Volatiles in Magmatic Systems
The physical and chemical setting of geothermal systems is dominated by waters of surfi- cial origin; nevertheless, the heat sources are believed to be magma-s, and there is also a high probability that magmatic fluids contribute heat and some dissolved components H2s, SO2, CO2, … ) to the modern hydrothermal systems that are tapped for energy. By the same token, epithermal and other fossil hydrothermal deposits may well have received contributions of metals from magmas. Because subsurface zones of magma influence are never directly observed during magmatic activity, the magmatic story is an after-the-fact interpretation of mineral assemblages; and it is an emerging story with many chapters still unwritten. This chapter will develop the basis necessary to deal with the magmatic equilibria responsible for some of the magmatic gases.
For the most part we shall deal with silicic magmas because they are much more commonly associated with both geothermal activity and hydrothermal ore deposits than are mafic ones, but the general calculations described here are relatively insensitive to rock type. It is the minor minerals — especially the titanium-iron oxides and pyrrhotite — that are most critical in this discussion.
At magmatic temperatures many species are associated as neutral complexes rather than ionic compounds. So, for example, HCl, NaCl, and KCl are not ionic, but neutral molecules at temperatures above about 500°C or so (Franck, 1956; Quist and Marshall, 1968). Similarly, the sulfurous gases are present as H2S and SO2 rather than ionized species. As a consequence, reactions at magmatic
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This text is designed to introduce you to the practical concepts and calculations involved in interpreting the chemistry of high-temperature fluids in geothermal systems and hydrothermal ore-forming environments. It is intended that the energetic reader will learn to understand chemical principles, handle routine calculations and follow specialized chemical studies involved in geothermal exploration and exploitation and in ore genesis.
Although the emphasis of the text is on the interpretation of the chemistry of active geothermal systems, the principles involved are equally relevant to the interpretation of fossil hydrothermal ore-forming environments. Many gold-silver ore deposits, for example, have been shown to have formed in the near-surface region of hydrothermal systems similar in fluid chemistry and setting to those active today (White, 1981; Henley and Ellis, 1983). Combination of a knowledge of the principle processes within the active geothermal systems, the thermodynamics of complex ion formation, mineral-fluid equilibria and stable isotope systematics provide a framework which may assist in reconstruction of the hydrological regime within a fossil hydrothermal system where ore deposition occurred. This in turn may become useful in ore search. A chapter dealing with the hydrothermal chemistry of magmatic systems is included later in order to encompass a wider range of ore depositing environments and perhaps the root zones of the active geothermal systems.
After a short introduction to the types of geothermal fluids and chemical calculations, successive chapters will address the interpretation of water and gas analyses from geothermal wells. When we understand the reservoir compositions of some geothermal fluids and their relations to rock chemistry and temperature, we will consider the chemical and isotopic changes that occur in the natural transport of this fluid to the surface, derive and use chemical geothermometers and mixing relations, and map the surface chemistry of a hot spring system. After these studies of natural fluids at depth and at the surface, we will study chemical changes that occur during the exploitation of geothermal fluids and how to anticipate and avoid some of the problems of scaling and corrosion.