6b. bombana orogenic
TRANSCRIPT
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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)