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Epithermal gold deposits form in hydrothermal systems related to volcanic activity. These systems, while active, discharge to the surface as hot springs or fumaroles. Thus, the study of active hydrothermal systems by the Mineral Resources Department provides information on hydrothermal processes that are related to metal transport and deposition. In turn, this information can be used to predict how gold deposits form, and where to find them.
Epithermal gold deposits occur largely in volcano-plutonic arcs (island arcs as well as continental arcs) associated with subduction zones, with ages similar to those of volcanism. The deposits form at shallow depth, <1 km, and are hosted mainly by volcanic rocks.
There are two end-member styles of epithermal gold deposits, high
sulfidation (HS) and low sulfidation (LS)(Table 1). The two deposit
styles form from fluids of distinctly different chemical composition
in contrasting volcanic environment (Fig. 1).
The ore of HS deposits is hosted by leached silicic rock associated
with acidic fluids generated in the volcanic-hydrothermal environment
(Fig. 1). In contrast, the fluid responsible
for formation of LS ore veins (Fig. 2)
is similar to waters tapped by drilling beneath hot springs (Figs. 3
and 4) into geothermal systems, waters
that are reduced and neutral-pH.
Many hydrothermal minerals are stable over limited temperature
and/or pH ranges. Thus, mapping the distribution of alteration
minerals in areas of epithermal prospects allows the thermal and
geochemical zonation to be reconstructed, leading to a model of
the hydrology of the extinct hydrothermal system. Alteration minerals
are also crucial to distinguish the style of deposit, LS or HS
(Fig. 5).
Boiling of liquid in the LS geothermal environment leads to precipitation
of gold in veins (Fig. 4), accompanied
by a variety of features such as adularia and bladed calcite (Table
1) cementing colloform and brecciated quartz; silica sinters may
be the surface expression of such veins (Figs. 3
and 4),
and may be accompanied by nearby zones of surficial steam-heated
acid alteration (Fig. 5).
Exploration for epithermal systems must be based on basic geologic
characteristics (Table 1), especially those that can be recognized
easily in the field, as these characteristics reflect ore-forming
processes. Our understanding of ore-forming processes will continue
to be improved through the study of active hydrothermal systems.
Interpretations of such processes can then be used to guide efficiently
mineral exploration.
Table 1 General characteristics of epithermal gold deposits associated with subaerial volcanic rocks
| . | Low sulfidation (LS) (Adularia-sericite) | High sulfidation (HS) (Acid-sulfate) |
| Deposit form | Open-space veins dominant, stockwork ore common Disseminated and replacement ore minor | Disseminated ore dominant, replacement ore common Stockwork ore minor, veins commonly subordinate |
| Textures | Veins, cavity filling (bands, colloforms, druses), breccias | Wallrock replacement, breccias, veins |
| Ore minerals | Pyrite, electrum, gold, sphalerite, galena (arsenopyrite) | Pyrite, enargite, chalcopyrite, tennantite, covellite, gold, tellurides |
| Gangue | Quartz, chalcedony, calcite, adularia, illite, carbonates | Quartz, alunite, barite, kaolinite, pyrophyllite |
| Metals | Au, Ag, Zn, Pb (Cu, Sb, As, Hg, Se) | Cu, Au, Ag, As (Pb, Hg, Sb, Te, Sn, Mo, Bi) |
For more information contact:
Masahiro AOKI
e-mail: aoki@gsj.go.jp