Alluvial tin mining in Karangan area was closely related to the mineralization of Bongsu Granite and its contact zone with sedimentary rocks of Semanggol Formation possibly through greisenization process. A gravity survey focusing on high tin anomaly area in Gunung Bongsu Forest Reserve was conducted to define the subsurface geology and structural patterns related to tin mineralization. The gravity data were corrected for drift, free air, Bouguer, latitude and terrain corrections to produce final gravity data of the study area. The data were processed and analyzed using Oasis Montaj to produce Bouguer, regional, residual, Total Horizontal Derivative (THD) regional and THD residual gravity maps for qualitative interpretations. Comparison of the produced gravity anomaly maps indicate occurrence of partial ring fractured-style model of granite emplacement initialized by laccolith development, doming of sedimentary bedrock, magmatic diking through ring fractures and finally sinking and melting of surrounded sedimentary rock before magma cooling and crystallizing to become granites bedrocks. Greisenization occurs within the inner ring granite as the bedrock developed at later phase. ABSTRAK Perlombongan bijih timah aluvium di kawasan Karangan adalah berkait rapat dengan pemineralan Granit Bongsu dan zon persentuhan antara granit dengan batuan sedimen Formasi Semanggol berkemungkinan melalui proses penggreisenan. Suatu kajian graviti telah dijalankan di kawasan Hutan Simpan Gunung Bongsu yang tertumpu kepada kawasan beranomali timah bagi menilai geologi subpermukaan dan corak berstuktur yang berkaitan dengan pemineralan timah Data graviti dibuat pembetulan hanyutan udara bebas, Bouguer, latitud dan rupa bumi bagi menghasilkan data graviti akhir kawasan kajian. Data akhir diproses dan dianalisis menggunakan perisian Oasis Montaj bagi menghasilkan peta graviti Bouguer, rantau, baki, terbitan mendatar keseluruhan (THD) rantau dan THD baki bagi tujuan pentafsiran kualitatif. Perbandingan anomali antara peta graviti yang dihasilkan menunjukkan wujudnya model pembentukan granit melalui rekahan separa berbentuk cincin yang dimulakan dengan pembentukan lakolit, pembentukan struktur kubah pada batuan sedimen, rejahan magma secara daik melalui rekahan cincin dan akhirnya penenggelaman dan pencairan batuan sedimen sebelum penyejukan dan penghabluran magma untuk menjadi batuan dasar granit. Penggreisenan berlaku pada granit dalam struktur cincin kerana batuan tersebut terbentuk pada fasa lewat.
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