西秦岭地区脉岩成因与金成矿关系——来自李坝金矿年代学、地球化学及Nd-Hf-S同位素的约束

Petrogenesis of dykes and its relationship to gold mineralization in the western Qinling belt: Constraints from zircon U-Pb age,geochemistry and Nd-Hf-S isotopes of Liba gold deposit

甘肃李坝金矿位于西秦岭北成矿亚带礼岷金矿带东部,矿区内脉岩主要包括斜闪煌斑岩、云斜煌斑岩、闪长(玢)岩、细晶闪长岩、石英闪长岩、花岗(斑)岩、花岗细晶岩等。根据各脉岩与金矿的产出关系可划分为成矿前脉岩、成矿期脉岩和成矿后脉岩3类。不同种类、不同期次脉岩的成因及其对金成矿作用的制约存在不同认识。LA-ICP-MS锆石U-Pb定年结果显示,李坝金矿区花岗斑岩脉的年龄为(210±1)Ma(MSWD=0.15),与中川岩体同属晚三叠世构造岩浆活动的产物。成矿期花岗(斑)岩脉富碱、高钾,属于过铝质、高钾钙碱性系列,与成矿密切相关的煌斑岩脉同属高钾钙碱性系列。Nd-Hf同位素结果显示,李坝金矿区花岗斑岩脉的143Nd/144Nd比值为0.512084±0.000007,147Sm/143Nd比值为0.0903,εNd(t)为-7.98,Nd同位素亏损地幔模式年龄TDM2为1.64Ga;锆石εHf(t)=-6.30^+1.47,TDMC为1.65~1.15 Ga,显示花岗斑岩脉的源区以古老的壳源物质为主,并有年轻物质的加入。硫同位素和矿床地球化学结果显示,黄铁矿的δ34S值变化于4.3‰~11.6‰,平均值为8.45‰,表明其成矿硫源具有深源特点,金矿成矿物质来源于与脉岩有关的深部岩浆。综上,笔者认为矿区内脉岩与金矿化具有深部同源性,成矿同期的煌斑岩可能为金矿化提供部分成矿物质或成矿流体。

The Liba gold deposit is located in the Lixian-Minxian gold belt,western Qinling.Geological characteristics show that dykes are widely developed in the mining area and are composed of camptovogesite,mica-plagioclase lamprophyre,diorite aplite,diorite porphyry,fine diorite,quartz diorite,granite porphyry and fine-grained granite.According to their cross-cutting relationship,they can be divided into premetallogenic,mineralization and postmetallogenic dykes.The granite porphyry from the Liba gold deposit yielded an age of 210±1Ma(MSWD=0.15)by using LA-ICPMS zircon U-Pb.Granite porphyry is alkali-rich and potassium-rich,and belongs to peraluminous and high-K calc-alkaline series,whereas lamprophyre belongs to high-K calc-alkaline series.In terms of Nd-Hf-S isotopes,the granite-porphyry dykes have the data 143Nd/144Nd=0.512084±0.000007,147Sm/143Nd=0.0903,εNd(t)=-7.98,TDM2=1.64Ga,andεHf(t)=-6.30^+1.47,and TDMC=1.65~1.15Ga,suggesting that granitoid was derived mainly from an ancient continental crust,with involvement of juvenile components.Theδ34SV-CDT values of sulfides from the Liba gold deposit vary from 4.3‰to 11.6‰,with a mean of-8.45‰,which indicates that the sulfur in the orefield mainly came from mantle source.Au was mainly derived from deep magma related to granite dykes.According to these data,the authors have reached the conclusion that dykes and gold orebodies exist together spatially and temporally and have the features of the same source and successive evolution.In some degree,the dyke rocks probably provided a channel for upward movement of ore-forming materials and dykes probably provided a small amount of ore-forming materials for gold deposits.