3:30 PM - 5:00 PM
[SCG48-P05] Hydrothermal alteration and high-grade gold mineralization in Ryuo deposit,
Kitami region, Hokkaido, Japan
Keywords:Epithermal deposit, Hydrothermal alteration, Fluid inclusion
The Ryuo deposit is located around the Nittapu river in the Ikutahara area, Kitami distinct, Hokkaido. And this deposit was mainly a gold and silver ore producer.
The deposit is often found to have localized area of very-high grade gold mineralization, and in2021, a high-grade zone of 1,395 g/t was identified in a drill core by Japan Gold Corp. a Canadian exploration company.
In this study, 5 drillholes were selected from the 18 drillholes completed by Japan Gold Corp., and core descriptions and sampling were conducted. The distribution of hydrothermal alteration minerals throughout the deposit, temperature and salinity of mineralized fluids revealed by fluid inclusion analysis at high-grade points.
In the northeastern part, rhyolitic volcaniclastic rocks, lake deposits, black mudstone, and rhyolitic lava layers are observed. On the other hand, in the southwestern part of the deposit, rhyolite lava layers are not observed, and lake deposits layers are found in the shallow part of the deposit. Volcaniclastic rocks and black mudstones are the most prominent mineralized rock layers in the deposit.
Sampling was conducted every 5 to 10 m in the cores, and a total of 298 samples were analyzed by X-ray diffraction. The shallow to deep part is sericite-altered, and the deepest part is strongly chlorite-altered.
The general-grade ores observed in the shallow part and the quartz in the high-grade part trapped deep underground show gold precipitation in fine-grained quartz + comb- adularia in the former, and rhombic adularia in the gold precipitation bands in the latter, starting with the crystallization of comb- quartz.
The fluid inclusions in the quartz veins of the high-grade section differ before and after the gold precipitation. In the early stage, the fluid inclusions are homogeneous with a predominance of liquid phase and vaper-liquid bilayer inclusions, while in the late stage, the vaper-liquid difference is observed with a vaper phase of 20-70%. Micro thermometry homogenization temperature and salinity measurements of fluid inclusions in the quartz veins of the high-grade section showed that the early stage of gold precipitation: 221.0-271.4°C, 1.9-2.7 wt% (NaCl eq.); late stage of gold precipitation: 155.1-213.9°C, 1.9-2.4 wt%. (NaCl eq.), and late gold precipitation stage: 155.1-213.9°C, 1.9-2.4 wt%. The hydrothermal fluid has a neutral composition based on the formation temperature of sericite and chlorite, which are widely observed in the deep part of the Ryuo deposit, and the temperature of the hydrothermal fluid.
The sulfur fugacity was calculated from the homogenization temperature of fluid inclusions and silver content in the electrum in the high-grade veins, logfS2=-11.878~-11.124, indicating that the veins were in a medium sulfide state.
The factors that caused the precipitation of gold based on the homogenization temperature distribution of fluid inclusions and the occurrence of quartz and adularia at the high-grade sites are that the non-boiling mineralized fluid underwent some kind of depressurization, which led to boiling and an increase in pH, resulting in the precipitation of gold. It is concluded that the gold precipitation ended when the boiling ended due to temperature decrease.
The deposit is often found to have localized area of very-high grade gold mineralization, and in2021, a high-grade zone of 1,395 g/t was identified in a drill core by Japan Gold Corp. a Canadian exploration company.
In this study, 5 drillholes were selected from the 18 drillholes completed by Japan Gold Corp., and core descriptions and sampling were conducted. The distribution of hydrothermal alteration minerals throughout the deposit, temperature and salinity of mineralized fluids revealed by fluid inclusion analysis at high-grade points.
In the northeastern part, rhyolitic volcaniclastic rocks, lake deposits, black mudstone, and rhyolitic lava layers are observed. On the other hand, in the southwestern part of the deposit, rhyolite lava layers are not observed, and lake deposits layers are found in the shallow part of the deposit. Volcaniclastic rocks and black mudstones are the most prominent mineralized rock layers in the deposit.
Sampling was conducted every 5 to 10 m in the cores, and a total of 298 samples were analyzed by X-ray diffraction. The shallow to deep part is sericite-altered, and the deepest part is strongly chlorite-altered.
The general-grade ores observed in the shallow part and the quartz in the high-grade part trapped deep underground show gold precipitation in fine-grained quartz + comb- adularia in the former, and rhombic adularia in the gold precipitation bands in the latter, starting with the crystallization of comb- quartz.
The fluid inclusions in the quartz veins of the high-grade section differ before and after the gold precipitation. In the early stage, the fluid inclusions are homogeneous with a predominance of liquid phase and vaper-liquid bilayer inclusions, while in the late stage, the vaper-liquid difference is observed with a vaper phase of 20-70%. Micro thermometry homogenization temperature and salinity measurements of fluid inclusions in the quartz veins of the high-grade section showed that the early stage of gold precipitation: 221.0-271.4°C, 1.9-2.7 wt% (NaCl eq.); late stage of gold precipitation: 155.1-213.9°C, 1.9-2.4 wt%. (NaCl eq.), and late gold precipitation stage: 155.1-213.9°C, 1.9-2.4 wt%. The hydrothermal fluid has a neutral composition based on the formation temperature of sericite and chlorite, which are widely observed in the deep part of the Ryuo deposit, and the temperature of the hydrothermal fluid.
The sulfur fugacity was calculated from the homogenization temperature of fluid inclusions and silver content in the electrum in the high-grade veins, logfS2=-11.878~-11.124, indicating that the veins were in a medium sulfide state.
The factors that caused the precipitation of gold based on the homogenization temperature distribution of fluid inclusions and the occurrence of quartz and adularia at the high-grade sites are that the non-boiling mineralized fluid underwent some kind of depressurization, which led to boiling and an increase in pH, resulting in the precipitation of gold. It is concluded that the gold precipitation ended when the boiling ended due to temperature decrease.