RESULTS

Of the three chemically analyzed core samples (CSIRO 142701, 142703, and 142705), two have significant contents of Cu but none has other than trace levels of Zn or Pb. Contents of Ca reflect relative amounts of anhydrite-gypsum gangue, whereas other lithophile elements (Si, Al, etc.) vary in accordance with abundance of quartz gangue and wallrock particles. The massive pyrite sample collected by the logging tool contains little other than Fe and S, whereas the sand collected on the hammer drill has moderate contents of Cu and Fe; elevated Zn, Pb, Ca, and Ba; and lithophile elements attributable to altered country rock particles. In terms of the dominant chalcophile elements, all five samples are distinctly poorer in Cu and Zn than sulfide chimneys from Roman Ruins (Fig. F1).

Table T5 lists normative mineralogical constitutions computed as follows. First, sulfur was allocated with Ca, Ba, Pb, and Zn to form anhydrite, barite, galena, and Fe-free sphalerite, respectively, then S with Cu and Fe to form chalcopyrite, and, finally, remaining S with remaining Fe to form pyrite or, where Fe is deficient, to pyrite and pyrrhotite. Normative silicate content (including any residual Fe) is calculated as the difference from 100% total, and thereby includes H2O, which was not determined. The results agree well with the observed mineralogy of the samples (Table T1) except for pyrrhotite, which is unrecorded microscopically but whose minor normative presence is explicable by levels of analytical precision.

Table T6 provides estimated compositions for the silicate gangue components of the samples, calculated after subtracting normative sulfides and sulfates and reconstituting the residue to total 100%. The indicated FeO and CaO contents are highly subject to analytical error and to assumptions made when calculating normative sulfides and sulfates and take no account of the disseminated pyrite present in wallrock fragments. As observed by optical microscope, quartz gangue is almost exclusive in semimassive pyrite sample CSIRO 142705 and predominant (with some aluminous wallrock) in mineralized volcaniclastic sample CSIRO 142701. In the sand from the hammer drill (CSIRO 142807), the bulk silicate composition (arising from wallrock fragments) is broadly comparable with that of an illite-dominated wallrock sample collected just below the casing in Hole 1189B (Sample 193-1189B-1R-1 [Piece 2, 10–14 cm]) (Shipboard Scientific Party, 2002, and new data), but for semimassive sulfide CSIRO142703, also from just below the casing, the indicated nature of wallrock contaminants is more chloritic. The computed composition for scarce wallrock particles in the logging tool sample of massive pyrite (CSIRO 142808) is uncommonly rich in Fe and Mn. The Fe value is probably a spurious outcome of the calculation compounded by low abundance of the silicate component (<5%), but Mn is unexplained.

Relative to the average composition of Roman Ruins chimneys (Table T3), Leg 193 massive and semimassive sulfides are enriched in Co, Te, and Bi and mildly enriched in Se. These chalcophile trace elements, apart from Bi, concentrate within pyrite in PACMANUS chimneys (Binns et al., 2002); therefore, their enrichment reflects subsurface abundance of this mineral. However, contents of Mo and Tl, also concentrated in chimney pyrite, are similar. The habitat of enriched Bi, normally concentrated in chalcopyrite, is unknown for the Leg 193 samples. The latter are depleted relative to chimneys in Ga, Ge, and Cd (which concentrate in chimney sphalerite), in In (concentrated in chimney chalcopyrite and tennantite), and in As, Ag, and Sb (concentrated in chimney sulfosalts, particularly dufreynosite). Au is a significant trace element in the three leg samples analyzed by INAA. Its abundance is less than the chimney average but falls within the lower range of the chimney population. Native Au has not been identified microscopically in any sample examined in this study. Contents of Sr correlate with anhydrite abundance, whereas those of other lithophile trace elements relate to abundance of wallrock fragments, though with some anomalies, such as the high U in CSIRO 142807, the sand recovered from the hammer drill from within the cased interval of Hole 1189B.

Rare earth element (REE) abundances in Leg 193 massive and semimassive sulfides span the higher range of Roman Ruins chimneys (Fig. F2). Chondrite-normalized profiles of the latter (Fig. F2B), where REEs are contained principally in barite or rarer anhydrite, show pronounced light REE (LREE) enrichment (LaN/YbN = ~70) and distinct positive Eu anomalies (Eu/Eu* = ~15). Leg 193 samples show similar LREE enrichment patterns but subdued Eu anomalies ranging from slightly positive to negative (Fig. F2A). The variability in Eu/Eu* arises from the combined presence of anhydrite, which displays variable but mostly positive Eu enrichment, and altered wallrock fragments that tend to show significant Eu depletion (Bach et al., 2003).

Lead isotope ratios of the three Leg 193 samples analyzed are identical within precision limits to those of Roman Ruins chimneys, but like the latter they differ slightly yet significantly from those of fresh volcanic glasses at Pual Ridge, ranging from andesite to rhyodacite in composition (Fig. F3). Sulfur isotope ratios of Leg 193 pyrites also span the range of values measured in Roman Ruins chimneys (Fig. F4). These results are consistent with a cogenetic relationship between the seafloor and subsurface mineralization, with the important implication that the source of Pb in both was more radiogenic overall than the lavas constituting Pual Ridge. Whereas some of the Pb in chimneys and the subsurface massive and semimassive sulfides was potentially leached from the volcanic sequence during alteration, some must also derive from another source, most likely by leaching of basement rocks underlying the volcano in the deeper reaction zone of the PACMANUS hydrothermal system. The sulfur isotope data are consistent with the interpretation that a proportion of S2– in the PACMANUS hydrothermal fluids was of igneous provenance (34S = ~0) mixed with a variable component derived by reduction of seawater sulfate (34S = ~+4 to +6 for equilibrium reduction at 350–400C; Shanks, 2001, fig. 9). Since fresh Pual Ridge lavas contain negligible S (<100 ppm), a deeper magmatic source is suggested for the igneous component rather than leaching from Pual Ridge lavas during their alteration within the PACMANUS hydrothermal system. Sr and S isotope studies of Leg 193 anhydrites also suggest mixing between seawater and an igneous component (Roberts et al., 2003).

NEXT