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

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PT GMT Indonesia Mewet Vein : Jelai Gold Project 7 7.1 MINERALIZATION Deposit Types The Jelai Gold Project covers a series of low sulphidation epithermal veins with anomalous precious metal values hosted within andesitic volcanics. These types of deposits are common throughout Indonesia and examples include Mount Muro, Gosowong, Pongkor, Muyup and Toka Tindung. The deposits are characterised by quartz veins and vein breccias, generally with an argillic alteration halo containing evidence of near-surface hydrothermal deposition of various silica phases, with or without the synchronous deposition of metallic minerals. The exploration programme conducted has specifically targeted this style of mineralisation. 7.2 7.2.1 Mewet Vein Geology General Geology The Mewet Vein crops out on the side of a small, north-south trending, ridge. The vein consists of small discontinuous outcrops of colloform banded chalcedony with adularia in the drilled area. Sections of the vein structures predominately consist of volcanic clasts and are commonly set within a calcite matrix which breaks down easily resulting in a subdued topographic expression. The veins or breccias do not form prominent topographic features in any part of the system. Weathered breccia outcrops may have a box-work appearance due to the leaching of the volcanic clasts. From drilling, alluvium generally occurs in the first 4 metres of each hole with a cobble layer commonly encountered. Each hole intersected a manifestation of the multiphase breccia and vein which crops out on the western side of the north-south trending ridge. Unaltered andesitic basalt dykes were intersected in the drilling but these are un-mineralised and appear to postdate the main mineralising event. Silicified volcanic clasts and, occasionally, clasts of mesocrystalline to chalcedonic quartz may be present. Clasts are generally angular. However, sub-rounded clasts are also observed indicating that the hydrothermal system has transported the clasts from a deeper level. Basement sediment clasts have not been recognised within the breccias. The vein is hosted by andesitic lava, with clasts of silica-pyrite altered wallrock are cemented by opaline quartz in places. The vein strikes generally north-south, dips steeply to the west at approximately 60° and is traceable on surface for at least 2.5 kilometres. SPOT imagery interpretation indicates that the Mewet Vein forms part of the prominent oblique subsidiary set of the major north-northeast Jelai Fault System. This could explain the considerable strike length. A northwest to west-northwest fracture pattern is also strongly developed in its footwall. 7.2.2 Mineralisation The mineralisation in the Mewet vein comprises a discrete lode structure containing mixed hydrothermal vein breccia, massive quartz veins and colloform-crustiform quartz veins in a zone trending generally north-south and dipping at approximately 60° to the west Over the strike length of the vein a pinch and swell effect is observed with the width being variable at a similar RL along the strike of the vein. The lode dips at approximately 60° with a late cross-cutting quartz-carbonate vein and fracture fill phase at 65-70°. Late phase veins are typified by glassy to milky, fine grained drusy quartz + carbonate which are generally barren with assays rarely exceeding 0.02 ppm Au. The late phase quartz is rarely weakly amethystine in nature and crosscuts all phases of brecciation and veining. 0065 JCM/GEO/2009.04 Page 22
PT GMT Indonesia Mewet Vein : Jelai Gold Project High gold values were obtained from finely banded, colloform-crustiform, quartz-adularia veins. The highest gold values occur in zones of multiphase brecciation of the colloform banded quartz which are healed by chalcedonic quartz and adularia. No gold mineralisation occurs in the footwall to the vein but low grade gold values occur within the breccia on the hanging wall to the quartz vein. Silver grades are sporadic with the most elevated values occurring with the high grade gold in finely banded, sulphidic, quartz veins (Photograph 1). Drilling to date has only been carried out over a 325 metre strike length of the Mewet Vein. Most of the drill holes targeted at the vein intersected mineralised sections of vein and vein breccia material with variable mineralisation reported from the analysis of the core samples. 7.2.3 Alteration Pervasive propylitic alteration represents the predominant alteration throughout the andesitic volcanic sequence. The fine grained mafics always show a certain degree of chloritic alteration. Haematite is often present in the fine grained matrix of volcanic flow breccias, fracture coatings and in vesicles as coatings to chlorite and calcite. Up to 1% fine disseminated pyrite occurs with the chloritic alteration. Sporadic epidote occurs in zones of intense propylitic alteration and preferentially replaces feldspar phenocrysts and also occurs rarely as fracture coatings. In zones of intense propylitic alteration epidote occurs in vesicles with calcite and chlorite. Epidote alteration was not observed in the vicinity of the Mewet vein. Sericite alteration occurs as an halo to the hydraulic brecciation and to the epithermal mineralisation. The intensity of sericite is a function of the proximity to the mineralisation. Sericite is in all cases observed overprinting proplytic alteration. Sericite alteration is not specifically restricted to any one single phase of quartz veining/silicification. The intensity of the sericite – silica alteration is also a function of the intensity of brecciation. The strongly brecciated hanging wall acted as a more permeable medium for the alteration fluids than did the relatively unfractured footwall; thus sericite alteration is moderate to strong in the hanging wall while it quickly dissipates to strong chloritic alteration in the footwall. An advanced argillic zone is not present at Mewet. This is due to the system being eroded down to the top of the precious metal zone, at the boiling zone. Adularia is found as fine disseminations within the colloform-crustiform banded quartz veins in this zone. 7.3 Interpreted Genesis of the Epithermal Veins Multiple phases of brecciation and vein formation are evident at the Mewet Vein. Similar systems in other parts of Indonesia, and indeed the world, follow similar patterns of development and, although no specific studies have been undertaken to collaborate the observations, the following section describes the development of a typical low-sulphidation quartz vein, within the context of the Mewet Vein. North-south structures, originating from the major Jelai Fault System to the south, provided a conduit for ascending near-neutral chloride fluids which led to rapid silicification. The silica solution, with the silica originating from the leaching of the wall rocks at high temperatures, remained saturated and was deposited slowly, resulting in the formation of mesothermal/crystalline quartz observed in the early phase of silicification in the Mewet Vein. Sealing of the system with silica resulted in an over-pressuring effect, which was relieved through explosive brecciation of the system. Brecciation of the earlier phase of quartz veining resulted with sub-rounding of some clasts indicating that there has been some transportation from depth. No clasts of the underlying sedimentary basement were encountered in the drilling. Mixing of CO 2, with meteoric water, lead to the deposition of rhombic and bladed carbonate as breccia matrix flood and cavity infill. Subsequent mixing of the supersaturated acidic fluids with meteoric water resulted in the rapid deposition of fine grained microcrystalline to chalcedonic 0065 JCM/GEO/2009.04 Page 23
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Geology and mineralisation of the Mewet Vein, Jelai Gold Project ...

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