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Alteration and Vein Textures Associated withGold Mineralization
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Jurnal Geologi Indonesia, Vol. 5 No. 4 Desember 2010: 247-261
247
Naskah diterima 21 Juni 2010, revisi kesatu: 13 Agustus 2010, revisi kedua: 02 September 2010, revisi terakhir: 26 November 2010
Alteration and Vein Textures Associated withGold 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 ser-icite 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, crusti-form 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
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. How-ever, 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, Cikondang-Cianjur, 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 inter-preted 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 materi-als and opaque minerals. This unit shows a strong argillic and/or prophilitic alteration and, locally, silicification. In some places, a weak magnetic na-ture 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 phe-nocrysts 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 apha-nitic 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 young-est unit in the area and is found in Puncak Cac-ing 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
249Alteration and Vein Textures Associated with Gold Mineralization at the Bunikasih Area, Pangalengan, West Java (A.S. Subandrio and N.I. Basuki)
Rancatungku
Soreang
Tmb
Kramat
Bandasari
Qd
Cangkuang
Cipeundeuy
G. Geulis
G. Koromong
1151
Muara
BarosBanjaran
PasirpariukCiteuteurpBojonglao 2
Qtl
G. Bubut GegerheosCianjur
PasirhuniGanjen
G. Tanuaktangsi
1514
1333
G. Tikukur
Cikalong
Kebontunggul
G. Puntang
G. Malabar
2621
G. HarumanLamajang
Gambung
alk o gCi n
C si e reuup
PuncakbebarG. Tilu
Rancatungku
B Qd
TmbQwb
Qmt
Qtl
Riunggunung
Pasirurug
G.Gambung
Rancamanyar
Qmt
Pangalengan
CibeureumQwb
Qopu
Wanasari
Barussalam
Cibunihayu
Gunungcepu
Banjarsari
Situ Cileunca G. Wayang
QwbG. Windu
KertasariCikowokQyw
2054
Qopu
Puncak Cacing
PR. Singahal
k
Cail
i
G. Cikepung
Qwb
Qwb
Cepu
388
Cibolang
Wanasuka
Qkl
Samosa
G. Malang1880
1390
G. Karancang
1563
0 1 2 3 Km
A
BunikasihN
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.
Note : Q=Quaternary Age; T=Tertiary Age; Q w b = W a r i n g i n -
Bedi l Andesi te , 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 and Tilu lavas: Andesitic lava and basaltic-andesitic lava of Mt. Tilu;
T m b = B e s e r Format ion:Tuf f sandstone, pumice tu f f , c lays tone, conglomerate and lignite;
Qmt= Malabar Tilu Vo l c a n i s : t u f f , laharic breccias contains minor of pumice and lavas;
Qyw=Young volcanic: E f f l a t a a n d 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 tuf f , tuf faceous breccias contains pumices and old andesitic-basaltic laharic deposits.
Qd=Lake deposit:Clay s i l t , f i n e t o coarse sands and gravel, commonly tuffaceous.
Studied Area
250 Jurnal Geologi Indonesia, Vol. 5 No. 4 Desember 2010: 247-261
G.Puncakcacing
Q6
Q1 500m
Q6
Q5
Q4
Q3
30
10
14
00
Vein Strike/Dip Average vein thickness
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
N
0 1 km
N
Puncak Cacing Andesite Lava Unit
Cikabuyutan Andesite Lava Unit
Cibaliung Andesite Lava Unit
Geological Cross SectionScale V:H=1:1 (m) 1600
1500
1400
1300
1200
1100
B
D
0776000 0777000 0778000 0779000 0780000
920000
9199000
9198000
9197000
B
C
A
D
1600
1500
1400
1300
1200
1100
1700
1600
1500
1400
A
C
G. Puncakcacing
Figure 2. Topography map of Bunikasih area. Mt. Puncak Cacing is the highest landmark of this area. Gold mining is distrib-uted on quartz vein group on southwestern area.
Figure 3. Simplified geological map of Bunikasih area (Chandra, 2009).
251Alteration and Vein Textures Associated with Gold Mineralization at the Bunikasih Area, Pangalengan, West Java (A.S. Subandrio and N.I. Basuki)
0 1 km
N
Prophilitic zone
1500
1400
1300
1200
1100
1000
E
G. Puncakcacing
Argillic zone
920000
9199000
9198000
9197000
F
0776000 0777000 0778000 0779000 0780000
F
Geological Cross SectionScale V:H=1:1 (m)
Figure 4. Simplified alteration map of Bunikasih area (Chandra, 2009).
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 altera-tion types found in the area, with argillic occur-rences are much more widespread than prophilitic (Figure 4). Argillic alteration on volcanic rocks is characterized by clay mineral alteration (prob-ably 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, ac-companying 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 consis-tent with the trends of faults in the researched area, suggesting structural control on vein formation. The faults have probably acted as conduits for hydro-thermal fluids that were responsible for alteration of volcanic rocks and formation of quartz veins.
252 Jurnal Geologi Indonesia, Vol. 5 No. 4 Desember 2010: 247-261
Table 1. Classification of Quartz Structure and Texture
Texture typeComb: 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 differ-ences in the maximum extinction position under the microscope (4)
Microcrystalline: aggregates of microcrystalline quartzFibrous: aggregates of fibrous quartz grains oriented perpendicular to the growth surfaceDendritic: branching patterns of quartzColloform: 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 typeComb: 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 pre-cipitated 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)
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 re-crystallized 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 research-ers since Adams (1920) first proposed a terminology for quartz. Recent studies have noted that quartz tex-tures 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.
253Alteration and Vein Textures Associated with Gold Mineralization at the Bunikasih Area, Pangalengan, West Java (A.S. Subandrio and N.I. Basuki)
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
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
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 Tertiary-Quarternary volcanic rock. The mineralization can be divided into six major vein groups with general north-south Q4-Q5-Q6 (Q4-6 group) and northwest-southeast 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, fur-ther 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, crustiform-colloform banding, and ghost bladed in abundant milky white chalcedonic quartz with some intensively manganese and iron oxide stainings (Figure 5). Lat-tice 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-Pb-Zn-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-
tice bladed (Figure 8). Cockade is a typical texture showing concentric crustiform bands of quartz, sur-rounding 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 chal-cedonic 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
vug
Q3 1
ka
0.3mm
P-31
li
ch
mn
ch
1cm
Q32
sc
ad
vug
qz
0.3mm
P- 3 2
mn
1cm
Q33
mn
qz
mn
0.3mm
P- 3 3
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.
Q31. Lattice bladed, Q32. Crustiform banding of comb structure, Q33. Ghost bladed with segmental zone for chemical and microprobe analyses; P31, P32 and P33 microphotographs series of petrography in cross polar-ized 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
255Alteration and Vein Textures Associated with Gold Mineralization at the Bunikasih Area, Pangalengan, West Java (A.S. Subandrio and N.I. Basuki)
Untitled: 2
Label:
kV: 25.0 Tilt: 0.0 Take-off: 35.0 Det Type: STD Res: 144 Tc: 40
FS: 637 Lsec: 20 4-Jul-3 17:01:41
Edax image Q3 vein (B-zone)
Ag= 32.35 Wt %Au= 56.28 Wt %S = 5.52 Wt %As= 0.78 Wt %
First chalcedonicgroundmass
Profile & point ofanalyses in goldnugets
Second chalcedonicgroundmass
Silver peak
Gold peak
B-zone
AgLaKa
AgLbAuLa
PbLbAuLb
AuLa
AsKb
AsKa
CuKbCuKa
SiKa
SiKb
S K
2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00
EDAX ZAF Quantification (Standardless)Element Normalized
Element Wt % At % K-Ratio Z A F
SiK 4.60 17.44 0.0268 1.2104 0.4797 1.0036S K 5.52 18.32 0.0225 1.1996 0.3371 1.0066AgL 32.35 31.91 0.1895 1.0203 0.5741 1.0000CuK 0.49 0.83 0.0053 1.1009 0.9166 1.0649AuL 56.26 30.40 0.5152 0.9099 1.0065 1.0000AsK 0.78 1.11 0.0081 1.0707 0.9676 1.0000PbL 0.00 0.00 0.0000 0.8908 1.0119 1.0000
Total 100.00 100.00
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.
Argentite
Firstchalcedonicgroundmass
Electrumgold
Secondchalcedonicgroundmass
Argentite
MINERAL PARAGENESIS
1 chalcedony+argentite 2 chalcedony+pyrite+electrumst nd
3 chalcedony+Mn-oxide rd
Edax image Q3 vein (Zone B)
40um
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
P 21
vug
qz
qz
0.3mm
Q22li
qz li
ka & rh
P 22
mn
qz
0.3mm
Q23 P 23
ka
ch
0.3mm
ka & cqz
ca
Q21
li
ch
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.
257Alteration and Vein Textures Associated with Gold Mineralization at the Bunikasih Area, Pangalengan, West Java (A.S. Subandrio and N.I. Basuki)
mn
ad
li
Q1qz mn
qz
P6
0.3mm
vug
qz
P5
0.3mm
cqz
P4
0.3mm
cqz
vug
li
Q5
Q4
li
Q1. Banded chalcedonic with manganese film Q4. Cockade with saccharoidal core , Q5. Crustiform-colloform 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
ch
py
Vug & druse
qz
Q6
cqz
P - 6
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.
(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 (Bu-chanan, 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
textural superzone and the base metal interval over-laps the crystalline chalcedonic zone, quartz, adular-ia 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 manga-nese 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 Q32 and P-32). Relative higher Cu-Pb-Zn-Mn is a good indicator for the presence of base metal in Q1-3.
259Alteration and Vein Textures Associated with Gold Mineralization at the Bunikasih Area, Pangalengan, West Java (A.S. Subandrio and N.I. Basuki)
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 epochBarren comb structure developed in stages Q5 (Figure 7) and Q6 (Figure 8)Fine-grained structure associated with manganocalcite (rhodochrosite) and johannseniteCockade structure developed throughout stages Later mineralization epoch (Q1-3)
Characterized by crustiform banding of comb-structure, fine-grained, platy, colloform, and cockade quartzComb structure recognized throughout stagesColloform structure commonly observed in quartz of crustiform banding in stage Q32 (Figure 5) and in quartz showing botryoidal surfaces at stage Q23Cockade 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 Q33 and not in quartz from other stagesMicrocrystalline 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 formationFibrous 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 Q23)Lattice bladed and ghost-sphere texture is partly observed in microcrystalline quartz in stages Q31 and Q33.
Table 3. Characteristic Features of Quartz Veins in the Bunikasih Area
Untitled: 2
Label:
kV: 25.0 Tilt: 0.0 Take-off: 35.0 Det Type: STD Res: 144 Tc: 40
FS: 547 Lsec: 20 24-Jul-3 11:34:34
Edax image Q4 (Zone H)
Ag= 69.04 Wt %Au= 3.71 Wt %S = 11.68 Wt %As= 2.25 Wt %
First chalcedonicgroundmass
Profile & point of analyses
Silver & Sulfur peak
AgLa
AgLb
PbLb
AuLbAuLa
AsKa
S Ka
SiKb
2.00 4.00 6.00 8.00 10.00 12.00 14.00
EDAX ZAF Quantification (Standardless)Element Normalized
Element Wt % At % K-Ratio Z A F
SiK 2.47 7.27 0.0132 1.1711 0.4490 1.0115S K 12.09 31.10 0.0888 1.1612 0.6184 1.0237AgL 69.04 52.81 0.5463 0.9710 0.8149 1.0001CuK 0.57 0.86 0.0048 1.0464 0.7950 1.0031AuL 3.71 1.55 0.0321 0.8583 1.0080 1.0000AsK 2.25 2.48 0.0221 1.0114 0.9689 1.0000PbL 9.87 3.93 0.0837 0.8374 1.0133 1.0000
Total 100.00 100.00
EDAX ZAF Quantification (Standardless)Element Normalized
Element Wt % At % K-Ratio Z A F
SiK 3.01 8.66 0.0162 1.1676 0.4546 1.0124S K 12.24 30.83 0.0951 1.1577 0.6539 1.0258AgL 71.49 53.52 0.5784 0.9672 0.8365 1.0001Mnk 0.29 0.43 0.0024 1.0428 0.7946 1.0024Fek 0.53 0.77 0.0048 1.0665 1.8355 1.0035Cuk 0.52 0.66 0.0050 1.0451 0.9168 1.0095Aul 0.00 0.00 0.0000 0.8545 1.0071 1.0000Ask 0.71 0.77 0.0070 1.0070 0.9682 1.0000 Pbl 11.20 4.37 0.0946 0.8336 1.0126 1.0000
Total 100.00 100.00
S Kb
SiKaPbLa
MnKbMnKa
Zone H
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 Jurnal Geologi Indonesia, Vol. 5 No. 4 Desember 2010: 247-261
Figu
re 1
2. S
cale
mod
el fo
r zon
ing
of te
xtur
es, a
ltera
tion,
ore
and
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gue
min
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cal b
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cit.
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rison
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tion
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s PR
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; ILL
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ER=S
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EL=C
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AR
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CH
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0
100 200
300
400
500
250
200
150
100
WA
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261Alteration 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 petrog-raphy and analyses of gold content by Energy Dispersive Analyses of X-ray (EDAX) studies suggest that boiling of the fluid occurred repeat-edly, leading to silica-supersaturated conditions with respect to quartz and resulting in the for-mation of the certain silica textures. Recrystal-lization of silica to quartz occurred throughout vein formation. The geochemistry data combined with parageneses, quartz textures, and petrog-raphy 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 tex-ture 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 Super-zone 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 Crustiform-Colloform Superzone (CC) and Chalcedonic Su-perzone (CH). These CC and CH zones dominated by milky white chalcedonic quartz and associated with carbonate or bladed pseudomorph after car-bonate, 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).
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 Ger-many 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.
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