A Historical Perspective on the Nature and Genesis of Epithermal Gold-Silver Deposits
Authors: Berger Byron R. 
Published: November 2019

Economic geologists have sought for most of this century to achieve a unified set of models for epithermal Au-Ag deposits to serve as springboards for exploration and new avenues of research. Prior to the 1860s, form and appearance served as the basis for ore deposit classification, with genetic concepts modifying these early theories in the latter part of the century. In 1913, Lindgren classified vein deposits into separate categories on the basis of pressure and temperature criteria gleaned from documented attributes of many different types of ore deposits and study of metamorphic rock assemblages. In 1922, he coined the term “epithermal” for the shallower, volcanic rock-related banded veins.

Elie de Beaumont (1847), LeConte (1883), Posepny (1893), Lindgren (1901), and others recognized the possible link of active thermal systems to the formation of vein deposits. It was noticed that H2S- and CO2-bearing thermal springs deposit gold and silver. The respective roles of magmatic and meteoric fluids in vein formation were hotly debated. However, it was not until the 1950s and the advent of new analytical technologies, including mass spectrometry, and the further study of active geothermal systems by such workers as D. E. White that quantitative evidence of the sources of fluids in veins could be obtained. Between Lindgren’s (1913) classification of ore deposits and World War II was, therefore, a time of expanding the empirical data base on actual deposits and increasing the theoretical data base for understanding wall-rock alteration reactions and silicate mineral stabilities.

The discovery in the 1970s of economic epithermal deposits with surficial features that unequivocally tie them to geothermal systems has led to the final proof that they are formed in the deposition-discharge regime of convective, meteoric water-dominated geothermal systems. Metalliferous systems are not randomly distributed, but are related to large, crustal-scale fault systems where there is an ample supply of readily available water. Also, we now recognize that there are two major types of epithermal systems—quartz + adularia + sericite and kaolinite + alunite ± pyrophyllite. Each type is due to differing geochemical inputs into the source regime of geothermal systems: reduced sulfur gases in the case of the adularia-sericite systems and oxidized sulfur gases in the kaolinite-alunite ± pyrophyllite systems.

Thermochemical data acquired in the past 30 yrs in combination with high-speed computers have led to numerical simulations of mineral-fluid reaction paths and fluid flow paths. This has put us in a position to unravel finally the causes, meaning, and significance of mineralogical zoning in epithermal systems. This knowledge will aid exploration efforts in establishing drilling targets. Some understanding of paleohydrology will be part of the explorationist’s bag of tools in the near future.

In a sense, it can be said that our understanding today is largely a honed perspective based on concepts developed in the early part of the twentieth century. It behooves us to use this ample body of knowledge and experience to set our sights on the most important research needs yet to be addressed.