The copper-gold skarn mineralization occurred at the Batu Hijau deposit, a world class porphyry copper-gold deposit, is located in the southwestern corner of Sumbawa Island, Indonesia. The deposit mainly consists of three major rock types: andesitic volcaniclastic unit (Early to Middle Miocene) and equigranular quartz diorite (Late Miocene to Middle Pliocene), which are intruded by the three phases of copper-gold bearing tonalite porphyries (Early to Middle Pliocene). The tonalite porphyries were emplaced along the lithological contacts of two pre-mineralization wall rocks. Skarn were newly found at the contact of calcium rich andesitic volcanic layer and intermediate tonalite porphyry intrusion. Skarn can be classified as calcic-exoskarn, and it consists of a Fe-rich and oxidized skarn locally controlled by faults and fractures. Although no limestone is known in the Batu Hijau deposit, Ca-rich volcanic host rocks favoring to become skarn alteration within the Batu Hijau deposit.
Based on the mineral assemblages, the skarn forming process can be divided into two major stages such as prograde and retrograde stages consisting of four sub-stages. Prograde skarn consists of clinopyroxene (diopside) and garnet (mainly andradite) ± magnetite formed at relatively high fO2 and fS2 and temperature of 340 ̊-515 ̊C while retrograde skarn alteration is dominated by Fe- rich minerals such as amphibole and epidote formed at lower fO2 and fS2 , temperature down to 200 ̊-396 ̊C. Opaque minerals include chalcopyrite, pyrite, sphalertite, minor galena and bismuth-telluride. Gold precipitates in the retrograde alteration associated bismuth-telluride minerals. A decrease in SiO2 corresponds to increase FeOt from porphyry intrusion towards skarn zones. CaO and FeOt behave comparably in the skarns suggesting that the ore mineralization is coincident with carbonization of prograde assemblages during retrograde alteration. Chemical analysis shows the concentration of gold mineralization of 4.15 g/t and 2.7 g/t from
the high grade ore zone.
Based on the result of fluid inclusion and δ34S value, the evolution of fluid origin can be suggested that the origin of skarn and porphyry from the Batu Hijau deposit was largely of an igneous sulfur source of magmatic origin caused by the early anhydrous skarn assemblage is dominated by denser and more saline, and has very high temperature and salinity. The Batu Hijau deposit evidently shows a porphyry-related skarn mineralization that is mineralogically and metallogenically zoned and that transition from one style to the next can be relatively rapid. The result of this research has indicated that the range of porphyry-related deposits, skarn (calc-silicate-magnetite rocks) and porphyry (potassic altered rocks) can form during a single prolonged hydrothermal event. read more

Tertiary Petroleum System along Sorong Fault in the Bird Head of Papua to South of Halmahera Area, Indonesia
Surjono, S.S^1,2 , Toha, B² & Winardi, S²

^{1, 2} Geological Engineering Department, Faculty of Engineering, Gadjah Mada University

Jalan Grafika No.2, Yogyakarta, 55281, INDONESIA

¹(sugengssurjono@gmail.com)

Abstract

Sorong Fault occurred in the northern part of Papua Island westward to the eastern part of Sulawesi Island is the boundary of Australian Continental and Pacific Oceanic Crusts. Close to the fault zone, there is the Salawati Basin that proofed as prolific tertiary basin in the eastern Indonesia developed among the Paleozoic and Mesozoic Basins. Sorong Fault which have initiated in the Late Miocene deformed northern part of Papua westward to form transtensional basin including Salawati. Several explorations in Sorong Fault zones have been done following the successful of Salawati which their petroleum system rely on the Miocene Carbonate reservoir of Kais Formation with the younger Klasafet shale as the sources rocks. Juxtaposed stratigraphic position is the main petroleum play for this basin. read more

Oleh: Salahuddin Husein, Ph.D.

Gempabumi besar di dasar kerak samudera (lebih tepat lagi: di bagian atas mantel litosfer) lempeng Indo-Australia tanggal 11 April 2012 telah banyak diulas. Skalanya duh Gusti: 8,6 Mw dan disusul 8,2 Mw.
Meski sudah banyak gempabumi sebelumnya pada patahan yang sama, terutama setelah Gempabumi Lepas Pantai Aceh 24 Desember 2004, gempa tempohari itu tetap saja mengejutkan dalam hal intensitas.

Bagi para peneliti gempabumi (atau pembelajar gempabumi, seperti juga penulis) Pulau Sumatera memiliki 4 karakter gempabumi tektonik mengacu pada mekanisme pemicunya, dimana dua kelompok pertama terkait pada datangnya lempeng Indo-Australia dan konvergensinya dengan Paparan Sunda, yaitu: (1) terpicu patahan geser sinistral berarah U-S di kerak samudera Indo-Australia, yang mendekati Palung Sunda dengan kecepatan sekitar 6 cm/tahun; (2) terpicu patahan anjak di kisaran Palung Sunda hingga daerah prisma akresi akibat gaya kompresif sangat kuat dari konvergensi lempeng di sepanjang palung; kelompok ini dipopulerkan oleh Prof. Kerry Sieh dkk sebagai megathrust Sunda/Sumatera.
Dua kelompok berikutnya berada di Pulau Sumatera, namun pada kedalaman dan mekanisme berbeda, yaitu:
(3) gempabumi dalam, umumnya sesar normal, terpicu oleh pergerakan turun lempeng samudera Indo-Australia akibat tarikan gravitasi (slab-pull) dibawah Pulau Sumatera; dan (4) gempabumi dangkal di sekitaran Bukit Barisan akibat pergeseran dekstral Sesar Sumatera.
Keempat kelompok mekanisme tersebut saling mempengaruhi satu sama lain, namanya juga masih keluarga gempa, bisa akrab juga seperti keluarga Cemara? 😀 read more