The lithospheric architecture of the Lower Yangtze Metallogenic Belt, East China: Insights into an extensive Fe–Cu mineral system

Abstract

Diverse seismic (active and passive) and magnetotelluric (MT) surveys have been conducted in the past decade over the Lower Yangtze Metallogenic Belt (LYMB) and adjacent areas, in eastern China. We present an integrated interpretation of these data describing the lithospheric architecture of an extensive Fe–Cu system in the LYMB. A combination of anomalous geophysical features has been observed in the sub-continental lithospheric mantle (SCLM) and asthenospheric mantle, which include: anomalously thin lithosphere (thickness ~70 km), low S- and P-wave seismic velocity in the upper mantle (70–150 km), and a dipping high-conductivity body extending from the lower crust to the asthenosphere (~150 km). We interpret these features to be the result of lithospheric delamination and asthenospheric upwelling during Late Mesozoic orogenesis. Melts and metal-rich (e.g., Cu, and Au) fluids produced by the asthenosphere would have metasomatized the lithospheric conduits (faults) and the lower part of the SCLM. Therefore, we suggest that these areas acted as the primary source regions for the Fe–Cu (Au) mineral system in the LYMB. Seismic reflection profiles across different parts of the LYMB reveal various large-scale structures, including crustal wedges, Moho stacks, mega-thrusts, and even lower crust underthrusting at block boundaries; these features are consistent with collisional orogeny. The spatial coincidence of major block boundaries with the distribution of deposits is evidence that deep-penetrating boundary faults acted as conduits for migrating magmas/fluids from lower crust to the upper crust. Conductive zones in the crust beneath the LYMB are interpreted as the result of alteration by fluids within the conduits. 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.