2:45 PM - 3:00 PM
[HRE12-04] Coexistence of geothermal and magmatic-hydrothermal systems at epithermal gold-silver deposits in the Iriki and Hoshino areas, Kyushu District
Keywords:epithermal gold-silver deposit, geothermal systems, magmatic-hydrothermal systems, alunite, fluid inclusion, Kyushu District
Epithermal environments consist mainly of two types of hydrothermal systems which induce gold-silver mineralization: geothermal and magmatic-hydrothermal systems. Geothermal (low to intermediate-sulfidation epithermal) systems are dominated by deeply circulated, near-neutral pH chloride waters which are mostly meteoric origin. Magmatic-hydrothermal (high-sulfidation epithermal) systems are dominated by acid hydrothermal fluids with much contribution of magmatic liquid and vapor. In this study, coexistence of both types of paleo-hydrothermal systems was newly found in the two areas of Iriki and Hoshino, located in Hokusatsu and northern Kyushu, respectively.
The Iriki area, located in northern Kagoshima Prefecture, known as Hokusatsu, is a historical gold mining area excavated from small deposits such as the Yamashita veins and the Gimpi pipe within the prominent Iriki kaolin mine. Another gold mineralization has also been confirmed underground on the west side of Lake Imuta through several exploration drilling programs, which show the mineral assemblages of intermediate-sulfidation state. This study found the common presence of crystalline alunite with an acicular or tabular shape in the acidic alteration zone near Lake Imuta, about 1 km east from the Iriki kaolin mine.
The Hoshino area, located in northern Kyushu, is a large-scale hydrothermal system extending over 15 km in length and 3 km in width and has historical gold-silver mining activities. This study focused on the west side of the area. Silica sinter, crustiform-banded and massive quartz veins and silicified host rocks were broadly observed. Bladed quartz and hydrothermal breccia indicating boiling condition of hydrothermal fluids were also observed. The veins of quartz and hydrothermal breccia have a combined strike length more than 1 km and have maximum width more than 5 m. The trapping temperature inferred from fluid inclusion microthermometry ranges from 160 to 190 °C. The mineralization and alteration characteristics in the Hoshino area are defining features of geothermal (low- to intermediate-sulfidation) systems. By contrast, a hydrothermal breccia zone with acidic alteration minerals such as alunite and kaolin minerals occurred about 2 km north from the main trend of the veins of quartz and breccia, where alunite has tabular shape and well-developed cleavage under microscope, suggesting magmatic-hydrothermal (high-sulfidation) systems.
The presence of crystalline alunite in the host rocks, as well as near-neutral pH gangue alteration mineral assemblages suggests the coexistence of geothermal and magmatic-hydrothermal systems in both the Iriki and Hoshino areas. The recognition of such side-by-side occurrences of contrasting hydrothermal systems might help to construct and improve conceptual models of the entire epithermal mineralization across Kyushu District.
The Iriki area, located in northern Kagoshima Prefecture, known as Hokusatsu, is a historical gold mining area excavated from small deposits such as the Yamashita veins and the Gimpi pipe within the prominent Iriki kaolin mine. Another gold mineralization has also been confirmed underground on the west side of Lake Imuta through several exploration drilling programs, which show the mineral assemblages of intermediate-sulfidation state. This study found the common presence of crystalline alunite with an acicular or tabular shape in the acidic alteration zone near Lake Imuta, about 1 km east from the Iriki kaolin mine.
The Hoshino area, located in northern Kyushu, is a large-scale hydrothermal system extending over 15 km in length and 3 km in width and has historical gold-silver mining activities. This study focused on the west side of the area. Silica sinter, crustiform-banded and massive quartz veins and silicified host rocks were broadly observed. Bladed quartz and hydrothermal breccia indicating boiling condition of hydrothermal fluids were also observed. The veins of quartz and hydrothermal breccia have a combined strike length more than 1 km and have maximum width more than 5 m. The trapping temperature inferred from fluid inclusion microthermometry ranges from 160 to 190 °C. The mineralization and alteration characteristics in the Hoshino area are defining features of geothermal (low- to intermediate-sulfidation) systems. By contrast, a hydrothermal breccia zone with acidic alteration minerals such as alunite and kaolin minerals occurred about 2 km north from the main trend of the veins of quartz and breccia, where alunite has tabular shape and well-developed cleavage under microscope, suggesting magmatic-hydrothermal (high-sulfidation) systems.
The presence of crystalline alunite in the host rocks, as well as near-neutral pH gangue alteration mineral assemblages suggests the coexistence of geothermal and magmatic-hydrothermal systems in both the Iriki and Hoshino areas. The recognition of such side-by-side occurrences of contrasting hydrothermal systems might help to construct and improve conceptual models of the entire epithermal mineralization across Kyushu District.