Chlorite mineralogy, geochemistry and exploration implications: A case study of the Xiaokelehe porphyry Cu-Mo deposit in NE China

Abstract The recently discovered Xiaokelehe porphyry Cu-Mo deposit (PCD) is situated in the northern part of the Great Xing’an Range in NE China. At Xiaokelehe, chlorite-illite alteration is characterized by chlorite, illite, disseminated chalcopyrite and pyrite assemblages that are widely developed in the granodiorite porphyry. The illite is altered from plagioclase or K-feldspar, and the chlorite from both igneous and hydrothermal biotite. The chlorite can be divided into three types (Chl1, Chl2 and Chl3): Chl1 is locally developed in the deeper part of the granodiorite porphyry, and is altered from hydrothermal biotite; Chl2 is pervasively distributed in the deeper part of the granodiorite porphyry, and is altered from igneous biotite and associated with abundant disseminated Cu-Fe-sulfide mineralization; Chl3 is widely developed in the shallower part of the granodiorite porphyry, and is altered from igneous biotite. Chl3 is overprinted by phyllic alteration and associated with minor disseminated Cu-Fe-sulfide mineralization. These Xiaokelehe chlorites resemble the chlorites from typical sericite-chlorite-calcic alteration (SCC) in PCDs, and are the first reported SCC-related chlorites in the Great Xing’an Range. Mineral geochemical results show that Chl1 has relatively high FeO, Mn, Ti, Li and Zn contents. Chl2 has relatively high FeO, Mn, Co, Ni, Zn and Sr contents, and Chl3 has relatively high MgO, K and Ni contents. These geochemical differences are likely controlled by temperature (e.g., for Ti), precursor minerals (e.g., for FeO and MgO) and fluid compositions (e.g., for FeO, MgO, Mn, Co and Zn). All Chl1 to Chl3 are classified as trioctahedral chlorite. The Fe-Mg, Tschermark and di-trioctahedral substitution mechanisms may have controlled the chemical differences of the Xiaokelehe chlorites. Empirical thermometer yielded chlorite formation temperatures of 209 to 314 °C, indicating that the Xiaokelehe chlorites were formed from intermediate-high temperature hydrothermal fluids. Compared with the chlorites from propylitic zones in other PCDs, the Xiaokelehe chlorites have higher FeO, Ti, V, Zn and Ga contents and Fe/(Fe + Mg) ratio, as well as lower MgO, K, Co and Sr contents, which distinguish chlorites in the SCC zone from those in the propylitic zone. Such differences may have also been influenced by temperature, and the protolith and fluid compositions. Compared with typical metamorphic chlorites, the Xiaokelehe chlorites have lower FeO content but higher MgO and Zn contents, of which the Zn difference is likely influenced by the fluid chemistry. We propose that the presence of Chl3 can be used as an effective vector toward mineralized zone. In addition, chlorite Mn and Zn contents in the mineralized zone are clearly higher than those away from it, and thus the chlorite Mn and Zn contents can also be potential exploration vectors at Xiaokelehe.