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Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 15-28
Characteristics of Alteration and Mineralization
at Randu Kuning - Wonogiri Project
Karakteristik Alterasi dan Mineralisasi
di Projek Randu Kuning, Wonogiri
Abdul Muthi1, I Gde Basten2, I Gede Made Suasta3, and Naomi E. W. Litaay3
PT. Best Clean Energy (Augur Resources), Jln. Warung Buncit Raya 99, Jakarta 12740
2
Geological Engineering, ITB, Jln. Ganesha 10, Bandung 40132
3
PT Oxindo Exploration (MMG), Jln. Ciputat Raya 99, Jakarta 12310
1
ABSTRACT
Randu Kuning prospect is one of the prospective areas within the Wonogiri property which is fully
owned by PT. Alexis Perdana Minerals (PT. APM). The area is often reffered to as Selogiri property
as it lies within Selogiri Subdistrict. Exploration work in Wonogiri commenced by PT. Oxindo Exploration (MMG) includes regional, detailed mapping, and surface geochemical sampling including
rockchip, channel, stream sediment, and grid soil sampling. Ground magnetic survey aided subsurface
exploration and a 3D magnetic inversion assisted to define targets for 5 holes scout diamond drilling
program to test coincident anomalous soil geochemistry, modelled high magnetic bodies, and exposed
sheeted Cu-Au bearing quartz veins. Augur continued the work in the area with surface sampling
comprising extensive trenching, diamond drilling, and detailed ground magnetic. Extensive drilling
program in 2011 completed 50 diamond holes in 2011 with the total of 15,588.15 m, mostly were
drilled into Randu Kuning prospect including the deepest hole (WDD30) that was drilled until 854.95
m. Lithological classification, alteration zonation, and mineralisation have been refined with the new
exploration results. Lithological classification differentiates the units based on genesis, relation to
mineralisation event, and grain size. Two main groups of lithology are diorites and breccias. Zoned
prograde hydrothermal model of Randu Kuning Porphyry system depicts the intersection of diamond
holes into outer and inner propylitic zones, porphyry style veins, and core potassic zones. Additional
alteration study was done by PIMA work on 102 samples taken from 6 holes of Cross Section TRK01.
Clay minerals, illite, and montmorillonite are the main minerals identified in the vein samples. KAlunite and gypsum in wallrock were identified near the end of WDD030 that suggests signatures of
advance argillic alteration and intermediate argillic respectively. Extensive study including alteration
mineralogy is required to confirm this. The gold-copper porphyry mineralization in Randu Kuning
developed within sheeted and stockwork quartz vein in microdiorite and in the peripheral of microdiorite intrusion. The system obtained its higher Cu-Au grades from overprinting episodes and block
faulting may have caused variations in alteration. Published resource of Randu Kuning is estimated
at 90.9 Mt at 0.53 g/t AuEq (0.35 g/t gold and 0.10% copper) using a cut off of 0.2 g/t AuEq.
Keywords: Randu Kuning, alteration, mineralization, magnetic survey, prograde hydrothermal,
porphyry system, Cu-Au
SARI
Randu Kuning merupakan salah satu area prospektif di Wonogiri yang sepenuhnya dimiliki oleh
PT. Alexis Perdana Minerals (PT. APM). Daerah ini sering disebut sebagai properti Selogiri karena
terletak di Kecamatan Selogiri. Pekerjaan eksplorasi di Wonogiri yang dimulai oleh PT. Oxindo
Exploration (MMG) meliputi pemetaan regional dan terperinci, serta pemercontohan permukaan
yaitu rockchip, saluran, sedimen sungai, dan pemercontohan tanah grid. Survei magnet permukaan
membantu eksplorasi bawah permukaan dan inversi magnet 3D membantu untuk menetapkan target
lima lubang pemandu dalam pemboran intan untuk menguji geokimia tanah anomali, model tubuh
bermagnet tinggi, dan urat kuarsa pembawa Cu-Au. Augur melanjutkan bekerja di area tersebut
Naskah diterima: 03 Januari 2013, revisi terakhir: 22 Maret 2013, disetujui: 28 Maret 2013
15
Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 15-28
dengan pemercontohan permukaan yang terdiri atas pemaritan ekstensif, pemboran intan, dan
pemagnetan permukaan terperinci. Program pemboran ekstensif pada tahun 2011 menyelesaikan 50
lubang intan dengan total 15.588,15 m, terutama di prospek Randu Kuning, meliputi lubang (WDD30)
sampai kedalaman 854,94 m. Klasifikasi litologi, zonasi dan mineralisasi alterasi telah diperbaiki
dengan hasil eksplorasi baru tersebut. Klasifikasi litologi membedakan unit-unit berdasarkan genesis,
hubungan dengan mineralisasi dan ukuran butir. Dua kelompok litologi tersebut adalah diorit dan
breksi. Model hidrotermal prograde sistem Porpiri Randu Kuning yang dizonakan menggambarkan
interseksi lubang intan ke zona propilitik luar dan dalam, urat tipe porpiri, dan zona potasium inti.
Studi alterasi tambahan dilakukan oleh PIMA terhadap 102 percontoh yang diambil dari 6 lubang
Cross Section TRK01. Mineral lempung ilit dan monmorilonit merupakan mineral utama yang
teridentifikasi di dalam percontoh urat. K-Alunit dan gipsum di dalam wallrock teridentifikasi dekat
ujung WDD030 yang menandakan alterasi argilik lanjut dan argilik intermediet. Studi ekstensif yang
meliputi mineralogi alterasi diperlukan untuk mengkonfirmasikan hal ini. Mineralisasi porpiri emastembaga di Randu Kuning berkembang dalam urat kuarsa stockwork dalam mikrodiorit dan dalam
tepi intrusi mikrodiorit. Sistem yang memperoleh kadar Cu-Au yang lebih tingginya dari episode
pertimpasan dan pensesaran blok ini mungkin menyebabkan variasi dalam alterasi. Sumber daya
Randu Kuning yang dipublikasikan diperkirakan sebanyak 90,9 Mt pada 0,53 g/t AuEq (0,35 g/t emas
dan 0,10 % tembaga menggunakan cut off 0,2 g/t AuEq.
Kata kunci: Randu Kuning, alterasi, mineralisasi, survei magnetis, hidrotermal prograde, sistem
porfiri, Cu-Au
INTRODUCTION
Randu Kuning Prospect is a prospective
area within the Wonogiri property which is
fully owned by PT Alexis Perdana Minerals
(PT APM). The area is often referred to as
Selogiri property as it lies within Selogiri
Sub-regency. The property is located in the
southeastern part of Central Java Province
and approximately 30 km south of Solo/
Surakarta City. An initial exploration activity in this area was conducted by PT Oxindo
Exploration (MMG) in partnership with PT
APM starting in first semester of 2009. The
work and results of this exploration activity is
summarized in Suasta and Sinugroho (2011).
Exploration work on this project continued
with further surface and drilling program by
Augur Resources from Australia (Augur) that
added more knowledge and understanding of
the mineralization and prospectivity of the
area. Much works have been concentrated on
Randu Kuning area to follow up interesting
results in initial exploration program. This
paper will discuss the results of recent work
carried out by Augur Resources on Wonogiri
16
project with emphasis on characteristics of alteration and mineralization at Randu Kuning.
REGIONAL GEOLOGY
The Sunda-Banda volcanic arc developed
during subduction of the north-moving
Indo-Australian Plate beneath the Asian
continental plate margin. The Sunda - Banda
arc of Middle Miocene to Pliocene age is
thought to have initiated by subduction reversal following an Oligocene compressive
event that was associated with the northward
emplacement of ophiolite and island arc
assemblages onto the Sunda margin and
associated formation of melanges, ophiolite
fragments, and deformation zones offshore
from western Sumatra (Daly et al., 1991;
Harbury and Kallagher, 1991 in Hellman,
2011). The initiation of northward subduction beneath the Sunda - Banda arc migrated
eastward following this collision event. The
western segment of the arc, west of central
Java, developed on continental crust on the
southern margin of Sundaland whilst the arc
Characteristics of Alteration and Mineralization at Randu Kuning - Wonogiri Project
(Abdul Muthi et al.)
east of Central Java developed on thinner
island arc crust (Carlisle and Mitchell, 1994
in Hellman, 2011).
There are also variations in dominant styles
of mineralization along the arc. In northern
Sumatra in the Aceh Province, mineralization is characterized by porphyry Cu-Mo
systems and high-sulphidation deposits (e.g.
Miwah and Martabe). In contrast, southern
Sumatra, west Java, and central Java are
typified by a lack of known porphyry systems but an abundance of low-sulfidation
epithermal deposits or prospects with vein
systems. Examples include Tambang Sawah,
Rawas, Lebong Donok, Lebong Simpang,
and Seung Kecil in southern Sumatra, also
the Cikotok and Jampang districts, Gunung
Pongkor and Cikondang in west Java, and
Trenggalek in central Java. Further the
east, in east Java and then through Lombok
and Sumbawa, there is a reappearance of
porphyry and high sulfidation epithermal
systems along the eastern arc segment,
including the the Tumpangpitu high-sulfidation epithermal and porphyry system on
Intrepid’s Tujuh Bukit project, The Selodong
high-sulfidation and porphyry district including the Motong Botek porphyry system
on Lombok, and the Batu Hijau porphyry
Cu - Au system on Sumbawa (Hellman,
2011). Location of the Wonogiri Project
with respect to other porphyry projects is
shown in Figure 1.
The Sunda - Banda arc comprises both
Miocene to Pliocene volcanics and younger
Quaternary volcanics. The arc has migrated
not only from west to east over time but also
from south to north (Whitford et. al., 1979;
Katili, 1989; and Claproth 1989 in Hellman,
2011). This migration is clearly evidenced
by the east-west alignment of deeply dissected Miocene to Pliocene volcanic centers
along the south coast of Java, Lombok and
Sumbawa, and a parallel E-W alignment of
juvenile and active Quaternary volcanoes
that define the present active arc further
north along central Java and northern Bali,
Lombok, and Sumbawa (Figure 2).
The Sunda-Banda arc is segmented by a series of arc-normal structures that trend NNE
and which are evident in topographic-data
set. Tectonic factors appear to have local-
N
FLORES SEA
0
300
km
Selodong
(Soutern arc)
Wonogiri JV
Tujuh Bukit (Intrepid)
1.700 Mt @ 0.41% Cu
and 0.46 g/t Au
Oligocene Miocene Arc
0
o
sar
Quaternary Sunda Arc
Significant Project/deposit
Elang
(Newmont)
Bangka
Sumatra
Belitung
Borneo
Banjarmasin
Ma
ka
Augur’s Wonogiri Project
Batu Hijau (Newmont JV)
914 Mt @ 0.53% Cu
and 0.40 g/t Au
Sulawesi
I N D O N E S I A
J AVA S E A
Wonogiri Project Location
Java, Indonesia
Jakarta
Bandung
INDIAN OCEAN
10
Java
Banyumas F L O R E S
Bali Lombok
Sumbawa
Malang
SEA
Flores
Timor
o
Figure 1. Locality map of Wonogiri Project with respect to SundaArc and other major deposits (Augur Resources, 2012).
17
Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 15-28
110 E
112 E
o
114 E
o
o
7S
o
7S
8S
8S
o
o
o
Selogiri Research Area
Physiography
North Serayu-Kendeng zone
Alluvial plain
Domes and ridges
Quaternary volcano
Rembang-Madura
anticlinorium
Depression
Southern Mountain
Range
0
112 E
o
N
75
150
km
114 E
o
Figure 2. Regional physiography map of Java (after Imai et al., 2007).
ized volcanic centers of the Miocene arc at
positions near the southwestern margins of
these transfer structures. Contemporaneous
continental to ocean deep clastic sediments
were deposited on the margins of the volcanic centers (Hellman, 2011).
The project location is surrounded by
several Quartenary Volcanoes, such as Gunung Lawu, Merapi, Merbabu, and others.
Stratigraphically the Wonogiri Project area
consists of from young to old: Alluvium,
Merapi Volcanic Rock, Lawu Volcanic
Rock, Wonosari-Punung Formation, Oyo
Formation, Nglanggran Formation, Semilir Formation, Mandalika Formation and
Gamping Wungkal Formation (Figure 3).
From this regional geological map, Wonogiri Project area is dominated with Mandalika
Formation, which consists of dacite-andesitic lavas and dacitic tuff with dioritic dykes.
South area is covered by Semilir Formation
comprising tuff, dacitic pumice breccia, and
tuffaceous sandstone and shale. North area
is covered by alluvium deposits composed
18
of loose material of sands and clays with
varieted of grain size. Local geology shows
that the project area consists of volcanic
breccias, lithic tuff, andesite cut by quartz
diorite and microdiorite intrusions, and in
parts is overlain by alluvial deposits. Local
geological structures in the project area are
dominated by northeast southwest strike-slip
fault and east-west thrust fault that appears
to be the effect of the north south subduction movement from Australian Plate at the
Eurasian Plate collision.
M ETHODOLOGY A N D R ECENT
EXPLORATION
Surface work in Wonogiri by PT Oxindo
Exploration (MMG) includes regional and
detailed mapping and surface geochemical sampling including rock chip, channel,
stream sediment, and grid soil sampling.
Ground magnetic survey aided subsurface
exploration and a 3D magnetic inversion
assisted to define targets for 5 holes scout
Characteristics of Alteration and Mineralization at Randu Kuning - Wonogiri Project
(Abdul Muthi et al.)
110o45' E
110o50' E
7o45' S
7o45' S
Wonogiri Project Location
7o50' S
7o50' S
N
110o45' E
110o50' E
Legend:
Qa
Alluvium: Clay, mud, silt, sand, gravel, pebble, marbble.
Qvl
Lawu Volcanic Rocks: Volcanic breccia, lava, and tuff
Tms
Semilir Formation: Tuff, dacitic pumice breccia, tuffaceous sandstone and shale
Tomm Mandalika Formation: Dacite-andesitic lavas and dacitic tuff with dioritic dykes
Tpdi
Pendul Diorite: Diorite
Figure 3. Geology and broad stratigraphic succession of the area as defined on the 1:100,000 geological map
of the Surakarta and Giritontro (Surono et al., 1992).
diamond drilling program to test coincident
anomalous soil geochemistry, modelled high
magnetic bodies and exposed sheeted CuAu bearing quartz veins.
Augur continued the work in the area with
surface sampling comprising extensive
trenching which are assayed for 10 elements. Total of 9,783 m of trenching was
completed in 2011, mainly within Randu
Kuning and the surrounding area with the
aim to understand the distribution of Au
and Cu in this area (Figure 4). This surface
program was also aimed to define epithermal
vein targets which were not evaluated by
MMG. The trenches were sampled using
2 or 4 m composite channel samples with
total of 2,931 samples for the whole project.
These trenches were mapped and aided the
interpretation of surface and sub-surface
geology and drilling program. In addition,
a detail ground magnetic survey completed
over approximately 50% tenement area.
Extensive drilling program in 2011 completed 50 diamond holes in 2011 with the total
of 15588.15 m. They were mostly drilled
into Randu Kuning prospect including the
19
Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 15-28
486.000 mE
486.200 mE
486.400 mE
486.600 mE
486.800 mE
487.000 mE
487.200 mE
487.400 mE
9.138.600 mN
9.138.800 mN
485.800 mE
9.138.400 mN
Bukit Piti
9.137.800 mN
9.138.000 mN
9.138.200 mN
Randu Kuning
9.137.600 mN
South Randu Kuning
9.137.000 mN
9.137.200 mN
9.137.400 mN
Geblak
Jangglengan
Legend:
Alluvial
Diorite
Trench Line
Breccia
Volcanic Breccia
Drill Holes
Collar
Microdiorite
Tuff-Sediments
N
Figure 4. Trenching location on geological map (modified after Corey, 2010).
deepest hole (WDD30) that was drilled until
854.95 m. These drillholes together with 5
previous holes by MMG contributed to a
better understanding of mineralization and
alteration of Randu Kuning prospect.
GEOLOGY OF RANDU KUNING
The areas of interest in the Wonogiri project have a high topographic relief trending
NW-SE (Figure 5). This area occurs in
the northern part of tenement boundary
comprising a series of hills extends to the
north at lower elevation interpreted to form
a series of intrusion body. The area has been
sub-divided into several prospects based on
geology and type of mineralization (Figures
4 and 5).
Locally, geology of the Wonogiri project
comprises a series of multiple diorite in20
trusion intruding into the early volcanic
sequence and with dominant structures of
NE-SW strike-slip fault and E-W thrust
fault. The surface geology of Wonogiri
project is dominated by hydrothermal alteration (clay-chlorite-magnetite±epidote
and carbonate). This alteration is typical of
a propylitic alteration that is overprinted by
later argillic-phyllic alteration.
Most of the work has been concentrated on
Randu Kuning area to follow up on encouraging results in the previous work. A total of
38 drill holes including two historical holes
of MMG were completed in this area. These
drill holes have confirmed that the Randu
Kuning prospect is a typically porphyry
system with Cu-Au mineralization.
The prospective area occurs as a steep west
dipping NS trending zone characterized by
polyphasal porphyry intrusions separated
by intrusive and fault contact parallel to the
Characteristics of Alteration and Mineralization at Randu Kuning - Wonogiri Project
(Abdul Muthi et al.)
9.138.500 mN
486.000 mE
486.500 mE
487.000 mE
487.500 mE
Bukit Piti
9.138.000 mN
Randu Kuning
Gawe
9.137.500 mN
South Randu Kuning
Geblak
9.137.000 mN
Jangglengan
N
0
50
100
m
Scale 1:10.000
Figure 5. Distribution of prospects of the Wonogiri project on topographical relief.
mineralized trend. Many intrusion and fault
contacts were recognized between different
intrusive bodies, with some diorite and microdiorite intrusion types are recognized and
standardized in geology logging of drilling.
Most contacts are between different phases
of diorite and microdiorite with prominent
mafic phenocrysts herein summarized in
cross section TRK01 (Figure 6).
Lithology
Lithological classification of the Wonogiri
Project has been modified in more detail to
differentiate the units based on genetic and
grain size. The plates showing textural variation of the lithology can be seen in Figures 7
and 8. In summary, the intrusive phases that
are currently recognized at Randu Kuning
prospect, described from oldest to youngest, are:
Coarse Grain Diorite (CDIO)
Pre-mineral, coarse-grained, porphyritic,
>1 mm subhedral - anhedral of feldspar
phenocrysts set in fine-grained or crystalline
groundmass, finer size of mafic phenocrysts,
non to poorly mineralized.
Medium Diorite (DIO)
Pre-mineral, dominantly medium-grained,
<1 - 2 mm plagioclase or mafic phenocrysts
(variably fine-to slightly coarse-grained
within intensely altered and intrusive/
crackle breccia zone), equigranular to subporphyritic, non to poorly mineralized (in
microdiorite margin).
Microdiorite (MDR)
Syn-mineral, fine-grained, <1 mm phenocrysts size, aphanitic (?), variation of fine
feldspar porphyry to prominent mafic, variably to strongly mineralized.
21
Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 15-28
486.000mE
486.100mE
486.200mE
486.300mE
486.400mE
65.6 m @ 1.24 g/t Au & 0.29% Cu
30
W
DD
129.5 m @ 0.83 g/t Au & 0.24% Cu
100mRL
100mRL
123.5 m @ 1.34 g/t Au & 0.22% Cu
79 m @ 0.57 g/t Au & 0.14% Cu
89 m @ 0.56 g/t Au & 0.13% Cu
6 m @ 0.59 g/t Au & 0.06% Cu
5 m @ 0.28 g/t Au & 0.04% Cu
81 m @ 0.87 g/t Au & 0.15% Cu
0mRL
24 m @ 0.58 g/t Au & 0.06% Cu
67 m @ 0.68 g/t Au & 0.19% Cu
0mRL
20
D
D
W
W
DD
10
W
W
DD
48
DD
05
200mRL
W
DD
TRK01
938165mN
200mRL
01
485.900mE
98 m @ 0.33 g/t Au & 0.1% Cu
9 m @ 0.23 g/t Au & 0.3% Cu
18 m @ 0.31 g/t Au & 0.07% Cu
-100mRL
-100mRL
27 m @ 0.66 g/t Au & 0.1% Cu
118 m @ 0.75 g/t Au & 0.13% Cu
-200mRL
-200mRL
19 m @ 0.23 g/t Au & 0.03% Cu
9.1 m @ 0.38 g/t Au & 0.02% Cu
-300mRL
-300mRL
5 m @ 0.21 g/t Au & 0.03% Cu
27 m @ 0.20 g/t Au & 0.11% Cu
12 m @ 0.34 g/t Au & 0.13% Cu
-400mRL
5 m @ 0.22 g/t Au & 0.04% Cu
5 m @ 0.20 g/t Au & 0.05% Cu
3 m @ 0.20 g/t Au & 0.03% Cu
Microdiorite
Medium Grained Diorite
-400mRL
6 m @ 0.20 g/t Au & 0.14% Cu
6 m @ 0.28 g/t Au & 0.09% Cu
Coarse Grained Diorite
Intrusive Breccia
Hydrothermal Breccia
-500mRL
-500mRL
Fault zone
485.900mE
486.000mE
486.100mE
486.200mE
486.300mE
486.400mE
Figure 6. Cross section TRK01 showing lithology and significant intersections in the centre of Randu Kuning
Prospect.
Figure 7. Textural differences between dioritic intrusions in Randu Kuning (Muthi, 2012).
22
Characteristics of Alteration and Mineralization at Randu Kuning - Wonogiri Project
(Abdul Muthi et al.)
Figure 8. Two types of breccia in Randu Kuning (Muthi, 2012).
Porphyritic Plagioclase Diorite (PDIO)
Post mineral, coarse-grained, porphyritic,
>2 mm plagioclase, plagioclase rich, nonmineralized, (Not common in Randu Kuning but observed from intercepted drill holes
in surrounding Randu Kuning prospect).
In addition of multiple intrusions as described above, Randu Kuning drill holes
intersected two types of breccia that are
referred to as Intrusive Breccia (IBX) and
Hydrothermal Breccia (HBX). Intrusive
breccias are commonly observed in drill core
in contact with, and immediately above the
deep central microdiorite intrusion. They exhibit a distinct fragmental texture; generally
increasingly polymictic clast assemblages
at higher levels. This breccia is mostly
comprised of diorite and microdiorite clasts
which deformed during the intrusion phase.
Occasionally, the presence of clasts of mafic
dominant rock with disseminated pyrite is
noted. The source of these clasts is not yet
known.
Hydrothermal breccia was identified in
some drill intercepts with increasing silica
and clay alteration. This breccia is also
characterized by additional sulphide (pyrite)
and partly rimmed clasts. It is interpreted
that this unit is structurally controlled; hydrothermally generated silica and sulphide
deposited in the fault zone.
Corbett (2011) noted lithological variations;
include fine and coarser grained equivalents
as well as different styles of alteration; between different phases of diorite porphyry
with prominent mafic phenocrysts as seen
in the photos below. Shales, skarn type of
alteration and intrusive dykes in the eastern
part were intersected in WDD15 which appear to be the basementof thestratigraphy.
RESULT AND DISCUSSION
Alteration
Hydrothermal alteration and zonation is
modelled by Corbett (2011) as s hown in
Figure 9. Outer propylitic zone is typified
by magnetite-chlorite ± epidote alteration.
Interestingly, this relatively weak alteration
hosts elevated Cu-Au grade mineralisation
(Hole DDH10IWG002) and the barren
sheeted laminated quartz-magnetite veins
(Hole WDD19 and WDD20).
The zone of magnetite-epidote ± chlorite
at lower temperature or actinolite towards
the heat source is characteristic of inner
propylitic alteration which is best displayed
in hole WDD22. Fracture epidote within
chlorite alteration in the upper portion passing down hole to pervasive epidote flooding
of host rock.
Potassic alteration is the most prevalent
type of alteration in Randu Kuning mineralisation and it is indicated by pervasive
magnetite - biotite ± secondary K-feldspar.
23
Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 15-28
dilatant structure
WDD22
Possible settings
of the barren
quartz-magnetite
veins in WDD19
and WDD20
bottom of WDD15
OUTER PROPYLITIC
tes
oli te
ze ori te
l
ch ido lite
ep ino
t
ac
INNER PROPYLITIC
M vein
apophyses
POTASSIC
A vein
magnetite
biotite
k-feldspar
stock
magmatic source
Figure 9. Zoned prograde hydrothermal alteration model (Corbett, 2011).
Crackle brecciation developed in this alteration zone as an overprint comprising
magnetite ± actinolite ± chalcopyrite. Strong
positive magnetic anomaly as identified by
detail ground magnetic survey results is a
reflection of this magnetic flooding of the
hydrothermal alteration and brecciation.
The Randu Kuning porphyry Cu-Au hydrothermal system is localized within an 800 m
long NW- NNW trending magnetic feature
which declines in intensity to the SE.
The zone of silica-sericite-pyrite is a characteristic of phyllic alteration herein is adjacent to mainly the later stage porphyry style
B veins, collapsing down structures and
low temperature stage epithermal quartzsulphide veins.
Total of 102 PIMA samples were taken from
six holes of Cross Section TRK01. These
samples are collected from veins and pervasive or in the wallrock alteration zones
as well as clasts and matrix of the breccia
to see if there is any pattern in alteration
assemblage. Clay minerals illite and montmorillonite are the main minerals identified
in the vein samples throughout the section
24
suggesting structurally controlled argillic
alteration. Minor siderite (carbonate) occurs at depth. NH-Alunite is identified in
one of the sample and is present at centre
of the magnetic anomaly.
In wallrock of altered samples, apart
from illlite and montmorillonite, chlorite,
kaolinite and tourmaline were also identified in WDD030. K- Alunite and gypsum
were identified at depth of 743 m and 792
m respectively. The presence of alunite
indicates more acid fluid environment
and typical of advance argillic alteration
(Corbett and Leach, 1998). Gypsum after
anhydrite in porphyry setting is a characteristic of intermediate argillic as exampled
by Middleton et al. (2004). The presence
of these minerals might suggest another
zone(s) of porphyry or proximity of major
structure that channels hot acid fluid. However, more extensive study including alteration mineralogy is required to confirm this.
Mineralization
The gold-copper porphyry mineralization
in Randu Kuning area developed within
Characteristics of Alteration and Mineralization at Randu Kuning - Wonogiri Project
(Abdul Muthi et al.)
sheeted and stockwork quartz veins along
microdiorite and in the peripheral of microdiorite intrusion (Figure 10). Overprinting events and style of mineralization are
clearly visible in Randu Kuning. The CuAu grades developed as an accumulated
affect of these as depicted in Figure 11.
nated chalcopyrite is associated with clots
of magnetite and/or chlorite - epidote
and contains low grade Au and and some
elevated (?) Cu. It is interpreted to be locally derived from cooling intrusion (Corbett, 2011). The early quartz veins are the
veins generally known as the A veins with
sinusoidal gradational vein margins and
occasional contains disseminated chalcopyrite. Saccharoidal quartz texture and
minor magnetite are common features of
these veins. Majority of quartz veins with
Style of mineralization can be grouped as
disseminated chalcopyrite, early quartz
veins, most quartz veins with sulphides,
and barren laminated and sheeted quartz
magnetite (Corbett, 2011). The dissemi486.000mE
486.100mE
486.200mE
486.300mE
486.400mE
W
DD
01
485.900mE
30
DD
W
DD
W
129.5 m @ 0.83 g/t Au & 0.24% Cu
100mRL
100mRL
123.5 m @ 1.34 g/t Au & 0.22% Cu
79 m @ 0.57 g/t Au & 0.14% Cu
89 m @ 0.56 g/t Au & 0.13% Cu
6 m @ 0.59 g/t Au & 0.06% Cu
5 m @ 0.28 g/t Au & 0.04% Cu
81 m @ 0.87 g/t Au & 0.15% Cu
0mRL
24 m @ 0.58 g/t Au & 0.06% Cu
67 m @ 0.68 g/t Au & 0.19% Cu
0mRL
20
D
D
W
200mRL
48
10
W
W
DD
DD
200mRL
05
65.6 m @ 1.24 g/t Au & 0.29% Cu
98 m @ 0.33 g/t Au & 0.1% Cu
9 m @ 0.23 g/t Au & 0.3% Cu
18 m @ 0.31 g/t Au & 0.07% Cu
-100mRL
-100mRL
27 m @ 0.66 g/t Au & 0.1% Cu
118 m @ 0.75 g/t Au & 0.13% Cu
-200mRL
-200mRL
19 m @ 0.23 g/t Au & 0.03% Cu
9.1 m @ 0.38 g/t Au & 0.02% Cu
Propylitic
5 m @ 0.21 g/t Au & 0.03% Cu
Potassic
Base of Weathering
-300mRL
Base of Oxidation
-300mRL
Phyllic
27 m @ 0.20 g/t Au & 0.11% Cu
12 m @ 0.34 g/t Au & 0.13% Cu
6 m @ 0.20 g/t Au & 0.14% Cu
-400mRL
5 m @ 0.22 g/t Au & 0.04% Cu
5 m @ 0.20 g/t Au & 0.05% Cu
3 m @ 0.20 g/t Au & 0.03% Cu
No Qtz vein
Low Density Qtz Vein (0.1-1%)
Low - Medium Density Qtz Vein (1-2.5%)
-400mRL
6 m @ 0.28 g/t Au & 0.09% Cu
Medium Density Qtz Vein (2.5-5%)
High Density Qtz Vein (>5%)
Qtz-Mt “M-Vein” Zone
-500mRL
-500mRL
Qtz-Mt “A-Vein” Zone
Stockwork Vein
Sheeted Vein
485.900mE
486.000mE
486.100mE
486.200mE
486.300mE
486.400mE
Figure 10. Cross Section TRK01 at the center of Randu Kuning showing alteration and mineralization.
25
Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 15-28
Figure 11. Plates showing different veining style and mineralization of Randu Kuning (Corbett, 2011).
sulphides were the result of cooling of the
system at depth including M veins, massive
stockwork of A veins, linear A veins, and
later the AB veins taking over the earlier A
veins with deposition of chalcopyrite. The
barren laminated and sheeted quartz magnetite veins in WDD19 & 20 resemble M veins
and sheeted A style and both generally has
no sulphide content.
Overprinting of low temperature epithermal
veins on the earlier porphyry system can be
summarized in Figure 12, showing two possible mechanisms: cooling magmatic source
and later magmatic event.
Randu Kuning South
Randu Kuning South is located about 300
m south of main Randu Kuning prospect
Epithermal veins
Epithermal veins
x
x
x
+ +
++ +
+ + +
+ + +
+ + +
+ + +
+
+
+ + +
x
x
x
x
x
later
magmatic
event
x
x
x
cooling magmatic source
x
x
x
x
x
x
x
+ +
++ +
+ #+ +
#
# +
# # # #
+
# #
+
#
+ + +
#
+ + +
x
x
x
#
x
x
x
x
x
x
x
x
x
x
Figure 12. Alternative models for the origin of late stage epithermal veins (Corbett, 2011).
26
x
Characteristics of Alteration and Mineralization at Randu Kuning - Wonogiri Project
(Abdul Muthi et al.)
and was tested by four drill holes. Crackle
brecciation is common in this area filled
mostly by sulphide. Alteration on this area
is dominated by early prophylitic alteration
which is overprinted by phyllic alteration, possibly controlled by intense faulting
e.g. in WDD25. Infill clay and sulphide in
the crackle breccia in WDD25 is believed to
be a signature of epithermal system which
is controlled by fault zone with typical assemblage of quartz-carbonate and pyrite
veins. This area is interpreted as a fault
controlling extension of porphyry system
in Randu Kuning.
E-W trending block faulting may cause variations in alteration. Pervasive K-feldspar is
common in central Randu Kuning while the
drilling shows extensive epidote alteration
to the north with epidote flooding intersected
in WDD22. This may be indicative of buried
porphyry mineralized system. Some higher
Au grade mineralization is related to epithermal veins. Encounter of older basement of
sedimentary sequence defines the boundary
of this mineralization system in the east. To
the west, the system is not closed off and
it requires more work to define as bornite
veins have been intersected in hole WDD24
further west than expected.
CONCLUSIONS
Further work is required to define the western limit and postulated buried porphyry
as well as investigation of advance argillic
and intermediate argillic alteration at depth.
Based on the data grained from geology and
recent drilling, the mineralization at Randu
Kuning indicates extensive gold copper
porphyry mineralization from surface to
400 m depth below surface. It is related to
near vertical gold copper porphyry deposits
within a large eroded volcanic centre, appear to be related to a northward migrating
Oligocene to Miocene volcanic arc.
The system obtained its higher Cu-Au
grades from overprinting episodes as seen
in the overprinting of earlier chalcopyrite
within disseminations and magnetitebearing fracture by sheeted quartz veins
and later dilatant fractures filled with
chalcopyrite. Some intrusions are particularly enriched with early sulphides. Barren
laminated and sheeted quartz-magnetite
veins within chlorite- magnetite altered
diorite seems to be the upper unmineralized
portion of a porphyry and the potential for
vertical extension of these veins at depth is
unknown. Current resources of Randu Kuning is estimated at 90.9 Mt at 0.53 g/t AuEq
(0.35 g/t gold and 0.10% copper) using a
cut off of 0.2 g/t AuEq (Augur Resources,
2012).
ACKNOWLEDGMENTS
The paper has been presented in the MGEI, Banda
and East Sunda Seminar 2012. The authors would
like to acknowledge the MGEI Committee who give
a permission to publish the paper in MGI.
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