Jeffrey W. Hedenquist

Epithermal environments and styles of mineralization: Variations and their causes, and guidelines for exploration

Abstract Epithermal precious- and base-metal deposits are diverse, reflecting the different tectonic, igneous and structural settings in which they occur, the complexities of their local setting, and the many processes involved in their formation. Most epithermal deposits form at shallow crustal levels where abrupt changes in physical and chemical conditions result in metal deposition and attendant hydrothermal alteration. The principal factors that influence the conditions prevailing in the epithermal environment, and which ultimately determine the sites and character of mineralization, include: geology (structure, stratigraphy, intrusions and rock type, which affect the style and degree of permeability and the reactivity of the host); pressure and temperature (which in the epithermal environment are related on the boiling point with depth curve); hydrology (the relationship between permeability and topography which governs fluid flow, and discharge/recharge characteristics, as well as access of steam-heated waters); chemistry of the mineralizing fluid (which determines the metal-carrying capacity, as well as the associated vein and alteration assemblage); and syn-hydrothermal development of permeability and/or changes in hydraulic gradients. Many attempts have been made to classify epithermal deposits based on mineralogy and alteration, the host rocks, deposit form, genetic models, and standard deposits. All have their strengths and weaknesses. We prefer a simple approach using the fundamental fluid chemistry (high or low sulfidation, reflecting relatively oxidized or reduced conditions, respectively) as readily inferred from vein and alteration mineralogy and zoning, together with the form of the deposit, and using comparative examples to clarify the character of the deposit. Guidelines for exploration vary according to the scale at which work is conducted, and are commonly constrained by a variety of local conditions. On a regional scale the tectonic, igneous and structural settings can be used, together with assessment of the depth of erosion, to select areas for project area scale exploration. At project area scale, direct (i.e. geochemical) or indirect guidelines may be used. Indirect methods involve locating and interpreting hydrothermal alteration as a guide to ore, with the topographic and hydrologic reconstruction of the system being of high priority. These pursuits may involve mineralogic, structural, geophysical or remote sensing methods. On a prospect scale, both direct and indirect methods may be used; however, they can only be effective in the framework of a sound conceptual understanding of the processes that occur in the epithermal environment, and the signatures they leave.

Contemporaneous formation of adjacent porphyry and epithermal Cu-Au deposits over 300 ka in northern Luzon, Philippines

There is commonly a close spatial relation between porphyry Cu (± Au) and high-sulfidation epithermal Cu-Au deposits throughout the world, although a genetic association has not been proven. Nowhere is this spatial association better seen than in northern Luzon, Philippines, where the Lepanto epithermal Cu-Au deposit overlies the Far Southeast (FSE) porphyry Cu-Au deposit, both world-class orebodies. Fresh rock and hydrothermal mineral separates yield K/Ar ages indicating that premineralization and postmineralization volcanism occurred at 2.2–1.8 Ma and 1.2–0.9 Ma, respectively, and that the hydrothermal system was active from ∼1.5 to ∼1.2 Ma. K/Ar ages of alunite from Lepanto have the same range as those of hydrothermal biotite and illite from the FSE deposit, indicating that both epithermal and porphyry mineralization formed from an evolving magmatic-hydrothermal system that was active for about 300 ka. This temporal relation strengthens the argument for a genetic link between these two styles of ore deposit, and has implications for exploration. Where one style of mineralization is found, there is potential for the other nearby.

Flux of volatiles and ore-forming metals from the magmatic-hydrothermal system of Satsuma Iwojima volcano

The Satsuma Iwojima volcano, southwest Japan, degasses 5 x 10 6 t/yr H 2 O, 9 x 10 4 t/yr S, and 6 x 10 4 t/yr Cl from high-temperature (≤880 °C) fumaroles atop a now-altered rhyolite dome that erupted 1200 yr ago. Acidic hot springs (pH 1.1-1.8) discharge ∼20 x 10 6 t/yr H 2 O to the sea; this water is composed of ∼1 part magmatic vapor absorbed by 6 parts meteoric ground water. The Cl and SO 4 in solution originate from the vapor, whereas cation components are derived largely by dissolution of the rhyolite. The flux of Pb, Zn, Cu, and Mo in the vapor and acidic springs ranges from 0.1 to 10 t/yr each, whereas the Au flux is 10 -5 and 10 -3 t/yr, respectively. The low concentrations of NaCl and metals in the vapor are due to the condensation of a hypersaline liquid from the vapor during ascent and depressurization, meaning that the atmospheric-pressure vapor does not reflect the composition of the fluid exsolving from the magma. Neither this low-pressure vapor nor the acidic waters can account for high-sulfidation Cu-Au ore deposits deduced to have formed in this environment; such mineralization requires the subsequent ascent of a metal-rich fluid.

White Island, New Zealand, volcanic-hydrothermal system represents the geochemical environment of high-sulfidation Cu and Au ore deposition

The White Island volcanic-hydrothermal system, New Zealand, is thought to closely represent the chemical conditions that lead to the formation of high- sulfidation Cu-Au ore deposits. The amounts of Cu and Au produced over a 10 ka period of activity, largely from degassing magma, are calculated to be 106 and 45 t, respectively. Altered andesite blocks ejected from recent vents contain alunite, anhydrite, and pyrite. Their S isotopic composition indicates vein filling at ∼380 °C. At this temperature, Cu and Au are highly soluble in acid solutions, which may explain the depletion of Cu and absence of Au in the ejecta. Mass-balance calculations, however, suggest that Cu and Au are precipitated in cooler zones before the acid solutions discharge at the surface.

Meteoric interaction with magmatic discharges in Japan and the significance for mineralization

The andesitic volcanoes Kirishima, Kyushu, and Esan, Hokkaido, discharge 94 to 225 °C HCl-bearing vapors from summit fumaroles. At Kirishima a geothermal system exists on the flanks of the large volcanic massif (1500 m relief and 12 km radius). In contrast, Esan is a small dome (600 m relief and 1 km radius) with ephemeral hot springs. The chemical and isotopic compositions of the fumarole gases and condensates, and of waters from hot springs, indicate that Esan discharges are dominated by magmatic water and gases, whereas those at Kirishima are mainly meteoric. The Kirishima geothermal system contains acid fluids that are neutralized by interaction with the host rock and dilution by meteoric ground water; the acidity is probably of magmatic origin. A large ground-water carapace at Kirishima condenses a majority of magmatic volatiles and metals before they can discharge to the surface, in contrast to Esan, where the volatiles (including metals) degas to the atmosphere. This suggests that a meteoric system may be necessary to condense metals in this high-level volcanic environment to provide a situation conducive to hydrothermal mineralization at epithermal depths.

Metals in deep liquid of the Reykjanes geothermal system, southwest Iceland: Implications for the composition of seafloor black smoker fluids

Seafloor hydrothermal systems precipitate Cu, Zn, and Fe sulfides at and below black smoker vents on the seafloor; as a result, the metal concentrations in the vent fluids are minimum values. We sampled deep, unboiled liquids from the Reykjanes geothermal reservoir, Iceland, and measured the metal concentrations. This active, seawater-dominated system, situated on the Mid-Atlantic Ridge, is the subaerial equivalent to mid-ocean-ridge hydro thermal systems. The liquids, collected at 1350–1500 m depth and 284–295 °C, contain 154–2431 μ M Fe (9–140 ppm), 207–261 μ M Cu (14–17 ppm), 79–393 μ M Zn (5–27 ppm), 0.6–1.4 μ M Pb (120–290 ppb), 6–31 n M Au (1–6 ppb), and 250–960 n M Ag (28–107 ppb). Fluids discharged at surface from the same wells have orders of magnitude lower metal concentrations due to precipitation caused by boiling and vapor loss during depressurization. The concentrations of Cu, Zn, and Pb in the high-temperature reservoir liquids at Reykjanes are similar to those in the highest-temperature black smoker discharges, whereas Au and Ag concentrations are one to two orders of magnitude higher at Reykjanes; lower-temperature seafloor fluids have lower metal contents, suggesting subseafloor deposition before discharge. The Reykjanes heat flux of 130 MW requires a liquid flux of ~100 kg/s; over 104 yr, the minimum life of the system, 0.5 Mt each of Cu and Zn may have precipitated at depth.

Magmatic contributions to hydrothermal systems

Although there is agreement that many hydrothermal systems in the upper crust derive their thermal energy from magmas, debate continues over the extent to which magmas contribute water, metals, and sulfur to hydrothermal systems. A multidisciplinary seminar was held November 10–16, 1991, in Ebino and Kagoshima, Japan, to establish current understanding about this topic and to explore the major unanswered questions and the most promising research directions. The thirty-eight participants were from Japan (eighteen), the U.S. (thirteen), Canada and New Zealand (two each), and England, the Philippines, and Russia (one each). Disciplines represented were volcanology, geochemistry (volcanic-gas, water, isotopes, experimental, and modeling), igneous petrology, geothermal geology, economic geology, fluid-inclusion study, geophysics, and physical modeling.

One Hundredth Anniversary Volume

From the first issue in 1905 onward, Economic Geology has been the main publication for those who study mineral deposits; indeed, it is now difficult to imagine economic geology without Economic Geology. It is interesting to ask, therefore, Who were the farsighted people who founded the journal, and Why did they think a specialized publication devoted to mineral deposits was needed? Let us first address the question, Who were the founders? They were the 12 men who collectivelydecided a new publication was needed, who then planned the financial structure to support the venture, and who served as the original editorial group. All were employed by, or associated with, the U.S. Geological Survey. Josiah Edward Spurr suggested the need for a journal sometime in November or December 1904. After informal discussions, nine of the founders met in the office of Waldemar Lindgren in the headquarters of the U.S. Geological Survey in Washington, D.C., on May 16, 1905, and founded the Economic Geology Publishing Company. The sole purpose of the company was the publication of a journal ‘...devoted primarily to the broad application of geologicprinciples to mineral deposits of economic value, and to the scientific description of such deposits, and particularly to the chemical, physical, and structural problems bearing on their genesis.’ Initial financing for the new company was raised by the sale of 80 shares at a cost of

The Society of Economic Geologists Awards for 2015 R. A. F. Penrose Gold Medal for 2015Citation for Richard H. Sillitoe

25 per share. Eight of the men at the founding meeting formed the first board of directors; Spurr was president, Frederick L. Ransome, secretary, and George O. Smith, treasurer. Other members were Arthur H. Brooks, Marius R. Campbell, Walter H. Weed, Waldemar Lindgren, and a young academic from Lehigh University in Pennsylvania, John D. Irving. Theninth man at the meeting was H. Foster Bain. Irving was appointed editor. Lindgren, Ransome, and Campbell from the U.S. Geological Survey, together with three academics, James F. Kemp of Columbia University, Heinrich Ries ofCornell University, and Charles K. Leith of the University of Wisconsin, were appointed associate editors. The initial board members, the editor, and associate editors are the people we now recognize as the founders of Economic Geology. Two others, Frank D. Adams, of McGill University in Canada, and John. W. Gregory, of Glasgow University in Scotland, were subsequently added as associate editors, and a third person, W. S. Bayley of the University of Illinois, was appointed as business editor, but

Society of Economic Geologists Ralph W. Marsden Award for 2010 Citation of William Xavier Chávez, Jr.

President Foster, SEG members, and friends: Introducing Richard Sillitoe to economic geologists is an easy task, since most are familiar with his published record on ore deposits. Many here today have heard Dick speak at previous conferences, and the fortunate ones have spent time in the field with him on assignments, assessing the potential of prospects around the world. Dick got started in economic geology when he began Ph.D. studies in 1965 at London University, on supergene oxidation and enrichment of Chilean copper deposits and exploration implications. His first publication, in 1968, was on the chronology of landform evolution and supergene mineral alteration in the southern Atacama Desert, followed by several other papers on supergene minerals. On completion of his thesis, he took up a position in 1968 with the Instituto de Investigaciones Geologicas of Chile, the predecessor of SERNAGEOMIN, to develop exploration models for porphyry copper deposits throughout Chile. He decided to discontinue this work in 1971 because of difficulties created by the changed political situation. In retrospect, this was very fortunate, both for Dick and the broader economic geology community, as …