Indonesian Journal on Geoscience

9

Authors: Hananto kurnio, Ildrem Syafri, Adjat Sudradjat, Mega Fatimah Rosana

Geochemical analyses of selected coastal and seafloor samples from Sabang Area revealed abundances of trace and rare earth elements. The selected samples of element abundances were mostly taken from seafloor in the vicinities of active fumaroles either by grab sampler operated from survey boat above fumarole point or by diver directly took the samples on the seafloor especially at Serui - Sabang Bay. Results show that samples closed to seafloor fumaroles demonstrate plenty of trace and rare earth elements. The trace and rare earth elements mean values (n=10) are: Nb (4.33 ppm), La (16.52 ppm), Ce (38.82 ppm), Nd (19.15 ppm), Ce (38.82 ppm), Pr (4.907 ppm), Nd (19.15 ppm), Sm (4.04 ppm), Gd (3.95 ppm), Dy (3.38 ppm), Th (6.432 ppm), and U (4.335 ppm). Negatively, statistical correlations between Fe, Zn, and Ni as the main sulphide elements with sulphur is interpreted that sulphide minerals do not form in the Sabang Sea. Sea water influence in the mineralization process was shown by the good correlations between Fe, Zn, Pb, Ni, and Ba.

19

Authors: Heryadi Rachmat, Mega Fatimah Rosana, Ade Djumarma Wirakusumah, Gamma Abdul Jabbar

After the catastrophic 1257 caldera-forming eruption, a new chapter of Old Rinjani volcanic activity beganwith the appearance of Rombongan and Barujari Volcanoes within the caldera. However, no published petrogeneticstudy focuses mainly on these products. The Rombongan eruption in 1944 and Barujari eruptions in pre-1944, 1966,1994, 2004, and 2009 produced basaltic andesite pyroclastic materials and lava flows. A total of thirty-one sampleswere analyzed, including six samples for each period of eruption except from 2004 (only one sample). The sampleswere used for petrography, whole-rock geochemistry, and trace and rare earth element analyses. The Rombonganand Barujari lavas are composed of calc-alkaline and high K calc-alkaline porphyritic basaltic andesite. The magmashows narrow variation of SiO2 content that implies small changes during its generation. The magma that formedRombongan and Barujari lavas is island-arc alkaline basalt. Generally, data show that the rocks are enriched in LargeIon Lithophile Elements (LILE: K, Rb, Ba, Sr, and Ba) and depleted in High Field Strength Elements (HFSE: Y, Ti,and Nb) which are typically a suite from a subduction zone. The pattern shows a medium enrichment in Light REEand relatively depleted in Heavy REE. The processes are dominantly controlled by fractional crystallization andmagma mixing. All of the Barujari and Rombongan lavas would have been produced by the same source of magmawith little variation in composition caused by host rock filter process. New flux of magma would likely have occurredfrom pre-1944 until 2009 period that indicates slightly decrease and increase of SiO2 content. The Rombongan andBarujari lava generations show an arc magma differentiation trend.

10

Authors: Thomas Putranto, Thomas Rüde

In Semarang City, groundwater has been exploited as a natural resource since 1841. The groundwater exploited in deep wells is concentrated in confined aquifers. The previous hydrogeological model was developed in one unit of aquifer and refined then by using several hydrostratigraphical units following a regional hydrogeological map without any further analysis. At present, there is a lack of precise hydrogeological model which integrates geological and hydrogeological data, in particular for multiple aquifers in Semarang. Thus, the aim of this paper is to develop a hydrogeological model for the multiple aquifers in Semarang using an integrated data approach. Groundwater samples in the confined aquifers have been analyzed to define the water type and its lateral distribution. Two hydrogeological cross sections were then created based on several borelog data to define a hydrostratigraphical unit (HSU). The HSU result indicates the hydrogeological model of Semarang consists of two aquifers, three aquitards, and one aquiclude. Aquifer 1 is unconfined, while Aquifer 2 is confined. Aquifer 2 is classified into three groups (2a, 2b, and 2c) based on analyses of major ion content and hydrostratigraphical cross sections.

6

Authors: IJOG

The Template

15

Authors: Benyamin Sapiie

Most of the Cenozoic tectonic evolution in New Guinea is a result of obliquely convergent motion that led to an arc-continent collision between the Australian and Pacific Plates. The Gunung Bijih (Ertsberg) Mining District (GBMD) is located in the Central Range of Papua, in the western half of the island of New Guinea. This study presents the results of detailed structural mapping concentrated on analyzing fault-slip data along a 15-km traverse of the Heavy Equipment Access Trail (HEAT) and the Grasberg mine access road, providing new information concerning the deformation in the GBMD and the Cenozoic structural evolution of the Central Range. Structural analysis indicates that two distinct stages of deformation have occurred since ~12 Ma. The first stage generated a series of en-echelon NW-trending (π-fold axis = 300°) folds and a few reverse faults. The second stage resulted in a significant left-lateral strike-slip faulting sub-parallel to the regional strike of upturned bedding. Kinematic analysis reveals that the areas between the major strike-slip faults form structural domains that are remarkably uniform in character. The change in deformation styles from contractional to a strike-slip offset is explained as a result from a change in the relative plate motion between the Pacific and Australian Plates at ~4 Ma. From ~4 - 2 Ma, transform motion along an ~ 270° trend caused a left-lateral strike-slip offset, and reactivated portions of pre-existing reverse faults. This action had a profound effect on magma emplacement and hydrothermal activity.