The tension between CO 2 dissolved at relatively high atmospheric pressure in the Hadean ocean, and H 2 generated as ocean water oxidized ferrous iron during convection in the oceanic crust, was resolved by the onset of life. We suggest that this chemosynthetic life emerged within hydrothermal mounds produced by alkaline solutions of moderate temperature in the relative safety of the deep ocean floor. Exothermic reaction between hydrothermal H 2 , HCOO − and CH 3 S − with CO 2 was catalyzed in inorganic membranes near the mound's surface by mackinawite (FeS) nanocrysts and “ready-made” clusters corresponding to the greigite (Fe 5 NiS 8 ) structure. Such clusters were precursors to the active centers (e.g., the C-cluster, Fe 4 NiS 5 ) of a metalloenzyme that today catalyzes acetate synthesis, viz., the bifunctional dehydrogenase enzyme (ACS/CODH). The water, and some of the acetate (H 3 C.COO − ), produced in this way were exhaled into the ocean together as fluid waste. Glycine ( + H 3 N.CH 2 .COO − ) and other amino acids, as well as tiny quantities of RNA, generated in the same milieu were trapped within tiny iron sulfide cavities. Energy from the acetate reaction, augmented by a proton gradient operating through the membrane, was spent polymerizing glycine and other amino acids into short peptides upon the phosphorylated mineral surface. In turn these peptides sequestered, and thereby protected, the catalytically and electrochemically active pyrophosphate and iron/nickel sulfide clusters, from dissolution or crystallization. Intervention of RNA as a polymerizing agent for amino acids also led to an adventitious, though crude, process of regulating metabolism—a process that was also to provide genetic information to offspring. The fluxes of energy and nutrient available in the hydrothermal mound—commensurate with the requirements of life—encouraged differentiation of the first microbes into two separate domains. At the bifurcation the Bacteria were to specialize in acetogenesis and the Archaea into methanogenesis. Representatives of both these domains left the mound by way of the ocean floor and crust to colonize the deep biosphere. Once life had emerged and evolved to the extent of being able to reduce nitrogen for use in peptides and nucleic acids, light could have been used directly as an energy source for biosynthesis. Certain bacteria may have been able to do this, where protected from hard UV by a thin coating of chemical sediment produced at a sub-aerial hot spring operating in an obducted and uplifted portion of the deep biosphere. Embedded in fresh manganiferous exhalites, early photosynthetic bacteria could further protect themselves from radiation by adsorbing manganese on the membrane. Organization of the manganese with calcium, within a membrane protein, happened to result in a CaMn 3 O 4 cluster. In Mn(IV) mode this structure could oxidize two molecules of water, evolve waste oxygen, and gain four electrons and four protons in the process to fix CO 2 for biosynthesis. All these biosynthetic pathways had probably evolved before 3.7 Ga, though the reduced nature of the planet prevented a buildup of free atmospheric oxygen until the early Proterozoic.

Abstract The morphologies of base metal deposits controlled by a carbonate host but formed by quite different mineralizing processes are often confusingly similar. Various hypotheses have been used to explain the origin of the majority of these deposits, beginning with the artesian mechanism theory dominant 100 years ago (Chamberlain, 1882; Daubree, 1887; Siebenthal, 1915) and continuing through the magmatic hypotheses of the first half of this century (Emmons et al., 1927) to the more recent hypothesis of basin brine expulsion by compaction (Noble, 1963; Beales and Jackson, 1966). Evolved basin brines are still the favored candidate for the mineralizing solution, at least for the Mississippi Valley type, but the volume of water (≥10 18 g) required to generate the largest deposits in the midcontinent has forced reconsideration of the artesian mechanism (Garven and Freeze, 1984a and b; Bethke, 1986). The artesian-flow hypothesis explains why Mississippi Valley-type lead-zinc deposits occur in mineral districts with areal extents in excess of 100 km 2 at the distal margins of foreland sedimentary basins. However, large but more isolated lead + zinc + barite deposits, which are related to salt-gypsum domes in the young sedimentary prism of the Gulf Coast, do appear to form without the benefit of aqueous recharge (Kyle and Price, 1986). Mississippi Valley-type deposits in which fluorite dominates (Rosiclare-Pennine type) also occur in laterally extensive ore fields, but they are not limited to the margins of sedimentary basins. Consideration of deposits associated with rhyolite intrusions in northeastern Mexico (Kesler, 1977), along with those in other districts near vestigial igneous phenomena (Grogan and Bradbury, 1968), carries the implication that fluorine, in at least some of these districts, was of magmatic origin. The higher homogenization temperatures recorded in several such deposits (Sawkins, 1966) and the rare earth element contents of fluorite and other minerals (Smith, 1974) further support this possibility. These observations also suggest that the heat from deep intrusions may have driven convection cells involving ground water. The evidence does not exclude other hypotheses, such as the leaching of fluorine (via circulating ground water, driven by an artesian head), either from basement rhyolites and other felsic rocks or from phosphate horizons within the sedimentary sequence. Sedimentary exhalative orebodies in carbonate rocks (Irish type) normally occupy a surface area of <10 km 2 and are separated from their nearest neighbors by distances of 20 km or more. Such ores were probably produced by convection involving modified, highly saline seawater, initially circulating in cells 20 km across. Subsequent expansion to a diameter of 40 km and eventual occupation of the top 15 km of the crust are postulated. We reclassify the carbonate-hosted lead + zinc deposits into three types, dependent upon the enthalpy involved in the convective system. The low-enthalpy, or Mississippi Valley, type, operates in, and in some cases evolves to operate below, a mature sedimentary basin. In this type, an evolved basin brine is forced toward a basin margin, partly by compaction but mainly by a topographically controlled hydraulic head, with attendant large total-flow volumes. Sphalerite and galena (with or without barite) precipitate where the brine meets reduced sulfur (and sulfate) near the basin margin; this is perhaps effected by contained hydrocarbons. The medium-enthalpy, or Irish, type requires modified saline seawater convecting within the upper crust and leaching basement-derived metals; this process is thought to occur at the beginning of basin formation. The high-enthalpy, or Rosiclare-Pennine, type requires basinal brines, augmented by meteoric additions, in convection cells which are driven by their proximity to magmatic intrusion. In this last type, magmatic fluorine is postulated as the source for the tens to hundreds of millions of tons of fluorite occurring in association with the lead + zinc ores. We emphasize that the largest deposits of all three types contain>5 million metric tons of Pb + Zn, which requires ≥10 18 g of mineralizing aqueous fluid. This quantity of fluid is so vast that it makes the process of recharge generally inescapable, except in a limited number of cases (particularly some high enthalpy examples) where the fluid may be recycled.