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>

Bulk composition of a zoned rare-earth minerals-bearing pegmatite in the Pikes Peak granite batholith near Wellington Lake, central Colorado, U.S.A.

ABSTRACT A previously undescribed small lenticular (~5 × 5 × 5 m) pegmatite, located near Wellington Lake in the NW part of the 1.08 Ga ‘A-type’ (anorogenic) ferroan Pikes Peak granite batholith, ~15 km SW of the South Platte pegmatite district in central Colorado, is concentrically zoned around a mostly monomineralic quartz core with interconnected miarolitic cavities. Major constituents of the Wellington Lake pegmatite are quartz, perthitic microcline, albite (variety cleavelandite), hematite, and biotite. Accessory minerals include fluocerite, bastnäsite, columbite, zircon (var. ‘cyrtolite’), thorite, and secondary U phases. Fluorite is conspicuously absent, although it is a common phase in the South Platte district NYF-type pegmatites, which are rich in niobium (Nb), yttrium (Y), fluorine (F), and heavy rare-earth elements (HREE). Notable for the Wellington Lake pegmatite are a small quantity of well-developed tabular crystals of fluocerite that reach up to 4 cm in diameter, with sub-mm epitaxial bastnäsite overgrowths, suggesting formation from F- and CO2-bearing solutions rich in light rare-earth elements (LREE), with decreasing a(F-)/a(CO32-) during the last crystallization phase. An Nd-isotope value of εNd1.08Ga = -1.6 for the fluocerite is within the range of εNd1.08Ga = -0.2 to -2.7 of the host coarse-grained, pink K-series Pikes Peak Granite (PPG), indicating that REE and other pegmatite constituents derived from the parental PPG magma. A calculation of total pegmatite composition based on whole-rock chemistry and volume estimates of the different pegmatite zones reveals an overall composition similar to the PPG with respect to Si, Al, Na, and K. Yet the pegmatite is depleted in Fe, Mg, Ca, Ti, Mn, and P, the high-field-strength elements (HFSE; Zr, Hf, Nb, Y, Th), and, most significantly, total REE compared to the PPG. Despite containing the LREE minerals fluocerite and bastnäsite, the lack of a net overall REE enrichment of the pegmatite compared to the PPG reflects the large amount of REE-poor silicate minerals forming the wall, intermediate, and core zones of the pegmatite. The calculated total pegmatite composition suggests that the pegmatite formed by the separation from the PPG magma of an F-poor H2O-saturated silicate melt depleted in REE and HFSE compared to the F-rich melts, which formed the NYF-type HREE-rich (LaN/YbN &lt; 1) pegmatites in the South Platte district. Homogenization temperatures of &lt; 500°C for possibly primary fluid inclusions in large quartz crystals from the core of the Wellington Lake pegmatite are consistent with recent models of pegmatite petrogenesis leading to nucleation controlled mega-crystal growth resulting from supercooling.

Petrography and Geochemical Characterization of a Granite Batholith in Idanre, Southwestern Nigeria

Idanre granite batholith in southwestern Nigeria contain three rock types, namely, Older granite undifferentiated (OGu), Older granite porphyritic (OGp) and Older granite fine-grained (OGf). The granitoids intruded into a basement rock of primarily migmatite gneiss. Petrography indicates that quartz, orthoclase, hornblende, and biotite are common to all members while microcline is more prominent in OGp and plagioclase is poorly represented in OGf. Despite minor differences in petrographic features, the granite units generally have similar geochemical relationships. The average SiO2 contents in OGp (70.49%), OGu (68.7%) and OGf (65.8%) are comparable to similar Pan-African suites located in eastern and northern Nigeria. Na2O+K2O-CaO versus SiO2 diagram shows all the granite members are calcic, K2O vs SiO2 plot classify the granites as high-K calcic alkali to shoshonitic. ANK vs ACNK plot indicatesthey are peraluminous. Plot of A/CNK vs SiO2 and K2O vs Na2O diagrams classified the rock as S-type granite. The granitoids are calc-alkaline with elevated Na2O (>2.6%) and Al/(Na2O+CaO) contents (OGu, 2.1-3.4; OGp, 2.4-3.1 and OGf, 2.2-2.9). The tectonic diagram (Rb vs (Y+Nb) indicatesthatthe batholith is Within Plate Granite (WPG.

Dating of Guperas Formation rhyolites changes the stratigraphy of the Mesoproterozoic Sinclair Supergroup of Namibia

The volcanosedimentary Guperas Formation contains the youngest volcanic rocks of the Sinclair Supergroup in the Konkiep Terrane of southern Namibia. Precise U-Pb zircon microbeam dating shows that the Guperas Formation as mapped includes felsic volcanic rocks which belong to both the first (1.37 to 1.33 Ga) and the third (1.11 to 1.07 Ga) magmatic cycle of the Sinclair Supergroup. Volcanic rocks of the ‘true’ Guperas Formation are dated by three samples, with a combined age of 1108 ± 10 Ma. The sedimentary rocks mapped as Guperas Formation are also distinguished by two different detrital age spectra into the ~1 100 Ma true Guperas Formation and the Aruab Member of the ~1 217 Ma Barby Formation. Geochronology now resolves the previous stratigraphic separation of the very similar Nubib and Rooiberg (Sonntag) Granites. The two small outcrops of 1 334 ± 5 Ma Rooiberg Granite are now shown to be part of the regional 1 334 ± 8 Ma Nubib Granite batholith. The Konkiep Terrane was affected by faulting and shear zones, but was only gently folded and not involved in regional metamorphism, despite its proximity to the Namaqua-Natal Province to the southwest. This is due to the Konkiep Terrane having a thick and strong continental basement which may have formed as part of the mainly Palaeoproterozoic Rehoboth Province. However no Palaeoproterozoic rocks are exposed in the Konkiep Terrane, which is now interpreted as an unaffiliated terrane. The three cycles of extrusive and plutonic magmatism in the Sinclair Supergroup formed in chronologically distinct periods and different tectonic settings, which requires revision of the stratigraphic nomenclature. The Konkiep Group is replaced by three new groups which are separated by &gt;100 million-year unconformities. The Betta Group, represented by the mainly volcanic Kumbis, Nagatis and Welverdiend formations in the first magmatic cycle, probably formed in a passive continental rift setting due to breakup of the Rehoboth Province between 1 374 and 1 334 Ma. The Vergenoeg Group, represented by the sedimentary Kunjas and volcanic Barby and Haiber Flats formations, formed in a subduction setting at the margin of the Konkiep Terrane. This ~1 217 to 1204 Ma magmatic cycle ended with the accretion of Namaqua-Natal terranes to the Kalahari Craton. The ~1 100 Ma Ganaams Group, represented by the volcanic Guperas Formation and sedimentary Aubures Formation, was the result of interplay between the continental-scale Umkondo mantle heating event and movements between crustal blocks following the Namaqua-Natal collisional orogeny.

Granitoids and greenstones of the White Mfolozi Inlier, south-east Kaapvaal Craton

A Palaeoarchaean greenstone fragment and associated granitoid gneisses from an area south of Ulundi in KwaZulu-Natal is described. The fragment consists of an association of garnetiferous amphibolite and calc-silicate that was intruded at 3388 ± 4 Ma by tonalite and at 3275 ± 4 Ma by trondhjemite. Strong ductile deformation of the greenstones and granitoids under amphibolite facies conditions (7 kbar and 600 to 650°C) took place prior to uplift and emplacement of a granite batholith at ~3.25 Ga ago in which the granitoid gneiss-greenstone domain is now found. Magmatism 3.27 to 3.25 Ga ago was a direct response to regional metamorphism and anataxis, and gave rise to stabilization of the southeastern Kaapvaal Craton at that time, earlier than other parts of the craton. Deposition of quartz-arenites on stable granitic basement took place &lt;3.1 Ga ago. Contrasting ages in magmatic pulses and regional metamorphism reflect a different crustal growth history of the eastern and southeastern part of the Kaapvaal Craton.