Origin of the Jasperoids

The setting of jasperoids at Site 1189, as matrixes to breccias cutting altered volcanic wallrock (Fig. F1), precludes any likelihood they represent fossil exhalative Fe-Si deposits comparable with those associated with chimneys at the Roman Ruins seafloor. Their overall microfabric, particularly their possession of numerous drusy cavities and microcavities, is similar to that displayed by many quartz-rich veins and breccia matrixes nearby in the generally sulfidic stockwork zone intersected from the base of casing at 30 to ~100 meters below seafloor (mbsf) in Hole 1189B (Shipboard Scientific Party, 2002). The prominent difference between jasperoids and associated nonjasperoidal veins is the presence of early formed hematite-rich cores in jasperoid quartz grains and crystals, indicating an earlier phase of quartz growth from relatively oxidized hydrothermal fluids. In Sample 193-1189B-11R-1 (Piece 3, 14–16 cm; CSIRO 142714), frondlike and botryoidal aggregates of hematite (and possibly goethite) do not resemble the microbial filamentous structures common in seafloor Fe-Mn-Si deposits at Roman Ruins and reported from certain oxide deposits at modern submarine hydrothermal vents and in ancient ferruginous cherts and jaspers associated with massive sulfide ore deposits (Juniper and Fouquet, 1988; Duhig et al., 1992; Grenne and Slack, 2003). Conceivably, they formed by maturation of ferruginous gels, overgrown by quartz.

The tendency for jasperoid to occur centrally in veins and jogs within breccia (Fig. F1) would conventionally be interpreted to suggest late formation. However, petrographic features, including the clear quartz selvages on wallrock fragments locally extending as dilational veins into the latter, are more consistent with early crystallization of the jasperoidal quartz, perhaps at the initial stages of hydrothermal brecciation. The later selvages and veins possibly continue along Core 193-1189B-11R and adjacent cores to form more typical nonjasperoidal breccias, generally richer in pyrite, of the stockwork zone in Hole 1189B. Unfortunately, the thin section studied of Sample 193-1189B-11R-1 (Piece 3, 14–16 cm; CSIRO 142714) does not cover the full transition, and the jumbled nature of the rather poor core recovery renders this difficult to establish conclusively.

None of the jasperoidal or sulfidic quartz veins show crack-seal structures indicative of tectonic origin. Rather, they appear to have crystallized relatively rapidly and largely unimpeded within open spaces created by dilational hydrofracturing, leaving numerous small drusy cavities, but with limited replacement and silicification of enclosed wallrock fragments. The process was episodic, with an earlier generation of quartz crystallizing from oxidized Fe-bearing solutions simultaneously depositing hematite flakes on the expanding quartz surfaces and overgrowing some previously formed hematite aggregates.

This was followed fairly abruptly by continued growth, under more reduced conditions inhibiting hematite precipitation, of the clear margins on jasperoidal quartz grains, the linings to drusy cavities, and the selvages around wallrock fragments plus veinlets within them causing localized silicification. Pyrite crystallized within remnant cavities late in the sequence or replaced the jasperoid quartz, while limited anhydrite finally crystallized as overgrowths in or fillings of cavities. The pyrite is unlikely to be in equilibrium with hematite, having formed distinctly later as fluids became more reduced and sulfurous.

Relationships to Seafloor Fe-Mn-Si Deposits at Roman Ruins

The jasperoids lack Mn and have Si/Fe ratios distinctly higher than those of ferruginous exhalative deposits at the Roman Ruins seabed, apart from a few composed mainly of biogenic opaline silica (Fig. F18; Table T2). Relative to Fe contents they share enriched U, but the jasperoids are lacking in Zn, As, Sb, and Pb. Their REE abundances fall within the variable range displayed by seabed Fe-Mn-Si deposits at Roman Ruins (Fig. F17C), wherein REE abundances broadly correlate with Fe content, but their Eu anomalies are subdued by comparison and for two jasperoid samples the chondrite-normalized patterns differ by virtue of light REE depletion. Comparative geochemistry thus provides no definitive support for any contention that the jasperoids reflect subsurface passage of oxidized fluids depositing Fe-Mn-Si oxides at the seafloor, although the shared enrichment in U deserves note in that respect. Presence of Mn (a dominant component with Fe in chimney vent fluids) and elevated Zn, As, and Sb contents (elements also abundant in sulfide chimneys) instead suggest that the Fe-Mn-Si deposits at Roman Ruins precipitate from expiring chimney-forming fluids that cooled and deposited their sulfide load close to the seafloor. Nevertheless, the jasperoids establish that oxidized fluids passed through the stockwork zone of Hole 1189B and breccia zones in Hole 1189A at an early stage in their development, so it remains possible that some ferruginous oxide deposits on the seafloor formed as a consequence.