Variability of Alunite Quartzite Composition as a Reflection of the Characteristics of Its Genesis

Abstract

Based on 5000 analyses of geological prospecting samples from four alunite deposits (Began’, Ukraine; Haft Sanduq, Iran; Bolshaya Kremenyukha and Kachar, Kazakhstan) and 6000 microprobe analyses of alunite, the features of changes in three parameters of alunite quartzite composition have been studied: (1) alunite content, (2) mole fraction of K in alunite, and (3) correspondence of the amount of K and Na to that of alunite sulfur. All three parameters are highly variable. A linear correlation between them has been identified in over half of the exploration workings. This correlation is sometimes very strong, approaching a functional relationship, especially for two: the alunite content and mole fraction of K in alunite. In the first three of the above-mentioned deposits, this correlation is almost always positive, and in the last, it is always negative. In other words, as alunite content increases, it becomes higher-K in the first case and higher-Na in the second case. Alunite quartzite makes up a regular zone of the infiltration metasomatic column of sulfuric acid alteration in aluminosilicate rocks under conditions apparently close to isothermal. However, the highly variable contents of the basic minerals (alunite and quartz) and alunite composition point to a difference between the alunite quartzite formation model and the theoretical model for isothermal infiltration metasomatic rock. According to the theoretical model, the solution and rock compositions mainly change at the boundaries of the metasomatic zone, whereas the rock and mineral compositions are not characterized by significant variations within the zone. During alunite quartzite formation, the solution and rock compositions change not only at the boundaries, but also within the zone due to ongoing oxidation of sulfur compounds in the solution with the sulfate formation. An increasing sulfate concentration leads to the solution becoming supersaturated in relation to alunite and causes its precipitation, with filling of voids and displacement of quartz. Cooling, heating, or a change in the composition of the hydrothermal system solutions lead to temporal variations in the K/Na ratio in precipitated alunite. Alunite formation inside the alunite zone, accompanied by changes in the mole fraction of K in deposited alunite, is the reason for the high variability of the alunite quartzite composition and for the correlation between the alunite content and its mole fraction of K. If the contribution of higher-K alunite to the variability of its total content is higher than that of higher-Na alunite, a positive correlation is likely to appear, otherwise a negative correlation is more probable.