Occurrence of Zoned Epithermal to Porphyry Type Cu-Au Mineralistion at Wonogiri (1)
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Transcript of Occurrence of Zoned Epithermal to Porphyry Type Cu-Au Mineralistion at Wonogiri (1)
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PROCEEDINGS JCM MAKASSAR 2011
The 36th HAGI and 40
th IAGI Annual Convention and Exhibition
Makassar, 26 29 September 2011
OCCURRENCE OF ZONED EPITHERMAL TO PORPHYY TYPE CU-AU
MINERALISATION AT WONOGIRI, CENTRAL JAVA
I Gede Made Suasta, Iswahyudi Agus Sinugroho
PT Oxindo Exploration, Minerals and Metals Group, Gedung RPX 2nd Floor,
Jl. Ciputat Raya 99, Jakarta 12310, Phone:+62-21-75913237, Fax: +62-21-75913236
ABSTRACT
The Wonogiri prospect is located in the south eastern part of Central Java province approximately 30km
south of the city of Solo. Exploration by MMG commenced mid 2009 and included regional and semi
detailed geological mapping, stream and soil geochemistry, ground magnetic and completion in 2010 of a
5 hole scout diamond drilling program. A 3D magnetic inversion model was created from the magnetic
data and was used to define drill targets. Exploration of the property is currently being completed under a
Joint Venture between PT Oxindo Exploration - MMG, Auger Resources of Australia and the Indonesian
IUP company PT Alexis Perdana Minerals. Exploration results to date provide further evidence of the
porphyry potential of south eastern Java.
Porphyry type Cu-Au mineralization occurs at the Randu Kuning prospect and is manifest as quartz-
sulphide/oxide sheeted and stockwork vein zones. Minor disseminated chalcopyrite and trace bornite
occur associated with potassic and sodic (actinolite) alteration. Most veins are B type, commonly
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PROCEEDINGS JCM MAKASSAR 2011
The 36th HAGI and 40
th IAGI Annual Convention and Exhibition
Makassar, 26 29 September 2011
commenced in May 2009 and included regional
and semi detail mapping, stream and soil
sampling, ground magnetic and petrographic
studies.
PREVIOUS EXPLORATION
There was no previous exploration activities
conducted in Selogiri prior to Oxindo. The area
attracted the attention of University students since
1995 when illegal gold mining activity started in
the area. Academic investigations in the gold-
copper mineralization were completed by ESDM
Bandung in 1996 and 2002, Gadjah Mada
University in 2006 and Kyusu University in 2007.
These studies reported that copper and gold
mineralization at Wonogiri was associated with a
zone of sheeted quartz veins.
Prior to the scout drilling program, PT Oxindo
completed a surface exploration program of
detailed mapping, rock and soil sampling, and
ground magnetic. This work identified the Randu
Kuning area as the most significant drill target in
the Wonogiri IUP.
GENERAL GEOLOGY
The Wonogiri prospect area lies within the
Southern Mountain Range of Central Java,
situated in the present-day fore-arc region
between the Quaternary volcanic chain and the
Java trench (Figure 2). The Indo-Australian plate
is subducting northwards at the Java trench (Imai
et al, 2007). The geology of the Southern
Mountain Range comprises a series of volcanic
rocks and flysch-like deposits that unconformably
overlie the pre-Tertiary metamorphic rocks and
Eocene sedimentary formation of the Jiwo Hills
Complex. These rock units are in turn covered by
limestone formations which are the Wungkal
Formation and Gamping Formation (Imai et al,
2007).
Three types of dioritic andesitic intrusive rocks
are recognized in the prospect area (Figure 3),
these include hornblende microdiorite porphyry,
hornblendebiotite diorite and hornblende diorite.
The contact between hornblende microdiorite and
hornblende diorite is clearly observed at hole
IWG01 at 187 meter depth, where the
microdiorite was potassic-propylitic altered and
mineralized otherwise the diorite was propylitic
altered only. Petrographic study of surface
samples indicate Wonogiri intrusives comprise a
compositionally similar intrusive complex
comprising plagioclase, hornblende and
clinopyroxene arranged either as hypidiomorphic-
inequigranular or porphyritic textures. Magnetite
is the dominant primary Fe-Ti oxide mineral. The
prospect occurs within an interpreted eroded
caldera characterized by widely distributed
volcanic breccia and tuffaceous volcaniclastics.
Petrography of surface samples indicate the
dioritic rocks have variable retrograde alteration,
with most copper mineralization contained within
porphyry-style A/B quartz veins and associated
alteration dominated by actinolite/ tremolite, K-
feldspar/ albite, magnetite, biotite and chlorite.
The causative nature of these diorite rock types to
Cu mineralization is indicated by the common
occurrence of chalcopyrite with late-stage silicate
mineralogy indicating a transitional magmatic to
hydrothermal copper paragenesis.
Alteration at Wonogiri is characterized by a halo
of propylitic alteration manifest predominately as
chlorite, epidote and magnetite within adjacent
volcanic rocks (Figure 4). A zone of potassic
alteration indicated by secondary K-feldspar,
biotite and magnetite is coincident with the
mineralized Randu Kuning prospect area.
Superimposed on this area are structurally-
controlled zones of phyllic and argillic alteration.
Cu-Au mineralization is associated with zones of
potassic and phyllic alteration.
The copper-gold porphyry mineralization occurs
in Randu Kuning (RK) area. On surface, copper-
gold mineralization is hosted within zones of
sheeted to stockwork type quartz-sulphide/oxide
veins (Figure 5). Most veins are B type and
narrow (
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PROCEEDINGS JCM MAKASSAR 2011
The 36th HAGI and 40
th IAGI Annual Convention and Exhibition
Makassar, 26 29 September 2011
zones of propylitic (chlorite-magnetite-actinolite)
alteration.
Epithermal gold/base metal mineralization at
Wonogiri occurs mainly within the quartz-
carbonate-base metal veins, hosted within
intrusive and volcanic rocks. The veins have
exhibit druzy, colloform banding and cockade
type textures. Surface rock sampling returned up
to 24.7 g/t Au from pyritic vein material. Figure 6
below shows the occurrences of porphyry and
epithermal type of mineralization at Selogiri.
MMG exploration commenced with a property
wide soil sampling survey at 50m spaced intervals
along 100m spaced lines. A total of 384 soil
samples were collected. The soil geochemistry
images for Au, Ag, As, Cu, Mo, Pb, Sb and Zn are
shown in Figure 7. The maximum value of the
element is tabulated on Table 1. The results
indicate a well defined enrichment in Cu and Au
over the RK area with base metal enrichment
proximal to the prospect area and coincident with
the area of potassic alteration. There is also
indication of gold enrichment coincident with
mapped structural features and argillic alteration.
Exploration of the property is currently being
completed under a Joint Venture between PT
Oxindo Exploration-MMG, Auger Resources of
Australia and the Indonesian IUP company PT
Alexis Perdana Minerals. To date, Augur has
completed more than 2,000 meters of trenching
within RK and the surrounding area. The object is
to better understand the distribution of Au and Cu
at RK and to define epithermal vein targets for
drill testing proximal to RK. The epithermal gold
potential of the property was not evaluated by
MMG. The trenches were sampled using 2 or 4
meters composite channel samples and the results
reported by Augur indicate wide zones of gold
and copper mineralization including; 98 meters of
0.91 g/t gold and 0.29% copper in trench 5, 8
meters of 0.72 g/t gold and 0.13% copper and 12
meters at 0.59 g/t gold and 0.29% copper in trench
6 (Figure 8).
The trench results confirm the potential for near
surface, bulk mineable Au-Cu mineralization
within the Randu Kuning prospect contained
primarily within sheeted and stockwork type
quartz veins within diorite and andesite host
rocks. The occurrence of disseminated sulphide
mineralization is also suggested by the trenching.
GEOPHYSICS - MAGNETIC MODELLING
In the effort to better define potential porphyry
drill targets a 3D-magnetic inversion model of the
ground magnetic data was completed by MMG.
The inversion model indicated the presence of a
large (200x200mx300m) high magnetic body
immediately beneath RK (Figure 9) and the area
of anomalous Cu-Au in soils. The magnetic
model also indicated a possible structurally-
controlled feeder (conduit) connecting RK to the
deeper magnetic body (Figure 10). A smaller high
magnetic body was also modelled to the east of
RK underlying another area of Cu-Au anomalous
soils. A structural control to both intrusion and
subsequent alteration is clearly indicated by the
magnetic model.
SCOUT DRILLING RESULTS
A 5 hole (1,996.3m) scout drilling program was
completed by MMG during 2010. The drill
program was designed to test coincident
anomalous soil geochemistry, modelled high
magnetic bodies and exposed sheeted Cu-Au
bearing quartz veins (Figure 11).
The first 2 holes of the program were collared at
RK to test a coincident surface geochemistry and
magnetic target in an area of previous illegal
mining and exposed mineralized sheeted quartz
veins. Rock chip samples from the vein zone
returned 2-6g/t Au and 0.3-0.6% Cu.
Significant drill intersections within WG-001
include: 40m at 1.12g/t Au, 0.29% Cu from 92
132m depth and 15m of 1.61g/t Au, 0.19% Cu
from 137m. Au-Cu enrichment occurs within
propylitic and potassic altered microdiorite
associated with quartz veining adjacent to
magnetite-rich quartz stockwork veins hosted in
diorite. The veins are characterized as
predominately B-type veins with centreline
chalcopyrite with rare bornite and chalcocite
(Figure 12). The modelled high magnetic
susceptibility body was determined to be a quartz-
magnetite vein rich (25-90%) diorite intrusive
containing pyrite but no significant Cu-Au
mineralization (Figure 13). The hole was
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PROCEEDINGS JCM MAKASSAR 2011
The 36th HAGI and 40
th IAGI Annual Convention and Exhibition
Makassar, 26 29 September 2011
terminated at 340.85m having intersected the
upper and lower contacts of the diorite. Narrow
(1m) zones of low grade Au mineralization were
also intersected in microdiorite proximal to the
footwall contact and include 1m of 0.53g/t Au and
0.74% Cu from 295m. The last 1m assay interval
graded 0.98g/t Au and 0.16% Cu, suggesting that
the zone remains open to depth.
WG-002 was drilled to test the Randu Kuning
zone at an orientation perpendicular to WG-001.
The hole intersected a weakly magnetic and
potassic altered diorite from surface to 120m. The
best intersection in this interval was 1.6m of 7g/t
Au and 0.23% Cu from 70.4m depth. The most
significant intercept in the hole was 37m of
1.77g/t Au and 0.23% Cu from 458m. It is unclear
if this zone is the downward continuation of the
Au-rich zone in DDH-1 or a deeper perhaps en
echelon zone. Interestingly, the sulphide
mineralization (chalcopyrite) in WG-002 is not
associated with quartz veining like WG-001, but
is instead predominately disseminated. The hole
was terminated at 537.65m within a weakly
propylitic fine-grained microdiorite containing
minor quartz-pyrite chalcopyrite veins.
WG-005 was collared to test an area of previous
local gold mining hosted within an interpreted
hydrothermal breccia at Jangglengan (Figure 14).
Intercepts of 4m of 0.72g/t Au and 0.56% Cu
from 109m; and 1m of 3.69g/t Au and 0.66% Cu
from 120m are associated with pervasive argillic
alteration and minor quartz-sulphide (py>cpy)
veins. The hole did confirm the breccia to be a
post-magmatic diatreme containing epithermal
type mineralization.
CONCLUSION
The results of the scout drill program confirm that
the Cu-Au mineralization at the Randu Kuning
prospect is porphyry-related and suggest that there
may be potential for a low-grade (~1g/t Au) bulk
tonnage gold resource. Furthermore there is
potential to augment a low grade Au resource by
higher grade epithermal gold-bearing veins
adjacent to Randu Kuning being exploited by
local miners. The near surface Cu porphyry
potential however is less favourable given that the
obvious targets were tested. Furthermore it
appears that while the mineralization is
convincingly porphyry-related it is also
structurally controlled with a source area likely at
depth.
However, of more significance to MMG, the scout
drilling results provide further evidence of the
porphyry potential of southern Java.
REFERENCES
Imai A., Shinomiya J., Maung Thiha Soe.,
Setijadji L. D., Watanabe K., Warmada I Wayan.,
2007, Porphyry-Type Mineralization at Selogiri
Area, Wonogiri Regency, Central Java, Indonesia,
Resource Geology Vol. 57, No.2.
Augur Resources Ltd, 2011, ASX News Release,
27 April 2011.
Corey, Mike., 2010. Scout Drilling Program 2010
Summary Report, Porphyry Copper-Gold Project,
Wonogiri, Central Jawa, Indonesia, Internal
Report.
Hall, R., 2002. Cenozoic geological and plate
tectonic evolution of SE Asia and the SW Pacific:
computer-based reconstructions, model and
animations, Journal of Asian Earth Sciences.
Hall, R., Clements, B, Smyth H.R, and Cottam,
M.A., 2007, A new interpretation of Javas
structure, Proceedings Indonesian Petroleum
Association 31st Annual Convention.
Hamilton, W., 1979. Tectonics of the Indonesian
region, U.S.G.S. Professional Paper.
Kusnanto, B., 2010, Technical Report of
Wonogiri Porphyry Copper-Gold Project,
Wonogiri Central Jawa, Indonesia, Internal
Report.
Surono., B. Thoha., I Sudarno., S. Wiryosujono.,
1992. Geology of the Surakarta-Giritontro
quadrangle, Geological Research and
Development Centre.
Van Bemmelen, R.W., 1949, The Geology of
Indonesia, Government Printing Office, Nijhoff,
the Hague.
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PROCEEDINGS JCM MAKASSAR 2011
The 36th HAGI and 40
th IAGI Annual Convention and Exhibition
Makassar, 26 29 September 2011
Warmada I Wayan., Maung Thiha Soe., Sinomiya
J., Setijadji L. D., Imai A., and Watanabe K.,
2007, Petrology and Geochemistry of Intrusive
Rocks from Selogiri Area, Central Java,
Indonesia.
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PROCEEDINGS JCM MAKASSAR 2011
The 36th HAGI and 40
th IAGI Annual Convention and Exhibition
Makassar, 26 29 September 2011
Description Au_ppm Ag_ppm As_ppm Cu_ppm Mo_ppm Pb_ppm Sb_ppm Zn_ppm
Max 2.46 34.20 880.00 2170.00 38.10 2060 98.80 4540
Min
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PROCEEDINGS JCM MAKASSAR 2011
The 36th HAGI and 40
th IAGI Annual Convention and Exhibition
Makassar, 26 29 September 2011
FIGURE 2: East Java Regional Physiography (after Imai et al 2007).
Wonogiri
Property
Tujuh
Bukit
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PROCEEDINGS JCM MAKASSAR 2011
The 36th HAGI and 40
th IAGI Annual Convention and Exhibition
Makassar, 26 29 September 2011
FIGURE 3: General Geology of Wonogiri Property
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PROCEEDINGS JCM MAKASSAR 2011
The 36th HAGI and 40
th IAGI Annual Convention and Exhibition
Makassar, 26 29 September 2011
FIGURE 4: Wonogiri alteration map
FIGURE 5: Sheeted quartz veins exposed on Randu Kuning
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PROCEEDINGS JCM MAKASSAR 2011
The 36th HAGI and 40
th IAGI Annual Convention and Exhibition
Makassar, 26 29 September 2011
FIGURE 6: Plan view Randu Kuning prospect geology showing mineral occurrence types.
RK porphyry
type?
epithermal Au
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PROCEEDINGS JCM MAKASSAR 2011
The 36th HAGI and 40
th IAGI Annual Convention and Exhibition
Makassar, 26 29 September 2011
FIGURE 7: Soil anomaly in Wonogiri with overlay interpretative major structures.
RK RK
RK RK
RK RK
RK RK
JG
JG
JG
JG
JG
JG JG
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PROCEEDINGS JCM MAKASSAR 2011
The 36th HAGI and 40
th IAGI Annual Convention and Exhibition
Makassar, 26 29 September 2011
FIGURE 8: Wonogiri trenching result by Augur Resources (Augur, 2011).
FIGURE 9: Randu Kuning modelled 3D magnetic susceptibility with Cu-in-soil anomalies. Looking north
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PROCEEDINGS JCM
The 36th HAGI and 40
FIGURE 10: Modelled 3D magnetic susceptibility with interpreted structur
FIGURE 11: Drill hole locations, with mapped structure and modelled 3D
PROCEEDINGS JCM MAKASSAR 2011
HAGI and 40th IAGI Annual Convention and Exhibition
Makassar, 26 29 September 2011
Modelled 3D magnetic susceptibility with interpreted structural map - View from below.
Drill hole locations, with mapped structure and modelled 3D magnetic susceptibility
view.
View from below.
susceptibility - Plan
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PROCEEDINGS JCM MAKASSAR 2011
The 36th HAGI and 40
th IAGI Annual Convention and Exhibition
Makassar, 26 29 September 2011
FIGURE 12: Sheeted quartz vein from WG-001 showing centreline chalcopyrite and minor bornite.
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PROCEEDINGS JCM MAKASSAR 2011
The 36th HAGI and 40
th IAGI Annual Convention and Exhibition
Makassar, 26 29 September 2011
FIGURE 13: Quartz-magnetite pyrite veins intersected in WG-001. Veins textures suggest early near
massive quartz with coarse (0.5cm) magnetite overprinted by quartz with centrelines and crack-seal (?)
type quartz-magnetite veins. This stage of veining is follow.
FIGURE 14: Diatreme breccia from WG-005