In between running magnetotelluric (MT) acquisition programs I recently managed to find some time to do a bit of inversion and interpretation work. The dataset I focused on was from the CF23 MT survey the Geological Survey of Queensland collected last year with the help of Geoscience Australia [link]. These MT profiles were conducted along the 14GA-CF2 and 14GA-CF3 seismic lines.  The results are in a newly submitted paper to the special edition of Exploration Geophysics about MT in Australia. The paper is titled Electrical structure and crustal architecture of the southern Mount Isa Province. The work images several conductive anomalies in the southern Mount Isa Province for the first time (Figure 1f).

I have also been modelling the Cloncurry Extension MT data [link], and a technical note and model release for this work is forthcoming. The modelling in the Cloncurry area (Figure 1b) extends our understanding of features previously imaged in inversion work [link]. The key to understanding the big picture relationships between the modelled conductive anomalies can be found in the new seismic interpretation report released late last year [link]. This is work I did with Karen Connors from UQ’s Sustainable Mineral Institute and Dominic Brown from the GSQ which provides a large-scale framework, with a consistent structural interpretation of all the historic deep crustal seismic across the Mount Isa region.

The key structure in the interpretation that helps us understand the importance and potential continuity of the modelled conductive anomalies is the Gidyea Structure. In the south of the province, the newly named Urandangi – Burketown Fault Zone (UBFZ) is also associated with conductive anomalies. These structures are both blind (meaning they don’t appear at the surface) but the seismic allows us to image them. The Gidyea Structure is at the interface between the Central Isa/Pitta Pitta and Numil basement terranes and the UBFZ is the contact between the Central Isa and Pitta Pitta basement terranes (Figure 1).

The Gidyea Structure and the UBFZ are both imaged on the 14GA-CF3 and 06GA-M6 seismic lines. There is a remarkable similarity in the electrical structure on these two lines despite them being over 100 km apart (note that the MT model along 06GA-M6 is from Jiang et al. 2019 and has a slightly different colour stretch). The Numil basement terrane is consistently more conductive than the Pitta Pitta and Central Isa basement terranes. There are two upper crustal conductors in approximately the same location on 14GA-CF3 and 06GA-M6, labelled C13 and C14 on Figure 1d. The western-most of the two structures on both lines appears to be contained within the Central Isa basement terrane between the UBFZ and the Black Mountain Fault. It also appears to be bounded on the west by the steeply dipping Pilgrim Fault. It is possible that the anomaly represents fluid movement through this highly faulted segment of crust.   

Stepping to the north around the Cloncurry region a comparison of the conductivity features from MT modelling to the seismic interpretation produces a different story. In this area, the upper crustal portion of the Gidyea Structure is conductive and there is a very strong mid-crustal conductor in the hanging-wall to the west (Figure 1b). Conductive finger-like structures extend from the conductive mid-crust into the more resistive upper-crust.

Comparison of the three different inversion models also offers an appreciation of structures able to be modelled from different resolution datasets. The CF2/3 data was collected at 10 km station spacing, the data along 06GA-M6 was collected at approx. 4 km station spacing, and the data along 07GA-IG1 is at 2 km station spacing. The black bar at the top of Figure 1b and the red bar at the top of Figure 1f are the same length. As you can see, even at the coarsest resolution it is possible to image the larger parts of these systems of interest, but the features modelled from the 10 km data are likely to resolve into more targetable features with a tighter station spacing. 

Based on the current modelling there are several unanswered questions. Are the features between 14GA-CF3 and 06GA-M6 continuous? Do the features imaged in the Cloncurry area extend further south, and if so how do they relate to the features on 14GA-CF3 and 06GA-M6? Which areas have interesting electrical features that warrant MT data acquisition at a similar resolution to that available around Cloncurry? The new CCAMT survey (Figure 2), kicking off in October will collect data to answer these questions. I’m so excited to see the results as they come in and look forward to sharing them with people towards the middle of next year! The survey is being supported by Geoscience Australia and will collect 340 new MT sites covering the Eastern Succession with a series of east-west profiles at approximately 10 km station spacing.

For more details, please contact Dr. Janelle Simpson.