Mississippi Valley-type (MVT) deposits are carbonate-hosted, Zn-Pb deposits with their type examples in the Mississippi Valley area of the USA. MVT deposits are: (i) stratabound but epigenetic in the nature of their mineralization; (ii) occur on the periphery of sedimentary basins in platform facies above basement highs; (iii) lack any association with igneous activity; (iv) have a simple mineralogy of marcasite, pyrite, galena and sphalerite. These deposits are the result of the passage of large volumes of metal-bearing brines (Sverjensky, 1986; Anderson and Macqueen, 1988), and thus provide a record of the large-scale movement of crustal hydrothermal fluids (Bethke and Marshak, 1990; Halliday et al., 1991). MVT deposits are controversial, with current scientific debate about: (i) the causes of large-scale movement of crustal hydrothermal fluids and the focussing mechanism necessary to form the deposits; (ii) the timing of mineralization; (iii) the nature of the deposition site; (iv) the source of the sulfur required for base-metal precipitation and whether the sulfur was acquired at the deposition site or was carried with the ore solution; (v) the nature and characteristics of any alteration halo to the mineralization. All of these controversies are discussed in this field guide, but two specific aspects are singled out for special attention. These are the nature of the deposition site and the timing of mineralization.

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