A History of Theoretical Developments in Carbonate-Hosted Base Metal Deposits and a New Tri-Level Enthalpy Classification

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 (≥1018 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 km2 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 km2 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 ≥1018 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.