Geophysics Pre 2018

Fathom undertook reprocessing of the located airborne geophysical data collected by the Federal Institute for Geosciences and Natural Resources, Federal Republic of Germany in 1986 and applied modern filtering routines to the gridded data; generating outputs far superior to what was previously available. Fathom. The re-processing and filtering highlighted geophysical anomalism within the survey area that is comparable to responses over known porphyry related deposits (Bata Hijau, Grasberg, Alumbrera).

The located data needed considerable pre-processing prior to making final grids to be filtered. Profiles were manually analysed and quality control measures applied [de-spiking, removal of ‘turns’ and deviations from an acceptable altitude range, de-corrugation]. The EM data were poor but the radiometric and magnetic data were able to be brought up to an acceptable standard for filtering; notwithstanding limitations on positional accuracy and signal noise.


Figure 1: Pseudogravity magnetic image. Data within EL 03 and EL 04 displayed. The ‘close-up’ image on the right highlights the magnetic anomalies well. A red response indicates a stronger magnetic signal. The general outline of the large magnetic complex is indicated; and the elevated responses around the ‘rim’ of the complex is highlighted well. Prospects have been annotated.

A good image of the magnetic response within EL 03 and EL04 is presented in Figure 1. The pseudogravity filter involves both pole reduction and vertical integration; which can be thought of as effectively shifting anomalies over their sources and removing the dipolar response of the field [assuming induced magnetization]. The result is a smoothed and simplified version of the magnetic response over the area. The large circular magnetic complex is well highlighted, with the strongest response on the rim of the complex at Melilup.


Figure 2: Combined residual magnetic image. Data within EL 03 and EL 04 displayed. The ‘close-up’ image on the right highlights the magnetic anomalies well. White in the image reflects a strong magnetic response in all residuals – shallow, intermediate and deep; indicating a higher likelihood at that location of depth extensive sources [ie, not just a shallow response]. The general outline of the large magnetic complex is indicated. Prospects have been annotated.

Two more useful magnetic images are presented in Figures 2 & 3, and the radiometric response in Figures 4 & 5.


Figure 3: Radial symmetry [intrusion detection] magnetic image. Data within EL 03 and EL 04 displayed. This filter highlights positive magnetic anomalies that are both radially symmetric and exhibit a strong amplitude. The minimum radius = 800m, which corresponds to a diameter range of 1.6km to 3.2km. The elevated discrete responses around the rim of the main magnetic complex are well highlighted using this filter.


Figure 4: Residual Potassium image. Data within EL 03 and EL 04 displayed. The radiometric data was difficult to level, so a broad residual has been applied to the data grid improving the delineation of discrete anomalies. The ‘close-up’ image on the right highlights the zones of elevated Potassium well. The black polygons outline the most anomalous responses; mostly around the rim of the large magnetic complex [which could be multiple features with rims].


Figure 5: Ternary radiometric image, where Potassium = Red, Thorium = Green and Uranium = Blue [standard combination]. The residual element grids were used in the ternary display [residual 200-1km]. Unfortunately the data are not of a quality high enough to manipulate the element ratios or generate useful radioelement ratio ternary images. This standard ternary image does however, highlight the anomalous radiometric zones well. It is the elevated Potassium polygons that have been overlain on the ‘close-up’ image on the right.

The Potassium and ternary radiometric images shown in Figures 4 and 5 respectively are useful for highlighting anomalism. There is a reasonable spatial correlation between discrete elevated magnetic and radiometric anomalies. This observation was combined with an analysis of the SRTM 30m topography data [subtle circular to elliptical depressions specifically] to carry out preliminary targeting runs to identify locations where favourable magnetic, radiometric and topographic responses coincide. Being highly aware of the variable magnetic response of porphyry systems and the limitations of the data by virtue of its vintage; a set of target areas were defined for ground follow-up. The results are presented in Figure 6.


Figure 6: Geophysical target zones. The geophysical and remote sensing data were used to generate exploration target zones. The porphyry Cu-Au model of Koschke [2011] was used. Specifically, the following layers were combined: MAGNETIC radially symmetric highs, RADIOMETRIC potassium highs, TOPOGRAPHY radially symmetric lows. Targets were categorized into one of three priority classes, with priority-1 being the highest priority targets.