Geology and mineralisation of the Mewet Vein, Jelai Gold Project ...

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PT GMT Indonesia Mewet Vein : Jelai Gold Project palaeogeographic domains. The characteristics of these domains are conditioned by the relationships between a continent, situated to the west and a fluvial drainage system draining into an ocean, located to the east (Lefevre et al., 1982). The lithologies observed are flysch type sequences, with a marine environment found to the north and east. Lefevre et al. (1982) estimate a combined thickness of 7,000 metres for the various units. For simplicity, the Mentarang Formation is preferred, with the dacitic tuff/lava component represented by the Geh sub-Formation. 6.2.2 Late Oligocene-Early Miocene Several corridors of north-northeast trending granitoid intrusive complexes occur within the project areas. One belt occurs at Long Laai, the other southwest of Long Bia. The Long Laai complex has been studied in some detail by the BRGM/DSDM (Le Bel et al., 1985 & 1986) and found to consist of leuco adamellite, with the development of exo and endo skarn facies, at the contact of the intrusives with the older sediments. The complex situated southwest of Long Bia has been termed the Yamuk Intrusive Complex by Sennitt et al. (1995). It consists of biotite and hornblende granodiorite, granodiorite porphyry, quartz diorite porphyry, andesite porphyry and a “dry” quartz-feldspar porphyry. On the margins of the Yamuk Intrusive Complex, the older sediments are highly fractured and deformed, with hornfels and collapse breccias occasionally developed. Within the intrusive complex, intrusion breccias, autobreccias and xenolith rich zones are found. Age dating by the BRGM (Le Bel et al., 1985) obtained a 22.6 Ma age for the Long Laai leucoadamellite, indicating a Late Oligocene-Early Miocene age. A similar age is preferred for the Yamuk Intrusive Complex. 6.3 Regional Structure The regional structure of the project area is dominated by the north-northeast trending Jelai Fault System, which can be traced for at least 200 kilometres south-southwest from the project area and defines the main trend of volcanics and intrusives in the area. This fault system is interpreted to represent an arc-parallel structure, reactivated on numerous occasions and initially formed during the Cretaceous subduction event below Borneo from the northwest. The northern extension of the fault system is hidden beneath younger sediments of the Tarakan Basin. The Jelai Fault System contains a number of jogs to the south of the project area and the major rivers in the area, such as the Kayan River, follow the main fault line as a series of kinked drainages extending to the south from the project area. Subsidiary faults lie parallel to the main fault system and a number of splays can be see extending from the fault line on the north-western side, one of which hosts the epithermal vein system at the Jelai Gold Project. These splays may actually be conjugate fault sets developed from the main Jelai Fault System. Other structures noted on a regional basis around the project include west-northwest trending arc-normal structures, again assumed to have developed during the Cretaceous subduction event beneath Borneo. 6.4 6.4.1 Description of Lithologies in the Project Area Andesitic Lava The predominant host rocks are fine to coarse andesitic lava (Figure 4). The rocks have a distinctive lava texture and comprise clasts containing crowded fine to coarse grained 0065 JCM/GEO/2009.04 Page 18
PT GMT Indonesia Mewet Vein : Jelai Gold Project plagioclase laths, rare fine grained subhedral granular quartz and scattered fine to medium grained opaques. These are hosted within a groundmass of broken feldspar, mafic minerals, minor quartz phenocrysts and glass. Irregular compressed cavities are common. Mafic phenocrysts are pervasively altered to chlorite and/or smectite. Plagioclase is commonly replaced with calcite and more rarely by chlorite or smectite. In close proximity to the veins the plagioclase displays weak to moderate sericitic alteration and occasional replacement by adularia. Vesicles within the groundmass are commonly infilled with calcite and in zones of strong propylitic alteration also with chlorite and epidote. Haematite staining is a common feature of the fine grained glassy groundmass. 6.4.2 Andesite Intrusives Andesite is characterised by a porphyritic texture of euhedral or partly resorbed phenocrysts set in a fine grained, glassy groundmass. The andesite generally displays pervasive alteration with a strongly bleached, pale green colouration. It commonly contains vesicles infilled with quartz and in zones of strong propylitic alteration, chlorite, haematite and calcite. Irregular patchy alteration gives rise to a pseudobreccia texture. 6.4.3 Andesitic Basalt The andesitic basalt is characterised by fine to medium grained mafic and felsic phenocrysts set in a fine grained, glassy groundmass. It is essentially unaltered but fine to medium grained hornblende phenocrysts often display weak chloritic alteration and trace haematite is often observed along microfractures. 6.4.4 Lithic Tuffs Lithic tuffs occur as rare thin units within the andesitic volcanic sequence. These units are typically poorly sorted and fine to coarse grained with subangular to well rounded clasts. Clasts are predominately silty mudstone of the Menterang Formation and mesothermal quartz. Approximately 30% of the larger clasts are composed of andesitic pyroclastics with 90% of the matrix being composed of very fine andesitic tuff clasts. Devitrified glass shards are also often observed within the matrix. The lithic tuffs are pervasively sericite altered. Part of this unit may represent thin intrusive breccias. 6.4.5 Dacite Porphyry The dacite porphyry occurs as irregular stocks, dykes and plugs in the area and comprises a grey to green-grey fine grained rock with prominent quartz eyes to 10 mm in diameter. The fine grained mafic component is occasionally chloritised and sericite is also common in some areas. In some areas, the epithermal vein structures are associated with the contact of the dacite porphyry, such as the Sembawang Vein but in other areas the drilling has intersected significant mineralisation without the presence of the dacite porphyry in close proximity and, therefore, there appears to be no consistent relationship between dacite porphyry intrusives and mineralisation, other than as a host. 6.4.6 Rhyolite The rhyolite is restricted to the east of the concession area, where the rock has intruded into the main fault line of the Jelai Fault System as a single stock. The rock is pink-brown, strongly flowbanded and is variably, albeit weakly, altered. Little is known about the genetic relationship between the rhyolite and the other intrusive and it is no known if there is a genetic relationship to any mineralization style in the area. 0065 JCM/GEO/2009.04 Page 19
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Geology and mineralisation of the Mewet Vein, Jelai Gold Project ...

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