LA-ICP-MS zircon U-Pb age and petrochemistry of the Samalong diorite in the Duolong metallogenic area of Tibet and its constraint on the metallogenic setting of the porphyry deposits

The Duolong ore concentration area is the most favorable potential prospecting area in the Bangong Co-Nujiang metallo-genic belt.Four porphyry Cu-Au deposits have been discovered in the Duolong area, but their metallogenic setting is still in contro-versy.The Samalong diorite is located in the central part of the Duolong area.Combined with the tectono-magmatic activation, the authors studied geochronology and bulk geochemistry of the Samalong diorite so as to constrain the metallogenic setting of the por-phyry deposits.25 zircon samples selected form the Samalong diorite intrusion were analyzed by LA-ICP-MS, which yielded a weighted ²⁰⁶Pb/²³⁸U age of 121.6±1.7 Ma (MSWD=1.9, n=23), which represents its intrusive age.According to the bulk petrochemi-cal results, the Samalong diorite intrusion belongs to the middle potassium and sodium-rich magmatic series.The Samalong diorite is also enriched in light rare earth elements and large ion lithophile elements and depleted in heavy rare earth elements and high fieldstrength elements, with high Sr and low Y concentrations, suggesting adakite-like affinity.The high initial ⁸⁷Sr/⁸⁶Sr values (0.7056~0.7073) and low initial ¹⁴³Nd/¹⁴⁴Nd (0.5124~0.5125) and ε_Nd(t)(-1.3~0.7) suggest that the Samlong diorite was generated from the lower crust, and its relative high MgO content and incompatible elements (Cr, Ni, etc.) concentrations indicate that there might have been some melt derived from the peridotitic mantle wedge mixed in the source region.The Samalong diorite was cut by the former strike-slip faults, which probably suggests that these later strike-slip faults controlled the formation of the porphyry deposits within 116~121 Ma, and the porphyry deposits in the Duolong ore concentration area were formed within the structural transferring period at the last stage of the northward subduction of Bangong Co-Nujiang Neo-Tethys Ocean.