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Vol. 2 No. March 06, 2024 https://ajosh.org/

Asian Journal of Social and Humanities, Vol. 2 No. March 06, 2024 1401

Stereographic For Development of Stricture Pattern of

Geological Alteration Zone in Atauro Island, Timor-Leste

Nazario Gomes

, Sutarto

, Joko Soesilo

, Heru Sigit Purwanto

, James

Christopher

Universitas Veteran Yogyakarta, Indonesia

E-mail: alvianogomes@gmail.com

*Correspondence: alvianogomes@gmail.com

KEYWORDS

ABSTRACT

Stereography,

Development Pattern,

Structure, Geological

Alteration Zone, Atauro

Island.

The geological structural patterns and hydrothermal

alteration zones of Atauro Island, Timor-Leste, which are

influenced by the collision mechanism between the Banda

Arc and the Australian Continent, are the subject of this

study. The study identifies fault types and alteration zones

through stereographic analysis. It indicates the possibility of

mineralisation in the area. Four alteration zones were

identified in this study: argillic, Propylitic, calcosite, and

geotite. The analysis results show that left-striking and

normal faulting were essential in forming these zones. The

mineralisation zones, mainly chalcopyrite and chalcocite,

are related to the north-northeast trending left horizontal

Fault. In addition, this study suggests additional research that

includes mineralogy, geochemical, and geophysical analyses

to understand more about the characteristics of hydrothermal

alteration zones and possible mineralisation on Atauro

Island. The results of this study are expected to improve our

understanding of mineral resources in the region, and it is

suggested that this information be considered by authorities

responsible for the management of potential natural

resources on Atauro Island, Timor.

Attribution- ShareAlike 4.0 International (CC BY-SA 4.0)

Introduction

Geological structure is a science that studies essential aspects to determine the stress

pattern in a research area. This paper discusses the geological structure of marginal basins

in the South Banda Basin. It includes the northern parts of Alor, Atauro, Liran, Kisar, and

Wetar Islands (Maneerat et al., 2022). The arc-continent collision mechanism between

the Banda Arc and the Australian Continent influences this structure.

The study found that four areas at the Makili, Villa, Berau, and Bikeli sites in Atauro

experienced hydrothermal alteration (Aisabokhae et al., 2023). Such changes are argillic;

outer Propylitic, phillic, and advanced argillic changes are the types of changes produced

by (Aisabokhae et al., 2023). The Villa, Makili, Pala, Berau, and Atauro sites are

lithologically dominated by porphyritic, partly andesitic, dacitic, andesite rocks with

Nazario Gomes, Sutarto, Joko Soesilo, Heru Sigit Purwanto, James Christopher

Asian Journal of Social and Humanities, Vol. 2 No. March 06, 2024 1402

silicic, propylitic, and argillic Filik alteration. Normal faulting is joint and shows a

radiation pattern around the Pliocene-Pleistocene volcanic bodies, which still look ideal).

Therefore, this four-region trend defines the left horizontal fault zone, which consists of

both a strike-slip and a strike-slip zone. The left horizontal Fault is a mineralisation

trapline that hosts significant sulphide mineralisation (Govil et al., 2021). Submarine

Hydrothermal Systems: Dispersal Centers and Island Arcs interest researchers because of

the hydrothermal fluid systems in the Island's arc (Pirajno, 2012). These characteristics

suggest that the general process leads to hydrothermal fluids forming and transporting

precipitates of metallic minerals such as Au, Ag, Cu, Zn, and Pb. However, these features

are not yet diagnostic enough to generate a widely accepted genetic model. In the

mineralisation zone map, two geological structure models are proposed to indicate the

origin of Cu, Ag, Pb, and Zn elements. This model is the hydrothermal mineralisation

zone model described in Figure 10 on the alteration and mineralisation zone model map

of the study area, where mineralisation originates from magma, and the mineralisation

model to determine the mineral elements originating from the earth's crust through

meteoric and hydrothermal water fluids formed in brittle fault structures. This fluid

pathway initiated the Cu, Ag, Pb, and Zn mineralisation in the Atauro Island study area.

In the weak zone, hydrothermal fluids pass through faults and brittle zones. Stereographic

programs are often used to determine the movement of structural patterns as traps in the

formation of mineralisation and faults that occur in the study area. The results of

geological structure modelling and analysis of general structural direction patterns, vein-

style Cu, Ag, Pb, and Zn mineralisation are associated with significant N-S and B-T

faults, particularly the Villa, Berau, and Makili Pala faults (Brookhian et al., 2020).

Geological structures are still used to name faults and breccias at Atauro, Timor-Leste, to

develop alteration and mineralisation models.

Subduction and accompanying volcanism spread systematically over an area that

progressively expanded away from the continents towards the oceans during the tectonic

evolution of the Indonesian Archipelago from the Late Paleozoic to the Pliocene Katili,

(1975).

Research Methods

In this study, the phenomena observed were the following: The analysis was

characterised by three specific zones: thrust zone, proto-thrust zone, and normal fault

zone, where the boundary between frontal thrust and frontal deformation separates each

zone.

Alteration zones: This study reports four altered zones at Makili, Villa, Pala, Berau,

and Atauro, and all four zones have undergone hydrothermal alteration. The resulting

alteration types are quartz and clay: argillic alteration, propylitic alteration, dolomitic

alteration, and chloritic alteration of quartz, clay, and sericite—Filik (Figure 7d) (Govil

et al., 2021) on Atauro Island, East Timor.

This research collects secondary data from several observation locations or samples to

describe specific characteristics or phenomena. In this study, the data collected are on

Stereographic For Development of Stricture Pattern of Geological Alteration Zone in

Atauro Island, Timor-Leste

Asian Journal of Social and Humanities, Vol. 2 No. March 06, 2024 1403

strike/dip measurements of bridles, faults, structural patterns, and geological alteration

zones on Atauro Island, Timor-Leste (Fu et al., 2023).

The samples in this study are four geological alteration zones selected at the

location and map of the alteration zone purposefully. A purposive sample was selected

considering the geological alteration zone has different characteristics.

The instruments used in this research are geological maps and DEM (Digital Elevation

Model) (Khalid et al., 2016). (Figure 8a dan 8b) to analyse kinematics and relate

structures to each other and the tectonic setting, making it possible to determine the

genetics of the geological structures developed in the study area. Fractures, volcanic

breccias, slickensides, and variations in dip and strike over small distances, as well as

uplift and depression structures, indicate the performance of faults acting in the area,

which play an essential role in the geometry and distribution of mineralised masses

(Brookhian et al., 2020) and satellite imagery and field data. Geological maps (Figure 11)

were used to determine the characteristics of rocks on Atauro Island, Timor-Leste.

Satellite imagery determines the pattern of geological structures on Atauro Island, Timor-

Leste. Field data was used to observe geological alteration zones on Atauro Island, Timor-

Leste, directly.

The research procedures carried out in this study are as follows:

Conduct a literature study to determine the characteristics of rocks and geological

alteration zones. I am analysing geological maps to determine the distribution of rocks on

Atauro Island, Timor-Leste, and satellite images to determine the pattern of geological

structures on Atauro Island, Timor-Leste. Conduct field observations of geological

alteration zones on Atauro Island, Timor-Leste. Data analysis will be conducted to

analyse the structural pattern of the geological alteration zone on Atauro Island, Timor-

Leste.

Results and Discussions

The geological structure pattern on Atauro Island, Timor-Leste, is dominated by

normal Fault and shear fault structures. From the results of the alignment analysis, Atauro

Island is dominated by the alignment with the general direction of east-northeast-west-

west-west-west-west-west- -west southwest, followed by the relative alignment of north-

south to northwest-southeast. The east-northwest alignment is due to the structure that

developed the Escarpment Villa segment. The northwest-southeast alignment is a valley

resulting from erosion along the fault line. The north-south alignment pattern may be

related to structures developed in the island's northern part. After the collision at the

Sunda-Banda Trough, the initial recession system formed from the mid-Miocene to the

Pliocene subduction of the lithosphere from the Jurassic and Cretaceous oceans (Figure

2). When the most recent Pliocene Scott Plateau of the Australian continental margin

entered the trench, accessory inflation increased significantly, transforming it into a new

continental front fold and thrust belt (Figure 1).

Over the last 5 million years, subduction occurred where the Australian Oceanic

Crust subducted beneath the Banda Arc and produced Banda Arc volcanism (Figure 2)

Tate et al., (2015). Subduction of the continental crust produced natural basaltic

magmatism, resulting in the volcanism of the Gunung Berau-Tutonair subduction

volcanism zone Harris, (2011). The stresses acting in this phase are compression forces

with a north-northeast-south-southwest orientation, such as the direction of movement of

the subducted plate, in the appendix images of outcrops 4, 5a, and 5b.

Nazario Gomes, Sutarto, Joko Soesilo, Heru Sigit Purwanto, James Christopher

Asian Journal of Social and Humanities, Vol. 2 No. March 06, 2024 1404

Moving from subduction to collision in the Banda Arc, the global positioning

system experiences a decrease in speed compared to the Sunda Shelf (backarc).

Earthquake hypocenters are known as blue dots due to wedge collapse, white continental

crust, black oceanic crust, Dutch striped terranes, green mélange, and yellow basins. The

hypocenter of an earthquake outlines the subducting slab (Figure 6).

This standard fault structure was formed due to a compression process that occurred

in the Pliocene to present-day Kuarte, where the four islands of Atauro, Alor, Liran, Kisar,

and Wetar in northern Timor were no longer volcanically active, and activity ceased

around 3 million years ago. This compression process causes the formation of

hydrothermal alteration zones rich in sulphur dioxide minerals, such as pyrite. The

geological structure pattern on Atauro Island, Timor-Leste, significantly influences the

formation of hydrothermal alteration zones. These hydrothermal alteration zones have the

potential to be a source of metallic minerals such as gold, silver, and copper.

Control Geological and geothermal structures in the Makili segment include hot

springs and altered rocks. In this segment, there are geological structures in the form of

the Kalehura Descending Fault (station-9) and the Makili Left Flat Fault (station-5), with

an additional Fatulela location Descending Fault produced due to loading (Figure 7c).

The structural control of geothermal heat in this segment is interpreted to be caused by

the Makili left-lateral fault system. The presence of the Kalehura Descending Fault allows

for permeable zones, where hot sprays with Cu, Ag, Pb, and Zn mineralisation are found

around the study site. At the same time, the mineralised hills are still cut by the Makili

Fault. It is interpreted that the Makili Fault creates fractures in the subsurface that become

permeable zones for geothermal fluids to propagate to the surface.

At the Makili study site, alteration types are argillic and externally propylitic, with

the argillic type consisting of quartz minerals and mineral oxides that have extensively

altered the lithology. (Figure 9) Quartz fills veinlets, where chalcopyrite and chalcocite

are mineralised, and serves as a base-mass replacement for clay minerals. The formation

temperature is thought to range between 100 and 300 degrees Celsius. Host rock ages

range between 3.65 ± 0.18 million years and 3.27 ± 0.18 million years.

Cross section of alteration zone incision A-A' Yellow Argillic alteration type:

composed of quartz and clay minerals that have altered the lithology (Figure 10).

Pervasively, quartz is present as a base mass replacement accompanying clay minerals. It

is also present in filling veinlets that host chalcopyrite and chalcocite mineralisation. The

formation temperature is interpreted to be 100–300  C Pliocene–Pleistocene horsrock age

Propylitic Outer Alteration Type in Green in Cross Section: consists of chlorite and minor

calcite that alter the lithology in a selective pervasive manner, where chlorite replaces

mafic minerals. Calcite is present as infilling in fractures. The interpretation of the

alteration formation temperature is 200-300  C.

Geological structure patterns on Atauro Island (Aisabokhae et al., 2023), standard

and thrust faults dominate Timor-Leste; normal faults form hydrothermal alteration

zones, While the descending fault horizontal fault plays a role in forming the argillic

alteration zone (Prabowo et al., 2018).

Based on the results of stereographic analysis, the hydrothermal alteration zone on

Atauro Island can be divided into two, namely argillic alteration type with pyrite +jerosite

-hematite mineralisation zone, propylitic another alteration type,

Chalcopyrite+calcosite+geotite mineralisation zone (Honarmand & Shahriari, 2021). The

mineralised zone develops on a northeast-northeast trending left horizontal fault plane. In

Stereographic For Development of Stricture Pattern of Geological Alteration Zone in

Atauro Island, Timor-Leste

Asian Journal of Social and Humanities, Vol. 2 No. March 06, 2024 1405

contrast, the silica-alkali alteration zone develops on a southwest-northeast trending

strike-slip and left horizontal fault plane (Aryanto & Kurnio, 2020).

Hydrothermal alteration zones on Atauro Island have the potential to be a source of

precious metal minerals such as gold, silver, and copper (Setijadji et al., 2010).

To further optimise the potential of mineral resources on Atauro Island, further

research must be done to determine the hydrothermal alteration zone in more detail

(Aisabokhae et al., 2023). Such research may include mineralogical, geochemical, and

geological analysis of structures. Here are some recommendations for further research

that can be done:

Mineralogical analysis to determine the type of minerals formed in the

hydrothermal alteration.

1. Geochemical analysis for precious metal content in hydrothermal alteration zones

(Province, (2023).

2. Geophysical analysis to determine the prospects for mineral resources in the

hydrothermal alteration zone Utama, (2020)

3. Further research is needed to determine the relationship between structural and

geological alteration patterns. Further research can be conducted using geochemical,

petrographic, and geophysical methods.

4. Further research is needed to determine the mineralisation potential on Atauro Island

(Inoh et al., 2023). Further research can be carried out using geological exploration

methods.

This research has provided an overview of the structural pattern of geological

alteration zones on Atauro Island. This research has also shown that Atauro Island has

mineralisation potential.

Conclusion

The results show that geological structure and fault patterns are essential in forming

hydrothermal alteration zones. This study confirms that Atauro Island has significant

mineralisation potential, including valuable mineral resources such as gold, silver, and

copper. However, further research is needed to gain a more in-depth understanding of the

types of minerals formed in the alteration zone, the precious metal content, and the overall

mineral resource outlook. Further research recommendations include mineralogical,

geochemical, and geophysical analyses to understand hydrothermal alteration zones

better. In addition, further geological exploration research could help identify the

mineralisation potential in detail. The data and information generated from this study have

great potential. They should be presented to the local government and the Timor-Leste

Energy and Mineral Resources office to take the mineral resource potential of Atauro

Island more seriously. As such, this study provides a solid foundation for further

development in the geological exploration field and an understanding of the potential

mineral resources in this region.

Nazario Gomes, Sutarto, Joko Soesilo, Heru Sigit Purwanto, James Christopher

Asian Journal of Social and Humanities, Vol. 2 No. March 06, 2024 1406

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