Advanced Argillic Alteration and Geochemistry of Alunite in an Evolving Hydrothermal System at Baguio, Northern Luzon, Philippines

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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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