Nine wells drilled in the Taranaki-Cook Strait-Westland area intersected preCenozoic basement or basement-derived material. Petrological examination of cores and cuttings from these wells suggests that the Rotoroa Igneous Complex, the Separation Point Batholith, the NW Nelson Paleozoic metamorphic rocks, and possibly the Karamea Batholith extend from the South Island in a N-NNE direction beneath the Cenozoic cover. The trend of these rocks parallels the western margin of the New Zealand Geosyncline, supporting the suggestion that they constituted a foreland region during the development of the Geosyncline in the North Island, and were an important source of clastic material within it.
Overturned Paleozoic quartzo.feldspathic and pelitic schists, amphibole schists and calc-silicate rocks were regionally metamorphosed to the lower amphibolite facies and intruded by granodiorite near a major thrust fault during the Cretaceous. Ex· tensive hydrothermal alteration and sulphide mineralization are associated with the grandodiorite intrusion.Alteration of the non-calcareous rocks resulted in the formation of an outer zone characterised by muscovite ± albite, and an inner zone characterised by microcline and muscovite; and in the addition of large amounts of potash and sulphur and minor amounts of molybdenum. Alteration of the calcareous rocks resulted in the formation of an outer zone characterised by tremolite-actinolite, epidote ± talc, an intermediate zone characterised by diopside, and an inner zone by andradite, magnetite ± hedenbergite; addition of large amounts of silica, iron oxide and sulphur and minor amounts of alumina and copper; removal of large amounts of lime and C02; and both redistribution and removal of some magnesia and ti tania.The mineralogy and oxygen isotope fractionation between co·existing magnetite and quartz suggest that the temperature in the more intensely altered rocks reached at least 500°c. Factors which influenced localisation of sulphide mineralization were the presence of (1) a major fault, (2) calcareous rocks, and (3) a granitic intrusive exposed near its apex.
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