Geological Characteristics of Epithermal Precious and Base Metal Deposits
Published:January 01, 2005
Stuart F. Simmons, Noel C. White, David A. John, 2005. "Geological Characteristics of Epithermal Precious and Base Metal Deposits", One Hundredth Anniversary Volume, Jeffrey W. Hedenquist, John F. H. Thompson, Richard J. Goldfarb, Jeremy P. Richards
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Epithermal deposits are important sources of gold and silver that form at <1.5-km depth and <300°C in high-temperature, mainly subaerial hydrothermal systems. Such hydrothermal systems commonly develop in association with calc-alkaline to alkaline magmatism, in volcanic arcs at convergent plate margins, as well as in intra-arc, back-arc, and postcollisional rift settings. Many important deposits are T ertiary and younger in age and are concentrated around the Pacific Rim and in the Mediterranean and Carpathian regions of Europe. Older deposits occur in the Tethyan arc from Europe to Asia and others are scattered in volcanic arcs of all ages with rare examples as old as Archean.
Precious metal mineralization develops in zones of high paleopermeability, hosted within sequences of coeval volcanic and underlying basement rocks. V eins with steep dips are common and these tend to host highest grade ores. Precious metal mineralization also occurs in breccias, coarse clastic rocks, and intensely leached rocks; such disseminated ore is much lower in grade but greater in total tonnage and may be amenable to bulk mining methods. Deposits and districts, comprising one or more orebodies, cover areas from <10 to -200 km2.
Epithermal deposits have been classified on the basis of alteration and gangue mineral assemblages, metal contents, sulfide contents, and sulfide mineral assemblages, and each classification scheme has its merits. Because ores are oxidized by weathering, we prefer a classification that utilizes gangue mineral assemblages. We describe two types of mineralization associated with quartz + calcite + adularia + illite and quartz + alunite + pyrophyllite + dickite + kaolinite assemblages, which reflect the pH of hydrothermal solutions.
Epithermal deposits associated with quartz + calcite + adularia + illite contain Au-Ag, Ag-Au, or Ag-Pb-Zn ores. Electrum, acanthite, silver sulfosalts, silver selenides, and Au-Ag tellurides are the main gold- and silver -bearing minerals, with generally minor sphalerite, galena, and chalcopyrite; in some deposits base metals dominate the metal assemblage. Quartz is the principal gangue mineral accompanied by variable amounts of chalcedony, adularia, illite, pyrite, calcite, and/or rhodochrosite, the latter in more Ag- and base metal-rich deposits. Distinctively banded crustiform-colloform textures, and lattice textures comprising aggregates of platy calcite and their quartz pseudomorphs, are common. Hydrothermal alteration is zoned and comprises deep regional propylitic alteration, which gives way upward to increasing amounts of clay, carbonate, and zeolite minerals, whereas quartz, adularia, illite, and pyrite form proximal alteration zones enveloping orebodies. Ore-grade mineralization commonly terminates upward, and where there has been minimal erosion, it can be concealed beneath regionally extensive blankets of clay-carbonate-pyrite or kaolinite-alunite-opal +pyrite alteration. Fluid inclusion data indicate salinities are commonly <5 wt percent NaCl equiv for Au-Ag deposits and <10 to >20 wt percent NaCl equiv for Ag-Pb-Zn deposits. Stable isotope data indicate that hydrothermal solutions were composed mostly of deeply circulated meteoric water, with a nil to small and variable component of mag-matic water.
Epithermal deposits associated with quartz + alunite + pyrophyllite + dickite + kaolinite assemblages contain Au + Ag + Cu ores. Native gold and electrum are the main ore-bearing minerals, with variable amounts of pyrite, Cu-bearing sulfides and sulfosalts such as enargite, luzonite, covellite, tetrahedrite, and tennantite, plus sphalerite and telluride minerals; enargite dominates the Cu sulfides and indicates a high-sulfidation state. Quartz (both massive and vuggy) and alunite are the main gangue minerals with kandite minerals (dickite and/or kaolinite) and/or pyrophyllite. Concentric patterns of hydrothermal alteration envelop thEzone of vuggy and massive quartz alteration, which hosts ore. Outward, these comprisEzones of quartz and alunite, dickite + kaolinite or pyrophyllite, and illite or smectite alteration, surrounded by regional propylitic alteration. Zones of illite or pyrophyllite alteration occur in the roots beneath some deposits. Fluid inclusion data indicate that salinities are typically <5 to 10 wt percent NaCl equiv but may be as high as >30 wt percent NaCl equiv. Stable isotope data indicate that the altering fluids are composed mostly of magmatic fluids with a minor to moderate component of meteoric water.
Critical genetic factors include: (1) at several-kilometers depth, the development of oxidized and acidic versus reduced and near-neutral pH solutions, controlled by the proportions of magmatic and meteoric components in solution, and the amount of subsequent water -rock interaction during ascent to the epithermal environment; (2) at epithermal depths, the development of boiling and/or mixing conditions which create sharp physical and chemical gradients conducive to precious and base metal precipitation; and (3) at shallow level, the position of the water table, which controls the hydrostatic pressure-temperature gradients at depth where epithermal mineralization forms.
Epithermal mineralization can occur in large areas, with orebodies that range in shape, size, and grade, and lie easily concealed beneath blankets of clay alteration or unaltered volcanic deposits. Efficient exploration requires integration of all geological, geochemical, and geophysical data, from regional to deposit scale. Vein mineralogy and texture, patterns of hydrothermal alteration, patterns of geochemical dispersion, and three-dimensional interpretation of related geophysical signatures are important guides. W illingness to drill is crucial, as surface features may not reliably indicate what is present at depth.
Figures & Tables
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 $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