Characteristics of pyrophyllite and hydrothermal alteration at Argotirto area, Malang Regency, East Java, Indonesia

Abstract

Abstract Argotirto area located in South Malang, East Java province, has abundant non-metallic mineral resources, e.g., pyrophyllite. The presence of pyrophyllite indicates that the area has undergone hydrothermal alteration processes. This research aims to determine characteristics of pyrophyllite and hydrothermal alteration at Argotirto and its surrounding area. The methods used in this research comprises geological and hydrothermal alteration mapping, thin section and X-ray diffraction (XRD) analyses from rock samples. There are five lithological units from the oldest to the youngest, namely altered basalt, altered andesite breccia, altered dacite intrusion, altered dacitic tuff breccia, and limestone units. In hand specimen, pyrophyllite has white to tan colour, massive and waxy/soapy textures. Furthermore, in thin section, pyrophyllite selectively replaces feldspar phenocrysts or it is disseminated in the matrix and intergrown with quartz. In the research area, there are four alteration zones, such as pyrophyllite-dickite-sericite-illite, pyrophyllite-vermiculite-alunite-tridymite, pyrophyllite-cristobalite-sericite-illite-smectite, and pyrophyllite-halloysite-sericite-illite zones. According to hydrothermal alteration mineral assemblages, it can be interpreted that those minerals are formed at low pH (approximately pH 4-5) and relatively low temperatures (100-<250°C).