Abstract

Ertsberg Intrusion is one of a number of Pliocene intrusions cutting a thick sequence of Tertiary limestone located in the Gunung Bijih Mining District (GBMD) of PT. Freeport Indonesia in Papua, Indonesia. This study is focused on detailed structural analysis of the Ertsberg Intrusion and its relationship to the tectonic evolution of the district (Figure 1).

The Ertsberg Intrusion cuts all of the Cretaceous-Miocene strata in the study area, consisting of Kembelangan Group and New Guinea Limestone Group. Three major faults, each of which is more than 10 meters in width, were measured in the study area: Wanagon Fault, Hanging Wall Fault, and West Fault. All of these faults show left-lateral strike-slip movement.

More than 2,000 structural measurements of minor faults, veins, shear fractures, and foliations have been collected from the study area. Minor faults, which have clay gouge less than 1 m in width, are divided into three groups based on kinematic analysis, and are interpreted as Riedel fracture systems related to the movement of the major faults. Kinematic analysis of these faults shows two different axial trends. Minor faults surrounding the Wanagon Fault and Hanging Wall Fault show NE-SW shortening and NW-SE extension, but the kinematic analysis of minor faults surrounding the West Fault shows N-S shortening and E-W extension.

Two deformation events are inferred from kinematic analysis. The first is related to the formation of major NW trending foldbelt with minor thrust. The second event is dominated by numerous evidence of strikeslip faulting. All of the extension fractures in the research area show NW-SE extension, suggesting they formed at the second deformation stage. This stage produced fractures that provided space for the emplacement of the Ertsberg Intrusion.