Carbonatites are essentially “igneous limestones”. That's pretty odd when you think about it. Carbonatites are rare igneous rocks - some are intrusive (formed deep underground by relatively slow cooling of magma), but some are demonstrably extrusive (formed at the Earth's surface by relatively quick cooling of lava). They are principally composed of carbonate minerals (hence the term carbonatite) - mainly calcite (calcium carbonate), but dolomite (calcium magnesium carbonate) and sodium & potassium & iron carbonate minerals are also known.
Carbonatites composed of calcite are known as calciocarbonatites. Those composed of sodium-rich carbonate minerals are natrocarbonatites. Those that have a significant dolomite component are magnesiocarbonatites. Those that have a significant iron carbonate component are ferrocarbonatites. Examples of all four carbonatite types are shown below.
The German rock shown below is famous among carbonatite researchers. It comes from the first ancient carbonatite ever demonstrated to have originally been surficial volcanic material (extrusive origin). As are most carbonatites, this rock is composed of calcite (calcium carbonate - CaCO3), and it bubbles in acid. The tannish-brownish-grayish portions of the rock consist of rounded to subrounded particles - those are carbonatite lapilli from an ancient volcanic eruption. The white portions of the rock represent calcite cement that binds the lapilli together.
This extrusive calciocarbonatite is from the ancient Kaiserstuhl Volcano, formed along a continental rift called the Upper Rhine Graben. The long-extinct volcano is now eroded away, but some of its extrusive rocks and much of the originally-underlying intrusive complex is still preserved.
Age: Langhian to Burdigalian, late Early Miocene to early Middle Miocene, 14-18 million years.
Locality: Kaiserstuhl Complex (a.k.a. Kaiserstuhl Volcanic Complex, Kaiserstuhl Carbonatite Complex), near Freiburg, southwestern Baden-Württemberg, far-southwestern Germany.
Extrusive calciocarbonatite (calciocarbonatitic lapillistone; welded calciocarbonatite) (7.4 cm across), erupted during the Miocene from Germany's extinct Kaiserstuhl Volcano.
MAGNET COVE CARBONATITE
Central Arkansas has a famous “igneous limestone” unit called the Magnet Cove Carbonatite. The Magnet Cove is a mid-Cretaceous ring dike complex in the Arkansas Alkaline Province. The samples shown below are calciocarbonatites (a.k.a. sövites), dominated by the mineral calcite.
Age: late Albian to Cenomanian, mid-Cretaceous, 96-102 million years.
Locality: Cove Creek, northern Hot Spring County, central Arkansas, USA.
Calciocarbonatite (sövite) (10.1 cm across) having calcite (whitish to very light grayish) plus apatite, magnetite, monticellite, phlogopite, and spinel.
Calciocarbonatite (sövite) (7.5 cm across), almost entirely composed of calcite.
Igneous calcite (left: 3.3 cm across; right: 3.6 cm across) - two large calcite rhombs from a Magnet Cove calciocarbonatite (sövite).
Some info. provided by Alexander Falster.
The Oka Carbonatite Complex is located in Quebec, Canada. It’s a large body of alkaline igneous rocks intruded through Precambrian metamorphics. The Oka occurs in the western part of the Canadian Shield’s Monteregian Hills Province. Published research indicates that Oka rocks cooled from magma produced by partial melting of upper mantle rocks (inferred to be metasomatized garnet lherzolites). The rocks in the complex contain some rare elements, including economic concentrations of niobium (Nb). Several mines exploit Oka rocks for their Nb content. Oka rocks include coarsely-crystalline calciocarbonatites (a.k.a. sövites; a.k.a. C1 calciocarbonatites), alnoites, ijolites, and okaites (see below).
Age: mid-Barremian Stage, mid-Early Cretaceous, 124-125 million years.
Locality: Oka Niobium Mine, southeastern part of the Oka Hills, Oka Hills Inlier, Deux-Montagnes County, just west of Montreal & Laval, far-southern Quebec Province, southeastern Canada.
Calciocarbonatite (sövite) (6.1 cm across), dominated by whitish calcite, plus some minor brownish pyrochlore (slightly uraniferous (Na,Ca)2Nb2O6(OH,F) - a niobium ore mineral & very mildly radioactive) and blackish biotite mica + pyroxene? or amphibole? and minor blackish magnetite.
Calciocarbonatite (sövite) (4.5 cm across), ~entirely composed of coarsely-crystalline calcite.
Calciocarbonatite (sövite) (5.1 cm across), dominated by calcite (whitish-yellowish), plus some niocalite (yellowish Ca14Nb2(Si2O7)4O6F2 - a niobium ore mineral) and pyrochlore (slightly uraniferous (Na,Ca)2Nb2O6(OH,F) - a niobium ore mineral & very mildly radioactive).
Calciocarbonatite (sövite) (5.3 cm across), dominated by calcite plus minor accessory minerals.
Calciocarbonatite (sövite) (5.5 cm across) with calcite plus minor accessory minerals (including titanite - CaTiSiO5).
Calciocarbonatite (sövite) (4.2 cm across) with calcite (whitish-yellowish) plus dark-colored perovskite (CaTiO3) & latrappite ((Ca,Na)(Nb,Ti,Fe)O3) - a niobium ore mineral). This rock is mildly radioactive.
Ilmenite & magnetite mass (4.0 cm across) with minor calcite. Large magnetite masses are known in other carbonatites as well, such as the Late Triassic-aged Shawa Complex of Zimbabwe and the Late Cretaceous-aged Catalão Carbonatite Complex of Brazil.
Okaite sample (7.3 cm across), a rare intrusive igneous rock, named after the Oka Complex. Okaite is a feldspathoidal ultramafic igneous rock composed of nepheline, melilite, biotite mica, plus minor accessory minerals. This sample is very mildly radioactive.
Ijolite sample (5.1 cm) with calcite, nepheline, pyroxene, and titanium mineral(s).
Alnoite sample (6.0 cm across). This is an alnoite lamprophyre (an intrusive porphyritic mafic igneous rock) from a breccia pipe. The rock contains melilite, phlogopite mica, plus other minerals.
PHALABOWRA COMPLEX CARBONATITE
One of the world’s largest copper mines exploits cupriferous carbonatites in the Phalabowra Complex (Palabora Complex) of South Africa. The copper ore sample shown below is a calcite-bearing carbonatite (= calciocarbonatite). It is richly mineralized with metallic oxide and sulfide minerals. The dominant mineral, apart from calcite, is blackish-colored magnetite (Fe3O4). Also present are the copper sulfide minerals chalcopyrite (CuFeS2 - brassy golden-colored), bornite (Cu5FeS4 - metallic purplish-bluish), and apparently cubanite (CuFe2S3).
Unit & age: core of Loolekop Pipe, central Phalabowra Complex, mid-Paleoproterozoic, 2.06 billion years.
Locality: copper mine on the Loolekop Pipe, northeastern Kaapvaal Craton, northeastern South Africa.
Cupriferous calciocarbonatite (copper ore) (8.6 cm across along the base) from the Phalabowra Complex of South Africa having iron & copper minerals. Whitish gray = calcite. Brassy gold = chalcopyrite. Black = magnetite. Metallic bluish = bornite.
OL DOINYO LENGAI
Ol Doinyo Lengai in eastern Africa (see pics; more pics) is only active volcano on Earth that erupts carbonatite lava. All other volcanic lavas on Earth are rich in silicate minerals. Ol Doinyo Lengai lava has essentially zero silicate content. Instead, it is dominated by sodium carbonate minerals (plus some potassium carbonate and calcium carbonate). The high sodium content makes this lava natrocarbonatite. Natrocarbonatite is the rarest lava type in the world.
Shown below are samples freshly collected from eruptions back in the 1960s. Natrocarbonatite has the unfortunate tendency to alter upon exposure to the atmosphere & water. Molten natrocarbonatite has the appearance of very dark flowing mud, but it does glow red at night. After cooling, it alters relatively quickly to a whitish, crumbly material. The samples below were collected before any significant chemical or physical alteration could occur.
Ol Doinyo Lengai natrocarbonatite is principally composed of the minerals gregoryite and nyerereite. Gregoryite (Na1.6K0.1Ca0.15CO3; a.k.a. (Na2K2Ca)CO3) forms dark-colored, platy-shaped, glassy-looking crystals in the lava. Nyerereite (Na0.8K0.2Ca0.5CO3; a.k.a. Na2Ca(CO3)2) forms small, grayish, rounded masses in the lava. See thin section photo of these two minerals together.
Ol Doinyo Lengai is one of many volcanoes in the East African Rift Valley, a long continental rift complex formed as the Afar Hotspot slowly rips Africa apart.
Locality: Ol Doinyo Lengai Volcano, ~9.5 miles south of Lake Natron, northern Tanzania, eastern Africa.
Natrocarbonatite (4.1 cm across) from a freshly flowing pahoehoe lava flow at Ol Doinyo Lengai Volcano back in 1963. This is from sample CML-9 of Peterson (1990) - Contributions to Mineralogy and Petrology 105: 143-155. Published chemical analyses on this very lava indicate that it has phenocrysts of gregoryite (77% sodium carbonate, 18% calcium carbonate, 5% potassium carbonate) and nyerereite (50% Ca-carbonate, 41% Na-carbonate, 9% K-carbonate), with a groundmass of 65% Na-carbonate, 20% Ca-carbonate, and 15% K-carbonate.
Natrocarbonatite ash and lapilli (field of view ~5.2 cm across) from a mid-August to late October 1966 ash eruption of Ol Doinyo Lengai Volcano. This is the rarest volcanic ash type on Earth. It occurs only at this volcano in northern Tanzania.
ICE RIVER COMPLEX
This is ferrocarbonatite, an uncommon variety of carbonatite. Traditionally, this rock type is simply characterized by having a high-iron content compared with calciocarbonatites. The main carbonate mineral of ferrocarbonatites is (or is supposed to be) iron-rich (versus the rock being merely hematitic or magnetitic). Iron-rich carbonate minerals documented in ferrocarbonatites include siderite (FeCO3), ankerite (CaFe(CO3)2), and ferroan dolomite (Ca(Mg,Fe)(CO3)2). What about hematitic carbonatites, or magnetitic carbonatites, or carbonatites rich in iron silicate minerals? These are called ferruginous carbonatites (see Gittins & Harmer, 1997).
The ferrocarbonatite shown below has calcite, ankerite, siderite, iron oxides and iron silicates (Peterson & Currie, 1994). It comes from British Columbia’s Ice River Complex. The Ice River is an 18-kilometer long, 29-square kilometer, backward J-shaped alkaline igneous intrusion emplaced in Cambro-Ordovician passive-margin limestones and shales.
Age: ~356 million years (Tournaisian Stage, Early Mississippian or Famennian Stage, Upper Devonian, depending on which geologic time scale one uses)
Locality: Ice River Intrusion, Yoho National Park, southeastern British Columbia, eastern Cordillera, southwestern Canada.
Ferrocarbonatite (8.1 cm across) from the Ice River Complex of British Columbia.
Peterson & Currie (1994) - The Ice River Complex, British Columbia. Geological Survey of Canada Current Research 1994-A: 185-192
Gittins & Harmer (1997) - What is ferrocarbonatite? A revised classification. Journal of African Earth Sciences 25: 159-168.
Magnesiocarbonatites have a significant component of dolomite (CaMg(CO3)2 - calcium magnesium carbonate). The rock below is a magnesiocarbonatite from southwestern Canada. This is from the Verity Carbonatite, a 15 to 31 meter thick intrusive sill forming part of the Omineca Alkaline Province’s central carbonatite belt. Rocks from the Verity have also been called beforsites (= medium- to finely-crystalline Mg-rich carbonatites) and rauhaugites (coarsely-crystalline Mg-rich carbonatites). The “beforsite” and “rauhaugite” rock names have been replaced by the more meaningful term “magnesiocarbonatite”.
This rock is dominated by calcite and dolomite (the whitish to light gray portions of the rock), plus magnetite (black) and apatite (see rounded whitish spots in the middle portions of the rock). The rock is now somewhat foliated, the result of Mesozoic metamorphism, but the alteration appears not to have changed the mineralogy. It also contains vermiculitized phlogopite mica, zircon, and pyrochlore. Mining companies have expressed interest in the rocks from this general area, principally for the niobium (Nb) and tantalum (Ta) content of the pyrochlore.
Locality: Mt. Lempriere, between Rt. 5 and Kinbasket Lake Reservoir, Monasheee Mountains, southeastern British Columbia, southwestern Canada. Vicinity of 52º 23’ 26” N, 118º 58’ 40” W.
Magnesiocarbonatite (7.4 cm across along the bottom part of the sample) from the Verity-Paradise Carbonatite Complex of British Columbia, Canada.
Most info. from Tony Peterson.
DREAMER’S HOPE CARBONATITE
This rock is from a carbonatite dike that is peripheral to the McClure Mountain-Iron Mountain Alkaline Complex in Colorado, USA. The northwestern side of the complex has a halo of radiating carbonatite dikes - the Dreamer’s Hope is one of these radiating dikes. I’m not sure of the mineral content of this rock, but there appears to be at least two carbonate minerals (the cream-colored areas & the reddish-brown areas). The reddish-brown material appears to be calcitic. The creamish-colored material may be dolomitic. Sphalerite and galena have been reported in rocks from here. The sample shown below does have galena on the flip side.
Locality: Road Gulch, northern Wet Mountains, southwestern Fremont County, south-central Colorado, USA.
Age: late Early Cambrian, ~524 m.y.
Dreamer’s Hope Carbonatite (cut surface; field of view: 5.3 cm across)
EAST AFRICAN CARBONATITES
Here are some carbonatite rocks from eastern and southeastern Africa. The first two are from old, eroded carbonatitic volcanic centers in Tanzania & Uganda in the East African Rift Valley. One active carbonatite volcano still exists in the area - Ol Doinyo Lengai, but quite a few ancient ones still dot the landscapes of eastern Africa.
Tororo Carbonatite (6.2 cm across at its widest) from Tororo Rock, south of town of Tororo, southeastern Uganda, eastern Africa. (~00º 41’ 08” North, ~34º 11’ 02” East). This rock is from the Tororo Carbonatite Complex of Middle Eocene age (40 m.y.). The Tororo Carbonatite Complex is an eroded volcanic plug, one of many alkaline volcanic centers in & near the East African Rift Valley.
None of the Tororo carbonatite literature I’ve examined is explicit on the mineral content of the carbonatites at this locality. However, this rock doesn’t readily bubble in acid, but does when powdered. This seems to indicate that the sample is a magnesiocarbonatite, dominated by dolomite.
Panda Hill Carbonatite (field of view 5.2 cm across) from Panda Hill, ~25 km SW of Mbeya, between Lake Rukwa & Lake Nyasa, southwestern Tanzania, eastern Africa (08º 59’ 30” South, 33º 14’ East). The Panda Hill Carbonatite (a.k.a. Mbeya Carbonatite) intruded through Proterozoic quartz-feldspar gneisses, which have been significantly altered as a result. Quartzo-feldspathic country rocks that have been contact metamorphosed by carbonatite intrusions are called fenites. The resulting country rocks are said to be fenitized. This Panda Hill rock is calcitic, so it's called a calciocarbonatite (a.k.a. sövite). The Panda Hill intrusion dates to about 113 million years (Aptian Stage, late Early Cretaceous).
Carbonatite (7.4 cm across at its widest) from Chilwa Island, western Lake Chilwa, southeastern Malawi, southeastern Africa. The rock doesn't readily bubble in acid. When powdered, it mildly effervesces in acid - I suspect it is dolomitic.
This is from the Early Creatceous-aged Chilwa Alkaline Province, an igneous complex consisting of intrusive igneous rocks (alkali granites, syenites, alkaline syenites, nephilinites, and carbonatites) and extrusive igneous rocks (lavas - basanites, phonolites). The complex was intruded in phases throughout the Early Cretaceous, from about 111 to 138 million years ago. The Chilwa Island carbonatite dates to 126 m.y. (late Barremian Stage, mid-Early Cretaceous).