A Practical Guide to the Thermodynamics of Geothermal Fluids and Hydrothermal Ore Deposits
R. W. Henley, K. L. Brown, 1985. "A Practical Guide to the Thermodynamics of Geothermal Fluids and Hydrothermal Ore Deposits", Geology and Geochemistry of Epithermal Systems, B. R. Berger, P. M. Bethke
Download citation file:
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
Figures & Tables
Geology and Geochemistry of Epithermal Systems
In the context of exploration for epithermal deposits, why study geothermal systems at all? After all, not one exploited system to date has been shown by drilling to harbor any economically significant metal resource--but then until recently not one had been drilled for other than geothermal energy exploration.* The latter involves drilling to depths of 500-3000 meters in search of high temperatures and zones of high permeability which may sustain fluid flow to production wells for steam separation and electricity generation. In many cases such exploration wells have discovered disseminated base-metal sulfides with some silver and argillic-propylitic alteration equivalent to that commonly associated with ore-bearing epithermal systems (Browne, 1978; Henley and Ellis, 1983; Hayba et al., 1985, this volume). In general, however, geothermal drilling ignores the upper few hundred meters of the active systems and drill sites are situated well away from natural features such as hot springs or geysers, the very features whose characteristics (silica sinter, hydrothermal breccias) are recognizable in a number of epithermal precious-metal deposits (see, for example, White, 1955; Henley and Ellis, 1983; White, 1981; Berger and Eimon, 1983; Hedenquist and Henley, 1985; and earlier workers such as Lindgren, 1933). Knowledge of the upper few hundred meters of active geothermal systems is scant and largely based on interpretation of hot-spring chemistry. Tantalizingly, in a number of hot springs, transitory red-orange precipitates occur which are found to be ore grade in gold and silver and which carry a suite of elements (As, Sb, Hg, Tl) now recognized as characteristic of epithermal gold deposits (Weissberg, 1969).