10.2 Mineral exploration geophysics

Mineral exploration geophysics applies physical measurements at the Earth's surface and in boreholes to detect subsurface ore bodies and alteration systems. Because most economic mineral deposits differ from their host rocks in density, magnetism, conductivity, or radioactivity, geophysical surveys can narrow vast search areas down to drill-ready targets. This guide covers the main survey methods, how to interpret the anomalies they produce, how different datasets are integrated, and the role of borehole geophysics.

Geophysical Methods for Mineral Exploration

Gravity, Magnetic, and Electromagnetic Surveys

Gravity surveys measure small variations in the Earth's gravitational field caused by lateral density contrasts in the subsurface. A massive sulfide lens (density ~4.5 g/cm³) sitting in felsic volcanic rock (~2.7 g/cm³) produces a measurable positive gravity anomaly. After applying standard corrections (free-air, Bouguer, terrain), the residual anomaly map highlights density contrasts that may correspond to ore bodies.

Magnetic surveys map variations in the total magnetic field intensity. The primary target mineral is magnetite, but pyrrhotite and other ferrimagnetic phases also contribute. Magnetic data are especially useful for tracing iron-oxide-rich alteration halos around deposits and for mapping subsurface geology under cover. One complication is remanent magnetization: magnetic minerals can retain a magnetization direction acquired during formation that differs from the present-day field, causing anomalies that don't align with the expected response.

Electromagnetic (EM) surveys detect electrically conductive or magnetically susceptible bodies by measuring how they interact with a transmitted EM field.

• Frequency-domain EM methods, such as controlled-source audio-frequency magnetotellurics (CSAMT), transmit a range of frequencies. Lower frequencies penetrate deeper, so sweeping across frequencies builds a conductivity profile with depth.
• Time-domain EM (TDEM or TEM) methods transmit a pulse and then measure the decaying secondary field. The decay rate and shape reveal the conductivity, size, and depth of a target. Massive sulfide bodies, for example, produce a slow, strong decay compared to weakly conductive overburden.

Induced Polarization and Radiometric Surveys

Induced polarization (IP) measures the chargeability of subsurface materials. When current is injected into the ground and then switched off, certain minerals cause the voltage to decay slowly rather than dropping to zero instantly. Disseminated sulfide grains (chalcopyrite, pyrite) are the classic source of this effect. IP is particularly valuable for porphyry copper and epithermal gold systems, where sulfides are finely disseminated through large rock volumes and may not produce strong gravity, magnetic, or EM anomalies on their own.

Radiometric surveys measure natural gamma radiation from the decay of potassium ($^{40}\text{K}$), uranium ($^{238}\text{U}$), and thorium ($^{232}\text{Th}$). These surveys are most effective in areas with thin or no overburden. They can identify:

• Potassic alteration zones (elevated K), which commonly surround porphyry copper deposits
• Uranium mineralization in sedimentary basins, where U anomalies directly indicate the target commodity
• Lithological boundaries, since different rock types have distinct K-U-Th signatures

Together, IP and radiometric surveys fill gaps left by gravity, magnetic, and EM methods, adding information about disseminated mineralization and alteration chemistry.

Interpretation of Geophysical Data

Anomalies and Their Significance

Every geophysical method produces anomalies, but not every anomaly means ore. The table below summarizes common anomaly sources and the ambiguities involved:

Geological Context and Interpretation

Geophysical data should always be interpreted within the geological framework. A strong EM conductor in a greenstone belt terrane is a high-priority massive sulfide target; the same conductor in a sedimentary basin may just be a graphitic shale horizon.

Steps for sound interpretation:

1. Establish the geological setting from mapping, stratigraphy, and known structural controls.
2. Identify coherent anomalies that stand out from regional background trends.
3. Cross-check anomalies across methods. A coincident gravity high, EM conductor, and IP chargeability high is far more compelling than any one alone.
4. Apply geophysical modeling. 2D and 3D inversion algorithms estimate the depth, geometry, and physical properties (density, susceptibility, conductivity) of the body causing the anomaly. These models are non-unique, so geological constraints are essential to guide them toward realistic solutions.
5. Rank targets for follow-up (infill surveys, drilling) based on the strength and coherence of multi-method anomalies and their geological plausibility.

Integration of Geophysical Data

Combining Geophysical, Geological, and Geochemical Information

No single dataset tells the whole story. Effective mineral exploration integrates several types of information:

• Geological mapping provides lithological, stratigraphic, and structural context. It tells you where certain deposit types are geologically permissible, so you don't chase anomalies in the wrong rock package.
• Geochemical sampling (soil surveys, stream sediment surveys, rock chip analyses) highlights areas with elevated pathfinder elements (e.g., arsenic and antimony around gold deposits). A geophysical anomaly that coincides with a geochemical anomaly jumps up the priority list.
• Petrophysical measurements (density, magnetic susceptibility, resistivity, chargeability) on rock samples and drill core provide the physical property link between geology and geophysical response. Without petrophysical data, modeling is poorly constrained.
• Geochronological data (U-Pb, Ar-Ar dating of intrusions and alteration minerals) establish the timing of mineralization events, helping you determine whether a geophysical anomaly is associated with the right-aged system.

Data Integration Techniques and Tools

• GIS platforms allow you to overlay geophysical grids, geological maps, geochemical point data, and structural interpretations in a common spatial framework. Simple overlay analysis can quickly highlight areas where multiple favorable indicators converge.
• 3D modeling software (e.g., Leapfrog, GOCAD) builds volumetric models that incorporate geophysical inversions, geological surfaces, and drill hole data. These models let you visualize how an anomaly relates to known geology at depth.
• Machine learning approaches, including unsupervised clustering and supervised classification, can identify subtle multi-variable patterns across large datasets that a human analyst might miss. These are tools to assist interpretation, not replace geological reasoning.

The goal of integration is to move from a large number of possible targets to a short list of well-supported drill targets, reducing exploration risk and cost.

Borehole Geophysics in Mineral Exploration

In-Situ Measurements and Characterization

Once drilling begins, borehole geophysical logging provides continuous, in-situ measurements of physical properties along the hole. These logs serve two purposes: they characterize the intersected geology directly, and they supply ground-truth data for calibrating surface geophysical interpretations.

Common borehole logging tools in mineral exploration:

• Gamma-gamma density: Measures bulk density by detecting back-scattered gamma rays. Essential for resource estimation (converting volume to tonnage) and for calibrating surface gravity models.
• Magnetic susceptibility: Identifies magnetic mineral distribution downhole. Particularly useful where remanent magnetization complicates surface magnetic interpretation.
• Resistivity and IP logs: Delineate conductive or chargeable zones tied to sulfide mineralization or alteration, at much higher spatial resolution than surface surveys.
• Natural gamma: Detects K, U, and Th concentrations, identifying potassic alteration or uranium-bearing intervals.

Structural Analysis and Resource Estimation

Acoustic televiewer (ATV) and optical televiewer (OTV) tools produce oriented images of the borehole wall. From these images you can pick fractures, veins, lithological contacts, and faults with their true orientation (dip and dip direction). This structural information is critical when mineralization is structurally controlled, because it tells you which direction to drill next.

Borehole geophysical data feed back into the broader exploration program in several ways:

1. Calibrate surface models. Density and susceptibility logs constrain inversion models, reducing ambiguity.
2. Refine resource estimates. Continuous physical property logs fill gaps between assayed intervals and improve geological domaining.
3. Guide future drilling. Structural data from televiewers, combined with updated 3D models, help optimize the location and orientation of the next holes, targeting extensions of known mineralization or testing new anomalies with greater confidence.