Integrated Petrophysical and Mineralogical and Structural Characterisation of the Golden Forty deposit, Tennant Creek, NT

Abstract

The Tennant Creek Inlier is geologically complex, and its geological history is subject to contrasting interpretations, which can be hard to resolve using conventional datasets. This integrated characterisation study provides co-located, quantitative, scale consistent assessment of the Golden Forty camp. Integrated characterisation data provide holistic insights into the coupling of metasomatism, structural development, metallogenic evolution, and geophysical expression, that would be almost impossible to decipher using any approach in isolation. The Golden Forty camp is comprised of numerous sources of magnetisation, which vary significantly in scale, and depth. These sources have a range of magnetisations as a consequence of several stages of alteration which precipitated magnetite in varying abundance and locally overprinted that magnetite with hematite. The style of alteration and its controls evolved in parallel with two distinct orogenic cycles, the Tennant and Murchison events, both of which followed a predictable evolution of: 1. sedimentation 2. ductile Inversion (folding/vertical extension), 3. ductile-brittle transpression, and 4. brittle transtension (extension). Early magnetite-chlorite alteration was controlled by WNW-trending strike-slip shear zones and focussed within WSW-trending structures, mainly beneath a shallow SW-dipping plane inferred to be an unconformity between the Warramunga and Ooradidgee groups. The chlorite ironstones only extend 100-200m beneath this horizon, suggesting it acted as a permeability control, a 'lid' on the system. This early Fe-metasomatism produced discrete WSW-trending zones of high magnetic intensity (e.g., GF East), but also broader stratigraphically controlled zones of much weaker intensity which define the two camp-scale anomalies at Golden Forty North and South. As the crust cooled ductile brittle transtensional tectonics caused vertical displacement and strike-slip movement on a range of fault orientations, displacing, and rotating the existing architecture. This resulted in earlier magnetic source bodies being cut up and rearranged, and greatly increased the complexity of the magnetisation at the camp scale. Structurally controlled Talc-hematite alteration, grading upward into Quartz-Fe-oxide alteration formed approximately at the Warramunga / Ooradidgee unconformity toward the start of the Murchison Event. The formation of Quartz Ironstones involved sub-grain-scale replacement of magnetite by hematite, which reduced magnetic domains, producing strong and highly stable remanent magnetisation, mainly in Quartz-ironstone. The Quartz Ironstones at Golden Forty North retains remanence sub-parallel to the Earth's Field, whereas those at Golden Forty East produced south oriented, shallow upward to sub-horizontal W- or E- oriented remanence directions, which appear to be paleomagnetic noise from pencil magnets. In general, significant remanence appears to be volumetrically minor and are unlikely to present a major problem for modelling. Directions from Golden Forty North may also suggest remanence may have been acquired (at least in part) due to relatively recent oxidation. Although the Golden Forty South anomaly is large and considered a bit of a 'geophysical mystery', there is not much evidence to suggest atypical geophysical phenomena are to blame. Modelling Golden Forty is difficult primarily because the geology and the petrophysical evolution of the system is complex. The importance of remanence, as implied by previous studies, is likely over estimated. Conversely, the importance of structural controls and the unconformity (i.e., the 'lid' of the system) may not have been fully recognised. Magnetisation and mineralisation at Golden Forty South should have the similar controls to Golden Forty North. It would likely sit in the same stratigraphic horizon and extend 100-200 m beneath the hypothesised unconformity and would require early magnetite localised at the junction of WNW- and WSW- oriented shear zones, overprinted by later faults, that upgraded the mineralisation, forming a quartz ironstone. Ultimately the coupling of specific metasomatic events with specific structural controls, focussed mineralisation at favourable sites, but that metallogenic evolution generally progresses toward magnetite destruction. Therefore, structural controls should be more predictive of mineralisation than magnetic bullseye targeting. Consideration of magnetic zonation and radiometric properties in conjunction with the confluence of the outlined structural controls, however, should prove fruitful for targeting in the TCMF.