33. Carbonatites Dan Barker July 2009 Cretaceous carbonatite dike (green) cutting Ordovician...

33. CarbonatitesDan Barker July 2009

Cretaceous carbonatite dike (green) cutting Ordovician limestone, Francon quarry, Ottawa, ONT

Closeup of carbonatite dike, Francon Quarry

Carbonatites are rare igneous rocks containing at least50% carbonate minerals (usually calcite or dolomite, or both). Intrusive examples are much more numerousthan volcanic, although more and more lavas and tuffsare being recognized.Most geologists rejected an igneous origin for theserocks until about 1960, when three things happened.1) Experiments showed that a calcite liquid could existat low pressure and 650° C. 2) Field relations showed that these rocks can intrudeand metamorphose their wall rocks.3. A volcano in Tanzania erupted carbonate-rich lavaand ash. The carbonate minerals, however, are rich inNa and K.

Carbonate compositions (mole proportions)in carbonatites.

A small volcanic field of tuff cones and lava at Fort Portal inwestern Uganda proved that calcite could dominate.

tuff cone

The Fort Portal field. For "cinder cone", read "tuff cone".

A crater in a small tuff cone, Fort Portal.

A lake within onecrater.

The tuff cones are armored with tightly cemented carbonatitetuff and lapilli tuff.

A small lava flow erupted from this fissure, producing a veritable cornucopia of minerals.

A quarry was opened in the lava flow. Why? Because in thisregion of Uganda the bedrocks are Precambrian, so lime foragriculture must be obtained from here or imported.

Spurrite (~0.5 mm) in groundmass of Fort Portal lava.

Tabular calcite (0.2 mm long) in groundmass

Apatite and biotite(~ 2 mm long) ingroundmass

Jennite tufts in vesicle, Fort Portal lava (field of view 0.8 mm)

The airfall tuff forms a hard, concrete-like surface. That is not surprising, because thebulk composition of thetuff is close to that ofPortland cement.

In places there are blowholes that vented gas after the hotash fell on wet ground.

The tuffs are also quarried, for lime and construction materials.

Pellets (they are too small to be called lapilli)in carbonatite tuff, Fort Portal. Pellets are cored by spinel, apatite, biotite and clinopyroxene. Field of view 3 mm high.

More cored pellets. Fieldof view is 3 mm.

White carbonatite dikes cut dark wall rock in the back yardof the Prime Minister's Residence, Meach Lake, Ontario.

Dolomite phenocrystin calcite groundmass(stained blue), MeachLake, Ontario. 5 mm.

Angular fragments of wall rock are enclosed in carbonatite.Meach Lake.

At some contacts of wallrock with carbonatite, coarse-grainedblack biotite and blue amphibole grew. Meach Lake again.

Carbonatite (white) and wallrock, Hydro's quarry, Fen, Norway

Hematite-rich carbonatite, on road to ancient Fen iron mine, Norway

Siderite-rich carbonatite in calcite carbonatite, Fen, Norway

Sodic clinopyroxene in calcite carbonatite, Holla, Fen. Width 3 mm.

Layers in carbonatite, Orberg quarry, Kaiserstuhl, Germany

Dendritic calcite, Kaiserstuhl. Height 3 mm.

Vertical branchingcalcite crystals, Orberg quarry, Kaiserstuhl

Carbonatite dike cutting gabbro, Oberbergen, Kaiserstuhl

Calcite, apatite, andpyrochlore, Kaiserstuhl.Height 3 mm. Pyrochloreis deep brown, nearly opaque.

Carbonatite tuff, Henkenberg, Kaiserstuhl

Nepheline-clinopyroxene rock (dark) in carbonatite,St Lawrence Niobium pit, Oka, Quebec

Thin plates of calcite in carbonatite, Oka,Quebec

Layers in carbonatite, Bond Zone, Oka, Quebec

Dike rock: platy dolomite and fluorite, GoudiniFarm, South Africa

Apatite-rich streaks in carbonatite, Spitskop, South Africa

Iron-rich cutting magnesium-rich carbonatite,Spitskop, South Africa

On the floor of the Palabora Mining Co. pit, South Africa. This has held the world record for the amount of rockremoved in one day. The primary product is copper, but profitable byproductsinclude gold, iron, uranium,thorium, zirconium, titanium, niobium, rare earths, vermiculite, phosphorus andsulfuric acid.

Carbonatite with copper sulfides, Palabora.

Banded carbonatite, Palabora

Dicker Willem carbonatite, Namibia. The name(Fat Billy) was given by German colonists who thought that the mountain looked like Kaiser Wilhelm II lying on his back.

Layered carbonatite, Dicker Willem

"Comb structure",crystals of calciteperpendicular tolayers, DickerWillem, Namibia.

Elongated branching calcite crystals, Dicker Willem.

Dolomite phenocrysts (dark) in calcite carbonatite,Dicker Willem

Iron Hill carbonatite CO

Xenoliths of DurnessLimestone in carbonatite, Loch Urigill, NW Highlands, Scotland