MAGNETIC STRUCTURE AND LINEAMENT ALSO ITS IMPLICATION TO THE MAGMATISM AND MINERALISATION IN LANUT DISTRICT, BOLAANG MONGONDOW, NORTH SULAWESI

• Main Author: MARIO CIPTA LUBIS, EKO
• Format: Theses
• Language: Indonesia
• Subjects: Geologi, hidrologi & meteorologi
• Online Access: https://digilib.itb.ac.id/gdl/view/15516
• Institution: Institut Teknologi Bandung

Lanut district in Bolaang Mongondow regency, North Sulawesi, has been recognized as a gold mineralization district since the 1990s during the Dutch colonialization period. Foreign and local mining companies Placer Dome-BHP and Utah Pacific-PT Aneka Tambang have conducted intensive exploration in the district since 1986. US-based mining company PT Newmont Indonesia gathered a number of important exploration data between the 1995 – 2002 period that continued by PT Avocet Bolaang Mongondow, which resulted in discovery of three high sulfidation epitermal prospects (Riska, Rasik, and Effendi). However, extensive exploration activities—both local and regional—have yet to be able recognize the system of geological structure. It is caused by the limitation of outcrop to be mapped and very high degree of weathering at the surface. The existing geological structure study was limited to (1) aerial photographic analysis and (2) interpretation of feeder structure from coring data. <br /> Fracture data were gathered using five scanlines and outcrop mapping over the Riska lantai tambang during the field research. The gathered fracture data comprised orientation, host rock, openness, spacing, continuity, healing, and filling gathered from 1,263 fractures. Aeromagnetic and groundmagnetic data have been used to analyze magnetic lineament and structure which appeared in regional and local scales. <br /> Statistical analysis of frequency to fracture spacing and openness and Fry analysis was done over all fracture data and point data of bench gold distribution. The analysis showed that the existing fracture network was not related to the primary control of gold mineralization in the Riska deposit. The mineralization was dominantly controlled by the hidrotermal breccia. Meanwhile, analysis of magnetic data led to the emergence of regional lineaments which previously were not recognized by surface mapping. The lineaments showed that regional control was east-west trending en enchelon in a northwest-southeast trending rigth-lateral fault zones and that it could be associated with the Inobonto fault in north coast. <br /> The 3D inversion model recognized high magnetic body under the Riska-Sinter prospect which was suspected as a pottasic alterasion zone of a Porphyry Cu system. The 2D forward model recognized northwest-southeast trending extensional structure which was assosiated with this high magnetic body and northeast-southwest trending structure where the advanced argillic alteration spread out continuously along the structure. The activity of this northeast-southwest trending structure believed to be related to the appearance of hidrotermal breccia that controlled gold mineralization over the surrounding prospects. The development of northwest-southeast rigth-lateral fault zones has controlled the magmatism and mineralization in the research area since late Pliosen and the magnetic structure under Riska-Sinter prospects at depth between 750 – 1,000 meters below the surface is a potential porphyry target for advance exploration.