Fig. 4.7 Schcmata for thc formalion of Mississippi Vallcy-typc dcposits. (a) Chcrprcssurcd, hot porc fluids cscapc from a shalc basin (pcrhaps aidcd by hydraulic fracturing) and movc up aquifcrs to form dcposits in coolcr strata, filling fracturcs or forming othcr typcs of orcbody. (b) Gravity-drivcn fluids flowing from a hydraulic hcad in a highland arca flush through a basin driving out and rcplcnishing thc formation watcrs. Thcsc diagrams draw on thc work of Garvcn (1985), Bcihkc (1986), Ohmoto (1986) and Ravcnhurst cl al. (1989).