Site 1191 is located at Satanic Mills, a site of high-temperature hydrothermal venting and sulfide chimney development located between the low-temperature Snowcap (Site 1188) and high-temperature Roman Ruins (Site 1189) hydrothermal sites. We therefore felt that Site 1191 would provide a good location to test whether a similar style of subsurface alteration to that observed at Snowcap and Roman Ruins hydrothermal sites could reasonably be assumed to underlie the bulk of the PACMANUS hydrothermal field. Unfortunately, drilling conditions proved difficult, and Hole 1191A was abandoned at a depth of only 20.1 mbsf without reaching the zone of pervasive hydrothermal alteration, meaning that this hypothesis remains untested.
The vesicular volcanic rocks that were recovered from Hole 1191A at Satanic Mills hydrothermal site were classified as a single lithologic unit, which ranges from fresh to moderately altered (Fig. F1) (see "Site 1191 Alteration Log").
Intervals 193-1191A-1R-1, 0-11 cm (0.00-0.11 mbsf), and 3R-1, 9-26 cm (14.79-14.96 mbsf), are fresh, glassy, aphyric lavas. Alteration of these rocks is restricted to thin films of clayey material and occasional Fe-oxyhydroxide coatings on fracture surfaces. The deeper interval is possibly composed of pieces that have fallen from higher in the hole.
All other samples from Hole 1191A exhibit slight to moderate alteration manifested by patchy to pervasive replacement of the glassy portion of the igneous groundmass by cristobalite (from X-ray diffraction [XRD] analysis) (Table T4), clay, and disseminated marcasite and minor pyrite. Aggregates of framboidal pyrite were observed in a thin section of Sample 193-1191A-2R-2, 103-106 cm. Microlitic plagioclase and scattered phenocrysts of plagioclase, augite, and magnetite are unaffected by the alteration (Fig. F6).
The vesicles of slightly to moderately altered rocks from Site 1191 are lined with a soft, drusy material that contains microcrystalline silica, sulfide, and possibly a clay mineral. Most vesicles also contain rosettes of zeolite overgrowing the silica-clay. The most abundant zeolite in vesicles is colorless and platy, often forming rosette-like aggregates. The mineral has an RI of ~1.48 and a platy habit suggesting that it may be clinoptilolite. Clinoptilolite has also been tentatively described in thin sections, where it appears as vesicle fill in the form of blocky to platy colorless crystals with low birefringence. A much less abundant second zeolite has been observed to form rosettes of radiating, milky, acicular crystals in vesicles. The crystal habit indicates that this zeolite may be phillipsite.
A small number of core pieces show slight brown weathering (see "Site 1191 Alteration Log"), and vesicles near weathered surfaces contain black botryoidal spots that are possibly hematite or goethite. These samples also contain, in addition to the other zeolites, blocky clear crystals of a third zeolite, with an RI of 1.494, thought to be heulandite.
Sulfide veins (marcasite and minor pyrite), 2 mm wide, are found at the fractured ends of many individual core pieces. One interval (193-1191A-2R-1, 103-109 cm) is cut by a 1- to 2-mm-wide marcasite-pyrite vein along which a 1-cm-wide pale alteration halo is developed. The alteration style in the vein halo is similar to that in other parts of the core (silica-clay with disseminated sulfides), but the intensity of alteration is significantly higher.
Alteration of volcanic rocks at Site 1191 (Satanic Mills hydrothermal site) is limited to moderate intensity cristobalite-clay alteration of the glassy portion of the groundmass, with marcasite-(pyrite) veining and zeolite vesicle fill. This alteration differs from that seen in the upper portion of Hole 1189A (Roman Ruins hydrothermal site), where more intense cristobalite-clay alteration is associated with anhydrite-pyrite veining. However, given the generally low recovery at both locations, some caution must be exercised when comparing core from the two sites.
The increase in intensity of cristobalite-clay alteration in the halos of marcasite-pyrite veins at Site 1191 indicates a genetic link between the alteration and the veining. Consequently, cristobalite-clay alteration and sulfide veining are interpreted to have formed by interaction of the rocks with a hydrothermal fluid (rich in SiO2, Fe, and H2S). Spotty iron oxides and zeolites are confined to vesicles and fracture surfaces and are thought to represent later low-temperature, seawater-dominated oxidative alteration that may have no direct relationship to circulation of hydrothermal fluids.
Anhydrite, a common vein and vesicle fill in the uppermost portions of the hydrothermal system at Sites 1188 and 1189, has not been identified in rocks from Hole 1191A. The lack of anhydrite in the shallow subseafloor at Satanic Mills hydrothermal site may be caused by one or more of the following reasons: