斑岩铜矿围岩蚀变绿帘石的光谱特征——以德兴铜矿富家坞矿区为例

斑岩型矿床具有十分广泛和明显的热液蚀变带,青磐岩化通常位于中心钾化带外侧,可分为三个亚带,绿帘石是青磐岩化带最重要的蚀变矿物之一,但目前对青磐岩化带内各亚带之中的绿帘石的特征研究较为薄弱。近年来,短波红外光谱技术(SWIR)已经广泛应用于热液蚀变矿物的识别,但由于短波红外光谱通过识别特定的基团进而识别含此基团的矿物,难以识别斑岩中部分关键蚀变矿物,如钾长石、钠长石和硬石膏。X射线衍射技术(XRD)能识别大部分蚀变矿物,可对红外光谱技术在斑岩矿床中的应用进行补充。本文以江西省德兴地区富家坞矿区为研究对象,提出应用SWIR和XRD分析辅助蚀变填图。富家坞矿区发育了十分广泛的绿帘石化,根据其共生矿物组合及空间分布,划分了三种绿帘石类型。第Ⅰ类:矿物组合为绿帘石-钠长石-石英-方解石,绿帘石和钠长石以脉状形式产出于钾化花岗闪长斑岩体内,穿切钾长石斑晶;第Ⅱ类:矿物组合为绿帘石±绿泥石-石英-方解石,绿帘石和绿泥石以集合体形式交代早期岩体内黑云母、斜长石的方式产出,并保留有原生矿物晶型,伴有方解石、石英等矿物;第Ⅲ类绿帘石共生矿物主要为石英、方解石、沸石等,含少量高岭石,主要呈不规则脉状,分布于蚀变花岗闪长斑岩外侧。通过对以上三类绿帘石进行系统SWIR和XRD分析,发现类型Ⅰ绿帘石大部分表现出较大的Fe-OH吸收峰位值(Pos2252 2255),类型Ⅱ和类型Ⅲ绿帘石大部分表现为较小的Fe-OH吸收峰位值(Pos2252 2255)。XRD结果显示绿帘石晶体特征更为明显,特征衍射峰位值{■13}晶面表现出相对集中的特征,但是次峰{022}晶面表现出一定的差异性,且绿帘石{022}晶面2. 40?衍射峰半高宽(FWHM-2)与绿帘石Dep2334/Dep2252值呈现负相关的关系。故本文认为绿帘石的次峰{022}晶面2. 40?衍射峰值及其半高宽(FWHM-2)可以作为讨论斑岩矿床围岩蚀变矿物绿帘石的结晶度的主要参数。三种绿帘石结晶指数的差异可能是热液流体演化过程中温度降低和距离岩体中心的远近造成的。

The hyperspectrum characteristics of epidote in wall rock alteration in porphyry Cu deposits: An example from Fujiawu deposit, Dexing district, Jiangxi Province

Porphyry deposits always have widespread and distinctive hydrothermal alteration zones, as typically shown by a potassic alteration zone in the central domain and passing outward laterally into the propylitic alteration zone which can be further subdivided into three subzones. Epidote, as one of the most pervasive distributed minerals in the propylitic alteration zone, has limited studies about its characteristics in the subzones. Hyperspectral technology (Short-Wave Infrared Spectroscopy, SWIR) has been widely used in the identification of hydrothermal alteration minerals. However, SWIR can''t identify many key alteration minerals in the porphyry environment, such as orthoclase, albite and anhydrite, because it is only sensitive to certain molecules and radicals. X-Ray Diffraction (XRD) technology instead can discern the major alteration minerals, which is a good supplemental tool for SWIR. In this study, SWIR and XRD analyses have been conducted to aid the alteration mapping of the Fujiawu deposit in Dexing district, Jiangxi Province. Epidote developed intensively at Fujiawu. According to mineral assemblages and their spatial distribution, we classified those epidotes into three types. Type I epidote mainly associated with albite, quartz and calcite, and hosted in veins within the potassic altered granodiorite porphyry, always crosscuting the K-feldspar phenocrysts. Type II epidote intergrown with chlorite, minor calcite and quartz, replacing early mafic minerals, with the primary rock textures preserved. Type III epidote associated with quartz, calcite, zeolite, and locally kaolinite in irregular veins, distributed in the outside of porphyry. Systematic analyses of the three types of epidotes have been conducted by SWIR. Type I epidote mostly shows high values of Fe-OH absorption peaks (Pos2252 > 2255), whereas the majority of type II and type III epidotes show lower values of Fe-OH absorption peaks (Pos2252 < 2255). All the three types of epidote show the clustered peaks in the primary characteristic diffraction{113}in the XRD results, whereas the second characteristic peaks {022}have good variations. The half-height width of the 2.40Å peak (FWHM-2) shows the negative correlation with Dep2334/Dep2252 value of epidote. Therefore, this study has selected the secondary diffraction peak{022}and its FWHM-2 values as the main parameters, which are the response of the crystallinity of epidote. The differences of the crystallinity index of three types of epidote may be caused by the physiochemical conditions and fluid chemistry such as temperature reduction during hydrothermal evolution and the distance from the center of the intrusion.