6b. Bombana orogenic

8/9/2019 6b. Bombana orogenic

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Arifudin Idrus*1, I Wayan Warmada1, Irzal Nur2, Fadlin3,

Franz Michael Meyer4 & Sukmandaru Prihatmoko5

THE METAMORPHIC ROCK-HOSTED GOLD MINERALIZATION

AT BOMBANA, SOUTHEAST SULAWESI:A NEW EXPLORATION TARGET IN INDONESIA

1Department of Geological EngineeringGadjah Mada University, Yogyakarta, INDONESIA

2Hasanuddin University, 3STTNas, 4RWTH Aachen University,

Germany & 5AGC Indonesia (Ivanhoe Mines Ltd)*E-mail addresss:[email protected]

The 34th IGC Brisbane, Australia, 5-10 August 2012


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Presentation Outline

Introduction Geological framework

The Langkowala Placer/paleoplacer Gold

The Characteristics of Primary Au Deposit

Host rock petrology

Gold-bearing quartz vein characteristics

Hydrothermal Alteration

Ore mineral and chemistry Mineralizing fluid characteristics

Concluding Remarks


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INTRODUCTION-1

Currently, in Indonesia gold has mostly beenmined from volcanic-hosted hydrothermal

deposits. In Sulawesi, gold mineralization is

also dominantly related to volcanic rocks,

which is extended along the western andnorthern Neogene magmatic arcs

However, some metamorphic-hosted

deposits e.g. Poboya (Epithermal), Awak Mas(Mesothermal) are also discovered. These

deposit styles become to be new target for

exploration in the future.


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INTRODUCTION-2

Placer/paleoplacer gold has been discovered inLangkowala plain (Bombana Regency), Southeast

Sulawesi.

Local geological framework indicates that thep acer go s no re a e o vo can c roc -re a e

hydrothermal systems.

This study is aimed to identify and characterizethe primary gold deposit type as a source of the

Langkowala (Bombana) placer gold.


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Traditional gold

mining in Bombana

2009

2011


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GEOLOGY OF INDONESIA

Study area


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GEOLOGY OF THE STUDY AREA

Sulawesi Island, Indonesia

(Hamilton, 1979; Carlile et al., 1990)

Three major tectonic units:

1. Western magmatic arc

2. Central metamorphic belt

Study area


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LOCAL GEOLOGY & STRUCTURAL CONTROL

Langkowala plain

Rumbia mountain range

Outcropped

quartz veinlocation


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PLACER/PALEOPLACER GOLD IN LANGKOWALA

Gold grain is present in stream sediment of the

present-day active rivers and in Mio-Pliocenesediments of Langkowala Formation.

Gold grains seems to be not so far transported fromits primary source. This is consistent withsubrounded-angular form of gold grains panned.

Abundance of gold grain decreases as its distancefrom the metamorphic mountain range increases.

Gold is also found in the colluvial materials alongWumbubangka mountain slope and isolated valleyof the mountain range.


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Placer gold not far transportedPrimary depositPlacer gold

Company office

Isolated valleyGold>>

Colluvial deposit


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Sub-angular gold

PLACER/PALEOPLACER GOLD IN LANGKOWALA

Gold & cinnabar


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SOME KEY GENETIC CHARACTERISTICS


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MICASCHIST

1. Host rock petrology

Mica Schist

Phyllite, meta sediment(meta sandstone)

Silicified

metasediments

Brecciated, crystalline

quartz vein (~2 m) in mica

schist


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Mica schist

petrography

Muscovite, chlorite Actinolite, albite, epidote &

sericite & opaque minerals

Mus

Qtz

Chl

Greenschist

facies (Yardley, 1989)

Mus


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2. Quartz Vein Characteristics

At least three generations of gold-bearing

quartz veins:

1. Parallel to the foliation

2. Crosscut to the foliation

(second generation)

3. Deformed laminated quartz+calcite veins

(third/late generation)


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1. First generation quartz vein:parallel to the

foliation of mica schist (N 300E/60)

Segmented/sigmoidal

structures of quartz veins

hosted by mica schist

Massive, crystalline quartz

vein (up to 2 m width),

parallel to the mica schist

foliation


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Deformed/sigmoidal

quartz vein (parallel

to the foliation)

Claysilicaalteration


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2. Second generation quartz vein:

crosscutting the foliation

3. Third generation quartz vein:

Laminated quartz+calcite veins

Highly oxidized/mineralizeddeformed quartz vein


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3. Hydrothermal Alteration

Silicification

Claysilica (argillic) alteration

-

Carbonization


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1. Silicification

2. Claysilica

(argillic)

alteration

Quartz+(clay) vein


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3. Chlorite-carbonate alteration

Carbonate

4. Carbonization

ChloriteCarbonate

Carbon(graphite?)


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4. Ore Mineralization

Native gold

Cinnabar (HgS)

(FeSbO4)

Arsenopyrite (FeAsS2) Pyrite (FeS2)


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Native gold (Au)

Gold

GoldGold

Qtz


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Cinnabar (HgS)

Stibnite-cinnabar-

mineralized rock

Cinnabar in

metasediment

layers


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Stibnite Stibnite

Qtz vein

Stibnite commonly as vein & disseminated

Stibnite microscopyTripuhyite (EDS)


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Stream sediment & ore chemistry (AAS)(Prihatmoko et al., 2010)

No. SS samples Au No. Rock/vein Au1 0.009 1 0.006

2 0.014 2 0.01

3 0.007 3 0.006

4 0.005 4 0.009

5 0.008 5 0.036

6 0.008 6 0.0127 0.007 7 0.012

8 0.006 8 0.016

9 0.008 9 0.036

10 0.006 10 5.3

11 0.008 11 0.228

12 0.033 12 8413 0.012 13 8.66

14 0.014 14 0.107

15 0.007

16 0.007

17 0.014

18 0.005

Variable: 0.005 - 84 g/t Au


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Ore chemistry (FA-AAS)

SampleCodes

Elements (ppm)

Au Cu Pb Zn Ag Hg As Sb

WB-01-B 0.02 13 34 27


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Mineralizing fluid characteristics

Temperature of homogenization (Th) of the firstgenerationof quartz vein varies from185 to 245 C,with relatively higher salinity ranging from5.3 to 9.1wt.% NaCl eq.

The second enerationof uartz veins is formed inmoderate temperatures of132 to 283 C (mean 158

to 209 C)and salinity of3.6-5.9 wt.% NCl eq.

The latest/third generation stageof veining wasoriginated at the lowest temperature of114-176 Cand salinity of 0.4 to 4.0 wt.% NaCl eq.


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Fluid characteristics

The evidences of the

contribution of metamorphic

fluid, hydrothermal magmatic

fluids and meteoric water

forming quartz veins are

H2O-NaCl-CO2 fluid

systemFirst quartz veins:

CO2-vapour?

2 - - 2fluids.

CO2-rich fluid, however, is

present in very small portion

(< 4% CO2) (personal

communication, Richard J.

Goldfarb, 2011).


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Second quartz vein

(H2O-NaCl+/-CO2)

Third quartz+calcite vein

(H2O-NaCl+/-CO2)


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H2O-NaCl-CO2 fluid

Goldfarb, 2009

Bombana


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Hydrothermal fluid evolution of three

generations of gold-bearing quartz veins

200

250

300

Quartz vein paralel to foliation (N=36)

Quartz vein crossing foliation (N=120)

Calcite+quartz vein (N=12)2

First vein

0

50

100

150

0 5 10 15

Th(C)

Salinity (wt % NaCl eq.)

1

Second vein

Third vein

Shepherd et al. (1985).


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Miniralizing fluid evolution

The first quartz vein generation underwent an

isothermal mixing with fluids of contrasting

salinity. It is interpreted that the vein is

dominantly originated from hydrothermal

magmatic fluid mixing with metamorphic fluids. ,

minor or relatively iso(thermal), but the salinity

decreases significantly.

The second and third quartz vein generationsare likely formed from mixing of the magmatic

and metamorphic fluids, and with cooler less

saline meteoric water.


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The Bombana orogenic deposit


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CONCLUDING REMARKS-1

Geology: Host-rock: greenschist facies type

of metamorphic facies mostly hosts the

orogenic gold deposits (Gebre-Mariam et

al., 1995; Goldfarb, 2009).

The quartz veins textures: massive andcrystalline, brecciated, drussy and

pseudomorph bladed carbonate textures. Vein

structures: deformed, occasionally sigmoidal

Hydrothermal alteration:

silicification, claysilica (argillic), chlorite-carbonate alteration, carbonization.


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CONCLUDING REMARKS-2 Typical minerals: cinnabar, stibnite, tripuhyite

& arsenopyrite: genetically indicates that theorogenic gold deposit transition between

epizonal and mesozonal (cf. Groves et al.,

1998, 2003; Goldfarb, 2009). 2-r c u nc us on s presen n very sma

portion the Bombana gold-bearing quartz

vein is situated at shallow level, in which the

pressure condition may not be sufficient topreserve CO2 in the hydrothermal fluids and it

may escape up to the surface (personal

communication, Volker Lueders, 2003).


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CONCLUDING REMARKS-3

By considering all key features discussedabove, the primary metamorphic-hosted gold

mineralization type at Bombana tends to meetthe criteria oforogenic gold type (cf. Groves et

al., 1998; 2003; Goldfarb, 2009). The primary

gold in the area.

The discovery of the metamorphic-hosted gold

deposit in the area has opened up more targetsand challenges for gold exploration in the

region, and other terrains in Indonesia that have

identical geological setting.


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METAMORPHIC ROCK-HOSTED GOLD DEPOSITS:

A New Exploration Target in Indonesia?

Study area

Buru Island


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ACKNOWLEDGEMENTS Gadjah Mada University, Yogyakarta, Indonesia

AGC Indonesia (Ivanhoe Mines Ltd), Jakarta

Panca Logam Makmur (National Private

Company)

RWTH Aachen University, Aachen, Germany

Dr. Richard J. Goldfarb (USGS)

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6b. Bombana orogenic