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[SCG47-P18] Study on differentiation of epithermal precious metal mineralization from base-metal mineralization: The example of Kitami region, Hokkaido, Japan
Keywords:Au-Ag mineralization, Cu-Pb-Zn mineralization, fluid inclusions, melt inclusions, apatite
Hydrothermal metal deposits are distributed in volcanic arcs, are classified into Au-Ag enriched deposits and Cu-Pb-Zn enriched deposits. The selectivity of the mineralization is attributed to the salinity of the hydrothermal fluids and the dissolved Cl/S ratio, as Au is transported as sulfide complexes and Ag and Cu, Pb, Zn are transported as chloride complexes in the hydrothermal fluids. In Japan arc, there is a similarity in the distribution of vein-type deposits and igneous rocks, and one of the factors causing the compositional differences in hydrothermal fluids may have been differences in the chemical composition of magma related to deposit formation. In order to demonstrate the relationship between the selectivity of mineralization and the composition of magma that formed the deposits, the Kitami Cu-Pb-Zn and Kitano-o Au-Ag deposits in the Kitami region, Hokkaido, were selected as the study area. The chemical composition of the melt was estimated, and the volatile composition of the melt was estimated by using quartz, clinopyroxene phenocrysts and ilmenite hosted melt inclusions and apatite.
The quartz hosted fluid inclusions show temperature range of ca. 207-286°C was obtained for the Kitami deposit and ca. 155-285°C for the Kitano-o deposit, and salinity NaCl eq. ranged from 4.2-5.1 wt.% for the Kitami deposit and 1.8-1.9 wt.% for the Kitano-o deposit. A significant difference in salinity was shown between the hydrothermal fluids that formed the two deposits.
The results of whole-rock chemical composition analysis show dacitic composition at the Kitami and rhyolitic composition at the Kitano-o.
Cl and S content obtained from melt inclusions and apatite showed that the representative values for each host phenocryst are 0.029 wt.%; 0.018 wt.% (clinopyroxene), 0.029 wt.%; <0.007 wt.% (ilmenite) at Kitami, and 0.12-0.22 wt.%; 0.011-0.073 wt.% (quartz), 0.18 wt.%; <0.013 wt.% (ilmenite) at Kitano-o. The partition coefficient of Cl and S between the apatite and the melt was used to calculate the Cl and S content in the melt, which was >0.42 wt.%; 1-11 ppm in Kitami and >0.43 wt.%; <3 ppm in Kitano-o.
The results of fluid inclusions study indicate that the hydrothermal fluids have different salinities, suggesting a stronger involvement of Cl in the formation of the Cu-Pb-Zn deposits, and that the Cu-Pb-Zn transported in the fluids as chloride complexes.
The estimated Cl and S content of the melts are plotted on a diagram where the Y axis is the Cl/S ratio, and the X axis is the evolution of the melt compositions. The results suggest that if the Cl/S ratio of the Kitami dacitic melt when the apatite crystalize and the Kitano-o rhyolitic melt when the quartz crystallize is maintained, and the Cl and S is separated as a fluid phase, it is suggested that the ore-forming fluids have sufficient Cl/S ratio to form Cu-Pb-Zn and Au-Ag deposits as experimentally shown in previous study. However, the Cl/S ratio of the melt at the time of apatite crystallization in the Kitano-o was too high for the formation of Au-Ag deposits.
The partition coefficients of Cl between the fluid and the melt may have been different in both deposits, semi-measured partition coefficients using the salinity obtained from fluid inclusions and Cl content in the melt during apatite crystallization were calculated. As a result, the semi-measured partition coefficient was calculated. The minimum values of the partition coefficients were 3.64-7.20 in Kitami and 1.35-2.68 in Kitano-o. The maximum pressures when fluid separation occur were 145-200 MPa in Kitami and 64-95 MPa in Kitano-o, suggesting that the depth of fluid separation from each melt may be different.
In addition to the salinity of the hydrothermal fluid, the selectivity of metal species is due to differences in the Cl/S ratio of the melt at fluid separation, the SiO2 content and the pressure dependent partition coefficient of Cl between the fluid and the melt.
The quartz hosted fluid inclusions show temperature range of ca. 207-286°C was obtained for the Kitami deposit and ca. 155-285°C for the Kitano-o deposit, and salinity NaCl eq. ranged from 4.2-5.1 wt.% for the Kitami deposit and 1.8-1.9 wt.% for the Kitano-o deposit. A significant difference in salinity was shown between the hydrothermal fluids that formed the two deposits.
The results of whole-rock chemical composition analysis show dacitic composition at the Kitami and rhyolitic composition at the Kitano-o.
Cl and S content obtained from melt inclusions and apatite showed that the representative values for each host phenocryst are 0.029 wt.%; 0.018 wt.% (clinopyroxene), 0.029 wt.%; <0.007 wt.% (ilmenite) at Kitami, and 0.12-0.22 wt.%; 0.011-0.073 wt.% (quartz), 0.18 wt.%; <0.013 wt.% (ilmenite) at Kitano-o. The partition coefficient of Cl and S between the apatite and the melt was used to calculate the Cl and S content in the melt, which was >0.42 wt.%; 1-11 ppm in Kitami and >0.43 wt.%; <3 ppm in Kitano-o.
The results of fluid inclusions study indicate that the hydrothermal fluids have different salinities, suggesting a stronger involvement of Cl in the formation of the Cu-Pb-Zn deposits, and that the Cu-Pb-Zn transported in the fluids as chloride complexes.
The estimated Cl and S content of the melts are plotted on a diagram where the Y axis is the Cl/S ratio, and the X axis is the evolution of the melt compositions. The results suggest that if the Cl/S ratio of the Kitami dacitic melt when the apatite crystalize and the Kitano-o rhyolitic melt when the quartz crystallize is maintained, and the Cl and S is separated as a fluid phase, it is suggested that the ore-forming fluids have sufficient Cl/S ratio to form Cu-Pb-Zn and Au-Ag deposits as experimentally shown in previous study. However, the Cl/S ratio of the melt at the time of apatite crystallization in the Kitano-o was too high for the formation of Au-Ag deposits.
The partition coefficients of Cl between the fluid and the melt may have been different in both deposits, semi-measured partition coefficients using the salinity obtained from fluid inclusions and Cl content in the melt during apatite crystallization were calculated. As a result, the semi-measured partition coefficient was calculated. The minimum values of the partition coefficients were 3.64-7.20 in Kitami and 1.35-2.68 in Kitano-o. The maximum pressures when fluid separation occur were 145-200 MPa in Kitami and 64-95 MPa in Kitano-o, suggesting that the depth of fluid separation from each melt may be different.
In addition to the salinity of the hydrothermal fluid, the selectivity of metal species is due to differences in the Cl/S ratio of the melt at fluid separation, the SiO2 content and the pressure dependent partition coefficient of Cl between the fluid and the melt.