Summary:
The Buffalo Reef and Selinsing gold deposits are thought to be mesothermal lode gold deposits, with auriferous quartz-pyrite- arsenopyrite±stibnite veining, with associated hydrothermal alteration. The two deposits are structurally controlled and thought to be part of the same structural trend, with the Buffalo Reef deposit occurring along strike and to the north of the Selinsing deposit.
Selinsing.
The Selinsing gold deposit is hosted by a 20 m to 100 m thick shear zone that dips 55° to 75° towards mine grid east (082° true grid). This zone or “envelope” of sheared rocks has been variably mineralised and intruded by gold-bearing quartz veins and stockworks. The quartz veins are likely to have been emplaced along fault surfaces, which are thought to be reverse thrusts caused by compression from the east. Strike-slip movement is not thought to be significant; however, a north-westerly trending structure, which post-dates the gold mineralisation, has been identified and may have a strike-slip component. The host rocks for the Selinsing shear zone consist of a series of finely interbedded argillites and very fine-grained arenites, along with sequences of quartz-rich, variably silicified sediments of likely tuffaceous origin, which are referred to as “felsic tuff” and a few thin beds of quartzite conglomerate. These host rocks are collectively known as “the mine sequence series”.
The true thickness of the mine sequence is not well understood. One interpretation is that the mine sequence has a true thickness of about 200 m; however, the footwall contact is not well defined, and it is difficult to distinguish between the mine sequences in the field due to the fine-grained nature of the host rocks. A second interpretation is that within the shear zone, repetition of these units by shearing creates a structural thickening of the sequence.
The hangingwall rocks at Selinsing are a distinctive sequence of predominantly competent, well bedded, dark-coloured limestones. Towards the base of the limestones is a narrow unit of black, well-bedded carbonaceous shales, which may be calcareous in places. The contact of these hangingwall units with the mine sequence below is thought to be a tectonic or faulted contact due to the unconformable nature of the bedding on either side of the contact. The contact itself is characterised by large water-filled clay-lined cavities. The footwall contact of the mine sequence is poorly understood as the base of the mine sequence has not been extensively explored. However, the footwall rocks consist of the same type of grey-black limestone as found in the hangingwall and it is thought that the footwall rocks are the same as the hangingwall, repeated due to faulting. This would mean that the less competent mine sequence rocks were more deformed by shearing due to rheological contrasts between the limestones and the argillites and arenites. The hangingwall limestones have locallydeveloped folding resulting from easterly compression.
Gold and sulphide mineralisation are associated with these rock types as well as intensive replacement by quartz and calcite gangue minerals.
The gold mineralisation at Selinsing is hosted within a shear zone that strikes at 350° and dips 60–70° to the east, with the higher-grade mineralised shoots within the main mineralised shear plunging to the southeast. The main shear zone is hosted within a sequence of felsic tuff and very fine clastic argillite with calcareous material and limestone in the hangingwall. High grade mineralisation is often associated with quartz stockworks and quartz-carbonate veins within highly deformed sedimentary rocks. Pressure and temperature studies on fluid inclusions in quartz from veins suggest that the mineralisation formed at a temperature in the order of 200–350°C and at a depth of between 2 km and 5 km.
The gold at Selinsing is generally in the form of fine-grained gold particles (<20 µm) commonly associated with pyrite and arsenopyrite and rarely with chalcopyrite. Visible (mm-scale) gold, although not common, occurs in quartz veins within the shear zone. The higher-grade quartz veins can be over a metre in true thickness and have been traced up to 300 m along strike and 200 m down dip. Lower grade gold mineralisation occurs as finely disseminated gold within intensely deformed envelopes around the quartz veins within the shear zone. Disseminated pyritisation also occurs within the deformed country rock within the shear zone, with the presence of euhedral arsenopyrite as a good indicator of elevated gold grades.
Buffalo Reef.
The Buffalo Reef deposit occurs approximately 1 km to the east of the Raub-Bentong suture. The area is dominated by argillite and limestone of Permian age to the east, with conglomerates and sandstones of Devonian age to the west. Low grade regional metamorphism up to Greenschist facies (locally up to Amphibolite facies) occurs throughout the area. The sediments have subsequently been intruded by granitic bodies of approximately Jurassic age. These intrusive bodies occur to the east of Buffalo Reef and generally form elevation highs.
The dominant structural feature present is a 200 m wide, north-south striking shear zone, with an apparent sinistral sense of displacement, which parallels the tectonic Raub-Bentong suture to the west. The shear zone is composed of graphitic shale with minor interbedded fine-grained sandstone and tuffaceous rock. Bedding within the sediments typically dips 65–75° to the east and strikes towards a bearing of 330–360°.
Gold mineralisation at Buffalo Reef is structurally controlled and associated with Permian sediments within a 200 m wide shear zone that parallels the north-south trending Raub-Bentong suture. Mineralisation occurs over a total strike length of approximately 2.6 km. Rocks within the Buffalo Reef shear zone have typically undergone silica-sericite-pyrite alteration to varying degrees.
The gold occurs within moderately to steeply east-dipping veins and fracture zones, which range in thickness from 1 m up to 15 m in thickness (average thickness is approximately 10 m in the main mineralised veins), although local flexures in the veins can host mineralisation up to 25 m in thickness. Veins, which are boudinaged in some areas, are generally composed of massive quartz with 1–5% (by volume) sulphide minerals, namely pyrite and arsenopyrite, along with varying amounts of stibnite. The stibnite generally occurs in association with elevated gold grades; however, the presence of gold does not necessarily indicate high stibnite levels (i.e. the stibnite tends to be associated with gold, rather than the gold being associated with stibnite).