Woodson County

Two lamproite sills intrude and dome Upper Pennsylvanian sedimentary rocks in Woodson County. The Silver City (Hills Pond) sill is exposed at the surface and drilling indicates it to be >20 m thick and about 1.7x0.3 km. The Rose Dome sill is not exposed at the surface but drilling indicates an area of about 1.9x0.3 km and a thickness of 1-5 m. Both sills have hornfelsed the surrounding sediments and Franks et al. (1971) have described the addition of potassium to the hornfelses and the alteration of granite xenoliths that are abundant in the Rose Dome sill. The Silver City lamproite consists of phenocrysts of phlogopite, diopside, potassic richterite, serpentinized olivine and chrome spinel in a groundmass principally of serpentine with accessory perovskite, apatite and magnetite. The Rose Dome lamproite is highly altered and contains serpentinized olivine pseudomorphs, partly replaced by carbonate and quartz, and phlogopite, mostly replaced by carbonate, in a groundmass of dolomite, minor quartz and accessory magnetite. Analyses for major and trace elements, including some REE, and Sr isotopic compositions (0.7042-0.7102) for lamproites and contact metamorphosed rocks are given by Cullers et al. (1985); K2O:Na2O ratios vary from 9.6-150.