|
Revised 1997
Ray, G.E. (1998):
Au Skarns, in Geological Fieldwork 1997, British Columbia Ministry of
Employment and Investment, Paper 1998-1, pages 24H-1 to 24H-4.
IDENTIFICATION
SYNONYMS: Pyrometasomatic, tactite, or
contact metasomatic Au deposits.
COMMODITIES (BYPRODUCTS): Au (Cu,
Ag).
EXAMPLES (British Columbia - Canada/International):
Nickel Plate (092HSE038), French (092HSE059), Canty (092HSE 064), Good
Hope (092HSE060), QR - Quesnel River (093A121); Fortitude, McCoy and
Tomboy-Minnie (Nevada,USA), Buckhorn Mountain (Washington,USA), Diamond
Hill, New World district and Butte Highlands (Montana,USA), Nixon Fork
(Alaska, USA), Thanksgiving (Philippines), Browns Creek and Junction
Reefs-Sheahan-Grants (New South Wales, Australia), Mount Biggenden (Queensland,
Australia), Savage Lode, Coogee (Western Australia, Australia), Nambija (Ecuador),
Wabu (Irian Jaya, Indonesia).
GEOLOGICAL
CHARACTERISTICS
CAPSULE DESCRIPTION: Gold-dominant
mineralization genetically associated with a skarn gangue consisting of Ca
- Fe - Mg silicates, such as clinopyroxene, garnet and epidote. Gold is
often intimately associated with Bi or Au-tellurides, and commonly occurs
as minute blebs (<40 microns) that lie within or on sulphide grains. The
vast majority of Au skarns are hosted by calcareous rocks (calcic subtype).
The much rarer magnesian subtype is hosted by dolomites or Mg-rich
volcanics. On the basis of gangue mineralogy, the calcic Au skarns can be
separated into either pyroxene-rich, garnet-rich or epidote-rich types;
these contrasting mineral assemblages reflect differences in the hostrock
lithologies as well as the oxidation and sulphidation conditions in which
the skarns developed.
TECTONIC SETTINGS: Most Au skarns form in
orogenic belts at convergent plate margins. They tend to be associated
with syn to late island arc intrusions emplaced into calcareous sequences
in arc or back-arc environments.
DEPOSITIONAL ENVIRONMENT / GEOLOGICAL
SETTING: Most deposits are related to plutonism associated with the
development of oceanic island arcs or back arcs, such as the Late Triassic
to Early Jurassic Nicola Group in British Columbia.
AGE OF MINERALIZATION: Phanerozoic (mostly
Cenozoic and Mesozoic); in British Columbia Au skarns are mainly of Early
to Middle-Jurassic age. The unusual magnesian Au skarns of Western
Australia are Archean.
HOST/ASSOCIATED ROCK TYPES: Gold skarns are
hosted by sedimentary carbonates, calcareous clastics, volcaniclastics or
(rarely) volcanic flows. They are commonly related to high to intermediate
level stocks, sills and dikes of gabbro, diorite, quartz diorite or
granodiorite composition. Economic mineralization is rarely developed in
the endoskarn. The I-type intrusions are commonly porphyritic,
undifferentiated, Fe-rich and calc-alkaline. However, the Nambija, Wabu
and QR Au skarns are associated with alkalic intrusions.
DEPOSIT FORM: Variable from irregular
lenses and veins to tabular or stratiform orebodies with lengths ranging
up to many hundreds of metres. Rarely, can occur as vertical pipe-like
bodies along permeable structures.
TEXTURE/STRUCTURE: Igneous textures in
endoskarn. Coarse to fine-grained, massive granoblastic to layered
textures in exoskarn. Some hornfelsic textures. Fractures, sill-dike
margins and fold hinges can be an important loci for mineralization.
ORE MINERALOGY (Principal and
subordinate): The gold is commonly present as micron-sized inclusions
in sulphides, or at sulphide grain boundaries. To the naked eye, ore is
generally indistinguishable from waste rock. Due to the poor correlation
between Au and Cu in some Au skarns, the economic potential of a prospect
can be overlooked if Cu-sulphide-rich outcrops are preferentially sampled
and other sulphide-bearing or sulphide-lean assemblages are ignored. The
ore in pyroxene-rich and garnet-rich skarns tends to have low Cu:Au
(<2000:1), Zn:Au (<100:1) and Ag/Au (<1:1) ratios, and the gold is
commonly associated with Bi minerals (particularly Bi tellurides).
Magnesian subtype: Native gold ±
pyrrhotite ± chalcopyrite ± pyrite ± magnetite ± galena ±
tetrahedrite.
Calcic subtype:
Pyroxene-rich Au skarns: Native gold
± pyrrhotite ± arsenopyrite ± chalcopyrite ± tellurides
(e.g. hedleyite, tetradymite, altaite and hessite) ±
bismuthinite ± cobaltite ± native bismuth ± pyrite
± sphalerite ± maldonite. They generally have a high
sulphide content and high pyrrhotite:pyrite ratios. Mineral and metal
zoning is common in the skarn envelope. At Nickel Plate for example, this
comprises a narrow proximal zone of coarse-grained, garnet skarn
containing high Cu:Au ratios, and a wider, distal zone of finer grained
pyroxene skarn containing low Cu:Au ratios and the Au-sulphide orebodies.
Garnet-rich Au skarns: Native gold ±
chalcopyrite ± pyrite ± arsenopyrite ± sphalerite ± magnetite ± hematite ±
pyrrhotite ± galena ± tellurides ± bismuthinite.
They generally have a low to moderate sulphide content and low pyrrhotite:pyrite
ratios.
Epidote-rich Au skarn: Native gold ±
chalcopyrite ± pyrite ± arsenopyrite ± hematite ± magnetite ±
pyrrhotite ± galena ± sphalerite ± tellurides.
They generally have a moderate to high sulphide content with low
pyrrhotite:pyrite ratios.
EXOSKARN MINERALOGY (GANGUE):
Magnesian subtype: Olivine,
clinopyroxene (Hd2-50), garnet (Ad7-30), chondrodite and monticellite.
Retrograde minerals include serpentine, epidote, vesuvianite,
tremolite-actinolite, phlogopite, talc, K-feldspar and chlorite.
Calcic subtype:
Pyroxene-rich Au skarns: Extensive
exoskarn, generally with high pyroxene:garnet ratios. Prograde minerals
include diopsidic to hedenbergitic clinopyroxene (Hd 20-100), K-feldspar,
Fe-rich biotite, low Mn grandite garnet (Ad 10-100), wollastonite and
vesuvianite. Other less common minerals include rutile, axinite and sphene.
Late or retrograde minerals include epidote, chlorite, clinozoisite,
vesuvianite, scapolite, tremolite-actinolite, sericite and prehnite.
Garnet-rich Au skarns: Extensive
exoskarn, generally with low pyroxene:garnet ratios. Prograde minerals
include low Mn grandite garnet (Ad 10-100), K-feldspar, wollastonite,
diopsidic clinopyroxene (Hd 0-60), epidote, vesuvianite, sphene and
apatite. Late or retrograde minerals include epidote, chlorite,
clinozoisite, vesuvianite, tremolite-actinolite, sericite, dolomite,
siderite and prehnite.
Epidote-rich Au skarns: Abundant
epidote and lesser chlorite, tremolite-actinolite, quartz, K-feldspar,
garnet, vesuvianite, biotite, clinopyroxene and late carbonate. At the
QR deposit, epidote-pyrite and carbonate-pyrite veinlets and coarse
aggregates are common, and the best ore occurs in the outer part of the
alteration envelope, within 50 m of the epidote skarn front.
ENDOSKARN MINERALOGY (GANGUE): Moderate
endoskarn development with K-feldspar, biotite, Mg-pyroxene (Hd 5-30) and
garnet. Endoskarn at the epidote-rich QR deposit is characterized
by calcite, epidote, clinozoisite and tremolite whereas at the Butte
Highlands Mg skarn it contains argillic and propyllitic alteration
with garnet, clinopyroxene and epidote.
WEATHERING: In temperate and wet tropical
climates, skarns often form topographic features with positive relief.
ORE CONTROLS: The ore exhibits strong
stratigraphic and structural controls. Orebodies form along sill-dike
intersections, sill-fault contacts, bedding-fault intersections, fold axes
and permeable faults or tension zones. In the pyroxene-rich and
epidote-rich types, ore commonly develops in the more distal portions of
the alteration envelopes. In some districts, specific suites of reduced,
Fe-rich intrusions are spatially related to Au skarn mineralization. Ore
bodies in the garnet-rich Au skarns tend to lie more proximal to the
intrusions.
GENETIC MODEL: Many Au skarns are related
to plutons formed during oceanic plate subduction. There is a worldwide
spatial, temporal and genetic association between porphyry Cu provinces
and calcic Au skarns. Pyroxene-rich Au skarns tend to be hosted by
siltstone-dominant packages and form in hydrothermal systems that are
sulphur-rich and relatively reduced. Garnet-rich Au skarns tend to be
hosted by carbonate-dominant packages and develop in more oxidising and/or
more sulphur-poor hydrothermal systems.
ASSOCIATED DEPOSIT TYPES: Au placers (C01,C02),
calcic Cu skarns (K01), porphyry Cu deposits (L04) and Au-bearing quartz
and/or sulphide veins (I01, I02). Magnesian subtype can be
associated with porphyry Mo deposits (L05) and possibly W skarns ( K05).
In British Columbia there is a negative spatial association between Au and
Fe skarns at regional scales, even though both classes are related to arc
plutonism. Fe skarns are concentrated in the Wrangellia Terrane whereas
most Au skarn occurrences and all the economic deposits lie in Quesnellia.
COMMENTS: Most Au skarns throughout the
world are calcic and are associated with island arc plutonism. However,
the Savage Lode magnesian Au skarn occurs in the Archean
greenstones of Western Australia and the Butte Highlands magnesian
Au skarn in Montana is hosted by Cambrian platformal dolomites. Note:
although the Nickel Plate deposit lies distal to the Toronto stock in the
pyroxene-dominant part of the skarn envelope, the higher grade ore zones
commonly lie adjacent to sills and dikes where the exoskarn contains
appreciable amounts of garnet with the clinopyroxene.
EXPLORATION GUIDES
GEOCHEMICAL SIGNATURE: Au, As, Bi, Te, Co,
Cu, Zn or Ni soil, stream sediment and rock anomalies, as well as some
geochemical zoning patterns throughout the skarn envelope (notably in Cu/Au,
Ag/Au and Zn/Au ratios). Calcic Au skarns (whether garnet-rich or
pyroxene-rich) tend to have lower Zn/Au, Cu/Au and Ag/Au ratios than any
other skarn class. The intrusions related to Au skarns may be relatively
enriched in the compatible elements Cr, Sc and V, and depleted in
lithophile incompatible elements (Rb, Zr, Ce, Nb and La), compared to
intrusions associated with most other skarn types.
GEOPHYSICAL SIGNATURE: Airborne magnetic or
gravity surveys to locate plutons. Induced polarization and ground
magnetic follow-up surveys can outline some deposits.
OTHER EXPLORATION GUIDES: Placer Au. Any
carbonates, calcareous tuffs or calcareous volcanic flows intruded by
arc-related plutons have a potential for hosting Au skarns. Favorable
features in a skarn envelope include the presence of: (a) proximal
Cu-bearing garnet skarn and extensive zones of distal pyroxene skarn which
may carry micron Au, (b) hedenbergitic pyroxene (although diopsidic
pyroxene may predominate overall), (c) sporadic As-Bi-Te geochemical
anomalies, and, (d) undifferentiated, Fe-rich intrusions with low Fe2O3/FeO
ratios. Any permeable calcareous volcanics intruded by high-level porphyry
systems (particularly alkalic plutons) have a potential for hosting
epidote-rich skarns with micron Au. During exploration, skarns of all
types should be routinely sampled and assayed for Au, even if they are
lean in sulphides.
ECONOMIC IMPORTANCE
TYPICAL GRADE AND TONNAGE: These deposits
range from 0.4 to 13 Mt and from 2 to 15 g/t Au. Theodore et al.
(1991) report median grades and tonnage of 8.6 g/t Au, 5.0 g/t Ag and 213
000 t. Nickel Plate produced over 71 tonnes of Au from 13.4 Mt of
ore (grading 5.3 g/t Au). The 10.3 Mt Fortitude (Nevada) deposit
graded 6.9 g/t Au whereas the 13.2 Mt McCoy skarn (Nevada) graded
1.5 g/t Au. The QR epidote-rich Au skarn has reserves exceeding 1.3
Mt grading 4.7 g/t Au.
IMPORTANCE: Recently, there have been some
significant Au skarn deposits discovered around the world (e.g.
Buckhorn Mountain, Wabu, Fortitude). Nevertheless, total historic
production of Au from skarn (more than 1 000 t of metal) is minute
compared to production from other deposit types. The Nickel Plate
deposit (Hedley, British Columbia) was probably one of the earliest major
Au skarns in the world to be mined. Skarns have accounted for about 16 %
of British Columbia's Au production, although nearly half of this was
derived as a byproduct from Cu and Fe skarns
REFERENCES
Billingsley, P. and Hume, C.B. (1941): The
Ore Deposits of Nickel Plate Mountain, Hedley, British Columbia;
Canadian Institute of Mining and Metallurgy, Bulletin, Volume 44,
pages 524-590.
Brookes, J.W., Meinert, L.D., Kuyper, B.A.
and Lane, M.L. (1990): Petrology and Geochemistry of the McCoy Gold Skarn,
Lander County, Nevada; in Geology and Ore Deposits of the Great
Basin, Symposium Proceedings, Geological Society of Nevada, April
1990.
Ettlinger, A.D. and Ray, G.E. (1989a):
Precious Metal Enriched Skarns in British Columbia: An Overview and
Geological Study; B. C. Ministry of Energy, Mines and Petroleum
Resources, Paper 1989-3, 128 pages.
Ettlinger, A.D., Albers, D., Fredericks, R.
and Urbisinov, S. (1995): The Butte Highlands Project, Silver Bow County,
Montana; An Olivine-rich Magnesian Gold Skarn; in Symposium
Proceedings of Geology and Ore Deposits of American Cordilleran,
Geological Society of Nevada, U.S. Geological Survey and Geological
Society of Chile, April 10-13, 1995, Reno, Nevada.
Fox, P.E., and Cameron, R.S. (1995):
Geology of the QR Gold Deposit, Quesnel River Area, British Columbia;
in Porphyry Deposits of the Northwest Cordillera of North America,
(editor) T.G. Schroeter, Canadian Institute of Mining, Metallurgy and
Petroleum, Special Volume 46, Paper 66, pages 829-837.
Hammarstrom, J.M., Orris, G.J., Bliss,
J.D., and Theodore, T.G. (1989): A Deposit Model for Gold-Bearing Skarns;
Fifth Annual V.E. McKelvey Forum on Mineral and Energy Resources, U.S.
Geological Survey , Circular 1035, pages 27-28.
McKelvey, G.E., and Hammarstrom, J.M.
(1991): A Reconnaissance Study of Gold Mineralization Associated with
Garnet Skarn at Nambija, Zamora Province, Ecuador, in USGS Research
on Mineral Resources - 1991, Program and Abstracts. Editors E.J. Good,
J.F. Slack and R.K. Kotra, U.S. Geological Survey, Circular 1062, page 55.
Meinert, L.D. (1989): Gold Skarn Deposits -
Geology and Exploration Criteria; in The Geology of Gold Deposits;
The Perspective in 1988; Economic Geology; Monograph 6, pages
537-552.
Mueller, A.G. (1991): The Savage Lode
Magnesian Skarn in the Marvel Loch Gold-Silver Mine, Southern Cross
Greenstone Belt, Western Australia; Part I. Structural Setting,
Petrography and Geochemistry; Canadian Journal of Earth Sciences,
Volume 28, Number 5, pages 659-685.
Orris, G.J., Bliss, J.D., Hammarstrom, J.M.
and Theodore, T.G. (1987): Description and Grades and Tonnages of
Gold-bearing Skarns; U. S. Geological Survey, Open File Report
87-273, 50 pages.
Ray, G.E. and Dawson, G..L. (1994): The
Geology and Mineral Deposits of the Hedley Gold Skarn District, Southern
British Columbia; B.C. Ministry of Energy, Mines and Petroleum
Resources, Bulletin 87, 156 pages.
Ray, G.E. and Webster, I.C.L. (1997):
Skarns in British Columbia;, B.C. Ministry of Energy, Mines and
Petroleum Resources, Bulletin 101, 260 pages.
Ray, G.E., Ettlinger, A.D. and Meinert,
L.D. (1990): Gold Skarns: Their Distribution, Characteristics and Problems
in Classification; in Geological Fieldwork 1989, B.C. Ministry
of Energy, Mines and Petroleum Resources, Paper 1990-1, pages 237-246.
Theodore, T.G., Orris, G.J., Hammarstrom,
J.M. and Bliss, J.D. (1991): Gold Bearing Skarns; U. S. Geological
Survey; Bulletin 1930, 61 pages. |
