INTRODUCTION

Sulfide chimneys at the Roman Ruins hydrothermal site in the PACMANUS field, Manus Basin, Papua New Guinea, are accompanied by low-lying, intermingled, and fringing deposits of Fe-Mn-Si oxide comprising variable mixtures of Fe oxyhydroxide, opaline silica (commonly with a filamentous microbial structure), and coatings or veinlets of birnessite and todorokite (Binns, Barriga, Miller, et al., 2002; Binns et al., 2002). Similar deposits, overlying dacite and rhyodacite lavas, occur at the Tsukushi and Rogers Ruins sites at PACMANUS, and others occur independently of chimneys at numerous smaller sites in the general vicinity. Some deposits actively vent clear fluids at temperatures in the range 29–73C (Binns et al., 2002). Small deposits composed principally of Mn oxide minerals also occur at Tsukushi and the nearby Snowcap site.

During Leg 193 we were alert to the possibility that equivalent fossil Fe-Mn-Si deposits at subsurface paleoseafloor horizons within the volcanic pile might denote earlier phases of hydrothermal activity. No convincing examples were found, but three significant occurrences of bright red hematitic material were encountered during drilling at Roman Ruins (Site 1189; Shipboard Scientific Party, 2002).

These occurrences are closely related to pyrite-bearing breccias and veins abundantly developed at Site 1189. Although superficially resembling jaspers commonly associated with ophiolite complexes and massive sulfide ore deposits, the term "jasperoid" is preferred here to avoid any connotation of an exhalative origin. This report describes investigations of the jasperoids undertaken in order to test their possible relationships to the seafloor oxide deposits at Roman Ruins, in the same sense that the stockwork zone in Hole 1189B represents a likely conduit for sulfide chimney-forming fluids (Shipboard Scientific Party, 2002; Binns, this volume).

Details of the samples studied are provided in Table T1. The setting of the three jasperoid occurrences within surrounding core material is as follows. The lithologic units cited are those set out in Binns, Barriga, Miller, et al. (2002).

Core 193-1189A-7R

A single 3-cm piece of jasperoid with brecciated inclusions of altered country rock, cataloged as lithologic Unit 14 in Hole 1189A, was present at the base of this core. Overlying Unit 13 in Core 193-1189A-7R and underlying Unit 15 in Core 193-1189A-8R both show hydrothermal brecciation and quartz-pyrite veining of pale greenish gray to tan altered volcanic rock, but neither has any development of jasperoid. The relationship between jasperoid and other rock types is therefore unknown for this intersection, although petrographic similarity of the jasperoid to that in the core next described indicates an equivalent nature.

Core 193-1189B-6R

Core 6R from the stockwork zone of Hole 1189B was a partial recovery of rubble, most of which consists of fragments of jigsaw-fit, hydrothermally brecciated, pale greenish gray altered volcanic rock with a fine-grained matrix or stockwork veining of quartz and pyrite (Fig. F1). Many fragments contain pods of red jasperoid located centrally in the veins, especially where they broaden into a breccia matrix. Since the rubble was clearly jumbled when extracted from the core liner, separate pieces of massive sulfide and altered amygdaloidal volcanic rock within it were moved to the end of the interval and cataloged as lithologic Units 6 and 7, respectively, while the remaining jasperoid-bearing fragments were assigned to Unit 5. Overlying Unit 4 in Core 193-1189B-5R and Unit 8 in underlying Core 193-1189B-7R are both hydrothermally brecciated altered volcanic rocks with stockwork-like matrixes of quartz-pyrite-(anhydrite) vein material. They are broadly similar in appearance to the nonjasperoid parts of Unit 5. This sequence of cores clearly indicates that jasperoid formation is an integral, though only locally developed, component of transgressive quartz veins (normally gray, with sulfides) within the stockwork zone in the upper part of Hole 1189B.

Core 193-1189B-11R

Most of the material recovered in this core comprised long pieces of altered vesicular volcanic rock curated as lithologic Unit 19 and representing commencement of the lower sequence of Hole 1189B (Shipboard Scientific Party, 2002). The upper part of Core 193-1189B-11R unfortunately consisted of fine rubble, containing a single 3-cm piece of red jasperoidal breccia with inclusions of altered wallrock. The remaining fragments in the rubble were of two kinds: breccia with randomly oriented clasts of altered, flow-banded volcanic rock in a quartz matrix with minor magnetite and polymict breccia or volcaniclastic rock with greenish gray, gray, and very pale clasts in a quartz matrix with minor pyrite and magnetite. Neither lithology shows any development of jasperoid. Although somewhat mixed, the two breccia types were, respectively, more abundant on either side of the jasperoid piece when extracted from the core liner. The rubble was rearranged for curating into Units 16 (flow-banded breccia), 17 (jasperoid), and 18 (polymict breccia). There is no certainty this represents the sequence drilled, indeed the rubble could well be derived from collapse higher in the hole. Accordingly, Core 193-1189B-11R provides no definite evidence of the relationship between jasperoid and other rock types. While superficially similar to the other two jasperoid intersections and likely to have the same structural relationship to wallrock, the sample collected from Piece 3 shows important microscopic differences.

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