The oxidation of sulfide-rich base metal mineral deposits results in residual, generally in situ, materials rich in iron oxyhydroxide and oxide minerals. Historically, these decomposed rocks have commonly been termed gossan. Gossans form through the oxidation of Fe sulfide minerals, typically pyrite and pyrrhotite. When pyrite is exposed to air and water, it undergoes a chemical reaction that produces Fe oxyhydroxide, oxide and sulfate minerals and sulfuric acid. The distinctive reddish-brown to yellow colour of gossans is due to the formation of goethite, jarosite, and Fe-hydroxy-sulfate phases. Acidic oxidizing solutions then oxidize the remaining sulfide minerals as well as primary silicate minerals. Neutralization and metal attenuation lead to formation of numerous secondary minerals. A portion of the metal content is generally precipitated lower down in the mineralized structure, commonly termed an enrichment zone, with ‘gossan’ generally referring to the upper leached zone. Miners historically used gossan as an indicator of potentially economic base and/or precious metal ore bodies in the underlying weathered rock and primary sulfide minerals. Although no longer targeted as ores themselves, gossans are still sought in modern exploration as a guide to concealed sulfide deposits.