Fig. 3

Variation in rock chemistry of SPW52. To depict the modification of the chemical composition due to the hydrothermal alteration, all oxide contents are standardized using that of Al₂O₃, which is considered the most immobile element during hydrothermal alteration. The samples from deep depths (≤ 858 m asl) form a cluster, whereas those from shallower depths scatter, which indicates higher alteration at shallower depths. The deeper and shallower depths indicate no significant difference for the Fe and Ti content (a); however, lower Na, Si (b), Ca, Mg (c), and K (d) indicate leaching of those elements at shallow depths. The higher sulfur content in the shallow depths indicates that sulfuric acid is a leaching agent (d)