Geochemical characteristics (C, N, and S) of low-grade metamorphic rocks in geothermal fields, northern Taiwan

Due to the shortage of non-regenerated energy resources and the development of advanced technology in geothermal energy exploration, the potential geothermal energy in Yilan is under revaluation. Three drilled cores from the Chingshui geothermal field and Yilan plain were examined for basic geochemical characteristics (C, N, S and S isotopes) of low-grade metamorphic rocks in slate belt and also for possible signature of geothermal resource. In order to nvestigate possible change of geochemical characteristics during progressive metamorphism, this study also examined a set of samples from sedimentary to greenschist-facies rocks in a transection of the central Taiwan mountain belt. The results indicated that the C, N, and S contents of slate in Yilan area might still inherit their sedimentary origin and did not be modified during low-grade metamorphism. The low C/N ratios demonstrated that their organic matter is predominant marine origin. The relationship between organic carbon and pyrite contents suggested that the original sedimentary environment may be euxinic, where sulfate, organic matter and reactive iron were available and the sulfate reduction rate was high. The pyrite can be classified into two categories, according to their textures and δ34S signatures. One is fine-grained with highly variable δ34S values, representing a general characteristic of sedimentary pyrite; the other is coarse-grained with a narrower range of δ34S values, inferring a homogenization during metamorphism or recrystallization of a new growth from a specific geothermal fluid. Our findings seems consistent with previous study that the higher geothermal gradient in Yilan may caused by a magma chamber. The samples from the transection of the central Taiwan mountain belt showed that their C, N, and S contents was gradually depleted and sulfur isotopic compositions seemed homogenized in rocks of polymetamorphosed greenschist facies. Since the numbers and types of samples were limited currently, the result is still inconclusive.