Gold and Silver
Gold and silver are commonly found in areas of igneous and metamorphic activity and are generally associated with silicic intrusives and Precambrian metamorphic rocks. When gold-bearing rocks weather, the liberated gold, because of its higher specific gravity, is mechanically separated from the accompanying lighter material and concentrated in the stream bottoms in the valleys. This type of mineral concentration is called placer deposit.
In Kentucky, the surficial geology is not favorable for the natural occurrence of gold or silver. Most of the surface and near-surface rocks are sedimentary, and there has been no igneous, metamorphic, or tectonic event to allow the gold or precious metal to be concentrated in economic quantities.
The most common mineral mistaken for gold is pyrite, an iron sulfide, commonly referred to as "fool's gold." Another mineral that often confuses an amateur rock hound is muscovite, often called white mica. It is nonmetallic, but has a shiny luster, and when weathered it turns brownish gold. Small flakes of muscovite are disseminated throughout some shales and sandstones in Kentucky. It is remotely possible that minute, noncommercial amounts of gold could be found in glacial outwash debris in southern Ohio near the Kentucky border. If this gold occurs in Kentucky, it was derived from sediments in the northeastern United States.
Some silver was recovered from galena concentrates in the Western Kentucky Fluorspar District in the 1960's, but the amount was very small and uneconomic. The silver occurred in the crystal lattice structure of galena.
Rift-related gold mineralization could occur in deep Proterozoic rocks (greater than 5,000 feet deep in central Kentucky and greater than 15,000 feet deep in western Kentucky) along the Grenville Front in parts of Kentucky and Ohio. The provincial boundary between the Grenville and Central Terranes could be an excellent area for syn- and epigenetic mineralization. Precise timing and location of tectonic events related to the Grenville Front are not known, but would play an important role in mineralization. The occurrence of several geologic events such as polymetamorphic events in the Grenville, post-Grenvillian igneous volcanic events, or significant metamorphism during and after Grenville events, could provide the mechanism for metal remobilization. The presence of iron and other metallic minerals, rift-related mylonites, CO2 degassing in oil fields, and suture zones provide much of the structural mechanics necessary for precipitation of base and precious metals in the eastern Midcontinent region. Numerous metasediments (sediments subjected to metamorphism), quartzites, and thick carbonate sequences, including marble, provide an effective host for these materials. Although these rocks have produced gold in Virginia, Georgia, North Carolina, and South Carolina, the depth of any potential deposits in Kentucky precludes them from being economic at this time.