Petrogenesis and Prospecting Significance of Ore-Bearing Rocks in Dajiacuo Silver Polymetallic Deposit, Tibet

Dajiacuo silver polymetallic deposit is the firstly and newly discovered deposit dominated by silver in the western Gangdese metallogenic belt. In order to find out the time of diagenesis and mineralization, genesis of ore-bearing rocks and metallogenic significance of the deposit, zircon U-Pb chronology, trace elements and Hf isotopes, whole rock geochemistry and molybdenite Re-Os chronology of the ore-bearing (sub-volcanic) volcanic rocks (rhyolitic crystal tuff and rhyolitic porphyry) exposed in the mine area are analyzed in this paper. The results show that the ore-bearing (sub-volcanic) volcanic rocks were formed at 66.6-67.4 Ma, and the late molybdenite isotopic Re-Os model age was 61.0±0.8 Ma. Combined with the mineralization characteristics of ore-bearing geological bodies and the interpenetration relationship between quartz-pyrite and molybdenite veins and ore-bearing rhyolite porphyry etc., it is confirmed that Dajiacuo has two stages of mineralization dominated by the high silicified (sub-volcanic) volcanic rock type and skarn type silver-polymetallic mineralization in the early stage and the quartz vein-type silver-polymetallic mineralization in the late stage. It has a metallogenic time limited of 66.6-61.0 Ma, forming the epithermal cryogenic metallogenic system related to the post-volcanic hydrothermal period. The geochemical data of the (sub-volcanic) volcanic rocks indicate that they belong to the high-K series to shoshonite series with metaluminous to peraluminous, enriched in LREE and large ion lithophile elements (K, Rb, Pb), and depleted in HREE and high field strength elements (Nb, Ta, Ti). The average temperature of zircon is 712 ℃, the ϵHf(t) values are strongly negative (-16.5 to -3.6) and the TDMC is 1.2-2.1 Ga. The geochemical characteristics indicate that the ore-bearing rocks were typical S-type granite, which were generated from the partial melting of the Paleo-Mesoproterozoic Lhasa crystallization basement. Their characteristics are consistent with those of the ore-forming magmatic rock associated with the Ag-Pb-Zn deposits in the central Lhasa subterrane, but are obvious different with erupted Ⅰ-type granite related copper polymetallic mineralization in the south Lhasa subterrane. It is revealed that the south Lhasa subterrane has the same metallogenic potential as the central Lhasa subterrane related to S-type granites and has great theoretical and practical significance to enrich the mineralization types of the south Lhasa subterrane and promote the further exploration of this type deposits. © 2022 China University of Geosciences. All rights reserved.