The crustal architecture and properties of a world-class Fe-Cu mineral system

Diverse geophysical and geochemical surveys have been conducted in the past decade over the Lower Yangtze Metallogenic Belt (LYMB) and adjacent areas, in eastern China. Here, we present an integrated interpretation of these data describing the crustal architecture, properties and tectonic processes controlling the formation of an extensive Fe-Cu system in the LYMB.

Seismic reflection profiles across the LYMB reveal various large-scale structures, including crustal wedges and duplexes, mega-thrusts, Moho stacks, and even lower crust underthrusting at block boundaries, reflecting a compressional tectonic regime. This tectonic event could have made the materials from different crust mixed physically at the boundary zone. A wide range of zircon crustal Hf-isotopic model ages confirmed the mixing of different basement material, possibly by Jiangnan and Yangtze basements. The spatial coincidence of block boundary zone with the distribution of deposits is evidence that deep-penetrating boundary structures, such as thrust faults, wedges, and duplexes acted as conduits for migrating magmas/fluids from lower crust to the upper crust. The corresponding conductive zones beneath the boundary zone between the Jiangnan orogen and Yangtze fold-and-thrust belt (YFTB) are interpreted as the result of alteration by fluids within the conduits.

The Vp velocity mode derived from the wide-angle seismic reflection indicate that there is 10km thick high velocity zone above the Moho beneath the YFTB, we interpreter this as juvenile lower crust, possible derived from underplating. The zircon Hf-isotope compositions of the Cu-Au-Mo deposits related high-K Calc-alkali intrusions, formed from 151 to 138 Ma, evolved from high to low, however, the ɛHf (t) of magnetite deposits related magmatic rocks, formed from 137 to126 Ma, evolved from low to high. These indicate that the concentration of S and Cu-(Au) in the mother magma might be diluted by a large amount of crustal material, and the Cu content in the magma was significantly reduced by magnetite formation.

Therefore, we conclude that the Late Mesozoic Fe-Cu (Au) mineral system of LYMB was formed by during an intra-continental orogeny with multi-level sources, processes, and conduits communicating each other within an exceptional lithospheric architecture.