Pseudosection and modelled profile sections of the Kingbig prospect (30 k)

The Kingbig geophysical prospect in central NSW was surveyed using time domain Sirotem Mk 3 equipment in May 1996. The main survey line (11600N) transects a localized EM anomaly of 250 metres strike length oriented slightly east of north. The initial detection of the feature was through regional geochemistry with high arsenic and gold values being detected in soil samples from local drainage. A small 25-30 nT magnetic anomaly is noted across the anomaly centred at 10800E. No mapped outcrop is noted in the area although regional aeromagnetics suggests the flank of a buried granitoid exists 3.2 kilometres to the south. Due to radiating magnetic features, minor sub-surface extensions towards the anomalous area are postulated.

The electromagnetic survey was undertaken in May 1996 along a series of east-west traverses. The survey used Sirotem Mk 3 equipment in In-Loop configuration traversing the survey lines with 50 metre sampling and 100 square metre transmitter loops. 50% overlap between transmitter setups was used. Currents up to 12 Amps using a SATX transmitter unit were obtained. The Sirotem was operated with a standard stacking of 512 samples per station and with composite frequency (53 channels) being recorded.

All results were processed using the EM Vision software visualization and analysis package. Data was read directly into the software with repeat readings being averaged. Both the late time asymptotic and 'Spiker' algorithms were used to convert the data from measured voltages to Apparent Resistivities. A resistivity parasection derived from the Spiker algorithm is shown above (see available graphic). This section clearly indicates a buried conductive sequence rising to a depth of approximately 40-45 metres. The depth of weathering in the prospect vicinity is known to be 40-50 metres below surface.

Decay analysis for stations located at 10650E and 10700E both suggest a conductor with time constant of approximately 12 Siemens contained within a host of high resistivity (800-1900 ohm-metres).

Inversion modelling results undertaken using the GRENDL program on line 11600N can be seen in the above section. A three layer seed model was used to initially represent the geoelectric section. Error analysis within the program indicate that a near surface conductive layer was well defined and the host resistivity and thickness results converged with errors of less than 2%. The conductive unit rising from basement appears less well resolved but still with errors less than 15%.

The high accuracy of the solution for this anomaly gave confidence to the depth and conductivity for the basement unit. Shallow drilling sited at 10850E and angled to the west intersected a highly fractured granite unit with significant mineralised veining. Low level sulphide mineralisation (chalcopyrite with minor small grain pyrhotite) associated with sub-economic copper and native gold where noted in the drill cores.

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Grid Protem 47 Arnhem Land Prospect (37 k)

This case study examines data acquired in an electromagnetic survey undertaken in the Arnhem Land of Northern Territory, Australia. Geologically, the area is located on Kombolgie Formation sandstones which lie unconformably above basement rocks of metamorphics, gneiss and granite. The Kombolgie can be up to 80-100 metres thick at this locality.

The area is highly prospective for uranium deposits and these are typically related to alteration halos. The prospect area was surveyed by regional magnetics, radiometrics and airborne time domain EM. Reconnaissance follow-up and geochemistry was undertaken to assess the possibility of a prospective alteration zone.

Geophysically, the Kombolgie sandstones are highly resistive (7-12,000 ohm-metres) but the basement rocks have resistivity between 1500-4000 ohm-metres. The aim of the EM survey was to investigate the area's electrical structure and test for the presence of an alteration zone which was thought to decrease the resistivity both in the basement and Kombolgie sandstones.

The survey was undertaken with a Protem 47 instrument along several east-west traverses. Separation between transmitter and receiver loops was at 50 metres and data was collected at 50 metre intervals. The logistics of the survey were difficult due to the plateau in the area being incised deeply along weathered joints and faults. This is a common feature of weathering in the Arnhem Land region. Preliminary tests undertaken by the contractor indicated that the decay rates for the area were extremely high and so a high frequency of 250 Hz was used.

The data after acquisition was converted to normalised voltage units (nV/Am2) and reformatted to AMIRA file format. The readings were then input to the EM Vision visualisation and modelling software package

Stacked profiles combined with a contour display of Channel 9 (0.0451 ms) are presented. The ultra-high frequency used for this survey by the Protem 47 has decay gates ranging from 0.00685 to 0.701 ms. The contour response clearly indicates a conformable zone of reduced resistivity. At later channels (beyond Channel 12 - 0.105 ms) the low resistivity feature breaks down as the 'smoke rings' dissipate and travel beyond the anomalous zone

Inversion modelling of line 6267600N provides a better understanding of the resistive nature of the Kombolgie sandstones above a more conductive 'basement'. The target low resistivity alteration zone is located between 474800E-475050E. Modelling used the GRENDL program which converged accurately with a three layer seed model to having final errors less than 12%.

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