FIELD MAPPING AND GEOCHEMICAL ANALYSIS OF DIAMOND-BEARING LAMPROITE AND ADJACENT SEDIMENTS FROM THE WORTHINGTON DIAMOND MINE, PIKE COUNTY, ARKANSAS

The Worthington Diamond Mine comprises approximately one acre of near-surface lamproite that was emplaced in the early Late Cretaceous Period. The mine is a part of the formation known as the Twin Knobs One Lamproite, located within Pike County, Arkansas, and is part of a northeast–southwest trending group of volcanic emplacements. Over the past ~100 Ma, sediments of the Trinity Group and others have been both deposited and eroded over the diamond-bearing lamproite, leaving the lamproite rock relatively unexposed beneath a minimal, unconsolidated layer of sedimentary overburden. Recent excavation has exposed an approximately 5 meters high by 10 meters wide wall along the north side of the mine pit that was the study area for this project.

Field relationships of rocks and sediments were examined and mapped, and samples were collected for petrographic and geochemical analysis from the lamproite and overlying sediments. Methods of analysis include: labeling, sketching, and photographing layers of sediments and the contact between sediments and brecciated lamproite; generating a stratigraphic column of the study area; preparation of thin sections; petrographic analysis; and geochemical analysis with use of scanning electron microscopy (SEM) and powder X-ray diffraction (XRD). These efforts have resulted in the first detailed description of the relationship between individual rock units at the Worthington Diamond Mine, and has revealed a previously unrecognized diversity of rocks, minerals, compositions and textures found within this unique igneous environment.