Celebes International Conference on Earth Science
(CICES) 2014
http://www.uho.ac.id/cices2014
Metamorphic facies and hydrothermal alteration
characteristics of the metamorphic-rock hosted gold at
Gunung Botak and Gogorea, Buru island, Indonesia
Irzal Nura, Sufriadina, Sri Widodoa, Ulva Ria Irfanb*
a
Mining Engineering Study Program, Hasanuddin University, Makassar 90245, Indonesia
Geological Engineering Study Program, Hasanuddin University, Makassar 90245, Indonesia
b
Abstract
This paper discusses a recent study on metamorphic facies, hydrothermal alteration mineral assemblages, as well as
geochemical characteristics (ore grades) of the metamorphic rock-hosted gold deposit at Gunung Botak and Gogorea,
Buru Island, Indonesia. Based on the petrographic study of the metamorphic rocks, it was identified that the rock
types are dominantly schist and less phyllite. The mineral assemblages include chlorite, muscovite, quartz, biotite,
clay, cordierite, and opaque. Based on these metamorphic rock and mineral assemblages, it is inferred that
metamorphic facies of the host rock is greenschist, with the formation temperature ranges from about 250 to 500oC
and the pressure of about 2 to 10 Kbar, or in intermediate metamorphic grade. Based on direct observation in the
field, the ore mineralizations are generally surrounded by argillic and chloritic alterations. Hydrothermal alteration
minerals identified from XRD analysis include muscovite, chlorite and quartz. Seven ore samples were analyzed
using AAS (fire assay) method, and indicates a grade range of Au 0.23-5.90 g/t. Other important elements were also
assayed using ICP-OES method, mostly high-grade ore samples also indicate significant concentrations of As and Sb,
whereas Ag, Cu, Pb and Zn are generally low.
© 2014 The Authors.
Selection and/or peer-review under responsibility of the CICES 20014 Editorial Board and supported by Halu Oleo
University, CRISU, HAGI, IAGI, HFI, IPREMS as co-hosts.
Keywords: metamorphic facies; hydrothermal alteration; gold; Gunung Botak; Gogorea
1. Introduction
Metamorphic rock-hosted gold mineralization at two areas in Buru island, Indonesia, namely Gunung
Botak and Gogorea, has been previously reported by Idrus et al. (2013) as a mineralization that shows
characteristic features of low sulfidation epithermal or orogenic gold deposit types. Until the end of 2012,
about 100,000 artisanal and small-scale miners are operating in Gunung Botak, and 5,000 in Gogorea
* Corresponding author. Tel.: +62-411-580202; fax: +62-411-580202.
E-mail address: irzal_nur@yahoo.com.
Author name / IJSESTech 00 (2014) 000–000
(Idrus et al., 2013). Administratively, Gogorea is included in Waeapo district and Gunung Botak in Teluk
Kayeli district, Buru regency, Moluccas Province, Indonesia. The Buru island is located in eastern part of
Indonesia, which geologically situated in outer part of the Tertiary-Quaternary Sunda-Banda magmatic
arc, not in a volcanic arc (Carlile and Mitchell, 1994; Fig. 1). This paper describes an update study of the
mineralization, based on field and laboratory data, which focused on metamorphic facies of the host rock,
hydrothermal alteration mineral assemblages, as well as its geochemical characteristics (ore grades).
Fig. 1. Location of Buru island in the distribution and polarity map of mineralized Late Cretaceous to Pliocene
magmatic arcs in Indonesia (redrawn from Carlile and Mitchell, 1994).
2. Geology
Tectonically, formation of Buru island, which is a part of the non-volcanic outer Banda arc, is related
to a process that involves mountain building and continental collision. So far, it is generally accepted that
Buru Island is a microcontinent derived from Australian continent that had been detached during the
Mesozoic, where its emplacement to the present position is still subject to debate (Guntoro, 2000).
Geology of Buru island has been described by Tjokrosapoetro et al., 1993 (Fig. 2). The oldest rock
units in the island is Permian moderate-grade metamorphic rocks of Wahlua Complex (Pzw) and Rana
Complex (Pzr). The Wahlua Complex consists of schist, phyllite, meta-arkosic sandstone, quartzite and
marble. This unit is widely distributed in the eastern part of the island, where Gunung Botak and Gogorea
areas are included. The younger Rana Complex consists of phyllite, slate, meta-arkose, meta-greywacke
and marble; occupies central (around Rana Lake) and southern parts of the island.
Clastics and carbonaceous sedimentary rocks ranging from Triassic to Pliocene in age are widely
distributed in the west and south, from the oldest to the youngest consist of Ghegan Formation (TRg),
Dalan Formation (TRd), Kuma Formation (MTk), Waeken Formation (Tomw), Wakatin Formation
Author name / IJSESTech 00 (2014) 000–000
(Tmw), Hotong Formation (Tmh), and Leko Formation (Tpl). These sedimentary formations are
dominated by sandstone, limestone and conglomerate.
Two units of volcanic rocks are distributed scarcely in the western part of the island, Jurassic Mefa
Formation (Jm) which consists of basaltic lava and tuff, and Miocene Ftau Formation (Tmfv) which
consists of andesitic lava, volcanic breccia and tuff. Intrusive rocks of diabase (JKd) and biotite-andesite
(Tpa) have intruded rock units in the island, where the first intruded rock units that older than Cretaceous,
and the second intruded rock units that older than Pliocene.
Quaternary sediments mostly occupy the lower elevations of the island, around beach, rivers and lake.
These include: reef limestone (Ql), terrace deposit (Qt), lake deposit (Qd), and alluvial (Qa). Qt and Qa
which distributed widely within valleys and rivers in the study area, are generally composed by boulder,
gravel, sand, silt, clay, and mud.
Geological structures in Buru island are dominated by normal fault and strike-slip fault which are
generally trend to north-south, northeast-southwest, and northwest-southeast (Tjokrosapoetro et al., 1993;
Fig. 2).
Tpl
Qt
Tmh
Pzw
NAMLEA
Pzr
GOGOREA
TRd
TRg
TRd
Qd
Qa
Tpl
MTk
Pzw
GUNUNG
BOTAK
TRd
TRg
Pzr
N
0
25 Km
Fig. 2. Geological map of Buru island (cropped and modified from Tjokrosapoetro et al., 1993). The study area
(Gogorea and Gunung Botak) and Namlea (the capital city of Buru island are situated in Pzw (Wahlua metamorphic
rock complex).
Author name / IJSESTech 00 (2014) 000–000
3. Methods
This study was conducted in several stages include desk study, field work and sampling, as well as
laboratory analyses. The field work was carried out in the early of June 2014, where observation and
sampling of rock, ore, and alteration were collected in the two mining areas, Gunung Botak and Gogorea.
Additionally, for study of host rocks, observation and sampling were also performed on metamorphic
rocks that exposed in several locations along the main road between Namlea (the capital city of Buru
regency) and Gogorea.
The laboratory analyses include petrography which conducted for studies of metamorphic facies and
hydrothermal alteration; sample preparation (thin sections) and the analyses were performed in
Laboratory of Petrography, Department of Geological Engineering, Hasanuddin University. XRD
analysis was also performed to study of hydrothermal alteration, where samples were sent to Department
of Geological Engineering, Gadjah Mada University to be analyzed. Before sent, the alteration samples
were previously prepared in powdered using agate mortar, in Laboratory of Technical Exploration,
Hasanuddin University. For geochemical study, mineralized (ore) samples were sent to a commercial
research laboratory, PT. Intertek Utama Services, Jakarta to be prepared and analyzed. In this laboratory,
the major oxides were determined by XRF method, the trace elements by ICP-OES method, and the Au
grades by AAS (fire assay) method.
4. Result and discussion
4.1. Metamorphic facies
Based on field observation, it is recognized that there are two types of metamorphic rock in the study
area, which are role as host rocks of the mineralization: schist and phyllite. These are the main members
of the Permian Wahlua Metamorphic Complex (Tjokrosapoetro et al., 1993). In the outcrops and hand
specimen samples, schist generally shows light grey to grey in color, and partly greenish. The schist
commonly contains quartz veins that some parallel and some cross-cut the foliation (Fig. 3.a).
Measurement in an outcrop between Namlea and Gogorea indicated orientation of the schist foliation is
N20oE/16o. At one location in Gunung Botak, strongly altered schist was found exposed, the surface color
is greenish reflecting chlorite alteration (Fig. 3.b), and showing indication of mineralization (contains
secondary quartz with reddish surface as a result of metal-contained oxidation). Orientations of schist
foliation strike in Gunung Botak and Gogorea are generally trend to north-south to northeast-southwest,
with dips range of 22 to 37o, trend to north to west-northwest. Outcrops of phyllite were found in two
locations in Gunung Botak. In the field, phyliite showed dark grey to black in color, and partly greenish.
Similar to schist, phyllite also showed foliation texture with orientations of the strike nearly north-south
with 29 to 37o dip to west and east. Phyllite in Gunung Botak also the host rock of the mineralization, it
contains clear and transparent quartz veins which parallel to the foliation (N185oE/37°).
Under the microscope, samples of schist were characterized by foliation and schistose textures, with
general mineral composition of major chlorite (10-40%), muscovite (25-35%), quartz (15-35%), and less
biotite, clay, cordierite, and opaque (Fig. 3.c and 3.d). Phyllite also showed foliation texture under the
microscope, with mineral composition of quartz, biotite and clay.
Conceptually, metamorphic facies is a group or series of metamorphic- rock and mineral assemblages
that formed in the same temperature and pressure conditions. Based on the genetic and scale of its
distribution, metamorphic facies is divided into two types: contact-metamorphic facies which generally
formed in lower pressure and distributed locally and narrow, and regional-metamorphic facies which
Author name / IJSESTech 00 (2014) 000–000
formed in a broad range of temperature and pressure and distributed extensively, always in form of
mountains (orogenic). The contact-metamorphic facies was further divided into relatively lower
temperature hornfels- and higher temperature sinidinite facies. Whereas regional-metamorphic facies was
divided into, in order of increasing temperature and pressure: zeolite, prehnite-pumpellyite, greenschist,
blueschist, amphibolite, granulite, and eclogite facieses (Winter, 2001; Best, 2003).
a
Ms
b
Qtz
Cly
Bt
Chl
Ms
Op
Chl
Crd
Qtz
0.2 mm
c
0.2 mm
d
Fig. 3. (a) Outcrop of light grey schist contains quartz veins that parallel and cross-cut foliation, exposed at the main
road between Namlea and Gogorea; (b) Altered schist at Gunung Botak, showing greenish color; (c) and (d)
Photomicrographs of schists from Namlea-Gororea area, showing foliation texture with mineral assemblages of
chlorite (Chl), muscovite (Ms), quartz (Qtz), biotite (Bt), clay (Cy), cordierite (Crd), and opaque (Op).
The characteristics of metamorphic mineral assemblages in the metamorphic rocks in study area
(schist and phyllite) indentified by petrographic study, particularly the dominant composition of chlorite,
muscovite and quartz with less cordierite and biotite, indicating greenschist facies, according to the list of
mineral assemblages in each facies summarized by Best (2003). The field of greenschist facies in the P-T
diagram as shown in Figure 4.a is the ranges of temperature of about 250 to 500oC and pressure about 2 to
10 Kbar. This suggests that the host rocks of the mineralization in the study area were formed in such
conditions. Numbers on curves in the figure refers to the reactions that represent the main mineral
assemblages in each adjacent facieses. The reactions are in follows (reactant phases on left are stable at
lower temperature): (1) Analcite + quartz = albite + H2O. (2) 2 lawsonite + 5 glaucophane = tremolite +
Author name / IJSESTech 00 (2014) 000–000
10 albite + 2 chlorite. (3) 6 tremolite + 50 albite + 9 chlorite = 25 glaucophane + 6 zoisite + 7 quartz + 14
H2O. Also 13 albite + 3 chlorite + quartz = 5 glaucophane + 3 paragonite + 4 H 2O. (4) 25 pumpellyite + 2
chlorite + 29 quartz = 7 tremolite + 43 zoisite + 67 H2O. (5) 4 chlorite + 18 zoisite + 21 quartz = 5 Alamphibolite + 26 anorthite + 20 H2O. Also 7 chlorite + 13 tremolite + 12 zoisite + 14 quartz = 25 Alamphibolite + 22 H2O. Also albite + tremolite = Al-amphibolite + quartz. (6) Hornblende =
clinopyroxene + orthopyroxene + Ca-plagioclase + H2O (Best, 2003). Note the main metamorphic
minerals identified in the rock samples in study area, chlorite and quartz, always present in reactions 3, 4
and 5, which are the borders of greenschist facies (Figure 4.a). These conditions, the metamorphic rocks
in greenschist facies or in intermediate metamorphic grade, are genetically related to continental orogen
(Figure 4.b, Best, 2003).
(a)
(b)
Fig. 4. (a) P-T diagram of metamorphic facies as discussed in the text. Abbreviations: Ab: albite, And: andalusite, Jd:
jadeite, Ky: kyanite, Pl: plagioclase, Qtz: quartz, Sil: sillimanite; (b) P-T diagram of tectonic setting related to
metamophism (Best, 2003).
4.2. Hydrothermal alteration and mineralization
Hydrothermal alteration study was performed by combining field data, petrography, and XRD
analysis. In Gunung Botak, proximal alteration distributed widely around ore mineralization operated by
artisanal miners is argillic, which is recognized mainly by its physical properties in the field. In the
outcrops this alteration is characterized by white color, partly reddish brown, and clay to sand in grain
size. Representative outcrops were found in sample point GB.05 and GB.07, where the alteration was
exposed by mining (Fig. 5.a) and construction of adit. XRD analysis of the two samples resulting
dominant composition of quartz. Whereas in petrographic analysis, under the microscope thin section
sample GB.07 which prepared from consolidated argillic shown domination of clay mineral which crosscutted by quartz veinlets; opaques also observed scattered (Fig. 5.b).
Author name / IJSESTech 00 (2014) 000–000
Quartz
veinlet
a
Clay
Opaques
0.2 mm
b
Fig. 5. (a) Argillic alteration in Gunung Botak; (b) Photomicrograph of thin section sample from outcrop in Fig. 5.a,
showing clay cross-cutted by quartz veinlets, disseminated opaques also shown.
Alteration samples were also collected from the more proximal sites to mineralization, in a 30 m depth
shaft operated by the artisanal miners in Gunung Botak (sample points GB.10 and GB.11). Here the
alteration is more intense and closely associated with mineralized veins. Identification by XRD analysis
resulting composition of chlorite, quartz and muscovite (Figure 6). This mineral assemblage indicating
chloritic alteration in mesothermal environment (Thompson and Thompson, 1996).
In Gogorea, proximal alteration samples also taken and analyzed using XRD mehod, and resulting
similar characteristics with the alteration in Gunung Botak. Thus, it can be suggested that the proximal
alterations in the study area are argillic and chloritic.
○ Muscovite
◇
Intensity (cps)
∆ Chlorite
◇ Quartz
◇
○ ∆
2
6
10
14
18
◇
∆
○
22
26
30
34
38
◇ ◇
◇
42
46
50
54
◇
58
62
2-theta (o)
Fig. 6. Diffractogram of alteration sample GB.11 from Gunung Botak resulting from XRD analysis showing mineral
assemblage of quartz, chlorite and muscovite.
Author name / IJSESTech 00 (2014) 000–000
Mineralization in the study area are generally formed in quartz vein; outcrops and floats of vein quartz
were ubiquitously found in the area. In specimen samples, vein quartz in the area between Namlea and
Gogorea are generally barren, showing transparent and clear in color, except in Gunung Botak, where in
several samples parts of the quartz surface showed reddish to dark grey colors, indicating oxidation or
supergene metals.
Observation conducted in the shaft (GB.11) which is the zone of high grade ore, showing vein type
mineralization. The main vein is about 40 to 50 cm thick, with crustiform banding-like texture where
there is an alternating between sulphide band and quartz band (Fig. 7.a-b). At the other part inside the
shaft, irregular cross-cutting veins also observed as shown in Fig 7.c. The mineralized veins are generally
strongly weathered and surrounded by intensive alteration, which is characterized by light green to green
and yellowish brown in colors (Fig. 7.d). As described above, the proximal hydrothermal alteration type
in this zone is chloritic.
Vein
a
b
Cross-cutting
veins
Weatehered
veins
c
d
Fig. 7. Veins and alteration in the shaft in Gunung Botak as described in the text.
Author name / IJSESTech 00 (2014) 000–000
4.3. Geochemical characteristics and ore grades
Seven selected mineralized samples from Gunung Botak were geochemically analyzed to determine
their major oxides, trace elements and gold concentration. The results, covering 13 major oxides
(including sulphur) and 36 trace elements are listed in Table 1.
Table 1. Results of geochemical analyses.
Sample code
GB.04
Sample type
Quartz float
Major oxides (wt.%)
SiO2
97.43
TiO2
<0.01
Al2O3
0.04
Fe2O3
2.61
MnO
0,026
MgO
<0.01
CaO
0.02
Na2O
<0.01
K2O
0.01
P2O5
0.01
Cr2O3
<0.05
LOI
-0.7
S
<0.002
Trace elements (ppm)
Au
<0.01
Ag
<0.1
Al
0.02
As
<2
Ba
1
Bi
<2
Ca
0.01
Cd
<0.2
Co
2
Cr
18
Cu
2
Fe
1.79
Ga
2
K
<0.01
La
<1
Li
<1
Mg
0.01
Mn
182
Mo
1
Na
0.01
Nb
<1
Ni
4
Pb
<2
Sb
1
Sc
<1
Se
<10
Sn
<5
Sr
1
Ta
<5
Te
<5
Ti
<0.01
V
1
W
<10
Y
<1
Zn
2
Zr
<1
GB.06
Vein quartz
GB.08
Vein
GB.09
Vein
GB.10
Vein
GB.11
Vein
GB.11.B
Vein
Detection
limit
96.71
0.02
0.24
2.65
0.021
<0.01
<0.01
<0.01
0.03
0.01
<0.05
-0.6
<0.002
82.84
0.49
10.49
1.43
0.010
0.15
0.01
0.06
0.94
0.014
0.036
3.4
0.006
79.66
0.47
11.62
2.37
0.005
0.22
<0.01
0.1
1.52
0.012
0.044
3.4
<0.002
82.72
0.42
9.49
1.87
0.006
0.27
0.02
0.08
1.64
0.015
0.028
2.8
<0.002
74.15
0.75
15.72
1.74
0.005
0.27
0.01
0.16
2.32
0.018
0.011
4.2
0.003
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
0.01
0.01
0.01
0.01
0.005
0.01
0.01
0.01
0,01
0.001
0.005
0.1
0.002
0.31
0.2
0.04
51
2
<2
<0.01
<0.2
2
16
2
1.87
2
0.01
<1
<1
<0.01
159
<1
<0.01
<1
3
<2
9
<1
<10
<5
<1
<5
<5
<0.01
1
<10
<1
2
<1
0.23
<0.1
1.19
92
21
<2
<0.01
<0.2
1
15
13
0.83
3
0.1
4
1
0.01
16
<1
0.01
<1
1
2
20
3
<10
<5
3
<5
<5
0.01
12
<10
2
2
<1
1.83
0.1
1.25
228
36
<2
<0.01
<0.2
2
15
35
1.66
4
0.2
4
2
0.02
22
1
0.01
<1
1
3
26
3
<10
<5
4
<5
<5
<0.01
12
<10
2
2
<1
0.85
<0.1
0.56
191
29
<2
<0.01
<0.2
1
7
16
1.3
3
0.15
10
1
0.01
8
<1
0.01
<1
<1
4
33
2
<10
<5
3
<5
<5
<0.01
10
<10
2
2
<1
0.76
<0.1
1.02
153
38
<2
<0.01
<0.2
1
11
18
1.02
4
0.22
21
1
0.02
10
1
0.01
<1
<1
6
26
5
<10
<5
7
<5
<5
<0.01
14
<10
4
2
<1
5.9
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
not analyzed
0.01
0.1
0.01
2
1
2
0.01
0.2
1
1
1
0.01
2
0.01
1
1
0.01
1
1
0.01
1
1
2
1
1
10
5
1
5
5
0.01
1
10
1
1
1
Author name / IJSESTech 00 (2014) 000–000
Table 1 shows that from the seven samples analyzed, Au grades is in the range of 0.23 to 5.9 ppm (g/t).
The highest grade (5.9 g/t) is in sample GB.11 that was collected from the main vein in the shaft in
Gunung Botak (Fig. 7.a and 7.b). Other significant gold grade is in sample GB.09 which was taken from
open pit mining near the shaft, the Au grade is 1.83 g/t. The enrichment of gold grades in the
mineralization were not followed by silver, where for all samples the Ag grades are generally low, mostly
below the XRF detection limit, < 1 ppm (Table 1). This also happens in base metals (Cu, Pb, Zn), which
are all showing low grades, less than 1% (Table 1). The higher percentages of SiO 2 (74.15 to 97.43%) in
all samples reflecting quartz vein gangue in the samples. It is also figured in Table 1 that concentrations
of arsenic and antimony in all samples are significant, where grade of As is up to 228 ppm and Sb 33
ppm. This geochemical signatures indicate that mineralization in the study area is a non polymetallic, it is
only characterized by gold and not associated with silver and base metals.
5. Conclusion
Idrus et al. (2013) suggested that the gold mineralization in the study area tends to meet the
characteristics low sulfidation epithermal or orogenic gold deposit types. Results of this study are
generally consistent with the suggestion, particularly for the orogenic gold deposit type. Based on the
study of metamorphic- host rocks and mineral assemblages, it is resulted that the metamorphic facies is
greenschist, which formed from a regional-intermediate metamorphic grade processes, that related to
orogeny (in a continental orogen setting). This host rock type, metamorphic processes and tectonic setting
are the main characteristics of orogenic gold deposits as described by Groves et al. (1998, 2003),
Goldfarb et al. (2001), and Goldfarb (2009). The inferred ranges of temperature and pressure formation of
host rocks defined by their greenschist metamorphic facies, 250 to 500oC and 2 to 10 Kbars respectively,
indicating that the mineralization formed in mesozonal to lower-epizonal environment. The significant
concentrations of As and Sb associated with Au in the mineralization also reflecting this environment
(Groves et al., 1998; Goldfarb, 2009). The proximal-ore associated hydrothermal alteration, chloritic,
which mainly consists of chlorite, quartz and muscovite, also indicating the typical mesothermal (recently
termed as orogenic gold deposits by Groves et al., 1998) alteration (Thompson and Thompson, 1996).
Acknowledgements
The authors wish to express a gratitude to Directorate of Higher Education, Department of Education
and Culture, Indonesia, as well as Institute for Research and Community Services Hasanuddin University
for the financial support through “Internal Competency Research Grant 2014” with contract number of
1454/UN4.20/PL.09/2014 granted to the first author as a principal researcher. Thanks to Alam Budiman,
Moh. Khaidir, Mashuri Alim and Ilham Badu for the assistances during field work.
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