CHARACTERISTICS AND INFLUENCE FACTORS OF ELEMENT MIGRATION OF HYDROTHERMAL ALTERED ROCK IN EASTERN MANUS BASIN

Five fresh and one altered volcanic rock samples from Eastern Manus Basin were chosen as research materials in this study. The major and trace elements of the bulk samples and the in-situ plagioclase of these rocks are analyzed. Comparative studies of altered rock and fresh rocks are carried out to explore the characteristics and influence factors of element migration during the hydrothermal alteration. As results, the hydrothermal sulfide of altered rock is a sort of copper-rich sulfide which mainly composed of chalcopyrite, sphalerite and ilmenite. Silicified alteration is the main type of alteration as the SiO₂ content is extremely high in the altered area. The contents of major and trace elements of the altered rock change around the average of the content of basaltic andesite and dacite. Andesite is believed the protolith of the altered rock. During the process of hydrothermal alteration, the mass of the altered rock is increased by 150%. The altered rock is most enriched by the elements of Cu, Zn, Ga, Sr, Cd, Ba and Pb, enriched by Ti, V, Mn, Co, Ni, Mo and U, less enriched by Si and Fe, and least enriched by Sc, Cr, Nb, Ta, W and Bi. The altered rock is most depleted by Be and Ca, more depleted by Li, Na, Mg, K, Rb, REE and Y, and least depleted by Zr, Cs and Th. Al, P and Hf are immobile elements. The influence factors of element migration of the hydrothermal altered rock may include mineralization, metasomatism (such as chloritization and silicification) and the activity of elements in fluids.