The Metamorphic Rocks-Hosted Gold Mineralization At Rumbia Mountains Prospect Area In The Southeastern Arm Of Sulawesi Island, Indonesia

Recently, in Indonesia gold exploration activities are not only focused along volcanic-magmatic belts, but also starting to shift along metamorphic and sedimentary terrains. The study area is located in Rumbia mountains, Bombana Regency, Southeast Sulawesi Province. This paper is aimed to describe characteristics of alteration and ore mineralization associated with metamorphic rock-related gold deposits. The study area is found the placer and primary gold hosted by metamorphic rocks. The gold is evidently derived from gold-bearing quartz veins hosted by Pompangeo Metamorphic Complex (PMC). These quartz veins are currently recognized in metamorphic rocks at Rumbia Mountains. The quartz veins are mostly sheared/deformed, brecciated, irregular vein, segmented and relatively massive and crystalline texture with thickness from 1 cm to 15.7 cm. The wallrock are generally weakly altered. Hydrothermal alteration types include sericitization, argillic, inner propylitic, propylitic, carbonization and carbonatization. There some precious metal identified consist of native gold and ore mineralization including pyrite (FeS2), chalcopyrite (CuFeS2), hematite (Fe2O3), cinnabar (HgS), stibnite (Sb2S3) and goethite (FeHO2). The veins contain erratic gold in various grades from below detection limit <0.0002 ppm to 18.4 ppm. Based on those characteristics, it obviously indicates that the primary gold deposit present in the study area is of orogenic gold deposit type. The orogenic gold deposit is one of the new targets for exploration in Indonesia.


Introduction
Gold is one of the most malleable, ductile, dense, conductive, nondestructive, brilliant, and beautiful of metals.This unique set of qualities has made it a coveted object for most of human history in almost every civilization, and there have been active gold markets for over 6000 years (Green, 2007(Green, in et al., 2015)), so many research and mining companies are trying to find economical gold reserves to explore.
Currently, gold exploration activities in Indonesia are focused on volcanic-hosted hydrothermal deposit.Several gold deposits were discovered are Erstberg, Kucing Liar, Deep Ore Zone (DOZ) in Papua including skarn type; Batu Hijau in Sumbawa Island (Idrus et al., 2007;Imai & Ohno, 2005) and Grasberg in Papua including porphyry type ; Pongkor in West Java (Warmada, 2003), Gosowong in Halmahera Island including epithermal type.In Sulawesi Island, gold is also predominantly related to volcanic rocks, which are extended along western and northern Neogene magmatic arcs of the island (Idrus, 2009) (Fig. 1).
The gold mineralization genetically occur in association with sedimentary rocks is Paningkaban, Banyumas Regency, Central Java (Idrus et al., 2015).
Rumbia mountaints (Fig. 2) is one of the location having a potential of mineral resources gold has been found in southeast Sulawesi.The gold is not associated with volcanic rock-related gold deposits but associated with metamorphic rocks.This research aims to explore the characteristics of hydrothermal alteration and mineralization hosted by metamorphic rocks at Rumbia mountains by the aplication petrography, ore microscopic, (X-Ray Diffraction) and FA-AAS (Fire Assay-Atomic Absorption Spectrophotometry) analyses.This study is an important stage for the next exploration of gold in the area or other areas that have an identical setting of geology.Metamorphic rockhosted gold deposits could represent the new targets for gold exploration particularly in Indonesia.Simandjuntak et al., 1993).Squared area indicates the location area of this study.

Geological Setting
The stratigraphy in the southeastern arm of Sulawesi consists of three constituent rocks are Sulawesi Molasse composed of clastic sediments and carbonate, ophiolite complex are dominated by mafic and ultramafic rocks and continental terrain composed of metamorphic rocks.Contaccts between the ophiolite complex and continental terrain, including their basement rocks are faulted.The Sulawesi Mollase uncorformably overlies both the ophiolite complex and continental terrain (Surono, 2013).The mountains Rumbia is a part of continental terrain is subsequently occupied by metamorphic rocks (Pompangeo Complex, Mtpm) consisting of mica schist, quartzite, glaucophane schist and chert.The continental terrain, which were firstly described by Surono, 1994.The metasediments and metamorphic rocks are of Permian-Carboniferrous in age and occupy the Mendoke and Rumbia Mountains.Mica schist and metasediments particularly meta-sandstone and marble are commonly characterized by the presence of quartz veins various width up to 2 meters, containing gold in some places (Idrus et al., 2011).

Research Methods
Field investigation was carried out in Rumbia mountaints area of Southest Sulawesi.The samples consist of altered rock, veins and clay samples collected from different hydrothermal alteration zones and ore samples.This study is conducted in four stages including field work, laboratory analyses, data analyses and interpretation.Fieldwork includes mapping of surface geology, alteration and ore mineralization as well as sampling of representative rock types, altered rocks and gold bearing veins.Laboratory work includes slab, vein textural dan structural analyses and mineralogy (petrography, ore microscopy and XRD (X-Ray diffraction) and ore geochemistry.Mineralogical analysis was conducted at Department of Geological Engineering, Gadjah Mada University and ore geochemistry was done at AAS laboratory, ALS Canada Ltd in Canada and PT.Intertek Utama Services Jakarta.

The Quartz Vein
The quartz veins in the study area consist three generations.The first is parallel to the foliations, the second crosscuts the first generation of veins/foliations, and the third is of laminated deformed quartz+calcite veins at the late stage (Fig. 3).Gold grades in the second and third veins are relatively higher than that in first veins (Table 1).Characteristics of quartz veins are mostly sheared/ deformed, irregular vein, brecciated, and relatively massive with thickness from 1 cm to 15.7 cm.Based on data shows that gold-bearing quartz (Qz) veins/veinlets have been discovered in association with Paleozoic metamorphic rocks particularly mica schist, actinolite schist (green schist), phyllite and metasandstone.In addition, as observed by Prihatmoko et al. (2010), druzy/sugary and some pseudomorph bladed carbonate textures have also been recognized associated with quartz veins/reefs cross cutting foliation.In the Onggomate hill the veins formed a breccias zone composed of quartz as matrix, massive to crystalline, crackle to mosaic, with mica schist and phyllite fragments.In the Roko-Roko hill quartz veins (1-30 cm) hosted by mica schist and metasediment are commonly massive to crystalline quartz (druzy textures) with pseudomorph bladed carbonate textures.Therefore, at least 2 later stages of veinings (after the first generation veinings) could be identified, including (1) vein breccias and (2) later quartz veinlets, 1-10 mm, which are commonly crystalline and containing native gold (Prihatmoko et al., 2010in Idrus et al, 2012 ).

Hydrothermal Alteration
Hydrothermal alteration and mineralization underlie within all lythologic units found in the research area.Widespread zones of hydrothermal alteration are found on the surface.Alteration is typically pervasive and selective pervasive with intensity ranges from weak to strong, so it is very rare to observe good outcrops in the area.In general, the wallrocks are weakly altered.Strong alteration zone is only restricted surrounding quartz veins (like halos/selvage).There are six main alteration zones that are indentified, namely sericitization, argillic, inner propylitic, propylitic, carbonatization and carbonization alterations.Sericitization is characterized by the presence of sericite (or muscovite), chlorite and quartz while argillic alteration is typified by montmorilonit-kaolinite, chlorite and quartz.Inner propylitic alteration is characterized by actinolite, chlorite, quatz and epidot, while propylitic alteration is typified epidote and/or chlorite and calcite.Carbonization is probably represented by (rare) occurrence of graphite with commonly black in colour in the quartz vein and altered mica schist and carbonatization altera¬tion is typified by the presence of calcite veinlets/stringers while (Fig. 4).Ore mineralization is closely associated with seritisization, argillic, inner propylitic, propylitic and weak carbonation alterations.
The carbonization is considered to be one of the alteration type characteristics, associated with orogenic/ metamorphic-hosted gold deposit (Idrus, et al., 2012).Idrus et al. (2012)
Pyrite, hematite, cinnabar and stibnite are present abundantly in the primary mineralization gold deposits, present in the quartz veins and wallrocks; and commonly present at alteration rocks (Fig. 5).Pyrite occurs as isolated idiomorphic crystals, angular fragmens,, strongly brecciated fragments, anhedral shape, medium reflectance and isotropic.Some pyrite grains are partly enclosed by hematite, chalcopyrite and possibly stibnite.Fractures and brittle cavities in pyrite are often filled by hematite and chalcopyrite.Hematite is typically pinkish orange in color and commonly present in altered rocks, internal purple reflections, subhedral-anhedral crystals, present as sulfide mineral interactions with oxygen causing the oxidation of sulphide minerals, especially pyrite.Hematite is looked replacing pyrite.
Chalcopyrite is associated with pyrite while cinnabar is typically pinkish red in color and commonly occurred in the form of mineralized layers along foliations of the metamorphic rocks.
Pyrite, hematite, cinnabar and stibnite are genetically closely related to gold mineralization.Graphite (Gr) is present as a result of carbonization and associated with stibnite minerals.
Those sulfides could be pathfinder minerals for the exploration of the metamorphic-hosted gold deposit.In general, gold is very fine-grain, but occasionally native gold is visible in quartz veins.
Bulk-ore chemistry analyzed by AAS (Atomic Absorption Spectrometry) indicates a very broad and erratic variation of gold grade ranging from below detection limit <0.0002 ppm to 18.4 ppm Au (Table 1).

Conclusion
The primary gold grains hosted by Rumbia mountains are evidently derived from sheared and segmented, partly brecciated, relatively massive, laminated gold-bearing quartz±calcite veins/reefs with thickness from 1 cm to 15.7 cm hosted by Pompangeo Metamorphic Complex (PMC).The PMC particularly consists of mica schist (dominant rock type), actinolite schist, phyllite and metasedimen.Mica schist is abundantly composed of muscovite, chlorite quartz, actinolite, epidote with a small amount of, sericite, rutile, kyanite and opaque minerals.Hence, the metamorphic rock is categorized into green schist facies, which is noted as an important host rock facies for orogenic gold deposit worldwide The metamorphic rocks are strongly weathered, however trenching program has opened up the soil cover and exposes the hydrothermal alteration zones.In general, the wallrocks are weakly altered.Hydrothermal alteration types include sericitization, argillic, inner propylitic, propylitic, carbonatization and carbonization.The veins contain erratic gold in various grades from below detection limit limit <0.0002 ppm to 18.4 ppm.Mineralogically, gold is genetically related to consist of pyrite (FeS2), chalcopyrite (CuFeS2), hematite (Fe2O3), cinnabar (HgS), stibnite (Sb2S3) and goethite (FeHO2).Idrus et al. (2012) also reported the presence tripuhyite (FeSbO4) and rare arsenopyrite (FeAsS2) are present in the quartz veins and silicified metamorphic wallrocks.Gold is mainly identified in the form of 'free gold' among silicate minerals particularly quartz.Based on those characteristics, it obviously indicates that the primary gold deposit present in the study area is of orogenic gold deposit type (cf.Groves et al., 1998;2003 andGoldfarb et al., 2005).The orogenic gold deposit is one of the new targets for exploration in Indonesia.

Acknowledgements
This study was funded by RISTEKDIKTI research grant 2017 awarded to the author.Fieldwork was partly supported by PT.Panca Logam Makmur (PLM) which has given us permission to do this research in the Wumbubangka prospect area and its vicinity.Special thanks to my students from Department of Geological Engineering, Halu Oleo University for their assistance during the fieldwork.We also would like to thank the Head and staff the Geological Engineering
also reported the presence silicification alteration is represented by silicified metasediment and mica schist mostly present surrounding quartz veins or along structural zones.The presence of narrow clay-sericite alteration halo (tens cm to 1 m) around the quartz veins in the Roko-Roko hill(Prihatmoko et al., 2010in Idrus et al., 2012).