Gossan is defined as being an iron-bearing capping over a sulfide deposit. It is formed by the oxidation and leaching of sulfide minerals, leaving hydrated iron oxides such as limonite and goethite, along with manganese oxides. Hematite and jasper may also form. It forms in the zone of oxidation above the water table, and is sometimes referred to as an “iron hat.” If the underlying sulfide deposit contains gold, the gossan will also contain gold. In fact, gold is often enriched in gossan. Gossan is especially obvious at the surface in arid and semi-arid regions, often forming bold outcrops or “iron blowouts.”
One can rest assured that every obvious gossan in the West has been examined more than once by prospectors. Since assay work can be expensive, many prospectors crush fragments of gossan and pan them in a small pan or cup, using only a small amount of water. If there are any gold colors at all, the prospector will pursue it accordingly.
Often, a gossan contains cavities left by sulfide minerals that have been leached. Cubic and triangular cavities usually indicate the former presence of sulfide minerals. There may be boxwork or cellular masses, and there could be enrichments at depth. However, large cubic or triangular cavities in gossan could be a bad sign, because gold-bearing sulfides usually form as small crystals that would not create large cavities. Gossan is commonly found in areas where there are copper-gold ore bodies. There may be enough gold in gossan to form placer gold deposits in streamcourses, below the outcrops. The presence of unaltered pyrite, chalcopyrite, or other sulfides in gossan is a bad sign, because it indicates that leaching is incomplete, and that enrichments would not likely be found at depth. On the other hand, the presence of visible gold would be cause for rejoicing.