KIMBERLITES & LAMPROITES
Kimberlites and lamproites have tremendous economic importance because they are host rocks for gem-grade and industrial-grade diamonds. Kimberlites & lamproites are unusual igneous bodies having overall pipe-shaped geometries. Their mode of formation is only moderately understood because they have not been observed forming. Kimberlites & lamproites are known from scattered localities throughout the world - only some are significantly diamondiferous. Classic localities for diamonds are India and Brazil. Africa was also discovered to have many kimberlites and is world-famous for producing large numbers of diamonds. Other notable diamondiferous kimberlite-lamproite occurrences include Russia, China, northwestern Australia, and northwestern Canada.
Kimberlites are named for the town of Kimberley, South Africa. Several kimberlite pipes occur in the Kimberley area (see rock below).
Kimberlite from a Cretaceous-aged diamondiferous pipe at Kimberley, South Africa (CMNH 18991R, Cleveland Museum of Natural History, Cleveland, Ohio, USA).
Premier Kimberlite (4.5 cm across) from the Mesoproterozoic of South Africa. The largest gem-quality diamond ever found came from the Premier Kimberlite Pipe near the town of Cullinan, ~30 km northeast of Pretoria, in northeastern South Africa. When found in 1905, the Cullinan Diamond measured 10.5 cm (4.25 inches) across at its widest (about the size of a fist), and weighed ~1.5 pounds. Uncut, the Cullinan was 3026 carats. The Cullinan was cut into 105 gemstones, the largest of which is the Great Star of Africa (530 carats), on display with the British Crown Jewels in the Tower of London.
The Premier Kimberlite Pipe erupted about 1.2 billion years ago, during the Mesoproterozoic. It is significantly diamondiferous (some of the diamonds pictured here are from the Premier Kimberlite). Published inclusion dating studies have shown that Premier diamonds fall out into three age groups:
1) ~1.2 billion year eclogitic diamonds (Mesoproterozoic)
2) ~1.9 billion year lherzolitic diamonds (Paleoproterozoic)
3) ~3.2 billion year diamonds (Mesoarchean)
Koidu Kimberlite from the Jurassic of Sierra Leone, Africa. The large, rounded serpentinized olivine crystal at upper left is just over 4 mm across.
This bluish-green kimberlite is from the diamondiferous Koidu Kimberlite Complex of Jurassic age in eastern Sierra Leone, West Africa. The rounded, whitish- to pale green-colored grains are serpentinized olivine crystals having a mesh texture (reticulate texture) (just discernible in the photo). The sample is probably from one of the kimberlite dikes in the area (Dike Zone B, C, or D of Tompkins & Haggerty, 1984).
Location: Yengema-Koidu (Sefadu) area, eastern Sierra Leone, West Africa (approximately 8¡ ~35-40' North, ~11¡ West).
Info. mostly synthesized from: Linda Tompkins (pers. comm.), Stuart McRae (pers. comm.), Tompkins & Haggerty (1984 - Kimberlites I: kimberlites and related rocks, Proceedings of the "Third International Kimberlite Conference", volume 1, Developments in Petrology 11A).
PRAIRIE CREEK LAMPROITE
Prairie Creek Lamproite - weathered olivine lamproite (6.3 cm across) from Crater of Diamonds, Arkansas, USA. The light-colored grains are deteriorated olivine crystals.
The only publicly-accessible diamond collecting site in America is Crater of Diamonds State Park, just south of Murfreesboro, in Pike County, southwestern Arkansas, USA (34¡ 1' 55" to 34¡ 2' 5" North, 93¡ 40' 18" to 93¡ 40' 25" West). There, diamondiferous lamproitic rocks of the Prairie Creek Lamproite (a.k.a. Prairie Creek Pipe) are exposed (actually, it's just a field of rocky dirt). Diamonds are found by tourists on a daily basis. The Prairie Creek Lamproite is an ultramafic pipe that intrudes Proterozoic-aged (Grenvillian) basement rocks. Rocks of the Prairie Creek complex date to about 106 million years (mid-Albian Stage, late Early Cretaceous).
Prairie Creek Lamproite - bluish, lamproitic volcanic tuff (7.7 cm across) from western side of southern-most pipe at Crater of Diamonds State Park, Arkansas, USA (same locality as the next rock above). Three distinctive lithofacies occur at Crater of Diamonds - a lamproite pipe facies, a lamproitic volcanic breccia facies, and a lamproitic volcanic tuff facies. This bluish rock is from the tuff facies, although it does have obvious large fragments - not really what's supposed to be in a "tuff" - it's more of a volcanic breccia.
Age: about 106 million years, mid-Albian Stage, late Early Cretaceous.
BLUE BALL LAMPROITE
Blue Ball Lamproite (3.0 cm across) - this blackish rock comes from the Blue Ball Lamproite Dike of western Arkansas, USA. Rocks of the Blue Ball Lamproite consist of phlogopite and serpentinized olivine in a groundmass of mica, chromite, Ti-magnetite, perovskite, and carbonate. This dike intruded at about 106 million years (mid-Albian Stage, late Early Cretaceous).
Locality: northern side of Freedom Creek, ~0.45 km northwest of the Freedom Creek-Dutch Creek intersection, south of Rt. 80, north of Dutch Creek Mountain (ridge), between the towns of Union Hill and Blue Ball, far-eastern Scott County, western Arkansas, USA (about 34¡ 56' 14" North, 93¡ 45' 33" West).
Most info. provided by Jack King.
Jericho Kimberlite (8.4 cm across) - the diamondiferous Jericho Kimberlite is a cluster of 3 pipes discovered in the mid-1990s at Echo Bay, near the northern part of Contwoyto Lake, northern Slave Province, southwestern Nunavut Territory (near the border with the Northwest Territories), in the Canadian Arctic (~420 km north-northeast of the city of Yellowknife). The pipe complex intruded during the mid-Middle Jurassic, at about 172 million years. The rock has a bluish-gray color & is full of interesting fragments, including reddish garnets and nice phlogopite flakes.
Location: Echo Bay Mine of Canada (65¡ 59' 50-55" North, 111¡ 28' 30-45" West).
Northwestern Australia's diamondiferous Argyle Lamproite was discovered in October 1979 using the classic prospecting technique of identifying indicator minerals in stream sediments. It is a unique diamondiferous unit due to its relative abundance of pink diamonds. Elsewhere on Earth, pink diamonds are exceedingly rare (two famous examples of subtly pink diamonds were found at Golconda, India and Tanzania, Africa). Pink diamonds from the Argyle Lamproite can be intensely pink, unlike those from outside Australia.
Several different colors of diamonds are processed at the Argyle Mine, including clear ("white"), pink, brown, yellowish-brown, and a few unimpressive blue diamonds. About 5% of Argyle diamonds are gem-quality.
Diamond octahedral crystal from the Argyle Lamproite. The public has been successfully fooled into buying ugly light brown and dark brown Argyle diamonds by their being marketed under the names "champagne diamonds" and "cognac diamonds".
Argyle Lamproite (field of view ~3.7 cm across) - olivine lamproite lapilli-ash metatuff from the Mesoproterozoic of northwestern Australia.
Two major facies are present in the Argyle Lamproite (aka Argyle AK1 Lamproite, Argyle Diatreme, Argyle Lamproite Diatreme, Argyle AK1 Pipe): 1) olivine lamproite lapilli-ash metatuff; and 2) sandy lamproite tuff.
The first rock above is from the first facies, olivine lamproite lapilli-ash metatuff. The original olivines are gone, and have been replaced by the metamorphic minerals serpentine, talc, and chlorite (the serpentine & chlorite give the rock its greenish coloration). Mineral studies indicate that Argyle Lamproite material also contains sericite, K-feldspar (replacing original leucite), some carbonate, chromite, phlogopite, and several accessory minerals.
Argyle Lamproite (4.1 cm across) - sandy lamproite tuff from the Mesoproterozoic of northwestern Australia. This is a sample from the second facies. It contains ~50% quartz sand grains (= the small specks of white).
The Argyle Lamproite Pipe erupted during the mid-Mesoproterozoic, at about 1.178 billion years ago. The included diamonds are much older (~1.6 billion years). The Argyle Pipe is fortuitously preserved. Nearby pipes have eroded down to their roots. A quartzite ridge adjacent to the Argyle has protected it from erosion.
The pink coloration in some Argyle diamonds is apparently not the result of chemical impurity, which gives most diamonds their color variation. The pink color appears to be related to metamorphism-induced microfaulting or slippage of carbon atoms within the diamond crystals.
Geologic context: lamproite pipe emplaced along a splay of the Glenhill Fault, near-eastern margin of the Halls Creek Mobile Zone, eastern edge of the Kimberley Craton.
Location: Argyle Mine, near the headwaters of Smoke Creek, west of Lake Argyle, northwestern Western Australia (~16¡ 14' South, ~128¡ 23' East).
(Info. mostly synthesized from Karen Rice (pers. comm.), Boxer et al. (1989), Jaques et al. (1989a, 1989b), and Pidgeon et al. (1989). The latter four references are published in Kimberlites and related rocks, Volume 1, their composition, occurrence, origin and emplacement. Geological Society of Australia Special Publication 14.)
INDIAN GUIDE KIMBERLITE
Indian Guide Kimberlite (field of view ~4.7 cm across) from the Devonian of Wyoming, USA. The Wyoming Geological Survey has identified several kimberlites and kimberlite fields in Wyoming, including the Indian Guide Kimberlite Field in southeastern Wyoming. Above is a sample from an Indian Guide kimberlite dike that consists of serpentinized olivines and garnets having whitish reaction rims. Dike emplacement occurred during the Early Devonian. Kimberlites in this area have intruded through the Sherman Granite, an widespread Proterozoic unit (1.4 billion years) in southeastern Wyoming.
Location: undisclosed locality in the Indian Guide Kimberlite Field, Iron Mountain Kimberlite District, northeast of Laramie, southeastern Wyoming, USA.
Wyomingite (= peralkaline ultrapotassic lamproite) (~8.8 cm across) from the late Cenozoic of the Pumice Hills of Wyoming, USA. Some very unusual extrusive igneous rocks occur in southwestern Wyoming, USA. The sample shown above is a rare volcanic rock called wyomingite, but it's probably "better" called a peralkaline ultrapotassic lamproite (a.k.a. diopside-leucite-phlogopite lamproite). The principal minerals are reported to be leucite, diopside pyroxene, phlogopite mica, fluorapatite, and katophorite. This wyomingite is part of the Leucite Hills Volcanic Province, which experienced eruptions from the Late Pliocene to the Early Pleistocene (~1.4 to ~3.4 million years). I've not seen a more precise published date on this particular wyomingite occurrence.
Location: open-pit mine in the Pumice Hills, east of Zirkel Mesa, Leucite Hills, northeast of the town of Superior, north-central Sweetwater County, southwestern Wyoming, USA (~41¡ 47' 53" North, 108¡ 54' 54" West).
LAKE ELLEN KIMBERLITE
Lake Ellen Kimberlite (field of view ~7.2 cm across) from the Lake Ellen area, Upper Peninsula of Michigan, USA.
Michigan's Upper Peninsula has a well-known microdiamondiferous kimberlite body at Lake Ellen, appropriately called the Lake Ellen Kimberlite Pipe. Reported large fragments and xenoliths in Lake Ellen Kimberlite rocks include ilmenite, pyroxene, garnet, olivine, phlogopite, eclogite, pyroxenite, phyllite, amphibolite, gabbro, granulite, and dolostone.
I've not seen a precise published date on the Lake Ellen Kimberlite, but have heard second-hand that it was emplaced during the Jurassic (confirmations or corrections are welcome!).
Location: hill between Michigamme Reservoir and Lake Ellen-Little Lake Ellen (approximately N1/2, N1/2, NW1/4, Section 34, Kiernan 7.5' USGS topographic quadrangle), far-eastern Iron County, western UP of Michigan, USA (~46¡ 10' 27" North, 88¡ 10' 30" West).
ISON CREEK KIMBERLITE
Ison Creek Kimberlite (cut & polished surface; 4.5 cm across) from the Permian of Kentucky, USA. This is the Ison Creek Kimberlite (a.k.a. Ison Creek Pipe) from northeastern Kentucky. Petrographic studies report that Ison Creek rocks contain serpentinized olivine, perovskite, ilmenite, vermiculitized phlogopite, pyrope garnet, bronzite pyroxene, Cr-diopside, calcite, and xenoliths. Documented xenoliths include Pennsylvanian Breathitt Formation black shale, limestone, basement granite fragments, and upper mantle eclogite fragments. The Ison Creek Kimberlite is part of the Elliott County Peridotite and erupted at about 257 million years (Late Permian). Diamonds have not yet been reported from this unit.
Locality: Ison Creek valley area (vicinity of Rt. 409 & Hamilton Creek Road), just west of Stephens, east of Newfoundland, eastern Elliott County, northeastern Kentucky, USA (vicinity of 38¡ 07' 35" North, 82¡ 59' 44" West).
Info. mostly synthesized from Schulze (1984), in Kimberlites II: the Mantle and Crust-Mantle Relationships.
HAMILTON BRANCH KIMBERLITE
Hamilton Branch Kimberlite (cut & polished surface; field of view ~2.6 cm across) from the Permian of Kentucky, USA. This is a sample from northeastern Kentucky's nondiamondiferous Hamilton Branch Kimberlite (a.k.a. Hamilton Branch Pipe), part of the Late Permian-aged Elliott County Peridotite. It has obvious & abundant serpentinized olivine crystals (rounded to irregularly shaped green masses below). Small xenoliths of eclogite from the upper mantle have been reported from this pipe, but are rare.
Locality: between Ison Creek and Hamilton Branch (Hamilton Creek), between Rt. 409 and Hamilton Creek Road, just west of Stephens, east of Newfoundland, eastern Elliott County, northeastern Kentucky, USA (area of 38¡ 07' 42" North, 82¡ 58' 53" West).
Most info. from Schulze (1984), in Kimberlites II: the Mantle and Crust-Mantle Relationships.
Akluil‰k Dike (4.2 cm across) - microdiamondiferous minette from the late Paleoproterozoic (1.832 Ga) of the Gibson Lake area, northern Canada. This is an ultrapotassic minette, a rare intrusive igneous rock. Minettes are potassic mica lamprophyres. The dark crystals are biotite mica. The light-colored crystals are orthoclase feldspar. This particular minette is microdiamondiferous, and comes from the Akluil‰k Dike System of the Central Churchill Province, northern Canada. The Akluil‰k Dikes were intruded during the late Paleoproterozoic, at about 1.832 billion years ago. Microdiamonds found in these rocks are ~2 billion years old.
Locality: Gibson Lake area, Gibson-MacQuoid Lake Belt, ~120 km northwest of Rankin Inlet, District of Keewatin, Northwest Territories (sensu lato) ("Nunavut"), northern Canada (approximately 63¼ 35' North, 93¼ 30' West).
See MacRae et al. (1996) for more info.: MacRae, N.D., A.E. Armitage, A.R. Miller, J.C. Roddick, A.L. Jones & M.P. Mudry. 1996. The diamondiferous Akluil‰k lamprophyre dyke, Gison Lake area, N.W.T. in Searching for diamonds in Canada. Geological Survey of Canada Open File 3228: 101-107.
CHICKEN PARK KIMBERLITE
Chicken Park Kimberlite (3.0 cm across) - a decent number of kimberlite bodies have been identified in the Rocky Mountains along the Colorado-Wyoming border. This is a nice, small kimberlite sample from the diamondiferous Chicken Park Pipe near Red Feather Lakes in northern Larimer County, northern Colorado, USA. The Chicken Park Kimberlite Pipe is part of the State Line Kimberlite Field, traditionally considered to represent many kimberlite bodies emplaced at about the same time. Recent isotopic dating has indicated this is not the case. The Chicken Park Pipe is late Neoproterozoic in age (614 million years).
Picton Kimberlite (field of view ~2.1 cm across) from the Middle Jurassic of southeastern Ontario, Canada. The Picton Kimberlite (informal) is an igneous dike that intrudes the Ordovician-aged Verulam and Lindsay Formations in southeastern Ontario, Canada. It is exposed in the western part of the Picton Quarry in Prince Edward County, at the southeastern margin of Ontario. The unserpentinized olivine grains making up the bulk of this Picton Kimberlite sample are relatively small. Published research indicates that phlogopite, calcite, spinel, and serpentine are also present. This kimberlite dike was emplaced during the Middle Jurassic.
NOONKANBAH LAMPROITE FIELD
Leucite lamproite (wet, cut surface; field of view 5.6 cm across) from P Hill, Noonkanbah Lamproite Field, West Kimberley Lamproite Province, northwestern Australia (18¼ 29' 11" South, 124¼ 54' 56" East).
The northern part of Western Australia has many lamproite intrusions, some of which have been reported to be diamondiferous (usually microdiamondiferous). The rock shown above is from the Noonkanbah Lamproite Field, one of four fields in the West Kimberley Lamproite Province. The Noonkanbah Field consists of over two dozen individual lamproitic intrusions of late Early Miocene age (18-20 m.y.). The sample shown here is from the P Hill intrusion, which is not known to be diamondiferous.
This P Hill rock is a porphyritic leucite lamproite, which is the dominant lamproitic lithology in the West Kimberley Province. It consists principally of diopside clinopyroxene, leucite, and phlogopite mica (which glitters nicely in the proper light on both broken & cut surfaces).
Morin Kimberlite (3.9 cm across) from Quebec, Canada. Southwestern Quebec's TŽmiscamingue Kimberlite Field consists of several, recently-discovered kimberlite pipes. This is a friable sample from the Morin Kimberlite. It's from the pipe's crater facies, which is usually quickly eroded away and rarely preserved in most of the world's kimberlite bodies. The Morin Pipe is undated, as far as I know. Based on ages of other kimberlites in southeastern Canada, it wouldn't surprise me if the Morin was emplaced during the Mesozoic.
This rock appears to consist of serpentinized olivine (dirty greenish), phlogopite mica, and rotten clasts of something (orangish-brown). Pyrope garnet is reported from this material as well. The Morin Kimberlite does contain diamonds, but the mining company that owns the pipe has determined that it is not economically diamondiferous.
Location: Morin Kimberlite Pipe, SW of the southern end of Lac des Quinze, ENE of Ville-Marie, E of Lake Timiskaming, western TŽmiscamingue County, southwestern Quebec, southeastern Canada.
MIDDLE PARK LAMPROITE
(a.k.a. RABBIT EARS LAMPROITE)
Middle Park Lamproite (10.5 cm across) from the Oligocene of Colorado, USA. Here's a rock from an unusual, small igneous body west of Lake Granby in northern Colorado. This is from the Middle Park Lamproite (= Rabbit Ears Lamproite, but it's not from the Rabbit Ears Pass locality), an Early Oligocene intrusion (dike? sill? pipe?) having abundant, flow-aligned phenocrysts of phlogopite mica. The phlogopite (= glittery flakes) makes this a visually stunning rock (some appear bluish - that's not real - it's an artifact of scanner light reflectivity). Published literature indicates that the greenish-gray groundmass consists principally of analcime, biotite mica, richterite, apatite, ilmenite, and some diopside pyroxene. The Middle Park Lamproite is not known to be diamondiferous.
Age: early Early Oligocene, 33 m.y.
Location: 5 to 5.5 km west of Lake Granby (it's not from Rabbit Ears Pass), northeast-central Grand County, northern Colorado, USA.
Middle Park Lamproite (2.8 cm across) - close-up of the above rock. Bluish flakes are not bluish on rock - the phlogopite mica flakes are reflecting the scanner light.
Bellsbank Kimberlite (6.6 cm across) - here's a sample from South Africa's diamondiferous Bellsbank Kimberlite. It has the typical appearance of a kimberlite: a jumbled mix of crystals and clasts. Dirty green-colored, serpentinized & partially serpentinized olivines with dark greenish-gray reactions rims are common and conspicuous. Scattered phlogopite mica and garnet are also present. The Bellsbank Kimberlite intruded through Proterozoic-aged dolostones of the Ghaap Group during the late Early Cretaceous (Aptian Stage, 118 m.y.).
Locality: Bellsbank Mines, near Mount Rupert, north of Kimberley, north-central South Africa.
Mir Kimberlite - diamondiferous kimberlite from Siberia's Mir Diamond Mine. Identifiable large fragments in the matrix of this rock include serpentinized olivine (greenish) and garnet (very dark red).
The Mir Kimberlite Pipe was the first diamondiferous kimberlite found in Siberia and one of the most important diamond mines on Earth. The Mir Kimberlite is one of several ultramafic pipes in the Malo-Botuoba Kimberlite Field of the Siberian Platform. Additional diamondiferous kimberlite fields have been found elsewhere in Siberia.
Geochronologic studies on rocks and minerals from the Mir Kimberlite Pipe have given dates ranging from the Archean to the Phanerozoic. Ages from small inclusions in diamond crystals date well into the Precambrian, although Mir diamonds appear to have crystallized not long before eruption of the pipe in the Devonian (there is mounting evidence indicating diamond inclusion dates don't necessary reliably indicate diamond crystallization dates).
Available evidence indicates that the Mir Pipe erupted some time in the Devonian or Mississippian (Early Carboniferous). Devonian to Mississippian ages have also been obtained from other pipes in the Malo-Botuoba Kimberlite Field. Published eruption dates for the Mir Pipe range from 403 to 324 million years. Many studies report dates clustering around 354 to 360 m.y. (~latest Devonian to Early Mississippian). These dates roughly correspond to a time of intraplate basalt volcanism in Siberia.
The Mir Kimberlite's geometry is that of a gently tapering-downward pipe, as are most other kimberlite bodies. The pipe intrudes Cambrian and Ordovician carbonate rocks, which occur throughout the Siberian Platform.
Reported xenoliths in the Mir Pipe include Paleozoic sedimentary rocks, Paleozoic igneous rocks, Precambrian basement rocks, and mantle rocks (eclogites & several types of peridotites).
Location: Mirnyy, Yakutia/Sakha, Siberia, Russia (62û 31' 43.93" North, 113û 59' 38.60" East).
Specimen owned by Stan & Pris Woollams.
Kimberlite from the Beartooth Mountains, Stillwater County, southern Montana, USA.