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M E E T I N G   N O T E S

Carbonatites of Colorado and the World

Speaker: Pete Modreski

February 20, 2005

 Location: Woodland Park Library




 The origin of carbonatites has been the subject of significant debate. Once it was thought that carbonatites were a result of assimilation of sedimentary limestones by basaltic magmas, but in the lab it is difficult to dissolve much limestone by this mechanism. Also, the magma tends to freeze quickly.

A more accepted hypothesis is that carbonatites originate as fluids leaking out of the mantle, undergoing changes as they migrate through the crust.  This is consistent with the association of carbonatites with rift zones within continental crust.  Primordial carbon is well known to be present in the mantle (i.e., diamonds).  Carbonate minerals are also stable in the mantle, and may form supercritical fluids upon partial melting.  These fluids contain lots of rare earth and other unusual elements.  Lab experiments can explain the close spatial association of silicate bodies with carbonatites.  Likely starting as one magma that evolves into two immiscible melts, the silica melt incorporates more typical elements such as Fe and Al.

Carbonatites are probably rarer than kimberlites.  About 450 occurrences have been recognized globally, ranging in age from active-today to 2 billion years old.  The edges of many are fenitized; that is, the country rock is alkali-metasomatized.  Several are zoned.  They are likely the roots of eroded extrusive systems.  There is one occurrence of sodic carbonatite lavas in Tanzania along the East African Rift.  It is associated with an active volcano.   Pete presented the details of several global occurrences.

In Colorado, carbonatites occur in the Powderhorn district (a large titanium resource) and the Wet Mountains.  Colorado carbonatites represent a narrow time span from latest Precambrian to Cambrian.

Cathodluminescence is an excellent way to study the paragenesis, fabrics, etc. of carbonatites.  That is because many associated minerals—especially rare earths-- have activators causing luminescence.  Carbonatites can be thought of as “oxidized systems”, whereby ferric iron assists luminescence (while ferrous iron retards luminescence).

Pete brought several mineral specimens to the meeting, including several impressive giant phlogopites.   He concluded with a slide show of several carbonatite localities he had visited.