Geologic, Mineralogic, and Geochemical Characteristics of Volcanic-Hosted Epithermal Precious-Metal Deposits
Published:January 01, 1985
Daniel O. Hayba, Philip M. Bethke, Pamela Heald, Nora K. Foley, 1985. "Geologic, Mineralogic, and Geochemical Characteristics of Volcanic-Hosted Epithermal Precious-Metal Deposits", Geology and Geochemistry of Epithermal Systems, B. R. Berger, P. M. Bethke
Download citation file:
In Chapter 1, R. W. Henley summarized our understanding of the chemical and hydrodynamic structure and the transport properties of active hydrothermal systems, with particular emphasis on terrestrial magmatic-hydrothermal systems. Such an overview is especially valuable because active geothermal systems are modern “archetypes” of the ancient systems which concentrated metals in their upper portions to form epithermal ore deposits. More than any other factor, the study of active systems has provided the framework on which the observations on epithermal deposits have been arranged in the relatively recent development of comprehensive models of epithermal ore formation. The Principle of Uniformitarianism has served us well in this instance.
In this chapter, we focus on observations on epithermal ore deposits in continental silicic to andesitic volcanic terranes. Volcanic-hosted deposits offer the most direct comparison with many of the well-studied modern geothermal systems. We first compare the attributes from a number of epithermal ore deposits and show how they may be used to identify two important, and distinct volcanic-related hydrothermal environments. We then examine the best-studied deposit of each type: Creede and Summitville, both of which are located in the San Juan Mountains in southwest Colorado. In so doing, we are able to examine epithermal deposits for evidences of processes that are now occurring in geothermal systems. Finally, we use the observational base and interpretations derived from each deposit type to develop generalized “geothermal” models of mineralization. The models have been taken, in large part, from the excellent synthesis by Henley and Ellis
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).