Geology
The country rocks of the project area are part of the Ruffaer Metamorphic Belt and are locally known as the Derewo Metamorphics by Freeport geologists. They consist of graphitic phyllite and slate and were almost certainly formed from the Cretaceous Kembelangan shale and siltstone exposed in the thrust belt to the south.
Metamorphism is of high pressure greenschist facies as shown by the presence of chloritoid, paragonite and muscovite. Metamorphism is dated between 28ma and 20ma with the heat source likely to come from the Irian Ophiolite Belt to the north. The boundary of the Ruffaer Metamorphics and the southern fold and thrust belt is gradational (Warren, 1995). In the project area, the Derewo Fault, which forms a WNW?ESE valley for almost 200km through the highlands, separates slate and phyllite from folded Kembelangan shale, siltstone and sandstone on its southern side (#Figure 14).
The extremely high rates of uplift which continue to the present day, combined with the equatorial climate to produce very high rates of erosion in the steep topography in Papua. Together with the gold prospectivity of the bedrock, it is a perfect environment to produce alluvial gold deposits and probably the least explored one of significance in the world.
Figure 14. Geology of the project area (Roberts, 2011)