Generating 4D Geological Maps from Regional Geophysics

Examples: Lumwana and the Eastern Goldfields

Geological interpretation is still the basic skill underlying the mining and exploration industries. Historically, exploration activities have tended to be dominated by geochemical prospecting methods. But it is becoming more common to interpret results by applying a much broader geological understanding of the mineralisation processes than geochemistry alone allows.

The key driver for the exploration industry at present is the discovery of mineral deposits under cover, meaning hidden beneath surficial deposits or in the subsurface of other geological formations. The range and scale of datasets available to explorers and the way they are used is changing. A wide range of datasets from global remote sensing to local and regional geophysical surveys are now being tested in studies at local scale. Regardless of the type of data or the area it covers, all data interpretation requires sound geological reasoning.

Geological input is critical during drill targeting and resource estimation, where the primary risk factor is the effective utilisation of the underlying geological model. A resource model is a numerical expression of a geological entity; however the relationship between it and the underlying geology is rarely evaluated or communicated critically. Similarly, a geophysical or geochemical target requires evaluation in its geological context to ensure the drilling budget is allocated effectively.

Our understanding and visualisation of geological settings is greatly impacted by technology. Using advanced computer graphics and processing improves our ability to picture the geological model and, more importantly, to check the validity of our interpretations. Unravelling the complex geological history of an area to understand the interaction between mineralising events and structures that pre- and post-date them, is fundamental to exploration and resource geology.

SRK’s approach to targeting and resource geology has always been to apply the top geological skills to the best data using the most appropriate technology. In the early 1990s SRK partnered with Fugro to generate regional geological interpretations of Western Australia using high-quality aeromagnetic data. Serial cross-sections were used to control 3D continuity, and time factors were worked into interpretations of structures by analysing the timing and origins of the major structures in the Archaean Cratons. This work led to an understanding of the extensional nature of the sedimentary basins, and the recognition of structures associated with it. It was recognised that extensional shear zones are located at major granite margins and are associated with gold deposits.

Using geophysical inversion methods to construct three-dimensional geometry has further enhanced interpretation, and allowed us to expand regional potential field datasets to 3D with added confidence.

Leapfrog has greatly advanced SRK’s ability to relate detailed structural analysis to resource geology. Leapfrog can model both geochemical and geological information directly in three dimensions, which provides an immediate visualisation of the geometrical links between particular structures and alteration systems, and the grade of mineralisation.

Despite the expansion of tools, and the increased range of problems that can now be tackled realistically using computer models, the difficulty of generating high-quality geological interpretation remains the main limitation to applying modeling to the wide variety of geological problems at a range of scales. Geological interpretation is still the fundamental skill needed in exploration and mining. Fostering this vital skill within industry requires on-going training both at the institutional level and within the workplace for continued development of professional geologists.

Peter Williams:
Peter Gleeson:


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