Active Geothermal Systems and Hydrothermal Ore Deposits
Authors: White Donald E. 
Published: May 2021

Abstract
During the past 25 years our understanding of hydrothermal ore deposits has progressed remarkably because of combined approaches through detailed study of actual deposits, laboratory experimental study of ore and gangue minerals and fluid inclusions, and study of active geothermal systems. My review emphasizes the active systems, which have recently become a focus of interest in a worldwide search for alternative energy sources.
Sulphur Bank, California, and Ngawha, New Zealand, have provided several keys for understanding the generation of many mercury deposits. Major requirements probably are: deep source regions of fluids and Hg at temperatures &gt;200°C; metamorphic environments above subduction zones on continental margins; a through-going (rather than local) vapor phase enriched in CO2 or other gases, migrating along with liquid water; and instability of HgS at high temperatures, decomposing to Hg0 and S0, with the migrating vapor required for major transport of Hg at temperatures &lt;200°C; a coexisting liquid phase is generally required to transport SiO2 and other nonvolatile constituents. This two-phase mechanism best explains the general absence of other significant ore metals. Vapor-phase transport of the Hg associated with other metals at higher temperatures is probably not essential.
Epithermal precious metal ore deposits are probably the fossil equivalents of high-temperature geothermal systems like Broadlands, New Zealand, and Steamboat Springs, Nevada. The evidence suggests that the fossil and active systems are similar in their rare chemical elements, ranges in temperature, pressure, compositions of fluids, isotope relationships, and mineralogy of ore, gangue, and alteration minerals. Broadlands and Steamboat Springs show a depth zoning of the “epithermal” chemical elements, Au, As, Sb, Hg, Tl, B, and some Ag, that selectively concentrate near the surface. Much Ag, base metals, and probably Se, Te, and Bi precipitate at somewhat greater depths and higher temperatures.
Nolan (1933) divided the epithermal precious metal deposits into a gold-rich group (Au &gt; Ag by weight) and a silver-rich group. The concepts of depth zoning in active geothermal systems, if applied to epithermal deposits, suggest that some gold-rich deposits (including the recently recognized Carlin-type) form at relatively shallow depths and low temperatures. These may grade down into deposits enriched in Ag and base metals, perhaps in places separated by a relatively barren zone resulting from changes in the dominant complexing agent, Cl vs. S. This possibility, even if remote, justifies close examination.
Active systems that might form base metal ore deposits were virtually unknown 25 years ago. Discovery of the Salton Sea, Red Sea, and Cheleken thermal chloride brines in the early 1960s focused on Cl as the probable critical agent, permitting transport of base metals as metal-chloride complexes. Also, some oil field waters were found to have Pb and Zn contents in the range of a few parts per million (ppm) to many tens of ppm. The low-temperature brines have no sulfide within detection limits; only at temperatures &gt;200°C can small quantities of sulfide coexist with the base metals in solution. All of these metal-bearing brines are deficient in sulfide; most of their metals can precipitate as ore deposits only where supplemental sulfide can be provided by any one of several proposed mechanisms. Comparable brines in the past probably formed low-temperature epigenetic deposits like those of the Mississippi Valley, as well as many marine sediment-hosted syngenetic and early diagenetic ore deposits.
Ore fluids rich in both base metals and reduced sulfide species probably require very high salinity, high temperature, and rock-water reactions buffered at low pH (thus, with little free S−2 immediately available). Hostile environments of extreme temperature and salinity, such as those indicated in generating porphyry copper deposits, cannot be drilled by present methods, even if we knew where to drill. Visual observation of comparable environments seemed unlikely until early in 1979, when Cu and Zn sulfides were found to be precipitating from spring vents on the spreading axis of the East Pacific Rise at temperatures exceeding 350°C. Low-temperature discharges on the sea floor had been known for a few years, but the activity in this hydrothermal area, known as Twenty-One North, is the first that bears directly on the origin of volcanogenic massive sulfide deposits (and indirectly on other deposits formed at extreme temperatures and salinities).
Brines of many origins can form base metal deposits; origin of the water may be less important than the physical and chemical environments of the brines and source rocks. Ocean water alone, ocean and fresh waters plus evaporites, evolved connate waters of marine sedimentary rocks, and magmatic waters are all effective solvents of base metals in suitable environments. Precipitation of metals from these brines can occur by decreasing temperature, mixing with low-salinity water, access of supplemental sulfide, and neutralizing reactions with wall rocks, as well as various combinations of these.
Suggestions for exploration for concealed deposits of the major groups considered here are offered, resulting from improved understanding of various genetic models.