Abstract. The emergence of SEDEX ores in the rock record at about 1.8 to 2.2 Ga can be related to the hydrosphere and atmosphere pro- gressively becoming oxidized and therefore sulfide-poor and sulfate- rich, evolution of sulfate-reducing bacteria, and fundamental changes in the Earth crust. The reason for the apparent change in tectonic setting of SEDEX deposits from intracratonic rifts in the Proterozoic to passive margins is unclear. Phanerozoic SEDEX deposits formed in paleolatitudes that mirror modern evaporative belts, suggestive of a brine reflux origin for passive margin SEDEX deposits. Keywords. Sediment hosted lead-zinc deposits, SEDEX 1 Introduction The reasons for the uneven distributions of certain classes or ore deposits in the geological record are generally attri- buted to secular variations in geological processes that con- trolled the formation and destruction of these deposits (e.g. Meyer 1981; Sangster 1990; Hutchinson 1992; Barley and Groves 1992; Titley 1993). We examine the secular distribution of SEDEX lead-zinc deposits (Fig. 1) that is based on the metal resource data (Leach et al. 2005) for 148 SEDEX (sedimen- tary exhalative deposits) lead-zinc deposits. Despite the in- herent “exhalative component” in the term “SEDEX”, direct evidence of an exhalite component in the ore or alteration assemblage is not recognized for many SEDEX deposits. The age of mineralization is commonly considered to be synsedimentary, however, some deposits formed in a sub- seafloor diagenetic environment (i.e. Red Dog, Alaska) and some formed during burial diagenesis (i.e. Century, Aus- tralia). Nevertheless, the age of formation is assumed to be the same or very close to the age of the host rocks. SEDEX ores are mainly restricted to two groups: one in the Proterozoic and another in the Phanerozoic (e.g. Goodfellow 1994 and references therein, Large et al. 2005 and references therein, Leach et al. 2005 and references therein). The Proterozoic group (at about 1.4 to 1.8 Ga) includes the large deposits in the Mt. Isa-McArthur basin of Australia (Large et al. 2005) the Sullivan deposit in Canada and the deposits in the highly metamorphosed rocks of the Aravalli-Delhi belt of northwestern India. The Phanerozoic ores includes the large Red Dog, Alaska deposits and the deposits in the Selwyn basin, Canada. The older age group corresponds to SEDEX deposits in failed continental rifts (rift-sag basins), whereas depos- its of the younger age group formed along passive conti- nental margins (e.g. Large et al. 2005 and references therein, Huston et al. in prep.). We discuss the possible reasons for the emergence of SEDEX deposits and examine the tectonic factors that may have influenced their distribution in the rock record. Re- cent fluid inclusion studies of the Red Dog deposit to- gether with analysis of the distribution of SEDEX depos- its using GIS and plate reconstructions provide insights into why some Phanerozoic basins are fertile for SEDEX and others barren. Chapter 2-18 The distribution of SEDEX Pb-Zn deposits through Earth history D. Leach, E. Marsh U.S. Geological Survey, MS973, Box 25046, Denver, CO 80225, USA D. Bradley U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508, USA S. Gardoll University of Western Australia, Centre for Global Metallogeny, SEGS, Crawley, WA 6009 Australia D. Huston Geoscience Australia , GPO Box 378, Canberra 2601, Australia 2-18