滇东南老君山锡-钨-锌-铟多金属矿集区含矿矽卡岩成因研究

Genesis of the ore-bearing skarns in Laojunshan Sn-W-Zn-In polymetallic ore district,southeastern Yunnan Province,China

滇东南老君山矿集区广泛分布的矽卡岩是本区锡-钨-锌-铟多金属矿床的主要赋存围岩。长期以来,该区含矿矽卡岩的成因争议较大,由此也制约了对该区锡钨多金属成矿规律的认识。本文以区内代表性的都龙和南秧田矿区含矿矽卡岩为研究对象,在对其地质特征详细研究的基础上,运用电子探针和ICP-MS分别测定了上述两个矿区含矿矽卡岩的矿物成分、微量和稀土元素组成,探讨了它们和多金属矿床的成岩成矿机制的关系。结果表明,区内同时存在与地层产状一致的"层状"含矿矽卡岩和明显切割层理的穿层含矿矽卡岩。都龙矿区含矿矽卡岩富Fe、贫Al,主要矿物端元成分为钙铁榴石(And52-69Gro28-45Spe1-4)、钙铁辉石(Di11-41Hd51-73Jo0-28)和铁阳起石等,从干矽卡岩到退化蚀变阶段,形成环境由酸性的弱还原环境向偏碱性的相对氧化环境变化。南秧田矿区含矿矽卡岩富Mg、Al,贫Fe,主要矿物端元成分为钙铝榴石(Gro82-89Alm7-13And2-5)、透辉石(Di55-81Hd18-42Jo0-5)和透闪石(阳起石)等,形成于相对还原的环境。都龙和南秧田矿区含矿矽卡岩与花岗岩都显示出相似的、LREE相对富集的右倾型稀土配分模式,多具有中等-弱Eu负异常,与典型的热液交代成因矽卡岩特征相似。综合分析认为,该区含矿矽卡岩主要形成于燕山晚期花岗岩浆热液与围岩的交代作用,"层状"矽卡岩可能是热液沿层间构造、岩相突变带等有利位置进行交代的结果。

In the Laojunshan ore concentrated district,southeastern Yunnan Province,skarns are widely distributed,which are the main host rocks of Sn-W-Zn-In polymetallic deposits. For a long time,the genesis of ore-bearing skarns in this area is controversial,which restricts the further understanding of metallogenic regularity. This paper focus on the ore-bearing skarns from the representative Dulong and Nanyangtian deposits in Laojunshan ore district. Based on the study of their geological characteristics,together with their mineral compositions,trace and rare earth element compositions determined by EPMA and ICP-MS,respectively,the formation mechanism of skarns and polymetallic deposits are discussed. The results show that there are two types of ore bearing skarns in Laojunshan ore district,namely,the"stratiform"type which are consistent with the occurrence of the strata,and the cross-layer type.The ore-bearing skarns in the Dulong mining district are rich in Fe and poor in Al,and the endmembers of main minerals are andradite( And52-69Gro28-45Spe1-4),hedenbergite( Di11-41Hd51-73Jo0-28) and ferroactinolite. Compositions of the skarn minerals record the dynamic evolution process of redox environment in the Dulong deposit. From the dry skarn stage to the degraded alteration stage,the formation environment changes from acidic weak reduction environment to alkaline relative oxidation environment. The ore-bearing skarns in Nanyangtian mining district are rich in Mg and Al,while poor in Fe. The endmembers of main minerals are grossularite( Gro82-89Alm7-13And2-5),diopside( Di55-81Hd18-42Jo0-5) and tremolite,which are formed in a relative reduction environment. The ore-bearing skarns and granites in the Dulong and Nanyangtian mining district show similar right-dipping REE distribution patterns with relative LREE enrichment,mostly with medium weak Eu negative anomalies,similar to typical hydrothermal metasomatic skarns. According to the comprehensive analysis,the ore-bearing skarn in this area is likely formed by the metasomatism between the magmatic hydrothermal fluids form the Late Yanshanian granite and the carbonate rocks,and the stratiform skarns are the result of the metasomatism of the hydrothermal fluids along favorable positions such as the interlayer structures and the abrupt lithofacies zones.