Chlorite alteration in porphyry Cu systems: New insights from mineralogy and mineral chemistry

Chlorite, an important mica-like clay mineral formed in a variety of geological environments, is sensitive to bulk rock composition and physicochemical conditions during formation, and thus an ideal mineral to investigate the nature of fluid-rock reactions and mass transfer during hydrothermal fluid interaction. Samples of chlorite from alteration systems around three large porphyry Cu deposits, Tuwu (NW China), Atlas (Philippines) and Xiaokelehe (NE China), were used to investigate element transfer from pyroxene, amphibole, and biotite to chlorite. By calculating the chlorite formation temperatures using the chlorite empirical thermometer, the temperature ranges for biotite, hornblende, and pyroxene chloritization in this study were broadly the same (around 270 °C). Contacts are gradational between biotite and chlorite grains along biotite cleavage but abrupt between hornblende/pyroxene and chlorite. Concentrations of K2O, Na2O, TiO2, and SiO2 decreased gradually from biotite to chlorite, whereas MgO, FeOT, Al2O3, and MnO concentrations increase. Different forms of hydrothermal titanite grains occurred commonly in the biotite, hornblende, and pyroxene chloritization processes in the porphyry systems studied, and titanite formed during chloritization of biotite had higher Al2O3 and F contents compared to titanite that formed during chloritization of hornblende and pyroxene. Given the high Al2O3 and F contents in biotite, this suggested the concentrations of these components in titanite may be controlled by the composition of the precursor minerals. The FeOT and MgO contents of chlorite correlated with the composition of the precursor mineral, whereas their TiO2 and Al2O3 contents did not, with all chlorite having similar contents. This suggested that FeOT and MgO contents of chlorite were more likely controlled by the precursor minerals, whereas TiO2 and Al2O3 were probably controlled by the similar formation temperature of chlorite without obvious influence of precursor minerals. Thus, TiO2 and Al2O3 contents of chlorite could be more suitable for use as a mineral geochemical vector for hydrothermal center in porphyry deposits. •Chlorite is more likely to inherit part of biotite structure compared to hornblende and pyroxene.•Different forms of hydrothermal titanite grains occur commonly in the chloritization processes in the porphyry systems.•FeOT and MgO contents of chlorite are likely controlled by precursor minerals.