Stamp sand

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Stamp sand is a coarse sand left over from the processing of ore in a stamp mill. It is often black or dark grey, and may contain dangerous trace metals.

Stamp sand, photographed in Gay, Michigan

In the 19th and early 20th centuries, many ore mines (especially copper and iron) used stamp mills to process ore-bearing rock. The rock was brought to a stamp mill to be crushed into small pieces. After crushing the material may hav been heated, causing the ore to melt out of the rock, or mechanically separated to extract metallic ore, or chemically treated if the metal could be leached out. The latter two cases would require large quantities of process water so stamp mills would often be located near water sources. Once the rock has been processed, the remaining small (1 - 2 mm) rock fragments, called stamp sand, would not have enough ore content to warrant further processing at the time. The sand was then usually disposed near the mill. As mills often relied on steam power to operate, they were built on the shore of lakes and rivers. The stamp sand was thus dumped into the water, sometimes growing deep enough to create entirely new land.

In some cases, during successive waves of exploitation, previously discarded stamp sand has been reprocessed to extract additional economic value in the form of the original metal, or other minerals.

Stamp sand is not entirely sterile, since it contains trace amounts of ore and related elements (such as arsenic). Thus, land created from stamp sand is frequently poisonous to plant life, and pollutes nearby water as well. For example, the Keweenaw Waterway, near the Keweenaw copper mines of Michigan, is virtually dead near stamp sand deposits, while reasonably healthy in other areas. Several stamp sand dumps have been designated as superfund sites to remove or contain the sands. Some stamp sand land has been covered with clean fill dirt and used for housing developments.

The coarseness of the sand has led to its use in place of (or in combination with) road salt in some areas, such as the Copper Country of Michigan. In addition, some companies have developed methods to reprocess stamp sands to reclaim their small mineral content.