Petrology and Alteration of Lari Mountain in Arinem Area, West Java, Indonesia Fatoni Adyahya 1 *, Euis T. Yuningsih 1, Ildrem Syafrie 1, H. Matsueda 2, A. Hardiyono 1 1 Faculty of Geology, University of Padjadjaran, Bandung, West Java, Indonesia 2 The Hokkaido University Museum, Hokkaido University, Sapporo, Japan *Fatoniady@gmail.com Abstract Lari Mountain is located in Garut region, West Java, Indonesia as a part of the Arinem deposit. Arinem deposit is epithermal low-intermediate sulfidation with Au-Ag-base metal mineralization.. This mineralization was resulted from Arinem and Bantarhuni veins with relatively northwest southeast direction. These veins hosted by Oligocene Middle Miocene Jampang formation volcanic rocks. The formation consists of andesitic tuff, breccia, and lava. The younger Pliocene Pleistocene volcanic rocks was covered this formation. Lithology of Lari Mountain is Porphyry Dacite which intruded older volcanic rocks. Petrology and alteration studies has been undertaken from the outcrops of Lari mountain region. Lari Mountain is an northern most of the Arinem and Bantarhuni veins. This research was held using thin and polish sections analyses, then the result was correlated with the XRD data to gain interpreted of lithology and alteration types in the area. Type of alteration in Lari Mountain according to the alteration minerals assemblage which found in the rocks samples is Argillic. Vuggy quartz and kaolinite group mineral indicate that formation condition of alteration is in acidic condition (ph 3-4). Keywords :alteration, argillic, kaolinite, Lari Mountain, XRD. Introduction Lari Mountain is located in Wangonjaya Region, Pakenjeng Subdistrict, Garut District, West Java Province. This location known as a part of the epithermal low-intermediate sulphidation Arinem Deposit with Au-Ag-base metal mineralization Arinem and Bantarhuni veins with relatively northwest southeast direction (Yuningsih & Matsueda, 2014; Yuningsih et al., 2012). The deposit is situated at the southwest of Papandayan active volcano. The mineralization in southern part of West Java is related with volcanic intrusion which formed in Oligocene Miocene. The lithology act as mineralization host rock of the area is Oligocene Middle Miocene volcanic rock of Jampang Formation. The formation consist of andesitic tuff, breccia, and lava. The younger Pliocene Pleistocene volcanic rocks was covered this formation (Alzwar, et al., 1992). Geology of Lari Mountain area consist of lithologies from volcanic activity in Tersier such as tuff and others volcanic rocks. This lithology is overlain by Pleistocene Andesit Lava. Then tuff and Andesit lava were intruded by younger Dacite. (Tanssa et al.,,, 2014). Figure 1. Map of West Java, red box showns the location of study area 313
Mineralization in Lari Mountain is unique, because in this area were found vuggy silica which commonly formed in epithermal high sulphidation system. The aim of this study is to characterized of lithology and alteration types in the area. The type of the alteration were determined according to the alteration minerals assemblage which found in the rock samples. This research will determine the characteristics of Lari Mountain comparing to other region in Arinem Deposit. Analytical Method Figures 2. Geological map of Wangonjaya and Surrounding (modified after Tanssa, et al., 2014). Location of Lari Mountain mark by yellow line circle. Petrography is a branch of petrology that focuses on detailed descriptions of rocks, the mineral content and the textural realationship within the rock are described in detail. Detailed analysis of minerals was conducted by optical mineralogy in thin section. Optical mineralogy of every minerals is characteristic and most of minerals can be identified under a microscope, but it is very difficult to identifiy finer grained minerals such as clay minerals. To identify this clay minerals, X-Ray Diffraction has been used. The samples was collected in outcrop of the research area. Total of three samples were prepared with thin sections and one sample with polished section for microscopic observation. An additonal 2 samples (R8 and R9) were analyzed by XRD (X - Ray Diffraction) and PIMA (Portable Infrared Mineral Analyzer) for clay mineral identification. 314
Figures 3. Photo of Hand Specimen samples in R25 (a) and R28 (b). Both samples has light grey colour, in R25 showing a few and small vugs. (c) Under microscope in transmitted light showing phenocryst of quartz in fine grained groundmass, rocks already strongly intense altered it showed by replacement of feldspar to secondary quartz and kaolinite. (d) R28 in transmitted light photomicrographs showing the occurences of vugs textures. Thin and polished sections analyses were using microscope in Laboratory of Petrology-Mineralogy UNPAD, for XRD and PIMA analysis conducted at the Laboratory of Geological Resources Centre (PSDG). Determination of alteration then could be identified using these petrology, xrd and PIMA analyses. Results Macroscopic Dacite as shown in hand specimen of R25 and R28 (Figure 3a and 3b) have light grey colour, porphyritic texture with fine grained groundmass, the degree of crystallization is hypocrystalline, dominantly by subhedral crystal shapes, inequigranular textures, this samples composed of quartz and plagioclase, hardness of this rock is very hard due sillification alteration, a few vugs found in samples. Microscopic Under microscope R25 shown dark grey colour, porphyritic texture with fine grained groundmass, quartz occurred as phenocryst in microcrystalline groundmass, the degree of crystallization is hypocrystalline, subhedral crystal shapes, inequigranular textures, this samples has been altered by secondary quartz and kaolinite which may replace feldspars, even this samples has been altered but the original textures still can be identifed. Based on classification of igneous rocks by Travis (1955) 315
the name of this rocks is Porphyry Dacite (Figure 3c and d). In samples number 09, alteration has been very intense, all of the feldsparreplaced by secondary quartz. It can be shown in macroscopic with white colour and some vuggy silica occurs in samples. Figures 4. Polished Section photomicrograph of pyrite in R25 In polished section, the ore minerals can be identified is pyrite (Figure 4), this mineral is fine grained, with granular textures and yellow in colour and subhedral anhedral crystal shape. X-Ray Diffraction XRD analyses result of R8 and R9 samples indicated the presence of quartz, kaolinite, halloysite and montmorillonite (Figure 5). Figures 5. Graphic from XRD Analyses in R8 Sampls showing the presence of quartz, kaolinite, and montmorillonite Discussions Petrographic observation indicate alteration minerals in Lari Mountain are quartz and kaolinite. Ore minerals can be observed is pyrite. Secondary quartz and kaolinite may occurs in dacite by replacing feldspars, the presence of kaolinite mineral can be shown by result of XRD analyses. Kaolinite has indicate if this alteration is formed at a ph of around 3-4 (Thompson, 1996). Kaolinite is formed at shallow depths under low temperature conditions (<150-200 o C), Halloysite occurs mainly as a supergene weathering product. (Corbett and leach, 1997). Montmorillonite may occur as alteration of glasses contained in rocks, it may be produced at 150 o C. Vuggy quartz alteration forms by reaction of low-ph aquaeous fluid or vapours with rocks. It is indicate that the alteration were occurred in acidic condition. The present of quartz (vuggy quartz) - kaolinite-halloysite-montmorillinite-pyrite alteration minerals assemblage indicated that Lari mountain alteration type is Argillic. Alteration mineral association of quartz (with vuggy texture) kaolinite-halloysite-montmorillonite-pyrite in Lari Mountain is indication for the occurrence of argillic type of alteration This type of alteration is occurred in epithermal sulphidation, but there is still some doubts if this system is indication for high or low sulphidation epithermal. Because Kaolinite group minerals formed in environment with 3-4 ph, a key mineral alteration presence such as alunite group is needed to clarifiedto make sure which environment is this system belongs to. Conclusions Lari mountain is formed by Porphyry Dacite which intruded the older volcanic rocks. Type of alteration found in Lari mountain is argillic. Vuggy quartz texture and kaolinite group indicate if deposit characteristic formed in acidic condition (3-4 ph). Lari mountain is an intersting to study further in related to discover the relation with epithermal lowintermediate sulphidation mineralization in Arinem and Bantarhuni veins. Acknowledgments The authors are very grateful to PT. Antam Tbk. for access to the exploration area and valuable discussion with exploration team of Arinem. Mr. Reqy Tradyla Tanssa is also thank for additional data. 316
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