Near- and sub-solidus magma/fluid reaction, Coromandel Volcanic Zone, and implications for deep reservoir geothermal systems

Abstract

The Paritu Pluton is located in the northern Coromandel Volcanic Zone (CVZ), which was tectonically and magmatically active in the Pliocene from 18 - 2 million years ago. This was a consequence of the rotation of the convergence between the Australian Plate and the Pacific Plate. This convergence promoted magma generation, volcanism and the initiation of geothermal systems. Today, the sites of former geothermal systems are observed as veins of mineralized quartz containing Au and Ag and outcrops of variously altered plutonic rocks that once formed the deep heat source of a once active system. The Paritu Pluton exhibits a range of compositions from diorite to granodiorite which are a result of fractional crystallisation. The pluton has a crystallisation age of 16.3 ± 0.1 Ma that was defined by undertaking U-Pb geochronology. This study combines x-ray fluorescence analysis, electron microprobe analysis and oxygen isotope analysis to determine the high-temperature fluid interactions between the rocks and the circulating fluids. Fluid infiltration is a major process that occurs in a cooling, evolving magma body and at Paritu there is an abundance of evidence to show the water/rock interactions that occurred. Petrographic observations show that between the samples there are varying degrees of alteration, where sericitisation is abundant in some plagioclase crystals whilst it is minimal in others. Magmatic hornblende is altered to tremolite and exhibits an average temperature of crystallisation of about 530 °C. This alteration suggests high-temperature fluid infiltration in the pluton. There has also been oxygen isotope exchange between the pluton and the circulating fluids. The whole rock oxygen isotope values are variable from 6.2 to 3.6 ‰ and they exhibit zoning in the pluton from high oxygen isotope values from the margin to lower values towards the centre. This is a direct result of high-temperature infiltration. The penetration of the fluids is interrupted by two localised zones exhibiting low oxygen isotope values < 4 ‰. This localisation is a result of long-lived low-temperature fluid infiltration via faults/fractures. The mineral separates display a similar situation as they increase in oxygen isotope value with increasing whole rock oxygen isotope value. This is indicative of high-temperature fluid infiltration. Thermal modelling of the Paritu Pluton showed that the pluton cooled very quickly reaching full solidus in 5000 years. The high-enthalpy geothermal system that was associated with the emplacement of Paritu did not last more than 40,000 years. The pluton reached thermal equilibrium with the local geothermal gradient in about 100,000 years. The high-temperature fluid interactions that occurred at Paritu can contribute to understanding the heat engines driving the geothermal systems in the TVZ, such as Ngatamariki, which has the first and only intercepted in situ dioritic pluton