西藏打加错银(多金属)矿床含矿岩石成因及其找矿意义

打加错银多金属矿床是在冈底斯成矿带西段新发现的首例以银为主的多金属矿床.为了查明该矿床成岩成矿时限、含矿岩石成因及成矿意义，对该矿区出露的含矿（次）火山岩（流纹质晶屑凝灰岩与流纹斑岩）开展了锆石U-Pb年代学、微量元素及Hf同位素、全岩地球化学、辉钼矿Re-Os年代学等分析.研究表明，含矿（次）火山岩形成时代为66.6~67.4 Ma，晚期辉钼矿同位素Re-Os模式年龄为61.0±0.8 Ma，结合含矿地质体的矿化特征、石英-黄铁矿-辉钼矿脉与含矿流纹斑岩的穿插关系等，证实打加错矿区存在两期成矿作用，即早期形成的强硅化（次）火山岩型和矽卡岩型银多金属成矿作用及晚期石英脉型银多金属成矿作用，成矿时限介于66.6~61.0 Ma，构成了与（次）火山期后热液相关的浅成低温热液成矿系统；（次）火山岩地球化学数据表明其属高钾钙碱性-钾玄岩系列，具准铝质-过铝质特征，富集轻稀土元素和大离子亲石元素（K、Rb、Pb）、亏损重稀土元素和高场强元素（Nb、Ta、Ti），锆石Ti的平均温度为712 ℃，ε_Hf（t）值均为负值（-16.5~-3.6）且T_DMC为1.2~2.1 Ga；成矿岩石具低氧逸度和贫含水性特征，属于典型S型花岗岩，源于中-古元古代古拉萨地壳的部分熔融，并与中拉萨地体发育的银铅锌矿床成矿岩体特征一致，而与南拉萨地体中爆发的、与Ⅰ型花岗岩相关的铜多金属矿化明显不同，据此揭示出南拉萨地体也与中拉萨地体一样具有与S型花岗岩相关的成矿潜力，这对丰富南拉萨地体的成矿类型及促进该类型矿床的进一步找矿新发现具有重要的理论及现实意义. 

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 ε_Hf(t) values are strongly negative (-16.5 to -3.6) and the T_DMC 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.