Mineralization at Mount Owen, central Nelson

The Lower Paleozoic sequence at Mt. Owen, Nelson, hosts gold and base metal mineralization. The succession comprises the Wangapeka Formation (a black shale and sandstone unit), the underlying Arthur Marble Formation, which in turn rests on a metamorphosed and hydrothermally altered sequence of quartzite, dolomite, and black shales, informally defined here as the Owen Formation. The succession dips steeply westward, away from the adjacent intrusive batholith of Cretaceous Separation Point Granite. Tectonic deformation, metamorphism, and hydrothermal alteration are related to two principal controls: (a) granite intrusion, and (b) confinement of fluid processes beneath the marble. Study of the disseminated sulphide minerals in the sedimentary rocks shows that whereas the sequence within and above the marble contains only crystalline pyrite, the more altered Owen Formation contains a complex suite of minerals, exhibiting the paragenesis: pyrite → pyrrhotite → marcasite → pyrite. Base metal ores form both replacement and infilling bodies within, and near, the marble, and comprise argentiferous galena, sphalerite, and minor chalcopyrite; in one deposit a silver-rich tetrahedrite occurs, together with more complex minerals of the fahlore group. Gold is substantially confined to quartz-carbonate veins in the Owen Formation, where it occurs in a hypogene assemblage of marcasite, pyrite, and quartz, which represents late stage deposition. Some gold, which is highly argentiferous, occurs partly in idiomorphic form and is considered to be hypogene; supergene gold is also present. An earlier report of native bismuth appears to be based on the misidentification of native lead. Mineralization extended over a wide range of generally declining temperatures and changing fluid composition; precise reconstruction of physico-chemical conditions is difficult due to the effects of retrograde overprinting, and the lack of equilibrium data for the sulphide, carbonate, and silicate systems examined. A study of fluid inclusions provides some precise data on the hydrothermal system; evidence for the existence of a CO₂-saturated fluid during mineral deposition permits the use of filling temperatures without correction for sealing pressure. The measurement of both CO₂ content and salinity for one inclusion permits complete P-T-X evaluation of the local fluid system at the time of sealing: XCO2₂ = 15 wt.% XNaCl = 9.3 wt.% T = 312°C P = 790 b All data are combined in an interpretative model for the mineralization, in which a fluid is produced by intrusion-induced dewatering and dehydration of sediments, and the mobilisation and transport of ore metals is promoted by decarbonation and desulphurization reactions. Structural control by the impermeable marble produces an up-dip, non-recirculatory fluid flow, resulting in intensified alteration and mineralization in the upper Owen Formation