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 {<(1)over bar>13} in the XRD results, whereas the second characteristic peaks {022} have good variations. The half-height width of the 2. 40 angstrom 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.