Robert L. Brathwaite

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.

Geology, mineralogy and geochemistry of the rhyolite-hosted Maungaparerua clay deposit, Northland

A rhyolite dome complex at Maungaparerua, Ar/Ar dated at 3.7 ± 0.04 Ma (Early Pliocene), is bounded on the west by a sequence of hydrothermally altered basalt flows intercalated with several non-marine siltstone and rhyolitic tuff units. This basalt sequence and the rhyolite dome complex are overlain by younger unaltered basalt flows. Within the dome complex, several small pits have been worked in the past for china clays. Recent drilling has outlined a halloysite-rich ‘Southern Area’ extending to a depth of up to 24 m below the present-day erosion surface. Primary sanidine and plagioclase phenocrysts are completely leached in halloysite-rich rhyolite, but are only partially leached at greater depth. Halloysite-rich rhyolite is characterised by relative enrichment in loss on ignition (LOI; 5–9%) and Al2O3 (18–24%) and depletion in K2O (<0.5%), compared with 2.0% LOI, 15.0% Al2O3 and 4.1% K2O in least-altered rhyolite. Oxygen and hydrogen isotope compositions of halloysite samples indicate that it is of supergene rather than hydrothermal origin. This is consistent with weathering type clay profiles in halloysite-rich zones. Although there is earlier hydrothermal alteration in the form of silicified rhyolite 800 m to the west of the Southern Area and kaolinite-pyrite alteration in the adjacent basalt, we conclude that the dominant process in the formation of the halloysite was deep weathering of sanidine rhyolite under water-saturated subtropical conditions.

Pyrite-coated granite cobbles at Lee Bay, Stewart Island

On the west side of Lee Bay on the northeast coast of Stewart Island, ventifact cobbles of pyrite-coated granite occur on the beach near the high tide mark and appear to be derived from a sand-cemented gravel deposit that forms a low bank at the back of the beach. The pyrite coat (up to 1 mm thick) completely covers the granitic cobbles and is zoned, with an inner zone of fine-grained colloform pyrite and an outer framboidal zone. Framboidal pyrite is typically formed in anoxic sedimentary environments. Subrounded grains of hematite, ilmenite with hematite blebs, magnetite, feldspar, biotite, quartz and zircon are present in the outer framboidal zone, with some ilmenite and hematite grains being partially replaced by pyrite. The assemblage of ilmenite-hematite-magnetite-biotite-zircon is similar both in mineralogy and size range to that found in heavy mineral beach sands. Sulphur isotope values of the pyrite coat are consistent with formation of the pyrite by microbial sulphate reduction of seawater sulphate. The framboidal texture together with the presence of grains of beach sand in the pyrite coating indicate that it was deposited in a low-temperature sedimentary environment.

Gold mineralisation in the polymetallic Sams Creek peralkaline microgranite, South Island, New Zealand

Abstract At Sams Creek, a peralkaline microgranite dyke intrudes Lower Palaeozoic greenschist facies metasediments. The granitedyke has been extensively silicified and hosts stockwork veins composed of quartz, siderite, and arsenopyrite + pyrite ± gold ± galena ± sphalerite ± chalcopyrite ± pyrrhotite. Alteration and gold mineralisation are confined to the granite, and these features together with the style of veining and the sulphide mineralogy indicate that mineralisation is related to granite rather than to an external metamorphic source. Calculated δ18OH2O values from vein quartz and siderite range between 6 ℵ and 9 ℵ, and between 6 ℵ and 8 ℵ (VSMOW), respectively. δ13C values of siderite are all about − 5 ℵ (VPDB) and δ34S values of the sulphides have a narrow range with an average of +9.0ℵ (CDT). These oxygen, carbon, and sulphur isotope values of the vein minerals (quartz, siderite, and sulphides) are consistent with a magmatic-hydrothermal fluid source, but they could also be derived from a metamorphic source.

Chromite, platinum group elements and nickel mineralisation in relation to the tectonic evolution of the Dun Mountain Ophiolite Belt, east Nelson, New Zealand

ABSTRACT In the Dun Mountain Ophiolite Belt of east Nelson, New Zealand, chromite segregations (chromitites) occur in a lower zone of serpentinised harzburgite and dunite from Wooded Peak (Dun Mountain) to Little Ben, and in dunite-rich zones in the Red Hills massif and Baldy Ridge outlier. Electron probe microanalyses of chromite grains from the chromitites and also the Red Hills peridotite reveal a range in chromite compositions that are consistent with depletion of peridotite and reaction with island arc tholeiite and boninitic melts in a forearc setting of a supra-subduction zone. Podiform chromitite samples in the harzburgite zone show elevated Ir and Ru values, similar to the levels of enrichment of these platinum group elements (PGE) found in podiform chromite deposits in ophiolites worldwide. The PGE values in dunite-hosted chromitite from Baldy Ridge (Matakitaki) have elevated Pt, Pd and Rh, as is found in cumulate dunite zones in some other ophiolites. Disseminated Ni mineralisation as awaruite ± magnetite ± ferritchromit ± heazlewoodite ± pentlandite ± native Cu occurs in serpentinised peridotites, and was likely produced as a by-product of hydration as a result of detachment faulting allowing penetration of seawater into underlying mantle peridotite.