浙东南后坑酸性蚀变岩帽地质及矿物学特征

Geological and mineral characteristics of the Houkeng lithocap in the southeastern Zhejiang

蚀变岩帽在地表一般表现为陡倾的正地形,是高温、高氧逸度流体对围岩进行酸性淋滤,形成的一系列高级泥化和泥化等蚀变矿物组合。这种蚀变是浅成低温热液蚀变系统的一部分,其深部可能具有寻找斑岩型矿化的潜力。浙东南陆相火山岩地区普遍发育此种蚀变岩帽,但该类型蚀变常被前人定义为"次生石英岩"。本文选择其中典型的后坑蚀变岩帽,通过大比例尺蚀变填图、岩心编录,利用短波红外(ASD)、电子探针(EPMA)、扫描电镜(SEM)、能谱仪(EDS)等分析技术手段,发现蚀变岩帽具有垂直方向和水平方向的蚀变矿物分带。垂直方向由深到浅依次发育刚玉-绢云母,绢云母-叶蜡石,明矾石-高岭石-地开石具分带特征;水平方向上从核部到边部依次发育多孔状石英-金红石、高级泥化带(明矾石-叶蜡石-高岭石-地开石-水铝石-Aluminum Phosphate and sulphates矿物)、泥化带(高岭石-伊利石-蒙脱石)。并得出以下几点认识:①后坑蚀变岩帽中明矾石短波红外特征吸收峰在1480~1491nm之间,以岩浆-热液成因的钾明矾石为主,含少量钠明矾石;②叶蜡石短波红外特征吸收峰在2167nm左右,分为两种成因:早期高级泥化阶段的绢云母经后期热液退变质蚀变而成以及早期高岭石经后期热液进变质作用形成;③蚀变岩帽根部的刚玉被后期热液蚀变形成绢云母,刚玉-绢云母蚀变带之下可能是黄铁绢英岩化带;④早期水铝石被后期热液蚀变形成地开石,代表了热液的多期次性;通过对比后坑蚀变岩帽与其他斑岩-浅成低温热液矿床(如Horse-Ivaal和Vuda等矿床)的蚀变矿物分带后,发现后坑蚀变岩帽蚀变矿物分带与这些典型矿床相似。本文认为后坑蚀变岩帽属于该成矿系统上部的浅成低温热液蚀变部分,其深部具有寻找斑岩矿床的潜力,同时提出浙东南其他发育在陆相火山岩中的蚀变岩帽也属于该蚀变系统,具有寻找斑岩型铜金矿床的较大潜力。

Lithocaps generally show steep topography on the surface.They are formed by high temperature and high oxygen fugacity acid hydrothermal fluid leaching the surrounding rocks to form a series of advanced argillic and argillic mineral assemblages.These alteration minerals are a part of the epithermal alteration system and show the potential to search for porphyry mineralization underneath."Secondary quartzite",a term defined by predecessors to describe alteration systems,are widely developed in volcanic systems in the southeastern Zhejiang Province.In this paper,we investigate the Houkeng lithocap using large-scale alteration mapping,core logging,and using short-wave infrared(ASD),Electron probe microanalyzer(EPMA),scanning electron microscopy(SEM),Energy Dispersive Spectrometer(EDS)and other analytical techniques.We found that the lithocap shows vertical and horizontal alteration zoning.Vertical alteration grading occurs from corundum-sericite at depth,through sericite-pyrophyllite,to alunitekaolinite-quartz zoning at shallow levels.Horizontally,vuggy quartz-rutile developed in the core,grading to advanced argillic(alunite-pyrophyllite-kaolinite-diaspore-diaspore-Aluminum phosphate and sulphate minerals),and argillic(kaolinite-illite-montmorillonite)alteration.We reached the following conclusions:①The short wavelength infrared(SWIR)spectrum absorption peak of the Houkeng alunite ranges from 1480 nm to 1491 nm.These alunite,originating from magmatic-hydrothermal alteration,are mainly composed of potassium alunite,with a small amount of sodium alunite;②Pyrophyllite,whose short wavelength infrared(SWIR)spectrum absorption peak at 2167 nm,shows two origins:sericite retrograde alteration and kaolinite metamorphism;③corundum is altered to sericite by hydrothermal alteration in the later stage,and phyllic alteration may develop below the corundum-sericite zone;④ diaspore is transformed into dickite by later hydrothermal fluid,which indicates multiple stages of fluid alteration.Comparing the lithocap with the other porphyry-epithermal deposits’ alteration zonation(like Horse-Ivaal,Vuda et al),we propose that the Houkeng lithocap is the upper part of the porphyry-epithermal alteration system and shows the potential for locating porphyry deposits beneath the base of the lithocap.At the same time,it is suggested that other lithocaps developed in volcanic rocks in southeastern Zhejiang also belong to this alteration system and hold great potential for prospecting for porphyry copper-gold deposits.