SIGNIFICANCE OF TERMINATED AND DOUBLY TERMINATED QUARTZ CRYSTALS, LOWER MISSISSIPPIAN BOONE FORMATION, SOUTHERN OZARK REGION, ARKANSAS, OKLAHOMA AND MISSOURI

Lower Mississippian (Osagean) chert-bearing limestones of the Boone Formation are the thickest post-Ordovician, pre-Atokan (Middle Pennsylvanian) succession in the southern Ozark region. Silica development is represented by three distinctive types: 1) nodular, penecontemporaneous, black chert deposited with carbonate calcisiltites during maximum flooding as the lower Boone (Reeds Spring); 2) later diagenetic, white chert deposited as a post-depositional groundwater replacement of the upper Boone (Burlington-Keokuk) prior to Upper Mississippian (Chesterian) deposition; 3) white, post-depositional, hydrothermal silica replacement of basal upper Boone lacking obvious bedding and exhibiting restricted distribution both vertically and regionally (Elsey). Tripolite forms as decalcitization of the later diagenetic chert, but in the southern Ozarks that same process affected the hydrothermal silica of the Elsey. The lower Boone (Reeds Spring) interval has not been tripolitized.

Silica for the penecontemporaneous and later diagenetic cherts is likely volcanic ash from an island arc system developed along the southern margin of Laurussia as it was deformed by its collision with Gondwana. The main tripolitic interval (lower portion of upper Boone) reflects both pre-tripolitic and post-tripolitic hyrothermal lateral secretion produced by that same collision. The pre-tripolite hydrothermal movement replaced the Elsey interval confined by tighter Reeds Spring/lower Boone below, and the Burlington-Keokuk/upper Boone above. Post-tripolite hydrothermal movement allowed precipitation of terminated quartz crystals and druse in the micro-cavities of the tripolite created by decalcitization. The hydrothermal fluid movement is likely Pennsylvanian age, and the same event that deposited the Mississippi Valley type lead and zinc deposits across the Ozark Dome.