Saline County (Bauxite)

Saline County is an outcropping portion of the essentially buried Little Rock intrusion (No. 120), consisting of a number of separate outcropping areas isolated by overlying Tertiary sands, gravels, clays and silts of the Saline Formation. Almost the whole area of the intrusion is occupied by nepheline syenites within which are a few small areas of Palaeozoic sedimentary rocks. The extent of the nepheline syenite is in many areas obscured by overlying bauxite deposits. The nepheline syenite comprises perthite, up to 25% nepheline, which is usually interstitial but occasionally euhedral, analcime, which replaces nepheline, sodalite, biotite and aegirine-augite zoned to aegirine and often surrounded by arfvedsonite, which also occurs independently. Some varieties contain a sodic plagioclase which may be more abundant than the perthite and these rocks contain aegirine as the only mafic mineral. Small areas occur of an alkali feldsparphyric syenite free of nepheline; it contains arfvedsonite and a little biotite and rare sodic diopside. Dykes of pegmatitic nepheline syenite and aegirine tinguaite cut the nepheline syenite, and monchiquitic dykes of several varieties are numerous. Some alteration of Palaeozoic sediments included in the nepheline syenite has been observed, with the formation of astrophyllite and aegirine.