Chalcophile Element Contents of Coeval Volcanic Rocks Derived from Heterogeneous Mantle Beneath the Eastern North China Craton

Abstract

The metasomatized subcontinental lithospheric mantle (SCLM) is progressively thought to have played a pivotal role in the generation of gold deposits. However, the metasomatized SCLM is generally heterogeneous and its melting often yields magmas with diverse chemical compositions. Whether such source heterogeneity displays various metal contents in the mantle and derivative magmas remains poorly understood. The lithospheric mantle underneath the eastern North China Craton (NCC) displays two different mantle domains (EM1 and EM2) during the Early Cretaceous at the western and eastern parts of the trans-lithospheric Tanlu Fault system (TLF). Based on systematic petrological studies, we report platinum group element (PGE), Au, and Cu contents of coeval volcanic rocks (the Qingshan Group, 130~120 Ma, from 5 locations, n=36) along the TLF zone and secondary fault systems. These rocks display arc-like trace element signatures and distinctly enriched Sr-Nd isotopes with EM1-type (the intraplate) or EM2-type (the cratonic margin) affinity. Sulfide petrography and the lack of correlation between chalcophile element contents, Cu/Pd and MgO contents indicate negligible sulfide segregation during ascent for EM1-derived magmas, whereas EM2-derived magmas show sulfide separation and increasing Cu/Pd with progressive evolution. The EM1-derived samples display MORB-like PGE (e.g., Pd: 2~6 ng/g), Au (< 1~4 ng/g) and Cu (< 5~100 μg/g) contents. The EM2-derived samples also display similarly low PGE (e.g., Pd: < 1~3 ng/g), Au (< 1~3 ng/g) and Cu (< 5~40 μg/g) contents. The broad correlations of metasomatism indicator Ba/Nb and the value of Au/Cu and Au/Pd suggest weak Au elevation relative to Cu and PGEs during mantle metasomatism and the introduction of some metals related to subduction. Magmatic differentiation also leads to a slight increase in Pd, Au and Cu contents for some EM1-derived magams. However, regardless of spatial and chemical heterogeneity, these early-stage EM1-derived and EM2-derived magmas (MgO > 5.5 wt.%) display comparable Au and Cu contents. Therefore, mantle heterogeneity may not be the key cause leading to diverse metal contents in mantle sources and magmas. Instead, the crustal thickness and fault systems of the intraplate and margin areas are distinct during cratonic destruction. We thus propose that the distinct evolution pathways of mantle-derived magmas induced by the regional tectonic settings rather than mantle heterogeneity in metal fertility are key for spatial variations of metal distribution and final mineralization in the crust.

Keywords: Volcanic rocks, Chalcophile elements, Magmatic pathways, Lithospheric mantle, North China Craton