|Andrew Watson opines that in his article called - Grasberg Part 1 - he discussed the social and cultural aspects of the Grasberg deposit and outlined its long and fascinating history. In this article he summarizes the geology of this deposit.|
The first striking fact is how young the deposit is. The mountain spine of New Guinea formed in the Middle Miocene and is less than 12 Million years old. The deposit itself is 3 million years old. He suggests that porphyry copper deposits generally form at 3-4 kilometers ( about 2 miles) at depth, which implies some 3,000-4,000 meters of erosion in little over 2 million years (given that deposits can take up to 1,000,000 years to form). This is due to the extremely rapid movement of the Pacific and Australian plates (100 km per 1 million years) which are colliding and crushing a series of smaller plates (micro plates) together. Prior to that time the entire area was a deep to shallow ocean basin with thick sequences of clastic (sand/silt/mud) sediment and carbonates (e.g. limestone) deposited from the Triassic to Early Miocene. In the Grasberg area the carbonates and other sedimentary rocks play a critical role in controlling the nature and style of mineralization.
Mineralization in New Guinea is controlled by the conjunction of long orogen parallel thrust faults, and cross cutting transform faults. These serve to localize intrusions and channel the mineralizing fluids into distinct regional corridors. Also these mega structures are long lived and can be tied back in some cases to the Pre-Cambrian basement of the orogenic belt.
The immense size and richness of the GIC and Ertsberg is a function of repeated intrusion by igneous rock in the same area over a period of 1 million years.
Good reading ... read on.