Magnetic Survey

Maps variations in the magnetic properties of rocks. It’s primarily used to understand the structure and geology of the site—identifying faults, shear zones, lithological contacts, and intrusive rocks that often host gold deposits.
IP Survey: Maps the chargeability and resistivity of the subsurface. It’s used to detect the physical alteration and mineralization itself—specifically, the presence of disseminated sulfide minerals (like pyrite and arsenopyrite) that are very frequently associated with gold.
Together: They answer two critical questions: 1) Where is the right geological setting? (Mag) and 2) Within that setting, where are the actual sulfides that may contain gold? (IP). This combination dramatically improves the success rate of exploratory drilling.

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1. Magnetic Survey

Principle
Measures small variations in the Earth’s magnetic field caused by the magnetic mineral content (primarily magnetite, pyrrhotite, and maghemite) in rocks.

Application in Gold Mining
Gold itself is not magnetic. Therefore, the magnetic survey is an indirect targeting method.

Mapping Geology and Structure:
Identifying Shear Zones and Faults: These are often pathways for gold-bearing fluids. They can appear as linear lows in the magnetic data because the intense deformation and alteration often destroy magnetite (“magnetite destruction”).
Lithological Contacts: Different rock types have different magnetic signatures. A magnetic survey can map out volcanic sequences, sedimentary basins, and intrusive bodies (like granites) that may act as heat engines for gold-depositing hydrothermal systems.
Alteration Mapping: Certain types of alteration (e.g., potassic alteration) can sometimes be associated with an increase in magnetite, while advanced argillic alteration can destroy it. The magnetic pattern can serve as a guide to altered and potentially mineralized areas.

Data Presentation
Total Magnetic Intensity (TMI) Map: The raw measured data.
Reduced-to-Pole (RTP) Map: A processed version that centers anomalies over their source, making interpretation easier.
First Vertical Derivative: Enhances shallow, near-surface features and defines geological contacts sharply.
Analytic Signal: Helps identify the edges of magnetic bodies.

What a gold explorer looks for: Linear magnetic lows (potential shear zones) adjacent to or cutting through magnetic highs (potential iron-rich host rocks or intrusions).

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2. Induced Polarization (IP) Survey

Principle
Measures the ability of subsurface materials to hold an electrical charge (Chargeability) and their resistance to electrical current (Resistivity).

Chargeability: High chargeability is a direct indicator of metallic minerals (e.g., pyrite, arsenopyrite, chalcopyrite, graphite). These minerals polarize—they temporarily hold an electrical charge when current is applied.
Resistivity: Measures how much a material resists electrical flow. Altered rocks (e.g., silicification, clay alteration) often have a different resistivity than unaltered host rock.

 Application in Gold Mining
This is a direct method for detecting the common hosts of gold.

Detecting Sulfide Minerals: Most gold deposits, especially intrusion-related and orogenic types, are associated with sulfide minerals. A high chargeability anomaly is a prime target for gold exploration.
Mapping Alteration Zones: Hydrothermal alteration can change the rock’s resistivity. For example:
Silicification (common in gold systems) creates very high resistivity.
Clay alteration (e.g., argillic alteration) creates very low resistivity.
Differentiating Mineralization Types: The relationship between chargeability and resistivity can help interpret the style of mineralization (e.g., disseminated vs. massive sulfide).

Data Presentation
Chargeability Section (Pseudosection): A cross-section showing the distribution of chargeability with depth. High values (hot colors like red/white) are targets.
Resistivity Section (Pseudosection): A cross-section showing resistivity distribution. Used for geological context.
Plan Maps: Chargeability or resistivity values mapped at a specific depth or time window.

What a gold explorer looks for: A coherent, sub-surface chargeability anomaly (high IP response) that coincides with a favorable structural or lithological feature identified by the magnetics.

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The Integrated Workflow on a Gold Site

1. Regional Reconnaissance (Airborne Mag): A large-area airborne magnetic survey is flown to identify major structures and geological domains. This helps select the specific project area.
2. Project-Scale Ground Survey: Once a area of interest is selected, a higher-resolution ground magnetic survey is conducted to refine the structural interpretation.
3. IP Survey Targeting: The magnetic data is used to place IP survey lines directly over the most promising structural traps (e.g., a specific magnetic low representing a shear zone).
4. Drill Target Definition: The IP survey identifies which of these structural features actually contains significant sulfide mineralization. The highest-priority drill target is where a strong IP chargeability anomaly coincides with the interpreted favorable structure.
5. Drilling: Drill holes are planned to test the depth and extent of the IP anomaly. Core logging will then confirm the presence of sulfides and assay will determine the gold grade.

Limitations and Considerations

Not All That Glitters Is Gold: A high IP response can be caused by non-economic sulfides or, worse, graphite or clays, which can create false positives. Careful geological modeling is essential.
Depth of Investigation: Both methods have depth limitations. Ground IP is generally effective to a few hundred meters. Magnetic surveys can see deeper but with lower resolution.
Terrain and Access: Ground IP surveys require laying out cables and electrodes, which can be challenging in rugged, forested, or culturally busy (e.g., near villages) terrain.
Cost: IP surveys are significantly more expensive and slower to conduct than magnetic surveys. This is why magnetics are used first to narrow down the area.