THE GENERATION AT HOT-SPRINGS OF SEDIMENTARY ORE-DEPOSITS, MICROBIALITES AND LIFE

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.