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Simandl, G.J., Paradis, S. and Birkett,
T. (1999): Schist-hosted Emeralds; in Selected British Columbia Mineral
Deposit Profiles, Volume 3, Industrial Minerals, G.J. Simandl, Z.D. Hora
and D.V. Lefebure, Editors, British Columbia Ministry of Energy and Mines,
Open File 1999-10.
IDENTIFICATION
SYNONYMS:
Emerald deposits commonly described as "suture zone-related", "pegmatite-related
schist-hosted" or "exometamorphic", "exometasomatic", "biotite schist-type",
"desilicated pegmatite related" and "glimerite-hosted" are covered by this
model.
COMMODITIES (BYPRODUCTS):
Emerald (industrial grade beryl, other gemstones, such as
aquamarine, chrysoberyl, phenakite, tourmaline).
EXAMPLES (British Columbia -
Canadian/International): Socoto and Carnaiba
deposits (Brazil), Habachtal (Austria), Perwomaisky, Mariinsky, Aulsky,
Krupsky, Chitny and Tsheremshansky deposits (Russia), Franqueira (Spain),
Gravelotte mine (South Africa), Mingora Mines (Pakistan).
GEOLOGICAL
CHARACTERISTICS
CAPSULE DESCRIPTION: Emerald
deposits principally related to mafic and ultramafic schists or
unmetamorphosed ultramafic rocks in contact with felsic rocks, either
pegmatoid dykes, granitic rocks, paragneisses or orthogneisses. Such
contacts may be either intrusive or tectonic.
TECTONIC SETTING: Found
in cratonic areas as well as in mobile belts. In many cases related to
major Phanerozoic or Proterozoic suture zones that may involve island
arc-continent or continent-continent collision zones. The lithological
assemblages related to suture zones commonly form a "tectonic mélange" and
in some areas are described as "ophiolitic melange".
DEPOSITIONAL ENVIRONMENT /
GEOLOGICAL SETTING: Mainly in greenstone belts, but
also in other areas where Cr-bearing rocks may be adjacent to pegmatites,
aplites, granites and other felsic rocks rich in beryllium. Metamorphic
grade is variable; however, it typically reaches green schist to
amphibolite facies.
AGE OF MINERALIZATION: The
deposits are hosted by Archean age rocks or younger. The age of
mineralization is typically linked to either a period of tectonic activity
or a time of pegmatoid emplacement.
HOST/ASSOCIATED ROCKS: Biotite
schists ("biotites", "phlogopitites" and "glimerites") are a particularly
favourable host. Other favourable hosts are metamorphosed mafic volcanic
rocks, such as epidote-chlorite-actinolite-bearing rock, chlorite and
chlorite-talc schists, talc and talc-carbonate schists, white mica schists,
mafic schists and gneisses and amphibolites. Less commonly emeralds occur
in unmetamorphosed mafic or ultramafic rocks and possibly listwaenites.
Pegmatites or quartz veins in the contact zone between granitic rocks and
mafic rocks may in some cases host emeralds. A wide variety of rocks can
be associated with schist-hosted emerald deposits, including granite,
syenite, tonalite, granodiorite, a variety of orthogneisses, marbles,
black phyllites, white mica schists, mylonites, cataclasites and other
metasedimentary rocks.
DEPOSIT FORM: Most
of the mineralization is hosted by tabular or lenticular mafic schists or
"blackwall zones". Favourable zones are a few metres to tens of metres
wide and follow the contacts between felsic and mafic/ultramafic
lithologies for distances of tens to hundreds of metres, but economically
minable portions are typically much smaller. For example, minable bodies
in the Urals average 1 metre in thickness and 25 to 50 metres in length.
Pegmatoids, where present, may form horizontal to steeply dipping pods,
lens-shaped or tabular bodies or anastomosing dykes which may be zoned.
TEXTURE/STRUCTURE: In
blackwall or schists lepidoblastic texture predominates. The individual,
discrete emerald-bearing mafic layers within the favourable zones may be
complexly folded, especially where the mineralization is not spatially
associated with pegmatites. Emeralds are commonly zoned. They may form
porphyroblasts, with sigmoidal orientation of the inclusion trails; beryl
may form the rims separating phenakite form the surrounding biotite schist;
or emerald crystals may be embedded in quartz lenses within the biotite
schist. Chrysoberyl may appear as subhedral porphyroblasts or skeletal
intergrowths with emerald, phenakite or apatite.
Where disseminated beryl crystals also occur within pegmatites, they are
short, commonly fractured, prismatic to tabular with poor terminations;
but may be up to 2 metres in length and 1 metre in cross section. Long,
prismatic, unfractured crystals occur mainly in miarolitic cavities.
ORE MINERALOGY:
Emerald and other beryls (in some cases aquamarine or
morganite), ± chrysoberyl and industrial grade beryl. Spodumene gems (in
some cases kunzite) may be found in related pegmatites.
GANGUE MINERALOGY [Principal and
subordinate]: In the schist: biotite and/or
phlogopite, talc, actinolite, plagioclase, serpentine,
± fuchsite, ± quartz, ± carbonates, ±
chlorite, ± muscovite, ±
pyrite, epidote, ± phenakite, ± milarite and other beryllium species, ± molybdenite, ±
apatite, ± garnet, ±
magnetite, ± ilmenite, ±
chromite, ± tourmaline, ±
cassiterite.
In the pegmatoids: feldspars (commonly albite), quartz, micas; ±
topaz, ± phenakite , ± molybdenite, ± Sn
and W-bearing minerals, ± bazzite, ± xenotime, ±
allanite, ± monazite, ±
phosphates, ± pollucite, ±
columbite-tantalite, ± kyanite, zircon, ± beryllonite, ±
milarite and other beryllium species. Emerald crystals may contain
actinolite-tremolite, apatite, biotite, bityite, chlorite, chromite,
columbite-tantalite, feldspar, epidote, fuchsite, garnet, hematite,
phlogopite, pyrrhotite, rutile, talc, titanite and tourmaline inclusions.
ALTERATION MINERALOGY: Limonitization
and pyritization are reported in the host rocks. Kaolinite, muscovite,
chlorite, margarite, bavenite, phenakite, epidimyte, milarite, bityite,
bertrandite, euclase are reported as alteration products of beryl.
WEATHERING: Weathering
contributes to the economic viability of the deposits by softening the
matrix, and concentrating the beryl crystals in the overlaying soil or
regolith.
ORE CONTROLS:
1) The principal control is the juxtaposition of
beryllium and chromium-bearing lithologies along deep suture zones.
Emerald crystals are present mainly within the mafic schists and in some
cases so called "blackwall zones" as described ultramafic-hosted talc
deposits (M07). In this settings it may be associated with limonite zones.
2) This often occurs near the contacts of pegmatoids with mafic
schists. Emerald crystals are present mainly within the mafic schists,
although in some cases some of the mineralization may be hosted by
pegmatoids.
3) Another prospective setting is along fracture-controlled
glimmerite zones.
4) Mineralization may be concentrated along the planes of regional
metamorphic foliation, especially in cores of the folds where the
relatively high permeability favors chemical exchange and the development
of synmetamorphic reaction zones between chromium and beryllium-bearing
lithologies.
5) Serpentinite roof pendants in granites are prospective.
GENETIC MODELS: The
origin of schist-hosted emerald deposits is controversial as is the case
with many deposits hosted by metamorphic rocks. All emerald deposits
require special geological conditions where chromium (±
vanadium) and beryllium coexist. Where pegmatoids or plagioclase-rich
lenses occur within ultramafic rocks, the crystalization of emeralds is
commonly explained by interaction of pegmatites or
pneumatolytic-hydrothermal, Be-bearing fluids with Cr-bearing
mafic/ultramafic rocks. In other cases, emeralds in schists form by syn-
or post-tectonic regional metamorphic chemical exchange (metasomatism)
between felsic rocks, such as felsic gneisses, garnet mica schists or
pre-metamorphic pegmatoids, with the adjacent Cr-bearing rocks such as
schists, gneisses or serpentinites. Contacts between Cr- and Be-bearing
source rocks may be tectonic, as is the case for "suture zone-related"
deposits.
ASSOCIATED DEPOSIT TYPES: Feldspar-quartz
and muscovite pegmatites (O03, O04). Mo and W mineralization may be
associated with emeralds. Some porphyry W deposits (L07)
have associated beryl. Tin-bearing granites are in some cases associated
with emeralds. Gold was mined at Gravelotte Emerald Mines (no information
about the gold mineralization is available).
COMMENTS:
Recently, microprobe studies have shown that the green color of some
beryls is due to vanadium rather than chrome. In most cases both Cr and V
were detected in the beryl crystal structure. There are two schools of
gemmologists, the first believes that strictly-speaking the vanadium-rich
beryls are not emeralds. The second school believes that gem quality
beryls should be named based on their physical, and more particularly,
color properties. It is possible that pegmatoid-related or suture
zone-related emerald deposits hosted by black shales or other chromium
and/or vanadium-bearing rocks will be discovered. In those cases it will
be difficult to decide if these deposits are schist-hosted or
Columbia-type (Q06)
emeralds.
EXPLORATION GUIDES
GEOCHEMICAL SIGNATURE:
The presence of beryl in eluvial and alluvial deposits is
good pathfinder. The distribution of beryllium in stream sediments proved
to be useful in Norway when coupled with identification of the individual
drainage basins and knowledge of the geological environment.
GEOPHYSICAL SIGNATURE:
A portable field detector that uses 124Sb as a
gamma radiation source, the berylometer, is used to detect Be in outcrop.
The instrument should be held less than 4 cm from the sample. Radiometric
surveys may be useful in detecting associated radioactive minerals where
pegmatites are involved. Magnetic and electromagnetic surveys may be
useful in tracing suture zones where ultramafic rocks and felsic rocks are
faulted against each other.
OTHER EXPLORATION GUIDES: Any
Be occurrences in a favorable geological setting should be considered as
positive indicators. If green, chromium and/or vanadium-bearing beryls are
the main subject of the search then ultramafic rocks, black shales or
their metamorphic equivalents represent the most favorable host rocks. If
exploration is focused on a variety of gem-quality beryls (not restricted
to emerald), or if the targeted area is not mapped in detail, then Be
occurrences without known spatial association with Cr- or V-bearing
lithologies should be carefully considered. Minerals associated with
emeralds in the ores may be considered as indirect indicators. A wide
variety of field-tests based on fluorescence, alkalinity, staining,
density and refractive index have been used in the past to distinguish
beryl.
ECONOMIC FACTORS
TYPICAL GRADE AND TONNAGE:
The grade and tonnage of these deposits is difficult to
estimate due to erratic emerald contents (gram/tonne), episodic nature of
the mining activity which often results in high grading, and variability
in the quality of gemstones (value/carat). For example, at the Mingora
mines in Islamia Trench two, 15 to 30 centimetres thick layers of
talc-rich rock surrounding quartz lenses contained 1000 to 5000 carats of
good stones up to 30 carats in size. Some of the individual pits in the
area produced less than 1000 carats. The cumulative production of the
Mingora emerald mines was reported between 20 000 to over 50 000
carats/year between 1979 and 1988. At Gravelotte Emerald Mine, at least 23
000 kg of emeralds of varying grades have been produced since 1929 from
several zones. For the same mine promotional literature states that "
conservative estimates" of ore within the Cobra pit are 1.69 million
tonnes that could result in production of 17 000 kg of emeralds (
approximately 1gram /tonne). It is estimated that about 30% of the
emeralds could be sold, but only 2-3% of these are believed to be gem
quality. In the Urals the Mariinsky deposit was explored to a average
depth of 500 metres by boreholes and underground workings. To determine
emerald content, bulk samples as large as 200 tonnes are taken
systematically at 100 metres interval along the favourable zone. No grade
and tonnage are available.
ECONOMIC LIMITATIONS:
Mining of precious stones in underdeveloped countries and smaller
deposits is done using pick and shovel with limited use of jackhammers and
bulldozers. Larger schist-hosted emerald deposits, may be successfully
exploited by a combination of surface and underground mining. The
Mariinsky deposit was mined by open pit to the depth of 100 metres and is
exploited to the depth of 250 metres by underground methods. "Low impact"
explosives, expanding plastics or hydraulic wedging are used to break the
ore. The ore is milled, screened and manually sorted.
END USES:
Transparent and colored beryl varieties, such as emerald, morganite and
aquamarine, are highly valued gemstones. Industrial grade beryls commonly
recovered as by-products are a source of Be oxide, Be metal alloys used in
aerospatial and defence applications, Be oxide ceramics, large diameter
berylium-copper drill rods for oil and gas, fusion reactors, electrical
and electronic components. Berylium metal and oxides are strategic
substances, and may be substituted for by steel, titanium and graphite
composites in certain applications. Phosphor bronze may replace
beryllium-copper alloys. However, all known substitutes offer lower
performance than Be-based materials.
IMPORTANCE: Schist-hosted
deposits are the most common source of emeralds, although the largest and
most valuable gemstones are most frequently derived from the Colombia-type
deposits. Besides schist-hosted deposits and pegmatites, beryl for
industrial applications may be also be present in fertile granite and
syenite complexes that may be parent to pegmatites. A major portion of the
beryl ore used in the U.S.A. as raw material for beryllium metal is
recovered as a byproduct of feldspar and quartz mining from pegmatites.
REFERENCES
Beus, A.A. (1966): Geochemistry of Beryllium
and Genetic Types of Beryllium Deposits; W.H. Freeman, San
Francisco, 401 pages.
Brinck, J.W. and Hofmann, A. (1964): The
Distribution of Beryllium in the Oslo Region, Norway - a Geochemical,
Stream Sediment Study; Economic Geology, Volume 59, pages 79-96.
Frantz, G., Gilg, H.A., Grundmann, G. and Morteani, G.
(1996): Metasomatism at a Granitic Pegmatite-Dunite Contact in
Galicia: The Franqueira Occurrence of Chrysoberyl (alexandrite), Emerald,
and Phenakite: Discussion; Canadian Mineralogist, Volume 34, pages
1329-1331.
Giuliani, G., Silva, L.J.H.D. and Couto, P. (1990):
Origin of Emerald Deposits of Brazil; Mineralium Deposita,
Volume 25, pages 57-64.
Grundmann, G. and Morteani, G. (1989): Emerald
Mineralization during Regional Metamorphism: The Habachtal (Austria) and
Leydsdorp (Transvaal, South Africa) Deposits; Economic Geology,
Volume 84, pages 1835-1849.
Kazmi, A.H., Anwar, J. and Hussain, S. (1989):
Emerald Deposits of Pakistan; in Emeralds of Pakistan, Geology,
Gemology and Genesis, A.H. Kazmi and L.W. Snee, Editors, Van Nostrand
Reinhold Co., New York, USA. Pages 39-74.
Kazmi, A.H., Lawrence, R.D., Anwar, J., Snee, L.W. and
Hussain, A.S. (1986): Mingora Emerald Deposits (Pakistan):
Suture-associated Gem Mineralization; Economic Geology, Volume 81,
pages 2022-2028.
Kramer, D.A., Cunningham, L.D. and Osborne, S. (1997):
Beryllium Annual Review-1996; Mineral Industry Surveys; United States
Geological Survey, 7 pages.
Laskovenkov, A.F. and Zhernakov, V.I. (1995):
An Update on the Ural Emerald Mines; Gems and Gemology, Summer
issue, pages 106-113.
Martin-Izard, A., Paniagua, A., Moreiras, D., Aceveddo,
R.D. and Marcos-Pasqual, C. (1995): Metasomatism at a Granitic
Pegmatite-Dunite Contact in Galicia: The Franqueira Occurrence of
Chrysoberyl (alexandrite), Emerald and Phenakite; Canadian Mineralogist,
Volume 33, pages 775-792.
Martin-Izard, A., Paniagua, A., Moreiras, D., Aceveddo,
R.D. and Marcos-Pasqual, C. (1996): Metasomatism at a Granitic
Pegmatite-Dunite Contact in Galicia: The Franqueira Occurrence of
Chrysoberyl (alexandrite), emerald, and phenakite: Reply; Canadian
Mineralogist, Volume 34, pages 1332-1336.
Muligan, R. (1960): Geology of Canadian
Beryllium Deposits, Geological Survey of Canada; Economic Geology
Report, Number 23, 109 pages.
Robb, L.J. and Robb, V.M. (1986): Archean
Pegmatite Deposits in the North-eastern Transvaal; in Mineral
deposits of South Africa, C.R. Anhaeusser, and S. Maske, Editors,
Geological Society of South Africa, Johannesburg, Volumes 1 and
2, pages 437-449.
Sinkankas, J. (1959): Gemstones of North
America; D. Van Nostrand Company, Inc., Princeton, 75 pages.
Sinkankas, J. (1981): Emerald and other Beryls;
Chilton Book Company, Radnor, Pennsylvania, pages 1-665. |
