Veinlette Mineralogy and Chemistry in a Nepheline Syenite Dike, Augusta County, Virginia

Several post Triassic igneous features occur in the Central Appalachians of Virginia and West Virginia and the mineralogy and origin of many of these features are poorly understood. Particularly problematic is an extensive set of Jurassic dikes in western Virginia.

One of the most typical of these dikes intrudes Ordovician sedimentary rocks in Augusta County just north of the city of Staunton, Virginia. The dike is approximately 10 meters wide along the exposure on US route 11 and relatively well exposed in cross section. The contact with the limestone of the Edinburg Formation is sharp with no visible indication of metasomatism. Outer margin textures are finer grained than those at the center. Described as a nepheline (natrolite) syenite, the dike has been previously reported to be composed of orthoclase, nepheline, analcime, acmite, biotite and hornblende with accessory natrolite, apatite and opaques.

Previously unreported textures include mineralized veinlettes that are approximately 4 mm thick, cutting through samples from the center of the dike. Veinlettes are coarser grained than the surrounding syenite and mineralogically zoned, appearing to be regions of higher water content. Radiating arfvedsonite with anhedral to subhedral analcime and anhedral feldspars dominate the core of the veinlettes. Energy dispersive spectroscopy (EDS) has determined accessory phases in the veinlettes to be enriched in Th and the rare earth elements La, Ce and Nd, while other phases are Zr and Nb enriched. Chemical compositions of some of these grains are consistent with monazite and zircon.

The newly recognized textures indicate a complex mineralization. Mineralogical constraints may provide refined interpretations of whole rock chemical compositions and isotopic investigations. The newly identified suite of minerals may provide further PT estimations and geochronological controls.