西藏铁格隆南铜（金银）矿床地质特征及勘查模型--西藏首例千万吨级斑岩-浅成低温热液型矿床

摘要

铁格隆南矿床(荣那矿段)位于多龙矿集区多不杂斑岩型铜(金)矿的北东中部,铜(金)矿体的主要赋矿围岩为早白垩世中酸性斑岩及早中侏罗统色哇组(J1-2s)复理石或类复理石建造；下白垩统美日切错组(K1m)陆相中基性火山岩建造(110 Ma)覆盖在斑岩-浅成低温热液成矿系统之上,是该矿床得以良好保存的重要原因。通过详细的地质填图、地质编录、岩石地球化学、镜下鉴定、电子探针分析、流体包裹体分析、同位素年代学、岩石地球化学测量、音频大地电磁测深和综合研究,确定了矿体形态整体上为一个似筒状体,铜(金、银)矿体总体呈北东向,控制矿体北东延长约2000 m,南东—北西向约1200 m, Cu平均品位大于0.5%,伴生Au品位0.08 g/t,伴生银品位2 g/t左右,单个矿床铜资源量约1100万吨,远景资源量超过1500万吨。矿石以典型的细脉浸染状为主,从浅部至深部,金属矿物由铜蓝、蓝辉铜矿、斯硫铜矿、雅硫铜矿、久辉铜矿、斜方蓝辉铜矿及黄铜矿、斑铜矿(微量辉钼矿),主要非金属矿物有明矾石、地开石、高岭石、水铝石、绢云母、石英、硬石膏等组成(图2-17),矿物和蚀变组合显示高硫型浅成低温热液矿体叠加于斑岩型矿体。成矿岩体为120 Ma侵位的花岗闪长斑岩,辉钼矿Re-Os同位素年龄显示斑岩成矿形成于119 Ma左右,明矾石Ar-Ar同位素测年结果为116 Ma左右,表明浅成低温热液型矿化略晚于斑岩型矿化。由于羌塘地体南缘的不断隆升,导致浅成低温热液矿化持续向深部迁移,形成巨厚典型的浅成低温热液矿物组合。流体包裹体研究表明,流体包裹体均一温度显示存在2套明显不同温度的流体,即形成斑岩及其围岩钾硅化与绢英岩化带矿化的中高温流体以及高级泥化带中的中低温流体,高温、高盐度的流体代表了斑岩系统的流体特征,而中低温、低盐度的流体代表了高硫化型浅成低温热液系统的流体特征。铁格隆南矿区的岩体侵位较浅,浅部岩体已经在 ZK1604与美日切错组呈角度不整合接触,表明在美日切错组火山岩喷发覆盖于矿体上之前,含矿岩体已经出露地表,根据其估算形成深度(约1.2 km),可以判断在矿床形成之后地壳已抬升了一定高度,斑岩-浅成低温热液系统上部的矿体和独立金矿体可能已被剥蚀。岩石微量元素和稀土元素特征表明,均为高钾钙碱性,富集大离子亲石元素,亏损高场强元素,具弧岩浆地球化学特征；在Y-Th/Ta和Ta/Yb-Th/Yb图解中多龙地区主要侵入岩均位于活动大陆边缘,在Th-Co-Zr/10和Th-Sc-Zr/10微量元素构造图解中,大部分样品落在陆缘弧区域。区域上构造岩浆活动也表明多龙地区岩浆岩形成与班公湖—怒江特提斯洋早白垩世北向俯冲至南羌塘地体之下密切相关,类似于南美安第斯成矿带洋陆俯冲背景下的陆缘弧环境。锆石铪同位素落在球粒陨石与亏损地幔之间,地壳模式年龄相对年轻与多龙地区主要侵入岩一致,表明岩浆主要来源亏损地幔,同时也有部分新生地壳物质的加入,岩浆表现出角闪石源区作为主要源区熔融矿物的特征,表明班公湖-怒江特提斯洋壳俯冲至50~70 km下发生相变,从而导致类似角闪石等矿物脱水引发的楔形地幔部分熔融。短波红外光谱填图和岩石地球化学剖面测量显示在尕尔勤、地堡那木岗还有寻找铁格隆南(荣那矿段)式矿床的潜力,区域找矿潜力极大,多龙矿集区铜资源量可以达到2500万吨以上。%The Tiegelongnan deposit is located in the middle of the northwest Duobuza porphyry Cu-Au deposit, and its primary ore host rock are Early Cretaceous porphyries and a suite of flysch formation or fly-schoid formation of Middle-Lower Jurassic strata, named Sewa Formation (J1-2s). A suite of Meiriqiecuo intermediate-basic volcanic formation (K1m), which was formed at 110 Ma in the post metallogenic period, lies upon the porphyry metallogenic system and is proved to be an essential element of ore preservation. With such means as detailed geological mapping, geological logging, lithogeochemical measurements, microscope observation, electron microprobe analysis, fluid-inclusion analysis, isotopic geochronology, audio magnetotelluric sounding and comprehensive research, the authors detected that the Cu-Au-Ag deposit is generally like a cylinder, trending northeast and stretching for about 2 000 meters in the northeast-southwest direction and 1 200 meters in the northwest-southeast direction. The average grade of Cu is greater than 0.5 percent and the average grade of associated Au and Ag is about 0.08 g/t and 2 g/t, separately. The resource of Cu is estimated to be about 11 megatons, and the prospective resources could be more than 15 megatons. Ores are mainly of typical veinlet disseminated structures. From top to bottom, the metallic minerals vary in a certain sequence, i.e., covellite, digenite, spionkopite, yarrowit, djurleite, anilite, chalcopyrite, bornite and a small amount of molybdenite, and the nonmetallic minerals vary in the same way, i.e., from top to bottom there are alunite, dickite, kaolinite, diaspore, sericite, quartz, anhedritite and so on. The typical minerals and the alteration mineral assemblage indicate that the deposit belongs to a porphyry-epithermal metallogenic system. The isotopic dating of different minerals gives us a clear mineralization sequence that the mineralization of epithermal system is later than porphyry’s. To be specifically, the data of granodiorite-porphyry yields a diagenesis age of 120 Ma, which is older than the Re-Os age of molybdenite of the porphyry stage which is about 119 Ma, and the Ar-Ar age of alunite of the epithermal stage is about 116 Ma which is younger than the two ages mentioned above. The location of epithermal deposits was likely to move to a lower altitude continually due to the continual uplifting of the south side of Qiangtang Terrane, which could make a contribution to the formation of the typical extremely thick orebodies of the epithermal type. Two different kinds of fluid were recognized through the homogenization temperature in fluid inclusion study. The porphyry metallogenic system is characterized by high temperature and high salinity fluid, whereas the epithermal metallogenic system is characterized by low temperature and low salinity fluid. The emplacement location of the intrusions in the Tiegelongnan deposit is fairly shallow and the boundaries between the intrusions and the upper strata of Meiriqiecuo Formation are unconformable in the drill hole of ZK1604, which could be an evidence of the outcropping of the orebodies in the post ore-forming period, and in the meantime it is probable that the upper part of epithermal Au orebodies of the porphyry-epithermal deposit system was eroded, which is based on the metallogenic depth estimation that the upper orebodies exposed at surface had formed at the depth of about 1.2 km. The characteristics of the trace elements and the rare earth elements in the intrusions indicate that the intrusions belong to high-potassium calc-alkaline series, which are enriched in LILE and depleted in HFSE and can represent the geochemical characteristics of arc magma. In the Duolong ore concentration area, almost all the samples fall in the active continental margin zone in Y-Th/Ta and Ta/Yb-Th/Yb diagrams, and most of the samples fall in the continental marginal arc zone in Th-Co-Zr/10 and Th-Sc-Zr/10 trace element tectonic diagrams. The tectonic-magmatic activities also suggest that the formation of Duolong intrusions was closely related to the southward subduction of Bangong-Nujiang Tethys Ocean toward the Qiangtang terrane in Early Cretaceous, which was similar to the situation of tectonic setting of Andes metallogenic belt. The fact that the magma originated from depleted mantle and was contaminated by the crustal materials is primarily based on the Hf isotopic studies, which could be known from the Hf isotope data that they fall between chondrite and depleted mantle evolutionary lines and the Hf crustal modal ages are approximately the same as the intrusions there. Besides, it is likely that the hornblende might have been the main source of the melt magma, which indicates that the Bangong-Nujiang Tethys Ocean crust might have undergone a phase transition in the 50~70 km depth, which could induce the partial melting of the mantle by dehydration of such water-rich minerals as hornblende. It turns out that more deposits resemble Tiegelongnan or Rongna type could be uncovered in Gaerqin and Dibiaonamugang according to the short infrared spectra mapping and petrogeochemical profile measurement. There is a great potential for finding more Cu and the resource quantity of Cu is assessed to be more than 25 megatons in the Duolong ore concentrated area.