Indonesian Journal on Geoscience

7

Authors: Danny Zulkifli Herman

Lepadi area situated in the Regency of Dompu, West Nusa Tenggara, is occupied by limestone unit comprising limestone and sandy limestones; and sandstones which are occasionally found as lenses within units of tuffaceous sand and tuff breccia. Indication of metallic mineralization is shown by the occurrence of the base metal-bearing quartz veins penetrating a host rock of limestone. A pervasive hydrothermal alteration was formed as chlorite-calcite-muskovite/sericite-quartz and kaolinite/haloysite series with veins consisting of massive quartz veinlets (millimeter to 2.0 cm maximum width) and very thick vuggy quartz veins (± 3.2 m width). The first quartz type contains ore minerals such as galena, sphalerite, chalcopyrite, pyrite and iron oxide; whilst the second type has a predominant galena content with an association of, chalcopyrite, covelite, pyrite and iron oxide which occurred as tiny vugs filling and distributed sporadically within the quartz body. The result of studies by using microscope (petrography and mineragraphy), infra-red analysis (PIMA), chemical analysis and fluid inclusion on entirely altered rock/ore mineral/quartz vein samples reveals that the alteration and mineralization processes might occur in a mineralization system involving hydrothermal fluid of near neutral at the temperature ranging from 275º - 200oC. It brings to a suggestion that deposition of quartz veins and associated ore minerals has taken place in a depth ranging from 620 – 150 m beneath paleosurface. It is based on hydrothermal fluid’s salinity ranging from 1.2 – 2.2 equivalent wt.% NaCl detected from fluid inclusion within quartz crystal samples. Keywords: bioclastic limestone, mineralization, metallic sulphide ore

9

Authors: S. Poedjoprajitno, J. Wahyudiono, dan A. Cita

Kaligarang Fault which cuts across Semarang City in a north-south direction has been activated for long periods. During Tertiary time, the Kaligarang Fault experienced a dextral wrench movement, which was triggered by a tectonic activity during that time, with a maximum principal stress (σ11) in a northeast - southwest direction. While on Quaternary time, this fault was reactivated as a sinistral wrench fault. Reactivation of this fault was caused by a principal stress (σ12) in a north east - south last direction. Keywords: reactivation, main principal stress, tectonic, active fault

11

Authors: S. Bronto, S. Mulyaningsih, G. Hartono, dan B. Astuti

Pillow lava flows of pyroxene basalt containing 50 wt.% SiO2 are exposed at Opak River, west of Watuadeg Village, Sleman - Yogyakarta. The length of flow structures is between 2 – 10 m, with diameter of 0.5 – 1.0 m and it has a glassy skin at the surface body. Flow directions vary from N70oE in the northern side, through N 120oE in the middle to N 150oE in the southern side. About 150 m away from the river to the west, there is a small hill about 15 m high, that has a similar composition with the pillow lavas. Both lava flows and the small hill are composed of pyroxene basalt, dark grey in color, hypocrystalline vitrophyre to porphyritic texture, with fine-grained phenocrysts of pyroxene (10 %) and plagioclase (25 %) set in glassy groundmass. These data indicate that the small hill was the eruption source of the basaltic pillow lavas. The lavas are overlain by pumice-rich volcaniclastic rocks, composed of tuff, lapillistones and pumice breccias, that are known as the Semilir Formation. Near the contact with lavas, the volcaniclastic rocks contain some fragments of pyroxene basalt, similar composition with the pillow lavas. This fact, together with analyses of petrology, volcanology, and radiometric dating show that the basaltic pillow lavas are unconformably overlain by the Semilir Formation. Keywords: pillow lava, eruption source, stratigraphy, Watuadeg