Abstract

The Gunung Bijih (Erstberg) Mining District of PT Freeport Indonesia is located 4 km to the northwest of Puncak Jaya (4884 m). The geographic position of this study area lies between 4° 04' 00" - 4° 05' 16" South Latitude and 137° 05' 25" - 137° 07' 20" East Longitude, in the Timika administrative district of Irian Jaya.

The study is focused on the stratigraphic relationship between the Kembelangan Group and the New Guinea Limestone Group, structural analysis and the correlation to mineralization in the research area.

The Kembelangan Group in this area, of Cretaceous age, is divided into the siliciclastic Piniya Formation and Ekmai Formation. The Tertiary New Guinea Limestone Group was deposited conformably above the Kembelangan Group and consists of the Paleocene Waripi Formation, the Eocene Faumai Formation, the Oligocene Sirga Formation and the Early Miocene Kais Formation. These groups were intruded by porphyritic diorite in the Late Pliocene, and are unconformably covered by recent alluvium deposits.

Structural analysis of 629 lineaments from aerial photographs, 298 minor faults, 978 shear fractures, and 42 extension fractures which cut all of the lithological units except the recent alluvium deposits, shows a very significant strike slip fault system. The fractures were divided into six structural domains and interpreted as Riedel fractures. The ENE - WSW trending sinistral faults were interpreted as synthetic Riedel fractures, the ENE - WSW trending dextral faults were interpreted as antithetic Riedel fractures, and the WNW - ESE trending sinistral faults were interpreted as 'P' Fractures.

Kinematic analyses on minor faults and extension fractures of these structural domains show consistently NE - SW shortening strain axes and NW - SE extension strain axes.

The fluid flow in the research area is controlled by fractures with high concentrations of veins in the dolostone of the Waripi Formation. These mineralization zones were emplaced on the hanging wall of the Wanagon Fault. These faults were interpreted as a thrust faults that were rejuvenated as sinistral strike-slip faults.