Mineralogy of carbonatite-breccia: Amba Dongar carbonatite diatreme, Gujarat, India

Carbonatite-breccia forms a major unit in the carbonatite-alkaline diatreme of the Amba Dongar carbonatite ring complex. In addition to the innermost part of the ring strucure, small and large plugs outside the ring structure are also formed in the form of discontinuous rings around the sövite. It mainly comprises rounded to sub-angular fragments of basement metamorphics, Bagh sandstones, pre-carbonatite basalt, nephelinite and sövite set in the carbonatitic matrix. Besides rock fragments, it also shows the presence of xenocrystal minerals. Carbonatite breccia is later invaded by sövite, alvikite and ankeritic carbonatite. Thorite, pyrochlore, barite, apatite, fluorite and a small amount of REE-minerals were introduced in carbonatite breccia by these later intrusives. The zircon, however, seems to have been caught up from metamorphic gneisses. Microprobe analyses of all these minerals in the carbonatite-breccia indicate the presence of important REEs. Carbonatite breccia has been silicified in places by invading hydrothermal solutions rich in fluorite and silica.