Acid alteration related to epithermal Au mineralization in the Baguio district, northern Luzon, Philippines, reflects the nature of the past hydrothermal activity. The acid alteration is characterized by an assemblage of pyrophyllite, diaspore and dickite, sometimes with strongly zoned alunite solid solution. This style of alteration is associated with a large zone of leached rock, mainly residual silica now recrystallized to quartz. The alunite has cores rich in Ba, Sr, and PO₄; together with³⁴S values of +15 to +24 permil, these data suggest formation from a high temperature fluid which contained reactive magmatic components such as H₂SO₄ and HCl. The extensive ridge-forming residual silica and surrounding advanced argillic alteration are underlain by district-wide propylitic alteration and Au-bearing quartz veins. Some of the veins have halos of acid alteration containing quartz, sericite and pyrophyllite, with or without alunite, diaspore and dickite. The pervasive acid alteration formed about 1.4-0.9 Ma, 1 m.y. after the intrusion of microdiorite in the district. At about 0.6 Ma, quartz veins formed from a subsequent hydrothermal system. The neutral pH waters of this system may have had a local input of magmatic acid components, particularly in the vicinity of the. Kelly vein system. This younger system was responsible for epithermal Au mineralization in the district.

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Kuroko and kuroko-type deposits are distributed in the Shakotan-Toya district, southwest Hokkaido. The mode of occurrence of kuroko-type deposits is somewhat different from the typical kuroko deposits in the Hokuroku district. For example, the kuroko-type deposits lack any bedded ore-body. K-Ar age determination for hydrothermally altered rocks from the Kunitomi, Yoichi, Meiji, Horobetsu, Akaiwa and Otaru-Matsukura deposits has been carried out in order to compare mineralization ages of the typical kuroko deposits with the kuroko-type deposits from the view point of mineralization age.
Sericite and potash feldspar from the No.3 bedded ore-body in the Kunitomi showed same K-Ar age, 13.02±0.34 Ma and 13.24±0.31 Ma. Sericites from ore-bearing vein in the Yoichi and from a disseminated ore-body in the Meiji showed K-Ar ages of 12.39±0.32 Ma and 12.56±0.31 Ma, respectively. Ore veins within the Horobetsu deposit have ages from 15.41±0.38 Ma to 14.51±0.33 Ma. K-Ar ages obtained from hydrothermal minerals from the Kunitomi, Yoichi, Meiji and Horobetsu deposits are of middle Miocene age. These ages coincide with the typical kuroko deposits of the Hokuroku district (15.6 to 10.7 Ma).
K-Ar ages of 9.78±0.24 Ma and 9.51±0.46 Ma were obtained from a stockwork ore-body in the Akaiwa and a massive barite ore-body of the Otaru-Matsukura deposit. These results would indicate that different mineralization from main stage kuroko mineralization occurred during late Miocene time.

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In this study we report the occurrence of gold mineralization in the Noya geothermal area, located in the ENE-WSW trending Beppu-Shimabara graben, central Kyushu, Japan. The Noya geothermal area is located on the marginal zone of the Shishimuta caldera. The area also falls on the shoulder of a positive Bouguer gravity anomaly which is centered approximately 3 km northeast of drill hole 51-WT-1. In the vicinity of the Noya area there are several lava flows of Pleistocene age which erupted during the mineralizing stage of the Noya geothermal system. Basement rocks in the study area are not exposed. However, they are thought to be composed of Cretaceous metamorphic rocks and granitic rocks, overlain by volcanic rocks and lacustrine deposits of Pliocene to Pleistocene age.
Many hot springs and fumaroles as well as zones of hydrothermal alteration are present in the graben. Gold bearing quartz-calcite-adularia veins were found in drill cores 51-WT-1 and 51-WT-2 from the Noya area. The host rock is mainly composed of andesitic lapilli tuff which has been altered to propylite. The main alteration minerals are kaolinite and smectite which occur from surface down to 125m below ground level in the 51-WT-1 core. These minerals are replaced by chlorite, a chlorite/smectite mixed-layer mineral, and an illite/smectite mixed-layer mineral below the 125m level. Silica minerals include cristobalite and tridymite from surface to the 100m level, however, a phase change to quartz occurs at deeper levels. A K-Ar age of 0.37±0.01 Ma was obtained from an early-stage adularia-rich vein sample with a gold content of 5.5ppm. This age is relatively young compared with the formation ages of most epithermal deposits in Japan.
In drill hole 51-WT-1, several quartz-calcite-adularia veins occur at a depth of 164 to 215m below ground surface. The veins are white in color, and are virtually sulfide-free. High calcite contents (70 wt.% on average) are characteristic of the gold-bearing veins (up to 20ppm Au) in drill hole 51-WT-1. The average Ag/Au ratio is 0.8 in the veins.
In contrast to drill hole 51-WT-1, drill hole 51-WT-2 contains adularia-bearing silicified rock with 0.1 to 0.4 ppm of gold. Many quartz-calcite-adularia veinlets, which contain a few ppm of gold, cut the silicified body at a depth of 25 to 61m. Veins in drill hole 51-WT-2 can be divided into two types based on their color; gray-colored veins and white-colored veins. The gray-colored veins consist of quartz, adularia, calcite and sulfides with associated high concentrations of As, Sb, Hg and S. The white-colored veins consist of quartz, adularia and calcite, and cut the gray-colored veins. Quartz is the dominant vein mineral followed by lesser amounts of adularia and calcite, respectively. The average Ag/Au ratio is 4.9 in the veins, which is significantly greater than that in the veins of 51-WT-1.
The present down hole temperature of the hydrothermal fluid in drill hole 51-WT-1 is -150°C at a depth of 200m. Homogenization temperatures of fluid inclusions in early-stage vein calcite average -190°C from drill core samples of the same depth. The fluid inclusion homogenization temperatures are considered to be related to early hydrothermal activity in the Noya geothermal system with fluid temperatures at least 40°C higher than today. This early-stage of hydrothermal activity is coincident with gold mineralization in the Noya geothermal system.

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