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

Abstract

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.

U‐Pb geochronology and geochemistry of molybdenum‐bearing granodiorite porphyry at Copperstain Creek, west Nelson, New Zealand

“…Major element compositions of three samples of the granodiorite porphyry reported by Wodzicki (1972) show similar Al2O3 and Na2O contents to those analysed here (Table 1). Potassium contents are high (7.27-7.65% K 2 O), except for P31470 (2.31% K 2 O), which is unaltered Copperstain Creek granodiorite from a small outlying body 100 m southeast of the main body in Moly Creek (Wodzicki 1972, fig. 2).…”

“…K-Ar dates for samples of granite and metamorphic rocks from the Onekaka area, which includes Copperstain Creek, have been reported by Wodzicki (1972, details in Hulston & McCabe 1972, and are listed in Table 2. These dates, together with two others (samples R15713 and R3076), are also included in the geochronology database for New Zealand rocks (Nathan et al 2000).…”

“…1), a pyritic copper skarn deposit is hosted in calc-silicate rocks that are intruded by small granodiorite stocks, which carry porphyry molybdenum style mineralisation (Wodzicki 1972). The calc-silicate rocks are intercalated with quartzofeldspathic, pelitic, and amphibolite schists of the Onekaka Schist, which is in thrust fault contact with a tectonic sliver of the Upper Ordovician Arthur Marble (Grindley 1961(Grindley , 1971.…”

“…1). The granodiorite is weakly schistose and has been hydrothermally altered in association with molybdenum mineralisation, which occurs as minor molybdenite along joints and in quartz veins in the granodiorite and adjacent quartzofeldspathic schists (Wodzicki 1972). The mineralisation is defined geochemically by molybdenum in-rock values of 100-500 ppm within a zone 450 x 180 m (Smale 1970), and a drillhole intersected 91m of mineralised granodiorite averaging 300 ppm Mo (Wodzicki 1972).…”

“…The granodiorite is weakly schistose and has been hydrothermally altered in association with molybdenum mineralisation, which occurs as minor molybdenite along joints and in quartz veins in the granodiorite and adjacent quartzofeldspathic schists (Wodzicki 1972). The mineralisation is defined geochemically by molybdenum in-rock values of 100-500 ppm within a zone 450 x 180 m (Smale 1970), and a drillhole intersected 91m of mineralised granodiorite averaging 300 ppm Mo (Wodzicki 1972). Alteration of the calc-silicate country rocks adjacent to the granodiorite has produced a sequence of skarn assemblages, with an inner garnet (andradite)-magnetite ± hedenbergite zone, an intermediate zone characterised by diopside, and an outer tremoliteactinolite-epidote ± talc zone.…”