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Pell, J. (1998):
Lamproite-hosted Diamonds, in Geological Fieldwork 1997, British Columbia
Ministry of Employment and Investment, Paper 1998-1, pages 24M-1 to 24M-4.
IDENTIFICATION
SYNONYMS: None.
COMMODITY: Diamonds.
EXAMPLES (British Columbia (MINFILE #) -
Canada/International): No B.C. examples; Argyle, Ellendale (Western
Australia), Prairie Creek (Crater of Diamonds, Arkansas, USA), Bobi (Côte
d'Ivoire), Kapamba (Zambia), Majhgawan (India).
GEOLOGICAL
CHARACTERISTICS
CAPSULE DESCRIPTION: Diamonds occur as
sparse xenocrysts and in mantle xenoliths within olivine lamproite
pyroclastic rocks and dikes. Many deposits are found within funnel-shaped
volcanic vents or craters. Lamproites are ultrapotassic mafic rocks
characterized by the presence of olivine, leucite, richterite, diopside or
sanidine.
TECTONIC SETTING: Most olivine lamproites
are post-tectonic and occur close to the margins of Archean cratons,
either within the craton or in adjacent accreted Proterozoic mobile belts.
DEPOSITIONAL ENVIRONMENT / GEOLOGICAL
SETTING: Olivine lamproites are derived from metasomatized lithospheric
mantle. They are generally emplaced in high-level, shallow "maar-type"
craters crosscutting crustal rocks of all types.
AGE OF MINERALIZATION: Any age except
Archean. Diamondiferous lamproites range from Proterozoic to Miocene in
age.
HOST/ASSOCIATED ROCK TYPES: Olivine
lamproite pyroclastic rocks and dikes commonly host mineralization while
lava flows sampled to date are barren. Diamonds are rarely found in the
magmatic equivalents. Lamproites are peralkaline and typically
ultrapotassic (6 to 8% K2O). They are characterized by the
presence of one or more of the following primary phenocryst and/or
groundmass constituents: forsteritic olivine; Ti-rich, Al-poor phlogopite
and tetraferriphlogopite; Fe-rich leucite; Ti, K-richterite; diopside; and
Fe-rich sanidine. Minor and accessory phases include priderite, apatite,
wadeite, perovskite, spinel, ilmenite, armalcolite, shcherbakovite and
jeppeite. Glass and mantle derived xenocrysts of olivine, pyrope garnet
and chromite may also be present.
DEPOSIT FORM: Most lamproites occur in
craters which are irregular, asymmetric, and generally rather shallow (often
the shape of a champagne glass), often less than 300 metres in depth.
Crater diameters range from a few hundred metres to 1500 metres. Diamond
concentrations vary between lamproite phases, and as such, ore zones will
reflect the shape of the unit (can be pipes or funnel-shaped). The
volcaniclastic rocks in many, but not all, lamproite craters are intruded
by a magmatic phase that forms lava lakes or domes.
TEXTURE/STRUCTURE: Diamonds occur as
discrete grains of xenocrystic origin that are sparsely and randomly
distributed in the matrix of lamproites and some mantle xenoliths.
ORE MINERALOGY: Diamond.
GANGUE MINERALOGY (Principal and
subordinate): Olivine, phlogopite, richterite, diopside, sanidine;
priderite, wadeite, ilmenite, chromite, perovskite, spinel, apatite,
pyrope garnet.
ALTERATION MINERALOGY: Alteration to talc
carbonate sulphide or serpentine -septechlorite + magnetite has been
described from Argyle (Jacques et al., 1986). According Scott Smith
(1996), alteration to analcime, barite, quartz, zeolite, carbonate and
other minerals may also occur. Diamonds can undergo graphitization or
resorption.
WEATHERING: Clays, predominantly smectite,
are the predominant weathering product of lamproites.
ORE CONTROLS: Lamproites are small-volume
magmas which are confined to continental regions. There are relatively few
lamproites known world wide, less than 20 geological provinces, of which
only seven are diamondiferous. Only olivine lamproites are diamondiferous,
other varieties, such as leucite lamproites presumably did not originate
deep enough in the mantle to contain diamonds. Even within the olivine
lamproites, few contain diamonds in economic concentrations. Controls on
the differences in diamond content between intrusions are not completely
understood. They may be due to: different depths of origin of the magmas (above
or below the diamond stability field); differences in the diamond content
of the mantle sampled by the lamproite magma; differences in degrees of
resorption of diamonds during transport; or some combination of these
factors.
GENETIC MODEL: Lamproites form from a small
amount of partial melting in metasomatized lithospheric mantle at depths
generally in excess of 150 km (i.e., within or beneath the diamond
stability field). The magma ascends rapidly to the surface, entraining
fragments of the mantle and crust en route. Diamonds do not crystallize
from the lamproite magma. They are derived from harzburgitic peridotites
and eclogites within regions of the sub-cratonic lithospheric mantle where
the pressure, temperature and oxygen fugacity allow them to form in situ.
If a lamproite magma passes through diamondiferous portions of the mantle,
it may sample them and bring diamonds to the surface provided they are not
resorbed during ascent.
ASSOCIATED DEPOSIT TYPES: Diamonds can be
concentrated by weathering to produce residual concentrations or by
erosion and transport to create placer deposits (C01, C02, C03).
Kimberlite-hosted diamond deposits (N02) form in a similar manner, but the
magmas may be of different origin.
EXPLORATION GUIDES
GEOCHEMICAL SIGNATURE: Lamproites can have
associated Ni, Co, Ba and Nb anomalies in overlying residual soils.
However, these may be restricted in extent since lamproites weather
readily and commonly occur in depressions and dispersion is limited.
Caution must be exercised as other alkaline rocks can give similar
geochemical signatures.
GEOPHYSICAL SIGNATURE: Geophysical
techniques are used to locate lamproites, but give no indication as to
their diamond content. Ground and airborne magnetometer surveys are
commonly used; weathered or crater-facies lamproites commonly form
negative magnetic anomalies or dipole anomalies. Some lamproites, however,
have no magnetic contrast with surrounding rocks. Various electrical
methods (EM, VLF, resistivity) in airborne or ground surveys are excellent
tools for detecting lamproites, given the correct weathering environment
and contrasts with country rocks. In general, clays, particularly
smectite, produced during the weathering of lamproites are conductive; and
hence, produce strong negative resistivity anomalies.
OTHER EXPLORATION GUIDES: Heavy indicator
minerals are used in the search for diamondiferous lamproites, although
they are usually not as abundant as with kimberlites. Commonly, chromite
is the most useful heavy indicator because it is the most common species
and has distinctive chemistry. To a lesser extent, diamond, pyrope and
eclogitic garnet, chrome spinel, Ti-rich phlogopite, K-Ti-richterite,
low-Al diopside, forsterite and perovskite can be used as lamproite
indicator minerals. Priderite, wadeite and shcherbakovite are also highly
diagnostic of lamproites, although very rare.
ECONOMIC FACTORS
TYPICAL GRADE AND TONNAGE: When assessing
diamond deposits, grade, tonnage and the average value ($/carat) of the
diamonds must be considered. Diamonds, unlike commodities such as gold, do
not have a set value. They can be worth from a few to thousands of $/carat
depending on their quality (evaluated on the size, colour and clarity of
the stone). Argyle is currently the only major lamproite-hosted diamond
mine. It contains at least 75 million tonnes, grading between 6 and 7
carats of diamonds per tonne (1.2 to 1.4 grams/tonne). The Prairie Creek
mine produced approximately 100 000 carats and graded 0.13 c/t. Typical
reported grades for diamond-bearing lamproites of <0.01 to .3 carats per
tonne are not economic (Kjarsgaard, 1995). The average value of the
diamonds at Argyle is approximately $US 7/carat; therefore, the average
value of a tonne of ore is approximately $US 45.50 and the value of total
reserves in the ground is in excess of $US 3.4 billion.
END USES: Gemstones; industrial uses
such as abrasives.
IMPORTANCE: Olivine lamproites have only
been recognized as diamond host rocks for approximately the last 20 years
as they were previously classified as kimberlites based solely on the
presence of diamonds. Most diamonds are still produced from kimberlites;
however, the Argyle pipe produces more carats per annum (approximately
38,000 in 1995), by far, than any other single primary diamond source.
Approximately 5% of the diamonds are good quality gemstones.
SELECTED BIBLIOGRAPHY
Atkinson, W.J. (1988): Diamond Exploration
Philosophy, Practice, and Promises: a Review; in Proceedings of the
Fourth International Kimberlite Conference, Kimberlites and Related Rocks,
Volume 2, Their Mantle/Crust Setting, Diamonds and Diamond Exploration, J.
Ross, Editor, Geological Society of Australia, Special Publication
14, pages 1075-1107.
Bergman, S.C. (1987): Lamproites and other
Potassium-rich Igneous Rocks: a Review of their Occurrence, Mineralogy and
Geochemistry; in Alkaline Igneous Rocks, J.G. Fitton and B. Upton,
Editors, Geological Society of London, Special Publication 30,
pages 103-190.
Fipke, C.E., Gurney, J.J. and Moore, R.O.
(1995): Diamond Exploration Techniques Emphasizing Indicator Mineral
Geochemistry and Canadian Examples; Geological Survey of Canada,
Bulletin 423, 86 pages.
Griffin, W.L. and Ryan, C.G. (1995): Trace
Elements in Indicator Minerals: Area Selection and Target Evaluation in
Diamond Exploration; Journal of Geochemical Exploration, Volume 53,
pages 311-337.
Haggerty, S.E. (1986): Diamond Genesis in a
Multiply-constrained Model; Nature, Volume 320, pages 34-37.
Helmstaedt, H.H. (1995): "Primary" Diamond
Deposits What Controls their Size, Grade and Location?; in Giant Ore
Deposits, B.H. Whiting, C.J. Hodgson and R. Mason, Editors, Society of
Economic Geologists, Special Publication Number 2, pages 13-80.
Jacques, A.L., Boxer, G., Lucas, H. and
Haggerty, S.E. (1986): Mineralogy and Petrology of the Argyle Lamproite
Pipe, Western Australia; in Fourth International Kimberlite
Conference, Perth, Western Autralia, Extended Abstracts, pages 48-50.
Jennings, C.M.H. (1995): The Exploration
Context for Diamonds; Journal of Geochemical Exploration, Volume
53, pages 113-124.
Kjarsgaard, B.A. (1996): Lamproite-hosted
Diamond; in Geology of Canadian Mineral Deposit Types, O.R.
Eckstrand, W.D. Sinclair and R.I. Thorpe, Editors, Geological Survey of
Canada, Geology of Canada, Number 8, pages 568-572.
Levinson, A.A., Gurney, J.G. and Kirkley,
M.B. (1992): Diamond Sources and Production: Past, Present, and Future;
Gems and Gemmology, Volume 28, Number 4, pages 234-254.
Macnae, J. (1995): Applications of
Geophysics for the Detection and Exploration of Kimberlites and
Lamproites; Journal of Geochemical Exploration, Volume 53, pages
213-243.
Mitchell, R.H. (1991): Kimberlites and
Lamproites: Primary Sources of Diamond; Geoscience Canada, Volume
18, Number 1, pages 1-16.
Mitchell, R.H. and Bergman, S.C. (1991):
Petrology of Lamproites; Plenum Press, New York, 447 pages.
Nixon, P.H. (1995): The Morphology and
Nature of Primary Diamondiferous Occurrences; Journal of Geochemical
Exploration, Volume 53, pages 41-71.
Scott Smith, B.H. (1992): Contrasting
Kimberlites and Lamproites; Exploration and Mining Geology, Volume
1, Number 4, pages 371-381.
Scott Smith, B.H. (1996): Lamproites; in
Undersaturated Alkaline Rocks: Mineralogy, Petrogenesis and Economic
Potential, R.H. Mitchell, Editor, Mineralogical Association of Canada,
Short Course 24, pages 259-270.
Scott Smith, B.H. and Skinner, E.M.W.
(1984): Diamondiferous Lamproites; Journal of Geology, Volume 92,
pages 433-438. |
