MINERALOGICAL EVIDENCE FOR CARBONATITE AND NEPHELINE SYENITE IN ALKALINE ULTRAMAFIC ROCKS IN THE WESTERN KENTUCKY FLUORSPAR DISTRICT

The igneous intrusive complex in the western Kentucky Fluorspar District indicates a diverse and complex mantle-derived magma source and fractionation history. Minerals in igneous groundmass and phenocrysts in core indicate a polyphase fractionated facies of an alkaline silicate magma, a carbonatite and nepheline syenite. These minerals in dikes, determined by X-ray diffraction include:

a) Titanium garnets (schorlomite and titanium andradite) occur in the northern part of the district. These garnets are usually associated with carbonatite deposits such as the Magnet Cove, Ark., igneous complex which is the type locality of schorlomite.

b) Astrophyllite, a sodium, potassium, iron, titanium, niobium silicate, was detected in an intrusive breccia mantle xenolith which is indicative of a nepheline syenite magma, similar to the astrophyllite at Lovozero, Russia complex.

c) Abundant rare earth element-bearing perovskites were identified and are similar to REE rich loparite at the Lovozero Complex in Russia, further suggesting a nepheline syenite and carbonatite facies.

d) Villiaumite, a sodium fluoride; natrite, a sodium carbonate; and spodumene, a lithium silicate, each indicative of nepheline syenite-carbonatite lithology occur at the southern end of the district.

e) Niobium rutile (anatase, brookite), detected in several dikes in the district, occurs in other worldwide alkali ultramafic rocks including both carbonatite and nepheline syenite facies.

f) An REE-bearing fluorescent mineral called fluocerite, was detected at the Columbia dike, in addition to several REE fluorides at the Columbia, Davidson North, Midway, and Hutson dikes. These REE bearing fluoride complexes were within the dike or breccia facies, usually associated with carbonatites. Other REE bearing fluorophosphates also occur in dikes in the district.

Within the mid-continent craton, near the boundaries of the New Madrid Rift Complex, mantle intrusions occur along basement fractures and predetermined lines of weakness. This mineralogy from several core in opposite ends of the district suggest multiple fractionated mantle facies in the district.