worm is possibly secreting significant quantities of proteins 

 into the vent environment that actively complex heavy metals 

 such as Cd, Zn and Cu which are locally very abundant. 



FILAMENTOUS FE/SI DEPOSITS: MINERAL OR MICROBIAL ORIGIN? 



Orange-red deposits of iron/silica are formed by low- 

 temperature venting at a number of sites in the northeast 

 Pacific and along the length of the EPR (Juniper and Fouquet 

 1988). Vent-specific organisms such as Vestimentif era, 

 alvinellid polychaetes and bivalves are not usually found in 

 association with this type of venting, as the fluids contain 

 little or no H2S, the major energy source for vent faunal- 

 microbial symbioses. Although well known to geologists this form 

 of hydrothermal activity has received little attention in the 

 literature, likely because of the highly oxidized nature of the 

 venting fluids, the lack of economically interesting minerals and 

 the total absence of vent fauna. 



Iron/silica deposits form either on top of sulfides 

 deposited by previous higher temperature venting or directly on 

 basalt where they can occur as chimney structures (Fig. 8). The 

 iron is mainly in a highly oxidized form, giving the deposits 

 their distinct orange-red color. Associated silica is an 

 amorphous oxide, Opal A. An active 1.5m high chimney on Explorer 

 Ridge was composed of 73 percent silica, 7 percent iron, and 1.3 

 percent organic carbon along a number of minor constituents 

 (Juniper and Fouquet 1988). The chimney was emitting a 

 27°C fluid enriched in Fe and Si over ambient seawater but 

 containing no detectable H 2 S (Tunnicliffe et al. 1986). In a 

 routine scanning electron microscopic survey of geological 

 samples from Explorer Ridge, we discovered this chimney to be 

 formed of hollow microbial-like filaments. This observation 

 together with the presence of organic carbon led to an 

 investigation of the possible role of micro-organisms in mineral 

 precipitation. Work to date has been limited to re-examination of 

 geological samples from the northeast Pacific and the EPR. 



All examined samples were porous and consisted of 

 aggregates of branching filaments of iron frequently covered in 

 amorphous silica (Opal A) (Fig. 9). Frequently distinct 

 mineralogical zonation was visible, indicating local variation in 

 redox conditions during deposit formation. Consistent among all 

 samples was the fact that iron was deposited in a filamentous 

 arrangement before the deposition of silica. Filaments of iron 

 were occasionally observed to be coated in concentric layers of 

 amorphous silica (Fig. 9e). Silica often filled inter-filament 

 spaces as well, further consolidating the deposits. 



No strictly mineralogical explanation satisfactorily 

 accounts for the filamentous morphology of these deposits. 

 Spherical and tubular structures are known to form during the 



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