In-situ mineralogical interpretation of the mantle geophysical signature of the Gangdese Cu-porphyry mineral system

Abstract

The lithosphere provides a reservoir for ore-forming elements introduced by repeated fertilisation during asthenosphere-lithosphere interaction by the slab subduction. Here we use the Gangdese copper metallogenic belt to show how mineralogical study can help to interpret these mantle signatures. We present insights into the in-situ geochronological, geochemical and whole-rock isotopic analyses of minerals from mantle-derived lamprophyres that intruded the Bairong Cu-porphyry deposit in the Gangdese metallogenic belt. Zircon U-Pb dating yields weighted-mean concordant 206Pb/238U ages of 11.69 ± 0.27 Ma to 12.41 ± 0.83 Ma. These zircons show juvenile εHf (t) (+1.0 ∼ +10.9) and low δ18O (5.5‰ ∼ 7.7‰) values. Variable initial 87Sr/86Sr ratios (0.7073–0.7080), εNd(t) values (−6.3 ∼ −5.0) and initial 187Os/188Os ratios (0.1382–0.1591) suggest hybridization between lithosphere mantle and asthenospheric melts. Phlogopites in the lamprophyres reflect partial dissolution and incipient melting with low Mg# values but high Al2O3 contents in core. And these phlogopites are surrounded by a zone with high Mg# values. We interpret this as superheating and hybridization of refractory lithosphere mantle and asthenosphere. The 3-D magnetotelluric and seismic signature of the Gangdese mineral belt reflect a mantle source region comprising asthenosphere, old refractory lithosphere, ponded mafic melts near the crust-mantle boundary, and higher-level magma chambers where more extensive mixing between crust- and mantle-derived melts occurs. The asthenospheric upwelling provides a mechanism for heat transfer and the expulsion of volatile elements. It is generating a new young, fertile lithosphere beneath the southeast Lhasa block, which has driven the mineralization processes in the Gangdese metallogenic belt.