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Summary
The Carajás Mineral Province in Brazil hosts the highest concentration of major IOCG deposits in the world. Airborne and ground electromagnetic (EM) methods are frequently applied when exploring for these deposits using relatively high operating frequencies (25Hz - 30Hz). This frequency range limits the survey to effectively detect shallow, weakly to moderately conductive sources.
An airborne EM survey (VTEM) using a 30Hz base frequency has been flown by Falconbridge Ltd over the Pedra Branca deposit in 2005, identifying two discrete anomalies in the area. The high frequency nature of the VTEM system did not provide good depth resolution of the anomalies and follow-up ground EM surveys were conducted.
An orientation fixed loop EM (FLTEM) survey was carried out at Pedra Branca to compare the effectiveness of using lower frequency (2Hz) ground EM against existing VTEM and previous high-frequency ground EM surveys. The application of EM methods using frequencies of 2Hz and below enables the detection of late time conductors by recording much later in time compared to high frequency systems. These typically detect stronger, deeper conductors which are commonly related to higher grade and/or thicker massive sulfide mineralisation.
The results of this new survey significantly increased the resolution of the conductor. Modelling of the FLTEM data shows excellent correlation with the high-grade mineralisation surface, down to a depth of approximately 550m.
An FLTEM survey was also completed over the Azevedo prospect and provided a significant improvement in resolution compared to the historical, poorly defined VTEM anomaly. The FLTEM data provided an 80m lateral adjustment to the initial anomaly, allowed detailed modelling, and reduced drill testing uncertainty.
Ground and downhole EM surveys at lower frequencies are now being used extensively to effectively explore for massive sulfides with a greatly increased depth of investigation compared to historical airborne and ground EM surveys.