Notwithstanding the current fashion which favours an epigenetic origin
for what used to be termed SEDEX deposits, there are several lines of
evidence to indicate that Phanerozoic base-metal orebodies of this ty
pe have at least some exhalative aspects. The fossil polychaete worms,
which occur in Lower Carboniferous pyrite mounds at Tynagh and Silver
mines in Ireland, have affinities to Paralvinella, an organism that li
ves attached to hydrothermal chimneys at the Juan de Fuca hot spring s
ite in the Northeast Pacific. In addition, fossil tube worms, and thei
r moulds, occur both in silica masses underlying the Carboniferous gia
nt Red Dog sulphide orebody in Alaska and in Devonian barite and base-
metal deposits in North America and in Russia, respectively. The devel
opment of sulphide and carbonate fossil microbialites over exhalative
centres further supports generation of some mineral deposits on sea or
lake floors. Carbonate microbialite mounds are also developing today
over warm springs and seepages. The existence of an environment in whi
ch sulphide mineralisation developed at the sea floor has implications
also in a different sphere. Life itself may have emerged in a similar
milieu at similar to 4.2 Ga from iron monosulphide bubbles. A primiti
ve metabolism could have been driven by the high, long-lived and const
ant, redox potential of similar to 300 mV made available across an iro
n monosulphide membrane which would have been spontaneously generated
where sulphide-bearing, submarine, alkaline springs issued into the ac
idic, iron-bearing, Hadean ocean. The alkaline spring provided bisulph
ide to the iron-rich (carbonic) acid ocean for the precipitation of ir
on-monosulphide bubbles (probotryoids), as well as acetate (Shock, 199
2) - the feeder to the biochemical Krebs cycle, driven in reverse by t
he high partial pressure of CO2. In addition to its scientific signifi
cance, an understanding of these beginnings may well benefit research
into many aspects of economic geology. Even more extreme odor contrast
s are revealed by the presence of sedimentary jasper or iron formation
in three of the major Carboniferous sulphide orebodies in Ireland. Py
ritic sulphide microbialites also grew over some of the associated fos
sil hot-spring sites and may be recognised by their bacteriogenic delt
a(34)S values (-20 to -40 parts per thousand). Recognition of such fos
sil hot-spring sites could lead to further discoveries of SEDEX deposi
ts.