Summary:
Bukit Besi mine comprises the East, Valley and West Deposits and is situated in the district of Dungun, state of Terengganu in Malaysia.
The Bukit Besi’s mining areas straddle the contact between Palaeozoic sediments and granite, which is presumed to be of late Cretaceous age. Granite tongues have intruded the sediments up to 100 m beyond the main line of the irregular contact. Additionally, blocks of shale have been caught up and lie within the body of the granite.
Almost all the magnetite skarn mineralisation at Bukit Besi occurs as replacements in the sediments along or within 100 m of their contact with the granite. Magnetite and haematite replacement can also be seen within the granite. Here, fragments of altered sedimentary rock in this ore suggest that the ore has completely replaced bodies of shale engulfed by the granite. The orientation of the mineralisation is controlled by NE–SW, NW–SE and N–S controlling structures.
SRK defined the 3D magnetite estimation domains based on geochemical and magnetic susceptibility data, with boundaries being defined by step-changes in magnetic susceptibility, Fe%, S, SiO2% and TiO2%. The 3D geometry was observed to be relatively consistent and predictable over the extent of the drill coverage, with reasonable continuity between drill holes, although pinching and swelling of the veins was evident in both down dip and along strike directions.
West
At the West area, the six mineralised veins have an average strike length of 350 m, a combined width of 75 m and dip sub-vertically. The wireframes extend from the surface to 90 m depth.
The main lithologies in the Project area are exposed in the northern half of the West pit. The primary unit in the pit is fine-grained calc-silicate hornfels (CSH). The northern wall of the pit exposes, from left to right, the bounding quartzite, Western magnetite skarn (SKM), actinolite-tremolite skarn (SKA), Middle SKM, SKA, Eastern SKM, SKA, sulphide skarn (SKS) and then a brecciated zone. The bounding quartzite dips between 65° and 80° towards the east. It has a sharp contact with the CSH and is outcropping in the north of the pit. Biotite granite outcropping is visible to the south and east of the pit.
In the pit, the CSH is identified as being fine grained, often silicified and light grey to green in colour. Due to its higher iron content, the SKA is a darker green to black and is coarser grained. Under hand lens, the fibrous radiating lathes of actinolite can be seen in the fresher rock. The later brecciation is polymictic, and matrix supported with angular fragments composed of granite, CSH and SKS.
Valley
The main mineralisation at the Valley area has an average strike length of 175 m. The veins are thinner (approximately 5 m) surface and in the north. Closer to the southern granite contact they are 25 m wide at surface, with the western vein bulging to 100 m wide at depth. The three new veins interpreted northeast of the main Valley mineralisation strike at N560E and dip steeply to the southeast. The veins vary in true thickness between 8 m and 15 m.
The lithologies at the Valley area are exposed by earthworks removal at the northern part and pit development at the Valley prospect. Recent surface mapping by Fortress geologists at the north eastern part of the Valley prospect led to a new discovery.
Drillhole logging indicates that the iron mineralisation is within two NNE trending deformation zones and is hosted primarily in SKA, with minor veins in CSH and in small intrusions of biotite-granite (GBG). The interpretation of the wider deformation zone being surrounded by GBG is supported by results of the ground magnetic and radiometric surveys conducted for Fortress in 2017, and recent drilling.
Most of the iron mineralisation logged in the drill core and chips is magnetite (SKM) with minor occurrences of haematite (SKH), pyrrhotite (SKS), and pyrite (SKS). The main SKM mineralisation follows the western edge of the southern deformation zone along its strike length. There are two veins 50 m apart, with an average thickness of 25 m, with the western vein bulging to approximately 100 m thickness at depth. Due to the lack of down-dip drilling intersections, this vein has been interpreted as being conformable to the deformation zone, thus trending NNE with a steep easterly dip.
In 2018, seven DD holes were drilled 50 m to the west of the two main veins and intersected magnetite mineralisation. These results were followed up in 2019 with eight drill holes, drilled to an average depth of 100 m. Only minor mineralisation was intersected; thus, development of a mineralisation model was not warranted at this location.
Approximately 150 m east of the existing Valley mineralisation, a line of drilling trending N-NE over a 900 m strike length was drilled in 2019. The 21 drillholes had an average depth of 95 m and were spaced 50 m to 100 m apart. Three drillholes northeast of the existing deformation zone intersected significant magnetite mineralisation. This mineralisation is interpreted as being continuous with an outcrop of SKS at the west side of drillhole BJ187. At this location, the vein is 5 m wide and hosted in GBG with argillic alteration. The three veins strike N560E and dip steeply to the southeast.
East
The eight mineralised veins at the East area have an average strike length of 200 m, width of 5 m and extend vertically from surface to 100 m.
In the East mining area massive magnetite mineralisation that has a sub-vertical contact with a 14 m to 23 m wide pyrrhotite vein is exposed. Locally, primary and secondary haematite mineralisation is also prevalent. Late-stage cross-cutting soft pyrite veins occur on the north of the exposure.
In 2018, mineralisation 250 m to the west of the main East area mineralisation was discovered – three massive magnetite veins exposed on surface with thicknesses between 1 m and 3 m.
In 2019, Fortress geologists significantly updated the interpretation of the East and Eastern East geology. At the East area, the main magnetite vein strikes northwest and dips steeply to the southwest. The veins are offset and truncated by predominantly N–NW structures. The mineralisation in the Eastern East area is more discontinuous and structurally complex. Fortress geologists mapped various outcrops of SKM, SKA, SKS and sandstone in the deposit area. The bedding plane orientation is N110-115E and dipping is between 45° and 65°. Recent drilling of five RC holes to test the vertical and lateral continuity of the massive magnetite veins on the southern part of the West area and northern part of the East area intersected mineralisation.