Pinkish colored epidotes found in a geothermal exploration well NB–1, Noboribetsu, Hokkaido

Abstract

Occurrence and mineralogical properties of pinkish colored epidotes that were found in core samples from a geothermal exploration well NB–1, Noboribetsu, Hokkaido, Japan are described together with estimating the formation conditions. The minerals occurred mainly as vein– and druse–fillings in volcaniclastic rocks of the Miocene Osarugawa Formation. They coexisted with brownish chlorites, K–feldspar, illite, and quartz, and associated with minor hematite, titanite, and/or apatite. The minerals were produced by local K–alteration that overprinted early–formed regional propylitization associated with ordinary greenish epidotes and chlorites. The cell parameters of pinkish epidote were: a = 8.884(6), b = 5.614(1), c = 10.149(7) Å, β = 115.5(8)°, and V = 456.9(4) ų. The Ca₂Mn³⁺₃Si₃O₁₂(OH) contents of the pinkish epidotes were generally less than 2.4 mol%, while the Ca₂Fe³⁺₃Si₃O₁₂(OH) contents attained 28 mol%. It is considered that the low Mn contents and relatively low Fe contents resulted in giving the characteristic pinkish to pale yellow colors for epidote. Associated brownish chlorites were characterized by low total Fe and relatively high Fe³⁺ contents. Their formation temperatures were estimated to be 230–300 °C, using semi–empirical chlorite geothermometers. These data indicate that pinkish epidotes coexisted with brownish chlorites formed under oxidative conditions and probably low Fe contents in hydrothermal fluids.