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. © 2019 Penerbit Universiti Kebangsaan Malaysia. All rights reserved.