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In studies of epithermal precious and base metal ore deposits, estimates of salinity (total dissolved salts) are frequently in error when based on fluid inclusion ice melting measurements in the absence of an independent determination of the CO2 content of the inclusion fluid. For a fluid of known composition, the melting point of ice (Tin) may be calculated from Tm= --• Kimi where Ki is the molal freezing (or melting) point depression constant and mi is the molality of a component i (i = Na +, K +, CI-, CO2, etc.). For fully dissociated solute species such as CI-, in the salinity range considered here, K = 1.72 Kelvin/molal, and for undissociated, nonpolar species such as CO•, K --1.86 Kelvin/molal. Fluid inclusion ice-melting data from New Zealand geothermal fields correlate well with values calculated using the above equation and the measured compositions of discharges from wells from which the inclusion samples were obtained. Loss of the dominant dissolved gas, CO•, during boiling at depth results in large, systematic decreases in apparent salinity (in terms of Tm) in the Broadlands field. Misinterpretation of fluid inclusion freezing data may lead to substantial errors in the reconstruction of the physico-chemical environment of ore formation in fossil systems. For example, in the absence of CO• analyses, inclusion fluids similar in gas content to the Broadlands geothermal fluid (NaC1 = 0.2 wt %, CO2 up to 4.4 wt %) may be interpreted to have salinities of 0.85 wt percent NaC1, leading to errors of 23 percent in the estimated depth of formation at 280øC, -0.5 unit in the estimated pH of the ore fluid, and on the order of 200 times in the estimated solubility of an ore component such as lead. Such errors may be transmitted into subsequent estimation of fluid flux or duration of ore formation.A review of published fluid inclusion studies shows that true salinities of inclusion fluids from epithermal precious metal deposits are coincident with those from the majority of explored geothermal systems but that the salinities of inclusion fluids from epithermal base metal deposits are substantially higher, indicating the presence of an evolved, high-salinity fluid during ore metal transport. In the case of low-salinity epithermal deposits, the extreme range in gold to silver ratios between deposits can be accounted for by variations in the H•S to chloride ratio in the original hydrothermal fluid. These variations (primarily in the total gas) occur between active geothermal systems and can be recognized in epithermal deposits through fluid inclusion freezing and crushing studies.

Porphyry copper deposits, all showing similar geological characteristics, occur in Tertiary and older orogenic-volcanic belts around the w•orld. Recent isotope and fluidinclusion studies have shown that in a number of deposits the development of the characteristic ore alteration pattern, at some stage, involved the interaction of meteoric ground waters with saline fluids evolved from a magma. A fluid dynamic model is proposed for porphyry copper emplacement which focuses on the interaction of a buoyant low-salinity magmatic vapor plume with surrounding ground water. As the magmatic vapor rises and cools, high-salinity liquid condenses in a two-phase plume core, drains under gravity, and is diverted to vertical lower salinity stream lines tangential to the two-phase core boundary. Cool ground water is entrained into the rising fluid, giving rise to a buoyant dispersion plume. The potassic core and inner part of the phyllic alteration envelope of the porphyry copper system is regarded, in compliance with isotopic data, as the remnant imprint of the plume on the ground-water regime.Although the model may be modified to a ground-water source for the "magmatic fluid," the authors favor an orthomaglnatic hypothesis by which w, ater and essential ore components are derived from a cooling magma column convecting lighter, more volatile components from a deeper level. The temperature profile of the steady-state plume is calculated using empirical data for permeability and heat input from the active Wairakei and Broadlands geothermal systems. Chemical implications of the physical model are in accord with the observed alteration-mineralization patterns and available high-temperature solubility data. Metals enter the system as hydroxyl or chloride complexes in the low-salinity magmatic gas precipitating in response to ground-water entrainment, temperature, and wall-rock induced pH and ]:o2 variations. Some transport analogies are tentatively drawn with the observed chemistry of volcanic gases.The plume model also provides an interpretation of the characteristics of the deep portion of active geothermal systems and may be extended to other ore-forming systems such as epithermal veins and massive sulfides. In the majority of such hydrothermal systems, if ore formation occurred below around 350øC, the magmatic input may be marked by the then predominant entrained ground-water component.

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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.

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Large bulk-tonnage high-sulfidation gold deposits, such as Yanacocha, Peru, are the surface expression of structurally-controlled lode gold deposits, such as El Indio, Chile. Both formed in active andesite-dacite volcanic terranes. Fluid inclusion, stable isotope and geologic data show that lode deposits formed within 1500 m of the paleo-surface as a consequence of the expansion of low-salinity, low-density magmatic vapor with very limited, if any, groundwater mixing. They are characterized by an initial 'Sulfate' Stage of advanced argillic wallrock alteration ± alunite commonly with intense silicification followed by a 'Sulfide' Stagea succession of discrete sulfide-sulfosalt veins that may be ore grade in gold and silver. Fluid inclusions in quartz formed during wallrock alteration have homogenization temperatures between 100 and over 500°C and preserve a record of a vapor-rich environment. Recent data for El Indio and similar deposits show that at the commencement of the Sulfide Stage, 'condensation' of Cu-As-S sulfosalt melts with trace concentrations of Sb, Te, Bi, Ag and Au occurred at N 600°C following pyrite deposition. Euhedral quartz crystals were simultaneously deposited from the vapor phase during crystallization of the vapor-saturated melt occurs to Fe-tennantite with progressive non-equilibrium fractionation of heavy metals between melt-vapor and solid. Vugs containing a range of sulfides, sulfosalts and gold record the changing composition of the vapor. Published fluid inclusion and mineralogical data are reviewed in the context of geological relationships to establish boundary conditions through which to trace the expansion of magmatic vapor from source to surface and consequent alteration and mineralization. Initially heat loss from the vapor is high resulting in the formation of acid condensate permeating through the wallrock. This Sulfate Stage alteration effectively isolates the expansion of magmatic vapor in subsurface fracture arrays from any external contemporary hydrothermal activity. Subsequent fracturing is localized by the embrittled wallrock to provide highpermeability fracture arrays that constrain vapor expansion with minimization of heat loss. The Sulfide Stage vein sequence is then a consequence of destabilization of metal-vapor species in response to depressurization and decrease in vapor density. The geology, mineralogy, fluid inclusion and stable isotope data and geothermometry for high-sulfidation, bulk-tonnage and lode deposits are quite different from those for epithermal gold-silver deposits such as McLaughlin, California that formed near-surface in groundwater-dominated hydrothermal systems where magmatic fluid has been diluted to less than about 30%. High sulfidation gold deposits are better termed 'Solfataric Gold Deposits' to emphasize this distinction. The magmatic-vapor expansion hypothesis also applies to the phenomenology of acidic geothermal systems in active volcanic systems and equivalent magmatic-vapor discharges on the flanks of submarine volcanoes.