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

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PT GMT Indonesia Mewet Vein : Jelai Gold Project mapping, rock chip sampling, stream sediment sampling, ridge and spur soil sampling, grid based soil sampling, geophysics (limited), trenching and drilling. The program utilised a dual-engined man-portable Drill Technics DT450P contracted from P.T. Radial Drilling Indonesia, which was capable of drilling 250 metres HQ diameter diamond core, with a maximum capability of 450 metres of NQ core. Holes were precollared by the dry plugging method and diamond coring was undertaken once competent rock was encountered. Diamond core was nominally HQ, however, reduction to NQ was sometimes necessary due to poor ground conditions or machinery problems. Down hole surveys were undertaken at 50 metre intervals using an Austmine single shot camera. Orientating of the drill core was undertaken using the ‘spear and pencil’ method. Orientations were attempted on regular intervals, before the inferred vein interception depth and after intercepting the vein. Orientation readings were generally unsuccessful within the high grade epithermal veins due to their fractured or brecciated nature. Drilling was carried out on a 2x12 hour shift per day basis. The general project sampling statistics are shown in Table 2. Programme Quantity Mapping Total detailed mapping at 1:50,000 to 1:2,000 scale, approximately 100 km² Stream sediment sampling 251 samples of -80#, 6 BLEG samples Rock chip sampling 1,680 rock chip outcrop and 871 float samples from the main project area plus surrounding samples from other projects Soil sampling 9,393 samples, ridge and spur plus grid based, some deep augering Geophysics Magnetometer survey, grid based Trenching Approximately 40 hand-dug trenches, interesting zones channel sampled Drilling Twenty-six fully cored drill holes for 3,901.52 metres. Interpretations Various, geological, geochemistry, geophysics Survey control Various differential GPS surveys to fix base stations and grid survey for soil sampling and drilling, Entopos Table 2. Exploration summary by Ivanhoe Mines Ltd at the Jelai-Mewet Project area. The project was closed in December 2000, along with all other Ivanhoe Mines Ltd operations in Indonesia. 0065 JCM/GEO/2009.04 Page 16
PT GMT Indonesia Mewet Vein : Jelai Gold Project 6 6.1 GEOLOGICAL SETTING Regional Geology 6.1.1 Tectonic History Recent geological mapping by AGSO and GRDC (Pieters, 1993) indicates that a suture formed in Borneo, by the collision of two continents. The collision occurred during the Cretaceous to the Late Tertiary, with possible after effects continuing until the Quaternary. Gold deposits are widespread in Borneo, forming a 100 kilometre wide belt trending eastwest in West Kalimantan and north-northeast in east Kalimantan. The deposits conform to an uplifted, faulted basement high, that arose during post-collisional uplift and extensional tectonism. Recently recognized epithermal gold-silver mineralization was most likely deposited as a result of hydrothermal activity associated with this volcanism, coeval with the uplift of the basement high (e.g. Kutai Basin-Kelian, Barito Basin-Mt. Muro, Mt. Wullersdorf in Sabah). The principle tectonic features of northeast Kalimantan are the Tarakan Basin and the Kuching High (a pre-Tertiary core complex). During the Miocene, the region was a zone of major extension, resulting in the formation of the Tarakan Basin and uplift of the Kuching High core complex. Dacitic volcanic activity commenced in the Pliocene, during the waning stages of extension, related to tectonic movement marginal to the Tarakan Basin. A suite of alkali ‘hot granites’ containing high concentration of U and Th, were emplaced in Late Cretaceous time, associated with a transitional tectonic environment from subduction to extension (Pieters, 1993). The BRGM (Le Bel et al., 1985 & 1986) postulate that the ‘hot granites’ resulted from arc accretion phenomena. The ‘double convection model’ proposed by Katili (1973) explains the formation of volcanic arcs and related basins up to the Early Eocene, the north west subduction zone being located at Sarawak, the southeast zone at the Mangkalihat Peninsula (Mangkalihat Ridge). The transition from this regime to a collisional regime took place when the southeast subduction gently stopped, while the northwest was still active. Partial melting of arc-derived material at depth, generated the ‘hot granites’. 6.2 Stratigraphy The geology of northeast Kalimantan is still poorly understood due to the lack of detailed mapping programmes and published data. Several petroleum companies have compiled their own stratigraphic subdivisions, including Pertamina-Beicip and Total Indonesia. However, the most useful work remains that of the various BRGM/DSDM surveys undertaken between 1979- 1986. 6.2.1 Late Cretaceous - Early Eocene Mapping by the BRGM/DSDM (reports by Lefevre et al., 1982, Le Bel et al., 1985 & 1986 and Nagel et al., 1987) has indicated the widespread distribution of a thick sequence of laminated siltstone, carbonaceous siltstone, shale and micaceous feldspathic sandstone. A minor dacitic tuff component is evident in some areas and a thin intra-formational breccia grit/quartz pebble conglomerate unit is also observed. These rocks are well laminated, generally steeply dipping and tightly folded and display varying degrees of metamorphism, ranging from unaltered through contact hornfels to lower greenschist facies. The metamorphic grade and degree of deformation are probably a function of proximity to granitoid intrusions and major structures. The BRGM/DSDM team has named the Mentarang, Paking, Lurah, Malinau, Long Bawan and Long Geh formations as what is an essentially synchronous sedimentary cycle, but in different 0065 JCM/GEO/2009.04 Page 17
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