安徽沙溪斑岩铜(金)矿区闪长斑岩形成机制

Genesis of Diorite Porphyry in the Shaxi Porphyry Cu(Au)Orefield

沙溪铜(金)矿床是目前长江中下游成矿带中已探明储量最大的斑岩型铜矿床。文章通过对沙溪矿区内闪长斑岩的年代学、全岩化学成分、同位素地球化学等综合研究,探讨了闪长斑岩的形成机制。闪长斑岩的LA-ICP-MS锆石U-Pb加权平均年龄为128.3±1.5 Ma,属于早白垩世岩浆活动产物。岩石具有富碱(ALK=6.12~7.53 wt%)、富钾、富镁(Mg#=38.99~51.53)、准铝质(A/CNK=0.90~0.99)等特征,属于钙碱性—高钾钙碱性系列岩石;岩石轻稀土元素富集,重稀土元素亏损(LaN/YbN=12.63~17.63),无明显的Eu异常(δEu=0.84~1.14);大离子亲石元素(Ba、Sr)和Pb等富集,高场强元素(Nb、Ta、Ti等)亏损。闪长斑岩的全岩(87Sr/86Sr)t值为0.7052~0.7056,εNd(t)值为-6.09^-3.42,(206Pb/204Pb)t值为17.51~18.16、(207Pb/204Pb)t值为15.49~15.63、(208Pb/204Pb)t值为37.57~38.45,表明成岩物质源于壳幔混合。130 Ma左右,平移的郯庐断裂的伸展引发了大规模岩浆作用,来自深部的岩浆沿着郯庐断裂带上侵,并混染了大量的地壳物质,形成了沙溪闪长斑岩及与铜矿有关的石英闪长斑岩。

The Shaxi Cu(Au)deposit is the largest porphyry copper deposit with proved resource reserve in the Middle-Lower Yangtze River Valley metallogenic belt till present.This paper discusses the genesis of the diorite porphyry by integrating the analysis of chronology,whole-rock chemical composition and isotopic geochemistry.The LA-ICP-MS zircon U-Pb dating results reveal that the diorite porphyry was the product of early cretaceous magmatic activities,with a weighted mean 206 Pb/238 Pb age of 128.3±1.5 Ma.The diorite porphyry is rich in alkali(ALK=6.12~7.53 wt%),K and magnesium(Mg#=38.99~51.53),and present metaluminous characteristic(A/CNK=0.90~0.99),which can be grouped into calc-alkalic-high-K calc-alkalic rock series.Light rare earth elements(LREEs)are enriched,heavy rare earth elements(HREEs)are depleted(LaN/YbN=12.63~17.63),and almost no Eu anormaly(δEu=0.84~1.14)is shown.In addition,an enrichment of large ion lithophile elements(LILEs)(e.g.,Ba,Sr)as well as Pb,and a depletion of high field strength elements(HFSEs)(e.g.,Nb,Ta,Ti)are observed.The diorite porphyry have(87 Sr/86 Sr)t values of 0.7051~0.7056,andεNd(t)values range of-6.09^-3.42;(206 Pb/204 Pb)t values are 17.51~18.16,(207 Pb/204 Pb)t values are 15.49~15.63,and(208 Pb/204 Pb)t values are 37.57~38.45,which indicate the rock-forming materials were derived from melted crust with involvement of mantle materials.Around 130 Ma,the stretching of the translational Tanlu fault caused large-scale magmatism.The magma from the deep intruded along the Tanlu fault belt and experienced contamination of considerable amount of crustal materials,finally the magma emplaced at relative shallow depth and resulted in the formation of the Shaxi diorite porphyry and quartz diorite.