brood

Brood is the fine, sticky mass of clay, silt, and worthless rock that clings to tin and copper ore during extraction. It is not ore itself, but rather the overburden and weathered host rock that must be separated before the ore can be concentrated. In practice, brood forms where water has worked into fractured rock, turning quartz, feldspar, and other waste minerals into a cohesive paste that coats and binds ore particles together.

The composition varies by deposit and depth. Near the surface, brood is predominantly clay and oxidized minerals, sometimes with sand. Deeper down, it may contain more quartz and partially decomposed feldspathic rock. The stickiness matters greatly: ore caked with brood will not wash clean easily, and the fine clay particles clog screens and interfere with gravity separation in dressing sheds. Brood-heavy ore is slower and more expensive to process than clean ore from deeper workings.

Separation and disposal

Miners traditionally removed brood by hand before hauling ore to the dressing works, using water jets and scrubbing. Mechanical scrubbers and spiral classifiers later took over this task, using agitation and water flow to separate clay from heavier ore particles. The brood slurry was run to settling ponds or tailing lagoons. In some operations, particularly in cornish tin mining, brood was discarded in dedicated waste pits rather than mixed with other tailings, because its high clay content could choke drains in nearby ground.

The term is rooted in the image of worthless matter clustered around valuable ore, much as a brood of unwanted creatures might cluster around food. It is regional: cornish and devon miners used it freely, but the word rarely appears in american or australian mining records for the same material, which was more often called slime or clay burden. Modern usage has largely faded as open-pit mining and mechanical extraction have reduced the proportion of ore reaching the surface in such poor condition.