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Simandl, G.J, Birkett, T. and Paradis,
S. (1999): Vermiculite; 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.
IDENTIFICATION
COMMODITY (BYPRODUCT):
Vermiculite (± apatite).
EXAMPLES (British Columbia -
Canadian/International): Joseph Lake, Sowchea Creek vermiculite
; Libby (Montana, USA), Waldrop Pit, Enoreeq area (South
Carolina, USA), Blue Ridge deposits (North Carolina, USA),
Palabora deposit (Republic of South Africa).
GEOLOGICAL
CHARACTERISTICS
CAPSULE DESCRIPTION:
These near surface vermiculite deposits may also contain recoverable
apatite. World-class vermiculite deposits occur mainly within zoned
ultramafic complexes or carbonatites. Smaller or lower grade deposits are
hosted by dunites, unzoned pyroxenites, peridotites or other mafic rocks
cut by pegmatites and syenitic or granitic rocks.
TECTONIC SETTING:
Deposits hosted by carbonatites and ultramafic complexes are commonly
related to rifting within the continental platform or marginal to the
platform in geosynclinal settings.
DEPOSITIONAL ENVIRONMENT /
GEOLOGICAL SETTING: Mafic and ulramafic igneous or metamorphic
rocks exposed to intense weathering and/or supergene, low temperature
alteration.
AGE OF MINERALIZATION:
Most deposits are derived from rocks of Precambrian to Jurassic age.
Deposits post-date emplacement of intrusive host and regional metamorphism.
Their age may be linked to periods of intense weathering which show up as
erosional surfaces, paleo-regolith or unconformities.
HOST/ASSOCIATED ROCK TYPES:
For major deposits the main hosts are biotitites, pyroxenites,
phlogopite-serpentine rock, phlogopite-diopside±apatite rock and
peridotites. Associated rock types are magnetite pyroxenites, foscorite,
carbonatites, and variety of serpentinites that are in contact with alkali
granites, syenites, fenites or pegmatites.
For smaller or marginal deposits located in highly metamorphosed settings
the typical host rocks are amphibolite and biotite schists in contact with
pyroxenites or peridotite dykes or lenses, sometimes cut by pegmatites.
DEPOSIT FORM: Variable
shapes, a function of the geometry of the favourable protolith and zone of
fluid access. Semi-circular surface exposures found with deposits
associated with ultramafic zoned complexes or carbonatites, usually near
the core of the intrusion. Lenticular or planar deposits of vermiculite
are found along serpetinized contacts between ultramafic rocks and
metamorphic country rocks. Individual lenses may be up to 7 metres thick
and 30 metres in length. Smaller lenses may be found along fractures and
the margins of pegmatites crosscutting ultramafic lenses within high grade
metamorphic terranes. The degree of alteration and vermiculite grade
generally diminishes with depth. Vermiculite grades of economic interest
rarely extend more than 40 metres below the surface.
TEXTURE/STRUCTURE:
Vermiculite may be fine-grained or form books up to 20 cm across ("pegmatitic").
Serpentine can form pseudomorphs after olivine.
ORE MINERALOGY [Principal and
subordinate]: Vermiculite ± hydrobiotite; ± apatite.
GANGUE MINERALOGY [Principal and
subordinate]: Biotite, chlorite, phlogopite, clinopyroxene,
tremolite, augite, olivine, hornblende, serpentine. In some of the
deposits acicular tremolite and asbestos are reported.
ALTERATION MINERALOGY:
Vermiculite is probably, in part, a low temperature alteration product of
biotite.
WEATHERING: At least in
some deposits, weathering is believed to play an important role in
transformation of mafic minerals, mainly biotite, into vermiculite.
Weathering also weakens the ore making blasting unnecessary; in extreme
case it results in formation of semi- or unconsolidated, residual
vermiculite deposits.
ORE CONTROLS: 1) The
existence of a suitable protore, commonly dunite or pyroxenite rock
containing abundant biotite or phlogopite which may be of late magmatic to
hydrothermal origin. 2) Deposits occur mainly at surface or at shallow
depths, but in some cases as a paleoregolith along an unconformity. 3)
Vermiculite develops from periods of intense weathering or near surface
alteration. 4) The maximum depth extent of the ore zone depends on the
permeability, porosity, jointing and fracture system orientation which
permit the circulation of meteoric fluids.
GENETIC MODELS:
Vermiculite can form from variety of mafic minerals, but biotite or
Fe-bearing phlogopite are deemed key components of the protore within
economic deposits.
Most of the early studies suggest that
vermiculite is a late magmatic, low temperature hydrothermal or deuteric
alteration product.. Currently, the most accepted hypothesis is that
vermiculite forms by supergene alteration due to the combined effect of
weathering and circulation of meteoric fluids.
ASSOCIATED DEPOSIT TYPES:
Palabora-type complexes or other carbonatites contain vermiculite mineralization. Ultramafic-hosted
asbestos, ultramafic-hosted talc-magnesite , nepheline-syenite (R13), Ni and platinoid showings, some sapphire deposits
associated with so called "crossing line" pegmatites and placer platinoid
deposits may be associated with the same ultramafic or mafic complexes as
vermiculite deposits.
COMMENTS: In British
Columbia, vermiculite is reported from surface exposures of granite,
granodiorite and quartz diorite at the Joseph Lake and Sowchea Creek
showings in the Fort Fraser/Fort St. James area (White, 1990). Low grades
in combination with the preliminary metallurgical studies indicate that
these occurrences are probably subeconomic (Morin and Lamothe, 1991).
Similar age, or older, mafic or ultramafic rocks in this region may
contain coarse-grained vermiculite in economic concentrations.
EXPLORATION GUIDES
GEOCHEMICAL SIGNATURE:
Vermiculite in soil.
GEOPHYSICAL SIGNATURE:
Ultramafic rocks that host large vermiculite deposits are commonly
characterized by strong magnetic anomalies detectable by airborne surveys.
Since vermiculite is an alteration product of ultramafic rocks,
vermiculite zones are expected to have a negative magnetic signature.
However, no detailed geophysical case histories are documented.
OTHER EXPLORATION GUIDES:
The largest commercial deposits usually form in the cores of ultramafic or
alkaline complexes (mainly pyroxenites and carbonatites). The roof
portions of these complexes have the best potential because they may be
biotite-rich. Deposits derived from biotite schist are typically much
smaller. All these deposits are commonly associated with some sort of
alkali activity, be it only alkali granite or syenite dykes. Vermiculite
deposits may have a negative topographic relief. A portable torch may be
used to identify vermiculite in hand specimen since it exfoliates and
forms golden flakes when heated. Therefore, an excellent time to prospect
for vermiculite is after forest fires. Fenitization halos associated with
alkaline ultramafic complexes and carbonatites increases the size of the
exploration target. Horizons of intense paleo-weathering that exposed
mica-bearing ultramafic rocks are particularly favourable.
ECONOMIC FACTORS
TYPICAL GRADE AND TONNAGE:
Deposits with over 35% vermiculite (<65 mesh) are considered high grade.
Most of the economic deposits contain from few hundred thousand to several
million tonnes; although clusters of small, high-grade, biotite
schist-hosted deposits ranging from 20 000 to 50 000 tonnes were mined in
South Carolina.
ECONOMIC LIMITATIONS:
World vermiculite production in 1995 was estimated at 480 000 tonnes.
Major producing countries were South Africa (222 000 tonnes, mainly from
Palabora), USA (170 000 tonnes) and Brazil (41 500 tonnes). In the early
half of 1996 the prices of South African vermiculite imported to USA
varied from US$127 to 209 per tonne. Deposits must be large enough to be
amenable to open pit mechanized mining. Large flake size (more than 65
mesh) is preferred. Both wet and dry concentrating methods are in use.
Crude vermiculite is moved in bulk to exfoliation plants that are
typically located near the markets. In commercial plants expansion of 8 to
15 times the original volume is typical, but up to 20 times may be
achieved. The higher the degree of expansion (without decrepitation) the
better the concentrate. The concentrates from those deposits where
vermiculite coexists with asbestos or "asbestiform" tremolite are
difficult to market because of the concerns over related health risks.
END USES: Agriculture
40%, insulation 23%, light weight concrete aggregate 19%, plaster and
premixes 13%, other 5% (USA statistics). Other applications include
carrier substrate for predatory mites in pest extermination, additive to
fish feed, removal of heavy metals from soils and absorbent in poultry
litter.
IMPORTANCE: Some
vermiculite is derived from laterite-type deposits. Vermiculite may be
substituted in concrete applications by expanded perlite or by expanded
shale. Recently the use of vermiculite in cement compounds has reduced due
to substitution by polystyrene. In agricultural applications it may be
substituted by peat, perlite, sawdust, bark, etc. In ion exchange
applications it may be substituted by zeolites.
REFERENCES
Anonymous (1991): The
Economics of Vermiculite, 6th edition, Roskill Information
Services Ltd. London, 152 pages.
Anonymous (1997):
Vermiculite; Metals and Minerals Annual Review, Mining Journal Limited,
pages 95-96.
Bates, R.I. (1969):
Geology of Industrial Rocks and Minerals; Dover Publications Inc.,
New York, 459 pages.
Boettcher, A.L.(1967):
The Rainy Creek Alkaline-ultramafic Igneous Complex near Libby, Montana;
Journal of Geology, Volume 75, pages 526-553.
Bush, A.L. (1968):
Lightweight Aggregates; in Mineral Resources of the Appalachian
Region, U.S. Geological Survey, Professional Paper 580, pages
210-224.
Bush, A.L. (1976):
Vermiculite in United States; 11th Industrial Minerals Forum,
Montana Bureau of Mines, Special Publication 74, pages 146-155.
Hagner, A.F. (1944):
Wyoming Vermiculite Deposits; Geological Survey of Wyoming,
Bulletin 34, 47 pages.
Hindman, J.R. (1994):
Vermiculite; in Industrial Minerals and Rocks, D.D. Carr, Editor, 6th
Edition, Society for Mining, Metallurgy, and Exploration, Inc.,
Littleton, Colorado, pages 1103-1111.
Morin, L. and Lamothe, J.-M.
(1991): Testing on Perlite and Vermiculite Samples from British
Columbia; B.C. Ministry of Energy, Mines and Petroleum Resources,
Geological Fieldwork 1989, Paper 1990-1, pages 265-268.
Palabora Geological and
Mineralogical Staff (1976): The Geology and the Economic
Deposits of Copper, Iron, and Vermiculite in the Palabora Igneous Complex:
A Brief Review; Economic Geology, Volume 71, pages 177-192.
White, G.V. (1990):
Perlite and Vermiculite Occurrences in British Columbia; B.C. Ministry
of Energy, Mines and Petroleum Resources, Geological Fieldwork 1989,
Paper 1990-1, pages 481-268. |
