Indonesian Journal on Geoscience

9

Authors: Moeh. Ali Jambak

This research aims to determine the facies and diagenetic level of limestone of the Upper Cibulakan and Parigi Formations, and also aims to determine the structural correlation between surface and subsurface limestones. Based on thin section analyses taken from the core and outcrop samples, there are four types of lithofacies on the Upper Cibulakan Formation, i.e. mudstone-wackestone, wackestone-packstone, packstone-grainstone, and grainstone facies, and also four types of lithofacies on the Parigi Limestone Formation, i.e. mudstone-wackestone, wackestone-packstone, packstone-grainstone, and lower mudstone-wackestone facies. The analysis of surface and subsurface limestone facies of the Upper Cibulakan and Parigi Formations led to the knowledge of the proportionality and variation of the limestone characteristics on both positions. Limestone of the Upper Cibulakan Formation was deposited locally and discontinuously, whilst the Parigi Formation limestone was deposited evenly and continuously. The structural correlation between the surface and subsurface limestone indicates that these formations were uplifted/exposed due to a local force, likely caused by the intrusion of igneous rocks, as happened in the Kromong Complex. The presence of residual hydrocarbon on the surface of the limestone samples suggests the possibility of potential hydrocarbon trapped in the limestone beneath the surface.

12

Authors: Agus Haris Widayat

Geochemical and petrographical analyses were carried out to investigate the occurrence of calcite in theformer Ombilin lacustrine lake. The study involves eight samples taken from a 56 m long drill core of Sangkarewangoil shale. Geochemical investigation showed that the samples consist of varied terrigenous input represented by Si, Al, K, and Ti, and autochthonous input represented by S, total organic carbon (TOC), and d13C of bulk organic matter. Along the drill core profile the abundance of autochthonous input decreases upwards, while that of terrigenous input oppositely increases upwards. Petrographical analysis revealed that calcite is a major mineral in the samples. In this study, the abundance of calcite could be represented by the abundance of Ca, as calcite is the only significant Ca containing mineral. Ca is abundant in the samples (8.4% in average) and its concentration varies similarly with those of S, TOC, and d13C, suggesting that the element as well as calcite incorporates the autochthonous input. Thevariation of calcite abundance in the drill core profile is considered to be related with primary productivity changes during the development of the former lake. Higher primary productivity represented by more positive of d13C value(-24.8‰) during the deposition of the lower part of the drill core profile promoted the higher amount of deposited organic matter. In such environment, the supersaturation of carbonate ion in lake water was also reached and significant precipitation of authigenic calcite occurred. As the lake developed, the primary productivity decreased as indicated by more negative of d13C value (eventually -26.8‰). This condition led to the decreases of deposited organic matterand calcite in the lake sediments.

17

Authors: Bhakti Hamonangan Harahap

Strong mineralized carbonate rock-bearing Pb-Zn-Cu-Ag-(Au) ores are well exposed on the Latong River area, Madina Regency, North Sumatra Province. The ore deposit is hosted within the carbonate rocks of the Permian to Carboniferous Tapanuli Group. It is mainly accumulated in hollows replacing limestone in the forms of lensoidal, colloform, veins, veinlets, cavity filling, breccia, and dissemination. The ores dominantly consist of galena (126 000 ppm Pb) and sphalerite (2347 ppm Zn). The other minerals are silver, azurite, covellite, pyrite, marcasite, and chalcopyrite. This deposit was formed by at least three phases of mineralization, i.e. pyrite and then galena replaced pyrite, sphalerite replaced galena, and pyrite. The last phase is the deposition of chalcopyrite that replaced sphalerite. The Latong sulfide ore deposits posses Pb isotope ratio of 206Pb/204Pb = 19.16 - 20.72, 207Pb/204Pb = 16.16 - 17.29, and 208Pb/204Pb = 42.92 - 40.78. The characteristic feature of the deposit indicates that it is formed by a sedimentary process rather than an igneous activity in origin. This leads to an interpretation that the Latong deposit belongs to the Sedimentary Hosted Massive Sulfide (SHMS) of Mississippi Valley-Type (MVT). The presence of SHMS in the island arc such as Sumatra has become controversial. For a long time, ore deposits in the Indonesian Island Arc are always identical with the porphyry and hydrothermal processes related to arc magmatism. This paper is dealing with the geology of Latong and its base metal deposits. This work is also to interpret their genesis as well as general relationship to the regional geology and tectonic setting of Sumatra.