内蒙古北山老硐沟金多金属矿床闪长玢岩年代学、地球化学及其成矿意义

Geochronology and geochemistry of diorite porphyrite from Laodonggou gold-polymetallic deposit,Beishan,Inner Mongolia,and its metallogenic significance

内蒙古北山老硐沟金多金属矿床原生矿化与闪长玢岩脉时空-成因关系密切,部分闪长玢岩形成工业矿体,但其地球化学特征、岩石成因及精确的成岩时代仍不清楚。笔者在详细野外地质调研基础上,选取3个闪长玢岩体进行全岩主量元素、微量元素、锆石U-Pb年龄及Lu-Hf同位素研究。闪长玢岩具斑状结构,斑晶主要为斜长石、角闪石,少量黑云母,基质主要为斜长石和角闪石,w(SiO_(2))介于62.87%~65.33%,地球化学特征显示其属于高钾钙碱性或超钾质系列。矿化闪长玢岩、蚀变未矿化闪长玢岩、弱蚀变闪长玢岩中锆石^(206)Pb/^(238)U年龄分别为(243.0±1.0)Ma、(237.8±1.2)Ma和(233.8±0.9)Ma,表明闪长玢岩的岩浆活动持续了至少9 Ma,并可能发育多幕侵位,成矿时代约为243 Ma。其锆石Hf同位素组成均一,ε_(Hf)(t)值基本一致,介于0.05~5.67,暗示矿化与未矿化的闪长玢岩具有相同的岩浆源区。二阶段模式年龄(t_(DM2))介于905~1258 Ma,表明岩浆可能起源于中新元古代地壳的部分熔融。三叠纪受造山后地壳伸展减薄影响,基性岩浆底侵带来的热导致新生地壳的部分熔融。岩浆经历分离结晶作用并受到地壳物质的混染,顺区域断裂上升侵位至构造薄弱地段形成闪长玢岩并沉淀矿质。而不同期次的闪长玢岩可能是岩浆持续脉动侵位的结果。

The diorite porphyrite from the Laodonggou gold-polymetallic deposit has close temporal-spatial and genetic relationship with primary mineralization.However,its geochemical characteristics,petrogensis and precise age remain unclear.Based on detailed field work,the authors selected three diorite porphyrite rocks for investigation of whole-rock geochemistry,zircon U-Pb geochronology and Lu-Hf isotopes.The diorite porphyrite has porphyritic texture.The phenocrysts are mainly plagioclase and hornblende as well as minor biotite,and the matrix includes plagioclase and hornblende.The content of SiO_(2)ranges from 62.87%to 65.33%,and geochemical characteristics show high-K calc-alkaline and shoshonitic characteristics.Zircon U-Pb geochronological dating of the mineralized diorite porphyrite,the altered diorite porphyrite and the weakly altered diorite porphyrite are(243.0±1.0)Ma,(237.8±1.2)Ma and(233.8±0.9)Ma,respectively.The magmatic activity lasted for at least 9 Ma,with multiple emplacement,and the ore-forming age is approximately 243 Ma.The ε_(Hf)(t)values of different diorite porphyrite vary from 0.05 to 5.67,implying similar magma source.Two-stage Hf modal ages range from 905 Ma to 1258 Ma,indicating that the magma was derived from partial melting of Meso-Neoproterozoic crust.Affected by the lithospheric thinning and crustal extension in the Trrassic,the basic magma underplated and caused the partial melting of juvenile crust.Accompanied by fractional crystallization and crustal materials,the diorite porphyrite of different stages might have been formed by magma impulsing emplacement.