False-color backscattered electron images
Finnx1; section from inner conduit (300 ° C) to middle of section
Finnx2; from middle of section to contact with 2 ° seawater at outer wall
Mineralogy
Reflected light microscopy indicates that the chimney inner wall-vent interface
(302 ° C vent fluid) is composed of about 1.2 cm of massive chalcopyrite (CuFeS2),
succeeded
by various intergrowths of sub- to euhedral pyrite (FeS2), chalcopyrite, and sphalerite
(ZnS). The outermost section of the chimney, in contact with 2 ° C seawater, is dominated
by euhedral to subhedral sphalerite and minor amounts of pyrite. Minor barite and anhydrite
are also present throughout the section.
As you move from the interior of the vent towards the outer wall, the section transitions
from massive chalcopyrite into eu- to subhedral pyrite grains and from there into a large
void space. The void space gives way to a zone of relatively large, elongate sphalerites
which host small inclusions of chalcopyrite, placed like ribs along the spine of the elongate
grains. The second half of the cross-section contains dominantly sphalerite and pyrite.
The pyrite grains are largely sub- and euhedral and occur near the beginning of the second
cross-section. The region nearest the outer wall of the vent is dominated almost exclusively
by sphalerite grains. In addition, void space increases in abundance as one moves closer to
the outer wall, where the chimney contacts 2 ° C seawater.
The cross-section appears to contain at least two overprinting episodes, evident in the
false color backscattered electron image. Pyrite is overwhelmingly dominant in both regions,
overprinting minerals previously present. The first case occurs just outside of the massive
chalcopyrite, where a large void zone is evident. The second case occurs near the split
between the two slides. The zonation present in the latter case is termed "hydrothermal
reworking" and involves dissolution of low-temperature sphalerite by high-temperature fluids
and the redeposition of the sphalerite towards the outer portions of the deposit [Hannington
et al. (1995)]. This latter process is termed "zone refining" and is supported by the % Fe
content of the sphalerites found in the outer wall. While these sphalerites should contain
the lowest % Fe in the section (lowest temperature), they actually increase in % Fe relative
to sphalerites in the inner portion of the cross-section.
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