Mineralogy, geochemistry and origin of hydrothermal alteration and sulphide mineralization in the disseminated molybdenite and skarn‐type copper sulphide deposit at Copperstain Creek, Takaka, New Zealand

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. Extensive 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 CO 2 ; and both redistribution and removal of some magnesia and titania. 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.