URANIUM ANOMALIES IN THE OMBILIN INTERMONTANEBASIN,WEST SUMATRA, INDONESIA Hadisuwito Head of Exploration Division Center Development of Nuclear Geology National Nuclear Energy Agency
Location Map of Indonesia We have around 17,500 islandsThe big fives are Sumatra, Java, Borneo (Kalimantan), Celebes (Sulawesi) and Papua.
INTRODUCTIONUranium Mineralization in IndonesiaIn Indonesia there are four types of Uranium mineralizations :1. Sandstone Type in the Tertiary Sediments of Sibolga Basin at North Sumatra Province2. Intrusive Type in the Cretaceous Pegmatite Granite at Lampung Province3. Vein type in the Jurassic Metamorphic at West Borneo Province4. Volcanic type in the Tertiary Volcanic at East Borneo Province
INTRODUCTION (cont)Location Map of Uranium Mineralization in Indonesia
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.
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.
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
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.
Faults in Study Areaamong others:Takung faultMangani FaultBarisan Tengah faultPakis fault
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)
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.
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 Conglomerateswith a muddy to sandy matrix, formed as alluvial fan deposits. The unit isinterbedded with and partly time-equivalent to the sediments of theSangkarewang FmSangkarewang Fm (Eocene)Dark bluish grey to black laminated shales, including oil shalesSawahlunto Fm (Early Oligocene)Shale, siltstone, quartz sandstone and coalSawahtambang 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
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)
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
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.
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.
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.
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.
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.