igneous

Gossan (Iron Hat)

Limonitic Gossan (Fe-oxide/hydroxide complex)

Hardness: 4.0-5.5 (variable); Color: Rusty orange, yellow-brown, red, and dark brown; Luster: Dull, earthy, to sub-metallic; Structure: Porous, cellular, or 'vuggy' texture; Cleavage: None; Specific Gravity: 2.7-4.3.

Hardness: 4
Color: Rusty orange, yellow-brown, red, and dark brown
Luster: Dull, earthy, to sub-metallic

Identified April 6, 2026More igneous →

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Physical properties

Formation & geological history

Formed by the oxidation (weathering) of sulfide-rich ore bodies, usually occurring at the surface of an igneous or metamorphic deposit. Heavy metals are leached out by rainwater, leaving behind an iron-rich cap.

Uses & applications

Primarily used as an exploration tool to find underlying deposits of copper, gold, or silver minerals; historically used as a low-grade iron ore.

Geological facts

Gossan is often called an 'Iron Hat' because it sits directly on top of valuable ore deposits. Mining engineers look for the cellular 'boxwork' patterns in gossan to predict what minerals lie deep underground.

Field identification & locations

Identify in the field by its intense rusty staining, lightweight porous texture, and proximity to volcanic or metamorphic rocks. It often leaves a yellow or reddish-brown streak on a porcelain plate.

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Fluorapatite (Ca5(PO4)3F)

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Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous

Magnetite

Iron(II,III) oxide (Fe3O4)

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Apatite (Group), Ca5(PO4)3(F,Cl,OH)

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Magnetite

Magnetite (Fe3O4)

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