Petrology and Alteration in 402: an Acid Well in the Margin of the Tongonan Geothermal Field

Abstract

One of the significant aspects of the geochemistry of well is the acid nature of its discharges considering most Tongonan wells tap and discharge neutral sodium chloride brines. Cores and cuttings recovered from this well showed variably altered lithologies and mineral alteration zones-unique to Weakly altered clinopyroxene lavas and breccias were intersected down t o near sea level, intensely altered hornblende andesite to m a partly serpentinised ultramafic rock t o -1760 and bottomed i n clinopyroxene andesite(-1887 m). Several alteration zones and hydrothermal regimes are rethey are generally prograde, viz ;a) smectite cristobalite micaceous clay d) advanced e) talc-carbonate, and f ) calc- silicate zones. The interlayered clays show a pronounced zonation with depth and temperature. Above epidote, anhydrite, chlorite,illite , albite and wairakite comprise t h e neutral alteration suite. Alunite , covellite and natrojarosite occur within the supergene zone near sea level and are typical of a 'leached cap over some porphyry and base-metal deposits. Narrow zones of highly aluminous acid-stable diaspore-pyrophyllite alteration appear intermittently i n 402 cross-cutting vein relationships suggest the occurrence of both relict and current acid alteration due to episodic incursion of acid fluids. Talc is hosted by propylitised andesites and diorites in the wells of highest temperatures (-300?C) and salinities above the main Mahiao upflow zone, i n wells and 404; however a lower(?) temperature occurs i n as an alteration of the serpentinised ultramafic rock not previously reported i n other wells. The author proposes a model f o r t h e occurrence of acid alteration i n the 'margins' of the Tongonan geothermal field