摘要
近年来,大兴安岭地区众多有色金属矿床的发现与开发使得该区成为矿床学界研究的热点地区。研究表明,该区具有良好的成矿潜力,拜仁达坝银多金属矿床作为一个大型银铅锌多金属矿床即位于该地区。本文在综合分析前人工作成果的基础上,以该区的大地构造体制为框架,以岩浆热液成矿理论为指导,通过对矿区及其外围进行翔实系统的野外地质调查,详细研究了矿区的地质特征,结合岩石学、岩体地球化学、矿床地球化学、矿物学、年代学等综合研究手段,对矿床的成矿物质来源、成矿流体特征、控矿因素以及矿床的成因等一系列科学问题做了较为深入的研究与探讨,并建立了矿床的成矿模型,这对于矿床深部及外围的找矿勘查具有重要的应用价值。
研究表明,拜仁达坝银多金属矿床与区域内众多的燕山期有色金属矿床一样,受控于大兴安岭地区区域地质构造活动历史及其所在的大地构造环境,即古生代末期至中生代早期该区域内的压性构造体制向中生代伸展构造体制的转换过程,区域上包括拜仁达坝矿床在内的众多多金属矿床,其成因主要与中生代期间区域内地壳伸展的成矿作用有关。
通过对围岩地质、岩石学及岩石地球化学等方面的详细研究表明,片麻岩的原岩主要为正常沉积碎屑岩,并有部分火山碎屑物质的加入。SHRIMP锆石U-Pb测年表明,锡林浩特杂岩、白音高勒石英闪长岩及北大山花岗岩体的形成年龄分别为409～437Ma、326±3Ma和140±3Ma,杂岩中锆石的增生边形成于323～350Ma之间,很可能代表杂岩受到区域变质作用的时代,这与白音高勒石英闪长岩的侵入时代能够对应,显示二者可能是同一次构造岩浆作用的产物。
研究表明,拜仁达坝矿床是明显受构造控制的后生脉状充填交代矿床,矿体以断裂构造为赋矿空间,并明显经历了岩浆热液蚀变与成矿作用,矿床具有多阶段成矿的特点,可以将岩浆热液成矿期划分为四个阶段,即:毒砂—石英阶段、多金属硫化物—石英阶段1、多金属硫化物—石英阶段2以及脉石—黄铁矿阶段。硫化物电子探针数据分析表明,方铅矿是矿床中最重要的银的载体矿物,银黝铜矿则是矿床中主要独立银矿物,并计算了矿物的平均分子式。
矿床中金属硫化物的~(206)Pb/~(204)Pb值变化于18.333～18.515之间,~(207)Pb/~(204)Pb值变化于15.532～15.656之间,~(208)Pb/~(204)Pb值变化于38.057～38.610之间,分布范围较窄,这些铅同位素数据可以拟合成一条异常铅混合线,显示铅同位素为两端元混合来源。对这些数据的研究表明,拜仁达坝银多金属矿床成矿物质的两端元之一可能是矿区围岩锡林浩特杂岩,另一端元可能位于深部,具有相对较低的μ值。矿床的成矿热液为岩浆热液,并以东矿段为热液成矿的中心。通过成矿物质对比发现,大兴安岭地区燕山期众多的多金属硫化物具有类似的成矿物质来源,具有共同的两端元混合历史。矿床中硫化物的δ~(34)S值为-4‰～1.6‰,显示硫来源于深部岩浆。石英中δ~(18)O值为11.6‰～14.3‰,萤石样品的δ~(18)O值为-15.8～-5.2‰,δD值为-75～-106‰,氢氧同位素研究认为成矿期的热液为发生过水岩反应的岩浆水,成矿期末热液主要为大气降水。
综合所有研究内容表明,拜仁达坝银多金属矿床是一个与岩浆热液有成因关系的中低温脉状矿床。
In recent years, the discovery and exploitation of plenty of polymetallic deposits in Daxing'anling area makes it a hot area for metallogenic research. It shows that there is a huge potential for polymetallic mineralization, and the Bairendaba Ag polymetallic deposit with million tons of base metal and thousands tons of silver is located in this area. We made systematic field study in Bairendaba ore district and its nearby area to get the fist hand data to describe the geological features of this deposit, and report new data of mineralogy, petrology, petro-geochemistry, geochronology and geochemistry of mineral deposits. Incorporating the recent research data from literature, we inspect the deposit with the latest hydrothermal metallgenic theory based on the geodynamic setting and plate tectonic model aimed at unveil the ore-forming process through the detailed studies on ore-forming substance source tracing, ore-forming fluid characters, mineralization controlling factors and metallogenic model, and so on. It will be useful for the further mineral exploration in this area.
The studies show that the distribution of abundant polymetallic deposits in Daxing'anling area including Bairendaba Ag polymetallic deposit is restricted in Mesozoic and related to the regional geodynamic evolution, particularly to the transformation from compressional tectonic mechanism in pre-Mesozoic to extensional tectonic mechanism in Mesozoic.
The detailed studies of geology, petrology and petro-geochemistry indicate that the protolith of Xilinhot complex were mainly clastic sediments, with a small part of volcanic debris. SHRIMP zircon U-Pb ages of Baiyingaole quartz diorite, Xilinhot complex and Beidashan granite are 409～437Ma, 326±3Ma and 140±3Ma, respectively. The ages of the overgrowth on the cores of zircon in Xilinhot complex range from 323Ma to 350Ma, and are consistent with the intrusive age of the Baiyingaole quartz diorite. It may suggest that the metamorphism of the Xilinhot complex and the intrusion of Baiyingaole quartz diorite were formed under the same tectonic-magmatic event.
Studies on the ore deposit show that the Bairendaba Ag polymetallic deposit is an epigenetic vein deposit. The orebodies are controlled by nearly E-W trending structures. Four stages of the ore-forming process can be recognized in Bairendaba deposit, i.e.: arsenopyrite- quartz stage, polymetallic- sulfide stage 1, polymetallic- sulfide stage 2 and gangue- pyrite stage. Electron probe analysis reveals that the galena is the most important silver bearing mineral, and the freibergite is the primary independent silver bearing mineral.
The sulfides in Bairendaba Ag polymetallic deposit have narrow range of ~(206)Pb/~(204)Pb ratios from 18.333 to 18.515, ~(207)Pb/~(204)Pb ratios from 15.532 to 15.656, and ~(208)Pb/~(204)Pb ratios from 38.057 to 38.610. All these data can fit a straight line, which indicate the lead was anomalous lead and was the mixture of two end-members. The ore-forming fluid was derived from magma, and the east part of the Bairendaba deposit was the center of hydrothermal ore-forming process. Theδ~(34)S values of sulfides in Bairendaba deposit vary from -4‰to 1.6‰, suggesting that the sulfur probably came from the deep magma. Theδ~(18)O values of quartz from ores range from 11.6‰to 14.3‰, and theδ~(18)O values of fluorite in gangue-quartz stage range from -15.8‰to -5.2‰, theδD values range from-75‰to -106‰, suggesting that the ore-forming fluid was magmatic water in the early stage, and evolved to meteoric water in the late stage.
In view of all the studies in this paper, the Bairendaba Ag polymetallic deposit was regarded as a meso- epi-thermal vein deposit related to magmatic fluids.
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