Geology and Mineralization of the Bald Hill Molybdenum Occurrence, Buller District, West Nelson, New Zealand

<p>Molybdenite mineralization occurs within the Bald Hill Prospect (West Nelson) in brecciated and hornfelsed Greenland Group slates and metagreywackes and associated quartz trondhjemite porphyry minor intrusions (Lyell Porphyry). Potassium argon (K-Ar) ages of the Lyell Porphyry, several granites forming part of the adjacent Karamea Granite batholith (Bald Hill Granites) and mineralized hornfelsic country rocks fall in the range 102-120 Ma (mid-Cretaceous). Adjacent lower Ordovician Greenland Group slates yielded four K-Ar ages in the range 112-226 Ma indicating partial argon outgassing of these older metasediments. The Bald Hill Granites and the Lyell Porphyry granitic rocks belong to separate petrogenic provinces. Bald Hill Granites forming the western margin of the Karamea Granite batholith occur as a suite of foliated, medium-grained, muscovite-bearing leucogranites, pink microgranites and biotite-granites. Chemically these rocks are peraluminous-potash granites with 72-75% SiO2, MgONa2O with Rb > Sr and always contain more than 30% normative quartz and 3% normative corundum. In contrast, the Lyell Porphyry rocks intruding both Greenland Group and Bald Hill Granite country rocks, form a series of small, high-level plutons and cross-cutting dykes of quartz trondhjemite, granodiorite, quartz diorite, lamprophyre and quartz-bearing gabbroporphyry. Chemically the Lyell Porphyry intrusive rocks are soda-rich calc-alkaline granitoids containing 46-70% SiO2, >1% MgO, >2.2% CaO, with Na2O>K2O and Sr>Rb with less than 28% normative quartz and less than 2% normative corundum. From their studies of granite batholiths in southeastern Australia, Chappell and White (1974) recognise two contrasting granitoid types called I-type and S-type granites. The Lyell Porphyry and several other intrusive stocks associated with molybdenum mineralization in West Nelson and North Westland are shown to correspond to I-type granites, in contrast to the Karamea batholith granites (including Bald Hill Granites) which conform to S-type granites. Sulphur isotopic analyses of mineralization for ten molybdenum prospects in West Nelson indicate uniformly high temperatures of mineralization in the range 400° to 500°C, with a probable magmatic source for sulphur. The Bald Hill and other S-type granites forming the Karamea batholith were probably formed by the ultrametamorphism of crustal sedimentary material. The Lyell Porphyry and other molybdenum-bearing calc-alkaline intrusive stocks represent melt phases of deeper origin intruding the overlying granites and sediments. The emplacement of these stocks appears to equate with north-south lineaments and large scale circular features in the granite terranes of West Nelson. The geological setting, age, petrological characteristics and molybdenite mineralization of the Lyell Porphyry and Bald Hill Granites are similar to that of other West Nelson occurrences. All are associated with mid-Cretaceous minor granitic porphyry intrusions, emplaced in Paleozoic metasediments, close to the margins of the Karamea and Separation Point batholiths.</p>

Geology and Mineralization of the Bald Hill Molybdenum Occurrence, Buller District, West Nelson, New Zealand

<p>Molybdenite mineralization occurs within the Bald Hill Prospect (West Nelson) in brecciated and hornfelsed Greenland Group slates and metagreywackes and associated quartz trondhjemite porphyry minor intrusions (Lyell Porphyry). Potassium argon (K-Ar) ages of the Lyell Porphyry, several granites forming part of the adjacent Karamea Granite batholith (Bald Hill Granites) and mineralized hornfelsic country rocks fall in the range 102-120 Ma (mid-Cretaceous). Adjacent lower Ordovician Greenland Group slates yielded four K-Ar ages in the range 112-226 Ma indicating partial argon outgassing of these older metasediments. The Bald Hill Granites and the Lyell Porphyry granitic rocks belong to separate petrogenic provinces. Bald Hill Granites forming the western margin of the Karamea Granite batholith occur as a suite of foliated, medium-grained, muscovite-bearing leucogranites, pink microgranites and biotite-granites. Chemically these rocks are peraluminous-potash granites with 72-75% SiO2, MgONa2O with Rb > Sr and always contain more than 30% normative quartz and 3% normative corundum. In contrast, the Lyell Porphyry rocks intruding both Greenland Group and Bald Hill Granite country rocks, form a series of small, high-level plutons and cross-cutting dykes of quartz trondhjemite, granodiorite, quartz diorite, lamprophyre and quartz-bearing gabbroporphyry. Chemically the Lyell Porphyry intrusive rocks are soda-rich calc-alkaline granitoids containing 46-70% SiO2, >1% MgO, >2.2% CaO, with Na2O>K2O and Sr>Rb with less than 28% normative quartz and less than 2% normative corundum. From their studies of granite batholiths in southeastern Australia, Chappell and White (1974) recognise two contrasting granitoid types called I-type and S-type granites. The Lyell Porphyry and several other intrusive stocks associated with molybdenum mineralization in West Nelson and North Westland are shown to correspond to I-type granites, in contrast to the Karamea batholith granites (including Bald Hill Granites) which conform to S-type granites. Sulphur isotopic analyses of mineralization for ten molybdenum prospects in West Nelson indicate uniformly high temperatures of mineralization in the range 400° to 500°C, with a probable magmatic source for sulphur. The Bald Hill and other S-type granites forming the Karamea batholith were probably formed by the ultrametamorphism of crustal sedimentary material. The Lyell Porphyry and other molybdenum-bearing calc-alkaline intrusive stocks represent melt phases of deeper origin intruding the overlying granites and sediments. The emplacement of these stocks appears to equate with north-south lineaments and large scale circular features in the granite terranes of West Nelson. The geological setting, age, petrological characteristics and molybdenite mineralization of the Lyell Porphyry and Bald Hill Granites are similar to that of other West Nelson occurrences. All are associated with mid-Cretaceous minor granitic porphyry intrusions, emplaced in Paleozoic metasediments, close to the margins of the Karamea and Separation Point batholiths.</p>

Layered Lithospheric Mantle Beneath the Ontong Java Plateau: Implications from Xenoliths in Alnöite, Malaita, Solomon Islands

A varied suite of mantle xenoliths from Malaita, Solomon Islands, was investigated to constrain the evolution of the mantle beneath the Ontong Java Plateau. Comprehensive petrological and thermobarometric studies make it possible to identify the dominant processes that produced the compositional diversity and to reconstruct the lithospheric stratigraphy in the context of a paleogeotherm. P–T estimates show that both peridotites and pyroxenites can be assigned to a shallower or deeper origin, separated by a garnet-poor zone of 10 km between 90 and 100 km. This zone is dominated by refractory spinel harzburgites (Fo91–92), indicating the occurrence of an intra-lithospheric depleted zone. Shallower mantle (∼Moho to 95 km) is composed of variably metasomatized peridotite with subordinate pyroxenite derived from metacumulates. Deeper mantle (∼95–120 km) is represented by pyroxenite and variably depleted peridotites that are unevenly distributed; the least-depleted garnet lherzolite (Fo90–91) lies just below the garnet-poor depleted zone (∼100–110 km), whereas the presence of pyroxenite is restricted to the deepest region (∼110–120 km), together with relatively Fe-enriched garnet lherzolite (Fo87–88). This depth-related variation (including the depleted zone) can be explained by assuming that the degree of melting for a basalt–peridotite hybrid source was systematically different at each level of arrival depth within a single adiabatically ascending mantle plume: (1) the depleted zone at the top of the mantle plume, where garnet was totally consumed in the residual solid; (2) an intermediate part of the plume dominated by the least-depleted garnet lherzolite just above the depth of the peridotite solidus; (3) the deepest pyroxenite-rich zone, whose petrochemical variation is best explained by the interaction between peridotite and normative quartz-rich basaltic melt, below the solidus of peridotite and liquidus of basalt. We explain the obvious lack of pyroxenites at shallower depths as the effective extraction of hybrid melt from completely molten basalt through the partially molten ambient peridotite, which caused the voluminous eruption of the Ontong Java Plateau basalts. From these interpretations, we conclude that the lithosphere forms a genetically unrelated two-layered structure, comprising shallower oceanic lithosphere and deeper impinged plume material, which involved a recycled basaltic component, now present as a pyroxenitic heterogeneity. This interpretation for the present lithospheric structure may explain the seismically anomalous root beneath the Ontong Java Plateau.

Early Proterozoic granite of the Taplejung Window, far eastern Lesser Nepal Himalaya

The Lesser Himalayan Sequence of the Taplejung Window in the far eastern Nepal Himalaya can be divided into Taplejung Formation, Mitlung Augen Gneiss and Linkhim Schist (from bottom to top respectively). The window is a large domal shaped anticline plunging to the east. Two-mica granite bodies (Amarpur Granite, Kabeli Khola Granite and Tamor River Granite) have intruded the metasediments of Taplejung Formation. The granite bodies are discordant to subconcordant in relation to the country rocks. Quartz, alkali feldspar, plagioclase, muscovite, biotite and tourmaline are the main constituent minerals of the granite. Generally, the core of granite bodies is undeformed, whereas the marginal part is gneissfied with S-C mylonitic texture showing the top to south sense of shear. This sense of shear is related to the movement along the Main Central Thrust (MCT). All the samples from the granitic bodies fall under the granite field in the normative quartz-alkali feldspar-plagioclase (QAP) triangular diagram. The mineral composition shows that the granite is peraluminous in nature. The Kabeli Khola Granite has yielded a 40Ar/ 39Ar muscovite age older than 1.6 Ga indicating its magmatic age. The granites of the study area can also be correlated with the 1.8 Ga Ulleri type augen gneiss of central Nepal.

Origin of quartzofeldspathic gneisses at Montauban-les-Mines, Quebec

Quartzofeldspathic gneisses near zinc–lead–gold mineral deposits at Montauban-les-Mines, Quebec, have been examined geochemically to determine whether their protoliths were dominantly sedimentary or igneous. The gneisses are generally similar in average compositions to both rhyolitic rocks and sandstones (greywackes in particular). The most useful methods of protolith discrimination were found to be: (1) log (SiO2/Al2O3) versus log [(CaO + Na2O)/K2O]; (2) Niggli si versus mg; (3) normative quartz; (4) Shaw's discriminant function. Application of these criteria indicates that the Montauban quartzo-feldspathic gneisses were derived dominantly from quartzose sandstone or greywacke protoliths (or both).

On the electrolysis of molten basalt

SummaryThe effects to be expected from the interaction of possible electric currents at depth with bodies of basaltic magma have been investigated experimentally. Trough-shaped molten specimens were produced in the surface of basalt blocks by heating from above ; the melts were electrolysed and the products analysed chemically.Si, Al, Ti, P, Fe2+, and probably Fe3+ are concentrated towards the anode, apparently in the form of drifting lattice remnants; oxygen gas is liberated. Na, K, Ca, Mn, and Mg ions concentrate towards the cathode.Relatively to basalt, the cathodic product is an alkaline and femic rock with normative nepheline and a more acid plagioclase. The anodic rock is distinctly calc-alkaline and salic, with normative quartz and a plagioclase that is more basic. The precise ‘rock-type’ developed depends on the amount of electricity passed, but the trends are distinct. In that increasing basicity and alkalinity (cathodic rocks) are accompanied by both increasing Na/K and Mg/(Fe2+ + Fe3+) ratios (which ratios decrease with decreasing basicity and alkalinity) the electrolytic series developed from a basalt magma appears to have few counterparts among natural rocks.

Calculation of the C.I.P.W. norm

SummaryA modification of the rules for the cMeulation of C.I.P.W. norms when no normative quartz is present is suggested.