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 type have at least some exhalative aspects. The fossil polychaete worms, which occur in Lower Carboniferous pyrite mounds at Tynagh and Silvermines in Ireland, have affinities to Paralvinella, an organism that lives attached to hydrothermal chimneys at the Juan de Fuca hot spring site in the Northeast Pacific. In addition, fossil tube worms, and their moulds, occur both in silica masses underlying the Carboniferous giant Red Dog sulphide orebody in Alaska and in Devonian barite and base-metal deposits in North America and in Russia, respectively. The development 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 which sulphide mineralisation developed at the sea floor has implications also in a different sphere. Life itself may have emerged in a similar milieu at 4.2 Ga from iron monosulphide bubbles. A primitive metabolism could have been driven by the high, long-lived and constant, redox potential of 300 mV made available across an iron monosulphide membrane which would have been spontaneously generated where sulphide-bearing, submarine, alkaline springs issued into the acidic, iron-bearing, Hadean ocean. The alkaline spring provided bisulphide to the iron-rich (carbonic) acid ocean for the precipitation of iron-monosulphide bubbles (probotryoids), as well as acetate (Shock, 1992) — the feeder to the biochemical Krebs cycle, driven in reverse by the high partial pressure of CO₂. In addition to its scientific significance, an understanding of these beginnings may well benefit research into many aspects of economic geology.Even more extreme redox contrasts are revealed by the presence of sedimentary jasper or iron formation in three of the major Carboniferous sulphide orebodies in Ireland. Pyritic sulphide microbialites also grew over some of the associated fossil hot-spring sites and may be recognised by their bacteriogenic δ³⁴S values (−20 to −40‰). Recognition of such fossil hot-spring sites could lead to further discoveries of SEDEX deposits.