Geneses of high chlorine and silver-lead-zinc-mineralized granitoids in Tsushima, Japan

Miocene granitoids of the Tsushima Islands have unique characteristics that cannot be seen in other major granitic plutons in the Japanese Islands as follows: (1) They are granitic in composition but contain synplutonic mafic dikes, abundant mafic enclaves, and intermediate facies between granite an d mafic enclaves. (2) They are mixture of magnetite-bearing and -free facie s, but generally magnetite-free in the marginal part. (3) They are high in K2O content (K-65=3.1) and intermediate in normative corundum (C-65=0.1) an d delta O-18 value (+9 parts per thousand at SiO2 70 %), which may be compa rable with those of the Miocene Outer Zone granitoids. (4) Yet the initial Sr ratio is low as 0.7037. (5) They are high in Cl and S, which occur in fl uid inclusions and as pyrrhotite>pyrite, respectively. Two genetic models are considered for the source of the unique granitoid ma gmas: the continental crust or the upper mantle fertilized with Si, K and O -18. The latter may be the case for the Tsushima granitoids, because of the low initial Sr ratio. The age of the granitoids (16 Ma) indicates the magm atism related to the opening of the Sea of Japan. Tt is suggested that both basaltic and granitic magmas were generated in the continental lithosphere under an extensional tectonic setting: the two magmas could have been part ly mingled. The mingled magma was originally an oxidized type, but reduced during the e mplacement by repeated inflow of S and C-bearing gases from the pelitic wal l rocks. Because of the: reduction, SO3 sulfur is almost nil in the rock-fo rming apatite, and most of sulfur remained in fluid phase of the magma as r educed species. Cl content was high in the original magma and concentrated in the fluid phase of the residual system which dissolved silver, lead and zinc metals. Such a fluid migrated into the Taishu fracture systems, as the magma crystallized, and formed the silver-lead-zinc deposits.