Our detailed three-dimensional model of the natural flow regime of the Cerro Prieto geothermal field, before steam production began, i s based on p a tterns of hydrothermal mineral zones and l i g h t stable isotopic ratios observed i n rock samples from more t h a n f i f t y deep wells, together w i t h temperature gradients, wireline logs and other data. A t the level so far penetrated by drilling, this hydrothermal system was heated by a thermal plume of water close t o boiling, inclined a

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... 4 5 O , rising from the northeast and discharging t o the west. thermal plume. Fission track annealing studies shows t h a t the reservoir To the east a zone of cold water recharge overlies the inclined reached 170°C only lo4 years ago. Oxygen isotope exchange d a t a indicate t h a t a 12 km3 volume of rock subsequently reacted w i t h three times i t s volume of water hotter t h a n 200OC. Averaged over the duration' of the heating event this would require a flow velocity o f a b o u t 6 mlyear through the pores of a typical cross section of the reservoir having an average porosity of 10%. The heat i n storage i n t h a t part of the reservoir hotter t h a n 2 O O O C and shallower t h a n 3 km depth i s equivalent t o t h a t w h i c h would be released by the cooling of about one or two cubic kilometers of basalt or gabbro magma. A l t h o u g h this i s an extensional tectonic environment of "leaky" transform f a u l t i n g i n w h i c h repeated intrusions of basalt magma are likely, for simplic i t y of computation we have modelled possible heat sources as simple two I dimensional basalt intrusions o f various sizes, shapes and locations. W e have cal cul ated a series of two-dimensi onal convect1 ve heat transfer model s , w i t h d i fferen,t' heat sources and permeabi 1 i t y distributions.