The radiometric data (figure 2) shows the skarn zone as a band of dark colour (little to no radiometric response) sandwiched between the bright colours of the flanking granites. The radiometric signature is different for the two granites indicating a more potassic phase to the west. The meta-sediments to the north and south also show a low radiometric response.

Several filtering attempts were made in an endeavour to isolate the magnetic response of the skarn zone from the regional magnetic gradients. These filters included first vertical derivative (1VD), horizontal derivative (Hx) and automatic gain control (AGC). Some comparisons of the results of these operators are shown in figures 3 and 4. Figure 3 shows the application of AGC and 1VD which can isolate the anomaly on line 8,041,400N due to the skarn zone from the regional effects. In figure 4, the filter products for line 8,041,200N show a weak reflection of multiple anomalies probably due to several lenses of skarn, however, the discrimination is weak due to the attenuation effects of the (50m) flight height.

The grey scale image (figure 5) shows the skarn zone, as inferred from drilling, superimposed with the northern limb of folds coinciding with the magnetic trend. The southern limbs of the proposed structure are not evident in the data.

The nature of the trend, due to the variable magnetic properties along strike and the spacing of the flight lines. This presentation also indicates a possible extension of the trend to the north and south, but with reduced amplitude perhaps due to depth or plunge of the structure. Quantitative analysis (figure 7) of the airborne magnetic data of the Gillian prospect, was undertaken with the aim to use actual line data to ascertain if the airborne data could resolve the dip and altitude of multiple skarn lenses in a fold structure or define a series of lenses across bedding. However, modelling of the northern fold shows the change in magnetic character for each line is due to several effects,

*the overlap of anomalies from line to line *multiple sources in other cases

*the 20nT gradient due to the two granites confuses the profile with the Gillian prospect being virtually at the contact.

Conclusion

The quantitative evaluation of line magnetic data from an airborne geophysical survey of Gillian

prospect, Mt. Garnet, Qld. showed;

A discontinuous magnetic NE trend that corresponds with the trend of skarn mineralisation located between two granitoids, which have different susceptibilities causing a regional gradient, which obscures the detail of the skarn anomalies.

The magnetic trend, as shown by enhanced magnetic images, coincides with only one (north-western) limb of the inferred folds and this has been checked using actual field data. The data modelling supports the proposal that detailed ground magnetic traversing of the prospect would be necessary to evaluate the dip of magnetic lenses and define the zone of interest beyond areas of outcrop and to site future drill holes.

Recommendations

It is recommended that a detailed ground magnetic survey of the Gillian prospect be undertaken to allow assessment of the nature of the skarn bodies, whether folded or a series of lenses across the stratigraphy. The survey grid should be 1.5km long with traverses lines of 500m length orientated at (AMG) 135º – 315º, to cross normal to the trend. Readings should be taken at 2 – 5m spacing along traverses spaced 50m apart. A decision to record closer line or station spacings can be made, where necessary, after examination of initial data.