33. Carbonatites Dan 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 least 50% carbonate minerals (usually calcite or dolomite, or both). Intrusive examples are much more numerous than volcanic, although more and more lavas and tuffs are being recognized. Most geologists rejected an igneous origin for these rocks until about 1960, when three things happened. 1) Experiments showed that a calcite liquid could exist at low pressure and 650° C. 2) Field relations showed that these rocks can intrude and metamorphose their wall rocks. 3. A volcano in Tanzania erupted carbonate-rich lava and ash. The carbonate minerals, however, are rich in Na and K.

Carbonate compositions (mole proportions) in carbonatites.

A small volcanic field of tuff cones and lava at Fort Portal in western 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 one crater.

The tuff cones are armored with tightly cemented carbonatite tuff 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 this region of Uganda the bedrocks are Precambrian, so lime for agriculture 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) in groundmass

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 the bulk composition of the tuff is close to that of Portland cement.

In places there are blowholes that vented gas after the hot ash 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. Field of view is 3 mm.

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

Dolomite phenocryst in calcite groundmass (stained blue), Meach Lake, Ontario. 5 mm.

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

At some contacts of wallrock with carbonatite, coarse-grained black 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 branching calcite crystals, Orberg quarry, Kaiserstuhl

Carbonatite dike cutting gabbro, Oberbergen, Kaiserstuhl

Calcite, apatite, and pyrochlore, Kaiserstuhl. Height 3 mm. Pyrochlore is 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, Goudini Farm, 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 rock removed in one day. The primary product is copper, but profitable byproducts include gold, iron, uranium, thorium, zirconium, titanium, niobium, rare earths, vermiculite, phosphorus and sulfuric 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 calcite perpendicular to layers, Dicker Willem, Namibia.

Elongated branching calcite crystals, Dicker Willem.

Dolomite phenocrysts (dark) in calcite carbonatite, Dicker Willem

Iron Hill carbonatite CO

Xenoliths of Durness Limestone in carbonatite, Loch Urigill, NW Highlands, Scotland