[gos-uhn, goz-]
a rust-colored deposit of mineral matter at the outcrop of a vein or orebody containing iron-bearing materials.
Also called iron hat.

1770–80; orig. dial. (Cornwall) < Cornish, derivative of gōs blood; akin to Welsh gwaed

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rust-coloured oxide and hydroxide minerals of iron and manganese that cap an ore deposit. Gossans form by the oxidation of the sulfide minerals in an ore deposit and they thus may be used as clues to the existence of subsurface ore deposits, especially if distinctive boxworks are present.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
When a saprolite layer is produced at a sulfide ore body the leached zone is
  often referred to as a gossan.
Gossan in shallow, caved pits locally is coated with malachite.
Sulfide-bearing gossan in one pit contains tetrahedrite.
The iron-stained hornfels, which is locally gossan, contains disseminated
  pyrite and small quartz-pyrite veinlets.
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