Selinsing, Buffalo Reef
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The Selinsing and Buffalo Reef deposits are distributed over a NNE trending, 4.5 km long interval, located at Bukit Selinsing Koyan, ~65 km north of Raub and 30 km west of Kuala Lipis on the Raub-Bentong Suture in NW Pahang State, Malaysia, ~120 km NNE of Kuala Lumpur (#Location: 4° 14' 55"N, 101° 47' 12"E).
Peninsular Malaysia has been divided into two main regional domains, the Gondwana-derived Sibumasu (Western) and the Indochina-related Manabor (Eastern) terranes, separated by a major shear zone the Raub-Bentong suture zone.
The Sibumasu Terrane includes parts of western Yunnan (the Baoshan and Tenchong Blocks), the Shan States of Burma, northwest Thailand, Peninsular Burma and Thailand, western Peninsular Malaysia and northwest Sumatra (Metcalfe, 1988).
Peninsular Malaysia to the east of the Bentong-Raub Suture, has a different pre-Jurassic tectonostratigraphy and evolution to the Sibumasu terrane, and is regarded as a southwards extension of the Indochina Terrane (Metcalfe, 1998). This terrane is bounded to the NE by the Song Ma Suture Zone, and to the west by the Uttaradit Nan-Sra Kaeo and Bentong-Raub sutures in Thailand and Malaysia, respectively. It is taken to include what has previously been referred to as the “East Malaya Block” (excepting Borneo) of Stauffer (1974, 1983) and Metcalfe (1988).
The Bentong-Raub Suture zone represents remnant segments of the main Devonian to Middle Triassic Palaeo-Tethys ocean. In Peninsular Malaysia, it has traditionally been recognised as an ~20 km wide zone bordering the eastern limit of the Main Range granitoids and comprises mélange, oceanic ribbon-bedded cherts, schist, and discontinuous, narrow, elongate bodies of serpentinised mafic-ultramafic rocks, interpreted as ophiolites (Hutchison, 1975, 1989; Tjia, 1987, 1989). Palaeotethyan oceanic ribbon-bedded cherts, preserved in the suture zone, range in age from Middle Devonian to Middle Permian, with mélanges that includes chert and limestone clasts that range in age from Lower Carboniferous to Lower Permian. These rocks are taken to indicate that the Palaeotethys ocean opened during the Devonian, when the Indochina and Chinese blocks separated from Gondwana. According to Metcalfe (2000), tectonostratigraphic, palaeobiogeographic and palaeomagnetic data indicate that the Sibumasu Terrane subsequently separated from Gondwana as a microcontinental sliver in the late Sakmarian (early Permian), and then drifted rapidly northwards during the Permo-Triassic, away from Gondwana, towards the Indochina Terrane (Metcalfe, 2000; 2006). The Bentong-Raub Suture is taken to be the result of northwards subduction of the Palaeotethys ocean beneath Indochina in the Late Palaeozoic, and the Triassic collision (and underthrusting) of the Sibumasu Terrane with the Indochina-East Malay block (Metcalfe, 2000; 2006).
During the Permian subduction phase, the East Malaysia volcano-plutonic arc, with I-Type granitoids and intermediate to acidic volcanism, was accreted to the margin of the Indochina Terrane (Metcalfe, 2000; 2006).
The main structural discontinuity in Peninsular Malaysia is between Palaeozoic and Triassic rocks, with orogenic deformation apparently initiated in the Upper Permian to Lower Triassic, when Sibumasu began to collide with Indochina. During the Early to Middle Triassic, A-Type subduction and crustal thickening generated the Main Range S-type granites syn- to post-orogenic granites, which range from Late Triassic (230±9 Ma) to earliest Jurassic (207±14 Ma) in age. The suture zone is covered by latest Triassic, Jurassic and Cretaceous, mainly continental, overlap sequence (Metcalfe, 2000; 2006).
Traditionally, Peninsular Malaysia has been divided into a Western Tin Belt (west of the Bentong-Raub Suture), a Central Gold Zone (including the suture zone and eastern Manabor Terrane) to the NNW-SSE trending Lebir Fault Zone, and the Eastern Tin Belt occupying the remainder of the eastern peninsular. Four narrower, distinct gold belts are recognised within this framework, which are i). immediately to the west of the suture zone, ii). immediately to the east of the suture zone; iii). along the Lebir Fault zone, and iv). within the eastern tin belt.
Historical gold production has predominantly come from, the Central Gold Belt of Malaysia, particularly the Raub-Bentong Suture zone, with the largest production from the Penjom and Raub Australian Gold mines, 25 km SE and 65 km south of Selinsing respectively, each producing an estimated ~30 t of gold bullion.
The Selinsing gold deposit is hosted by a 30 to 50 m thick shear zone dipping at 55 and 75°E. The mineralised envelope of sheared rocks has been variably mineralised and intruded by gold-bearing quartz veins and stockworks. The quartz veins are generally emplaced along individual fault surfaces. The faulting is interpreted to be essentially dip-slip reverse thrusting caused by compression from the east. Strike-slip movement is not generally significant, although a post-mineralisation NW structure with a possible strike-slip component is evident (Odell et al., 2013).
The host rocks within the shear zone comprise of a series of finely interbedded argillites and very fine-grained arenites. Also present are sequences of quartz-rich, variably silicified sedimentary rocks of likely tuffaceous origin, which are referred to as 'felsic tuffs', and a few thin beds of quartzite conglomerate. These country rocks are collectively known as 'the mine sequence series'.
The sedimentary rocks of the mine sequence are of deep marine epiclastic origin, deposited in quiet conditions and are thought likely to be of volcanogenic origin. The mine sequence has undergone low-grade regional burial metamorphism, which is seen by the development of chlorite in some of the country rocks, more notably the felsic volcanic rocks (Odell et al., 2013). The true thickness of the mine sequence has been estimated at ~200 metres, although the position of the footwall contact is difficult to distinguish due to the fine-grained nature of the host and underlying rocks. A second interpretation is that structural repetition within the shear zone, created a thickening of a thinner sequence (Odell et al., 2013).
The hanging wall is occupied by a distinctive sequence of predominantly 'dirty', competent, well-bedded, dark coloured limestone rocks. The base of these limestones is a small unit of black, well-bedded, carbonaceous shales, which are locally calcareous. The contact with the mine sequence is thought to be tectonic or faulted, due to the unconformable nature of the bedding on either side. The actual contact is characterised by large water-filled clay-lined cavities (Odell et al., 2013).
As stated previously, little is known about the footwall contact, although the footwall comprises a similar grey-black limestone as found in the hanging wall. It is suspected that these units may be the same, repeated by faulting. This would mean that the less competent mine sequence units were more deformed by shearing due to rheological contrasts between the limestones and the argillites and arenites. The hanging wall limestones have locally-developed folding resulting from easterly compression (Odell et al., 2013).
Within the shear zone, there are distinctive tectonic-deformed rock types, the most noticeable of which are cataclastics and mylonites. Local variation in shearing has produced a set of tectonic rocks from both brittle regimes (cataclastics) and ductile regimes (protomylonites or foliated cataclasites through to recrystallised mylonites) which may represent the brittle-ductile transition (originally at 10 to 15 km depth). Gold and sulphide mineralisation is associated with these tectonised rock types as well as intensive replacement by quartz and calcite gangue minerals. Pressure/temperature studies on fluid inclusions in quartz confirm a depth of about 10 km (Odell et al., 2013).
The mineralised envelope at Selinsing strikes at 350° and dips 60 to 70°E. High grade ore shoots within this envelope plunge to the SE, normally associated with quartz stockwork and quartz carbonate veins within the highly deformed sedimentary rock (Odell et al., 2013). The gold mineralisation at Selinsing occurs as very fine gold particles commonly associated with pyrite and arsenopyrite, and rarely with chalcopyrite. Coarse visible gold occurs in quartz veins within the shear zone, with grades that may be well in excess of 30 g/t Au. These high-grade quartz veins may be more than a metre in true thickness, and continuous along strike and down-dip. Some veins have been traced for up to 300 m along strike and >200 m down-dip. Lower grade gold mineralisation is found as fine disseminations within intensely deformed envelopes surrounding the quartz veins within the shear zone. Disseminated pyrite also occurs within the crushed country rock in the shear zone, with euhedral arsenopyrite as a good indicator of higher gold grades (Odell et al., 2013).
The Buffalo Reef deposit is ~1 km to the east of the structure within the Raub-Bentong Suture that hosts the Selinsing deposit, and is dominated by an eastern assemblage of Permian argillite and limestone in faulted contact with a western assemblage of Devonian conglomerates and sandstones. Low-grade regional metamorphism, up to greenschist (locally to amphibolite) facies, occurs throughout the area (Naidu, 2005). The sedimentary rocks have subsequently been intruded by granitic to intermediate intrusive rocks rocks of ~Jurassic age. Where these intrusive bodies outcrop to the east of Buffalo Reef, they generally form topographic highs (Odell et al., 2013).
The dominant structural feature, which hosts the gold mineralisation, is a 200 m wide, north-south striking shear zone, with an apparent sinistral sense of displacement, paralleling the Raub-Bentong Suture zone. The host rocks within the shear zone are composed of graphitic shale with minor interbedded fine-grained sandstone and tuffaceous rocks (Naidu, 2005). Bedding within the sedimentary rocks typically dip 65 to 75°E and strike at 330 to 360° (Flindell et al., 2003).
Mineralisation occurs in three main zones, Buffalo Reef North, South and Central, which are distributed over a total strike length of 2.6 km. Buffalo South is <1 km NNE of Selinsing. The gold occurs within moderately to steeply east-dipping veins and fracture zones, which range in thickness from 1 to 15 m (averaging ~10 m in the main mineralised veins), although in local flexures, the veins can host mineralisation over a thickness of up to 25 m. Veins, which are boudinaged in some areas, are generally composed of massive quartz with 1 to 5 vol.% of sulphide minerals, principally pyrite and arsenopyrite, along with varying amounts of stibnite. The stibnite generally occurs in association with elevated gold grades, although the presence of gold does not necessarily indicate high stibnite levels. Rocks within the Buffalo Reef shear zone have typically undergone silica-sericite-pyrite alteration to varying degrees (Flindell et al., 2003)
Reserves and resources
Remaining reserves and resources at Selinsing and Buffalo Reef are as follows (August 2012, Monument Mining website, 2015):
Proved + probable reserves, Oxide ore - 2.857 Mt @ 0.9 g/t Au
Proved + probable reserves, Sulphide ore - 2.034 Mt @ 2.2 g/t Au
TOTAL proved + probable reserves - 4.890 Mt @ 1.4 g/t Au
Measured + indicated resources, Oxide ore - 2.949 Mt @ 0.9 g/t Au
Measured + indicated resources, Sulphide ore - 3.358 Mt @ 1.9 g/t Au
TOTAL measured + indicated resources - 6.307 Mt @ 1.4 g/t Au
Inferred resources, Oxide ore - 0.268 Mt @ 1.2 g/t Au
Inferred resource, Sulphide ore - 0.801 Mt @ 0.5 g/t Au
TOTAL inferred resource - 1.070 Mt @ 1.4 g/t Au
TOTAL measured + indicated + inferred resources - 7.377 Mt @ 1.4 g/t Au
This summary is partly drawn from Odell, M., Swanson, K., White, M. and Fox, J., 2013 - Selinsing Gold Mine and Buffalo Reef Project Expansion,
Pahang State, Malaysia, a 43-101 Technical Report prepared for Monument Mining Limited, by Practical Mining LLC, 206p. Regional setting is taken from papers by Metcalfe, 2000; 2006.
The most recent source geological information used to prepare this summary was dated: 2013.
This description is a summary from published sources, the chief of which are listed below.
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