Two holes were cored 30 m apart at Site 1189. Both are close to sulfide chimneys in the Roman Ruins hydrothermal site as is LWD Hole 1189C. Because of the close proximity of the two cored holes, a similarity in their alteration and mineralization styles was anticipated.
Pyrite is the most common sulfide mineral within Hole 1189A, generally in trace amounts but infrequently with modes >10%. Other sulfide phases are sphalerite, chalcopyrite, marcasite, and galena. Magnetite is the major oxide phase, with trace amounts of hematite and possibly maghemite. The downhole distributions of pyrite, chalcopyrite, sphalerite, and magnetite are shown in Figure F48 (see "Hydrothermal Alteration").
The style of mineralization includes disseminated pyrite (Type 1) (see Table T6 in the "Explanatory Notes" chapter), pyrite within anhydrite-quartz veins (Type 2), and pyrite lining vesicles (Type 2a). Sulfide rocks, containing by definition >5% sulfide minerals (see "Sulfide and Oxide Petrology" in the "Explanatory Notes" chapter), consist predominantly of Type 2 anhydrite—quartz veins containing pyrite with some trace chalcopyrite. These are present in seven of the 24 units in Hole 1189A. Examples are shown in Figures F71 and F72. The pyrite occurs as very fine to fine-grained granular euhedral and subhedral crystals. Chalcopyrite occurs as very fine grained anhedral grains.
In contrast to Site 1188, quartz is more abundant than anhydrite in the veins, and in some cases, such as Sample 193-1189A-11R-1 (Piece 5, 25-30 cm), the veins are monomineralic quartz. Others range from 7% to 65% quartz or other forms of silica. Figure F73, on the other hand, shows a vein with cockade structure that is entirely anhydrite with ~5% pyrite between some of the layers. Most of the veins are very narrow (<0.5 mm) although Sample 193-1189A-12R-1 (Piece 16, 120-127 cm) is 5 cm wide.
Semimassive sulfide mineralization (Unit 21 of the lithologic log), comprising ~50% sulfides, was recovered in Sample 193-1189A-12R-1, 120-128 cm (Fig. F74). Aggregates of pyrite and chalcopyrite are observed within a very fine grained quartz-pyrite (±clay) matrix, which envelops angular clasts of altered vesicular volcanic rock (Fig. F75). The complete enclosure of the angular clasts with sulfide mineralization running along fractures into the clasts suggests that this is a sulfide-rich Type 2 vein. Within the matrix of the sample, chalcopyrite is intergrown with quartz and partially to completely encloses pyrite. There are few sphalerite grains within the pyrite.
Type 1 disseminated pyrite is widespread within Hole 1189A (Fig. F76). In some samples, Type 1 euhedral pyrite is crowded with inclusions of microlites, which have a similar morphology and distribution as those in the surrounding groundmass (Fig. F77). This pyrite has grown within the groundmass at the expense of the very fine grained material that is the matrix for the microlites. The most spectacular example of pyrite replacing the preexisting volcanic groundmass is found in Sample 193-1189A-8R-1 (Piece 7, 103-105 cm), where the original elongate laths of plagioclase, probably representing an igneous quench texture, are replaced by quartz and pyrite. The pyrite tends to locate along the cores of the replaced laths (Fig. F78). However, the host rock is xenolithic, and the relation of the replacement phenomena with the alteration described for Hole 1189A is uncertain.
Pyrite is commonly associated with Type 2 quartz-anhydrite veins, with the pyrite tending to be located near the selvages of these veins. Where anhydrite is present in the veins, there is a suggestion that the pyrite takes a coarser form. Pyrite grains within the veins commonly contain inclusions of magnetite.
Pyrite is also observed as vesicle fill associated with chalcopyrite (Type 2a). There are several examples in which quartz has coated the wall of a vesicle and pyrite has precipitated on the quartz (Fig. F79). Marcasite is found as a rare mineral enclosed in pyrite (Fig. F80). Framboids, assumed to be pyrite but too small to photograph at 0.006-mm diameter, are within quartz in Sample 193-1189A-2R-1 (Piece 15, 113-115 cm).
Chalcopyrite is commonly present in trace amounts, especially in Section 193-1189B-12R-1, where it is typically located within the clay groundmass. Chalcopyrite is estimated to represent 15% of the semimassive sulfide in Sample 193-1189A-12R-1 (Piece 16, 122-125 cm) (Fig. F74).
Chalcopyrite is locally intergrown with pyrite within quartz veins and may occur as vesicle fill (e.g., interval 193-1189A-7R1-1, 72-92 cm), where chalcopyrite occurs in central cavities of quartz masses within small amygdules (Fig. F81). Where observed as cavity fill, chalcopyrite locally replaces pyrite.
Sphalerite was seen sporadically in thin sections of Type 2 veins, where it is commonly associated with chalcopyrite, which partially replaces it. In such associations, the sphalerite typically develops a chalcopyrite disease texture (Figs. F82, F83).
One grain of galena was encountered, enclosed within pyrite in a Type 2 vein (Figs. F82, F83).
Magnetite is present as a primary igneous phase in the groundmass of fresh and some altered dacites, as scattered grains in Type 2 quartz-pyrite veins and Type 2a quartz-filled amygdules, and as fine inclusions in pyrite. Some of the magnetite has broken down to leucoxene, indicating that it was titanium bearing. A mineral tentatively identified as maghemite rims magnetite grains in Sample 193-1189A-8R-1 (Piece 17, 103-105 cm).
Hematite is found sporadically as an oxidation product (replacement) of magnetite and, in one instance, as a jasper. The bright red jasper or hematitic chert (Fig. F84) contains disseminated grains of pyrite and chalcopyrite. Because the sample is so small (~10 cm3), it is not certain whether this is a vein or a cavity filling in the dacite.
Detailed study of polished thin sections has revealed a number of key sulfide and oxide mineral associations. There is evidence for at least two generations of pyrite, most likely within a continuous mineralizing process, rather than separated by a significant gap in time. In Sample 193-1189A-10R-1 (Piece 9, 75-77 cm), pyrite is present as tiny inclusions in rare magnetite and magnetite is included in anhedral pyrite. In Figure F85, tiny crystals of either sphalerite or magnetite (too small to identify with certainty) have precipitated on early pyrite and, in turn, are overgrown by younger pyrite. There is a rare example (Fig. F86) of sphalerite inclusions with chalcopyrite disease within the second-generation pyrite. In Figure F80, euhedral pyrite crystals are overgrown by marcasite that, in turn, is overgrown by more pyrite. The age relation of the framboids in Sample 193-1189A-2R-1 (Piece 15, 113-115 cm) to the other pyrite is not known.
Figure F87 shows a quartz vein with pyrite intergrown with chalcopyrite-diseased sphalerite and chalcopyrite. A close-up view of another area in this same thin section (Fig. F83) reveals a pyrite crystal containing an inclusion of galena.
Figure F88 shows a magnetite-hematite-pyrite assemblage. The magnetite appears to be partially replaced by both pyrite and hematite pseudomorphs.
A paragenetic sequence that is consistent with the microscopic observations is given in Figure F89. There are at least two generations of pyrite and possibly two generations of sphalerite with chalcopyrite disease. The origin of chalcopyrite disease is somewhat contentious, with both replacive and coprecipitative models supported. Current research suggests that both mechanisms may be important (Bortnikov et al., 1991; Nagase and Kojima, 1997). Textural features are sometimes useful in distinguishing between the two models, although these are strongly influenced by the Fe content of the sphalerite (Nagase and Kojima, 1997). The composition of the sphalerite from Hole 1189A is unknown; consequently, it was not possible to use textures to discriminate between replacement and coprecipitation. The following paragenetic sequence is based on the replacive model for chalcopyrite disease.
Early sphalerite is encapsulated in late pyrite, which would have protected this sphalerite from subsequent reaction, so its chalcopyrite disease is considered to have been caused by an earlier chalcopyrite for which there is no other evidence (i.e., this chalcopyrite has not been observed as a separate phase). The time of deposition of magnetite and quartz is poorly constrained, other than it followed the deposition of early pyrite. The timing of the oxidation of this magnetite to hematite is also not known, nor is the relationship of this hematite, if any, to the hematitic chert (jasper) in Figure F84. The framboids must have formed at low temperature (<100°C). Because they are encapsulated in quartz, their formation must have preceded the quartz that filled the amygdules.
Chalcopyrite in seafloor hydrothermal systems is deposited from fluids with temperatures generally >300°C. A similar temperature is required for the replacive formation of chalcopyrite disease in sphalerite, but sphalerite is typically deposited at much lower temperatures. The paragenetic interpretation presented in Figure F89 suggests a progressive increase in temperature of the hydrothermal system through time, and that this has occurred at least twice (Fig. F89).
Hole 1189B exhibits the widest range of sulfide phases encountered during Leg 193. Minerals recognized are pyrite, chalcopyrite, sphalerite, and rare tennantite and covellite (or bornite). In addition, the oxides magnetite, hematite, and possibly ilmenite (one example) are present.
The majority of sulfide-bearing assemblages in Hole 1189B have trace to 5% sulfides, as disseminations in altered rock (Type 1; in Table T6 in the "Explanatory Notes" chapter), in anhydrite-quartz veins (Type 2), as vesicle linings with quartz and anhydrite (Type 2a), and with magnetite in anhydrite-quartz veins (Type 3). Hole 1189B also has examples of semimassive sulfides (25%-75% sulfides) and an ~1-cm3 sample of massive sulfide (Sample 193-1189B-06R-1 [Piece 6]: 90% pyrite, trace chalcopyrite and sphalerite, 7% quartz, and 3% anhydrite). The latter, the only massive sulfide encountered on Leg 193, is sufficiently small that it is possible that it is a piece of sulfide from a Type 2 vein that has become separated from most of its quartz and anhydrite gangue during drilling. The samples of semimassive sulfide were observed in Samples 193-1189B-1R-1 (Piece 1), 3R-1 (Pieces 2 and 4), and 5R-1 (Pieces 1, 2, and 5). Additional samples from an unknown depth were recovered on the logging tool during wireline operations. See the "Site 1189 Sulfide Log" for details of individual samples and Figure F48 for modal distributions of pyrite, chalcopyrite, sphalerite, and magnetite downhole. Overall, the uppermost sections of Hole 1189B show elevated sulfide (pyrite) concentrations, with modes commonly >5% in cores above Section 193-1189B-11R-1 (127.6 mbsf). Below 130 mbsf, the modal pyrite falls away to <5% and is commonly present in only trace amounts.
Oxides are present in trace amounts with the exception of Section 193-1189B-14R-1, which contains ~5% hematite. Magnetite first makes an appearance in Section 193-1189B-11R-1 at 127.74 mbsf and is present mostly in trace concentrations sporadically to the bottom of the hole.
Pyrite occurs as anhedral and subhedral discrete grains and as aggregates within the semimassive and massive sulfide samples. It typically is present within veins of anhydrite and quartz (Type 2) and disseminated within the clay-altered matrix of the volcanic protoliths (Type 1). Within the veins, the greatest concentrations of pyrite are commonly observed near the vein margins, where it is intergrown with quartz. The veins tend to show a pronounced "stockwork" texture, although their thickness rarely exceeds 5 mm. Pyrite commonly contains inclusions of silicates, chalcopyrite, and sphalerite.
Sample 193-1189B-1R-1 (Piece 1A) deserves special mention. In thin section, this semimassive sulfide from 31.0 mbsf contains arcuate bands of pyrite ~0.5 mm thick, which form a nearly complete ovoid structure about 6 mm long × 1.5 mm wide. The center of the ovoid is filled with chalcopyrite (Fig. F90). Immediately adjacent is a dismembered structure with arcuate fragments of pyrite defining a crude and incomplete ovoid shape, ~11 mm × 6 mm in diameter, filled with chalcopyrite and, in the center, anhydrite (Fig. F91), some of which is altered to gypsum. These structures are very similar to mineralized worm tubes in sulfide chimneys (cf. Ixer, 1990, fig. 24d), which act as conduits for fluid flow from the interior to the exterior of the chimneys. This texture has not been described from veins. If the interpretation is correct that this structure represents a fluid conduit in a chimney or a chimney fragment within a sulfide mound and given the presence of overlying dacites, it would suggest the sample is derived from a buried exhalative sulfide deposit. Alternatively, these relationships may simply represent subsurface sulfide textures. For the most part, the coarse granular fabric of the anhydrite gangue is unlike the frondescent or acicular texture of sulfate minerals that are rapidly deposited in chimneys. Furthermore, parts of the sample contain relatively closely packed angular fragments of pale, altered dacite glass containing disseminated pyrite. This is not characteristic of chimneys, except rarely at their basal contact with volcanic substrate. The anhydrite gangue resembles vein occurrences from which the sample differs chiefly in proportion and grain size of sulfide and anhydrite. It separates volcanic clasts breccia-style, as observed throughout much of Hole 1189B. Elongate bands of chalcopyrite are observed and overgrown by euhedral pyrite along the margins of the anhydrite veins.
Framboidal pyrite is present in Hole 1189B in Samples 193-1189B-15R-1 (Piece 11, 86-88 cm) and 16R-1 (Piece 2, 11-14 cm) (Fig. F92). The framboids are 0.01-0.05 mm in diameter with individual elements that are 0.001-0.002 mm. Given that the accepted origin of framboidal pyrite is within the water column, the framboidal forms observed within these veins possibly comprise or originated as the similar-looking Fe sulfospinel greigite, which is stable under hydrothermal conditions (Wilkin and Barnes, 1997).
Chalcopyrite is more enriched in Hole 1189B than in other holes drilled during Leg 193, achieving modes >10% in Sample 193-1189B-1R-1 (Piece 1) and even 20% in thin section (Fig. F48). Within this sample of semimassive sulfide is the arcuate pyrite-chalcopyrite aggregate described above. In contrast, there are other examples in the same section of arcuate pyrite aggregates whose convex surfaces are mantled by chalcopyrite. Elsewhere within this sample, chalcopyrite is present as anhedral 0.1-mm inclusions within pyrite.
After the initial core (Section 193-1189B-1R-1), chalcopyrite is present sporadically to near the bottom of the hole mostly in trace amounts; lining vesicles, on fracture surfaces, associated with covellite (possibly bornite) in three instances (e.g., Sample 193-1189B-16R-1 [Piece 11]); and rarely as disseminated grains within the altered volcanic groundmass (e.g., Sample 193-1189B-17R-1 [Piece 19]).
One sample (Sample 193-1189B-13R-1 [Piece 6]) contains what appears to be tennantite. The mineral has a tetragonal form, a bright gray luster, and is relatively soft (Mohs hardness = ~3). It is present within microveinlets associated with pyrite and sphalerite.
Covellite also is present as a trace mineral. In hand specimen, there is a suggestion that covellite may locally coat pyrite grains (e.g., Sample 193-1189B-3R-1 [Piece 5]), recognized as an iridescent indigo blue coating. This coating might also be bornite. Elsewhere, covellite is positively observed along fracture surfaces associated with chalcopyrite (e.g., Sample 193-1189B-16R-1 [Piece 11]) and associated with drusy pyrite within quartz-lined vesicles (e.g., Samples 193-1189B-16R-1 [Piece 18] and 17R-1 [Piece 17]).
Sphalerite is found sporadically throughout the hole, mostly in trace amounts (Fig. F48). Honey-yellow sphalerite is observed in quartz amygdules (Fig. F93), within the massive sulfide sample (Sample 193-1189B-6R-1 [Piece 6]), disseminated with pyrite in the quartz matrix of volcanic breccia, and within an anhydrite-pyrite vein with trace amounts of tennantite. In Sample 193-1189B-14R-1 [Piece 15], colloform masses of sphalerite, ranging in color from brownish yellow (Fe poor) in the center to dark brown and black (Fe rich) on the outside, line the wall of an anhydrite-filled cavity. The sphalerite shows local intergrowth with chalcopyrite and chalcopyrite disease toward the margins of grains (Fig. F94). Also in this sample, pyrite encloses sphalerite free from chalcopyrite disease, whereas elsewhere in the section pyrite is included within coarse (1.4 mm) chalcopyrite-diseased sphalerite.
Magnetite is present predominantly in Type 3 pyrite-anhydrite-quartz veinlets but also as a sparse dissemination in altered volcanic rock and in dark halos of larger veins. Within the veins, magnetite is most commonly observed as fine inclusions within quartz (Sample 193-1089B-8R-1 [Piece 3, 7-9 cm]). Groundmass magnetite is associated with pyrite in a few samples (Fig. F95).
Hematite is present as bladed masses within quartz veins (Fig. F96) and as acicular masses within amygdules (Sample 193-1189B-15R-2 [Piece 10, 61-64 cm]). Hematite also partially replaces magnetite in the groundmass (Fig. F97).
The parageneses of oxides and sulfides observed within Hole 1189B is similar to that already reported in Holes 1188A, 1188F, and 1189A. Magnetite is observed as Ti magnetite within the volcanic precursors to the alteration assemblage. Secondary magnetite is present within early quartz veins or as small grains locally enclosed within pyrite. Hematite, the other oxide present, rims magnetite (Sample 193-1189B-15R-2 [Piece 10, 61-64 cm]) and also occurs as acicular masses within quartz veins; it therefore most likely postdates the magnetite. However, establishing a paragenetic sequence for hematite relative to the sulfides is problematic within Hole 1189B because the mineral does not occur in close association or within any of the sulfides.
Chalcopyrite generally is present with quartz, although it can be observed as isolated grains within clay-altered groundmass of the volcanic rock. Chalcopyrite is also observed in some late crosscutting veins (Sample 193-1189B-8R-1 [Piece 10, 37-40 cm]). The mineral commonly is present as chalcopyrite disease within sphalerite and locally replaces sphalerite (Sample 193-1189B-13R-1 [Piece 10, 52-54 cm]).
Pyrite is both intergrown with and contains inclusions of chalcopyrite. Sphalerite is more evident within Hole 1189B than in any other core to date and provides convincing evidence for two generations of sphalerite deposition, before and after a period of pyrite formation.
The paragenetic data for Holes 1189A and 1189B are combined in a schematic paragenetic sequence in Figure F98. The diagram suggests at least two similar episodes of sulfide precipitation, probably as a continuous process. In particular, the contrasting relationships between pyrite and sphalerite and pyrite and chalcopyrite are consistent with at least two sulfide mineralization events. Both events are evident in Hole 1189A. It is not known if the parageneses in Hole 1189B belong to the first or second event.
Given the proximity and broadly similar depth interval over which these two holes were drilled, broad coherence in the sulfide setting and parageneses is to be expected. Within the two holes, pyrite is the dominant sulfide. It is disseminated within the groundmass of the volcanic rocks (Type 1), within quartz-anhydrite veins (Type 2), and finally, as linings and cores to vesicle fill (Type 2a). However, the accessory sulfides, sphalerite and chalcopyrite, are more abundant in Hole 1189B than in Hole 1189A. The relationships between the accessory sulfides and the pyrite provide further evidence for at least two generations of pyrite-quartz precipitation within the holes. Nevertheless, the overall sulfide content of this site is low. However, given the exceptionally low recovery rates in the upper 100 m of Hole 1189B, the meaning of this is uncertain.