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Alunite is used as a representative mineral for indicating deposits in lithocaps, and lithocaps are generally related to the porphyry–(high-sulfidation) epithermal mineralization system. The study of alunite is of theoretical and exploration significance for prospecting potential underlying porphyry and epithermal deposits. Studies on alunite geochemistry have made breakthroughs, but there is little research on alunite mineralogy, for example, using scanning electron microscopy (SEM) images, differential thermal analysis (DTA), and Fourier-transform infrared spectroscopy (FT-IR). This study mainly focuses on alunite micromorphological characteristics, weight loss changes with temperature, and ionic group structure, aiming to identify the relationship between these features and indications for prospecting. The Fanshan lithocap is located in the northwest part of the Luzong basin, Anhui province of China, and it can potentially be used for exploring porphyry and epithermal deposits. Fanshan alunite is formed in two stages with three types of alunite. IA alunite is formed in the early hydrothermal stage and replaces felsic minerals in the Zhuanqiao Formation, IB alunite is formed in the later hydrothermal stage and fills in open spaces with bladed particles, and II alunite is the product of pyrite oxidation and reaction with other minerals in the supergene stage. Alunite electron microprobe data and energy-dispersive spectroscopy data further confirm temperature decreases with hydrothermal evolution, and the presence of a high-sulfidation epithermal system in the Luzong basin. Aside from the forming environment, SWIR, and geochemistry of alunite, there are other indication indexes; for example, the larger peak values at 3480 cm−1 and smaller peak values at 1080 cm−1 in FT-IR spectra and the deeper exothermic valleys at 750 °C and steeper weight loss slopes in the DTA curve suggest a favorable formation environment for alunite and provide valuable indications for deposit exploration and assessments of mineralization potential. [ABSTRACT FROM AUTHOR]

Characteristic samples from the No. 1 ore body of the Jinlong bauxite deposit in Guangxi Province, China, were assessed to determine their chemical composition using whole-rock geochemical analysis, their mineral components using X-ray diffraction, their thermal characteristics via differential thermal analysis, thermogravimetry and differential thermogravimetry, and their dissemination characteristics by scanning electron microscopy/energy-dispersive spectrometry. The results show that the average AlO content in the Jinlong bauxite deposit is 41 % and that the AlO/SiO ratio is approximately 2.17, and thus, the ore may be defined as low-grade bauxite. The main minerals found in the ore were diaspore (20.3-43 %), gibbsite, clays and hematite. In differential thermal analysis results, the bauxite samples exhibit two distinct endothermic peaks, with the dehydration of gibbsite occurring in the 200-260 °C range and the dehydration of diaspore occurring in the 524-550 °C range. Thermogravimetry and differential thermogravimetry indicate four decomposition stages: evaporation of adsorbed water (50-160 °C), gibbsite decomposition (190-280 °C), goethite decomposition (190-280 °C) and diaspore dehydroxylation (460-580 °C). The diaspore belongs to the orthorhombic dipyramidal class and exists primarily in three dissemination forms-euhedral and subhedral grain, bean and oolitic shaped, and aphanitic and microcrystalline aggregates, and is always accompanied by kaolinite and pyrophyllite. [ABSTRACT FROM AUTHOR]

Abstract: In recent decades, more than 0.5 billion tons of ores scattered in the Quaternary laterite in western Guangxi, China have been explored. The ores were derived from a bauxite horizon in Permian via physical break-up and re-sediment process. Utilizing various test methods, i.e., XRD, DTA, TG/DTG, SEM/EDS and EPMA, the mineralogical characteristics of the Quaternary bauxite ores in Jingxi and Debao counties were investigated. XRD was used together with TG/DTG to obtain relatively accurate ore mineral abundance. Diaspore is the major phase, whereas hematite, kaolinite, anatase, chamosite, gibbsite, goethite, illite and rutile are minor. Diaspore is characterized by a small particle size, low degrees of crystallinity and complex chemical composition. Both gibbsite and goethite have a varied particle size, and goethite crystals contain high Al substitution and Si. It is clarified that diaspore, chamosite and anatase were formed in a mildly reduced and alkaline depositional environment in Permian, while gibbsite, hematite, goethite and part kaolinite were precipitated from Al3+-, Si4+- and Fe3+-enriched solutions within an Quaternary oxidized environment. The ions Al3+, Si4+ and Fe3+ are mostly released from chamosite in its dissolution process. The different physicochemical conditions between the Permian depositional and the Quaternary weathering periods resulted in a complex mineral assemblage in the Quaternary bauxite. [Copyright &y& Elsevier]