2.5 HYDROTHERMAL DEPOSITS - Steve Scott

There are two types of hydrothermal deposits in the caldera of Axial Volcano: Fe-oxyhydroxides and sulfide-sulfate. Both were investigated and sampled during NeMO 99.

Fe-oxyhydroxide deposits

Analyses of Fe-oxyhydroxide samples taken from Steve Mound, Gollum Vent and south of ASHES in 1998 show the material to be silica poor ferrihydrite (nominally Fe₅HO₈.4H₂O) together with amorphous silica. Ferrihydrite is commonly found in soils, oxidized mine wastes and other iron-rich environments, and now also on the deep seafloor (Boyd and Scott, 1999, Can. Mineralog.). The material is essentially amorphous although it can have short-range crystallographic order. It is of biogenic origin, clearly coating bacteria as seen in SEM and STEM images.

During NeMO 99, ferrihydrite deposits were sampled at Oxide Vent (inactive); at Naaz, south of ASHES (inactive); and, at the west wall of the caldera near ASHES (active). Red iron oxides and bacterial fluff were also observed at a few other places, most notably on the new lava in the south rift zone. The Naaz site is a new discovery of clusters of 10-15 cm diameter x 20-60 cm high conical structures that look like termite mounds made of ferrihydrite and of anhydrite coated by ferrihydrite. Naaz covers an area of about 5m (east-west) x 15m (north-south) and lies about 5 m south of Crack Vent. The distribution of the mounds within a cluster and in the peripheral regions where mounds are widely dispersed appears to be controlled by fractures in the otherwise relatively smooth sheet flow surface. Both vent fluids (6^oC maximum temperature) and consanguineous oxide-coated bacteria were sampled at the west caldera wall. This will enable a determination of partitioning of elements between fluid and solids, an important step in the study of this biomineralization process.

Sulfide-sulfate spires

The CASM T & S spires, discovered during the 1998 NeMO expedition, were sampled. The five samples include different mineralogical types and both actively venting and inactive areas. A short distance north of T & S, on the east wall of the CASM rift, there is a large pile of oxidizing sulfide talus whose source was not thoroughly investigated. There is also evidence (red staining) of hydrothermal activity in some of the talus blocks at the foot of the north wall of the caldera. One sample each from Castle and Flat Top were lost. The Castle sample, taken from the top of the structure, appeared to be oxidized.

Two recovered HOBOs (Hell Vent and CASM T & S Vent) had ~1 mm thick sulfide deposits on their probe sheath. By determining the mass of precipitated material/ unit area/time the HOBO was deployed, the precipitation rate can be calculated, assuming it is linear with time. Furthermore, this rate can be known as a function of fluid chemisty (the vent fluids were sampled) and temperature (from the HOBO record).

Suction sampling within the Marker 33 vent revealed the presence of sulfides (pyrite + ?chalcopyrite) and anhydrite coating a cm-size piece of basalt and impregnating the basalt=s vesicles. The anhydrite and possible chalcopyrite indicate much higher temperatures than are now observed at Marker 33. It is postulated that the vent system was much hotter just after the January 1998 eruption than it is now.