INTEGRATION OF GRAVITY METHOD, GEOCHEMISTRY, AND MINERALOGY IN GOLD DEPOSIT PROSPECTING AT KUTA USANG AREA, DAIRI REGENCY, NORTH SUMATERA

• Main Author: Steven, Frederic
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/85376
• Institution: Institut Teknologi Bandung

This study integrates gravity, geochemistry, and mineralogy methods as an efficient and cost-effective combination for prospecting primary gold deposits in areas with limited geological information. The Kuta Usang area in Dairi Regency, North Sumatra, is a research target suspected of having primary gold deposits shown from local mines with gophers and panning methods in the upstream area of the river. Therefore, the complete Bouguer anomaly (CBA) map from open access gravity data, for example Global Gravity Model Plus (GGMPlus), were interpreted to identify the source of local gravity anomalies related to gold deposits, such as faults and intrusive rocks. This identification was obtain by extracting high value of Total Horizontal Derivative (THD) and Analytic Signal (AS). Furthermore, the extraction result were combines with lineament from the gravity contrast boundaries indicated by the CBA residual map, as well as regional geological informations to determine the location of rock chip and pan concentrate sampling. The resulting map successfully identified the presence of faults and granite intrusions at several points and validated by sampling activities at 28 points. Petrographic observations, X-ray diffraction (XRD) and X-ray fluorescence (XRF) carried out on rock chip samples from previous activities. Alterations presents in this area occurred due to the development of fractures in the rock, then filled with carbonate minerals such as dolomite and calcite, as well as other alteration minerals such as sericite, epidotee, and minor chlorite. In addition, sulfide mineralization occurred after the fracturefilling alteration minerals were replaced by sulfide minerals such as pyrite, sphalerite, galena, and minor chalcopyrite. Gold and silver enrichment from samples enriched by sulfide minerals was supported by Atomic Absorption Spectrometry (AAS) and colorimetry tests with Au levels above two ppb and Ag levels above three ppm. From these tests, the two locations identified had Au grades above three ppm and Ag levels of 20 ppm in the mica schist and oxidized phyllite. Based on SEM-EDS observations, Au-Ag is enriched as an electrum in the mica schist after replacing pyrite. In addition, cassiterite is present in this gold mineralization as an accessory mineral, which also replaces pyrite. Identification of cassiterite at some river pan sample points also supports this. Based on the high AS and THD extraction maps, the two locations with high Au-Ag content are on the edge of the high THD zone. Those indicate that the location of high Au-Ag enrichment is outside the primary fault zone