Mineral paragenesis and zoning in the central Kentucky mineral district

The Central Kentucky mineral district occupies the greater part of the Blue Grass lowland of Kentucky, which lies astride the Lexington dome and the Cincinnati arch. The oldest exposed rocks of the mineralized area are limestones of Middle Ordovician age. The paragenesis throughout the district is constant and traceable, although the relative abundance of minerals varies from vein to vein. The mineral association, areal distribution of the ore minerals, and comparison with other epithermal deposits indicate that the Central Kentucky veins are epithermal. The ore deposits have a district-wide lateral zoning in a concentric pattern. Near the middle of the district, and within the Lexington dome, are two centers of fluorite-calcite-dark sphalerite veins that form Zone 1. Surrounding these centers is a much larger Zone 2 in which nearly all the veins contain fluorite, barite, calcite, and two-colored (yellow-black) sphalerite. Galena is absent or present in very minute quantities in Zones 1 and 2. In Zone 3, which surrounds Zone 2, barite, galena, and bright-colored sphalerite are the principal minerals. Fluorite is absent, or present only in traces. Textures of the mineral veins change from bands of coarse comb crystals in Zone 1 to mostly colloform bands in the outer zones. Structural control of the ore is related to faults and fractures transecting the collapsed Lexington dome and is an important factor in the distribution of mineral veins. The district has many of the mineralogic, zonal, and structural features of the Central Tennessee, Kentucky-Illinois, and Pennine, England, mineral districts, which suggest that the origins of these districts were similar. The ores in the Central Kentucky district were probably deposited from ascending thermal solutions of slightly lower temperatures than those that formed the deposits in the Kentucky-Illinois district. The solutions are believed to have been derived from a deepseated magmatic source centered beneath the Lexington dome, but they may have been greatly diluted by ground waters within the dome. The innermost zone probably is nearest to the emanative centers.