Mineral exploration geophysics uses various methods to find valuable resources underground. Gravity, magnetic, and electromagnetic surveys detect density differences, magnetic minerals, and conductive bodies. These techniques help locate potential ore deposits.
Other methods like induced polarization and radiometric surveys provide additional insights. By combining these approaches with geological and geochemical data, explorers can better understand subsurface structures and pinpoint promising mineral targets.
Geophysical Methods for Mineral Exploration
Gravity, Magnetic, and Electromagnetic Surveys
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Gravity surveys measure variations in the Earth's gravitational field caused by density differences in subsurface rocks, which can indicate the presence of dense ore bodies
Magnetic surveys detect variations in the Earth's magnetic field due to the presence of magnetic minerals (magnetite), which can be associated with certain types of mineral deposits
Electromagnetic (EM) surveys measure the electrical conductivity and magnetic susceptibility of subsurface materials, which can help identify conductive ore bodies or alteration zones associated with mineralization
Frequency-domain EM methods (controlled-source audio-frequency magnetotellurics (CSAMT)) use a range of frequencies to investigate subsurface conductivity at different depths
Time-domain EM methods (transient electromagnetics (TEM)) measure the decay of induced magnetic fields to detect conductive bodies
Induced Polarization and Radiometric Surveys
Induced polarization (IP) surveys measure the chargeability of subsurface materials, which can indicate the presence of disseminated sulfide minerals (chalcopyrite, pyrite) often associated with certain types of mineral deposits (porphyry copper, epithermal gold)
Radiometric surveys measure the natural radioactivity of rocks and soils, which can help identify potassium alteration or uranium mineralization
These methods complement gravity, magnetic, and EM surveys by providing additional information about the subsurface geology and mineralization
IP surveys are particularly useful for detecting disseminated sulfide minerals that may not be detectable by other methods
Radiometric surveys can identify alteration zones (potassic alteration) or specific types of mineralization (uranium) that are not directly targeted by other geophysical methods
Interpretation of Geophysical Data
Anomalies and Their Significance
Gravity anomalies can indicate the presence of dense ore bodies (massive sulfide deposits, iron oxide copper-gold (IOCG) deposits), which have higher densities than the surrounding host rocks
Magnetic anomalies can be associated with magnetic minerals (magnetite), which may be directly related to the mineralization or indirectly related through alteration processes
Remanent magnetization, caused by the alignment of magnetic minerals during rock formation, can complicate the interpretation of magnetic anomalies
Electromagnetic conductors can represent sulfide-rich ore bodies, graphitic shale horizons, or water-bearing structures, requiring careful interpretation in the context of the geological setting
Induced polarization anomalies can indicate the presence of disseminated sulfide minerals (chalcopyrite, pyrite), which are often associated with porphyry copper or epithermal gold deposits
Radiometric anomalies can help identify potassic alteration zones, which are often associated with porphyry copper deposits, or uranium mineralization in sedimentary basins
Geological Context and Interpretation
Geophysical data should be interpreted in the context of the geological setting, considering factors such as the host rock lithology, structural controls, and alteration patterns
Understanding the geological framework is crucial for distinguishing between anomalies related to mineralization and those caused by other geological features (lithological contacts, faults, intrusions)
Integrating geophysical data with geological observations (field mapping, drill core logging) and geochemical data (soil sampling, rock chip sampling) can improve the interpretation and targeting of potential mineral deposits
Geophysical modeling techniques (2D and 3D inversion) can help refine the interpretation of anomalies by estimating the depth, shape, and physical properties of the causative bodies
Integration of Geophysical Data
Combining Geophysical, Geological, and Geochemical Information
Geological mapping provides the framework for interpreting geophysical anomalies, considering factors such as lithology, stratigraphy, and structural controls on mineralization
Geochemical sampling (soil surveys, stream sediment surveys) can help prioritize geophysical anomalies by identifying areas with elevated pathfinder elements or alteration signatures
Petrophysical data (density, magnetic susceptibility, electrical properties) of rocks and minerals can help constrain the interpretation of geophysical anomalies and guide modeling efforts
Geochronological data (radiometric dating of intrusions, alteration events) can help establish the timing of mineralization and its relationship to geophysical anomalies
Data Integration Techniques and Tools
Data integration techniques (geographic information systems (GIS), 3D modeling software) can facilitate the combined analysis of geophysical, geological, and geochemical datasets for target generation
Machine learning algorithms (unsupervised clustering, supervised classification) can assist in identifying patterns and anomalies in large, multi-disciplinary datasets
These tools allow for the visualization and analysis of complex datasets, helping to identify relationships between different data types and prioritize exploration targets
Integrated 3D models can incorporate geophysical, geological, and geochemical data to provide a comprehensive understanding of the subsurface geology and mineralization potential
Borehole Geophysics in Mineral Exploration
In-Situ Measurements and Characterization
Borehole geophysical logging provides in-situ measurements of physical properties (density, magnetic susceptibility, electrical conductivity, natural gamma radiation) along the length of a drillhole
Borehole logging can help characterize the lithology, alteration, and mineralization of the subsurface, providing ground-truth data for the interpretation of surface geophysical surveys
Density logging (gamma-gamma density tools) can help estimate the bulk density of ore zones and host rocks, which is essential for resource estimation and geotechnical studies
Magnetic susceptibility logging can identify magnetic minerals and assist in the interpretation of surface magnetic surveys, particularly in the presence of remanent magnetization
Electrical logs (resistivity, induced polarization) can help delineate conductive or chargeable zones associated with sulfide mineralization or alteration
Natural gamma radiation logging can identify potassium alteration zones or uranium mineralization, complementing surface radiometric surveys
Structural Analysis and Resource Estimation
Acoustic televiewer (ATV) logging provides oriented images of the borehole wall, allowing for the identification and analysis of structural features (fractures, veins, faults), which may control mineralization
Borehole geophysical data can be integrated with geological logging and geochemical assays to improve the understanding of the subsurface geology and mineralization, leading to more accurate resource estimation and targeting of additional exploration efforts
Borehole data can be used to calibrate and refine surface geophysical interpretations, improving the accuracy of 3D geological models and resource estimates
Borehole geophysics can also help in the planning and design of future drill programs by identifying high-priority targets and optimizing drill hole locations based on the integrated geophysical, geological, and geochemical data