高取タングステン鉱床周辺の八溝層群堆積岩中の炭質物のグラファイト化度 鉱床形成に伴った熱的影響の解析と鉱床探査への応用

Abstract

The late Cretaceous Takatori tungsten deposit, located in the Yamizo Mountains of Japan, is hosted by sedimentary rocks of the Mesozoic Yamizo Group. The degree of graphitization of carbonaceous material in the sedimentary rocks has been used to examine the thermal influence of the tungsten mineralization. Several hundred samples from surface outcrops (n=392) covering an area of 400km2, together with drill cores (n=134), have been analyzed. The degree of graphitization of carbonaceous material separated from the rock samples was measured by X-ray diffraction, whic is given by the GD value, defined as 100x(d002-3.7)/log(Lc(002)/1, 000); where Lc (002) and d002 are the crystallite thickness and the lattice spacing, respectively. The GD value is set as 1, 000 when Lc (002) exceed 1, 000. An empirical calibration of temperature for the GD parameter elsewhere suggests an approximate temperature of 400°C for a GD value of 30. Most of the samples from the surface outcrops have GD values around 14. In the southern part of the study area near a local granite pluton, high GD values have been measured indicating contact metamorphic effects. In an area covering about 2.5×1.5km surrounding the Takatori tungsten deposit, GD values are greater than 30. Moreover, GD values up to 1, 000 are found even closer to the deposit. The steep gradient of GD values associated with the Takatori deposit is in contrast to the shallow GD gradient related to the contact metamorphic aureole found in the southern part of the study area. High temperature hydrothermal fluids which formed the Takatori tungsten deposit are considered to be responsible for the elevated GD values around the deposit. The heat associated with hydrothermal activity can be transferred by infiltration of such fluids into the Yamizo Group. This study shows that an area affected by hydrothermal fluid can be detected by GD data with an appropriate sampling density. Therefore the degree of graphitiza-tion can be utilized for mineral exploration.