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published_or_final_version / abstract / toc / Applied Geosciences / Master / Master of Science

Typescript. Thesis (Ph. D.)--University of Hawaii, 1958. Bibliography: leaves [84]-87.

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A detailed methodology has been devised and tested for establishing metal zoning patterns in and about oreshoots within the plane of Ag-Pb-Zn veins of the Keno Hill - Galena Hill mining camp, Yukon Territory, using whole rock vein geochemistry of run-of-mine samples. These ideal metal distribution patterns are easily interpretable in more classical zoning terms as mineral distribution patterns. To establish a zoning model samples that span a wide range of grades are rearranged in order of decreasing silver contents. The resulting "rearranged" profiles for other elements are then evaluated relative to silver. Computer-based curve fitting methods are useful means of generalizing these "rearranged" metal profiles. The foregoing procedure has been used to develop a general model for the Keno Hill - Galena Hill camp based on analytical results from 3 main deposits (Keno, Husky, No Cash) including 6 veins. Essential character of the model is embodied in analyses of Ag, Pb, Zn, and Ca and the Zn/Ag ratio. Additional but in cases ambiguous detail is added to the model by Hg, and Co and/or Ni analyses. These elements allow vein mineralogy to be monitored in a quantitative manner and provide a practical zoning model that can be used as an exploration, tool in evaluating underground workings for proximity to oreshoots in the more than 60 deposits known in the camp. A companion study involving whole rock K-Ar age determinations of small stockwork zones adjacent to Ag-Pb-Zn veins indicated an age of mineralization of about 87±2 Ma. for the Ag-Pb-Zn veins. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

The Komati Formation of the Barberton Greenstone Belt is the type locality of the rock type known as Komatiites. Komatiites are ultramafic lavas that were generated and erupted mainly during the Archaean. They give insight into volcanism on the early Earth as well as the nature of the mantle and melting processes. During 2010-2011 the International Continental Drilling Programme (ICDP) Barberton Project drilled two cores (BARB 1 and BARB 2) into the Komati Formation to obtain continuous sections of the komatiite strata. These cores were drilled to gain knowledge about the structure, textures, compositions, processes and contact relationships of komatiite flows, which could not be obtained from surface outcrop because of lack of continuity and relatively poor exposure. The drill holes also intersected a volcanic tumulus unit, the first of its type recognized in komatiite lava flows, allowing insight into the processes that created the tumulus and the processes responsible for creating the differentiated komatiite flows. The core was logged in detail, revealing a variety of rock-types and styles of volcanism. The rock-types encountered range from massive and differentiated ultramafic komatiites, through komatiitic basalts to mafic basalts. Some minor later intrusions of gabbros and dolerite are also present. The komatiites and komatiitic basalts are extrusive lavas and represent continuous eruptive sequences. The gabbros are typically intrusive, but can represent late stage crystallization. Contact relationships are evident in the core, where they have not been eroded by overlying flows, and are used to distinguish 85 individual flows in BARB 1 and 65 in BARB 2. Chill margins are typically between 5 and 50 cm thick and brecciated contacts are usually 5- 15 cm thick where present. Only rare examples of original mineralogy (olivine and pyroxene) are preserved because of pervasive alteration of the rocks. Alteration minerals are serpentine, chlorite, tremolite and magnetite. Early serpentine veining was followed by later stage magnesite veining. The opaque minerals – chromite with secondary magnetite overgrowth – are mostly located at olivine grain boundaries. The tumulus unit in the BARB 1 core was created by upward doming of the upper skin of a lava tube. The unit is 90 m thick and consists of five textural sections: basal cumulates, harrisite, pyroxene spinifex, gabbro-pyroxenite and a hyaloclastite unit that caps the sequence. The cumulates contain macrocrystic olivine grains that reach 15 mm in length; they are elongated and rounded, aligned in certain horizons and are tightly packed, with a maximum of 20 % matrix. The harrisite is a form of skeletal olivine (with crystals up to 5 cm in length) that grew upwards from the underlying cumulate layer. Between the skeletal olivine crystals are small (< 2 mm) crescent-shaped pyroxenes contained within the melt residue. Pyroxene grains in the spinifex lava reach 20 cm in length and are surrounded by a fine-grained matrix. The gabbro and pyroxenite layers contained within the spinifex layer are interpreted to represent the last stage of crystallization within the structure since they are chemically related to the tumulus and no chill margins are present between the gabbro and surrounding pyroxene spinifex. Both the spinifex and gabbro contain unaltered pyroxene crystals and the gabbro also contains relatively fresh plagioclase. The hyaloclastite breccia consists of fragmented fine-grained chill margin blocks derived from the upper crust of the lava tube. The fragments are surrounded and supported by a glassy shard-like matrix. Inflation processes are evident in the tumulus and gave rise to multiple layers of large elongated olivine cumulates together with the upward and outward bulging of the upper crust to form a hyaloclastite breccia. Chemically the tumulus exhibits a Fo93 olivine control. Fractionation processes are clear in MgO vs. depth, binary diagrams and REE plots. Element concentrations are between 1 and 11 times chondrite with a very small LREE enrichment. Differentiated komatiite flows are composed of three lithologies: basal cumulates, olivine spinifex and chill margin zones. The cumulus olivines have a crystal size of 0.5-1 mm, are euhedral and enclosed by a 30 to 60 % melt component. The spinifex olivine forms random or parallel sheets on a centimetre scale and is completely altered to serpentine. Between the olivine spinifex are chemically more evolved pyroxene spinifex blades, which are smaller (millimetre to centimetre scale) and altered to a combination of serpentine-chlorite-tremolite. The fine-grained chill margins of the flows are typically 1-10 cm wide and in some cases contain contact breccias or hyaloclastites. Three packages of differentiated komatiite flows occurring at several stratigraphic intervals where sampled in detail. The lowest package BARB 1 (89-118 m) exhibits chemical trends that are interpreted to indicate a combined crystallization control by olivine and pyroxene. This is evident in the rock compositions and by petrographic studies that reveal the presence of two cumulus phases. The chemical compositions of the other two differentiated packages, BARB1 (378-420 m) and BARB 2 (252-274 m) are controlled by crystallization or accumulation of Fo93 and Fo94, respectively. These packages have olivine cumulates which are surrounded in some cases by pyroxene oikocrysts. In some samples the pyroxenes have unaltered cores. The REE plots are well constrained between 2 and 9 times chondrite values, and have slight LREE enrichment. The tumulus structure and differentiated flow packages BARB 1 (378-420 m) and BARB 2 (252-274 m) have similar mineralogies and compositions but contain different textures. This is attributed to the size of the tumulus in comparison to the differentiated flows and implies similar magma processes and origins for all three units. The BARB 1 (89-118 m) interval appears to have undergone slightly different processes, as indicated by the presence of the two cumulus phases. This package is also altered to a greater extent than the others and no unaltered pyroxene is present.

The Bandelierkop Formation of the Southern Marginal Zone (SMZ) of the Limpopo Belt consists of metamorphosed ultramafic, mafic and sedimentary rocks. Metamorphic conditions indicated by the petrography of these different rock groups are consistent with upper amphibolite to granulite facies. The ultramafic and mafic rocks are geochemically akin to peridotitic-pyroxenitic intrusive rocks and high-Mg basalts respectively. Metamorphism of these two rock groups in the SMZ was an essentially closed system process, except for the highly volatile phases such as H₂O and CO₂. The metasediments appear to represent a sequence of high (Mg+Fe)-greywackes and/or deep-water shales with minor amounts of iron formation material. The unusually mafic character of the metasediments can be ascribed to a high ultramafic + mafic source component to the original sediment. Some modification of the major and trace element compositions of the pelitic rocks has been caused by the removal of partial melts and the metamorphism of the pelitic rocks is not therefore considered to have occurred under closed system conditions. Variable extraction of partial melts is implied by the chemical variations shown by the pelitic rocks and is also suggested by the presence of large pegmatitic felsic bodies which are commonly found close to the pelitic rocks. Detailed petrographic study of the Bandelierkop Formation rocks suggests an increase in metamorphic grade, and/or a decrease in water activity, from south to north within the Southern Marginal Zone. Peak metamorphic conditions of 730°C + 65°C at pressures of 6.1 ± 1.5 kbars may be deduced from a careful application of several cation thermometers and barometers on selected mineral analyses. A rigorous application of garnet-biotite thermometry to the pelitic rocks indicates that the transition from orthoamphibole gneisses in the south to orthopyroxene-bearing rocks in the north of the SMZ terrane, is a function of changing biotite composition and/or decreasing water activities rather than an increase in metamorphic temperatures. In contrast to the major and trace element data, the stable isotope data for the ultramafic and mafic rocks in the SMZ indicate open system behaviour for some of these rocks, but closed system behaviour for the pelites. Extraction of SO to 70% partial melts from the pelitic rocks, should not, however, have affected the δ¹⁸O value of the restite. Petrological and stable isotope data in the SMZ rocks are consistent with retrogression in all these rocks and open system behaviour for oxygen in some ultramafic and mafic rocks, being caused by the crystallization and accompanying fluid release of melts produced during prograde metamorphism of the pelitic rocks. Small scale (-5 to 30m's) heterogeneity is implied by both oxygen and carbon stable isotope compositions of closely spaced rock samples, even for those collected from within large "shear zones", suggesting small fluid/rock ratios for most of the samples in the high-grade terrane or heterogeneous stable isotopic compositions of the fluids. Furthermore, a similarity in mineral-mineral stable isotope fractionation factors is observed for all the pelitic rocks, including the orthoamphibole gneisses found immediately south of the orthopyroxene isograd. These features preclude the presence of pervasive fluid infiltration after peak metamorphism. Two implications are, that granulite facies metamorphism in the SMZ terrane was not caused by an influx of mantle derived CO₂-rich fluid and, that the orthoamphibole gneisses are not retrogressed equivalents of the granulites found to the north of the orthopyroxene isograd. It is suggested that this isograd represents a change in the water activities of the rocks during metamorphism. Apparent disequilibria in mineral - mineral stable isotope fractionations observed amongst different minerals within any one pelitic rock, may be explained by a combination of the crystallization of residual melt within these rocks and oxygen diffusion amongst minerals which have not reached their oxygen-closure. The concordant quartz-plagioclase, quartz-biotite and plagioclase-biotite oxygen isotope equilibration temperatures are taken to represent minimum crystallization temperatures for the melts (-560°C), while successively higher quartz-orthopyroxene, quartz-amphibole and quartz-garnet oxygen isotope equilibration temperatures are a function of increasing closure temperatures for the orthopyroxene, amphibole and garnet respectively. The minimum estimate to peak metamorphic temperatures is given by the quartz-garnet oxygen isotope temperature averaging 736 + 52°C. If oxygen diffusion experiments performed in hydrothermal systems are appropriate for the SMZ rocks, then cooling rates for the SMZ terrane could have been as low as 12 to 25°C/My over a temperature range of 480 to 600°C. Stable isotope modeling of hypothetical fluids that may have been in equilibrium with the high-grade rocks, suggests that a mixture of CO₂ and H₂O, with CO₂/H₂O mole ratios &gt; 0.1 can precipitate both quartz and carbonate of stable isotope composition similar to those determined for quartz and carbonate from the mineralized Archaean lode gold deposits of the Murchison and Pietersburg greenstone belts. A model involving granulite facies metamorphism, partial melt extraction and subsequent release of fluids, Au, K and S upon crystallization of such melts, appears to be viable for gold mineralization occuring in the adjacent lower grade greenstone belts and in the high-grade terrane of the Southern Marginal Zone.

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