hydrothermal system is within this interval, at ~116 mbsf.
  6. Pervasive silicification is the dominant style of alteration in the remainder of Hole 1188A (126-185 mbsf, discounting the final two cores, interpreted to be fallen rubble) and throughout Hole 1188F (218-375 mbsf). Quartz is the most abundant alteration mineral, whereas anhydrite generally decreases in abundance with depth and is mostly confined to late veins. Disseminated and less abundant vein-related magnetite is present in units between 140 and 185 mbsf in Hole 1188A, and again below a depth of 322 mbsf in Hole 1188F. The XRD analyses (Tables T8, T9) indicate that, with the exceptions of a 20-m interval (165-185 mbsf) at the bottom of Hole 1188A, where corrensite is the only clay detected, and a 9-m interval (247-256 mbsf) of possible brittle mica within Hole 1188F, illite and chlorite (particularly in the lower part of Hole 1188F) are the dominant clay minerals associated with silicification at Site 1188. The style of hydrothermal alteration in Hole 1188F gradually changes from pervasive quartz-illite with late vein-related anhydrite-pyrophyllite to quartz-chlorite-illite-magnetite-dominated between 280 and 300 mbsf. Associated with this change is a general trend of decreasing extent of alteration from 100% to 65% going down the hole. Whereas igneous plagioclase is rarely preserved within silicified rocks in Hole 1188A or the upper portion of Hole 1188F, at depths >300 mbsf relatively calcic (An51-An63) plagioclase is commonly present as a remnant igneous phase. This apparent decrease in alteration intensity with depth may alternatively reflect increased stability of calcic plagioclase under the prevailing alteration conditions.


The pervasive nature of hydrothermal alteration at Site 1188 indicates that the overall fluid fluxes below the Snowcap hydrothermal site must have been very high, implying prolonged periods of vigorous circulation of hydrothermal fluids. The spatial and temporal relationships between the different types of alteration encountered in the two successfully cored holes at Site 1188 allow a number of preliminary hypotheses to be advanced regarding the nature of the hydrothermal system at Snowcap.

In the upper portion of Hole 1188A (34-125 mbsf), pervasive GSC alteration is postdated by gray to white anhydrite-related silica-clay bleaching. Zones of pervasive bleaching and GSC alteration alternate, sometimes within single sections of core, and XRD results (Fig. F35; Table T8) indicate patchy distribution of different phyllosilicates (illite, smectite, and chlorite) and pyrophyllite. The transition from opaline silica (at 34 mbsf) to cristobalite to quartz (below 116 mbsf) also is developed in this zone. In the interval from 105 to 125 mbsf, locally developed quartz-pyrite veining and associated patchy silicification clearly overprint both these styles of alteration.

In the remainder of Hole 1188A (below 125 mbsf) and throughout the entire cored interval of Hole 1188F (218-375 mbsf), pervasive silicification is the dominant alteration type, showing a general trend from a quartz-illite to a quartz-illite-chlorite-dominated assemblage with increasing depth. The presence of domains of remnant GSC altered rocks (particularly between 150 and 185 mbsf in Hole 1188A) and microtextural observation of relatively coarse grained quartz overgrowing earlier pervasive clay alteration of igneous groundmass suggest that silicification developed as an overprint on an earlier GSClike alteration assemblage. Similarly, sporadic magnetite-enriched lithologies show clear evidence that the iron oxides (magnetite, hematite, and/or maghemite) and, in two cases, spinel are overgrown by quartz.

Pervasive bleaching is not developed below a depth of 125 mbsf at Site 1188. However, an alteration style with a similar mineralogy to the bleached units in the uppermost 125 m is developed throughout the cored interval below this depth, as zoned pyrophyllite-quartz alteration halos along late anhydrite ± pyrite ± quartz veins, which crosscut all earlier alteration types. Similar veins are observed to cut quartz veins within bleached rocks between 105 and 125 mbsf, clearly indicating that although these veins appear to have carried similar fluids to those responsible for the pervasive bleaching in the upper portion of Hole 1188A, they represent a later phase of hydrothermal activity.

Significance of GSC Alteration and Silicification

Although individual samples may reveal textural evidence for pervasive silicification postdating earlier GSC alteration, both alteration styles are intimately related throughout the lowermost 250 m of crust intersected at Snowcap. It is usually very difficult to decide which secondary clay phase was associated with either of those alteration events. For the sake of simplicity, we group these alteration styles together in the following discussion and separately consider the mineralogically similar, but temporally distinct, pervasive bleaching and anhydrite vein halo quartz-pyrophyllite alteration.

Above 125 mbsf, alteration is complex. Pervasive GSC alteration without an overprint is only represented in a few units. Below 125 mbsf, a quartz-illite-chlorite alteration assemblage is very common, sporadically accompanied by magnetite. In Hole 1188A, illite is the dominant phyllosilicate phase in GSC altered and silicified rocks to a depth of ~165 mbsf, after which corrensite and chlorite are more abundant and magnetite-bearing lithologies are more common. A similar transition is seen in Hole 1188F at a depth of 280 mbsf, below which the phyllosilicate mineralogy changes from illite to illite-chlorite to chlorite-illite. As in Hole 1188A, magnetite-rich alteration assemblages are restricted to the lowermost section of Hole 1188F. Also similar is the preferred preservation of igneous plagioclase in magnetite- and chlorite-rich units near the bottom of both holes (Fig. F35).

Magnetite, hematite, and spinel are most abundant as inclusions in quartz. Magnetite in these intervals often occurs as remnant inclusions in pyrite and as vesicle linings that have pyrite in their centers. From these observations, we infer that magnetite-bearing alteration occurred prior to the formation of late-stage quartz and pyrite. The magnetite impregnation appears to postdate an early pervasive stage of alteration, possibly equivalent to GSC alteration higher in the sequence. However, it was clearly followed by a later stage of quartz and pyrite formation, interpreted to be the main silicification event. It is interesting to note that the formation of Fe oxides did not result in significant Fe gains of the altered rock (see "Geochemistry"). Conceivably, magnetite might substitute for pyrite (which is more abundant in vein halos at shallower depths) in the alteration assemblage as a consequence of variations in the H2S(aq) and H2(aq) activities of the altering fluids. The roughly antithetical relationship between pyrite and magnetite abundance in the rocks from Site 1188 (Fig. F68) is consistent with this assertion.

The transition from illite-rich to chlorite-rich rocks with increasing depth at Site 1188 is similar to a gradation from paragonitized to chloritized rocks that was documented for the basement underneath the TAG hydrothermal mound at the Mid-Atlantic Ridge (26°N) (Humphris et al., 1995). However, the appearance of chlorite/corrensite-rich and illite-poor rocks midway in the sequence as well as the sporadic developments of chlorite and illite in the upper part of Hole 1188A suggests that the basement alteration at Snowcap hydrothermal site may be more complex than underneath the TAG mound.

A general gradation upward from chlorite-rich to illite-rich rocks would be consistent with the generally accepted compositional evolution of hydrothermal fluids at mid-ocean ridges. 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 may stabilize illite shallower in the system (Seyfried et al., 1999). The alteration mineralogy of the basement underneath Snowcap hydrothermal site does not reveal such a simple gradation in phyllosilicate mineralogy, which may suggest multiple episodes of fluid flow along dynamically changing fluid pathways. Alternatively, the low Fe and Mg contents of the dacitic volcanic sequence at the PACMANUS hydrothermal field, when compared to mid-ocean-ridge basalts, may explain the overall minor and intermittent occurrence of chlorite.

Significance of Bleaching

Pervasive and vein-related bleaching are developed throughout the altered volcanic sequence at Site 1188, showing a general decrease in intensity with increasing depth. Although sharing a similar mineralogical assemblage (quartz or cristobalite, anhydrite, illite, and pyrophyllite), the pervasive bleaching at depths <125 mbsf in Hole 1188A predates silicification and is clearly earlier than the vein-related bleached alteration halos, which are most strongly developed between 220 and 280 mbsf in Hole 1188F.

The large amount of anhydrite precipitated in bleached rocks at Snowcap hydrothermal site suggests that entrained seawater must play a major role in the evolution of the hydrothermal system. Anhydrite shows highly variable concentration downhole, particularly in the upper 250 m of the 375-m section drilled at Snowcap hydrothermal site (Fig. F68), suggesting that seawater flux within the volcanic sequence is fracture controlled and highly variable. The fact that pervasive bleaching is confined to the upper 125 m of the sequence and the generally low abundance of anhydrite below 280 mbsf (Fig. F68) indicates, unsurprisingly, that seawater circulation and mixing were more pronounced at shallow depths.

Complex crosscutting relationships between various sets of quartz-, anhydrite-, and pyrite-bearing veins (see "Structural Geology") imply episodic anhydrite-pyrite-quartz formation and consequently indicate repeated influx of different types of fluid. The width and complex banding of halos adjacent to many anhydrite-pyrite veins in Hole 1188F suggest that these veins could have been formed during repeated episodes of fluid flow with fluctuations in fluid chemistry and/or temperature.

Mixing of entrained seawater with hydrothermal fluids can be inferred from the widespread co-occurrence of anhydrite, quartz, and pyrite. These three phases will only co-precipitate in significant quantities if sulfate is supplied by seawater and if hydrothermal fluids supply Fe, SiO2, and H2S. The pyrophyllite present in bleached vein halos and in massive bleached units at Site 1188 requires low-pH fluids. This provides some support for the hypothesis that, despite the obvious importance of seawater, magmatic fluids may play a role in the hydrothermal system at Snowcap.