A common question: if gold is not radioactive, how would gamma-ray spectrometry help find it? The answer lies in the hydrothermal alteration that accompanies many gold deposits — and that alteration leaves a radiometric signature.
Start with the prospective model
Before interpreting any map, define the deposit model you are looking for. Gold's airborne-geophysics signature is not unique: it changes with the type of mineralizing system. Knowing which model you are pursuing determines which products to prioritize and how to read them — a shear-hosted orogenic target and a porphyry-related system leave very different footprints in magnetometry and radiometrics.
Main gold deposit models and their airborne-geophysics response
- Orogenic (lode) gold: strongly structurally controlled, hosted in shear zones and faults. Magnetometry is decisive — lineaments and edge filters (Tilt, Horizontal Gradient) map the hosting structures, while radiometrics can pick up potassic/sericitic alteration along them.
- Porphyry-related and epithermal systems: show alteration zoning (potassic, phyllic, argillic). Magnetite may be created (potassic core) or destroyed (phyllic/argillic halos), producing magnetic highs and magnetic "lows"; radiometrics highlights potassium enrichment or depletion.
- IOCG (iron oxide–copper–gold): typically a strong magnetic signature (magnetite/hematite) coinciding with alteration — here magnetometry is the primary vector.
- Paleoplacer and lateritic/supergene gold: geomorphologically controlled; the concentration follows paleochannels or weathering profiles, so radiometrics must be combined with the DEM (terrain) to make sense.
Because the signature depends on the model, defining your target model first turns the same datasets into very different — and much sharper — reading strategies.
What radiometrics actually detects
Gamma-ray spectrometry measures surface potassium (K), thorium (eTh) and uranium (eU). Gold itself does not appear, but the hydrothermal processes that transport and deposit it frequently add or remove potassium in the host rocks. Mapping these changes indirectly vectors the mineralization.
Potassic alteration: the classic vector
In many systems (porphyry, epithermal, orogenic), the passage of mineralizing fluids generates potassic alteration — potassium enrichment in the rocks near the ore. On the radiometric map, this appears as a potassium anomaly that can surround or coincide with the mineralized zone.
Caution: not every potassium high is alteration. Granites are naturally rich in K. That is why the interpreter never looks at K in isolation.
Ratios that separate signal from noise
To distinguish alteration from lithology, ratios and indices are used:
- A high K/eTh suggests potassium addition (possible alteration), since thorium tends to be immobile;
- The anomalous potassium index (K normalized by expected behavior) enhances enrichments that depart from the rock's pattern;
- Analysis per lithological unit avoids comparing granite with metasediment.
When a K/eTh anomaly aligns with a structural lineament (seen in magnetometry) and with known occurrences, you have a consistent target.
Combination beats a single method
Gold deposits rarely reveal themselves through a single dataset. The strength comes from convergence:
- Structure (magnetometry and derivatives) for the fluid conduits;
- Potassic alteration (radiometrics) for the hydrothermal footprint;
- Geochemistry and geology to close the model.
Accelerating vectoring
Generating the radiometric channels, ratios and per-lithology analysis manually is time-consuming. By automating processing and integration, the geologist receives the potassic-alteration products ready, cross-referenced with structure and geology — shortening the path to gold targets.
How Lithogeo helps
This is exactly the kind of work the Lithogeo platform automates. From an ANM process number or a polygon of your area, it processes and integrates the public data — magnetometry, gamma-ray spectrometry, geology, terrain and mineral occurrences — and delivers the products described here ready for analysis, in minutes and right in the browser.
Educational content. The radiometric signature is a support vector; confirming gold mineralization depends on geochemistry and drilling.