The exploration target at Saxby is magmatic sulphide Ni-CuPGE mineralisation hosted within a large mafic-ultramafic intrusive complex. The target rocks are buried beneath 400m of younger and conductive sedimentary rocks and as such, exploration is challenging. The textural variations, presence of magmatic sulphide and olivine-rich nature of the rocks all indicate the possibility for potential economic massive sulphide mineralization to be present at Saxby.
Airborne magnetic, ground gravity, AMT and ground electromagnetic surveys were conducted before to facilitate mapping of prospective units under deep cover followed by diamond drill testing of selected targets. However, the large size of the gravity anomaly and the thickness of cover contributed to the mafic-ultramafic intrusive complex being a very difficult target to explore. Consequently, when the JESSY DEEP SQUID EM receiver became available it was deployed in the field.
The gold discovery was made in 2008 with drill hole SXD005. It intersected 17 m of mineralization grading at 6.75g /t Au at 631 m depth and includes a high-grade core of 5 m grading at 19.3 g/t Au from 635 m. This hole also intersected a second zone of lower-grade gold mineralization (7m at 1.98 g/t Au from 614 m). The clients consider the Saxby gold prospect an important greenfields discovery an named it Lucky SQUID.
Anglo American used JESSY DEEP to target massive sulphides Ni-Cu-PGE mineralization at depth and this technique has, for the first time, allowed the definition of strong EM anomalies that were never previously identified by conventional geophysical techniques. The advantage of a SQUID sensor over other EM field sensors is that the SQUID sensor can record late or weak EM fields that are below the back-ground noise (or “detection level”) of a conventional EM field sensor like a fluxgate magnetometer or coil. JESSY DEEP can be used with confidence to define drill targets where other geophysical techniques have failed or have only been partially successful.
Anglo American completed a 276.4 line km JESSY DEEP TEM survey over the main gravity anomaly in 2007. The survey highlighted several large, well-defined conductors in the basement, previously undetected. The results of the SQUID survey were very encouraging; with the system readily mapping strong bedrock conductors beneath at least 400 m of very conductive (200 Siemens) cover sediments.
The excellent data of the JESSY DEEP survey contrast markedly with the data of other geophysical methods in the same terrain. When the data of the SQUID survey on line 7,877,000 mN are compared with previous AMT data on the same line (Figure 3), the SQUID data clearly shows bedrock conductors that can be accurately targeted by drilling.
- Kneeshaw, A., Haker,A., Collaborative Drilling Initiative Proposal: Saxby Project, NW Qld, Anglo American Memorandum, 2008
- Anglo American Annual Reports 2007-2009
- Falcon Minerals Website: www.falconminerals.com.au