The construction of plausible geological models
The 3D interpretation and modelling process is of a cyclical nature; where testing geological ideas with potential fields forward modelling and using inversions to update parts of a geological model is crucial. We had a chat with Dr. Mark Duffett and Daniel Bombardieri from Mineral Resources Tasmania (MRT) on the importance that cross-validation between geological ideas and geophysical data had when developing the 3D model of Alberton-Mathinna “Gold Corridor”, northeast Tasmania. They also shared how GOCAD Mining Suite (GMS) and its Potential Fields Module (VPmg) was integral to coming up with a plausible model. Here is an excerpt:
Q: Can you provide more details about the workflow you used?
Daniel: Initially, due to complexity of Tasmanian geology, we constructed our [geological] model explicitly. Using [the Potential Fields Module], we start with forward modelling to determine how reasonable our reference geological model is. We then conduct homogeneous inversions to get a handle on bulk rock property information, and a series of geometry inversions producing calculated magnetic and gravity responses which agree with observations. This process is iterative. We follow a workflow, but this will vary depending on what we’re trying to achieve. If new geological information becomes available, we test this and go through the process again with the aim of always trying to produce a reasonable geological model which satisfies the geophysics.
Q: Did the forward modelling and inversion results push against the geology
Daniel: In the case of the Alberton–Mathinna 3D model we were fortunate to have a granitoid model produced from previous 2D forward modelling attempts. We used this granitoid surface as a reference with the aim of slightly varying the geometry. Remember, current generation software allows us to truly model in the 3D domain, and at higher resolutions, compared to previous generation software. So yes, GMS and its potential fields module allowed us to produce a more detailed granite surface.
Mark: What we’ve largely come up with so far is: there’s been more second order and adding details to the existing 3D model of previous work; and that itself, there is value in that. The [cyclical nature of the workflow] did test and challenge the interpretations, but it’s not like we are coming into virgin territories here.
Caption: Post inversion granitoid volume (all plutons, pink) with superposed magnetic plutons from initial modelling (salmon) and historic gold occurrences plotted at the ground surface (yellow spheres). Granitoid below gold deposit points is generally 1-3 km.
Courtesy of Mineral Resources Tasmania
Q: Can you describe how GMS was valuable for dealing with sparse data?
Daniel: I think that one of the key abilities of GMS and the [Potential Fields Module] is its ability to deal with sparse data. This is why we use it. Mostly we just have cross sections with very limited drillhole control. Validating the geological model using geophysics… I think is the only way you can actually handle sparse data. That’s why GOCAD and VPmg are important tools to use in this regard.
Mark: Referring to sparse data was in the sense of having very little drilling control, particularly deep drilling control.
Q: What were the key revelations or realizations regarding the geology model? and Has the modelling heightened prospectivity of the area?
Mark: The homogeneous unit inversion was quite critical in characterizing the bulk signature of the geological domains. It highlights a definite need to subdivide the intrusive granite in some areas. This was not identified previously and the economic and exploration significance of this needs to be investigated. The modelling also impacted the geometry of the granitoid volume which leads to the generation of new exploration targets.
The plan going forward is to bring 3D modelling generally in sync with our traditional Geological Survey mapping and map updates. In these areas it is virgin terrain and there is an expectation of more insight from the process of 3D modelling.
Caption: Geological volumes, with one country rock unit (Sideling Sandstone) removed to reveal 3D fault surfaces.
Courtesy of Mineral Resources Tasmania
Q: Has using GMS facilitated those findings?
Mark: The whole exercise got value in seeing geological concepts that work and don’t work when tested as 3D. Insight start to come even before the geophysical modelling starts. It allows the construction of plausible geological model for which concepts on single cross-section would not have been tenable.
Daniel: Yes, it’s important to remember that geology is 3D in nature and GMS coupled with its VPmg potential field module was crucial in creating the new Alberton-Mathinna 3D model and providing new insights into the geometry of geological objects at depth.
After studies at the Universities of Adelaide and Tasmania, Mark Duffett has worked at Charles Darwin University, the Northern Territory Geological Survey and the University of Tasmania on projects ranging from saltwater crocodile nesting habitat to regional potential field acquisition and interpretation in the African Copperbelt. Since 2009 he has been Senior Geophysicist at Mineral Resources Tasmania, the state's geological survey.
After completing a PhD in physics at the University of Tasmania in 2008, Daniel Bombardieri was employed as a geophysicist with Mineral Resources Tasmania to develop high precision three dimensional geological models and validating these against existing magnetic and gravity observations using 3D potential field inversion codes. (e.g., GOCAD® and Geomodeller TM inversion codes.)