Gold in Gossan - March 2003 Issue

Gold in Gossan

by Edgar B. Heylmun, PhD

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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.

Contact between gossan and slates--photo courtesy of Dept. de Cristalografía y Mineralogía, UCM, Spain at http://www.ucm.es/info/crismine/

Color is another potentially useful characteristic, if used with caution. Massive dark brown to black gossan, sometimes iridescent, is formed by the leaching of pyrite. Reddish gossan may indicate the former presence of chalcopyrite. Both pyrite and chalcopyrite can contain gold. Ocher colors may indicate the possible presence of molybdenite at depth. Color is not always reliable and should be used with care.

Gossan that forms over a porphyry copper deposit, or a massive sulfide deposit, can be sizable in area. Streamcourses that drain such areas may contain placer gold.

Examples

Bingham Canyon. The Kennecott open-pit operation at Bingham Canyon, Utah, is one of the world’s largest porphyry copper deposits. Large areas of gossan led early prospectors to the district. Significant amounts of gold were produced from the gossan and associated placers before open-pit mining commenced. It has been said that enough gold has been produced to pay for the copper mining operations.

Iron Mountain Mine. This mine was about 10 miles northwest of Redding, California, in the Shasta copper-zinc belt. The gold-bearing gossan was noted in the 1860s, and mined during the 1930s. Some 520,000 ounces of gold were produced from the district as a whole.

Lavender Pit. This open-pit porphyry copper mine, at Bisbee, Arizona, was originally capped by an extensive red gossan. Over 75 million tons of copper ore were mined between 1954 and 1975. Small amounts of gold were found in the copper ore, as well as in the overlying gossan. This applies to most porphyry copper deposits.

Siskon Mine. The Siskon Mine in the Dillon Creek district, in the Klamath Mountains southwest of Happy Camp, California, is in a large gossan associated with massive sulfide deposits. Placer gold was found in the 1850s, and the Siskon Mine was operated on a large scale between 1951 and 1995, mostly as an open cut. Several million dollars in gold were recovered.

Many ore deposits throughout the world are capped by gossan, and in many places, they have been worked for gold. The gossan may be lean in gold, but substantial quantities may be present.

Summary

Gold should be looked for in all gossan. Many will be barren, but some could be of commercial interest. Some mining operations have been confined to enriched gossan. Two men, Augustus Locke and Roland Blanchard, did extensive work on gossan. The latter’s work has been published by the Nevada Bureau of Mines as Bulletin 66.

Spanish speaking readers can find several on-line articles regarding mineralogy, ore deposits, environmental issues regarding mining, and a full course on mining geology at:

http://www.ucm.es/info/crismine/TEXTOS_MONOGRAFICOS.htm

One of them is specifically on gossans:

http://www.ucm.es/info/crismine/gossan/gossanapuntes2.htm