November 10, 2008
6:00 - 7:30 pm
State Technical Community College
10915 Hardin Valley Road, Knoxville
J.L. Goins Administration Building, Cafeteria Annex
(A Double Feature)
Kimberlites: What Are They and Why Do We Care?
Dr. Kula Misra
University of Tennessee, Department of Earth and Planetary Sciences
Kimberlites are pipe-like bodies of ultramafic (SiO2 <35%) ultrapotassic (molar K2O/Al2O3 > 1) volcanic rocks, which are characterized by a pronounced inequigranular texture, a distinctive mineralogy and chemical composition, and the presence of diamonds in some cases. The inequigranular texture arises from the presence of mantle xenoliths, phenocrysts, and xenocrysts in a much finer grained matrix. Diagnostic primary minerals include Mg-rich olivine, Cr- and Ti-rich pyrope, Cr-diopside, Mg-rich ilmenite, and phlogopite. Noteworthy features of chemical composition include near-primitive contents of compatible elements (Ni > 400 ppm; Cr >1000 ppm; and Co >150 ppm) and enrichment in large lithophile and rare-earth elements.
Kimberlites constitute a very small fraction of the igneous rocks in the Earth&rsquos crust, but they are lucrative targets of study because of their diamond potential and because they offer a window into the mantle through the mantle xenoliths they bring to the surface. The occurrence of most of the kimberlites, especially the diamondiferous ones, is restricted to Archean cratons.
Geology and Mineralogy of the Sharps Chapel Area, Union County, Tennessee
Aggregates USA, LLC
The Sharps Chapel area in Union County, Tennessee, contains several notable geologic features which will be visited during an upcoming ETGS field trip. The Norris Lake (Clarks Hollow) kimberlite lies southwest of Sharps Chapel and consists two kimberlite bodies exposed in Silurian-Devonian strata. The kimberlite is sheared and altered (serpentinized), in part due to an adjoining thrust fault. Brecciation and xenoliths are recognizable. Minerals commonly observed include two colors of garnet, phlogopite phenocrysts, serpentine, and magnetite crystals up to 15 millimeters wide. Two diamonds were found several miles downstream of the kimberlite pipes prior to construction of the lake. The pipes were first reported in 1869 by Safford.
The New Prospect Mine at Leadmine Bend lies northwest of Sharps Chapel. Lead-zinc ore was mined sporadically from the 1880s until the 1940s. The ore was shipped by barge down the Powell River to a zinc smelter in Clinton, Tennessee. The lead ore component was shipped by rail to smelters in the northern U.S. The ore deposit is stratabound and fault bounded. It is hosted in Cambrian-age carbonate rocks. Primary minerals include are yellow sphalerite and galena. Secondary minerals include cerrusite, hemimorphite, smithsonite, and limonite.
During periods of low water levels, middle-Ordovician carbonate units are exposed at the Claiborne County boat launch east of Sharps Chapel. This exposure contains a group of large stromatolites, as well as bryozoa, brachiopods, and occasional trilobite fragments.
A previous trip to the kimberlite and New Prospect Mine are documented here: http://mcrocks.com/ftr07/StreeterFebruary2007.html.
Page updated November 10, 2008