In situ mineral chemistry of chlorite in Donghua area, Dehua‐Youxi‐Yongtai ore district, Fujian Province, south‐east China: Elemental characteristics and their implications for exploration

Abstract

As an ordinary alteration mineral in the process of fluid–rock interaction, chlorite (most especially its chemical composition) has the potential to become an effective tool to reveal the physicochemical conditions during alteration and help exploration. However, its elemental characteristics during the chloritization process are yet to be clearly understood. The Dehua‐Youxi‐Yongtai (DYY) ore district in south‐east China is one of the potential areas of gold and polymetallic deposits. Lying in the north‐west section of the DYY ore district, the Donghua area is notable for its remarkable metallogenic potential, and porphyry‐epithermal systems might have developed in the area. This study focuses on the elemental characteristics of chlorite in the Donghua area as footprints of the alteration process and mineralization vectoring. According to the geological features and occurrences, the chlorite from Donghua can be divided into two generations: (a) Chlorite I is closely related to chloritization developed in intrusive and volcanic rocks (Permian quartz monzobiorite and Jurassic volcanic rocks of Changlin Formation), and (b) chlorite II is accompanied by superimposed hydrothermal overprinting. The alteration process suggested by overprinting chlorite II can be regarded as almost coeval with the ca. 154–153 Ma magmatic event, and chlorite II is later than chlorite I generation. According to the geothermometry of the chlorite in Donghua, chlorite I might have crystallized on the temperature of 180–240°C with a peak of ~200°C, and the overprinting chlorite II might have experienced two episodes of hydrothermal/epithermal fluid pulsing. The mineral geochemistry of the trace elements in the two generations of the chlorite in Donghua shows different characteristics. The replacement of Mg2+ by Fe2+ plays an important role for ionic substitution in the octahedral position, especially for chlorite II. The other occasion might be Mg and Fe jointly entering the octahedral position as well as Mg‐ and Fe‐AlVI substitution mechanisms. The enrichment of Mg especially in chlorite II suggests low‐grade oxidation and acid conditions, which might be beneficial for the transportation of metallogenic substances. The obvious differences of Co and Ni are remarkable aspects of the trace elements of chlorite in Donghua, resulted by the ion substitution and function of different octahedral site preference energy. The mineral chemistry of chlorite II from Donghua can be helpful for targeting and exploration vectoring in the DYY ore district.