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In trying to understand the depositional processes which led to ore deposition in fossil hydrothermal systems, we attempt to reconstruct the chemistry of the fluid phase from observation of its relics (e.g., alteration minerals, fluid inclusions). We may also attempt to thermodynamically model the chemical changes experienced by this fluid as it passes upward through a vein, vents to the seafloor, boils or mixes with other waters, etc. A number of important assumptions are made; one is the assumption of equilibrium and another is that the thermodynamic data base is sound.

Analyses of fluids discharged from geothermal wells, together with drill-core data, allow the opportunity to independently check the validity of the thermodynamic data base and to observe directly, chemical processes leading to the deposition of gold, base-metal sulfides and common gangue minerals like quartz and calcite. The calculations involved are not trivial, but are essential to the understanding of epithermal or any other type of hydrothermal ore deposit.

To illustrate these procedures, we shall examine the discharge of one production well in the Broadlands geothermal field in New Zealand. Through the use of thermodynamics, the amount of information we shall retrieve about the reservoir and depositional processes is quite astonishing. We shall then turn to som e review questions to consider implications for the formation of some hydrothermal ore deposits.

In this chapter we have tried to follow a pragmatic course, avoiding the temptation to overindulge in the (essential) nuances of thermochemistry at the expense of the proscribed

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