南秦岭柞水—山阳矿集区园子街岩体岩石地球化学与成矿潜力探讨

Petrogeochemistry and mineralization potential of the Yuanzijie intrusion in the Zhashui-Shanyang ore deposit cluster in southern Qinling

柞水-山阳矿集区是南秦岭构造-岩浆活动强烈地区,广泛出露园子街等浅成-超浅成的花岗岩类侵入体及与之有关的夕卡岩-斑岩型矿床(化)。对该矿集区内的园子街岩体进行了详细的岩石学、元素地球化学、锆石U-Pb年代学、Lu-Hf同位素及微量元素分析,利用锆石微量元素组成计算了岩浆结晶温度及氧逸度条件,在此基础上探讨了园子街岩体的成因、成岩地质背景及成矿潜力。LA-ICP-MS锆石U-Pb测年和微量元素测定表明,花岗闪长玢岩的加权平均年龄为(141.7±1.7)Ma,锆石稀土元素普遍显示显著的正Ce异常和弱的负Eu异常,暗示了锆石在相对氧化的环境下形成。锆石εHf(t)值为-1.58^+2.18,表明园子街岩体的岩浆源区可能为地壳熔融岩浆与地幔重熔岩浆的混合,并且具有较高的混合均一化程度。锆石Ti温度计计算的园子街岩浆结晶温度为598~841℃(平均656℃),岩浆氧逸度lgf(O2)平均为-18.0,ΔFMQ集中于0~5,与柞水-山阳矿集区内的小河口夕卡岩型工业铜矿床相似。园子街岩体显著富集成矿元素Cu(170~5939μg/g,平均1 300μg/g),明显高于南秦岭花岗岩(11μg/g)和世界花岗岩(20μg/g)平均值;全岩RbY+Nb、Nb-Y以及锆石Yb-U、Hf-U/Yb、U/Yb-Y判别图解显示,园子街岩体形成于后碰撞造山环境。总之,园子街矿化点在成岩成矿大地构造环境、矿化地质特征、围岩蚀变类型和岩浆氧逸度条件等方面均与国内外及矿集区内小河口夕卡岩型铜矿床相似,暗示该岩体具有形成夕卡岩型铜矿床的成矿潜力。园子街岩体与秦岭造山带燕山期岩体形成于同一地球动力学背景下,即晚侏罗世与早白垩世之交造山带应力状态由挤压转向伸展,引发强烈壳-幔相互作用和花岗质岩浆活动,并随之形成了含矿岩体及其夕卡岩型矿化。

The Zhashui-Shanyang ore deposit cluster is a southern Qinling area with strong tectonic-magmatic activities.Many hypabyssal-super hypabyssal granitoids,such as the Yuanzijie intrusion,are widely distributed in this area.In this contribution,we performed detailed petrological,whole-rock geochemical,LA-ICP-MS zircon U-Pb isotope,trace element and Lu-Hf isotope analyses on the Yuanzijie granitoid.Using trace and rare earth elemental(REE)compositions of zircon,we calculated the magma temperature and f(O2)conditions,and subsequently evaluated the petrogenesis,geodynamic setting and metallogenetic potential of the granitoid.The Yuanzijie corcovadite yielded zircon 206Pb/238U weighted average age of 141.7±1.7 Ma,with significant positive Ce and weak negative Eu anomalies,suggesting an oxidized environment for zircon formation.Zircon εHf(t) values of -1.58 to +2.18 indicated a mixed origin of crust-and mantle-derived materials with high degree of homogenization for the formation of the Yuanzijie intrusion.The corcovadite displayed Ti-in-zircon temperatures of 598-841 ℃ (averaging 656 ℃),with an average lgf(O2)value of-18.0 andΔFMQ values concentrating between 0 and 5-all similar to the values of Xiaohekou skarn copper deposit in the ore cluster.The intrusion also had significantly high Cu content(170-5939μg/g,averaging 1300μg/g),obviously higher than that of granitoids in South Qinling(11μg/g)and the world(20μg/g)on average.Rock Rb-Y+Nb,Nb-Y and zircon Yb-U,Hf-U/Yb and U/Yb-Y discriminant diagrams further suggested that the Yuanzijie intrusion was formed in a post-collisional orogenic setting.We conclude that the Yuanzijie mineralized spot is similar to those of typical porphyry-skarn deposits in the Zhashui-Shanyang ore cluster and the world in diagenesis and metallogenic setting,mineralization characteristics,wall rock alteration,and magma f(O2)condition,implying its mineralization potential.The Yuanzijie intrusion and other Yanshanian intrusions in the Qinling Orogen occurred in a same geodynamic setting of a transitional regime from compression to extension during J3-K1,triggering a intense crust-mantle mixing and granitic magmatism,and forming the ore-bearing intrusions and skarn-type mineralization.