Propylite and Alteration Halo Around the Pb-Zn Vein of Toyoha Mine, West Hokkaido

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Propylite of Miocene epoch, host rock of the Pb-Zn vein of Toyoha Mine, is characterized by the presence of chlorite, calcite and albite. This rock was studied petrographically as well as chemically and it was clarified that this rock belongs to calc-alkali rock series. Based on the mineral assemblages, and the relative abundance of the minerals the alteration halo around the Tajima vein of this mine is divided into the following four zones of altered rock facies from outer zone to inner one: (a) Calcite-chlorite-albite (b) Calcite-sericite-chlorite-quartz (c) Calcite-chlorite-sericite-quartz (d) Quartz-sericite-pyrite The chlorite to sericite ratio in facies (b) is higher than that in facies (c), then these two facies are distinguishable. The zones of facies (b), (c), and (d) develop in both sides of the ore vein, while the distribution of the facies (a) is regional. The rocks of four altered rock facies were chemically examined and the following results were obtained: 1) Facies (a) is compositionally characterized by the presence of aboundant H2O compared with unaltered propylite. 2) In the zones of the facies (b) and (c), migration of various kinds of rock forming elements is remarkable though the original rock fabrics have been kept. 3) The zone of facies (d) is distinguishable from the preceding three zones in notable addition of SiO2, FeS2 and alkalis. It seems to the authors that the facies (a) alteration played a role of harbinger for the alteration around the ore vein. Facies (b) and (c) are gradational alteration zones perhaps formed by the reaction between ore forming fluid and wall rock. Facies (d) is a zone so intensely affected by the ore forming fluid that original rock fabrics were obliterated.
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Propylite and Alteration Halo Around the Pb-Zn Vein of Toyoha Mine, West Hokkaido

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