cerita seram

Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 51-63
Scratching at the Surface: Hidden Mineralization at Wetar?
Guratan Pada Permukaan Tanah: Mineralisasi Tersembunyi di Wetar?
1
Herman Seran and 2Chris Farmer
Batutua Tembaga Raya, 2Finders Resources
1
ABSTRACT
Wetar Island is one of the prominent mineralization nests lying along the Sunda - Banda Magmatic
Arc. The initial stage of modern exploration commenced in 1986 and resulted in gold mining at the
Kali Kuning and Lerokis deposits, producing approximately 20 tonnes of gold and 600 tonnes of silver.
Currently, the old pits are being reopened for copper mineral extraction by PT. Batutua Kharisma Permai
in cooperation with Batutua Tembaga Raya as part of the flagship Wetar Copper Project. Apart from the
deposits of Kali Kuning, Lerokis and Meron, elsewhere on the island there has been limited conversion
of hundreds of geophysical and geochemical anomalies to economic mineral resources. Having secured
more than 8 million tonnes mineable copper reserve, Wetar Copper Project has a strategic advantage
to build a production pipeline beyond the current ten-year-mine life. New exploration strategies and
technologies have been considered to revisit prospects that occur all over the island. Lesson learning
from past exploration results and methodologies, knowledge of mineralization and alteration styles,
and success stories on the north coast indicate that Wetar is still very prospective for new mineable
deposits. Hence, a new Wetar exploration strategy targets the discovery of other a typical volcanogenic
massive sulfides (VMS) gold - base metal deposits concealed by younger formations. In addition,
high sulfidation epithermal alteration that assemblages in the context of the Sunda-Banda Arc may
suggest the presence of porphyry style deposits that are related to the known massive sulfides and the
extensive unexplored anomalies spreading throughout the island. A cutting edge deep geophysical
method is considered to be an effective option to trace deep buried mineralization at target depths
unattainable by using technology available in the 1990’s. This approach has potential to unlock the
true mineral inventory on Wetar Island.
Keywords: mineralization, gold, copper, Wetar Island, Sunda-Banda Magmatic Arc
SARI
Pulau Wetar merupakan salah satu lokasi mineralisasi utama yang terletak di sepanjang Busur Magma
Sunda-Banda. Tahap awal eksplorasi modern dimulai tahun 1986 dan memunculkan penambangan
emas di cebakan Kali Kuning dan Lerokis, yang menghasilkan sekitar 20 ton emas dan 600 ton perak.
Saat ini, sumuran-sumuran tua dibuka kembali untuk mengekstraksi mineral tembaga oleh PT. Batutua
Kharisma Permai bekerja sama dengan Batutua Tembaga Raya sebagai bagian dari Proyek Tembaga
Wetar. Terlepas dari deposit Kali Kuning, Lerokis, dan Meron, dimana pun di pulau tersebut ada
keterbatasan konversi ratusan anomali geofisika dan geokimia bagi sumber daya mineral ekonomis.
Proyek Tembaga Wetar memiliki lebih dari 8 juta ton cadangan tembaga, dan juga memiliki keuntungan
strategis dalam membangun jalur pipa produksi di luar sepuluh tahun masa produksi pertambangan
saat ini. Strategi eksplorasi dan teknologi baru telah meningkatkan prospek yang terdapat di seluruh
pulau tersebut. Belajar dari hasil eksplorasi dan metodologi masa lalu, pengetahuan mineralisasi
dan tipe alterasi, serta cerita sukses di pantai utara mengindikasikan bahwa Wetar masih prospektif
untuk deposit tambang baru. Karena itu, strategi eksplorasi Wetar yang baru menargetkan penemuan
sulfida masif tipe vulkanik yang lain (VMS) deposit logam dasar emas yang tersembunyi oleh formasi
yang lebih muda. Selain itu, kumpulan alterasi epitermal sulfida tinggi dalam konteks Busur SundaBanda mengindikasikan keberadaan deposit tipe porpiri yang berkaitan dengan sulfida masif yang
telah dikenal dan anomali ekstensif yang tak tereksplorasi yang tersebar di seluruh pulau tersebut.
Naskah diterima: 03 Januari 2012, revisi terakhir: 10 April 2013, disetujui 12 April 2013
51
Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 51-63
Metode geofisika cutting edge deep dianggap sebagai opsi yang efektif untuk menjejaki mineralisasi
yang terkubur dalam yang tak dapat dicapai menggunakan teknologi yang ada pada tahun 1990-an.
Pendekatan ini memiliki potensi untuk benar-benar menemukan mineral di Pulau Wetar.
Kata kunci: mineralisasi, emas, tembaga, Pulau Wetar, Busur Magma Sunda-Banda
INTRODUCTION
The Wetar Copper Project is located on the
northern coast of Wetar Island approximately 400 km NE from Kupang, Nusa Tenggara
Timur and 100 km west of the nearby capital
city of Maluku Barat Daya Regency, Kisar
Island. The project was previously exploited
for gold and silver metals resulted in the
production of approximately 20 tonnes of
gold and 600 tonnes of silver from high
sulfidation volcanogenic massive sulfides
(VMS) ore, supplied by two pits namely
Kali Kuning and Lerokis pits. The mine
was operated by PT. Prima Lirang Mining
(PLM). Production ceased in 1997 even
though underlying copper sulfide bodies
were known, yet deemed uneconomic given
processing options and low copper prices at
that time.
Between 2004 and 2006, PT. Batutua Kharisma Permai acquired nine tenements over
both Lerokis and Kali Kuning prospects and
several other known PLM prospects targeting gold - silver and basemetal mineralization. The company built a strategic alliance
with PT. Batutua Tembaga Raya (Finders
Resources), as part of the Wetar Copper
Project flagship, to explore and exploit the
metals within the assigned tenements. The
main cooperation has been centered on the
introduction of new processing technology to enhance the feasibility of copper
extraction from known copper resources
mainly from the Kali Kuning, Lerokis, and
Meron deposits. Accordingly, the project
is now feasible to produce up to 25 kt/year
LME “Grade A” copper cathode for a 10
year mine life from approximately 8 Mt of
52
copper ore, employing copper sulfide heap
leaching and solvent extraction - electro
winning (SX-EW) processing technology.
With the first pure copper cathode processing facility in Eastern Indonesia and
technical leaching know-how developed
over the past 5 years, it is strategic to
extend and expand the current operation.
There are two routes to underpin growth,
namely aggressive exploration on Wetar
in BKP tenements and cooperation with
other tenement holders or to source raw
materials from wider afield. Accordingly,
defining a new exploration strategy based
on existing Wetar exploration knowledge
in combination with new advances in
geophysical technology provides a significant opportunity to re-assess the mineral
potential of Wetar and enhance the Wetar
Copper Project.
Therefore, the paper’s ultimate objective is
to discuss exploration strategies to increase
the value of the project to its stakeholders.
The discussion touches firstly on pertinent
geological aspects of Wetar Island, then
places high sulfidation VMS mineralization
found on Wetar in a wider context, before
commenting on the current status of the
project and possible paths forward.
GEOLOGY
Regional Geology
The Wetar Island is located along the Sunda
- Banda magmatic arc resulted from continual
subduction of the Australia - Indian Ocean
Plate below the Eurasian Plate. Further to
Scratching at the Surface: Hidden Mineralization at Wetar? (H. Seran and C. Farmer)
the south lies an accretionary terrain, with
Timor as the biggest island among others.
The magmatic arc extends somewhat 3.800
kms (Garwin et al., 2005) from Sumatra in
the northwest to Java. Lesser Sunda Islands
and ending up in Banda Archipelago (Figure
1). The arc hosts a large number of mines
or exploration targets for precious and base
metal resources, mainly in the form of epithermal and porphyry systems, but occasionally
in the case of Wetar, a volcanogenic massive
sulfides (VMS) style. Several notable projects
lying along the arc have been Martabe gold copper mine and Lebong Donok in Sumatra,
Pongkor and Tujuh Bukit Porphyry Project in
Java, Batu Hijau Porphyry in Sumbawa, and
Kali Kuning and Lerokis in Wetar (Figure 1).
Geology of Wetar
Wetar rises more than 1.875 m above sea
level, formed mainly by submarine volcanic products with a lesser degree of marine
sediments (Sewell and Wheatley, 1994).
Geology in Wetar started with the formation
of the basement dated 12 Ma in terms of
basaltic pillow lavas, overlain by younger
submarine volcanic rocks between 7.78 to
3.03 Ma in the forms of dacite, diorite, and
basaltic andesite. Meanwhile, sedimentation
processes also occurred forming Globigerina bearing limestones dated 5.2 to 3.9 Ma.
Paleontological analysis reveals deposition
process formed no less than 2,000 m below
the paleo-sea water level. The island started
to emerge from the sea approximately 3
million years ago (Scotney et al., 2005).
A complete stratigraphic column can be
observed in Figure 2.
Wetar Island is a product of extensional
oceanic crust rifting, resulted from the collision process further to the south. The land
masses are continuously uplifted with a rate
of 0.5 - 5 mm per annum. The volcanic arc
islands are actually isolated and off-setted
from each other by transform faults resulting
from the tectonic activities along the Timor
- Tanimbar trough (Figures 3 and 4). For instance, a strike-slip fault separates the Wetar
from Alor Island to the west. Meanwhile,
Wetar thrust plays significant role in absorbing collisional forces of 75 mm per annum
between SE Asian and Indian-Australian
Plates (Scotney et al., 2005).
Wetar Copper - Gold Deposits
More than a hundred anomalies and prospects were identified during the previous
exploration works within the island of Wetar
by means of remote sensing, geophysical,
and geochemistry works (Sewell and Wheatley, 1994). Epithermal style of mineraliza-
G. Pani
Martabe
Lebong Donok
Awak Mas
N
Tujuh Bukit
Gosowong
Lanut
MT. Muro
Pongkor
Toka Tidung
Grasberg
Romang
Batu Hijau
Wetar Copper
Figure 1. Locality map of Sunda - Banda Volcanic Arc and notable mining projects.
53
Lithology
Thickness
(m)
Continued uplift of
both the inner and
outer arcs
Pleistocene
QUATERNARY
Sequence
Epoch
Tectonic Setting
Age
Holocene
Period
Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 51-63
Massive grey
reef
limestones
20
Lahars+
dykes
70
Mineralization
Date
Late veins +
disseminations
Spherulitic
rhyolites
Rapid arc exhumation
related to continued
shortening of the
collision zone
Debris flows
250
Dacite
2.39 Ma
1
Pliocene
Late volcanism in the
south + uplift of the
Piacenzian inner Banda arc related
to the locking of the arc
Arrival and collision
between the
Australian contnental
margin and the outer
Banda arc + possible
arc reversal in the
Zandian
Limestone
15
Chert/
gypsum
3
Syenogranite
Extension+faulting,
preparation of sites
for mineralization
Tortonian
Extensive volcanism on
Wetar. Collisions of the
micro-continental
fragment art -8 Ma with
the subduction zone
Multiple lavas
(bimodal)+
intrusive rocks
Serravalian
Subduction related
volcanism+early back
arc rifing (evolution of
Wetar edifice)
Miocene
TERTIARY
Messinian
4.7 Ma3
(altered
footwal)
4.93 Ma1
4.73 Ma3
3
450 Stockwork zones 5.05 Ma
Andesitic
pillow lavas
7.783
6.52 Ma
Basaltic
pillow lavas
Langian
Waning stages
As rich sulfides
with flanking
Cu-Au-Ag-Hg
rich barite
mounds
Basaltic
andesites+
dacites
Felsic domes
5.2-3.9
4
Ma
12 Ma3
?
Basalt
Figure 2. Northern Wetar stratigraphic column.
N
Continental Crust
Banda Sea
Wetar
Basin
Accretionary Terrane
Active Volcano
(AP)
Flores Thrust
Alor
Thrust
Savu
h
125o E
Bali
Babar
Sermata
g
rou
rT
o
Tim
Roti
120o E
Papua
0
Sunda Arc
200 km
Teun
Wetar
Timor
m
Sunda Tren
Seram
Banda
Island
Alor
Su
Seram Through
Ambon
Sumbawa
Lombok
N
Moa
Buru
Savu
Basin
Java
Wetar
Atauro
Sum
ba
Damar
Basin
Wetar Thrust
ba
Serua
Nila Tanimbar
Damar
Island
Romang
Banda Arc
gh
rou
rT
o
Timor Sea
Tim
ch / Java Tr
ench
Inner Volcanic Banda-arc
0
500 km
120o E
Outer Banda-arc
Kai
Manuk
130o E
Aru
Island
Arafura Sea
Australia
Figure 3. Banda Volcanic Arc with Wetar tectonic setting (from Scotney et al., 2005).
54
gh
u
ro
rT
ba
m
ni
Ta
Aru T
rough
Oceanic Crust
Scratching at the Surface: Hidden Mineralization at Wetar? (H. Seran and C. Farmer)
Inner
Banda-arc
Outer
Banda-arc
SE
Damar
NW
Horizontal
lava flows
Romang
Kisar
extension
Wetar
Strait
Wetar
Timor
SE Asia Plate
200 km
compression
Crustal shortening
and thickening beneath
trench
Wetar Basin
0
gh
Moa
rT
rou
Banda
Sea
Tim
o
Mount
Api
75 mm/a
80 km
c
asi
Tri ent h
o
t
m
i
t d benc
cen se
Re rine nd to
ma ell a
sh
on
120 km
Oceanic
Crust
Down to
300 km
an f
ali hel
str l S
Au enta
n
nti
Co
ine
all t
yst men
r
C se
Ba
Sub detachment
at depth ?
Figure 4. Wetar tectonic setting in three dimensions (from Scotney et al., 2005).
tion, mainly volcanogenic massive sulfide
mineral deposits, predominantly occupies
the island. However, studies of the known
deposits reveal that Wetar VMS mineralization is rather atypical compared with classic
VMS deposits, giving its high gold content
and high sulfidation alteration signatures and
ore mineral associations (Herrington, 2007).
The mineralization in Wetar was formed
as the result of the escape of hydrothermal
solutions on the sea floor within an extensional crustal plate. The mineralization bodies appear in the form of lenses or mounds
of massive sulfide, forming in paleotopographic lows in seamounts, that have
sharp zoned alteration halos (vertically and
horizontally) and are often capped by a Au/
Ag bearing baritic sand which is a chemical
precipitate formed during a later hydrothermal overprint. The deposits were formed
at depths greater than 2.000 m in an ocean
environment some 4.7 - 4.9 Ma. Research
by Scotney and Herrington (amongst others)
on the Kali Kuning, Lerokis, and Meron
deposits, suggests that the high volatility of
the ore forming hydrothermal fluids resulted
from contamination by continental material
representing the source of elevated levels of
gold in the Wetar mineralized bodies. This
is drawn from the opinion that typical midoceanic ridge basalt (MORB) contain less
gold compared to continental crust material
based on typical mass balance ratios and
the proximity of Wetar to nearby sources
of continental crust. Hence, hydrothermal
fluids are believed to have scavenged gold
from the nearby continental crust as they
percolated through the relatively gold-rich
continental crust before reaching the sea
floor as VMS deposits.
As observed in northern coast deposits, the
VMS mineralization bodies are controlled
by NW-SE and NE-SW trending structures.
The massive sulfides bodies are discordant
to the host rocks, mainly basaltic andesite
and ryodacite flow units, rooting deeper
within the fault zones, and then covered by
younger marine sediments like limestone,
cherts, and gypsum. Due to high erosional
rates on Wetar Island, the new exploration
focus is to discover preserved (hidden) VMS
systems since previous exploration has been
55
Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 51-63
successful in discovering outcroppingsystems and/or partially exposed alteration
halos or eroded root systems.
For a preserved deposit to be located on
Wetar, three key steps are required to form
an ideal VMS target. Firstly, the seafloor
formation of typical low sulfidation black
smoker associated with predominantly prophylitic - argilic alteration zoning. Secondly, an overprinting stage of barite and gold
deposition along the periphery and utilizing
available fractures within the black smoker
bodies to form white smokers, associated
with more acidic and oxidized hydrothermal
fingerprints producing argilic and advanced
argilic alteration halos, that may overlap
and destroying some elements of the previous alteration. Finally, post hydrothermal
“protective” sedimentary or volcanic rock
units which are typically marine sediments
or dacitic flows. The alteration zonation
for typical VMS deposits reveals intense
silicification (silica) proximal to the sulfide
bodies, following by clay minerals (illite smectite) then towards distal portion where
exhibiting chloritic alteration. The alteration distribution for Wetar mineralization
is extensive, intensive and zoned (Scotney,
et al., 2005, p.835).
The sulfide bodies, which can take various styles including pipes, mounds, and
stockworks, are dominated by arsenian
pyrite (~90% by volume). Chalcopyrite
and sphalerite with minor galena are associated primary sulfides and are observed
as a replacement of and inter-grown with
pyrite. The main other copper sulfide species are covellite, chalcocite (dignite), and
tenantite. Mercury and silver minerals are
also present in the form of cinnabar and
moschellandsbergite. Barite is the main
gounge mineral forming sulfides mounds,
talus breccia, barite ore, stockworks, and
veins. Later phase were barite cementing by
56
iron oxides, hydroxides, complex arsenates,
and sulfates.
PROJECT STATUS AND
METHODOLOGY
Project Strategic Advantages
Wetar Copper Project has four unique advantages that positively influence successful project development and future growth.
Firstly, the Wetar Copper Project is aligned
well with the new mining law and produces
pure metal, with 99.99% pure copper cathode from the first SX-EW plant Indonesia.
Secondly, the project has been established
using proprietary technology developed
over the past 5 years, which not only suits
Wetar metallurgical characteristics but also
has potential applications for other ore types.
Thirdly, underpinning the economic attractiveness of the project, is an 18,000 tonnes
per annum SX-EW plant already imported
in Indonesia, whose replacement value is of
the order of $30 - 40 m. Finally, the project
location is isolated but within 4km of an
existing port and located on an unpopulated
Island where the previous mining history
in the area has resulted in a very aware and
very supportive set of local stakeholders.
By way of background, emphasizing how
technological advances assist new mineral
project development and discovery, the exploration and mining history of Wetar sets
the scene for both the current project and
growth opportunities. The copper deposits
lying beneath gold mineralization at the Kali
Kuning and Lerokis have been known for
more than 20 years. They were discovered
by drilling, but it soon became apparent that
the massive sulfides were all in stratigraphically similar position and could be readily
detected using electromagnetics. PT. Prima
Lirang Mining did not develop the copper
deposits because there was no economically
Scratching at the Surface: Hidden Mineralization at Wetar? (H. Seran and C. Farmer)
proven technology available during the time.
Airborne EM surveys conducted in the mid90’s were able to confirm the presence of
massive sulfides at Kali Kuning, Lerokis,
and Meron but realistically could only detect
to depths of 25 - 50 m due to the limitations
of airborne systems at that time.
The Wetar Copper project is the first economic copper sulfide heap leach in the
world. Heap leaching of copper oxides is
not new in the copper industry with about
20% of global copper production by SXEW, but leaching of sulfide was not thought
to be economic particularly due to slow
recoveries from chalcopyrite and to a lesser
extent covellite. However, after a series of
laboratory tests and a pilot project using
full height heaps, the careful analysis of the
interactions between 34 parameters allowed
optimization of the process and boost economic performance of the project.
RESULTS AND DISCUSSION
Mineral Reserves and Resources
Wetar Copper Project has a mining license
for 20 years from bupati Maluku Barat Daya
covering approximately 2,733 hectares. The
production plan is founded on 8.2 million
tonnes @2.5% Cu proven and probable
mining reserves from both Kali Kuning and
Lerokis pits. The reserves serve a ten year
life of mine producing up to 25,000 tonnes/
year LME grade A copper cathode by sulfide
heap leaching and solvent extraction - electro winning (SX-EW) technology.
Exploration focus #1: Additional massive
sulfide resources
First priority for the Wetar Copper Project is
to extend mine life by means of discovery
of additional massive sulfide resources. The
exploration philosophy has two priorities:
firstly, known prospects in the immediate
district surrounding Kali Kuning and Lerokis will be upgraded by drill testing and,
secondly, the potential of the rest of the
island will be evaluated using new exploration approaches. Despite extensive first/
second pass geochemistry and geophysical
surveys completed island-wide since the
mid-1980’s (Sewell and Wheatley, 1994),
the potential for further discovery on Wetar
is driven by knowledge of the character and
size of VMS mining districts elsewhere
in the world and that post-mineralization
cover masks in excess of 60 - 70% of the
Wetar Island land mass. Comparing with
notable VMS districts in Canada and Japan
(Figure 5), where typically massive sulfides
occur in clusters at a specific stratigraphic
horizon, the number of deposits and combined resources known on Wetar are still
very small. The three known deposits at
Kali Kuning, Lerokis, and Meron massive
sulfides account for approximately 10 Mt of
ore. Whilst the number of deposits and size
of VMS deposits in Canada and Japan is
significantly larger ranging from 34 Mt (10
deposits) in Matagami in Japan to 100 Mt
(19 deposits) at Noranda in Canada.
Roughly, 70 % of the Mining IUP is covered
by post mineral volcanic rocks, ranging from
<10 m to more than 100 m in thickness. In
places, where the mine sequence rocks are
exposed, strong alteration and evidence of
sulfide mineralization occur over an area
roughly 7 km east-west by 3.5 km northsouth. Both the Kali Kuning and Lerokis
deposits are strong electrical conductors,
and electromagnetic geophysical surveys
have proven to be effective in locating massive sulfide accumulations under the cover
sequence (Figure 6).
At Karkopang, a cluster of EM conductors
beneath younger cover was defined by an
airborne EM survey flown by the previous
57
Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 51-63
Potential for Futher VMS Discoveries
VMS Deposit Clusters in Canada and Japan
Snow Lake
~ 11 deposits
~ 40 mt
Flin Flon
~ 8 deposits
~ 80 mt
Noranda
~ 19 deposits
~ 100 mt
Matagami
~ 12 deposits
~ 90 mt
Hokuroku
~ 12 deposits
~ 90 mt
5 km
Wetar Island
~ 3 deposits
~ 10 mt
Figure 5. A comparison between Wetar and the VMS Projects in Japan and Canada (modified from Sangster,
1980).
COVER SEQUENCE
MINERALIZED ZONES
ADVANCED ALTERATION
CLAY ALTERATION
BASEMENT SEQUENCE
STRONG CONDUCTOR
MODERATE CONDUCTOR
WEAK CONDUCTOR
N
0.75
0
0.5
1 km
Kali Kuning
Meron
Lerokis
Figure 6. Northern Tenement Geology of the Wetar Copper Project.
58
Scratching at the Surface: Hidden Mineralization at Wetar? (H. Seran and C. Farmer)
owner.These anomalies have not been drill
tested, and mapping in the streams peripheral to the anomalies has identified a similar
alteration to the alteration halo surrounding
Kali Kuning.
are an order of magnitude higher than, and
unmistakable for, anomalies associated with
disseminated mineralization.
At Meron, only 31 of 85 of the historical
drill holes intersected sulfide mineralization as part of shallow drillings of the gold
cap known above the southern quarter of
the massive sulfide body. Based on this
information, it is possible that Meron may
contain between 1 and 5 million tonnes of
potentially economic copper mineralization
grading greater than 0.5% Cu. Since much of
the massive sulfide at Meron is untested by
drilling an initial 50 hole, ~5000 m program
is planned to bring the Meron prospect up
to Indicated and Inferred resource status.
In addition to the copper potential, there is
potential for discovery of new significant
additional gold mineralization above other
parts of the massive sulfide.
At Meron, a 500 m long strong electrical
conductor with similar size and geophysical characteristics as the known sulfide
deposit at Kali Kuning forms an immediate
focus for expanding the ore reserves. The
geology of Meron is similar to Kali Kuning, with a sequence of gold bearing barite
sands overlying a copper-bearing massive
sulfide deposit, with essentially the whole
prospect area covered by a thin (10 - 50 m)
sequence of younger volcanic rocks. The
Meron gold-silver resource, outcrops at
surface, and based on Billiton Indicated and
Inferred estimates contains 394,000 tonnes
grading 3.18 g/t Au and 143 g/t Ag (~80,000
Au Oz equivalent).
Elsewhere on Wetar Island, the exploration
strategy is to re-assess potential for hidden
extensions to known prospects or to discover
new deposits masked by cover sequence
rocks adjacent to exploration prospects discovered during the earlier exploration sur-
The massive sulfide can be “remotely”
mapped under the cover sequence using
geophysical techniques, specifically timedomain electromagnetics (Figure 7). Massive sulfides have “decay constants” which
TEM MAGNITUDE (uV/A) CHANEL-25, NORTH VALLEY - MERON PROSPECT
9147600N
NORTHINGS (m)
10
9147400N
9147200N
50
9147000N
0
uV/A
9146800N
205400E
205600E
205800E
206000E
0m
200m
400m
206200E
206400E
206600E
206800E
207000E
207200E
207400E
207600E
EASTINGS (m)
Scale : 1 : 10.000
600m
800m
Figure 7. Meron TDEM anomaly.
59
Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 51-63
veys since the mid-1980’s. In each of three
main prospect areas, known gold-silver
barite sand deposits were partially evaluated by scout drilling with only a cursory
evaluation of copper potential.
At the Pantai Merah Prospect (Figure 8),
11 scout drill holes (457 m) were drilled
in the vicinity of outcropping barite sands
with reported “potential” for 600,000 t @
1.6 g/t Au, 77 g/t Ag, 15.5% Ba; four of
the drill holes intersected the upper parts
of a massive sulfide (copper) body located
under gold barite mineralization. Extensive
geochemistry anomalism extends for + 3 km
north in an area with + 80 % cover sequence.
Similar opportunities occur at the W6 (Ilwaki) Prospect, where 8 drill holes assessed
gold mineralization adjacent to a limestone
cover sequence and at the Batu Duri (J91)
Prospect (Figure 9) where scout drilling
tested outcropping barite sands with 9 drill
holes (602.5 m) with a best Au result of 7 m
@ 1.1 g/t Au and only one hole intercepting
the underlying sulfides with 10 m @ 4.5%
Cu. Around 15 km2 of cover sequence immediately surrounds the Batu Duri prospect,
which is essentially a 0.5 km2 inlier of the
basement sequence.
LEGEND
Mineralization and
Alteration
Covered Sequences
Figure 8. Pantai Merah Prospect Drilling Target.
60
Exploration focus #2: Hidden Porphyry
Systems
The massive sulfide deposits in Wetar are
atypical compared to other VMS systems.
In addition to the significant gold-silver
cap, alteration assemblages comprise advanced argillic alteration (vughy silica with
enargite) more akin to those observed in
high-sulfidation systems (Figure 10). Thus,
the overall character of the Wetar deposits
is one of classic VMS systems with massive sulfides, occurring in clusters and at a
defined stratigraphic horizon/age, but there
are strong indications of a high sulfidation
fluid overprint.
On a district scale, the Lerokis, Kali Kuning,
and Meron are surrounded by large alteration zones that are connected. At Lerokis,
specifically, peripheral parts of the alteration halo are marked by a chloritic zone
with epidote, then closer to the center of the
alteration zone are magnetite stockworks,
enargite, and vughy silica. As elsewhere in
the Pacific Rim, a large number of porphyry
systems have high sulfidation caps including
Far South-East (Philippines), Frieda River
(Papua New Guinea), Dizon (Philippines),
Tujuh Bukit, Batu Hijau, and Grasberg in
Indonesia. Accordingly, future exploration
Scratching at the Surface: Hidden Mineralization at Wetar? (H. Seran and C. Farmer)
efforts will address the possible nature of
identified high sulfidation mineralization system telescoped on a deeper porphyry system.
New Exploration Applications: Deep
Aerial Geophysics
From the mid-1980s to mid-1990s, Wetar
Island was extensively explored using remote sensing, stream geochemistry, mapping, drilling, and geophysics. This stage of
exploration was successful with the discovery of at least 10 outcropping gold-bearing
barite sand deposits across the island (a new
deposit style for Indonesia). However, from
approximately a hundred anomalies derived
from geochemical and geophysical studies
only two deposits proven to be mineable.
A similar exploration approach and methodology to that undertaken in that previous
exploration period is unlikely to yield new
discoveries; since, as in many first/second
phases of exploration, the focus (and success) was to scratch at the surface.
Given the need for discovery of intact
mineralized systems, the next phase of
exploration focus is to explore for massive
or disseminated sulfide bodies hidden 50 200 m below the land surface. The airborne
EM technology of the 1990’s was unable to
detect sulfide at those depths, but since then
airborne EM technology has developed with
more powerful and deeper seeking systems.
Although ground based EM is capable,
given the topographical constraints at Wetar
Island, ground surveys will be very ineffective in terms of time and cost. Aerial
geophysics and particularly a new system
developed since 2007 may hold the key to
success. The Z-Tipper Axis Electromagnetism (Z-TEM) geophysical method (www.
LEGEND
Alluvium
Andesite-Hornblende
Limestone - Reef
Tuff-Silica
Lahar
Tuff-Barite
Basaltic Andesite
Barite Rock
Andesite-Silica
Figure 9. Batu Duri Geological Map.
61
Majalah Geologi Indonesia, Vol. 28 No. 1 April 2013: 51-63
styles of gold-copper VMS mineralization
and evaluating the potential of larger porphyry systems.
Figure 10. Photograph of vughy silica at Lerokis,
Wetar Island.
geotech.ca) has potential to be able to identify anomalous conductors down to 2.000
m depth. For instance, Z-ETM geophysics
method proves to be very accurate in locating the porphyry anomalies like in Fiji (Sabeto/Vuda Prospect) operated by Geopacific
Resources NL (www.geopacific.com.au)
(Figure 11) and Mandoos and Safwa Copper
VMS in Oman discovered another variant of
such geophysical method V-TEM (Syarief
et al., 2011).
Founded on an understanding of new processing and geophysical technologies developed in the past 5 years, the potential ability to identify the “hidden” mineral wealth
of Wetar for the benefit of all stakeholders
is an exciting opportunity for the “forgotten
islands” of Indonesia and could mark a new
successful chapter in the development of
Indonesia’s natural resources.
ACKNOWLEDGMENT
The paper has been presented in the MGEI, Banda
and east Sunda Seminar 2012. Thanks are expressed
to the MGEI Seminar Committe for permision to
publish the paper in MGI.
REFERENCES
Garwin, S., Hall, R., and Watanabe, Y., 2005. Tectonic Setting, Geology, and Gold and Copper Mineralization in Cenozoic Magmatic Arcs of Southeast
Asia and the West Pacific. Economic Geology 100th
Universary Volume, p.891-930, Society of Economic
Geologists, Inc.
Herrington, R.J., 2007. VMS Deposits and Modern
Seafloor Analogues. Irish Association for Economic
Geology Short course: Alteration and Geochemistry
in VMS Exploration, Dublin 31 March -1 April, 2007.
Figure 11. Helicopter Z-TEM survey in Geopacific
Resources NL from www.geopacific.com.au
CONCLUSSION
Despite being a known for over 20 years as a
mineralization hotspot on the Sunda - Banda
Arc, the true potential of the established
mining district on Wetar Island remains
untapped. With the imminent development
of the Wetar Copper Project, a new phase of
exploration can commence targeting unique
62
Sangster, D., 1980, Quantitative Characteristics of
Volcanogenic Massive Sulphide Deposits, in Metal
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Scratching at the Surface: Hidden Mineralization at Wetar? (H. Seran and C. Farmer)
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