Abstract:
A reconnaissance survey of Great Barrier Island in the Hauraki Gulf has been undertaken to determine thickness and structural relationship of the Miocene and Pliocene volcanic sequences to the underlying Jurassic greywacke basement and to define gross structural features within the volcanics.
The isolation of Great Barrier Island and the ruggedness of its terrain ruled out seismic methods for this type of investigation. Three other methods namely gravity, electrical resistivity, and airborne magnetics were used instead for the reconnaissance study. The gravity survey covered the whole Island whereas resistivity and magnetic surveys were less extensive. A Bouguer anomaly map has been compiled which shows the gravitational effect of the volcanics.
The interpretation of gravity anomalies has shown that the low density volcanic rocks of the Coromandel Group and the Whitianga Group have been deposited in an elongated NW trending depression. The minimum thickness of the volcanic sequences is at least 2km beneath Port Fitzroy. A number of basement features within this depression can be recognised in the residual Bouguer anomaly map. Evidence from gravity modelling, Landsat photo, and surface geology indicates that a gravity low (-20mgals) east of Mt.Hobson is associated with a caldera approximately 3 to 4km in diameter. A basement depression near Port Fitzroy is probably an old volcanic centre for volcanics of the Coromandel Group. An andesitic or dacitic intrusive body has been modelled for a gravity high at Medlands Beach which is also associated with a positive 120nT residual magnetic anomaly.
Interpretation of the magnetic data has shown that the volcanics of the Coromandel and Whitianga Groups exhibit low magnetisation of the order of 1.5Am-1. This magnetisation has been destroyed by thermal alteration in the central part of the Island. A significant portion of the residual total force magnetic anomalies over flight lines FL 2, 3, 6 and 7, are due to topographic effects of the near surface volcanics. The Mt.Young and Port Fitzroy rocks exhibit reverse magnetisation which is compatible with a paleomagnetic age of 10 to 11.6x106 years, a period with a dominantly reversely paleomagnetic field. Other reversely magnetic rocks belonging to the Coromandel Group occur near Okupu - Tryphena.
Resistivity measurements in the Kaitoke Valley have revealed a zone of low resistivity over which hot springs are located. It is likely that the hot springs are fed by fluids which ascend along ring fractures in the southern part of the Mt.Hobson Caldera.
Potential areas of mineralisation are the inferred ring fracture system of the Mt.Hobson Caldera and the thermally altered rocks to the west and south of the caldera rim.