J. SE Asian Appl. Geol., 2015, Vol. 7(2), pp. 75–81 GEOLOGY AND ORE MINERALISATION OF NEOGENE SEDIMENTARY ROCK HOSTED LS EPITHERMAL GOLD DEPOSIT AT PANINGKABAN, BANYUMAS REGENCY, CENTRAL JAVA, INDONESIA Arifudin Idrus*1, Fahmi Hakim1, I Wayan Warmada1, Mochammad Aziz2, Jochen Kolb3, and F. Michael Meyer4 1Department of Geological Engineering, Gadjah Mada University, Yogyakarta, Indonesia 2Department of Geological Engineering, Jenderal Soedirman University, Indonesia 3Geological Survey of Denmark & Greenland 4RWTH Aachen University, Germany Abstract argillic alteration. Vein structures such as massive, swarm and low angle veins, stockwork and veins Low suphidation (LS) epithermal gold deposits were dispersed in diatreme breccia are present. Normal recently found in the Paningkaban area, Central banded, cockade, crustiform, bladed carbonates as Java province, Indonesia, with more than five hun- well as, comb and saccharoidal features are the typi- dred artisanal gold miners currently operating in cal vein textures. It is noteworthy that the veins are the area. This study is aimed to understand the ge- basically composed of carbonate with minor quartz ological factors controlling the gold mineralization at gold grades of up to 83 g/t Au. Based on the and to characterize the alteration and ore mineral- vein structures and textures, four stages of ore min- ogy of the deposit. Several epithermal veins/veinlets eralization were developed consisting of (a) early trending N–S, NW–SE, and NE–SW are hosted by stage (fluidized breccia and quartz vein), (b) middle Tertiary turbiditic volcanoclastic sedimentary rocks stage (carbonate-base metal), (c) late stage (late car- of the Halang formation. This formation is com- bonate), and supergene stage. Gold mineralization posed of looping gradation of sandstone and silt- originated mainly during middle and late stages, stone units. Pre- and syn-mineralization struc- particularly in association with cockade, crustiform, tures such as extension joints, normal sinitral fault bladed carbonate-base metal veins. Based on those and sinitral fault control the gold mineralization. various features, the LS epithermal deposit in the Fault movements formed dilational jogs manifested study area is categorized as carbonate-base metal- by NW-SE-trending en-echelon tension gash veins. gold mineralization type. Four main alteration zones are identified: (a) phyl- lic, (b) argillic, (c) sub-propylitic and (d) weak sub- Keywords: Geology, Ore mineralization, Hy- propylitic. Ore minerals consist of native gold, elec- drothermal alteration, LS epithermal, Paningkaban trum, native silver, pyrite, chalcopyrite, sphalerite, area, Indonesia. galena, arsenopyrite, cubanite, marcasite, covellite and tennantite, which are commonly associated with 1 Introduction *Corresponding author: A. IDRUS, Department of Ge- Fifteen magmatic arcs are identified in In- ological Engineering, Gadjah Mada University. Jl. Grafika 2 Yogyakarta, Indonesia. E-mail: arifi[email protected] donesia which have a total extent in excess 75 IDRUS et al. of 15.000 km2. However 98% of the country’s gold-bearing veins. Laboratory work includes precious (gold, silver) and base metals (par- slab and vein textural and structural analysis, ticularly copper) are derived exclusively from mineralogy (petrography, ore microscopy and six arcs of mid-Tertiary or younger age (Carlile X-ray diffraction) and ore/mineral chemistry and Mitchell, 1994). These six major Neogene analyses of a total of 77 samples. Mineralogical mineralized magmatic arcs include the Sunda- analysis was conducted at Department of Ge- Banda, Central Kalimantan, Sulawesi-East Mi- ological Engineering, Gadjah Mada University, nandao, Halmahera and Central Papuan fold ore chemistry was done at AAS laboratory, In- and thrust belt. The Sunda-Banda arc is the tertek Jakarta, and gold chemistry was analysed longest of the magmatic arcs in Indonesia. It by EPMA at IML-RWTH Aachen University. extends nearly 4,000 km from northern Suma- tra through Sumbawa (Sunda arc) and onwards 3 Results and Discussion through Flores to its terminus in the Banda Islands (Banda arc). One of the most mineral- Geology ized segments of the Sunda-Banda magmatic arc is the southern part of Java Island. Our The study area is underlain by a series of sedi- research location, Paningkaban, is one of the mentary units, which from oldest to youngest gold-mineralized regions situated in Central are interlayered sandstone and siltstone, tuff, Java, 165 km west of Yogyakarta (Figure 1). calcareous sandstone, and limestone (Figure 2). Several primary gold and base metal de- The interlayered sandstone and siltstone unit is posits were discovered along the Sunda-Banda the host rock of LS epithermal gold mineraliza- magmatic arc, for instance, Kapur-Natas in tion in the study area. This unit is a member of North Sumatera (Idrus et al., 2007), Gunung the Miocene Halang formation. The feldspar- Pongkor in West Java (Warmada, 2003), Selo- rich rock unit is weakly to intensely altered giri in Wonogiri (Imai et al., 2007) and Batu Hi- from subpropylitic to argillic alteration styles. jau in Sumbawa (Garwin, 2000). All gold-base The age of the volcano-sedimentary rock unit metal deposits described above are hosted by is Late Miocene (Djuri et al., 1996). Younger volcanic rocks. Gold exploration activities in rock units are not mineralized and not altered. Indonesia are recently not only focused on vol- Pre- and syn-mineralization structures such as canic/magmatic belt, but also starting to shift extension joints, normal-sinitral fault and sini- along metamorphic and sedimentary terrains. tral fault controlled the gold mineralization and Primary gold mineralization occurring in as- alteration. Fault movements formed dilational sociation with metamorphic rocks are, for in- jogs manifested by N–S, NE–SW and NW–SE- stance, Awak Mas in South Sulawesi (Queru- trending en-echelon tension gash veins (Figure bin and Walters, 2011) and Gunung Butak in 3). Buru Island, Mollucas (Idrus et al., 2014). How- ever, no advanced studies have been conducted Hydrothermal alteration for hypogene gold deposits, particularly LS ep- Four main alteration zones are identified in- ithermal gold vein types hosted by sedimentary cluding: (a) phyllic alteration typified by rocks such as in the Paningkaban area. sericite-quartz-pyrite±carbonate±chlorite, (b) argillic alteration presented by illite-smectite- 2 Research methods illite/smectite-carbonate-quartz-sericite- dickite±chlorite, (c) sub-propylitic alteration This study is conducted in four stages includ- characterized by chlorite-chlorite/smectite- ing literature, fieldwork, laboratory work, data smectite-carbonate±quartz±zeolite, and (d) processing and interpretation. Fieldwork in- weak sub-propylitic alteration composed of cludes mapping of surface geology, alteration chlorite-smectite±carbonate mineral assem- and ore mineralization as well as sampling of blages, respectively. Ore mineralization is representative rock types, altered rocks and 76 © 2015 Department of Geological Engineering, Gadjah Mada University GEOLOGY AND ORE MINERALIZATION OF THE PANINGKABAN LS EPITHERMAL GOLD DEPOSIT Figure 1: Research location (Paningkaban area) plotted on the SRTM (Shuttle Radar Topography Mission) map of Java Island. closely associated with argillic and weak sub- propylitic alteration (Figure 4). Ore mineralization Table 1: Ore and gangue mineral paragenetic Vein structures such as massive, swarm and sequences of LS epithermal gold mineralization low angle veins, stockworks and diatreme at the Paningkaban area. breccia-hosted veins are present. Normal banded, cockade, crustiform, bladed carbon- ate, comb and saccharoidal are the typical open space filling textures of the LS epithermal veins. It is noteworthy that the veins are composed of carbonate with minor quartz and adularia and gold grades ranging from 0.09 to 83 g/t Au (8 vein samples). Electrum chemistry shows 73.9 wt.% Au and 23.5 wt.% Ag, respectively. Based on the vein structures and textures, four stages of ore mineralization in the study area were developed consisting of: (a) early stage (fluidized breccia and quartz vein), (b) middle stage (carbonate-base metal), (c) late stage (late carbonate), and supergene stage (Ta- ble 1). Gold mineralization chiefly occurred during Middle and Late stages particularly in association with cockade, crustiform, bladed carbonate-base metal veins (Table 1 and Fig- ure 5). Less abundant electrum is also identi- fied within banded quartz veins from the early stage. Precious metals identified consist of native gold, electrum and native silver (Figure 5), sul- phides consist of pyrite, chalcopyrite, spha- lerite, galena, arsenopyrite, cubanite, marcasite, covellite and tennantite. Gold-bearing veins are © 2015 Department of Geological Engineering, Gadjah Mada University 77 IDRUS et al. Figure 2: Geological map and cross section of the Paningkaban area. 78 © 2015 Department of Geological Engineering, Gadjah Mada University GEOLOGY AND ORE MINERALIZATION OF THE PANINGKABAN LS EPITHERMAL GOLD DEPOSIT Figure 3: NW–SE-trending en-echelon tension gash veins hosted by sandstone of Miocene Halang formation. Figure 4: Hydrothermal alteration map of the Paningkaban area. © 2015 Department of Geological Engineering, Gadjah Mada University 79 IDRUS et al. Figure 5: Photograph of a crustiform-bladed carbonate vein (top left) with related sketch (bottom), photomicrograph of ore sample showing the presence of native gold (Au) and base metal sulphides in