Sixty-one thin sections from the shipboard suite were examined for sulfide mineral presence and abundance, and an additional 37 thin sections were prepared from sulfide-bearing core samples from Site 1268. The results of petrographic examination of sulfide and oxide abundance are presented in Table T1. Sixteen samples with the most abundant and petrographically diverse sulfide mineral assemblages were selected for detailed mineral chemistry analysis. The electron microprobe analyses of sulfide minerals are presented in Table T2.
There is a distinct change in sulfide mineral abundance, morphology, and speciation downsection. The sulfide mineral suite present in these samples is shown in Figure F2, and the change in assemblage expected with decreasing oxygen and sulfur fugacity is shown in Figure F3. The uppermost samples from Hole 1268A contain millerite as lenses, blebs, and needles in pyrite. The pyrite contains Ni and Co for the most part in roughly equal proportions (<1–2 wt% each), although a single analysis has much more Co than Ni. Within samples from the second core taken starting at 15 mbsf, the assemblage includes chalcopyrite and the abundance of millerite decreases markedly, although millerite persists through Core 209-1268A-3R (~23 mbsf).
By Core 209-1268A-4R at ~25 mbsf, millerite and chalcopyrite are no longer present, but pyrrhotite is fairly common. Pyrite is still present, but it is found most commonly in disseminations and along late veins that crosscut all other structures. Core 209-1268A-9R, from 49 mbsf, contains the shallowest samples containing analyzed grains of Co-bearing pentlandite. The assemblage of magnetite, pentlandite, and pyrrhotite suggests sulfide mineral precipitation in lower oxygen and sulfur fugacity than the sulfide mineral assemblage in samples from shallower parts of the hole.
Section 209-1268A-12R-3 shows a distinct change in sulfide minerals. Millerite and chalcopyrite rejoin the sulfide mineral assemblage, and pyrrhotite and pentlandite were not detected. The higher sulfur and oxygen fugacity assemblage persists through Core 209-1268A-13R. This interval from Core 209-1268A-12R thorough 14R was identified by the shipboard scientific party as a breccia induced by intrusion of mafic magma. Curiously, although the intrusion breccia extends from the top of Core 209-1268A-12R to the bottom of Core 14R (~63–77 mbsf), the assemblage of millerite/chalcopyrite/±polydymite occurs only between Sections 209-1268A-12R-3 and 13R-1 (66.7–71.1 mbsf). Below this interval, the sulfide minerals rapidly return to the low-sulfur and -oxygen fugacity assemblage (predominantly pyrrhotite and pentlandite) and remain so through the deepest interval sampled. Most of the gabbroic rocks sampled below 100 mbsf did not contain any significant sulfide mineral abundance in the thin sections examined. Sphalerite was also present in three samples from the lower part of the gabbroic intrusion breccia (Sections 209-1268A-14R-3 and 15R-2 and 15R-3). In the lower part of the section, there are two chemically distinct pyrite compositions. One contains as much as 10 wt% Co, the other a few weight percent of Ni with no Co. There is no clear morphological difference between the occurrences of these phases; both occur as disseminated blebs and in veins. Inasmuch as pyrite is commonly indicative of a sulfur and oxygen richer environment, these phases are likely to be late-stage additions to the sulfide mineral assemblage.