GEOLOGY OF THREE ADDITIONAL LAMPROITE OCCURRENCES, PIKE COUNTY, ARKANSAS

Three previously unexplored Arkansas lamproites were delineated and evaluated during the 1980's. Exploration programs utilized detailed geophysics, heavy mineral sampling, and soil geochemistry. The three diatremes -- Black Lick (25 acres), Twin Knobs 2 (4 acres) and Timberlands (1 acre) -- were intruded into Early Cretaceous Trinity Fm. and overlain unconformably by the Late Cretaceous Tokio Fm. This stratigraphy is consistent with published isotopic ages of approximately 100 Ma for the largest diatreme within the lamproite province. Field relationships establish that about 75% of the diatremes consists of pyroclastic tuffs, typically with fine-grained sandy tuffs near the margins, and coarse-grained pyroclastic breccias near the cores of the diatremes. Approximately 25% of the diatremes consist of late stage magmatic olivine lamproite which may have reached the surface as extrusive flows. A 260 metric ton bulk sample taken from the two largest diatremes recovered 5 macro diamonds (>0.5mm) yielding a sub-economic average diamond grade of 0.04 carats per 100 metric tons. This diamond grade is slightly less than that reported for adjacent Prairie Creek and Twin Knobs 1 diatremes. The bulk sampling yielded a rare suite of mantle and crustal xenoliths. Crustal xenoliths include shale, sandstone, quartzite, gabbro, amphibolite, granite, granodiorite and gneiss. Mantle xenoliths include dunite, harzburgite, websterite, eclogite and spinel and garnet lherzolite.