Journal of Applied Geology, vol. 1(2), 2016, pp. 63–70 DOI: http://dx.doi.org/10.22146/jag.26962 Occurrences and Characteristics of Gold Mineralization in Rampi Block Prospect, North Luwu Regency, South Sulawesi Province, Indonesia Arifudin Idrus*1, Suaib Mansur2, Ahmad2, Rahmayuddin2, and Abdul Mahdi2 1Department of Geological Engineering, Faculty of Engineering, Gadjah Mada University, Indonesia 2Mining and Energy Agency, North Luwu District, South Sulawesi Province, Indonesia ABSTRACT. This study is aimed to identify the occurences, characterisatics and genetic type of gold mineralization, and to elucidate the preliminary gold potential in the study area. Research method includes field observation of prospect geology, hydrothermal alter- ation and ore mineralization. Laboratory analysis consists of petrography, ore microscopy and ore chemistry. Geologically, quartz ± gold veins were found in Rampi block prospect, which are mainly hosted by metamorphic and metasediment rocks. The quartz veins has structurally segmented with massive, brecciated and laminated textures. Orientation and distribution of veins is controlled by NW–SE and NE–SW trending structures. Sulfides are present with minor abundance (<1 %). Pyrite is obviously observed and partially oxidized. Arsenopyrite and stibnite are minor, while basemetal (Pb, Zn) sulfides are very rare. Gold mineralization occurred in quartz veins and closely related to silica-clay and silicic alter- ation. Gold grade varies from 0.1 to 11 ppm Au (29 samples). Quartz veins contained high gold grade (>9 ppm) commonly show laminated and brecciated texture and has been un- dergone a supergene enrichment. According to several characteristics above, gold deposit in Rampi block prospect tends to meet the criteria of orogenic/mesothermal gold type (cf. Groves et al., 2003). The deposit shares similarities with Awak Mas prospect, which is already confirmed to be a mesothermal type in Luwu district (cf. Querubin and Walters, 2011). Therefore, discoveries of economic mesothermal gold deposits are still open in the metamorphic terrains, and thus, it requires a systematic exploration based on the genetic model of the deposit. Keywords: Gold · Orogenic/mesothermal · Rampi block prospect · North Luwu · Su- lawesi · Indonesia. 1 INTRODUCTION Luwu Regency, located in northern part of Su- The needs and metal prices, especially pre- lawesi Province, are the area with the poten- cious metal and base metal in the world mar- tial of ore mineral resources, which are dom- ket are steadily increasing, “forcing” us to con- inated by gold hosted by metamorphic rock, duct an inventory, exploration and extraction volcanic rock and sedimentary/metasediment of those metal deposits. Moreover, with the rocks. Commonly, gold mineralization is asso- implementation of regional autonomy and de- ciated with quartz vein. With this background, centralized management of mineral deposits, a study was conducted with the focus on dis- the exploration activities become vital in plan- tribution and characterisation of ore mineral- ning and resource utilization policy to improve ization and interpretation of ore deposit ori- the regional economy. For example, North gin. The research is focused on Rampi block prospect (Figure 1). The objectives of this study *Corresponding author: A. IDRUS, Department of are to provide an overview of occurrences, char- Geological Engineering, Faculty of Engineering, Gadjah acteristics, deposit type and potential of gold Mada University. Jl. Grafika 2 Yogyakarta, Indonesia. E- mail: arifi[email protected] mineralization in research area and as support- 2502-2822/© 2016 Department of Geological Engineering, Gadjah Mada University IDRUS et al. area that consists of schist, gneiss, marble, ser- pentinite and metaquartzite, slate, phyllite and breccia. Latimojong Formation (Kls) is com- prised of interbedded of slate, phyllite, wake, limestone, quartzite and siltstone intercalated with conglomerate and chert. Kambuno gran- ite (Tpkg) consists of granite and granodiorite. Based on its lithological similarity with gran- ite in Pasangkayu map sheet, the Kambuno granite is estimated to be formed at 3.35 mil- lion years ago (Pliocene). Rampi tuff formation (Tmrt) is composed of tuffaceous sandstone, grey tuff and crystal tuff. This Formation is unconformably deposited on Latimojong For- mation and inter fingering with Tineba Volcanic Rock (Tmtv). Tineba Rock Volcanic consists of hornblende andesite lava, basalt, quartz latitte and breccia. This formation occupies Tineba plateau that extends northward Rampi area in northwestern part of Malili geological map. Al- luvium (Qal) is composed of river, swamp and coastal deposits. 3 RESEARCH METHODS Fieldwork methods applied to achieve the ob- Figure 1: Research area (Rampi block prospect) jectives of research consist of geological and hy- in Rampi sub-district, North Luwu regency drothermal alteration observation. Rock and located in northern part of South Sulawesi ore samples were collected for various labo- province. ratory analyses. Laboratory analysis includes mineralogical and ore geochemical analyses. ing information for exploration activities and Mineralogical analysis was performed for 15 development of ore mineral resources in the re- thin sections and 5 polished sections using gion. polarized- and reflected-light microscopes, re- spectively, at Mineral Resources laboratory, De- 2 REGIONAL GEOLOGY partment of Geological Engineering, Gadjah Rampi block prospect area is a plateau that ex- Mada University. Altered rock and ore petrog- tends from east to west. The highest peak of raphy was described based on Thomson and the mountain in the area is Temaduri Mountain Thomson (1996) and Marshall et al. (2004), (2,074 m). Among these mountains, lies rivers respectively. Twenty ore samples were anal- which commonly flow to Lariang River and To- ysed using AAS (Atomic Absorption Spectrom- boru River. Regional stratigraphy of research etry) at Energy and Mineral Resources (ESDM) area refers to the Geological Map of Malili sheet Agency of South Sulawesi province, where 9 (Simandjuntak et al., 1991) (Figure 2). Region- ore samples were analysed using AAS at In- ally, Rampi area is occupied by various litholo- tertek Utama Services laboratory in Jakarta. gies from oldest to youngest including Pom- Both fieldwork and laboratory analytical data pangeo Complex (MTmp), Latimojong (Kls), were integrated and interpreted to achieve the Kambuno Granite (Tpkg), Rampi Tuff Forma- objectives of research. tion (Tmrt), Tineba Volcanic Rock (Tmtv) and Alluvium (Qal). Pompangeo metamorphic rock complex (MTmp) is the oldest formation in the study 64 Journal of Applied Geology OCCURRENCES AND CHARACTERISTICS OF GOLD MINERALIZATION IN RAMPI BLOCK PROSPECT Figure 2: Regional geological map of Rampi block prospect area with 25.000 Ha plotted on Geolog- ical map of Malili, Majene, and western part of Palopo sheet (Simandjuntak et al, 1991). Sampling points and quartz veins are indicated on the map. 4 RESULTS AND DISCUSSION Hydrothermal alteration Geology of the study area Hydrothermal alteration types developed in re- On the basis of regional geology and ge- search area consist of propylitic alteration (chlo- ological observation, the studied area is rite - epidote - calcite), argillic alteration (clay predominantly occupied by sedimentary mineral) and silicification zone (quartz - clay). rock/metasedimentary rocks from Latimojong Field Observation indicates that ore mineraliza- formation (Kls), and a wide variety of volcanic tion in form of gold-bearing quartz veins are rocks including tuffs, breccias and andesite lava mostly enveloped by sillification and argillic from Rampi tuff formation (Tmrt) and Tineba zone. However, in some cases, quartz veins can volcanic rock formation (Tmtv). Schists, gneiss, be associated with propylitic alteration. In gen- marble, serpentinite, meta- quartzite, phyllite eral, prophyllitic zone developed in the periph- and slate, members of the Pompangeo com- ery of the mineralization system. Outcrop and plex (MTmp) are exposed in the eastern part petrographic microphotograph of silicified and of the research area. Gold mineralization and propylitic altered rocks are seen in Figure 3. base metals are found in the form of quartz Characteristics of gold mineralization veins hosted by metamorphic/sedimentary Ore mineralization in Rampi block prospect rocks/metasedimentary rock (Latimojong for- occurred in the form of quartz veins contain- mation), volcanic rocks (tuff) and also possi- ing gold and other metals and disseminated bly hosted in the Pompangeo metamorphic in wallrocks around the vein. The quartz vein complex. Geological structures particularly are commonly segmented with massive, brec- NW–SE, NE–SW trending strike slip faults ciated and laminated textures (Figure 4A–D). were interpreted to be a part of major Palu- Quartz veins are enveloped by silica-clay and Koro fault orders. The strike-slip faults were silicified altered wallrock. Several quartz probably the main geological aspect control- veins are hosted by prophyllitic altered rock. ling the formation of gold mineralization in the Quartz veins found in small scale mining at research area. Journal of Applied Geology 65 IDRUS et al. Figure 3: Outcrop/handspecimen and petrographic microphotograph of altered wall rocks in Rampi block prospect: (A) and (B) silicified rock; (C) and (D) propylitic altered rock. Tomonongka River and Salman Tunnel show Ore chemistry typical characteristics of strongly brecciated, Based