The Iron Oxide Ratio: Spotting Gossans and Rust From Space
Weathered, iron-stained ground — the rusty gossans that can cap a buried sulphide deposit — reflects red far more than blue. The iron-oxide ratio turns that simple contrast into a mineral-exploration vector.
The physical basis
The ratio is high where iron oxides such as haematite, goethite and jarosite absorb blue and reflect red, i.e. over oxidised, rust-coloured surfaces. It is the classic Landsat "iron" band ratio, dating back to the earliest satellite mineral mapping.
The math behind it
where Red is red reflectance, Blue is blue reflectance.
How to interpret the output
Elevated values map gossans and oxidised, iron-stained ground — surface indicators of buried sulphide deposits.
The bands it needs
| Sentinel-2 | Red B4 · Blue B2 |
| Landsat 8/9 | Red B4 · Blue B2 |
Real-world use
It maps gossans, oxidised outcrops and acid-drainage zones, and is a standard layer in exploration band-ratio composites.
Watch out for
Red soils, terracotta rooftops and other red-toned man-made materials also score high, so it is prone to false positives outside bare terrain — restrict interpretation to well-exposed, well-lit rock and validate in the field.
Compute Iron Oxide on your own study area
Skip the code. Draw or upload a boundary and Spatial Research Suite runs Iron Oxide on live Sentinel-2 or Landsat imagery — with cloud masking, exports and citations built in.
Run this analysis in GISforus →Frequently asked
What does the iron-oxide ratio map?
Oxidised, iron-stained surfaces such as gossans and acid-drainage zones — surface indicators of possible buried mineralisation.
Why does it produce false positives?
Any red-toned material, including red soils and terracotta roofs, raises the red/blue ratio, so it must be confined to bare rock and verified in the field.