Petrogenesis of Paleogene lamprophyres in the Ailaoshan tectonic belt, western Yangtze Craton: Implications for the mantle source of orogenic gold deposits

Abstract The Ailaoshan tectonic belt, western margin of Yangtze Craton, is an important orogenic gold belt with ore source derived from mantle. The Paleogene lamprophyre dykes are temporally and spatially associated with gold ore bodies in this belt. Here we research the geochemical features of lamprophyres aiming to better understand the features of mantle source for the orogenic gold deposit. These lamprophyres are characterized by enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), depletion in high field strength elements (HFSEs), high (87Sr/86Sr)i (0.7061-0.7093) and negative eNd(t) (-5.60 to -0.44). These features indicate arc-like geochemical signatures, collectively pointing to the derivation of lamprophyre magmas from an enriched lithospheric mantle. The low range of Nb/U (1.55-6.53), elevated ratios of Ba/La (13.95-62.94), and limited values of Th/Yb (5.25-9.94) suggest this enriched mantle source was metasomatized by subducted slab-derived fluids. Moreover, the lamprophyres exhibit light Mg isotopic compositions (δ26Mg = -0.44 to -0.26‰), low Ti/Ti* (0.11-0.27) and Fe/Mn (47.45-66.65), and high Na2O+K2O (6.01-9.38 wt%), further indicating the mantle was metasomatized by carbonate-rich melts. The Mg-Sr isotope binary mixing modeling highlights the metasomatism of clastic sediments. This study has demonstrated that the mantle source for the orogenic gold deposits has been modified by diverse metasomatic agents to develop the potential to produce a metal-bearing and CO2-rich aqueous fluid.