The Quartz Veins, Hydrothermal Alteration and Ore Mineralization of Orogenic Gold Deposit at Mendoke Mountains, Southeast Sulawesi, Indonesia

Abstract


Introduction
Gold in Indonesia is not only explored in volcanic and magmatic arcs, but also in sedimentary and metamorphic belts (Hasria et al., 2019).Many current discoveries of gold deposits with hosted metamorphic rocks such as Buru and Seram Islands orogenic gold deposits, in Mollucas (Idrus et al., 2014;Samalehu et al., 2021); LS-epithermal at Poboya, Central Sulawesi (Syafrizal et al., 2017); mesothermal at Awak Mas, South Sulawesi (Querubin and Walters, 2011) and orogenic gold deposits at Rumbia Mountains, Southeast Sulawesi (Idrus et al., 2012;Hasria et al., 2019).One of the most important types of gold resources in metamorphic belts that are formed by orogenic processes and contain more than half of the world's gold production ore is orogenic gold deposits (Goldfarb et al., 2019).Suitable geological environments for the formation of these deposits are the folded and orogenic belts and which occur on the active continental margins settings (Groves, 1993;Groves et al., 1998;2003;Goldfarb & Groves, 2015).Gebre-Mariam et al. (1995) stated that this deposit is occur in prehnite pumpellyite-greenschist-amphibolite-lower granulite transition facies.
The study of vein texture, hydrothermal alteration and ore mineralization is important to determine the type of gold deposit in the study area.This study is aimed to elucidate the main characteristics of gold mineralization in study area in term of quartz veins, hydrothermal alteration and ore mineralization.Furthermore, there has been no specific study about type of gold deposit at Mendoke Mountains.Currently, gold deposits with metamorphic rock are one of the deposits being explored in Indonesia and will be a new exploration target for primary gold resources.

Geological Setting
The Indonesian region is formed by three plates, the Australian Continental Plate, the Pacific Oceanic Plate or the Philippine Oceanic Plate, and the Eurasian Continental Plate.These tectonic dynamics affect the physical form of the Indonesian earth.The Australian Continental Plate is moving to north direction with speed of 7-8 cm/year and the Pacific Oceanic Plate or Philippine Oceanic Plate is moving to west direction with speed of 8-10,2 cm/year.The Eurasian Continental Plate is slowly moving to south direction (Hamilton, 1979;Simanjuntak & Barber, 1996) (Fig. 1).The Southeast Sulawesi Province is part of the Sulawesi Island which is located in the center of Indonesian region crotching the equator which is also affected by the interactions of these plates (Hamilton, 1979).Therefore, Southeast Sulawesi Province has a complex morphology, lithology and geological structure as well as a diversity of types of mineral deposits such as laterite nickel, laterite Fe, chromite and gold deposits (Van Leeuwen and Pieters, 2012;Surono, 2013;Hasria et al., 2019;Hasria et al., 2021a;2021b).The Mendoke Mountains are located in the central part of Southeast Sulawesi Province (Fig. 2).
Geomorphologically, the study area composed are flat hilly unit, flat hilly steeply very flat to moderately steep and moderately steep mountainous to extremely mountainous (Hasria et al., 2021c).Stratigraphically, composed are Mengkoka Complex, Pompangeo Complex, Ultramafic Complex, Langkowala Formation, Boepinang Formation, Alangga Formation and Alluvium.Geological structure controlled by a geological structure trending southeast-northwest and east-west (Simandjuntak et al., 1993).

Materials and Methods
The materials in this study were rock samples consisting of fresh rock, altered rock and quartz vein samples.Data were collected through observation and representative rock sampling in the field.The purpose of the petrographic analysis is to determine the mineral composition of rocks, while X-Ray diffraction (XRD) analysis is to identify of clay minerals (Thompson and Thompson, 1996) because Xrays are able to analyze the crystal structure and identify clay minerals in the form of crystals (Sutarno, 2022).XRD analysis has an advantage over petrographic analysis because it can clearly identify the type of clay mineral (Wicaksono et al., 2017).The purpose of ore microscopic analysis is to determine the optical properties and types of ore minerals.Determination of ore mineralization refers to Marshall et al. (2004) and Pracejus (2008).Laboratory analyzed samples consisted of 48 samples for petrographic, 5 samples for XRD and 28 samples for ore microscopy analysis.The laboratory used to analyze petrography, ore microscopy and XRD are the Laboratory at the Department of Geological Engineering, Universitas Gadjah Mada, Yogyakarta, Indonesia.

The Type and Texture of Quartz Veins
There are three types of quartz veins in the study area parallel to direction of foliation, crosscut to direction of foliation, and laminated quartz veins (Fig. 3).The texture of quartz veins is generally deformed, segmented, brittle, sheared, laminated, sheet, irregular veins, brecciated, relatively massive and sigmoidal.Based on the results of field observations, it shows that the quartz veins in metamorphic rocks found in mica schist, actinolite schist, and chlorite schist and graphite schist are petrologically included in the greenschist facies.The type and texture of quartz veins at Mondoke Mountains similar with quartz veins at Rumbia mountains (Idrus et al., 2012;Hasria et al., 2019).Commonly quartz veins of various minerals at Mendoke Mountains are be related with hydrothermal mineralization/ore deposits (Best, 2003). .massive quartz veins parallel to direction of foliation; c).Sigmoidal of quartz veins parallel to direction of foliation; d).Deformed-brecciated and massive of quartz veins crosscut to direction of foliation; e).irregular vein, brittle, sheeted, sheared, segmented of quartz veins crosscut to direction of foliation; f).high oxidation of deformed quartz veins crosscut to direction of foliation; g).sheeted veins parallel to direction of foliation; h).brecciated, deformed, sheared, laminated quartz veins; i).laminate quartz veins.

Hydrothermal Alteration
Hydrothermal alteration at Mendoke Mountains was determined based on data from surface geological observations, petrographic and XRD analysis.The determination of the alteration hydrothermal zone refers to the classification of Corbett and Leach (1997) and Pirajno (2009).The determination of clay minerals by XRD analysis refers to Chen (1977).There are four types of alteration at study area based on a combination of petrographic and XRD analysis, sericitization, argillic, propylitic and carbonization alterations.

Sericitization Alteration
Alteration of sericitization is indicated by the abundantly of sericite or muscovite minerals and the presence of other minerals such as chlorite and quartz (Fig. 4).This alteration is found in quartz veins associated with ore-bearing minerals.Sericite mineral is identified as a result of mineral crystallization process that occurs in hydrous magma conditions in the liquid phase (Thompson and Thompson, 1996;Hedenquist et al., 2000).Sericite generally replaces the minerals biotite, muscovite, and plagioclase and present as a primary mineral (Taylor, 1996;Thompson and Thompson, 1996;Hedenquist et al., 2000;Corbett and Leach, 1997).An opaque mineral, its presence is evenly distributed as an inclusion mineral among secondary quartz.Secondary quartz is present in filling veins in foliated altered rocks, mica schist, actinolite schists and chlorite schists.The control of geological structures such as faults and joints is major factor in the presence of this mineral, which is commonly found in foliated metamorphic rocks (Thompson and Thompson, 1996;Corbett and Leach, 1997;Hedenquist et al., 2000).Based on the presence of sericite minerals, this zone is formed at temperatures between 200-280 o C (Hedenquist et al., 2000;Corbett and Leach, 1997).

Argillic Alteration
Argillic alteration is indicated by the abundantly of clay minerals and the presence of other minerals such as chlorite, quartz and opaque minerals (Fig. 5).This alteration associated with mica schist, chlorite schist, calcite schist rocks.Argillic alteration paragenesis is a response of hydrothermal fluid deposited in the veins, causing the surrounding rock to have an alteration effect on the minerals in response to hydrothermal fluids (Thompson and Thompson, 1996;Corbett and Leach, 1997;Hedenquist et al., 2000;Pirajno 2009).This alteration is associated with quartz veins that form layered to massive textures.Chlorite as a primary mineral and secondary quartz are generally present together with clay minerals.The types of clay minerals identified by XRD analysis from argillic alteration are illite and kaolinite (Fig. 6).Based on the presence of kaolinite and illite minerals, hydrothermal alteration in this zone is formed at temperatures of 200-300 o C (Hedenquist et al., 2000;Corbett and Leach, 1997).

Propylitic Alteration
Propylitic alteration is indicated by the abundantly of epidote, chlorite and/or calcite mineral assemblages; and the presence minor of other minerals such as clay, quartz, and opaque (Fig. 7).This alteration is generally associated with mica schists, serpentinite and diorite rocks.Chlorite is present as a ground mass and is found in chlorite schist, chlorite phyllite, serpentinite, and diorite rocks.Epidote mineral is generally present as ground mass and is found evenly in serpentinite and diorite rocks, present replaces of the mineral hornblende and is generally associated with the mineral calcite (Fig. 7).Calcite is commonly found in diorite rocks, present evenly in the thin section and its presence replaces the mineral plagioclase Ca (Fig. 7) (Corbett and Leach, 1997;Hedenquist et al., 2000;Pirajno 2009).Based on the presence of mineral assemblages of chlorite, epidote, and calcite, the formation temperature of this alteration is 250-300 o C (Hedenquist et al., 2000;Corbett and Leach, 1997).

Carbonization Alteration
This alteration is indicated by the abundantly of graphite minerals and the presence minor of other minerals such as chlorite, muscovite, quartz and opaque minerals (Fig. 8).The presence of graphite minerals as are the result of direct precipitation from the fluid and are only scattered around the veins.This alteration is interpreted to be formed by the interaction of hydrothermal fluids with wall rocks, which are rich in minerals containing the element Carbon.This alteration is mostly associated with quartz minerals, which indicates the formation of this graphite mineral relatively near to the fluid heat source.Carbonization alteration is indicated by presence of graphite or carbon, generally black color, kind of brittle, associated with quartz veins and nearby to the altered wallrocks.Under the microscope (petrographic analysis), the graphite is evenly distributed in black colour, shows high relief with unobserved cleavage, and typically oriented parallel to the rock foliation.This alteration is one of the characteristics the type of alteration related with orogenic gold deposits by hosted metamorphic rock (Idrus et al., 2012).The presence of this alteration indicates that the formation of this graphite mineral is relatively adjacent to the fluid heat source.The hydrothermal alteration at Mendoke Mountains closely related to the presence of mineralization.Gold-bearing quartz veins are found in mica schist, actinolite schist, graphite schist and chlorite schist which are petrologically classified as greenschist facies and become host rocks of orogenic gold deposits (Groves, 1993;Gebre-Mariam et al., 1995;Groves et al., 1998;Goldfarb et al., 1998;2001;Idrus et al., 2012;Idrus et al., 2014;Goldfarb and Groves, 2015;Hasria et al., 2017;Samalehu et al., 2021).The characteristics of hydrothermal alteration in these altered rocks serve to character indicate the character and presence of the more focused ore deposits (Best, 2003).

Hydrothermal Mineralization
The hydrothermal mineralization at Mendoke Mountains generally occurs in metamorphic rocks associated with quartz veins, either parallel to direction of foliation, crosscut to direction of foliation, and laminated quartz veins.Based on results of ore microscopic analysis, the ore mineralization at Mendoke Mountains include native gold (Au) and presence of sulfide and oxide minerals such as chalcopyrite (CuFeS2), pyrite (FeS2), stibnite (Sb2S3), covellite (CuS), cinnabar (HgS), galena (PbS), arsenopyrite (FeAsS2), chrysocolla ((CuAl)2H2Si2O5nH2O), magnetite (Fe3O4), hematite (Fe2O3) and goethite (FeHO2) (Fig. 9).Native gold is found as free gold grain within quartz veins, subhedralanhedral and it is very small size < 0.10 mm.Chalcopyrite is commonly replaced by pyrite, covellite and chrysocolla.Pyrite is commonly associated with chalcopyrite, hematite, cinnabar, galena, arsenopyrite and magnetite.Pyrite commonly replaced by hematite and magnetite.Chalcopyrite, pyrite, hematite, goethite and cinnabar ore minerals are present abundantly while galena, arsenopyrite, chrysocolla, covellite, stibnite and magnetite, are presence minor.Stibnite is one of the specific pathfinder minerals related with orogenic gold deposits (Idrus et al., 2012).The presence of cinnabar and stibnite also genetically indicates that gold deposits at Mendoke Mountains are orogenic gold deposits which correspond to orogenic gold deposits at Western Australia and Canada (Gebre-Mariam et al., 1995).Hydrothermal mineralization which are ore minerals, present in quartz veins, wall rocks and altered rocks.The appearance of Hg and Sb in samples genetically show that the orogenic gold deposits at study area similar with orogenic gold deposits at Golden Mile, Lancefield, Racetrack, Mt.Charlotte in Western Australia; Dome, Kircland Lake, Ross Mine and wiluna in Canada; and Bombana Mountains, Southeast Sulawesi, Indonesia (Groves, 1993;Gebre-Mariam et al., 1995;Groves et al., 1998;Idrus et al., 2011;Hasria et al., 2019).Generally, the presence of ore minerals in the Mendoke Mountains is similar to ore minerals associated with orogenic gold deposits found around the world (Groves, 1993;Idrus et al., 2012;Hasria et al., 2019).The presence of ore minerals in the Mendoke Mountains is controlled by the geological structure trending southeast-northwest and east-west.The presence of ore minerals at Mendoke Mountains in accordance with the orogenic gold deposits at worldwide (Groves, 1993;Gebre-Mariam et al., 1995;Groves et al., 1998;Idrus et al., 2012;Goldfarb & Groves, 2015;Hasria et al., 2019).The hydrothermal mineralization at Mendoke Mountains is controlled by geological structure trending southeast-northwest and east-west.

Conclusions
Three types of veins identified in the study area include veins that parallel to the direction of foliation, crosscut to the direction of foliation and laminated quartz veins.The textures of quartz vein are generally deformed, segmented, brittle, sheared, laminated, sheeted, irregular, brecciated, massive and sigmoidal.Commonly quartz veins of various minerals at Mendoke Mountains are related to this mineralization hydrothermal.Hydrothermal alteration consists of four sericitization, argillic, propylitic and carbonization alterations.This hydrothermal alteration is closely related to the presence of mineralization and included in the greenschist facies.These categories of metamorphic facies generally become host rocks orogenic gold deposits worldwide.The common ore minerals assemblage of carrying hydrothermal mineralization are native gold, chalcopyrite, pyrite, stibnite, covellite, cinnabar, galena, arsenopyrite, chrysocolla, magnetite, hematite and goethite.Based on the characteristics of type and texture of quartz veins, alteration and hydrothermal mineralization, it is concluded that gold deposits in the Mendoke Mountains are orogenic gold deposits type with hosted metamorphic rock.Orogenic gold deposits in Mendoke Mountains mostly the same characteristics as the orogenic gold deposit in Rumbia Mountains because it has the same geological setting.

Fig. 3 .
Fig.3.The type and texture of quartz veins: (a).Sheared, deformed and single quartz veins parallel to direction of foliation; b).massive quartz veins parallel to direction of foliation; c).Sigmoidal of quartz veins parallel to direction of foliation; d).Deformed-brecciated and massive of quartz veins crosscut to direction of foliation; e).irregular vein, brittle, sheeted, sheared, segmented of quartz veins crosscut to direction of foliation; f).high oxidation of deformed quartz veins crosscut to direction of foliation; g).sheeted veins parallel to direction of foliation; h).brecciated, deformed, sheared, laminated quartz veins; i).laminate quartz veins.