Resultados

The geology of the Tasik Chini area is dominated by various phases of volcanic-plutonic rocks. The plutonic bodies are scattered in the northern and southern parts of the area, and several small intermediate intrusive bodies emplaced the Permo-Triassic succession to the south. Moreover, there are widespread occurrences of volcanic rocks of Permo-Triassic ages (Fig. 3.1; Minerals and Geoscience Department of Malaysia, 2004).

The granitoids in the northern and southern parts range from gabbro to monzogranite in composition and are dominated by the biotite granite (MacDonald, 1970; Schwartz et al., 1995). A few occurrences of granodiorite and minor diorite are present southwest of the Tasik Chini area (MacDonald, 1970). As also defined by MacDonald (1970), the volcanic rocks in the area mainly comprises rhyolite with only minor rhyodacite, dacite and trachyte. Petrology and geochemistry of these plutonic and volcanic rocks are not well documented in previous studies, and age of emplacement of the units was also unavailable or unknown prior to this study.

During this study only three granitoid bodies were found and sampled in the southern part of the mapped area. Due to the weathering and the lack of exposures, it is not possible to classify these granitoids into different rock types in the field. Thus, throughout this chapter the term granite is used to define the plutonic body, with the major characteristics of individual samples described as follows.

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The microgranite body exposed in the vicinity of Bukit Ridan is medium-grained and shows a porphyritic texture (Sampel BR317). The original igneous texture of this rock is well preserved, containing phenocryts of quartz and feldspars with a reddish clay dominanted groundmass. The feldspars have been mostly altered into clay minerals and are consequently difficult to identify (Figs. 3.6A and B).

The coarser granite body is exposed northeast of Bukit Ridan and is also mostly weathered, and yellowish in colour (Sampel BR326). This fine- to medium-grained rock is rarely porphyritic with phenocrysts of quartz and feldspars. Both the phenocryts and groundmass are intensely altered to clay and sericite (Figs. 3.6C and D).

The other igneous rock outcrop is located in the vicinity of Bukit Sembilan and is part of the small intermediate granitoid body exposed in the map area (Sampel BR318). The rock is intensely weathered, and is difficult to distinguish from the sedimentary rocks it intruded. Its texture is fairly homogeneous and all the plagioclase is a major mineral that has been altered to clay minerals. In addition, rare quartz and probably pyroxene phenocrysts are present and observed throughout this rock (Figs. 3.6E and F).

Four regional volcanic rock units were collected from the Tasik Chini area. Sample BR3 is a massive dacite/andesite subvolcanic unit from the vicinity of the Bukit Perah, west of the Bukit Botol deposit. The sample is greenish grey to dark green in colour and exhibits a porphyritic texture (Fig. 3.7A). It contains phenocrysts of plagioclase, hornblende (replaced by clay mineral and chlorite), quartz and opaque mineral. The groundmass is very fine- grained and consists of a feldspar, quartz and Fe-Ti oxide assemblage (Fig. 3.7B).

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Fig. 3.6. Outcrops of the granite bodies. A. Exposure of granite body in the vicinity of Bukit Ridan. B. The granite texture containing phenocryts of quartz and feldspars in a reddish clay dominant groundmass, Sample No. BR317. C. Highly weathered granite rocks exposed to the northeast from the Bukit Ridan. D. Hand specimen sample of the granite is yellowish in colour, fine to medium-grained, rarely porphyritic with phenocrysts of quartz and feldspars, Sample No. BR326. E. Intermediate igneous rock (diorite?) body outcrop near Bukit Sembilan. F. Texture of the diorite is fairly homogeneous, with all the plagioclase as major mineral and altered to clay minerals-only rare quartz and probably pyroxene phenocrysts are present and observed throughout this rock, Sample No. BR318.

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Fig. 3.7. A. Hand specimen of dacite/andesite subvolcanic unit in the vicinity of the Bukit Perah, west of the Bukit Botol deposit. B. Photomicrograph of the dacite/andesite comprising phenocrysts of plagioclase, hornblende (replaced by clay mineral and chlorite), quartz and opaque mineral. The groundmass is very fine- grained and consists of feldspars, quartz and Fe-Ti oxides, Sample No. BR3. C. Outcrop of the mixed felsic volcaniclastic and siliciclastic rocks. D. Hand specimen sample of felsic volcaniclastic rock, Sample No. BR325

E. Photomicrograph of the felsic volcaniclastic rock showing feldspar and quartz phenocrysts in a cryptocrytalline groundmass of feldspar and quartz assemblage, Sample No. BR325. Annotation: qtz=quartz, pl=plagioclase, hbl=hornblende and fs=feldspar.

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Sample BR325 is from a mixed sequence of felsic volcaniclastic and siliciclastic rocks, which occurs approximately 20 km southeast of the Tasik Chini deposit area. The felsic volcaniclastic rock is composed of feldspar and quartz phenocrysts in a cryptocrytalline groundmass of feldspar and quartz (Figs. 3.7C, D and E).

The third and fourth volcanic body outcrops are located in the vicinity of the Bukit Botol and Bukit Ketaya areas, which host the Bukit Botol and Bukit Ketaya deposits in the Tasik Chini area (Fig. 3.1). Both the volcanic bodies consist of rhyolite to rhyodacite flows and volcaniclastic rocks, and their characteristics are presented in Section 3.4.