Jcg19 Jun 2026

Gossan (Iron Gossan / Limonitic Gossan)

AI-suggested ID

The collector's 'iron gossan' identification is well-supported. The prominent copper-red patches are notable — this appears to be a cupriferous gossan, likely from a copper sulfide orebody. Check for malachite, azurite, or native copper in cavities. Gossans are excellent surface indicators for deeper ore. Heavy for its size due to iron oxide content.

oreSedimentary-exhalative (SEDEX)

Formation

Gossan forms by intense supergene oxidation of sulfide ore bodies (e.g., pyrite, chalcopyrite) near the surface. Sulfides oxidize to limonite/goethite boxwork; copper minerals may persist as native copper or malachite in the oxidized zone.

Geological origin

Keweenaw Peninsula, Michigan, USA

Properties

color

dark brown-black with copper-red patches

grade

Unknown — surface gossan, not primary ore

gangue

Limonite, goethite, hematite

commodity

Iron (Fe), potentially Copper (Cu)

host rock

Oxidized sulfide ore body / gossan cap

alteration

Argillic

Where else to find

Rio Tinto, Huelva, SpainMount Isa, Queensland, AustraliaNamaqualand, South AfricaIberian Pyrite Belt, PortugalAtacama Desert, ChileSudbury Basin, Ontario, Canada

Suggested tests

• Streak on unglazed porcelain: yellow-brown streak confirms goethite/limonite; reddish streak suggests hematite
• Scratch with steel knife (~5.5 Mohs): goethite/limonite will be scratched, confirming softness
• Check copper-red patches with a magnifier — native copper is malleable and will not crumble; malachite will show green powder
• Test with dilute HCl — malachite and calcite gangue will fizz; pure gossan iron oxides will not

Could also be

Cupriferous Gossan - The prominent copper-red metallic patches strongly suggest native copper or chalcopyrite/bornite remnants within the gossan matrix — this may be a copper-enriched gossan rather than a purely iron one

Limonite / Goethite Boxwork - The dark botryoidal to granular black-brown matrix is consistent with massive goethite/limonite — a common gossan component that can appear nearly identical without further testing

Slag (anthropogenic) - The highly vesicular, rough black matrix with copper-colored patches can superficially resemble copper smelting slag; weight, non-magnetic test, and field context would distinguish

prospecting-indicatohistorical-source-ircopper-bearing-gossageochemical-sampling