Introduction Heat and fluid flow are fundamental processes in the for- mation of all classes of hydrothermal ore deposits, including volcanic-associated massive sulfide (VMS) deposits. Com- puter models simulating heat and fluid flow in complex hy- drothermal systems provide considerable insight into how these systems operate to produce economic concentrations of metals. This contribution reviews recent models of active and fossil hydrothermal systems in five VMS settings, includ- ing bimodal-mafic (ultramafic) (many Precambrian de- posits), mafic (ophiolitic or mid-ocean ridge), mafic-silici- clastic (Besshi or Guaymas basin), bimodal-felsic (Kuroko or Mt. Windsor districts), and bimodal-siliciclastic (Bathurst or Iberian pyrite belt) settings. The models suggest that differ- ences in the style and pattern of alteration and mineraliza- tion are most likely due to differences in the geometry and depth of the intrusion that provides the heat, and the per- meability of the host rock that is intruded. There have been many excellent reviews of the processes of heat and fluid-flow modeling. This material will not be re- peated here, and the reader is referred to Cathles (1981), and Cathles et al. (1997). Heat and fluid-flow models are constructed by numerically requiring conservation of fluid mass, energy (heat), and momentum (Darcy’s Law). Free fluid flow is usually allowed into and out of the top surface in numerical calculations, corresponding geologically to recharge from the groundwater table, lakes, streams or oceans, and to discharge through the surface. Heat is in- troduced by magmatic intrusions which may be permeable or impermeable, and may be made more permeable by thermal cracking as they cool. The permeability of the host formation exerts an important control on circulation. If permeability is too low, no significant convection takes place and the intrusion cools by conduction. If the host per- meability is high, vigorous convection occurs and high tem- perature fluids are vented. Massive sulfide deposits are formed at or near the sea floor where non-boiling fluids are cooled by steep thermal gradients or by quenching in cold seawater. Metal precipitation is rapid if boiling occurs (Drummond and Ohmoto, 1985). For this reason boiling will induce subsurface metal precipitation in the form of vein deposits. VMS deposits require a sufficient (usually marine) water depth to prevent boiling (>~1.5 km for fluids at ~350°C). Although this review considers some hy- drothermal systems in subaerial settings, our main focus is on the marine settings that produce VMS deposits. The main controls on the vigor and pattern of convection are intrusion geometry and depth, and the permeability of the intruded environment. These factors are broadly re- lated to geologic setting and host-rock composition and thus to conventional classification of VMS deposits. Table 1 groups models of hydrothermal systems, first, according to whether they are submarine or subaerial, and secondly, ac- cording to their host-rock compositions and broadly associ- ated tectonic settings, following the classification given in Barrie and Hannington (1999). The VMS settings are or- dered from most primitive, that is, the highest proportion of mantle-derived material in the substrate, to most evolved, or those with the highest crustal (felsic or sedimentary or both) component: bimodal-mafic/ultramafic, mafic, mafic- siliciclastic, bimodal-felsic, and bimodal-siliciclastic settings. Table 1 summarizes the geometries of modeled heat sources, the permeability of the host environment, the re- sults of model calculations, and the principal conclusions reached for studies of the five VMS settings. The thickness of the crustal substrate beneath a VMS de- posit has an indirect but significant control on many of the parameters that affect the hydrothermal systems. If perme- ability decreases with depth or is temperature dependent, deep sills are insulated from rapid advective cooling in comparison to shallow sills. The bimodal mafic/ultramafic, bimodal felsic and bimodal-siliciclastic settings are com- monly underlain by rifted arc or continental crust ~10 to 30 km thick, whereas the mafic and mafic-siliciclastic settings have thinner, rifted oceanic crust 3 to 10 km thick. The most primitive, bimodal mafic/ultramafic setting has the highest proportion of magmas with the high liquidus tem- peratures (basalts, picrites, komatiites) and has a maxi- mum depth for hydrothermal circulation of ~15 km. Mod- eling has demonstrated that deep ultramafic sills are capable of sustaining hydrothermal venting for 500,000 to 1,000,000 years (Barrie et al., 1997; Cathles et al., 1998). The maximum sill depth for the mafic and mafic- siliciclastic VMS settings is <7 km, and models suggest shorter maximum durations for their hydrothermal vent- ing. This contribution reviews published models, and pre- sents the results of models constructed by Cathles and his 201 Chapter 9 Heat and Fluid Flow in Volcanic-Associated Massive Sulfide-Forming Hydrothermal Systems C. T. BARRIE, * Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, Canada K1A 0E8 L. M. CATHLES, A. ERENDI, H. SCHWAIGER, AND C. MURRAY Department of Geological Sciences, Cornell University, Ithaca, New York 14853 * Alternate address: Barrie & Associates, 23 Euclid Avenue, Ottawa, On- tario, Canada K1S 2W2.