The Mt Weld inversion model indicates the core of the Mt Weld...

The Mt Weld inversion model indicates the core of the Mt Weld feature is non‐magnetic and surrounded by
a magnetic annulus with variable magnetic susceptibility (possibly indicating variable depth of weathering).
The depth to top of the magnetic component of the Mt Weld feature appears to vary from ~200‐300 m
around parts of the northern half of the annulus to ~400‐600 m for the eastern and western extremities of
the annulus. These depth to top estimates are based on parts of the model with similar magnetic
susceptibility and are only indicative of the variations in the top of the magnetic lithologies; i.e. the
magnetite and or pyrrhotite content.????
The broken up appearance (variable magnetic susceptibility) of the upper parts of the model are probably
due in part to the low resolution, 400 m spaced line data used to generate the model, i.e. a more detaileddata set (e.g. 100 m spaced line data) should result? ?in a higher resolution, relatively more continuous and
smoother susceptibility model.??
The Mt Weld magnetic source appears complex in 3D. This suggests that 2D forward modelling technique
will have limited value in this situation.???
A weakly to non magnetic core is a prominent feature in the Mt. Weld TMI image and 3D model and is
considered to be a fairly reliable (within the limitations/resolution of the magnetic data) representation of
the actual distribution of magnetic material within the carbonatite intrusive system.? ?With the absence of
geological control, it is not known if this non magnetic core is a phase of the intrusive or country rock.????
If it can be sourced, drilling information from the magnetic and non magnetic parts of the carbonatite
system would be very useful for determining the actual depths to top of the magnetic source and the
relationship between the high magnetic susceptibilities and the REE mineralization. Unfortunately no
relevant drilling information could be sourced at the time of writing.


C O M A P R I S I O N O F 3 D M O D E L S
The two main differences between the Mt Weld and Mt Barrett magnetic features are their size and
magnetic susceptibility. The 3D magnetic models indicate Mt Weld and Mt Barrett have ~3 km and ~1.5 km
diameters respectively and maximum magnetic susceptibilities of ~0.30 SI and ~0.08 SI respectively.
The 3D model for Mt Weld indicates depths to top that range from ~200‐600 m below surface and the Mt
Barrett 3D model indicates depths to top between ~300 m and 500 m. These indications are somewhat
subjective as they depend on the magnetic susceptibility chosen to define the tops of the features.
However, both models show the bulk of the magnetic components of each feature are? ≥?250 m and show
variable depths to top. For both models, the depth to top/diameter ratio is less than ~1/3. This ratio is
favourable for constraining both the diameter and depth to top in the modelling process.??
The relatively small size of Mt Barrett may be contributing to the difference in the shape of the two models.
The Mt Weld 3D model shows a well developed, depth extensive non‐magnetic core surrounded by a
magnetic annulus with varying magnetic susceptibility and or depth to top. The Mt Barrett 3D model also
shows a similarly variable magnetic annulus although the magnetic core is far less well developed,
particularly at depths beyond ~500 m below surface. This may be an accurate reflection of the local
geology; e.g. the interpreted intrusive system is not as distinctly layered and differentiated as the Mt. Weld
system.? ?It also reflects the limitations or the magnetic data and modelling at these depths; i.e. an inability
to resolve magnetic sources with horizontal separations less than their distance from the magnetometer.
This is indicated in the east‐west cross section 69841970N ( Appendix C) where the non‐magnetic core in
the Mt Barrett model starts loosing definition and blending into the side wall of the annulus at ~700 m
below surface, i.e. at a depth of approximately the inner diameter of the magnetic annulus. Put simply it is
beyond the resolution of the magnetic data and the modelling process to reliably define magnetic
susceptibility distributions and depth extents beyond depths of ~600 m.? ? ? Geologically, a non magnetic
phase of the Mt. Barrett system may extend to considerable depth, similar to the situation inferred for Mt
Weld.Overall the 3D models indicate that the Mt. Barrett and Mt. Weld magnetic features both have variable
depths to top of? ≥? 200 m and variable magnetic susceptibilities. The models also in indicate the two
magnetic features may have similar basic geometry.???
It is strongly recommended that geological / drilling information from the Mt Weld be sourced to help
assess and interpret the 3D magnetic model. Ideally, this should clarify the relationship between the
magnetics and the REE distribution within the carbonatitie and help plan drill testing of the Mt Barrett
system and magnetics.??The limited available drilling in the Mt. Barrett area should also be reviewed.