云南北衙金多金属矿床富碱斑岩地球化学特征及其地质意义

Origin and geochemical characteristics of the porphyries in Beiya gold polymetallic deposit,Yunnan

北衙金多金属矿床位于金沙江—哀牢山富碱斑岩成矿带中段,是中国西南地区最大的金矿床。研究表明矿床的形成与喜马拉雅期富碱斑岩密切相关,以往研究集中于万硐山岩体,对外围笔架山、桅杆坡与红泥塘岩体研究甚少。本文以北衙矿区外围的笔架山、桅杆坡与红泥塘岩体为研究对象,通过系统的野外地质调查与全岩主量、微量和稀土元素分析,讨论了岩石成因以及岩浆源区特征,并与万硐山富矿岩体进行了对比。主量元素测试结果表明本次研究的所有岩体属于高硅、富碱的钾玄岩系列,且岩浆分异程度较高,同时P_(2)O_(5)与SiO_(2)呈现负相关关系,指示所有岩体均为Ⅰ型花岗岩。稀土元素配分模式为右倾型、无明显Ce异常和轻微的Eu负异常,显示有少量斜长石分离结晶。微量元素分析结果显示,所有岩体具有高Sr、低Y、高Sr/Y比值且富含大离子亲石元素、亏损高场强元素的特征。全岩Rb/Sr比值、Nb/Ta比值、La/Nb比值,均显示了地壳和地幔的双重特征。同时,在稀土元素配分模式图和微量元素配分模式图中,笔架山、桅杆坡和红泥塘以及前人测得万硐山石英正长斑岩均显示出与滇西加厚下地壳埃达克质岩石相似的地球化学特征,表明北衙矿床的岩浆均起源于加厚下地壳的部分熔融,并混有少量地幔物质。

The Beiya gold polymetallic deposit is located in the middle of the Jinshajiang Ailaoshan alkali rich porphyry metallogenic belt,and is the largest gold deposit in southwest China.The mineralization is closely related to the Himalayan alkali rich porphyry.Previous studies focused on the Wandongshan porphyry,and few studies were conducted on the surrounding Bijiashan,Weiganpo and Hongnitang porphyries.In this work,we take Bijiashan,Weiganpo and Hongnitang as research object,based on detailed field geological survey and analysis of major,trace and rare earth elements,and discuss the rock genesis,magma source and then compared with Wandongshan porphyry.The results show that all porphyries show characteristic of high silica asuelles alkali and belongs to shoshonite series with high degree of magmatic differentiation.SiO_(2) and P2O5 show obvious negative cor-relation,which indicates that all alkali rich porphyry belongs to I-type granite.All porphyries show a right dip distribution pattern rich in LREE,without Ce anomaly and slight negative Eu anomal,indicating that there is a small amount of plagioclase crystallization in the process of magmatic fractional crystallization.All alkali rich porphyries show high Sr,Low Y,high Sr/Y,relatively rich in large ion stone elements,and relatively deficient in high field strength elements.Rb/Sr ratio,Nb/Ta ratio and La/Nb ratio characteristics indicate that all porphyries have crust-mantle mixture characteristic.At the same time,rare earth and trace element distribution pattern show similar geochemical characteristics to the adakite rocks of the thickened lower crust in western Yunnan,indicating that their source are thickened lower crust mixed with a small amount of mantle material.