Sr-Nd-Pb isotopic compositions of volcanic rocks around the gold deposit, southwest Japan: implications for the contribution of a felsic lower crust

摘要

The Sr-Nd-Pb isotope compositions of Late Pliocene to Pleistocene volcanic rocks around the Hishikari mine in southern Kyusyu of Japan, one of the largest hydrothermal gold deposit in the circum-Pacific belt, were determined in order to elucidate their source characteristics and evolution and the characteristics of magma related to gold mineralization. The Hishikari volcanic rocks (HVR) are classified into the Kurosonsan group (2.4-1.0 Ma) in the northern area and the Shishimano group (1.7-0.5 Ma) in the southern area. Each group is composed of three subgroups of andesite and one subgroup of rhyodacite in a late-stage. In the Kurosonsan group, the Sr-87/Sr-86, Pb-206/Pb-204, Pb-207/Pb-204 and Pb-208/Pb-204 increase, while the Nd-143/Nd-144 decrease with decreasing age. The evolution pattern is opposite to this in the Shishimano group. On the Sr-Nd and Pb isotope diagrams, the HVR show a mixing array between two isotopically homogeneous components: a depleted component with a MORB-like composition, and an enriched component with radiogenic Pb and Sr and unradiogenic Nd. Regional comparison of isotope data from the HVR with those of rocks in and around Japan indicates that (1) the depleted component originated from low-K and high-alumina basaltic magma upwelling from the mantle, whereas (2) the enriched component was derived from rocks in lower crust of granodioritic composition, which we considered to have developed in the eastern margin of the Eurasian subcontinental lithosphere. According to a model calculation, the assimilation of the lower crust into the HVR increased with time from 51% to 77% in the Kurosonsan group, whereas decreased from 68% to 57% in the Shishimano group. We suggest that the Shishimano rhyodacite, which crystallized in the later stages of the Shishimano group and is considered to have been responsible for gold mineralization, was formed by intense fractional crystallization from andesite magma with a depleted Sr, Nd and Pb isotopic signature. (C) 2003 Published by Elsevier B.V. [References: 66]