Enu Deposit, Indonesia

Geological Settings

Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia.  The Indonesian archipelago comprises 17,508 islands, with over 238 million people.  It is the fourth most populous country of the world, making it a key country of Asia.


Regional Geology

Indonesia is an immense chain of islands located from east to west between 95° and 141°E, and crossing the equator from 6°N to 11°S.  Indonesia is one of the most dangerous crustal battlefield of the world.  The invasion from Eurasian continental plate, Australian continental plate, the Indian oceanic plate and Pacific oceanic plate have lasted for more than 300 million years, leaving a hundred of volcanoes active.

The most prominent fault is the Semango fault, indicated by the widespread Permian and Triassic granites, which is a dextral strike-slip fault underlain in the western subduction zone of Sumatra Island.  Another major fault is Palu-Koro fault, which controls the major structural feature in the central part of Indonesia.  It extends from Koro in central part of Sulawesi to Palu in the west coast of Sulawesi and extends across the Makassar strait to East Kalimantan.

Local Geology

The Enu deposit is located in the western Central Sulawesi fold-thrust belt, controlled by the prominent Palu-Koro strike-slip fault.  During the history of fault movement, the Palu-Koro Fault was dominated by a sinistral transpressional movement, giving rise to the up-lifting of the mountain ranges along the fault zone.

In Central Sulawesi, Late Miocene to Neogenepotassic calc-alkaline magmatism occurs notably along the left lateral Palu-Koro Fault Zone.  Metamorphic lithologies include amphibolites, eclogite, mica-schists, quartzites, chlorite-feldspar and graphite phyllites.  Geological mapping, paleomagnetic investigations, and GPS observations indicate complex Neogene deformation in Sulawesi, including extension, block rotations, and strike-slip faulting.  There are rapidly exhumed upper mantle and lower crustal rocks, and young granites, near to Palu-Koro fault.  During the Pliocene, coarse clastic sedimentation predominated across most of Sulawesi as mountains rose.  The Sulawesi fold-thrust belt has now propagated west into the Makassar Straits.  Neogenegranitoid containing fine and poor biotitzedmafic mineral is mainly composed of quartz, K-feldspar, plagioclase and muscovite.  They can be easily recognized as they present coarse equigranular or coarse and KF-megacrystals.



During the Early Miocene a calc-alkaline andesitic arc developed in relation to west-directed subduction.  Post-collision almagmatism in the North Sulawesi Arc produced felsic to mafic volcanic suites that are thought to be related primarily to rifting of the former arc rather than directly to subduction.  In the neck of Sulawesi, LILE and LREE-element enriched, potassic granites (Dondo suite) of continental affinity were generated.

The region is comparatively well mineralized with the presence of porphyry Cu-Au and porphyry Mo.  Mineralization related to porphyry Cu-Au districts includes Cu-Au skarn, polymetallic vein and high sulphidation epithermal Au.  Epithermal Au mineralization (typically quartz-calcite veins ± adularia) is associated mainly with the Early Miocene andesitic arc, the most important epithermal Au district (Kotamobagu) is inferred to be associated with a long-lived volcanic center represented by the Moat caldera.  Epithermal gold systems in the North Sulawesi Arc are generally eroded deeply, typically with chlorite-epidote wall rock alteration and little is known about their associated volcanic environment.