Geophysical detection of hydrothermal alteration footprints

Presented at AEGC2018 by John McGaughey, President, Mira Geoscience

The use of geophysical data is appealing when exploring deep or under cover because, although it does not directly respond to rock chemistry, it often provides uniform areal data coverage. In deep and undercover exploration contexts, direct recognition of footprint-scale hydrothermal alteration from geophysical data is the ultimate goal of geophysical interpretation—there cannot be any expectation of a direct ore deposit signature in exploration data, and understanding footprint-scale alteration signatures can lead directly to targets.

The key to geophysical recognition of alteration at the ore system scale is the assumption, typically met in practice, that the primary control on physical property variation across the system is formational and structural, with hydrothermal alteration a contributing secondary effect. Specialized interpretation workflows, such as described in the article above, can take advantage of this assumption to create physical property models composed of primary (formational and structural) and secondary (alteration) physical property signatures that are fully consistent with geophysical data and whatever level of geological data is available. The secondary physical property signatures are, in many cases, directly interpretable in terms of hydrothermal alteration domains.

3D geological model.

Magnetic data.

Case study: The Mutooroo Iron Project area, Australia

This is a great example of developing a geologically-based magnetic model using a variety of interpretation, modelling, and inversion techniques. Due to limited constraining data on the magnetic units, construction of the starting model was based predominantly on interpretation of magnetic data. In this instance, the goal was to first develop a geologically plausible 3D representation of homogeneous magnetic domains beneath non-magnetic cover that explains the majority of the measured magnetic response. This was supported by the sparse geological data and magnetic susceptibility provided by drilling under cover. The robustness of the magnetic domains are validated by assigning a homogeneous susceptibility to each domain, forward modelling, and observing a good correlation between the predicted and measured magnetic data. A final stage of inversion to solve for local susceptibility variations within the domains highlights magnetic anomalies that may be associated with alteration and therefore become potential targets or areas of geological complexity that require further investigation. This geologically-based model, consistent with geological constraints and geophysical survey data, provides a basis for confident decision making in technical and business realms, with the ability to adapt and accommodate new and evolving information as it becomes available.

Glenn Pears – Principal Geophysicist with Mira Geoscience

Glenn joined our team a little over 15 years ago! Glenn worked closely in the above project with his colleague James Alderman. Glenn is a highly experienced geophysicist providing expertise in data integration and interpretation projects. His strengths are in geologically-constrained geophysical interpretation and inversion using GOCAD Mining Suite, data assessment, QA/QC and executing integrated interpretations.

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