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Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 15-28 Characteristics of Alteration and Mineralization at Randu Kuning - Wonogiri Project Karakteristik Alterasi dan Mineralisasi di Projek Randu Kuning, Wonogiri Abdul Muthi1, I Gde Basten2, I Gede Made Suasta3, and Naomi E. W. Litaay3 PT. Best Clean Energy (Augur Resources), Jln. Warung Buncit Raya 99, Jakarta 12740 2 Geological Engineering, ITB, Jln. Ganesha 10, Bandung 40132 3 PT Oxindo Exploration (MMG), Jln. Ciputat Raya 99, Jakarta 12310 1 ABSTRACT Randu Kuning prospect is one of the prospective areas within the Wonogiri property which is fully owned by PT. Alexis Perdana Minerals (PT. APM). The area is often reffered to as Selogiri property as it lies within Selogiri Subdistrict. Exploration work in Wonogiri commenced by PT. Oxindo Exploration (MMG) includes regional, detailed mapping, and surface geochemical sampling including rockchip, channel, stream sediment, and grid soil sampling. Ground magnetic survey aided subsurface exploration and a 3D magnetic inversion assisted to define targets for 5 holes scout diamond drilling program to test coincident anomalous soil geochemistry, modelled high magnetic bodies, and exposed sheeted Cu-Au bearing quartz veins. Augur continued the work in the area with surface sampling comprising extensive trenching, diamond drilling, and detailed ground magnetic. Extensive drilling program in 2011 completed 50 diamond holes in 2011 with the total of 15,588.15 m, mostly were drilled into Randu Kuning prospect including the deepest hole (WDD30) that was drilled until 854.95 m. Lithological classification, alteration zonation, and mineralisation have been refined with the new exploration results. Lithological classification differentiates the units based on genesis, relation to mineralisation event, and grain size. Two main groups of lithology are diorites and breccias. Zoned prograde hydrothermal model of Randu Kuning Porphyry system depicts the intersection of diamond holes into outer and inner propylitic zones, porphyry style veins, and core potassic zones. Additional alteration study was done by PIMA work on 102 samples taken from 6 holes of Cross Section TRK01. Clay minerals, illite, and montmorillonite are the main minerals identified in the vein samples. KAlunite and gypsum in wallrock were identified near the end of WDD030 that suggests signatures of advance argillic alteration and intermediate argillic respectively. Extensive study including alteration mineralogy is required to confirm this. The gold-copper porphyry mineralization in Randu Kuning developed within sheeted and stockwork quartz vein in microdiorite and in the peripheral of microdiorite intrusion. The system obtained its higher Cu-Au grades from overprinting episodes and block faulting may have caused variations in alteration. Published resource of Randu Kuning is estimated at 90.9 Mt at 0.53 g/t AuEq (0.35 g/t gold and 0.10% copper) using a cut off of 0.2 g/t AuEq. Keywords: Randu Kuning, alteration, mineralization, magnetic survey, prograde hydrothermal, porphyry system, Cu-Au SARI Randu Kuning merupakan salah satu area prospektif di Wonogiri yang sepenuhnya dimiliki oleh PT. Alexis Perdana Minerals (PT. APM). Daerah ini sering disebut sebagai properti Selogiri karena terletak di Kecamatan Selogiri. Pekerjaan eksplorasi di Wonogiri yang dimulai oleh PT. Oxindo Exploration (MMG) meliputi pemetaan regional dan terperinci, serta pemercontohan permukaan yaitu rockchip, saluran, sedimen sungai, dan pemercontohan tanah grid. Survei magnet permukaan membantu eksplorasi bawah permukaan dan inversi magnet 3D membantu untuk menetapkan target lima lubang pemandu dalam pemboran intan untuk menguji geokimia tanah anomali, model tubuh bermagnet tinggi, dan urat kuarsa pembawa Cu-Au. Augur melanjutkan bekerja di area tersebut Naskah diterima: 03 Januari 2013, revisi terakhir: 22 Maret 2013, disetujui: 28 Maret 2013 15 Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 15-28 dengan pemercontohan permukaan yang terdiri atas pemaritan ekstensif, pemboran intan, dan pemagnetan permukaan terperinci. Program pemboran ekstensif pada tahun 2011 menyelesaikan 50 lubang intan dengan total 15.588,15 m, terutama di prospek Randu Kuning, meliputi lubang (WDD30) sampai kedalaman 854,94 m. Klasifikasi litologi, zonasi dan mineralisasi alterasi telah diperbaiki dengan hasil eksplorasi baru tersebut. Klasifikasi litologi membedakan unit-unit berdasarkan genesis, hubungan dengan mineralisasi dan ukuran butir. Dua kelompok litologi tersebut adalah diorit dan breksi. Model hidrotermal prograde sistem Porpiri Randu Kuning yang dizonakan menggambarkan interseksi lubang intan ke zona propilitik luar dan dalam, urat tipe porpiri, dan zona potasium inti. Studi alterasi tambahan dilakukan oleh PIMA terhadap 102 percontoh yang diambil dari 6 lubang Cross Section TRK01. Mineral lempung ilit dan monmorilonit merupakan mineral utama yang teridentifikasi di dalam percontoh urat. K-Alunit dan gipsum di dalam wallrock teridentifikasi dekat ujung WDD030 yang menandakan alterasi argilik lanjut dan argilik intermediet. Studi ekstensif yang meliputi mineralogi alterasi diperlukan untuk mengkonfirmasikan hal ini. Mineralisasi porpiri emastembaga di Randu Kuning berkembang dalam urat kuarsa stockwork dalam mikrodiorit dan dalam tepi intrusi mikrodiorit. Sistem yang memperoleh kadar Cu-Au yang lebih tingginya dari episode pertimpasan dan pensesaran blok ini mungkin menyebabkan variasi dalam alterasi. Sumber daya Randu Kuning yang dipublikasikan diperkirakan sebanyak 90,9 Mt pada 0,53 g/t AuEq (0,35 g/t emas dan 0,10 % tembaga menggunakan cut off 0,2 g/t AuEq. Kata kunci: Randu Kuning, alterasi, mineralisasi, survei magnetis, hidrotermal prograde, sistem porfiri, Cu-Au INTRODUCTION Randu Kuning Prospect is a prospective area within the Wonogiri property which is fully owned by PT Alexis Perdana Minerals (PT APM). The area is often referred to as Selogiri property as it lies within Selogiri Sub-regency. The property is located in the southeastern part of Central Java Province and approximately 30 km south of Solo/ Surakarta City. An initial exploration activity in this area was conducted by PT Oxindo Exploration (MMG) in partnership with PT APM starting in first semester of 2009. The work and results of this exploration activity is summarized in Suasta and Sinugroho (2011). Exploration work on this project continued with further surface and drilling program by Augur Resources from Australia (Augur) that added more knowledge and understanding of the mineralization and prospectivity of the area. Much works have been concentrated on Randu Kuning area to follow up interesting results in initial exploration program. This paper will discuss the results of recent work carried out by Augur Resources on Wonogiri 16 project with emphasis on characteristics of alteration and mineralization at Randu Kuning. REGIONAL GEOLOGY The Sunda-Banda volcanic arc developed during subduction of the north-moving Indo-Australian Plate beneath the Asian continental plate margin. The Sunda - Banda arc of Middle Miocene to Pliocene age is thought to have initiated by subduction reversal following an Oligocene compressive event that was associated with the northward emplacement of ophiolite and island arc assemblages onto the Sunda margin and associated formation of melanges, ophiolite fragments, and deformation zones offshore from western Sumatra (Daly et al., 1991; Harbury and Kallagher, 1991 in Hellman, 2011). The initiation of northward subduction beneath the Sunda - Banda arc migrated eastward following this collision event. The western segment of the arc, west of central Java, developed on continental crust on the southern margin of Sundaland whilst the arc Characteristics of Alteration and Mineralization at Randu Kuning - Wonogiri Project (Abdul Muthi et al.) east of Central Java developed on thinner island arc crust (Carlisle and Mitchell, 1994 in Hellman, 2011). There are also variations in dominant styles of mineralization along the arc. In northern Sumatra in the Aceh Province, mineralization is characterized by porphyry Cu-Mo systems and high-sulphidation deposits (e.g. Miwah and Martabe). In contrast, southern Sumatra, west Java, and central Java are typified by a lack of known porphyry systems but an abundance of low-sulfidation epithermal deposits or prospects with vein systems. Examples include Tambang Sawah, Rawas, Lebong Donok, Lebong Simpang, and Seung Kecil in southern Sumatra, also the Cikotok and Jampang districts, Gunung Pongkor and Cikondang in west Java, and Trenggalek in central Java. Further the east, in east Java and then through Lombok and Sumbawa, there is a reappearance of porphyry and high sulfidation epithermal systems along the eastern arc segment, including the the Tumpangpitu high-sulfidation epithermal and porphyry system on Intrepid’s Tujuh Bukit project, The Selodong high-sulfidation and porphyry district including the Motong Botek porphyry system on Lombok, and the Batu Hijau porphyry Cu - Au system on Sumbawa (Hellman, 2011). Location of the Wonogiri Project with respect to other porphyry projects is shown in Figure 1. The Sunda - Banda arc comprises both Miocene to Pliocene volcanics and younger Quaternary volcanics. The arc has migrated not only from west to east over time but also from south to north (Whitford et. al., 1979; Katili, 1989; and Claproth 1989 in Hellman, 2011). This migration is clearly evidenced by the east-west alignment of deeply dissected Miocene to Pliocene volcanic centers along the south coast of Java, Lombok and Sumbawa, and a parallel E-W alignment of juvenile and active Quaternary volcanoes that define the present active arc further north along central Java and northern Bali, Lombok, and Sumbawa (Figure 2). The Sunda-Banda arc is segmented by a series of arc-normal structures that trend NNE and which are evident in topographic-data set. Tectonic factors appear to have local- N FLORES SEA 0 300 km Selodong (Soutern arc) Wonogiri JV Tujuh Bukit (Intrepid) 1.700 Mt @ 0.41% Cu and 0.46 g/t Au Oligocene Miocene Arc 0 o sar Quaternary Sunda Arc Significant Project/deposit Elang (Newmont) Bangka Sumatra Belitung Borneo Banjarmasin Ma ka Augur’s Wonogiri Project Batu Hijau (Newmont JV) 914 Mt @ 0.53% Cu and 0.40 g/t Au Sulawesi I N D O N E S I A J AVA S E A Wonogiri Project Location Java, Indonesia Jakarta Bandung INDIAN OCEAN 10 Java Banyumas F L O R E S Bali Lombok Sumbawa Malang SEA Flores Timor o Figure 1. Locality map of Wonogiri Project with respect to SundaArc and other major deposits (Augur Resources, 2012). 17 Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 15-28 110 E 112 E o 114 E o o 7S o 7S 8S 8S o o o Selogiri Research Area Physiography North Serayu-Kendeng zone Alluvial plain Domes and ridges Quaternary volcano Rembang-Madura anticlinorium Depression Southern Mountain Range 0 112 E o N 75 150 km 114 E o Figure 2. Regional physiography map of Java (after Imai et al., 2007). ized volcanic centers of the Miocene arc at positions near the southwestern margins of these transfer structures. Contemporaneous continental to ocean deep clastic sediments were deposited on the margins of the volcanic centers (Hellman, 2011). The project location is surrounded by several Quartenary Volcanoes, such as Gunung Lawu, Merapi, Merbabu, and others. Stratigraphically the Wonogiri Project area consists of from young to old: Alluvium, Merapi Volcanic Rock, Lawu Volcanic Rock, Wonosari-Punung Formation, Oyo Formation, Nglanggran Formation, Semilir Formation, Mandalika Formation and Gamping Wungkal Formation (Figure 3). From this regional geological map, Wonogiri Project area is dominated with Mandalika Formation, which consists of dacite-andesitic lavas and dacitic tuff with dioritic dykes. South area is covered by Semilir Formation comprising tuff, dacitic pumice breccia, and tuffaceous sandstone and shale. North area is covered by alluvium deposits composed 18 of loose material of sands and clays with varieted of grain size. Local geology shows that the project area consists of volcanic breccias, lithic tuff, andesite cut by quartz diorite and microdiorite intrusions, and in parts is overlain by alluvial deposits. Local geological structures in the project area are dominated by northeast southwest strike-slip fault and east-west thrust fault that appears to be the effect of the north south subduction movement from Australian Plate at the Eurasian Plate collision. M ETHODOLOGY A N D R ECENT EXPLORATION Surface work in Wonogiri by PT Oxindo Exploration (MMG) includes regional and detailed mapping and surface geochemical sampling including rock chip, channel, stream sediment, and grid soil sampling. Ground magnetic survey aided subsurface exploration and a 3D magnetic inversion assisted to define targets for 5 holes scout Characteristics of Alteration and Mineralization at Randu Kuning - Wonogiri Project (Abdul Muthi et al.) 110o45' E 110o50' E 7o45' S 7o45' S Wonogiri Project Location 7o50' S 7o50' S N 110o45' E 110o50' E Legend: Qa Alluvium: Clay, mud, silt, sand, gravel, pebble, marbble. Qvl Lawu Volcanic Rocks: Volcanic breccia, lava, and tuff Tms Semilir Formation: Tuff, dacitic pumice breccia, tuffaceous sandstone and shale Tomm Mandalika Formation: Dacite-andesitic lavas and dacitic tuff with dioritic dykes Tpdi Pendul Diorite: Diorite Figure 3. Geology and broad stratigraphic succession of the area as defined on the 1:100,000 geological map of the Surakarta and Giritontro (Surono et al., 1992). diamond drilling program to test coincident anomalous soil geochemistry, modelled high magnetic bodies and exposed sheeted CuAu bearing quartz veins. Augur continued the work in the area with surface sampling comprising extensive trenching which are assayed for 10 elements. Total of 9,783 m of trenching was completed in 2011, mainly within Randu Kuning and the surrounding area with the aim to understand the distribution of Au and Cu in this area (Figure 4). This surface program was also aimed to define epithermal vein targets which were not evaluated by MMG. The trenches were sampled using 2 or 4 m composite channel samples with total of 2,931 samples for the whole project. These trenches were mapped and aided the interpretation of surface and sub-surface geology and drilling program. In addition, a detail ground magnetic survey completed over approximately 50% tenement area. Extensive drilling program in 2011 completed 50 diamond holes in 2011 with the total of 15588.15 m. They were mostly drilled into Randu Kuning prospect including the 19 Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 15-28 486.000 mE 486.200 mE 486.400 mE 486.600 mE 486.800 mE 487.000 mE 487.200 mE 487.400 mE 9.138.600 mN 9.138.800 mN 485.800 mE 9.138.400 mN Bukit Piti 9.137.800 mN 9.138.000 mN 9.138.200 mN Randu Kuning 9.137.600 mN South Randu Kuning 9.137.000 mN 9.137.200 mN 9.137.400 mN Geblak Jangglengan Legend: Alluvial Diorite Trench Line Breccia Volcanic Breccia Drill Holes Collar Microdiorite Tuff-Sediments N Figure 4. Trenching location on geological map (modified after Corey, 2010). deepest hole (WDD30) that was drilled until 854.95 m. These drillholes together with 5 previous holes by MMG contributed to a better understanding of mineralization and alteration of Randu Kuning prospect. GEOLOGY OF RANDU KUNING The areas of interest in the Wonogiri project have a high topographic relief trending NW-SE (Figure 5). This area occurs in the northern part of tenement boundary comprising a series of hills extends to the north at lower elevation interpreted to form a series of intrusion body. The area has been sub-divided into several prospects based on geology and type of mineralization (Figures 4 and 5). Locally, geology of the Wonogiri project comprises a series of multiple diorite in20 trusion intruding into the early volcanic sequence and with dominant structures of NE-SW strike-slip fault and E-W thrust fault. The surface geology of Wonogiri project is dominated by hydrothermal alteration (clay-chlorite-magnetite±epidote and carbonate). This alteration is typical of a propylitic alteration that is overprinted by later argillic-phyllic alteration. Most of the work has been concentrated on Randu Kuning area to follow up on encouraging results in the previous work. A total of 38 drill holes including two historical holes of MMG were completed in this area. These drill holes have confirmed that the Randu Kuning prospect is a typically porphyry system with Cu-Au mineralization. The prospective area occurs as a steep west dipping NS trending zone characterized by polyphasal porphyry intrusions separated by intrusive and fault contact parallel to the Characteristics of Alteration and Mineralization at Randu Kuning - Wonogiri Project (Abdul Muthi et al.) 9.138.500 mN 486.000 mE 486.500 mE 487.000 mE 487.500 mE Bukit Piti 9.138.000 mN Randu Kuning Gawe 9.137.500 mN South Randu Kuning Geblak 9.137.000 mN Jangglengan N 0 50 100 m Scale 1:10.000 Figure 5. Distribution of prospects of the Wonogiri project on topographical relief. mineralized trend. Many intrusion and fault contacts were recognized between different intrusive bodies, with some diorite and microdiorite intrusion types are recognized and standardized in geology logging of drilling. Most contacts are between different phases of diorite and microdiorite with prominent mafic phenocrysts herein summarized in cross section TRK01 (Figure 6). Lithology Lithological classification of the Wonogiri Project has been modified in more detail to differentiate the units based on genetic and grain size. The plates showing textural variation of the lithology can be seen in Figures 7 and 8. In summary, the intrusive phases that are currently recognized at Randu Kuning prospect, described from oldest to youngest, are: Coarse Grain Diorite (CDIO) Pre-mineral, coarse-grained, porphyritic, >1 mm subhedral - anhedral of feldspar phenocrysts set in fine-grained or crystalline groundmass, finer size of mafic phenocrysts, non to poorly mineralized. Medium Diorite (DIO) Pre-mineral, dominantly medium-grained, <1 - 2 mm plagioclase or mafic phenocrysts (variably fine-to slightly coarse-grained within intensely altered and intrusive/ crackle breccia zone), equigranular to subporphyritic, non to poorly mineralized (in microdiorite margin). Microdiorite (MDR) Syn-mineral, fine-grained, <1 mm phenocrysts size, aphanitic (?), variation of fine feldspar porphyry to prominent mafic, variably to strongly mineralized. 21 Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 15-28 486.000mE 486.100mE 486.200mE 486.300mE 486.400mE 65.6 m @ 1.24 g/t Au & 0.29% Cu 30 W DD 129.5 m @ 0.83 g/t Au & 0.24% Cu 100mRL 100mRL 123.5 m @ 1.34 g/t Au & 0.22% Cu 79 m @ 0.57 g/t Au & 0.14% Cu 89 m @ 0.56 g/t Au & 0.13% Cu 6 m @ 0.59 g/t Au & 0.06% Cu 5 m @ 0.28 g/t Au & 0.04% Cu 81 m @ 0.87 g/t Au & 0.15% Cu 0mRL 24 m @ 0.58 g/t Au & 0.06% Cu 67 m @ 0.68 g/t Au & 0.19% Cu 0mRL 20 D D W W DD 10 W W DD 48 DD 05 200mRL W DD TRK01 938165mN 200mRL 01 485.900mE 98 m @ 0.33 g/t Au & 0.1% Cu 9 m @ 0.23 g/t Au & 0.3% Cu 18 m @ 0.31 g/t Au & 0.07% Cu -100mRL -100mRL 27 m @ 0.66 g/t Au & 0.1% Cu 118 m @ 0.75 g/t Au & 0.13% Cu -200mRL -200mRL 19 m @ 0.23 g/t Au & 0.03% Cu 9.1 m @ 0.38 g/t Au & 0.02% Cu -300mRL -300mRL 5 m @ 0.21 g/t Au & 0.03% Cu 27 m @ 0.20 g/t Au & 0.11% Cu 12 m @ 0.34 g/t Au & 0.13% Cu -400mRL 5 m @ 0.22 g/t Au & 0.04% Cu 5 m @ 0.20 g/t Au & 0.05% Cu 3 m @ 0.20 g/t Au & 0.03% Cu Microdiorite Medium Grained Diorite -400mRL 6 m @ 0.20 g/t Au & 0.14% Cu 6 m @ 0.28 g/t Au & 0.09% Cu Coarse Grained Diorite Intrusive Breccia Hydrothermal Breccia -500mRL -500mRL Fault zone 485.900mE 486.000mE 486.100mE 486.200mE 486.300mE 486.400mE Figure 6. Cross section TRK01 showing lithology and significant intersections in the centre of Randu Kuning Prospect. Figure 7. Textural differences between dioritic intrusions in Randu Kuning (Muthi, 2012). 22 Characteristics of Alteration and Mineralization at Randu Kuning - Wonogiri Project (Abdul Muthi et al.) Figure 8. Two types of breccia in Randu Kuning (Muthi, 2012). Porphyritic Plagioclase Diorite (PDIO) Post mineral, coarse-grained, porphyritic, >2 mm plagioclase, plagioclase rich, nonmineralized, (Not common in Randu Kuning but observed from intercepted drill holes in surrounding Randu Kuning prospect). In addition of multiple intrusions as described above, Randu Kuning drill holes intersected two types of breccia that are referred to as Intrusive Breccia (IBX) and Hydrothermal Breccia (HBX). Intrusive breccias are commonly observed in drill core in contact with, and immediately above the deep central microdiorite intrusion. They exhibit a distinct fragmental texture; generally increasingly polymictic clast assemblages at higher levels. This breccia is mostly comprised of diorite and microdiorite clasts which deformed during the intrusion phase. Occasionally, the presence of clasts of mafic dominant rock with disseminated pyrite is noted. The source of these clasts is not yet known. Hydrothermal breccia was identified in some drill intercepts with increasing silica and clay alteration. This breccia is also characterized by additional sulphide (pyrite) and partly rimmed clasts. It is interpreted that this unit is structurally controlled; hydrothermally generated silica and sulphide deposited in the fault zone. Corbett (2011) noted lithological variations; include fine and coarser grained equivalents as well as different styles of alteration; between different phases of diorite porphyry with prominent mafic phenocrysts as seen in the photos below. Shales, skarn type of alteration and intrusive dykes in the eastern part were intersected in WDD15 which appear to be the basementof thestratigraphy. RESULT AND DISCUSSION Alteration Hydrothermal alteration and zonation is modelled by Corbett (2011) as s hown in Figure 9. Outer propylitic zone is typified by magnetite-chlorite ± epidote alteration. Interestingly, this relatively weak alteration hosts elevated Cu-Au grade mineralisation (Hole DDH10IWG002) and the barren sheeted laminated quartz-magnetite veins (Hole WDD19 and WDD20). The zone of magnetite-epidote ± chlorite at lower temperature or actinolite towards the heat source is characteristic of inner propylitic alteration which is best displayed in hole WDD22. Fracture epidote within chlorite alteration in the upper portion passing down hole to pervasive epidote flooding of host rock. Potassic alteration is the most prevalent type of alteration in Randu Kuning mineralisation and it is indicated by pervasive magnetite - biotite ± secondary K-feldspar. 23 Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 15-28 dilatant structure WDD22 Possible settings of the barren quartz-magnetite veins in WDD19 and WDD20 bottom of WDD15 OUTER PROPYLITIC tes oli te ze ori te l ch ido lite ep ino t ac INNER PROPYLITIC M vein apophyses POTASSIC A vein magnetite biotite k-feldspar stock magmatic source Figure 9. Zoned prograde hydrothermal alteration model (Corbett, 2011). Crackle brecciation developed in this alteration zone as an overprint comprising magnetite ± actinolite ± chalcopyrite. Strong positive magnetic anomaly as identified by detail ground magnetic survey results is a reflection of this magnetic flooding of the hydrothermal alteration and brecciation. The Randu Kuning porphyry Cu-Au hydrothermal system is localized within an 800 m long NW- NNW trending magnetic feature which declines in intensity to the SE. The zone of silica-sericite-pyrite is a characteristic of phyllic alteration herein is adjacent to mainly the later stage porphyry style B veins, collapsing down structures and low temperature stage epithermal quartzsulphide veins. Total of 102 PIMA samples were taken from six holes of Cross Section TRK01. These samples are collected from veins and pervasive or in the wallrock alteration zones as well as clasts and matrix of the breccia to see if there is any pattern in alteration assemblage. Clay minerals illite and montmorillonite are the main minerals identified in the vein samples throughout the section 24 suggesting structurally controlled argillic alteration. Minor siderite (carbonate) occurs at depth. NH-Alunite is identified in one of the sample and is present at centre of the magnetic anomaly. In wallrock of altered samples, apart from illlite and montmorillonite, chlorite, kaolinite and tourmaline were also identified in WDD030. K- Alunite and gypsum were identified at depth of 743 m and 792 m respectively. The presence of alunite indicates more acid fluid environment and typical of advance argillic alteration (Corbett and Leach, 1998). Gypsum after anhydrite in porphyry setting is a characteristic of intermediate argillic as exampled by Middleton et al. (2004). The presence of these minerals might suggest another zone(s) of porphyry or proximity of major structure that channels hot acid fluid. However, more extensive study including alteration mineralogy is required to confirm this. Mineralization The gold-copper porphyry mineralization in Randu Kuning area developed within Characteristics of Alteration and Mineralization at Randu Kuning - Wonogiri Project (Abdul Muthi et al.) sheeted and stockwork quartz veins along microdiorite and in the peripheral of microdiorite intrusion (Figure 10). Overprinting events and style of mineralization are clearly visible in Randu Kuning. The CuAu grades developed as an accumulated affect of these as depicted in Figure 11. nated chalcopyrite is associated with clots of magnetite and/or chlorite - epidote and contains low grade Au and and some elevated (?) Cu. It is interpreted to be locally derived from cooling intrusion (Corbett, 2011). The early quartz veins are the veins generally known as the A veins with sinusoidal gradational vein margins and occasional contains disseminated chalcopyrite. Saccharoidal quartz texture and minor magnetite are common features of these veins. Majority of quartz veins with Style of mineralization can be grouped as disseminated chalcopyrite, early quartz veins, most quartz veins with sulphides, and barren laminated and sheeted quartz magnetite (Corbett, 2011). The dissemi486.000mE 486.100mE 486.200mE 486.300mE 486.400mE W DD 01 485.900mE 30 DD W DD W 129.5 m @ 0.83 g/t Au & 0.24% Cu 100mRL 100mRL 123.5 m @ 1.34 g/t Au & 0.22% Cu 79 m @ 0.57 g/t Au & 0.14% Cu 89 m @ 0.56 g/t Au & 0.13% Cu 6 m @ 0.59 g/t Au & 0.06% Cu 5 m @ 0.28 g/t Au & 0.04% Cu 81 m @ 0.87 g/t Au & 0.15% Cu 0mRL 24 m @ 0.58 g/t Au & 0.06% Cu 67 m @ 0.68 g/t Au & 0.19% Cu 0mRL 20 D D W 200mRL 48 10 W W DD DD 200mRL 05 65.6 m @ 1.24 g/t Au & 0.29% Cu 98 m @ 0.33 g/t Au & 0.1% Cu 9 m @ 0.23 g/t Au & 0.3% Cu 18 m @ 0.31 g/t Au & 0.07% Cu -100mRL -100mRL 27 m @ 0.66 g/t Au & 0.1% Cu 118 m @ 0.75 g/t Au & 0.13% Cu -200mRL -200mRL 19 m @ 0.23 g/t Au & 0.03% Cu 9.1 m @ 0.38 g/t Au & 0.02% Cu Propylitic 5 m @ 0.21 g/t Au & 0.03% Cu Potassic Base of Weathering -300mRL Base of Oxidation -300mRL Phyllic 27 m @ 0.20 g/t Au & 0.11% Cu 12 m @ 0.34 g/t Au & 0.13% Cu 6 m @ 0.20 g/t Au & 0.14% Cu -400mRL 5 m @ 0.22 g/t Au & 0.04% Cu 5 m @ 0.20 g/t Au & 0.05% Cu 3 m @ 0.20 g/t Au & 0.03% Cu No Qtz vein Low Density Qtz Vein (0.1-1%) Low - Medium Density Qtz Vein (1-2.5%) -400mRL 6 m @ 0.28 g/t Au & 0.09% Cu Medium Density Qtz Vein (2.5-5%) High Density Qtz Vein (>5%) Qtz-Mt “M-Vein” Zone -500mRL -500mRL Qtz-Mt “A-Vein” Zone Stockwork Vein Sheeted Vein 485.900mE 486.000mE 486.100mE 486.200mE 486.300mE 486.400mE Figure 10. Cross Section TRK01 at the center of Randu Kuning showing alteration and mineralization. 25 Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 15-28 Figure 11. Plates showing different veining style and mineralization of Randu Kuning (Corbett, 2011). sulphides were the result of cooling of the system at depth including M veins, massive stockwork of A veins, linear A veins, and later the AB veins taking over the earlier A veins with deposition of chalcopyrite. The barren laminated and sheeted quartz magnetite veins in WDD19 & 20 resemble M veins and sheeted A style and both generally has no sulphide content. Overprinting of low temperature epithermal veins on the earlier porphyry system can be summarized in Figure 12, showing two possible mechanisms: cooling magmatic source and later magmatic event. Randu Kuning South Randu Kuning South is located about 300 m south of main Randu Kuning prospect Epithermal veins Epithermal veins x x x + + ++ + + + + + + + + + + + + + + + + + + x x x x x later magmatic event x x x cooling magmatic source x x x x x x x + + ++ + + #+ + # # + # # # # + # # + # + + + # + + + x x x # x x x x x x x x x x Figure 12. Alternative models for the origin of late stage epithermal veins (Corbett, 2011). 26 x Characteristics of Alteration and Mineralization at Randu Kuning - Wonogiri Project (Abdul Muthi et al.) and was tested by four drill holes. Crackle brecciation is common in this area filled mostly by sulphide. Alteration on this area is dominated by early prophylitic alteration which is overprinted by phyllic alteration, possibly controlled by intense faulting e.g. in WDD25. Infill clay and sulphide in the crackle breccia in WDD25 is believed to be a signature of epithermal system which is controlled by fault zone with typical assemblage of quartz-carbonate and pyrite veins. This area is interpreted as a fault controlling extension of porphyry system in Randu Kuning. E-W trending block faulting may cause variations in alteration. Pervasive K-feldspar is common in central Randu Kuning while the drilling shows extensive epidote alteration to the north with epidote flooding intersected in WDD22. This may be indicative of buried porphyry mineralized system. Some higher Au grade mineralization is related to epithermal veins. Encounter of older basement of sedimentary sequence defines the boundary of this mineralization system in the east. To the west, the system is not closed off and it requires more work to define as bornite veins have been intersected in hole WDD24 further west than expected. CONCLUSIONS Further work is required to define the western limit and postulated buried porphyry as well as investigation of advance argillic and intermediate argillic alteration at depth. Based on the data grained from geology and recent drilling, the mineralization at Randu Kuning indicates extensive gold copper porphyry mineralization from surface to 400 m depth below surface. It is related to near vertical gold copper porphyry deposits within a large eroded volcanic centre, appear to be related to a northward migrating Oligocene to Miocene volcanic arc. The system obtained its higher Cu-Au grades from overprinting episodes as seen in the overprinting of earlier chalcopyrite within disseminations and magnetitebearing fracture by sheeted quartz veins and later dilatant fractures filled with chalcopyrite. Some intrusions are particularly enriched with early sulphides. Barren laminated and sheeted quartz-magnetite veins within chlorite- magnetite altered diorite seems to be the upper unmineralized portion of a porphyry and the potential for vertical extension of these veins at depth is unknown. Current resources of Randu Kuning is estimated at 90.9 Mt at 0.53 g/t AuEq (0.35 g/t gold and 0.10% copper) using a cut off of 0.2 g/t AuEq (Augur Resources, 2012). ACKNOWLEDGMENTS The paper has been presented in the MGEI, Banda and East Sunda Seminar 2012. The authors would like to acknowledge the MGEI Committee who give a permission to publish the paper in MGI. REFERENCES Augur Resources Ltd., 2012. ASX News Release, 10 July 2012. Corbett G.J. and Leach T.M., 1998. Southwest Pacific Rim Gold-Copper Systems; Structure, alteration, and mineralisation. SEG Special Publication, 6, 236pp. Corbett, G.J., 2011. Comments on the exploration potential of the Wonogiri porphyry Cu-Au Project, Central Java, Indonesia, Gorbett Geological Services Pty. Ltd. 2011. 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