chat

Chat is the coarse rock waste ejected from lead and zinc mines, typically dumped in heaps near the mine entrance or scattered across the surrounding land. It consists of the barren host rock broken away during extraction, along with low-grade ore that was not economical to process. Unlike coal slack or colliery waste, chat has no fuel value and no secondary market; it is simply the cost of mining paid in tons of broken stone.

The material varies with the ore body. In the tri-state mining district (Kansas, Oklahoma, Missouri), chat is often reddish or ochre owing to iron oxide staining from weathered zinc and lead sulfides. Particle size ranges from dust to boulder, depending on how aggressively the ore was broken and whether any screening occurred. Most chat is coarse, angular, and poorly sorted. The density and hardness depend on the host rock: limestone chat is lighter and softer; dolomitic chat is denser and more durable.

Historical use and disposal

For decades, chat presented a disposal problem. Early mining operations simply built chat piles, which leached heavy metals into groundwater and runoff. By the late 20th century, some mine operators and contractors found secondary uses: chat was crushed for railroad ballast, aggregate for concrete, or fill material for road subgrades. In some regions it was spread over unpaved roads. These applications reduced environmental liability but did not eliminate the legacy piles dotting mining country.

The term is chiefly North American and most common in the former lead-zinc districts of the Ozarks and the Great Plains. It appears in mining reports, reclamation plans, and environmental surveys, though usage varies; some operators use the more formal term 'mine waste' or 'barren rock.' Regional variations exist: in some areas 'slack' or 'tailing' are used, though technically those terms have different meanings in other mining contexts.

Chat piles remain a feature of abandoned or closed mining sites and complicate brownfield redevelopment. Modern regulations require characterization of chat for heavy metal content, particularly lead and cadmium, before reuse or off-site transport is permitted. The material itself is inert and stable once settled; the environmental concern lies in what dissolved metals it may shed into percolating water.