MAGMAS, METALS AND DEEP ROOTS OF HYDROTHERMAL SYSTEMS: LESSONS FROM ACTIVE SYSTEMS OF NEW ZEALAND (Invited Presentation)

Hydrothermal systems of the Taupō Volcanic Zone (TVZ) are considered present day analogues to low-sulfidation epithermal ore deposits. Magmatism, tectonism and hydrothermalism are interrelated but the source of metals in the fluids is difficult to assess. We have undertaken a comparative study in the TVZ between two high gas active geothermal systems (Ohaaki and Rotokawa) and the fossil (~0.6 Ma) magmatic-hydrothermal system of Ngatamariki and its porphyry copper style hydrothermal halo. The current hydrothermal system at Ngatamariki is overprinting the fossil magmatic-hydrothermal system. We aimed to constrain the link between large magma degassing and dilute near neutral pH hydrothermal systems.

The active systems studied here have anomalous concentrations of Au (>1 ppb), As (>10 ppm) and Sb (>1 ppm) in whole-rock samples from depths shallower than 1,500 m (excluding the surface features), but only at Ohaaki are significant Au anomalies (>100 ppb) common. Arsenic, Sb and Au are enriched in rocks toward the surface in both fossil and modern hydrothermal systems, slightly positively correlated with sulfur, reflecting the bisulfide (HS-) complexation of these species. This is consistent with boiling as the principal mechanism responsible for the precipitation of these metals and metalloids.

Only the Ngatamariki rocks, altered by the magmatic-hydrothermal event, are enriched in Ag. Above the shallow Ngatamariki fossil intrusion, the altered rocks are also enriched in Cu, Te, Zn, Pb, Se, Bi and Mo. Therefore, the chemical zoning of altered rocks within the present-day geothermal systems is profoundly different from those of the old, high-temperature magmatic-hydrothermal altered rocks above the ca. 0.6 Ma Ngatamariki intrusive complex (2 km depth) and lack Cu, Zn, Pb, Mo, Se, Te and Bi enrichment.

There is no compelling chemical evidence in geothermal fluids or altered rock for water-chloride-rich fluids derived from shallow magmas (4-5 km), and we conclude that most of the TVZ geothermal systems are fed by heat conducted from deeper (>6 km) rhyolitic magmas. Here we will discuss the implications of this findings in term of future deep drilling and driving processes at the magmatic hydrothermal transition above large silicic magma bodies.