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Recognising geological similarities to their gold-barite mines on Wetar Island, Billiton explored and drilled the Lakuwahi prospect during the period 1988-1990. Despite clear indications of a very large mineralising system, and an 80 percent hit-rate in scout drilling, historic low metal prices and a corporate move away from gold found Billiton discontinuing the exploration programme.
Available geological evidence indicates that Lakuwahi is situated within the caldera of an extinct submarine volcano. A large, strong hydrothermal mineralising system developed within the caldera, which has been estimated from surface geology, geochemistry and aeromagnetic data to be approximately 20 square kilometres in area.
Mineralisation styles recognised at Lakuwahi include thick zones of lead-zinc-silver sulphide breccias, and, gold associated with massive barite. While both of these styles represent attractive exploration targets, the geological setting also lends itself to the formation of a third style, sea-floor massive sulphides, which usually contain high grades of copper, lead, zinc, gold and silver.
Significant drill intersections from the Billiton scout drilling include:
- LWD 02: 1 gram per tonne gold over its entire length of 73.9 metres including
- 2 metres at 8.1 grams per tonne gold
- 10 metres at 1.4 grams per tonne gold, 63 grams per tonne silver and 0.52 percent copper
- LWD 07: 1 gram per tonne gold over its entire length of 67.4 metres, including
- 4 metres at 2.3 grams per tonne gold
- 3 metres at 2.4 grams per tonne gold
- LWD 06: 4 metres at 2.5 grams per tonne gold, 150 grams per tonne silver, 3.1 percent lead and 3.7 percent zinc.
- LWD 08: 31 metres at 1.2 grams per tonne gold, 2.9 percent lead and 1.8 percent zinc.
Robust has currently identified three prospects within the Lakuwahi Caldera so far (Batu Mas, Batu Hitam and Batu Perak) and is working to identify further targets through geological mapping/sampling and planned electrical geophysical surveys to be conducted later in 2009.
Prospect Locations - Lakuwahi Project
Robust is continuing its Romang drilling programme with two owner-operated diamond drilling rigs, and construction of a third rig is has been completed and will be used to accelerate exploration further.
Robust also recently completed initial metallurgical testwork at the Batu Hitam and Batu Mas Prospects. The highlights of this testwork on 11 composite oxide samples were:
Details of the composite samples and metallurgical results are provided in the table below.
Detailed Results of Lakuwahi Metallurgical Testwork
These metallurgical tests are the first in a series of tests of increasing sophistication that will ultimately lead to the selection of the optimum process flow sheet for the Lakuwahi ore. The results are positive in that they clearly indicate that the near-surface Lakuwahi ore is free milling and that it will not require expensive treatment options such as a floatation and pressure oxidation process. It should be noted that the ores treated at Billiton's gold mines on nearby Wetar Island (Kali Kuning and Lerokis - now mined out) were also free milling and achieved very high gold and silver recoveries with a very simple and relatively inexpensive plant.
Based on the Lakuwahi results there is reason to expect that treatment of Lakuwahi ores will be low-cost, allowing the profitable mining and treatment of low grades.
Geophysics
Previously reported 3D modelling of the airborne magnetic has confirmed the existence of a large zone of low magnetic susceptibility which is interpreted to be due to a large (6km x 4km) magnetite-destructive hydrothermal alteration system. Much of this system is preserved beneath post-mineral reef limestone. As can be seen in the figure below, drilling has tested only a very small part (<0.2%) of the target system.
Contours of magnetic susceptibility from 3D modelling showing the large 13 km 2 alteration system within the Lakuwahi Caldera. Billiton drilling (in black) and Robust drilling (in red) has tested less than 0.2% of this system
To put the current exploration programme in perspective, only a very small proportion of the prospective Lakuwahi Caldera has been drilled. Interpretation of the remodelled magnetic survey data indicates a mineralising system approximately 13 km 2 in extent. Robust's drilling to date has only tested less than 0.02 km 2 or less than 0.2% of the system. In addition to systematic follow up of the positive drilling results obtained to date, Robust is planning an extensive geophysical programme including 3D induced polarisation (IP) to help refine additional targets within the Caldera. |