Hole 1188A

Sulfide minerals, dominantly pyrite in a matrix of anhydrite, silica, and clay minerals, are present throughout Hole 1188A core. However, the modal percent sulfide rarely exceeds 5%, designated as the threshold for "sulfide rocks" for Leg 193 (see "Sulfide and Oxide Petrology" in the "Explanatory Notes" chapter for details). The threshold value was achieved in nine of the 23 cores recovered. Three principal sulfide modes of occurrence were identified in core from Hole 1188A: disseminated pyrite (Type 1 in Table T6 in the "Explanatory Notes" chapter), anhydrite-silica veins with pyrite (Type 2), and silica-anhydrite-magnetite-pyrite veins (Type 3). See "Site 1188 Visual Core Descriptions" and "Site 1188 Sulfide Log" for details.

Within the hydrothermally altered rocks in Section 7R-1 and below, pyrite is predominantly present at concentrations of <5% (Fig. F68) within two distinct settings: disseminated throughout the variably altered igneous protoliths (Type 1) and associated with anhydrite-silica filled fractures (Type 2). Pyrite also lines some vesicles with anhydrite ± silica (Type 2a). The greatest concentrations of macroscopically visible pyrite are within fractures (Type 2). Extremely fine grained pyrite, suspected in hand specimen and confirmed by XRD, is also present within dark GSC altered rocks.

Although pyrite is by far the most abundant sulfide in Hole 1188A, sphalerite and chalcopyrite also are present in trace amounts associated with pyrite. The sphalerite is found both in veins and vesicle linings in the uppermost part of the core (e.g., Sections 193-1188A-7R-1 [Piece 15]; 8R-2 [Piece 3]; and 9R-1 [Piece 5]). Trace amounts of chalcopyrite are found within a strongly chloritized portion of the core (Section 193-1188A-12R-1 [Pieces 9-14]).

Magnetite is present as a relict primary igneous phase (Ti magnetite) within the less altered volcanic rocks. Within the altered rocks, magnetite first appears as inclusions within pyrite and quartz, becoming evident as a discrete phase deeper within the hole where the modal percent magnetite in the altered rocks achieves 5% (Fig. F68).

Mineralization Styles

Disseminated Pyrite (Type 1)

Traces of pyrite, typically <0.2 mm in size, are ubiquitous within pervasively altered rocks. Pyrite is present as disseminated euhedral to anhedral crystals and is commonly associated with very thin (<0.5 mm) Type 2 anhydrite ± quartz-pyrite veins. Extremely fine grained pyrite was encountered in the dark portions of altered rocks in Units 5, 7, 9, and 12 (e.g., Fig. F69). This pyrite, although not apparent in hand specimen, was initially identified by XRD and confirmed in thin section. From its peak height, it is estimated conservatively to constitute at least 5% of the mineral assemblage. A more accurate determination cannot be made, so all such samples are designated as containing >5% pyrite in the "Site 1188 Sulfide Log".

Type 1 pyrite appears to be part of the alteration assemblage of anhydrite, silica, and clay minerals and is found in varying proportions, with two distinct modes. The greatest abundance of pyrite is observed within breccia stratigraphically just below fresh dacite and is associated with cristobalite and anhydrite. A second mode, found lower in the core, is principally associated with quartz and magnetite (Fig. F68).

Anhydrite ± Silica-Pyrite Veins (Type 2)

Fine granular masses of pyrite are commonly found near the margins of anhydrite (±silica) veins in altered dacite (Fig. F70). The veins are typically 0.-1 mm wide but can be as much as 5 mm (Section 193-1188A-7R-2 [Piece 1]). The veins are found in the shallower sections of Hole 1188A between Sections 193-1188A-7R-2 and 16R-1. The veins have open space fill textures, with anhydrite, the main gangue mineral, showing fibrous growth at a high angle to the vein margins (Section 193-1188A-9R-1 [Piece 6B]). A particularly good example of veins with coarse-bladed anhydrite can be found in Section 193-1188A-15R-1 [Pieces 1 to 5], although these examples are devoid of pyrite. Two-phase (liquid + vapor) fluid inclusions are abundant in this anhydrite (Fig. F36), and one example of a three-phase inclusion (liquid + vapor + an unidentified birefringent daughter mineral) was observed. Such barren anhydrite veins postdate the anhydrite-pyrite-silica veins (Fig. F71). Microscopic examples of Type 2 mineralization are shown in Figures F72 and F73.

Pyrite cubes, 0.1-0.35 mm in size, are present with anhydrite and silica within the vesicles of altered dacite (Type 2a). The finer-grained pyrite is found toward the margins of the vesicles within anhydrite. The coarser pyrite crystals are a druse of open space fill toward the center of the vesicles. Rare chalcopyrite crystals are also found as open space fill. Quartz crystals, showing good crystal terminations, are commonly observed near the center of the vesicles.

Silica-Anhydrite-Magnetite ± Pyrite Veins (Type 3)

Magnetite showing occasional octahedral habit and typically 0.2 mm in diameter is present in Core 193-1188A-17R (146.16-146.53 mbsf) within GSC altered and silicified volcanic rocks. The magnetite is contained within microveins, associated with bladed anhydrite crystals and variable modes of pyrite; the pyrite to magnetite ratio varies between 0 and 1. Deeper in the section (Section 193-1188A-21R-1 [Pieces 4 to 18]; 183.37-184.25 mbsf), fine network veins of quartz-pyrite-(anhydrite) produce pervasive silicification, which obliterates a magnetite-rich GSC altered volcanic rock. This pronounced increase in magnetite near the bottom of Hole 1188A is evident in the plot of magnetite against curated depth in Figure F68.


Some of the larger pyrite crystals within the deeper sections of Hole 1188A contain extremely small inclusions of magnetite (Fig. F74). The pyrite has clearly overgrown the magnetite and has not reacted with it, suggesting that the two phases were in equilibrium, unless there was a kinetic inhibition (low temperature) for reaction. Magnetite with the same distinctive vermicular texture as the inclusions is also present nearby in a quartz matrix (Fig. F75). Further evidence for the magnetite predating pyrite is seen in Figure F76. Within this thin section, in a region of tan-colored alteration, rhomb-shaped pyrite crystals appear to engulf magnetite. The magnetite inclusions in the pyrite are smaller toward the center of the rhomb, suggesting that pyrite is replacing magnetite. Elsewhere, however, in a less silicified part of this same thin section, there are remnants of magnetite-free pyrite. Also, in this same less altered region, there is an example of magnetite grains on the margin of and within pyrite, suggesting that, here, magnetite postdates pyrite. Taken together, it appears that at least some of the magnetite and pyrite were penecontemporaneous and a reaction such as Fe3O4 + 3S2 = 3FeS2 + 2O2 may have proceeded in both directions at different times or places. Two gray minerals in this same thin section have a lamellar, latticelike texture reminiscent of magnetite-ilmenite exsolution (Fig. F77). However, from optical properties, the laths appear to be magnetite being replaced by maghemite or hematite.

Trace amounts of both black (Fe rich) and yellow (Fe poor; probably <1% Fe) subhedral sphalerite are locally developed as overgrowths on cavity-lining pyrite euhedra in vugs, and therefore are paragenetically later (Fig. F78). Thus, the paragenetic sequence observed for the opaque minerals in Hole 1188A is (early) magnetite pyrite ± magnetite sphalerite (late). The relative timing of deposition of the black and yellow sphalerite could not be determined. However, the apparent large differences in their iron contents, as indicated by their contrasting colors, indicate that they may have been precipitated at different times.

Hole 1188F

Pyrite is the dominant sulfide mineral within Hole 1188F. However, the modal percent sulfide never exceeds 5%; therefore, there are no new entries in the sulfide log (see the "Site 1188 Sulfide Log"). Instead, the information on sulfides and related minerals is recorded in the alteration log (see the "Site 1188 Alteration Log"). Macroscopic estimates of the abundance of pyrite range up to 4%, of which the vast majority (~88%) are 1%. The relatively elevated pyrite abundances (3% to 4%) are in Cores 193-1188F-8Z, 11G, 13Z, 17Z, and 34Z (see Fig. F68). The style of mineralization is similar to Hole 1188A. Pyrite is present as disseminated grains within altered volcanic groundmass (Type 1) (see Table T6 in the "Explanatory Notes" chapter), in thin veins with variable amounts of anhydrite and quartz (Type 2), and lining some vugs (former vesicles) together with clay minerals, anhydrite, and quartz (Type 2a). The veins are described in detail in "Structural Geology".

Other opaque minerals identified macroscopically are marcasite, both yellow and black sphalerite (or wurtzite), magnetite, and hematite. Marcasite is present with anhydrite and quartz as platelike bronzy radiating crystals on fracture surfaces. The mineral was not encountered in polished thin sections. Magnetite is present in trace amounts throughout the hole, increasing in abundance below 300 mbsf (Fig. F68).


Pyrite is found in Type 1, 2, and 2a settings (see Table T6 in the "Explanatory Notes" chapter), as observed in Hole 1188A; however, the modal percent pyrite in Hole 1188F is generally lower than that observed in the hydrothermally altered sections of Hole 1188A. Much of the pyrite, especially in vugs (Type 2a), is euhedral (Fig. F79). Some crystals have a brassy tarnish, which was assumed to be incipient oxidation. This oxidation is particularly prevalent in Cores 193-1188F-14Z and 15Z.

In thin section, pyrite typically contains small anhedral inclusions of magnetite and quartz, with lesser pyrrhotite and rare chalcopyrite (Figs. F80, F81, F82). Pyrite may also contain inclusions of hematite, both as rare replacive intergrowths with magnetite (Figs. F80, F83) and as platelets (Fig. F84).


Chalcopyrite is present in polished thin section as isolated anhedral grains and, in a few places, as partial replacements of pyrite (Fig. F85). Chalcopyrite is more obvious as a trace component within thin sections from Cores 193-1188F-6Z and below, where the mode of chalcopyrite peaked at 0.5% in Sample 193-1188F-37Z-2 (Piece 3, 31-33 cm). In this thin section, chalcopyrite is present as aggregates with pyrite and coarse quartz, and there is one example of chalcopyrite enclosed within pyrite. Elsewhere in this thin section, chalcopyrite was observed within the groundmass.


Pyrrhotite was not observed in hand specimen bu)t is a trace sulfide, typically as small, pink, anhedral to subhedral, 0.006- to 0.01-mm inclusions within pyrite grains. Pyrrhotite inclusions are observed alongside, but not intergrown with, inclusions of magnetite in the same pyrite grains (Fig. F82).


Present as only a trace component within much of Hole 1188F, magnetite achieves modes in excess of 3% in thin section with some hand specimen estimates as high as 10% at ~340 mbsf (Sample 193-1188F-36G-1 [Piece 3, 22-25 cm]). These magnetite-bearing samples have vugs filled with green clay, anhydrite, pyrite, and magnetite. Elsewhere, magnetite is present within veins of intergrown quartz, brown clay, and pyrite, with very rare extremely fine grained (0.005 mm) magnetite as inclusions in the quartz. These veins have a distinct dark halo, which comprises quartz and very fine granular magnetite (0.001- to 0.005-mm grains) with brown clay. This alteration type is discussed in "Hydrothermal Alteration" and "Structural Geology".

Detailed microscopic observations reveal a few examples of Ti magnetite-ilmenite exsolution (Fig. F86). This particular mineral assemblage with its characteristic texture is best represented in Sample 193-1188F-30Z-1 (Piece 2, 5-7 cm). Coarser ilmenite is intergrown with magnetite in Sample 193-1188F-37Z-2 (Piece 3, 31-33 cm) (Fig. F87). Magnetite also is present as remnants in leucoxene within the groundmass in many thin sections from Hole 1188F.


Most of the hematite seen in thin section is present as ruby red platy inclusions in quartz (Fig. F88). It is particularly abundant (1%) in Sample 193-1188F-37Z-2 (Piece 3, 31-33 cm). In this same thin section, magnetite contains inclusions of hematite, and ilmenite is intergrown with magnetite (Fig. F87). Tiny euhedral platelets of red hematite were encountered in a vug in Sample 193-1188F-42Z-1 (Piece 5, 98-123 cm).


A transparent to translucent spinel (Fig. F89) is enclosed within clear quartz in Samples 193-1188F-34Z-1 (Piece 9A, 45-47 cm) and 37Z-2 (Piece 3, 31-33 cm), respectively. The color of the mineral varies in plane-polarized transmitted light from bright apple green to a dark greenish brown. Crystals, many observed as perfectly formed and twinned octahedra with growth steps on their {111} face, are as large as 0.02 mm in size. The spinel contains tiny inclusions of what appears to be magnetite and is also rimmed by a thin film of magnetite and coarser grains of magnetite and ilmenite (Figs. F89, F90).


The dominant type of alteration within Hole 1188F is silicification (Sil), with a quartz-illite-dominated assemblage from 200 to 270 mbsf, grading into a quartz-chlorite-illite-dominated assemblage below 270 mbsf (see "Hydrothermal Alteration").

The parageneses of magnetite and hematite observed in samples from Hole 1188F are very complex. Magnetite (or Ti magnetite) is present both as a primary igneous phase and as a hydrothermal phase. Igneous magnetite is commonly enclosed by pyrite (Figs. F91, F92, F93), as was the case in Hole 1188A, and also as remnants in leucoxene. An excellent example of the presumed Ti magnetite precursor to the leucoxene-magnetite alteration assemblage was seen (Fig. F86). There are a few examples of igneous magnetite inclusions in pyrite that are partially replaced by hematite (Fig. F87). Elsewhere, magnetite in the groundmass contains inclusions of hematite (Fig. F90), but other magnetite grains are partially replaced by hematite. This magnetite, commonly enclosed within quartz, belongs to the hydrothermal mineralization suite that permeates some rocks.

Sample 193-1188F-26Z-1 (Piece 2, 20-23 cm) contains several examples of pyrite replacing an earlier quartz-magnetite assemblage. A first generation of quartz (quartz-1) forms euhedral crystals, some with a perfect hexagonal shape that are overgrown by magnetite (Fig. F94). Late quartz (quartz-2) overgrows this assemblage and encloses the magnetite (Figs. F94, F95). This quartz-2 has an irregular anhedral morphology. There are rare small inclusions of pyrite in quartz-2 in association with magnetite. The size and rarity of these pyrite inclusions creates problems in deciding if they represent an earlier generation of pyrite or if they are simply a consequence of a three-dimensional distribution within a larger pyrite crystal that surrounds this assemblage.

A late stage of pyrite formation overgrows and partially replaces this quartz-magnetite assemblage (Fig. F94). The pyrite contains small inclusions of magnetite crystals that have the same size and distribution as those within quartz-2. The cores of the pyrite crystals have inclusions of magnetite that are considerably smaller than those close to the edge, suggesting that there has been some replacement of magnetite as well as all of the quartz. The distribution of the magnetite inclusions in the pyrite, regardless of their size, serves to outline the shape of the original quartz-2-magnetite mass that was replaced (Fig. F95). Anhydrite that is present as thin veins with pyrite crosscuts quartz-1 (Fig. F94) and probably quartz-2.

Site 1188 Summary and Paragenetic Sequence

A paragenetic diagram for Site 1188 is presented in Figure F96. Different sulfide and oxide mineral assemblages are associated with the primary igneous suite (Ti magnetite and ilmenite) as well as the superimposed predominant alteration assemblages of cristobalite-illite-pyrophyllite and quartz-chlorite-illite.


Pyrite is the dominant sulfide mineral at Site 1188. The first appearance of pyrite correlates with the uppermost appearance of hydrothermal alteration in dacitic volcanics in Hole 1188A. Within the hydrothermally altered rocks, Cores 193-1188A-5R and below, pyrite abundance is predominantly <5%, with four distinct settings (see Table T6 in the "Explanatory Notes" chapter): disseminated throughout the variably altered igneous protoliths (Type 1); associated with anhydrite-silica-filled veins (Type 2); as vesicle linings associated with anhydrite ± silica (Type 2a); and in silica-anhydrite-magnetite veins (Type 3). The greatest concentrations of macroscopically visible pyrite are within fractures (Types 2 and 3). The downhole distribution of pyrite as observed in thin section (Fig. F68) is erratic with peaks of 5% to 10% at ~50, 140, 165, 255-270, and 345 mbsf.


Chalcopyrite is observed at Site 1188, but only as a minor phase reported in hand specimen and thin section. Chalcopyrite first appears in trace amounts within a GSC altered portion of Hole 1188A (Section 193-1188A-12R-1), as small inclusions in quartz. Deeper within the hole, chalcopyrite is observed in thin section as isolated anhedral grains and, in a few places, as inclusions in (Fig. F80) and overgrowths on pyrite (Fig. F85). Chalcopyrite is more obvious as a trace component observed in thin section from Section 193-1188F-6Z-1 and below, where its mode peaked at 0.5% in Sample 193-2288F-37Z-2 (Piece 3, 31-33 cm; 346.12 mbsf). In this sample, chalcopyrite is observed as aggregates with pyrite and coarse quartz, with one example of chalcopyrite enclosed within pyrite. Elsewhere in this thin section, chalcopyrite is within the groundmass. Chalcopyrite is also present in vesicle fill.


Pyrrhotite was not observed in hand specimen but is present in thin sections from Hole 1188F as a trace mineral. It is exclusively present as small (0.006 to 0.01 mm), pink, anhedral to equant subhedral inclusions within pyrite grains (Fig. F81). Pyrrhotite inclusions are found alongside but not intergrown with inclusions of magnetite in the same pyrite grains (Fig. F82). Pyrrhotite is first observed in Section 193-1188F-1Z-3 in lithologic Unit 28 at 221.9 mbsf (see "Igneous Petrology") and is found in many of the deeper samples. It was not observed in Hole 1188A.


Sphalerite is a rare mineral at Site 1188. It was observed only macroscopically in vesicles (Type 2a), in some cases perched on pyrite crystals that it clearly postdates. Fe-poor honey-yellow crystals are most common, but a black variety was also encountered, in one case in the same vesicle as the honey-yellow type. The mineral was not observed in thin section.


Ti magnetite is an accessory mineral and, in places, a liquidus phase of the relatively unaltered dacitic rock that caps the sequence of highly altered rocks. Hydrothermal magnetite, some with octahedral habit and typically 0.2 mm in diameter, first occurs in Core 193-1188A-17R (146.16-146.53 mbsf) within GSC altered and silicified volcanic rocks. The magnetite is observed within Type 3 veinlets, with hematite and hercynite, as described above, in vugs together with anhydrite, green clay, and pyrite and as inclusions within hydrothermal quartz. The downhole distribution of magnetite in thin section (Fig. F68) shows maxima at 150-165 mbsf in Hole 1188A and 355-375 mbsf in Hole 1188F.

Paragenetic Sequence

The mineral parageneses outlined in Figure F96 consider the sulfide and oxide assemblages within the context of the two major alteration mineral assemblages recognized in Hole 1188A. The uppermost parts of Hole 1188A show cristobalite-illite-pyrophyllite-dominated alteration assemblages, crosscut by veins of predominantly anhydrite + quartz and pyrite. Pyrite is the most abundant opaque phase. Lower in Hole 1188A, as chlorite becomes a significant alteration phase along with illite and the degree of silicification increases, magnetite is present as a hydrothermal phase with modal abundances as high as 5%. Magnetite is commonly overgrown by hematite and pyrite but magnetite can, in turn, overgrow earlier formed pyrite. Similar parageneses are repeated within Hole 1188F, although hematite is commonly observed to have replaced magnetite throughout, and with pyrite that commonly contains inclusions of magnetite and pyrrhotite. As was observed in Hole 1188A, the onset of chlorite formation marks a zone of enriched magnetite values.

Observations throughout Hole 1188A suggest that there are at least two phases of pyrite-quartz precipitation, if not more. This view is consistent with the development of crosscutting vein generations observed in Hole 1188A (see "Structural Geology") and with the observation that sulfide-oxide precipitation is predominantly a vein-controlled phenomenon. Whether or not the observed paragenetic sequence is a consequence of a single event or multiple events is open to question. Variable crosscutting vein relationships and variable mineral parageneses, especially between pyrite and magnetite, suggest that contrasting mineralization events may be coeval within the alteration sequence, depending on the nature of veining and fluid composition extant at the time of mineral precipitation.