Hydrothermal Solutions

Abstract

A hydrothermal fluid is defined as a hot (ca. 50->500°C), aqueous solution (hydro = water; thermal = hot), containing Na, K, Ca, Cl as major components, as well as many other elements (e.g. Mg, B, S, Sr, CO2, H2S, NH4, Cu, Pb, Zn, Sn, Mo, Ag, Au etc.) as minor constituents (Skinner 1979). The terms fluid and solution are here used interchangeably, although fluid in the strict sense refers to a phase at a supercritical temperature in which a liquid can no longer exist. The pressure required to cause condensation at a given critical temperature is called the critical pressure. There are certain known conditions in nature where a hydrothermal solution is in fact a fluid. We begin this chapter by having a look at those kinds of waters that may constitute a hydrothermal solution. A brief introduction is given on oxygen and hydrogen isotope systematics and their role in understanding the nature and interactions of hydrothermal fluids and wall rocks. As the only direct observation and sample material of hydrothermal solutions come from fluid inclusions and hot springs, an outline of the characteristics and methodology involved in the study of fluid inclusion is also given. It is emphasised that fluid inclusions provide important insights into the physical and chemical environments of ore deposition. This leads us to a discussion of dissolved constituents, where a series of tables provides some understanding of the composition of natural solutions. The transport of apparently insoluble metals had long baffled chemists until the role ofligands in the dissolution and transport of ore elements-and hence its importance in the study of hydrothermal mineral deposits-was discovered. Various mechanisms of deposition of metals from solutions conclude this chapter. 2.2 Water of Hydrothermal Solutions The water of hydrothermal solutions can be derived from the following sources: meteoric, sea water, connate, metamorphic,juvenile or magmatic. Most hydrother-mal solutions are of mixed origin in which one or more of the above sources can predominate. Meteoric waters include rain water, lake and river waters, and groundwaters. These waters can penetrate deep into the crust and may become heated and F. Pirajno, Hydrothermal Mineral Deposits