Ranger 3 Deeps – Orebody Knowledge and its Impact on Resource Modelling

摘要

The Ranger 3 Deeps (R3D) mineral resource is a structurally and lithologicallycontrolled uranium deposit localised by a NNW-trending zone of reversefaulting and deformation, the Deeps Fault Zone (DFZ). High-grade uraniummineralisation occurs within this zone when the right conditions of rockchemistry, breccia permeability and fault orientation are satisfied.The R3D uranium deposit was discovered in 2005 and has been drilledout in successive surface diamond drilling campaigns up to 2010. It is nowrecognised that the deposit, though hosted in similar stratigraphy, hassignificant differences in controls to that described in the existing model forthe Ranger 3 mineralising system observed in Pit 3, (some 400 m up-dip)where the host mineralised breccias were thought to be the product of astructurally modified ‘karstic collapse’ system. In 2012, a detailed structuralstudy utilising existing surface exploration drilling was commissionedby Energy Resources of Australia Pty Ltd (ERA) to develop a structuralstratigraphicunderstanding of the R3D system aimed ultimately at arationalisation of robust ore domaining for resource estimation prior to thecurrent R3D prefeasibility study.The majority of uranium mineralisation in R3D is hosted by an interconnectednetwork of brecciation developed within and around an upward-soling,brittle reverse fault system, the DFZ. The DFZ soling is controlled by thecompetency contrast of the local Cahill Formation mine stratigraphy. Thisstratigraphic sequence comprises lower mine sequence (LMS) carbonatesand upper mine sequence (UMS) chlorite schists and quartz-chlorite-biotiteschists (meta-arenites). The UMS chlorite schist (which hosts the majority of the resource) focuses the soling of the DFZ by acting as the weakest unitof the mine stratigraphy sandwiched between the underlying massive LMScarbonates and the overlying competent meta-arenites. This competencycontrast is hypothesised to directly reflect the depositional character of themine stratigraphy. Traditional use of a metamorphic lithology description,coupled with a reliance on geochemical alteration assemblage logging,has obscured the structurally important competency makeup of the minestratigraphy. The reintroduction of interpretive proto-lithology logging hasrefocused process analysis of the R3D system and has led to the currentstructural model.Structural logging to identify actual faults where movement was evidentand quantifying the associated ‘damage’ zones (where the uraninite ishosted) was also a key element in redefining the new structural model. Themapping of brecciation intensities and fault locations is now part of theroutine logging of drill core generated by the resource definition drillingprogram. The association of high-grade uranium mineralisation andbrecciation intensity is now unequivocal. In the core of the DFZ, multipleclose-spaced soling fault ‘strands’ coalesce these breccia zones to form themost continuous, highest grade parts of the resource that decreases up-dipas the system attenuates.Within the LMS carbonates, the DFZ is a steeply east dipping, NNWtrendingreverse fault that exhibits intense brecciation and silicification ofthe host carbonate. Significant high-grade uranium mineralisation occurswhere the fault intersects narrow (1–10 m thick) bedded, lenticular, highlydeformed chlorite schists within the massive carbonates. This mineralisationoccurs up-dip of the DFZ within the brecciated schist and suggests anascending fluid source that precipitates uranium when in contact with areactive lithology. Due to the limited drilling density within the LMS, morecomplex controls on mineralisation and constrained volumes of reactivestratigraphy when compared to the UMS, the highest confidence category inthe LMS resource is ‘Indicated’ within the current resource model.The resolution of the principal controls on uranium mineralisationhas allowed the understanding of the likely geometry of R3D. In turn,this has allowed increased confide