1. URANIUM ANOMALIES
IN THE OMBILIN INTERMONTANE
BASIN,WEST SUMATRA, INDONESIA
Hadisuwito
Head of Exploration Division
Center Development of Nuclear Geology
National Nuclear Energy Agency

2. Location Map of Indonesia
We have around 17,500 islands
The big fives are Sumatra, Java, Borneo (Kalimantan),
Celebes (Sulawesi) and Papua.

3. INTRODUCTION
Uranium Mineralization in Indonesia
In Indonesia there are four types of Uranium
mineralizations :
1. Sandstone Type in the Tertiary Sediments of Sibolga
Basin at North Sumatra Province
2. Intrusive Type in the Cretaceous Pegmatite Granite at
Lampung Province
3. Vein type in the Jurassic Metamorphic at West Borneo
Province
4. Volcanic type in the Tertiary Volcanic at East Borneo
Province

4. INTRODUCTION (cont)
Location Map of Uranium Mineralization in Indonesia

5. INTRODUCTION (cont)
• This paper reviews of Uranium exploration in the
Ombilin basin conducted by BATAN in 1974 and
continued by the joint cooperation works between
the Government of the Republic of Indonesia and the
Government of the Federal Republic of Germany in
1977 to 1978.
• Exploration carried out in the Ombilin basin because
the geological setting of this area considered as
favorable to occur the sandstone type uranium
deposits.

6. INTRODUCTION (cont)
• The purpose of this study is to obtain an overview
of the possibility of the sandstone type uranium
deposits in the Ombilin basin because in the period
of 1980’s to now, there was a lot of additional
surface and subsurface geological data and
information obtained by oil and gas as well as coal
companies that working in the Ombilin basin.
• In addition, the Government of Indonesia will start
to conduct uranium exploration in Ombilin Basin in
2012 which is focused on sandstone type uranium
deposits.

7. REGIONAL GEOLOGY
In Regional Lithology of the study area consists of :
Sedimentary rocks :
 Kluet and Kuantan sediments (Lower
Carboniferous – Lower Permian )
 Palepat, Silungkang and Mengkarang
sediments (Lower Permian)
 Kualu and Tuhur sediments (Triassic)
 Woyla Group (Upper Jurassic – Lower
Cretaceous)
 Eocene – Pliocene sediments
 Pleistocene – Holocene sediments
Intrusive Rocks :
 Late Triassic – Early Jurassic intrusives
 Middle Jurassic intrusives
 Eocene – Pliocene intrusives
Volcanic Rocks :
 Eocene – Pliocene volcanics
 Pleistocene – Holocene volcanics

8. REGIONAL FAULTS
 Faults in this area dominated
by the Sumatra Fault System,
is a large Strike Slip Fault, the
so-called Great Sumatran
Fault, running the entire
length of the island.
 This fault zone
accommodates most of the
strike-slip motion
associated with the
oblique convergence
between the Indo-
Australian and Eurasian
Plate.

9. Faults in Study Area
among others:
Takung fault
Mangani Fault
Barisan Tengah fault
Pakis fault

10. GEOLOGY OF THE OMBILIN
INTERMONTANE BASIN
Geomorphology
 Relatively small size of the basin,
1500 sq km, (25 x 60 km), the
basin fill is very thick Tertiary
sediments ( Up to 4,600 m)
 Mean elevation of the central
basin is 400 meters. In the
northern is Merapi (2891m) and
Malintang volcano (2292m)

11. Tectonic Setting
 The Ombilin Basin has a complex
history of reverse, wrench and
extensional tectonism. Initial basin
configuration and quantity of
sediment in the Ombilin Basin is due
to a north- south compression which
created a pull apart basin in the
Ombilin and Payakumbuh region.
 This compression was introduced by
the subduction of the Indian-
Australian plate beneath the Sunda
Craton Subduction started in the
early middle Eocene (Daly, 1991)
and created an extensional tectonic
regime which formed numerous
grabens in a back arc extensional
tectonic setting.

12. Stratigraphy of the Ombilin Basin

13. Basement (Pre Tertiary):
Pluton Granite, Tuhur Fm ( Phyllite, limestone), Silungkang Fm
(Limestone, Andesite , Basaltic), Kuantan Fm ( Marble, Slate, Quatrzite)
Brani Fm (Eocene)
A sequence of breccias and polymictic pebble to cobble Conglomerates
with a muddy to sandy matrix, formed as alluvial fan deposits. The unit is
interbedded with and partly time-equivalent to the sediments of the
Sangkarewang Fm
Sangkarewang Fm (Eocene)
Dark bluish grey to black laminated shales, including oil shales
Sawahlunto Fm (Early Oligocene)
Shale, siltstone, quartz sandstone and coal
Sawahtambang Fm (Late Oligocene)
Conglomeratic sandstones, siltstones and claystones.
Ombilin Fm (Miocene)
Characterised by dark grey shales, which are oftencalcareous.
Ranau Fm (pleistocene)
Tuff and Volcanic breccia

14. GEOCHEMICAL and RADIOMETRIC ANOMALIES
There are four geochemical and or
radiometric anomalies :
1. Taram Anomaly (Basement)
2. Harau Anomaly (Brani Fm)
3. Muara Anomaly (SawahTambang Fm)
4. Sumpur Anomaly (Granite)

15. 1. Taram Anomaly
 Anomalies located in the quartzite basement rock of the pre-
Tertiary Kuantan Formation.
 Radiometric reading (total count) range between 1,000 to 15,000
cps SPP2
 Uranium content range between 28 to 450 ppm.
 Thorium values range between 2000 to 10.000 ppm.
 Mineralization mainly occurs as fissure fillings and in close
association with iron oxides/hydroxides.
 Result of chemical analysis of the rock samples collected from
radiometric spot anomalies reveal that the high radiation rates
(radiometric total count) are mainly due to high thorium
contents.
 Anomalies are always related to Fe-/Mn-oxide and hydroxide
enrichments usually occurs in one of the following types : In the
matrix of brecciated quartzite, In mylonite of fault zones and as
joint and fissure fillings

16. 2. Harau Anomaly
 Harau anomaly is a radiometric anomalies with
radiometric readings (total count) range between 600
to 3500 cps SPP2, located at Harau river and its
tributary.
 Lithology of this area consists of conglomerate
interbedded with coarse sandstone. In regional
geology overview, it is included to Brani Formation
deposited in alluvial fan system.

17. 3. Muara (Upui) Anomaly
 Anomalies located in Tertiary
sediments of Sawahtambang
Formation, in the drainage area of
the Batang Upui north of Muara
Village.
 Maximum uranium values in
drainage samples reached 5,5 ppm
(background 0,19 ppm), the highest
radiometric reading at a bed rock
outcrop was 600 cps (background
50 cps).
 Systematic sampling of the trench
walls showed a relative increase of
uranium values within sandstone
beds towards the contacts with
shaly intercalations. Maximum
uranium values of rock samples
reached 17 ppm.

18. 4.Sumpur Anomaly
 The anomalies situated in the eastern portion of
the Sumpur Granite between Durian Gadang and
Menganti
 Radiometric reading up to 250 cps (background
150 cps).
 Uranium values in drainage samples ranging from
10,4 to 36,5 ppm (background 4,9 ppm).
 Bed rock samples showed maximum uranium
content of 11 ppm only.

19. DISCUSSION
 Tertiary sediment of Ombilin Basin generally deposited on the terrestrial
environment except Ombilin Formation that deposited in marine
environments. Brani Formation deposited in alluvial fan system,
Sangkarewang Formation deposited as a lake deposits, Sawahlunto
Formation deposited in river and floodplain system, Sawahtambang
Formation deposited in the braided stream setting. All of the four
formations, are prospect as the host rock for the sandstone type.uranium
deposits.
 In basinal area also founded the Pre Tertiary granites intrusion such as Lassi,
Sumpur, Singkarak and Tanjung Gadang granites. One of them is estimated
to be the same as a source of uranium found in the Sibolga basin, North
Sumatra (Kusumadinata and Sapardi, 1985)
 The results of the pre 1980’s survey, obtained four anomalies which are
Taram, Harau, Muara and Sumpur anomaly. Two of the anomalies is
predicted associated with the sandstone type uranium deposits (Harau and
Muara anomaly). Harau anomalies observed in the interval of alternating
conglomerate and sandstones of Brani Formation, deposited in alluvial fan
system while Muara anomaly found in shaly intercalations within the
predominantly sandy and conglomeratic sequence of Sawahtambang
Formation, deposited in fluvial braided stream environment.

20. CONCLUSION
Refer to the data and information of geologic,
radiometric and geochemical of the Ombilin basin
and its surroundings it is concluded as follow :
1. In general, the Tertiary sediment Ombilin basin
predicted has a prospect as host rock for sandstone
type uranium deposits, and the Pre Tertiary granitic
rocks plays a role as a source of uranium.
2. There are four uranium anomalies in the Ombilin
basin and its surroundings, which is Taram, Harau,
Muara and Sumpur anomaly. Two of them (Harau
and Muara) is predicted to be associated with
sandstone type uranium deposits and will be
followed by detailed investigation.

21. THANK YOU
FOR YOUR ATTENTION