The Onemana Peninsula is located on the eastern side of the Coromandel Peninsula north of the township of Whangamata. This study investigates the volcanology, structure and hydrothermal alteration of the northern half of the Onemana Peninsula. The study area is dominated by rhyolite domes and lava flows, with minor andesite lavas, rhyolite pyroclastics and lake sediments. The central region is extensively hydrothermally altered, with high levels of the original system preserved including hydrothermal eruption breccias and sinters.
The Onemana area is dominated by three rhyolite dome complexes: Pohakahaka, Pokohino, and Wharekawa. The Pohakahaka Dome Complex in the south of the study area consists of pyroxene rhyolite domes, lavas and autoclastic breccias. The Pokohino Dome Complex in the central region of the study area consists of biotite rhyolite domes, lavas and autoclastic breccias. Lavas appear to have erupted as coulees and have flowed in a SE direction from a NE-trending ridge. The Wharekawa Dome Complex and Eastern Flows are dominated by biotite rhyolites. Pyroclastic and epiclastic material is overlain by rhyolite lavas in coastal sections north of Pokohino Beach, suggesting explosive activity preceded the eruption of some lavas. Some Eastern Flow lavas have flowed into water and produced a hyaloclastite in the north of the area. The deposition of the Glassy Dome Pyroclastics is thought to have resulted from the collapse of a rhyolite dome. Andesite lavas occur in the west of the study area, and their compositions suggest that they are related to the McBeths Andesite which outcrops 6 km to the NW. A hornblende-biotite ignimbrite is exposed in the north of the area, and is thought to have been erupted from outside the area. Thick lake sediments occur between the Pokohino and Wharekawa dome complexes and infill a graben structure. A hydrothermal eruption breccia resulting from over-pressuring of a geothermal system overlies the lake sediments.
The structure of the area is dominated by NW, N, and NE striking faults, and minor EW striking faults. The local structure plays an important role in controlling the locations of volcanism and hydrothermal activity. Older N-striking structures appear to have controlled vent locations for the pyroxene rhyolites, while NE-striking structures have controlled the location the biotite rhyolites and hydrothermal alteration. Mineralised structures are controlled by NW and N striking faults between the NE striking Whitipirorua and Pokohino Faults.
XRF analysis of selected volcanic rocks from Onemana show the area to be dominated by medium-K to high-K rhyolites (75-78 wt% SiO₂), with minor andesite (56-58 wt% SiO₂). On the basis of Zr abundance the rhyolites can be divided into high Zr (pyroxene rhyolites) and low Zr (biotite rhyolites, Glassy Dome Rhyolite). The rhyolites are thought to be closely associated with partial melting of the continental crust in a rifting environment.
Pervasive to weak alteration occurs within a NE-trending corridor bounded by the Whitipirorua and Pokohino Faults. There are two main alteration types in the area which represent different hydrothermal environments: alkaline and acid alteration. Alkaline alteration consists of an inner core of quartz silicification, surrounded by quartz+ K-feldspar alteration, which grades out into epidote and weak clay alteration assemblages. Zeolites also occur as an overprinting alteration assemblage. Acid alteration consists of pervasive kaolinite alteration, with small areas of quartz + illite/smectite ± pyrophyllite alteration and opaline quartz silicification.
The Onemana area shows many volcanic and hydrothermal features that are typical of caldera settings and are comparable with the rhyolite volcanic centres in the TVZ. It is suggested that the rhyolitic volcanism at Onemana resulted from late stage volcanism along the ring fracture of a large caldera structure.
