Nature and tectonic setting of massive sulfide mineralisation and associated sediments and volcanics in the matakaoa volcanics, Raukumara Peninsula, New Zealand

Abstract

Volcanogenic massive sulfide (VMS) occurrences within the Matakaoa Volcanics on the Raukumara Peninsula, New Zealand, are associated with lenses of siliceous mudstones, cherts (radiolarites), micritic limestone, and rare volcaniclastic sandstone. The Matakaoa Volcanics represent the upper part of an ophiolite sequence of basaltic pillow and massive lavas and hyaloclastic breccias, locally with gabbro-textured concordant units that appear to be ponded lavas rather than sills. The lavas are tholeiitic basalts and basaltic andesites, with rare earth element and Ba, Rb, K, Th, Ta and Nb geochemical signatures indicating that they include both N-type mid-ocean ridge basalts and island-arc tholeiites. This is characteristic of basalts formed at a spreading centre in a back-arc basin with a subduction zone influence. Lenses of siliceous mudstones, cherts, micritic limestone, and rare volcaniclastic sandstone, ranging in size from a few metres to several hundreds of metres in length, are intercalated within the volcanic rocks, and are of Late Cretaceous and late Pale-ocene to early Eocene ages from foraminiferans and radiolar-ians. The mudstones and cherts are composed of continental and volcanic detritus and biogenic silica, consistent with a back-arc basin setting adjacent to a continental arc. The siliceous mudstones locally host some small bodies of massive sulfide and barite-sulfide mineralisation. At Upokon-garuru, a pyrite-pyrrhotite-chalcopyrite massive sulfide body, associated with andradite garnet-hedenbergite alteration, is similar to the high-temperature (350 to >400°C) interiors of sulfide chimneys found in basalt-hosted (ophiolite type) VMS deposits at spreading centres in the Pacific Ocean. A barite + pyrite + marcasite + sphalerite ± chalcopyrite ± galena ± gold occurrence at “B13 creek” is similar to VMS deposits found in volcanic arcs and immature back-arc basins in the western Pacific Ocean and to Kuroko-type VMS deposits. Thus, a spreading centre in an immature back-arc basin appears to be the best fit for the tectonic setting of the Matakaoa Volcanics and its associated sediments and mineralisation. © 2008 Taylor & Francis Group, LLC.