Premium
Carbon and Oxygen Isotope Compositions of Calcite and Rhodochrosite from Geothermal Exploratory Drills TH‐4 and TH‐6 near the Toyoha Deposit, Hokkaido, Japan
Author(s) -
NITTA Miyuki,
INOUE Atsuyuki
Publication year - 2011
Publication title -
resource geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.597
H-Index - 43
eISSN - 1751-3928
pISSN - 1344-1698
DOI - 10.1111/j.1751-3928.2011.00156.x
Subject(s) - rhodochrosite , calcite , geology , meteoric water , mineralogy , geochemistry , isotopes of oxygen , fluid inclusions , quartz , hydrothermal circulation , seismology , paleontology
We studied calcite and rhodochrosite from exploratory drill cores (TH‐4 and TH‐6) near the Toyoha deposit, southwestern Hokkaido, Japan, from the aspect of stable isotope geochemistry, together with measuring the homogenization temperatures of fluid inclusions. The alteration observed in the drill cores is classified into four zones: ore mineralized zone, mixed‐layer minerals zone, kaolin minerals zone, and propylitic zone. Calcite is widespread in all the zones except for the kaolin minerals zone. The occurrence of rhodochrosite is restricted in the ore mineralized zone associated with Fe, Mn‐rich chlorite and sulfides, the mineral assemblage of which is basically equivalent to that in the Toyoha veins. The measured δ 18 O SMOW and δ 13 C PDB values of calcite scatter in the relatively narrow ranges from −2 to 5‰ and from −9 to −5‰, respectively; those of rhodochrosite from 3 to 9‰ and from −9 to −5‰, excluding some data with large deviations. The variation of the isotopic compositions with temperature and depth could be explained by a mixing process between a heated surface meteoric water (100°C δ 18 O =−12‰, δ 13 C =−10‰) and a deep high temperature water (300°C, δ 18 O =−5‰, δ 13 C =−4‰). Boiling was less effective in isotopic fractionation than that of mixing. The plots of δ 18 O and δ 13 C indicate that the carbonates precipitated from H 2 CO 3 ‐dominated fluids under the conditions of pH = 6–7 and T = 200–300°C. The sequential precipitation from calcite to rhodochrosite in a vein brought about the disequilibrium isotopic fractionation between the two minerals. The hydrothermal fluids circulated during the precipitation of carbonates in TH‐4 and TH‐6 are similar in origin to the ore‐forming fluids pertaining to the formation of veins in the Toyoha deposit.