Re–Os and U–Pb geochronology of the Laochang Pb–Zn–Ag and concealed porphyry Mo mineralization along the Changning–Menglian suture, SW China: implications for ore genesis and porphyry Cu–Mo exploration

Abstract

Numerous polymetallic volcanogenic massive sulfide (VMS), vein, and replacement deposits are distributed along the Changning–Menglian suture zone in Sanjiang Tethyan metallogenic province, SW China. Laochang is the largest Pb–Zn–Ag vein and replacement deposit in this area, with a proven reserve of 0.51 Mt Pb, 0.34 Mt Zn, and 1,737 t Ag. Its age and relationship to magmatic events and VMS deposits in the region, however, have long been debated. In this paper, we present pyrite Re–Os and titanite U–Pb ages aiming to provide significant insights into the timing and genesis of the Pb–Zn–Ag mineralization. Pyrite grains in textural equilibrium with galena, sphalerite, and chalcopyrite from stratabound Pb–Zn–Ag and Cu-bearing Pb–Zn–Ag orebodies have a Re–Os isochron age of 45.7 ± 3.1 Ma (2σ, mean square weighted deviation (MSWD) = 0.45), whereas titanite grains intergrown with sulfide minerals yield a weighted mean 206Pb/238U age of 43.4 ± 1.2 Ma (2σ, n = 8). A Mo-mineralized granitic porphyry intersected by recent drilling below the Laochang Pb–Zn–Ag ores yields a zircon U–Pb age of 44.4 ± 0.4 Ma (2σ, n = 12). Within analytical uncertainties, the ages of the Pb–Zn–Ag deposit and the concealed Mo-mineralized porphyry are indistinguishable, indicating that they are products of a single magmatic hydrothermal system. The results show that Laochang Pb–Zn–Ag deposit is significantly younger than the host mafic volcanic rock (zircon U–Pb age of 320.8 ± 2.7 Ma; 2σ, n = 12) and Silurian VMS deposits along the Changning–Menglian suture zone, arguing against its origin as a Carboniferous VMS deposit as many researchers claimed. The initial 187Os/188Os ratio (0.540 ± 0.012) obtained from the pyrite Re–Os isochron suggests that metals were likely derived from the granitic porphyry that formed from a hybrid magma due to mixing of crustal- and mantle-derived melts, rather than from the mafic volcanic host rocks as previously thought. Our results favor that the Laochang Pb–Zn–Ag deposit is the shallow product of a porphyry Mo system. Thus, there is potential for discovery of porphyry Mo or Cu–Mo deposits below Laochang and similar Pb–Zn–Ag deposits in the Changning–Menglian suture zone.