CARBONATITES
Carbonatites are essentially Òigneous limestonesÓ.
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.
KAISERSTUHL CARBONATITE
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-WŸrttemberg,
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 Òigneous limestoneÓ 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. sšvites), 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 (sšvite) (10.1 cm across) having calcite (whitish to
very light grayish) plus apatite, magnetite, monticellite, phlogopite, and
spinel.
Calciocarbonatite (sšvite) (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
(sšvite).
Some info. provided by Alexander Falster.
OKA CARBONATITE
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.
sšvites; 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 (sšvite) (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 (sšvite) (4.5 cm across), ~entirely composed of
coarsely-crystalline calcite.
Calciocarbonatite (sšvite) (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 (sšvite) (5.3 cm across), dominated by calcite plus
minor accessory minerals.
Calciocarbonatite (sšvite) (5.5 cm across) with calcite plus minor
accessory minerals (including titanite - CaTiSiO5).
Calciocarbonatite (sšvite) (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 Catal‹o
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
photos) 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
FERROCARBONATITE
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.
Reference cited:
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.
VERITY CARBONATITE
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 ÒbeforsiteÓ and
ÒrauhaugiteÓ rock names have been replaced by the more meaningful term
ÒmagnesiocarbonatiteÓ.
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 52¼ 23'
26" N, 118¼ 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. (~0¼ 41' 08"
North, ~34¼ 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 (08¼ 59' 30" South, 33¼ 14' East). The
Panda Hill Carbonatite (aka 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.
sšvite). 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).