Occurrence of Zoned Epithermal to Porphyry Type Cu-Au Mineralistion at Wonogiri (1)

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





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 (





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





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.





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.





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





FIGURE 2: East Java Regional Physiography (after Imai et al 2007).

Wonogiri

Property

Tujuh

Bukit





FIGURE 3: General Geology of Wonogiri Property





FIGURE 4: Wonogiri alteration map

FIGURE 5: Sheeted quartz veins exposed on Randu Kuning





FIGURE 6: Plan view Randu Kuning prospect geology showing mineral occurrence types.

RK porphyry

type?

epithermal Au





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





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

PROCEEDINGS JCM


FIGURE 10: Modelled 3D magnetic susceptibility with interpreted structur

FIGURE 11: Drill hole locations, with mapped structure and modelled 3D


HAGI and 40th IAGI Annual Convention and Exhibition


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





FIGURE 12: Sheeted quartz vein from WG-001 showing centreline chalcopyrite and minor bornite.





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

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Occurrence of Zoned Epithermal to Porphyry Type Cu-Au Mineralistion at Wonogiri (1)

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