Alteration and Vein Textures Associated with Gold Mineralization at

Jurnal Geologi Indonesia, Vol. 5 No. 4 Desember 2010: 247-261
Alteration and Vein Textures Associated with
Gold Mineralization at the Bunikasih Area,
Pangalengan, West Java
A.S. SubAndrio and n.i. bASuki
Study Program of Geology – FITB – Institut Teknologi Bandung,
Jln. Ganesha 10 Bandung 40132
AbstrAct
The Bunikasih vein system in the Pangalengan district of West Java is a low-sulfidation, adularia sericite epithermal gold deposit. It is hosted by Late Miocene andesitic volcanic and volcanoclastic rocks
occurring in the south western margin of Malabar Volcano complex. Gold ore and alteration minerals
related to deposition of gold in Bunikasih deposits superimposed on Late Tertiary-Quaternary andesitic
formation that were altered and mineralized by some hydrothermal events. The veins consist almost
entirely of quartz, with small amounts of adularia, bladed calcite, pyrite, and gold. Gold ore shoots are
vertically restricted and are more continuous horizontally. The veins display complex and multi episodic
filling with texture characteristics of open space precipitation such us colloform, lattice bladed, crustiform banding, vugs, breccia, and cockade and comb texture. The presence of bladed calcite and silica
pseudomorph after bladed calcite suggests that the hydrothermal fluids boiled. In the Cibaliung section
of the area, anomalous gold is related to veins trending northeast - southwest, milky quartz with dark
grey to black manganese staining is found intermittently for a length of about 800m. The mineralized
andesite ore bodies exhibit broad alteration patterns adjacent to mineralization, passing from fresh rock
into anargillic, chlorite zone, and then sericite-silica close to mineralization. An argillic assemblage
composed of kaolinite with fine-grained pyrite bulb is present in the upper portions and surrounding of
the quartz vein system. The veins range from centimeter to meter in size. Of 24 vein samples collected,
gold averages up to 0.3 grams per tone ("g/t"), to a high of 24.6 g/t. The Bunikasih epithermal gold
deposit was mined by people for more than 10 years, mainly for the gold ore.
Keywords: alteration, vein texture, gold mineralization, Bunikasih
Sari
Urat-urat yang terdapat di Bunikasih, Kecamatan Pengalengan, Jawa Barat, merupakan bagian dari
suatu endapan emas tipe epitermal adularia serisit, sulfidasi rendah. Endapan emas tersebut terdapat
dalam batuan vulkanik andesit dan batuan klastik vulkanik berumur Miosen Akhir yang terdapat di
barat daya kompleks gunung api Malabar. Emas dan mineral alterasi penyerta dijumpai mengubah
satuan batuan andesit Tersier Akhir – Kuarter yang telah mengalami alterasi hidrotermal. Urat-urat
yang dijumpai tersusun oleh kuarsa dengan sedikit adularia, bilah-bilah kalsit, pirit, dan emas. Endapan
emas dalam urat penyebarannya terbatas secara vertikal, dan relatif lebih menyebar secara horizontal.
Urat-urat kuarsa memperlihatkan tekstur kompleks yang menunjukkan pengendapan berulang secara
episodik dalam ruang terbuka seperti koloform, mineral berbentuk bilah, perlapisan crustiform, breksi,
dan tekstur cockade and comb. Dijumpainya bilah-bilah kalsit dan pseudomorph silika hasil ubahan bilah
kalsit menunjukkan kemungkinan terjadinya pendidihan (boiling) pada larutan hidrotermal. Di daerah
Cibaliung, keterdapatan emas berasosiasi dengan urat-urat kuarsa berwarna putih susu berarah timur
laut – barat daya sepanjang sekitar 800 m, dengan bercak-bercak mangan berwarna abu gelap hingga
hitam. Batuan vulkanik andesit yang termineralisasi memperlihatkan pola alterasi yang berangsur dari
batuan tak terubah menjadi zona argilik dan klorit, dan kemudian menjadi zona serisit-silika mendekati
Naskah diterima 21 Juni 2010, revisi kesatu: 13 Agustus 2010, revisi kedua: 02 September 2010, revisi terakhir: 26 November 2010
247
248
Jurnal Geologi Indonesia, Vol. 5 No. 4 Desember 2010: 247-261
zona mineralisasi. Urat-urat kuarsa mempunyai lebar bervariasi dari sentimeter hingga meter. Dari dua puluh empat
percontoh yang dianalisis, kadar emas rata-rata adalah 0,3 g/t, dan dapat mencapai 24,6 g/t. Endapan emas Bunikasih
telah ditambang oleh penduduk setempat selama lebih dari 10 tahun.
Kata kunci: alterasi, teksture urat, mineralisasi emas, Bunikasih
IntroductIon
Bunikasih area is located about 60 km to the
south from Bandung, southwest of Situ Cileunca,
or about 15 km from Pangalengan (Figure 1). It is
part of West Java Southern Mountain Zone covered
by Quaternary (Pleistocene) volcanic rocks, known
as Waringin Andesite unit (Alzwar et al., 1992).
Based on a more detailed work by Chandra (2009),
the andesite unit can be subdivided into three
subunits; they are (older to younger): Cibaliung,
Cikabuyutan, and Puncak Cacing andesite lava
units. Gold mineralization was firstly discovered in
early 1990 by exploration of PT. Aneka Tambang.
Due to the small or subeconomic gold reserve, PT.
Aneka Tambang has not mined this prospect. However, relatively high gold grade (up to 20ppm Au)
in some quartz veins made a great interest to many
local people to dig and mine gold traditionally. The
Bunikasih epithermal gold deposit of Pangalengan,
is one of small scale oberating gold mines, located
in the southern part of West Java. The others being
Cineam and Salopa - Tasikmalaya, CikondangCianjur, and Gunungpeti - Sukabumi (Widi et al.,
1997). Based on these occurrences and the overall
volcanic setting, the southern belt of West Java
has a good potential for discovering new gold ore
deposits. For example, Pongkor Mount in Bogor
associated with the southern volcanic belt, is the
largest gold deposit in Java Island of which during
more than 10 years has been mined by PT. Aneka
Tambang (Warmada, 2003).
GeoloGy of the bunIkAsIh AreA
Based on a topographic map analysis and field
observation, there are five identified (strike-slip)
faults in the area. Two faults are NE-SW and the
others are NW-SE (Figure 1). They are interpreted
to be formed in post-Pleistocene, as they extend
across Quaternary (Pleistocene) volcanic rocks.
Based on regional geological study, the faults in
the researched area are most likely parts of, or
are influenced by, regional, NE-SW trend, dextral
strike-slip faults, as identified by Alzwar et al.
(1992). These regional and local faults are interpreted to be formed by a relatively N-S regional
stress, which has been active since Late Oligocene
– Early Miocene (Alzwar et al., 1992).
The Cibaliung andesite lava unit was observed
along the Cibaliung River (Figure 2 and 3) and is
characterized by whitish or greenish colour, and
porphyritic textures, comprising phenocrysts of
plagioclase and pyroxene in abundant aphanitic
groundmass. Petrographic observation revealed
that the aphanitic groundmass consists of minute
crystals of pyroxene, plagioclase, glassy materials and opaque minerals. This unit shows a strong
argillic and/or prophilitic alteration and, locally,
silicification. In some places, a weak magnetic nature of this unit can be detected by magnet. Quartz
veins in various sizes (1 cm to 3 m) are commonly
present, crosscutting this unit.
The Cikabuyutan andesite lava unit is present
along the Cikabuyutan River (Figure 2 and 3)
and is characterized by black colour, porphyritic
textures that show plagioclase and pyroxene phenocrysts in an aphanitic groundmass, and by its
very strong magnetic nature. Phenocrysts in this
unit are more abundant than those of the Cibaliung
andesite lava unit. Very fine pyroxene, plagioclase
crystals, and glassy materials constitute the aphanitic groundmass. The Cikabuyutan andesite lava
unit is weakly altered by clay mineral (argillic),
and unlike the previous unit, this unit is crosscut
by small-sized quartz veins (< 1 m-wide).
Puncak Cacing andesite lava unit is the youngest unit in the area and is found in Puncak Cacing Hill as well as along some branches of the
Cikabuyutan River near Puncak Cacing (Figure
2). This unit shows gray colour and comprises
abundant phenocrysts of pyroxene and plagioclase
in an aphanitic groundmass, which consists of fine
249
Alteration and Vein Textures Associated with Gold Mineralization at the Bunikasih Area,
Pangalengan, West Java (A.S. Subandrio and N.I. Basuki)
Rancatungku
B
Soreang
Note :
Q=Quaternary Age;
T=Tertiary Age;
Qwb=Warin ginBedil Andesite,
Old Malabar:
alteration of lava
breccias and tuffs,
pyroxene andesitic
and hornblende
a n d e s i t i c
composition;
Qtl= Kancana, Huyung
a n d Ti l u l a v a s :
Andesitic lava and
basaltic-andesitic
lava of Mt. Tilu;
T m b = B e s e r
F o r m a t i o n : Tu f f
sandstone, pumice
tuff, claystone,
conglomerate and
lignite;
Q m t = M a l a b a r Ti l u
Vo l c a n i s : t u f f ,
l a h a r i c b re c c i a s
contains minor of
pumice and lavas;
Qyw=Young volcanic:
Efflata
and
andesitic-basaltic
lava flows from Mt.
Wayang Windu.;
Qkl= Kencana lavas:
Andesitic lava and
basaltic anadesite
lavas.
QopuUndifferentiated
Efflata Deposits
of Old Volcanics:
Fine to coarse
dacitic crystalline
tuff, tuffaceous
breccias contains
pumices and old
andesitic-basaltic
laharic deposits.
Qd=Lake deposit:Clay
silt, fine to
coarse sands and
gravel, commonly
tuffaceous.
Qd
G. Koromong
G. Geulis
Tmb
Cipeundeuy
Qd
Kramat
1151
Tmb
Qwb
Cangkuang
Muara
Banjaran
Bandasari
Baros
Bojonglao 2
Pasirpariuk
Citeuteurp
Qtl
G. Bubut
Gegerheos
Cianjur
1333
Ganjen
Pasirhuni
Kebontunggul
G. Tanuaktangsi
Qmt
1514
Cikalong
G. Puntang
G. Tikukur
Lamajang
G. Haruman
G. Malabar
2621
Gambung
Qtl
Puncakbebar
G. Tilu
Rancatungku
Pasirurug
Qmt
Riunggunung
Pangalengan
Cibeureum
Qwb
G.Gambung
Rancamanyar
Wanasari
Qopu
Barussalam
G. Wayang
Situ Cileunca
Cibunihayu
Qwb
G. Windu
Gunungcepu
Banjarsari
Puncak Cacing
PR. Singahal
2054
Qyw
Cibolang
Cikowok
Kertasari
Qopu
Bunikasih
N
Wanasuka
Qwb
G. Cikepung
1390
Cepu
Qwb
0
Qkl
388
G. Karancang
1563
Samosa
1
2
3 Km
G. Malang
1880
A
Studied Area
Figure 1. Simplified map western part of Geological Map of Garut and Pameungpeuk Quadrangle (Alzwar et al., 1992). Bunikasih
(boxed area) is located near Lake Cileunca and Wayang Windu geothermal field. Mt. Malabar is located in northeastern part.
250
Jurnal Geologi Indonesia, Vol. 5 No. 4 Desember 2010: 247-261
G.Puncakcacing
Vein
Strike/Dip
Q1
90°/45°
1.3m
Q2
131°/83°
38cm
Q3
340°/83°
1.5m
Q4
108°/79°
1.0m
Q5
100°/70°
1m
Q6
232°/71°
0.7m
Average vein
thickness
Q6
Q1
500m
Q6
Q5
Q4
Q3
N
Figure 2. Topography map of Bunikasih area. Mt. Puncak Cacing is the highest landmark of this area. Gold mining is distributed on quartz vein group on southwestern area.
D
920000
G. Puncakcacing
C
N
9199000
0
1 km
B
9198000
A
Puncak Cacing Andesite Lava Unit
Cikabuyutan Andesite Lava Unit
Cibaliung Andesite Lava Unit
9197000
0776000
1600
0777000
0778000
0779000
Geological Cross Section
Scale V:H=1:1 (m)
07 80000
1600
1500
1500
1400
1400
1300
1300
1200
1200
1100
1100
A
B
1700
1600
1500
1400
C
Figure 3. Simplified geological map of Bunikasih area (Chandra, 2009).
D
251
Alteration and Vein Textures Associated with Gold Mineralization at the Bunikasih Area,
Pangalengan, West Java (A.S. Subandrio and N.I. Basuki)
plagioclase and pyroxene and glassy materials.
The lava unit is relatively unaltered and is weakly
magnetic.
AlterAtIon of VolcAnIc rocks
Argillic and prophilitic are the common alteration types found in the area, with argillic occurrences are much more widespread than prophilitic
(Figure 4). Argillic alteration on volcanic rocks
is characterized by clay mineral alteration (probably kaolinite) after primary minerals and gives
the altered rocks a feel like soap. Plagioclase and
pyroxene phenocrysts, as well as groundmass
materials show various degrees of alteration by
clay minerals, with plagioclase in general show
more intense alteration. Prophilitic alteration is
characterized by the presence of chlorite, epidote,
and calcite after phenocrysts of pyroxene, and
plagioclase, and groundmass. Quartz is commonly
present in argillic and prophilitic altered rocks, accompanying clay minerals, chlorite, epidote, and
calcite. In general, primary textures of the rocks
can still be identified. The intensity of alteration
decreasing from the Cibaliung lava unit (medium
to strong) to the Cikabuyutan lava unit (weak to
medium) to Puncak Cacing lava unit (weak), gives
an apparent systematic alteration intensity which
decrease from west to east.
Quartz veins in the area are mainly found in the
Cibaliung lava unit. The veins can have simple to
complex textures, occurring in various sizes (wide).
Micron-sized gold particles are known to be present
in quartz veins. The veins have been the primary
target for local miners to extract gold. The general
trends of some of the studied quartz veins are consistent with the trends of faults in the researched area,
suggesting structural control on vein formation. The
faults have probably acted as conduits for hydrothermal fluids that were responsible for alteration
of volcanic rocks and formation of quartz veins.
920000
G. Puncakcacing
N
0
1 km
9199000
Prophilitic zone
Argillic zone
9198000
F
9197000
0776000
1500
0777000
0778000
0779000
0780000
Geological Cross Section
Scale V:H=1:1 (m)
1400
1300
1200
1100
1000
E
Figure 4. Simplified alteration map of Bunikasih area (Chandra, 2009).
F
252
Jurnal Geologi Indonesia, Vol. 5 No. 4 Desember 2010: 247-261
MorpholoGy of QuArtz
In epithermal systems, silica may be deposited
as opal or amorphous silica, chalcedony or quartz
(Morrison et al., 1990). All these phases, except
quartz, were precipitated from solution that were
supersaturated with respect to quartz, and then recrystallized to quartz with time because they were
metastable at low temperatures (Fournier, 1985a).
The quartz texture in veins may reflect the original
conditions of silica saturation.
Classification of quartz textures in epithermal
vein systems have been presented by many researchers since Adams (1920) first proposed a terminology
for quartz. Recent studies have noted that quartz textures in epithermal gold veins can provide evidence
about mineralization processes (Morrison, 1990;
Dong et al., 1995).
Structural classes are defined by features such
as colour, grain size, and crystal form (comb, fine
grained, palty, colloform and cockade). Rhythmic
symmetrical and asymmetrical crustiform banding
is composed mostly of alternating band of quartz
showing various structures. Textures classes are
defined by features such as grain size and form of
quartz. A summary for structure and texture types
in quartz veins is provided in Table 1.
MethodoloGy
The studied samples were taken from outcrops
that represent different lithologic units and different
alteration types, and quartz veins (see map on Figure
3). Hand specimen and thin section petrographic
analysis were conducted to determine primary rock
textures and compositions, as well as secondary
minerals and textures. Several quartz vein samples
are cut and polished for detailed texture and mineral
paragenetic study. Geochemical analyses of selected
veins were done by XRF instrument and Energy
Dispersive Analysis of X-rays (EDAX), performed
on laboratory of Geological Department of Free
University Berlin – Germany.
Table 1. Classification of Quartz Structure and Texture
Texture type
Comb: a group of euhedral-subeuhedral crystals resembling the teeth of a comb under the microscope (2)
Feathery: a feathery and splintery appearance seen locally or throughout quartz crystals caused by slight differences in the maximum extinction position under the microscope (4)
Microcrystalline: aggregates of microcrystalline quartz
Fibrous: aggregates of fibrous quartz grains oriented perpendicular to the growth surface
Dendritic: branching patterns of quartz
Colloform: rhythmic bands of microcrystalline quartz on various scales (1)
Flamboyant: a radiant or flamboyant extinction of individual crystals with more or less rounded crystal outline'4'
Ghost-sphere: spherical distribution of impurities within microcrystalline quartz (4)
Pseudoacicular: linear arrangement of fine, elongate grains which could be caused by quartz replacement of calcite'4'
Structure type
Comb: a group of euhedral-subeuhedral crystals resembling the teeth of a comb (2)
Fine-grained: a group of anhedral quartz showing homogeneous grain shape; this structure is similar to the
massive quartz texture defined by Dong et al. (1995)
Platy: aggregates of radial, bladed crystals (3)
Colloform: rhythmic bands of chalcedonic silica grains with reniform habit (1)
Cockade: concentric crustiform bands of quartz, surrounding isolated fragments of host rock, or earlier precipitated quartz, or both (2)
The terminology of the quartz structures and textures is based on:
(1)
Rogers (1917)
(2)
Adams (1920)
(3)
Urashima (1956)
(4)
Dong et al. (1995)
253
Alteration and Vein Textures Associated with Gold Mineralization at the Bunikasih Area,
Pangalengan, West Java (A.S. Subandrio and N.I. Basuki)
results: VeIn petroloGy And GeocheMIstry
A classification of quartz veins at the Bunikasih
mine was made based on textures and cross cutting
relationship in the veins, adopting classification in
Shimizu et al. (1998). The epithermal quartz veins
are concentrated on the southwestern part of the
researched area (Figure 2). The deposit occurs in a
southwest-northeast shear zone within Late TertiaryQuarternary volcanic rock. The mineralization can
be divided into six major vein groups with general
north-south Q4-Q5-Q6 (Q4-6 group) and northwestsoutheast Q1-Q2-Q3 (Q1-3 group) strikes. Two
groups of major vein show variable internal structure
and texture as well as various gold and silver grade.
Based on crosscutting relationships and mineral
paragenesis, the veins appear to have been formed
during two mineralization epochs. The earlier event
is located in the northern part of River Cibaliung, further divided into three stages Q4-Q5-Q6, whereas the
rest in southern part can be distinguished in the later
event Q-Q2-Q3). The wide veins consist of multiple
mineralization stages. Relatively higher Au (4.0-24.6
ppm) and Ag-Cu-Pb-Zn contents are associated with
the Q1-3 stage. The earlier mineralization stages are
represented by Q4-6 vein group and characterized by
0.3-3.4 ppm Au and relatively lower base metal and
silver content (Table 2). Vein group of Q1-3 shows
an association texture of lattice bladed, crustiformcolloform banding, and ghost bladed in abundant
milky white chalcedonic quartz with some intensively
manganese and iron oxide stainings (Figure 5). Lattice bladed is a network intersecting blades of calcite
separated by polyhedral cavities which originate as
pseudomorph of lattice bladed calcite (Morrison et al.,
1990). This lattice texture indicates a boiling zone in
the uppermost level of epithermal system (Dong et
al., 1995). Adularia content on this Q3 vein (Figure
5) is also supported for the boiling zone environment.
Highest gold grade with relatively higher Ag-Cu-PbZn-As-Sb content (Table 1) is associated with vein
group of Q3 (20.1-24.6 ppm Au); this is characterized
by creamy white ghost bladed chalcedony with light
grey manganese oxide and sulfide staining (Figure
5). This highest gold content is detected by EDAX
analysis showing an electrum image with significantly
gold and silver peaks (Figure 6 and 7). Q2 vein shows
a lower gold content (7.2-15.0 ppm), characterized
by an associate texture of cockade and carbonate lat-
Table 2. Representative Composition of selected Quartz Veins
of Bunikasih Area by ICP Analyses (in ppm). Analyses were
performed in Geochemical Laboratory of Free University
Berlin – Germany
Sample
Au
Ag
Pb
Zn
Cu
As
Sb
Fe
Q11
4
8
25
25
8
0
0
605
Q12
3.7
7
30
25
7
0
0
554
Q13
1
90
40
40
13
8
5
7186
Q14
1
79
50
50
12
7
5
6111
Q21
15
545
45
55
60
24
6
1000
Q22
17.4
518
40
60
60
25
6
1096
Q23
7.2
617
20
111
61
21
6
2074
Q24
7.7
554
15
95
50
22
5
1795
Q31
24.6
618
40
55
65
22
6
954
Q32
20.6
493
35
45
55
23
7
1005
Q33
20.1
1164
30
131
96
38
9
1516
Q34
24.6
1056
33
123
85
40
8
1516
Q41
3.1
172
19
52
11
7
6
2669
Q42
3.4
160
70
55
12
7
7
2655
Q43
2.5
5
14
51
13
2
1
1168
Q44
2.8
5
14
53
12
2
1
1294
Q51
1
0
85
30
25
0
0
796
Q52
1
0
85
25
25
0
0
850
Q53
0.3
0
61
50
45
1
1
1362
Q54
0.4
0
80
50
40
1
0
1157
Q61
0.8
58
19
19
14
2
2
1313
Q62
0.7
55
19
19
12
2
2
1139
Q63
0.8
73
29
88
10
11
10
14325
Q64
2.7
71
10
25
15
22
4
10398
tice bladed (Figure 8). Cockade is a typical texture
showing concentric crustiform bands of quartz, surrounding isolated fragments of host rocks or earlier
precipitated quartz or both (Shimizu et al., 1998; see
Table 2). Rhodochrosite (Mn-carbonate) is often
present in this Q2 group. The lowest grade of gold on
group Q1-3 is Q1 vein, characterized by banded chalcedonic with abundant crystalline quartz and scarcely
manganese staining (Figure 9). Lowest gold contents
of the whole vein samples are associated with group
Q4-6. This vein group is characterized by 0.3 to 3.4
ppm gold content, banded chalcedonic, cockade with
saccharoidal core and crustiform-colloform bands
254
Jurnal Geologi Indonesia, Vol. 5 No. 4 Desember 2010: 247-261
Q3
P -31
1
vug
ka
0.3mm
Q32
sc
P-3 2
ch
mn
ad
vug
ch
qz
li
1cm
0.3mm
Q33
P-3 3
mn
qz
mn
mn
1cm
0.3mm
Q31. Lattice bladed, Q32. Crustiform banding of comb structure, Q3 3. Ghost bladed with segmental zone for
chemical and microprobe analyses; P3 1, P3 2 and P33 microphotographs series of petrography in cross polarized light of left side quartz veins. Note: ad=adularia, ca=calcite, ch=chalcedone, cqz=cryptocrystalline
quartz, ka=kaolinite, li=limonite (or limonitic staining or coating), mn=manganese oxide, qz=quartz,
rh=rhodochrosite
Figure 5. Series of macro- and micro photographs of vein group Q3. Selected geochemical and microprobe analyses of the
lowest photograph are available in Table 1 and Figures 6 and 7.
Alteration and Vein Textures Associated with Gold Mineralization at the Bunikasih Area,
Pangalengan, West Java (A.S. Subandrio and N.I. Basuki)
Untitled: 2
Edax image Q3 vein (B-zone)
Label:
kV: 25.0
Tilt: 0.0
FS: 637
Take-off: 35.0
255
Det Type: STD
Res: 144
Lsec: 20
Tc: 40
4-Jul-3
17:01:41
First chalcedonic
groundmass
Profile & point of
analyses in gold
nugets
Second chalcedonic
groundmass
AgLa
Ka
Silver peak
AgLb
SiKa
SiKb
AuLa
PbLb
AuLb
AuLa
CuKb
AsKb
CuKa
AsKa
SK
2.00
4.00
6.00
8.00
10.00
12.00
Gold peak
14.00
16.00
18.00
Ag=
Au=
S =
As=
32.35 Wt %
56.28 Wt %
5.52 Wt %
0.78 Wt %
B-zone
EDAX ZAF Quantification (Standardless)
Element Normalized
Element
Wt %
At %
SiK
SK
AgL
CuK
AuL
AsK
PbL
4.60
5.52
32.35
0.49
56.26
0.78
0.00
17.44
18.32
31.91
0.83
30.40
1.11
0.00
Total
100.00
100.00
K-Ratio
0.0268
0.0225
0.1895
0.0053
0.5152
0.0081
0.0000
Z
F
A
1.2104 0.4797
1.1996 0.3371
1.0203 0.5741
1.1009 0.9166
0.9099 1.0065
1.0707 0.9676
0.8908 1.0119
1.0036
1.0066
1.0000
1.0649
1.0000
1.0000
1.0000
Figure 6. Energy Dispersive Analysis of X-ray (EDAX) image of electrum founded B-zone of vein Q3 (inset). The profile
shows gold and silver peak significantly. Inset table displays relative percentage of the element.
Edax image Q3 vein (Zone B)
Argentite
40um
First
chalcedonic
groundmass
Electrum
gold
Second
chalcedonic
groundmass
Argentite
MINERAL PARAGENESIS
2 nd chalcedony+pyrite+electrum
1st chalcedony+argentite
rd
3 chalcedony+Mn-oxide
Figure 7. Detail microphotograph by EDAX imaging. The precious metals (Au & Ag) are associated with late chalcedonic
veinlets (light gray in colour).
256
Jurnal Geologi Indonesia, Vol. 5 No. 4 Desember 2010: 247-261
vug
Q21
qz
P 21
li
ch
qz
0.3mm
Q22
P 22
li
mn
qz
qz
li
ka & rh
0.3mm
P 23
Q23
ka & cqz
ka
ch
ca
0.3mm
Q21. Cockade, Q22. Cockade with rhodochrosite vug infill, Q23.Carbonate lattice bladed; P21, P22 and
P23 microphotographs series of petrography in cross polarized light of left side quartz veins.
Note: ad=adularia, ca=calcite, ch=chalcedone, cqz=cryptocrystalline quartz, ka=kaolinite, li=limonite
(orlimonitic staining or coating), mn=manganese oxide, qz=quartz, rh=rhodochrosite.
Figure 8. Series of macro- and micro photographs of vein group Q2. Selected geochemical analyses of vein are available in
Table 1.
Alteration and Vein Textures Associated with Gold Mineralization at the Bunikasih Area,
Pangalengan, West Java (A.S. Subandrio and N.I. Basuki)
Q1
257
P6
qz
mn
ad
qz
li
mn
0.3mm
Q4
P4
li
vug
cqz
0.3mm
Q5
P5
cqz
vug
qz
li
0.3mm
Q1. Banded chalcedonic with manganese film Q4. Cockade with saccharoidal core , Q5. Crustiformcolloform bands; P1, P4 and P5 microphotographs series of petrography in cross polarized light of left side
quartz veins. Note: ad=adularia, ca=calcite, ch=chalcedone, cqz=cryptocrystalline quartz, ka=kaolinite,
li=limonite (or limonitic staining or coating), mn=manganese oxide, qz=quartz, rh=rhodochrosite
Figure 9. Series of macro- and micro photographs of quartz vein group Q1, Q4 and Q5. Selected geochemical analyses and
microprobe profile and image of vein Q4 are available in Tabel 1 and Figure 6.
258
Jurnal Geologi Indonesia, Vol. 5 No. 4 Desember 2010: 247-261
(Figure 9 and 10). The relative higher gold grade
in this Q4-6 group is Q4 (3.4 ppm Au) detected
by EDAX as a smaller electrum included in pyrite
crystal (Figure 11). Significant different between
group of Q1-3 and Q4-6 is the presence of lattice
bladed texture and the colour as well as clarity of
chalcedonic veins. The colour of Q1-3 group that
has lattice bladed texture, shows milky white colour
with light grey fleck or staining, whereas veins of
group Q4-6 are dominated by crustiform texture
with abundant glassy transparence appearance
of crystalline quartz (Figure 10). A summary for
characteristics of quartz vein in the Bunikasih area
is provided in Table 3.
dIscussIon
In the Buchanan model of gold distribution (Buchanan, 1981) there are specific intervals that host
base and precious metal mineralization (Figure 12).
In the textural model, the precious metal interval
essentially corresponds to the crustiform-colloform
Q6
textural superzone and the base metal interval overlaps the crystalline chalcedonic zone, quartz, adularia and sulfide ore (Figure 12). Geochemical analyses
of certain texture samples have demonstrated that
within individual deposits there is a consistent grade
range for each texture assemblage. For example,
creamy white – light grey chalcedony with lattice
and ghost bladed texture on vein Q1-3 has highest
range of gold grade (> 1.0 to 24.6 ppm Au) and also
shows by elevated of Ag-Pb-Zn-Cu-As-Sb. Light
grey colour on the chalcedony is presumably due
to relative higher content of base metal and manganese oxide. In contrast, the vein assemblage of Q4-6
displays more clear and transparent chalcedony
or crystalline quartz with combination between
banded chalcedonic, crustiform and saccharoidal
textures showing relatively lower range of gold
grade (0.3 – 3.1 ppm). High grades Au of Q1-3
is also characterized by the presence of adularia,
sulfide bands, and manganese oxide in association
with crustiform banding texture (Figure 3 on Q3 2
and P-32 ). Relative higher Cu-Pb-Zn-Mn is a good
indicator for the presence of base metal in Q1-3.
P -6
cqz
Vug &
druse
py
ch
qz
0.3mm
Q6. Crustiform bands with vug and druse texture (lower right), P6 microphotographs series of petrography
in cross polarized light of left side quartz veins.
Note:
ad=adularia, ca=calcite, ch=chalcedone,
cqz=cryptocrystalline quartz, ka=kaolinite, li=limonite (or limonitic staining or coating),
mn=manganese oxide, qz=quartz,
rh=rhodochrosite
Figure 10. Q6 is a representative of barren quartz vein and its petrography image. The colour of quartz is more clear and
transparent than gold bearing milky quartz.
Alteration and Vein Textures Associated with Gold Mineralization at the Bunikasih Area,
Pangalengan, West Java (A.S. Subandrio and N.I. Basuki)
259
Table 3. Characteristic Features of Quartz Veins in the Bunikasih Area
Earlier mineralization epoch (Q4-6)
Characterized by development of barren comb texture quartz and few variations of quartz textures (comb-texture and
microcrystalline quartz)
Microcrystalline quartz partly associated with electrum in stage Q4 (Figure 7)
Characterized by less developed crustiform bandings of comb- and fine-grained structure quartz compared to the later epoch
Barren comb structure developed in stages Q5 (Figure 7) and Q6 (Figure 8)
Fine-grained structure associated with manganocalcite (rhodochrosite) and johannsenite
Cockade structure developed throughout stages
Later mineralization epoch (Q1-3)
Characterized by crustiform banding of comb-structure, fine-grained, platy, colloform, and cockade quartz
Comb structure recognized throughout stages
Colloform structure commonly observed in quartz of crustiform banding in stage Q32 (Figure 5) and in quartz showing botryoidal
surfaces at stage Q23
Cockade structure commonly developed in stages Q21 and Q22 (Figure 6)
Characterized by various textures: comb, feathery, microcrystalline, fibrous, dendritic, colloform, ghost-sphere, flamboyant, and
pseudoacicular (Figure 5)
Comb-texture quartz developed without ore minerals except stage Q32 and Q33 ; alternate precipitation of barren comb-texture
quartz and microcrystalline quartz; growth bands developed in comb-texture quartz of stage Q3 3and not in quartz from other stages
Microcrystalline quartz intimately associated with ore minerals and interstratified chlorite-smectite in stage Q32 (Figure 5)
Feathery texture apparently found as patches or zones in comb-texture quartz throughout vein formation
Fibrous texture with colloform structure quartz developed in stage Q5 and Q6 (Figure 7 and 8)
Colloform and flamboyant textures with fine-grained structure developed in stage Q32 (Figure 5)
Dendritic quartz partly observed in comb-texture quartz (stage Q6) and microcrystalline quartz Q2 3)
Lattice bladed and ghost-sphere texture is partly observed in microcrystalline quartz in stages Q3 1and Q3 .3
Untitled: 2
Label:
Edax image Q4 (Zone H)
kV: 25.0
Tilt: 0.0
FS: 547
Take-off: 35.0
Det Type: STD
Lsec: 20
Res: 144
Tc: 40
24-Jul-3
11:34:34
AgLa
First chalcedonic
groundmass
Profile & point of analyses
AgLb
S Ka
Silver & Sulfur peak
S Kb
SiKb
SiKa
PbLa
AsKa
MnKb
MnKa
2.00
4.00
AuLa
6.00
8.00
PbLb
AuLb
10.00
12.00
14.00
EDAX ZAF Quantification (Standardless)
Element Normalized
Element
Wt %
At %
SiK
SK
AgL
CuK
AuL
AsK
PbL
2.47
12.09
69.04
0.57
3.71
2.25
9.87
7.27
31.10
52.81
0.86
1.55
2.48
3.93
Total
100.00
100.00
K-Ratio
0.0132
0.0888
0.5463
0.0048
0.0321
0.0221
0.0837
Z
1.1711
1.1612
0.9710
1.0464
0.8583
1.0114
0.8374
A
F
0.4490 1.0115
0.6184 1.0237
0.8149 1.0001
0.7950 1.0031
1.0080 1.0000
0.9689 1.0000
1.0133 1.0000
Ag= 69.04 Wt %
Au= 3.71 Wt %
S = 11.68 Wt %
As= 2.25 Wt %
Zone H
EDAX ZAF Quantification (Standardless)
Element Normalized
Element
SiK
SK
AgL
Mnk
Fek
Cuk
Aul
Ask
Pbl
Total
At %
K-Ratio
Z
A
F
3.01
12.24
71.49
0.29
0.53
0.52
0.00
0.71
11.20
Wt %
8.66
30.83
53.52
0.43
0.77
0.66
0.00
0.77
4.37
0.0162
0.0951
0.5784
0.0024
0.0048
0.0050
0.0000
0.0070
0.0946
1.1676
1.1577
0.9672
1.0428
1.0665
1.0451
0.8545
1.0070
0.8336
0.4546
0.6539
0.8365
0.7946
1.8355
0.9168
1.0071
0.9682
1.0126
1.0124
1.0258
1.0001
1.0024
1.0035
1.0095
1.0000
1.0000
1.0000
100.00
100.00
Ag= 71.49 Wt %
Au= 0.00 Wt %
S = 12.24 Wt %
As= 0.71 Wt %
Figure 11. EDAX image of electrum founded H-zone of vein Q4 (inset). The profile shows gold and silver peak significantly.
Inset table displays relative percentage of the element.
260
0
100
ORE
GANGUE
Rare gold
usually in
pyrite
Zeolotes, Calcite
Clay (Agate)
Empty (Clays)
SI RESIDUE
ILL-CEL
100
SER
PR
Gold in pyrite
Ag-sulfosalts
PR
200
Pyrargyrite
Proustite
Argentite
Electrum
200
AD
300
PRECIOUS METAL
INTERVAL
Argentite
Electrum
BOILING LEVEL
SI
400
250
BASEMETAL
INTERVAL
Galena
Sphalerite
Chalcopyrite
Argentite
Calcite
Zeolites
Agate
Stibnite
Realgar
CRYSTALLINE CARBONATE (+ agate + parallel bladed + molds)
LATTICE BLADED + BLADED CARBONATE (+ agate + amethyst)
CH
MASSIVE CHALCEDONIC (+ lattice bladed + moss + agate)
Quartz
Calcite
Pyrite
(Bar, F1)
MOSS + CHALCEDONIC > CRYSTALLINE (+ lattice bladed
+ sulfide bands + moss adularia)
CC
Quartz
Adularia
Sericite
Pyrite
(Cal, Chl, Fl)
Quartz
Fluorite
Pyrite
CRYSTALLINE > MOSS + CHALCEDONIC (+ needle adularia
+ sulfide bands + disseminated sulfides)
X
CRYSTALLINE QUARTZ + ADULARIA + SULFIDE (+ crustiform)
Pyrhotite
Pyrite
Arsenopyrite
CRYSTALLINE QUARTZ + CARBONATE (+ crustiform)
500
DEPTH
T(°C)
(M)
Figure 12. Scale model for zoning of textures, alteration, ore and gangue mineralogy in a typical boiling zone epithermal vein (op cit. Morrison et al., 1990). Based on the
model of Buchanan (1981) with temperature reflecting the level for boiling under hydrostatic condition of a fluid containing 2.84% NaCl. Alteration zones PR=propylitic;
SI=Silica; AD=Adularia; ILL=Illite; SER=Sericite; CEL=Celadonite, AL=Alunite, kaolinite, pyrite. CH=Chalcedonic, CC=Crustiform-Colloform, and X=Crystalline,
Jurnal Geologi Indonesia, Vol. 5 No. 4 Desember 2010: 247-261
AL
WATER TABLE
150
TEXTURES
Alteration and Vein Textures Associated with Gold Mineralization at the Bunikasih Area,
Pangalengan, West Java (A.S. Subandrio and N.I. Basuki)
conclusIons
Quartz morphology combined with petrography and analyses of gold content by Energy
Dispersive Analyses of X-ray (EDAX) studies
suggest that boiling of the fluid occurred repeatedly, leading to silica-supersaturated conditions
with respect to quartz and resulting in the formation of the certain silica textures. Recrystallization of silica to quartz occurred throughout
vein formation. The geochemistry data combined
with parageneses, quartz textures, and petrography studies suggest the following model for
the Bunikasih gold-silver deposits. The veins
show two distinct mineralization epochs, an
earlier and a later one, which were responsible
for type 1 (vein group Q4-6) and 2 (vein group
Q1-3) hydrothermal fluids, respectively. Both
types are dominantly meteoric water in origin.
The most prospected gold in Bunikasih district
is associated with vein group Q1-3. It occurs in
the southwestern part characterized by the texture assemblage such as milky white chalcedony
with often grey staining of manganese oxide and
sulfide bands, lattice and ghost bladed, presence
of adularia, and a relatively higher range of gold
content between 1.0 to 24.6 ppm Au. Based on
the systematic evaluation of the vertical and
horizontal distribution of textures associated with
an epithermal system (Buchanan, 1981), the vein
group Q4-6 is associated with Crystalline Superzone or X-zone (Figure 12) and is characterized
by common crustiform bands, dominantly clear
crystalline and saccharoidal quartz, and also a
general decrease in the proportion of sulfides and
gold-silver content (see Table 1). The vein group
Q1-3 is presumably mineralized in a higher level
than Q4-6, deposited between the CrustiformColloform Superzone (CC) and Chalcedonic Superzone (CH). These CC and CH zones dominated
by milky white chalcedonic quartz and associated
with carbonate or bladed pseudomorph after carbonate, e.g. lattice bladed of sample Q3 (Figure
5) indicate the uppermost level and boiling zone
in the epithermal system (Morrison et al., 1990;
Dong et al., 1995).
261
Acknowledgements---The authors thank Prof. Dr. Peter
Halbach of Free University Berlin for the technical support
and permission to work in the laboratory for EDAX and ICP
analyses during our visiting study in Marine Geology in Germany at 2003. The authors especially thank Julius Chandra,
for the field work, report writing, and finishing thesis about
gold mineralization in Bunikasih area. The authors also
thank unit Geomin of PT. Aneka Tambang Tbk. for giving
permission to visit the mining concession area in Bunikasih.
references
Adams, S.F., 1920. A microscopic study of vein quartz;
Economic Geology, 15, p. 623-664
Alzwar, M., Akbar, N., and Bachri, S., 1992. Geologi Lembar Garut dan Pameungpeuk, Jawa Barat (1208-6),
Skala 1 : 250.000. Pusat Penelitian dan Pengembangan
Geologi, Bandung.
Buchanan, L.J., 1981. Precious metal deposits associated
with volcanic environments in the Southwest Arizona
Geological Society Digest, 14, p.237-262.
Chandra J., 2009. Geologi dan Prospeksi Emas Hidrotermal
Daerah Bunikasih, Pangalengan Jawa Barat, Tugas Akhir
Sarjana Strata-1 Prodi Teknik Geologi – FITB – ITB,
unpublished.
Dong, G., Morrison, G.W., and Jaireth, S., 1995. Quartz
Textures in epithermal veins, Queensland – classification, origin, and implication. Economic Geology, v.90,
p. 1841-1856
Fournier, R.O., 1985a. The behavior of silica in hydrothermal
solution, Reviews in Economic Geology, 2, p. 45-61
Morisson, G., Guoyi, D., and Jareith, S., 1990. Textural
Zoning in Epithermal Quartz Vein, Amira Project P247,
Gold Research Group, James Cook University of North
Queensland, p. 33
Shimizu, T., Matsueda, H., Ishiyama, D., and Matsubaya,
O., 1998. Genesis of Epithermal Au-Ag Mineralization
of the Koryu Mine, Hokkaido, Japan. Economic Geology, 93, p. 303-325.
Urashima, Y., 1956. “Bosa” quartz veins, especially the
fine-grained quartz aggregates, of the Konomai mine
in Hokkaido, Japan: Hokkaido University. Journal of
Faculty of Science, 9, p. 371–387.
Warmada, I.W., 2003. Ore mineralogy and geochemistry of
the Pongkor epithermal gold-silver deposit, Indonesia,
Clausthal-Zellerfeld. Papierflieger.
Widi, B.N., Sunarya, Y., Judawinata, K., and Setiawan, B.,
1997. The Epithermal Gold-Silver-Tellurides Deposits
of Citambal-Cineam, Tasikmalaya, West Java, Indonesia. Proceedings of Mineral Exploration Technology
in Indonesia, BPP-Technology, Jakarta, paper no. 10,
p. 1-18.