Monday, December 16, 2013

Porphyry Copper Deposits And Tectonic Plate Thickness

Just a follow up to my earlier post on porphyry copper deposits with an example from the Malanjkhand mines of Central India.

Nature Geoscience has a paper on the relationship between copper deposits and magmatic arc thickness. The full paper is behind a pay wall but the abstract is helpful enough:

Porphyry copper systems supply about 75% of the world’s copper. They form above subduction zones and are preferentially associated with calc-alkaline magmas. Such magmas result from continuous iron depletion during differentiation, in contrast to tholeiitic magmas that show initial iron enrichment during differentiation. The formation of calc-alkaline magmas is favoured by high water content and oxygen fugacity. These characteristics, as well as magmatic metal contents, are thought to be imparted in the mantle source by fluids of the subducted slab. Yet this process does not explain why porphyry copper systems preferentially occur in thicker arcs. Here I present a statistical assessment of more than 40,000 published geochemical analyses of magmatic rocks from 23 Quaternary-aged volcanic arcs worldwide. I find that magmas of thicker arcs are systematically more calc-alkaline and more depleted in copper than magmas of thinner arcs. This implies that the missing copper in the former accumulates as copper sulphides within or at the base of thicker arcs. Such copper accumulations are an essential step in forming porphyry systems. These results suggest that the thickness of the overriding plate provides a more important control on magma differentiation than the composition of the mantle source, and can explain the preferential association of porphyry copper systems with calc-alkaline magmas and thicker arcs.

Malanjkhand ores would have around 2.4 billion years ago been a copper enrichment at the base or at the deep levels of a magmatic arc system in a subduction zone setting as smaller cratonic blocks converged. The roots of this ancient magmatic arc mountain chain has since been exhumed due to subsequent tectonic movements and erosion to reveal its riches.

No comments:

Post a Comment