During Leg 169, the general stratigraphy of the Bent Hill deposit was defined (Fouquet, Zierenberg, Miller, et al., 1998). We also recognized the marked difference in mineralogy between the massive sulfide body and the Deep Copper Zone; in particular the abundance of isocubanite and chalcopyrite was significantly higher in the Deep Copper Zone than had been reported from any part of the massive sulfide body. The massive sulfide at Bent Hill is predominantly pyrite and pyrrhotite, with much lower abundance of sphalerite, chalcopyrite, and magnetite (Fig. F6). The relative proportions of pyrite and pyrrhotite vary antithetically; intervals rich in pyrite do not contain much pyrrhotite and vice versa. Figure F6 is an expansion of a similar graphic presented by Krasnov et al. (1994). Sulfide and magnetite abundance is normalized to 100% and plotted against depth in the section, including data from the additional penetration achieved during Leg 169. Note that in this graphic there are no data between ~125 and 200 mbsf, so the proportions of chalcopyrite and isocubanite in this interval are not known. The amount of sulfide mineralization in this interval is much less than 5% of the recovered core, but our visual inspection of the thin (<2 mm in most cases) sulfide veins and small nodules in the interval between 125 and 200 mbsf agree with the descriptions reported from Leg 169, where chalcopyrite (or at least copper-rich sulfide) is much more abundant over this interval than any other sulfide, and pyrrhotite is much more common than pyrite. Additionally, we note that the greater abundance of copper-bearing minerals is not restricted to the Deep Copper Zone, but, in fact, copper enrichment starts in the base of the massive sulfide body (present in Sample 169-856H-19R-1, 60-64 cm, from 94 mbsf), and there is significant copper enrichment in the upper part of the stockwork (Samples 169-856H-20R-1, 32-36 cm; 21R-1, 54-58 cm; 21R-1, 80-84 cm; 22R-2, 57-60 cm; and 23R-1, 94-98 cm).

The principal difference between the two massive sulfide deposits drilled during Leg 169 is the presence of multiple massive sulfide horizons separated by smaller, less well developed stockwork zones in ODP Mound (see Fig. F3). There is also a distinct difference in the sulfide minerals present. Sphalerite is much more abundant in ODP Mound, and copper-bearing sulfides are also common throughout the deposit (Fouquet, Zierenberg, Miller, et al., 1998). Our data indicate that there is also a difference in the chemistry of the individual massive sulfide bodies in ODP Mound (Tables T1, T4). Sphalerites from the uppermost massive sulfide body (recovered in the interval between 9 and 26 mbsf) are the most iron rich, on average nearly 20 wt% Fe. Sphalerite from the thin massive sulfide body recovered from between 75 and 84 mbsf has slightly lower iron enrichment (~16 wt% Fe). The lowermost massive sulfide body, recovered from between 127 and 160 mbsf, has variable (9-11 wt% Fe) but generally lower iron enrichment than the upper two massive sulfide intervals. Zinc content in isocubanite is high (nearly 1 wt%) throughout the deposit, except for samples from the lowest part of the section. In contrast to all the Juan de Fuca data, all our sulfide mineral analyses from the TAG hydrothermal mound are remarkably uniform, despite sampling from various depths and from several locations around the deposit (Tables T2, T5).

The lateral extent of the Deep Copper Zone is a subject of some speculation in the literature from Leg 169 (Fouquet, Zierenberg, Miller, et al., 1998). During the cruise, we surmised that the presence of copper mineralization at the same depth below sea level at both the BHMS deposit and ODP Mound indicated this may be a continuous horizon. Figure F7 illustrates the similarity in appearance of isocubanite from the Deep Copper Zone sampled at both the BHMS deposit and ODP Mound as well as from the base of the BHMS deposit. Elongated but relatively broad lamellae of chalcopyrite account for 20% to 30% of the exposed surface area of isocubanite. Although isocubanite from the Deep Copper Zone sampled at the BHMS deposit and ODP Mound are geochemically indistinguishable (see Table T1, Sample 169-1035H-19R-1, 58-62 cm; Table T2, Sample 169-856H-31R-2, 1-5 cm), isocubanite from the base of the BHMS deposit is significantly richer in iron and zinc.

Chalcopyrite disease (Barton and Bethke, 1987) was reported in samples from ODP Mound, although the intensity of this malady is highly variable even within a single sample. Figure F8 illustrates the morphology and distribution of chalcopyrite inclusions in sphalerite from Sample 169-1035H-16R-2. Figure F9 illustrates differences in paragenetic relationships in the massive sulfide bodies of ODP Mound. In Figure F9A, a corona of pyrrhotite surrounds anhedral sphalerite, which in turn exhibits variable amounts of exsolved chalcopyrite. Figure F9B shows the common relationship in these samples of massive pyrite dissected by anastomosing veins of pyrrhotite. It is also common to see pyrrhotite veins cutting pyrite veins, which cut through large anhedral sphalerite crystals (Fig. F9C). Rarely there is evidence of early pyrite crystallization as euhedral inclusions in sphalerite (Fig. F9D). More complex paragenetic histories are present in other samples, as shown in Figure F9E from a sample in the uppermost massive sulfide horizon. Vermicular embayments around the margin of chalcopyrite are in places filled with pyrite, and the entire grain is armored by sphalerite. Figure F9F (same horizon as the sample in Fig. F9E) has a similar mineral assemblage. Irregularly shaped but subrounded chalcopyrite is riddled with pyrite veins and inclusions and surrounded by sphalerite that also has pyrite-filled embayments.

We performed several analyses on grains that had anomalous appearances in reflected light. Although most of these resulted in totals significantly <100%, a few analyses were of submicroscopic inclusions of intimately intermixed phases. Listed under "Anomalies" in Table T1, these include grains that appear to be finely intergrown chalcopyrite and sphalerite. The analyses equate to a 75/23/2 mixture of chalcopyrite, pyrite, and sphalerite, and a 50/25/22/3 mixture of chalcopyrite, pyrite, pyrrhotite, and sphalerite, respectively. Note that these analyses were performed with a focused 1-µm beam, indicating the intimacy of these intergrowths. The last two analyses are from a euhedral galena inclusion in sphalerite (Fig. F10). Although petrographically there is no evidence of microscopic inclusions in the galena, these analyses included 12% and 7% antimony.