Tectonic controls on porphyry mineralization: Evidence from geochemistry and radiogenic isotopes
|Category||Tectonic & Seismotectonic|
|Location||International Geological Congress,oslo 2008|
|Author||Hollings, Pete۱; Cooke, David R.۲|
|Holding Date||11 October 2008|
In the last 20 m.y., the formation of giant porphyry copper-molybdenum and copper-gold deposits in the circum-Pacific region has been closely associated with subduction of aseismic ridges and seamount chains, and/or collision with oceanic plateaus beneath oceanic island and continental arcs. In continental margin settings such as Central and Northern Chile, there are recognizable changes in the geochemistry of the volcanic rocks prior to mineralizing events that produced giant porphyry copper deposits. These changes include abrupt increases in the La/Yb ratios of volcanic rocks associated with mineralization, and are interpreted to indicate a rapid change in the tectonic environment, likely associated with the subduction of an aseismic ridge.
As for continental margin settings, perturbations of subduction have also triggered porphyry mineralization in oceanic island arcs. The Plio-Pleistocene Baguio District of the Philippines is one of the world’s premier mineral provinces, containing >35 million ounces of gold and 2.7 million ounces of copper. It formed in a complex tectonic setting, sandwiched between two oppositely dipping subduction zones and associated with the subduction of the Scarborough Ridge beneath the island of Luzon. The tectonic complexity and gaps in the igneous record between 17 Ma and 5 Ma obscures geochemical signals of mineralization in the Baguio District. La/Yb ratios show a subdued increase in host rocks that predate and are coeval with mineralization, compared to the Andean examples. Similar subdued responses have been detected from other oceanic island arc porphyry systems. Published data for igneous rocks associated with porphyry mineralization in the Batu Hijau district show a subtle increase in La/Yb ratios at the time of mineralization with ratios increasing to levels comparable to those in the Baguio district. This implies that in oceanic arc systems, changes in the tectonic setting resulting in crustal thickening appear to be associated with mineralization events. The thinner crust beneath island arcs compared to that below continental arcs results in more subdued geochemical signatures of crustal thickening. Radiogenic isotopic data from rocks coeval with mineralization in the Baguio District can be interpreted to indicate that more primitive magmas were emplaced immediately prior to mineralization and may have acted as a heat or metal source. In both continental and oceanic arc systems, changes in the tectonic setting resulting in crustal thickening appear to be intimately associated with mineralization events. Crustal thickness probably influences the intensity of the syn-mineralisation geochemical signatures, with thicker crust producing more profound changes in magma compositions as the magmatic systems evolve towards mineralization.