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IDENTIFICATION
SYNONYMS: Epithermal massive
sulphide; subaqueous-hydrothermal deposits; Eskay- type deposit;
Osorezan-type deposit.
COMMODITIES (BYPRODUCTS): Ag,
Au (Cu, Pb, Zn, As, Sb, Hg).
EXAMPLES (British Columbia - Canada/International):
Eskay Creek (104B008), Lulu (104B376); Osorezan, Vulcano Islands and
Jade hydrothermal field (Japan), Mendeleev Volcano (Kurile Islands, Russia),
Rabaul (Papua New Guinea), White Island (New Zealand), Bacon-Manito and
Surigao del Norte (Phillippines).
GEOLOGICAL
CHARACTERISTICS
CAPSULE DESCRIPTION: Vein,
replacement and synsedimentary bedded sulphides are deposited in volcanic
rocks and associated sediments in areas of shallow lacustrine, fluvial or
marine waters or in glacial subfloors.
TECTONIC SETTING: Active volcanic
arcs (both oceanic island arcs and continental margin arcs) are likely
setting.
DEPOSITIONAL ENVIRONMENT / GEOLOGICAL
SETTING: 1) Water-filled reservoirs in active continental volcanic
areas (crater lakes, playa lakes, stream flood plains, glacier subfloors).
2) Sea-flooded, breached calderas, or unconsolidated shallow marine
sediments at the foot of a volcano.
AGE OF MINERALIZATION: Presumably
any age, oldest known example is Jurassic.
HOST/ASSOCIATED ROCK TYPES:
Mineralization hosted by intermediate to felsic flows and tuffs and minor
intercalated sedimentary rocks. Pillow lavas, coarse epiclastic debris
flows, and assorted subvolcanic feeder dikes are all part of the local
stratigraphic package.
DEPOSIT FORM: Highly variable.
Footwall stockwork or stringer-style vein networks. Large, textureless
massive sulphide pods, finely laminated stratiform sulphide layers and
lenses, reworked clastic sulphide sedimentary beds, and epithermal-style
breccia veins with large vugs, coarse sulphides and chalcedonic silica.
All types may coexist in a single deposit.
TEXTURE/STRUCTURE: Range from fine
clastic sulphides and "framboid"-like chemical precipitates to very coarse
grained sulphide aggregates in breccia veins. Structural styles include:
vein stockworks, major breccia veins, stratabound and stratiform sulphide
lenses and layers.
ORE MINERALOGY (Principal and
subordinate): Sphalerite, tetrahedrite, boulangerite, bournonite,
native gold, native silver, amalgam, galena, chalcopyrite, enargite,
pyrite, stibnite, realgar, arsenopyrite orpiment; metallic arsenic,
Hg-wurtzite, cinnabar, aktashite, unnamed Ag-Pb-As-S minerals, jordanite,
wurtzite, krennerite, coloradoite, marcasite, magnetite, scorodite,
jarosite, limonite, anglesite, native sulphur.
GANGUE MINERALOGY (Principal and
subordinate): Magnesian chlorite, muscovite (sericite),
chalcedonic silica, amorphous silica, calcite, dolomite, pyrobitumen,
gypsum, barite, potassium feldspar, alunite with minor carbon, graphite,
halite and cristobalite.
ALTERATION MINERALOGY: Massive
chlorite (clinochlore)-illite-quartz-gypsum-barite rock or
quartz-muscovite-pyrite rock are associated with the near-footwall
stockwork zones. Chlorite and pyrite alteration is associated with the
deep-footwall stockwork zones where alteration minerals are restricted to
fractures. Stratabound mineralization is accompanied by magnesian chlorite,
muscovite, chalcedonic silica, calcite, dolomite and pyrobitumen. At the
Osorezan hot spring deposits, pervasive silica and alunite microveinlets
are the dominant alteration phases.
GENETIC MODEL: Deposits are formed
by "hot spring" (i.e.: epithermal) fluids vented into a shallow water
environment. Fluids are magmatic in character, rather than meteoric. This
concept contrasts with some characteristics of the process model for
volcanogenic massive sulphides. Lateral and vertical zoning has been
recognized within a single lens. Lateral zoning shows changes from Sb, As
and Hg-rich mineral suites to Zn, Pb and Cu-rich assemblages. Vertical
zoning is expressed as a systematic increase in Au, Ag and base metal
content up-section. Fluid conduits are fissures generated by seismic shock,
aggradation of the volcano over a later expanding magma chamber, or
fracturing in response to regional compressional tectonics. A near-surface
subvolcanic magma body is an essential source of metals, fluids and heat.
ASSOCIATED DEPOSIT TYPES: Hot spring
Hg ,
hot spring Au-Ag , epithermal veins , volcanogenic exhalative massive sulphides
.
COMMENTS: This deposit type is the
shallow subaqueous analogue of hot spring Au- Ag, and both of these are
subtypes of the "epithermal" class of mineral deposits. Considering the
recent discoveries at Osorezan (1987) and Eskay Creek (1988), the brief
discussion by Laznicka (1985, p. 907) seems especially prophetic.
EXPLORATION GUIDES
GEOCHEMICAL SIGNATURE: Ag, Au, Cu,
Pb, Zn, As, Sb, Hg.
GEOPHYSICAL SIGNATURE: The pyrite
associated with stockwork mineralization and ubiquitous alteration should
produce a widespread induced polarization anomaly, but the best targets
may be local peaks within this broad anomalous 'plateau'. Airborne
magnetometer surveys may help delineate favourable strata and fault
offsets.
OTHER EXPLORATION GUIDES: The
geological deposit model and its regional setting may be the best
exploration tools available. Broad hydrothermal systems marked by
widespread sericite-pyrite alteration; evidence of a volcanic crater or
caldera setting; accumulations of felsic volcanic strata: 1) in a local
subaqueous setting in a regionally subaerial environment, 2) along the
near shore zone of a regional subaerial/subaqueous volcanic facies
transition (e.g.: the western margin of the Hazelton trough). Focus on the
sedimentary intervals within the volcanic pile.
ECONOMIC FACTORS
GRADE AND TONNAGE: These deposits
are not well known. The Eskay Creek deposit is attractive because of the
polymetallic signature and high precious metal contents. It contains an
estimated mining reserve of 1.08 Mt grading 65.5 g/t Au, 2930 g/t Ag, 5.7
% Zn, 0.77 % Cu and 2.89% Pb with geological reserves of 4.3 Mt grading
28.8 g/t Au and 1 027 g/t Ag.
IMPORTANCE: These deposits are
attractive because of their bonanza grades and polymetallic nature.
REFERENCES
Aoki, M. (1991): Gold and Base Metal
Mineralization in an Evolving Hydrothermal System at Osorezan, Northern
Honshu, Japan; Geological Survey of Japan, Report No. 277, pages
67-70.
Aoki, M. (1992a): Magmatic Fluid
Discharging at the Surface from the Osorezan Geothermal System, Northern
Honshu, Japan; Geological Survey of Japan, Report No. 279, 1992,
pages 16-21.
Aoki, M. (1992b): Active Gold
Mineralization in the Osorezan Caldera; 29th International Geological
Congress Field Trip, Epithermal Gold and Kuroko Mineralizations, in
Northeast Honshu, Shikazono, N., Aoki, M., Yamada, R., Singer, D.A., Kouda,
R. and Imai, A., Editors, pages 69-75.
Britton, J.M., Blackwell, J.D. and
Schroeter, T.G. (1990): 21 Zone Deposits, Eskay Creek, Northwestern
British Columbia; in Exploration in British Columbia 1989, B. C.
Ministry of Energy, Mines and Petroleum Resources, pages 197-223.
Izawa, E. and Aoki, M. (1991):
Geothermal Activity and Epithermal Gold Mineralization in Japan;
Episodes, Volume 14, No. 3, pages 269-273.
Laznicka, P. (1985):
Subaqueous-hydrothermal Deposits, in Empirical Metallogeny, Elsevier,
Amsterdam, 1758 pages.
Macdonald, A.J. (1992): Osorezan, in
Japan '92 - A Technical Report, Mineral Deposit Research Unit,
University of British Columbia, pages 29-67.
Mitchell, A.H.G. (1992): Andesitic
Arcs, Epithermal Gold and Porphyry-type Mineralization in the Western
Pacific and Eastern Europe, Institution of Mining and Metallurgy,
Transactions, Volume 101, pages B125-B138.
Roth, T. (1982): Eskay Creek 21A
Zone: An Update, in Iskut Project Annual Report, Year 2, Mineral Deposit
Research Unit, University of British Columbia May 1992. |
