Relative Effects of Groundwater and Magmatic Contributions in Hydrothermal Alteration Systems in the Philippines and New Zealand

Abstract

During the evolution of a hydrothermal system from immaturity to maturity, magmatic volatiles interact with rock and mix with invading groundwater In the Philippines, Recent volcanism in Cagua (1860) and Mahagnao (1895) rejuvenated a waning hydrothermal system, associated with groundwater-dominated cold (<180°C) water discharges and alteration- In each area, one well was drilled through the magmatic-hydrothermal interface at > 1500m while another, located in the cold outflow zone, was drilled through a groundwater-dominated part of the system. For comparison, the petrology and fluid chemistry of shallow (<300m) portions of Kawerau and the Poihipi sector, located at the western edges of the Wairakei hydrothermal system. New Zealand, were studied for groundwater effects. Fluids associated with active magmatic input have a distinct deuterium shift, in addition to the more common δ18O shift, ascribed to the addition of varying proportions of D-enriched arc-type magmatic waters. The proportion of magmatic water in Cagua is 65% and 90% in Mahagnao. Apart from isotopic compositions, relative N2, He and Ar contents in gases indicate considerable proportions of magmatic fluids in these two areas. Exceptionally high H2 and CO2 contents indicate temperatures >330°C, possibly due to proximity of magmatic environments. Within the upflow zone of magmatic fluids the Cagua and Mahagnao wells drilled through weakly altered microdiorite dikes, which are associated with a sudden increase in measured well temperatures, suggest that these latest heat source of these hydrothermal systems may be related to dike intrusion. Measured temperatures within the magmatic-hydrothermal interface are >330°C in both areas, although fluid inclusion homogenisation and mineralogical temperatures indicate temperatures up to 390°C in Mahagnao and up to >500°C in Cagua. Low permeability and high temperatures within the magmatic-hydrothermal interface caused the discharge of superheated steam from the wells within a few days of well testing, before waters representative of the deep reservoir could be sampled. Thus, the only way to characterise the altering waters within the interface is through detailed mineralogical and fluid inclusion studies. Magmatic waters in Cagua and Mahagnao contain 15000- 30000 mg/kg Cl equivalent, dissolved Cl2 ranging from 0.9 to 4 molal with high concentrations of Ca, Mg, F, B and S in reduced forms. Gold, Ag and Mo are introduced by magmatic fluids. Oxidation of S occurs only where magmatic volatiles dissociate within waterlogged fractures and form acid waters which produce advanced argillic cation-depleted alteration assemblages. The magmatic-hydrothermal interface is a narrow zone dominated by a vapour chimney, which connects the deep zones with surface fumaroles. Over the central parts of the vapour chimney, vapours may become superheated, causing evaporation of any earlier liquids and the formation of highly saline solutions and halite daughter minerals in fluid inclusions as well as resulting in critical point behaviour in fluid inclusions. Along the edges of this chimney is a narrow zone of acid Cl-SO4 condensate. As temperatures decrease away from this vapour core, coupled with the mixing of groundwater with residual fluids around the chimney, neutral Cl waters typical of mature hydrothermal systems form. With further cooling and increased dilution with groundwaters, bicarbonate waters form at the periphery of the system. Where faults are present, bicarbonate waters can percolate to deeper levels of the system, causing the formation of low temperature "groundwater-dominated" hydrothermal alteration. Alteration and fluid inclusion studies in the periphery of hydrothermal systems such as at Poihipi and wells CG-2D and MH-2D in Cagua and Mahagnao, respectively, and in the shallow portions of Kawerau show that these hydrothermal systems may have cooled by as much as 250°C. This is probably caused by the waning of the heat source followed by the invasion of cold bicarbonate waters to deep levels. These waters contain very low Cl but relatively high dissolved CO2, observed as clathrates in fluid inclusions. Measured, mineralogical and fluid inclusion temperatures in the groundwater-dominated regions of hydrothermal systems is <200°C even at depths of about 2800m, as in Mahagnao. The magmatic-hydrothermal interface is an ephemeral phenomenon and can only be sustained by low permeability and intermittent introduction of magmatic rock and volatiles. In contrast, groundwater influx is a protracted process occurring throughout the evolution of a hydrothermal system and requiring mainly the continued presence of permeable faults to act as channel ways to invade deeper regions of a hydrothermal system.