Source and redox controls on metallogenic variations in intrusion-related ore systems, Tombstone-Tungsten Belt, Yukon Territory, Canada
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Abstract:
The Tombstone, Mayo and Tungsten plutonic suites of granitic intrusions, collectively termed the Tombstone-Tungsten Belt, form three geographically, mineralogically, geochemically and metallogenically distinct plutonic suites. The granites (sensu lato) intruded the ancient North American continental margin of the northern Canadian Cordillera as part of a single magmatic episode in the mid-Cretaceous (96-90 Ma). The Tombstone Suite is alkalic, variably fractionated, slightly oxidised, contains magnetite and titanite, and has primary, but no xenocrystic, zircon. The Mayo Suite is sub-alkalic, metaluminous to weakly peraluminous, fractionated, but with early felsic and late mafic phases, moderately reduced with titanite dominant, and has xenocrystic zircon. The Tungsten Suite is peraluminous, entirely felsic, more highly fractionated, reduced with ilmenite dominant, and has abundant xenocrystic zircon. Each suite has a distinctive petrogenesis. The Tombstone Suite was derived from an enriched, previously depleted lithospheric mantle, the Tungsten Suite is from the continental crust including, but not dominated by, carbonaceous pelitic rocks, and the Mayo Suite is from a similar sedimentary crustal source, but is mixed with a distinct mafic component from an enriched mantle source.Keywords:
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In this study we examined Meso- to Neoarchean granitoids from the North Caribou Terrane within the Western Superior Province, Canada. Petrology, whole-rock geochemistry, zircon and titanite geochronology, and zircon trace element concentrations were analyzed. U–Pb ages from zircon and titanite are between 2·62 and 3·13 Ga. Although most of the granitoids in this study appear to record a complex magmatic history, about a third contain features that we interpret to be a result of hydrothermal alteration. Notable traits in rocks that contain altered zircons include K-feldspar overgrowths on plagioclase and compositional zoning in titanite. The altered zircon material itself occurs as CL-bright resorption shadows showing distinct chemical changes, including lower Th/U values and elevated LREE concentrations. The isotopic ages of the rims on the altered zircons (2835 Ma, 2760–2678 Ma) are similar to coexisting U–Pb titanite ages and regional U–Pb titanite and zircon ages. We propose that during the hydrothermal event, the affected areas of zircon re-equilibrated with fluid, which promoted Pb loss, resetting the isotopic clock. These results suggest that zircon rims might be useful for dating hydrothermal fluid flow episodes in addition to magmatic events and that a multi-element approach is useful for distinguishing ages that are magmatic from those that have been isotopically disturbed owing to alteration.
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Appendix A—Analytical methods; Appendix B—Supplementary tables B1 (summary of zircon characteristics), B2 (zircon U-Pb age deduction summary and notes), B3 (summary of titanite characteristics), and B4 (titanite U-Pb age deduction summary and notes); and Appendix C—zircon and titanite U-Pb data.
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Abstract: I– and S‐type granites differ in several distinctive ways, as a consequence of their derivation from contrasting source rocks. The more mafic granites, whose compositions are closest to those of the source rocks, are most readily classified as I– or S–type. As granites become more felsic, compositions of the two types converge towards those of lowest temperature silicate melts. While discrimination of the two is therefore more difficult for such felsic rocks, that in no way invalidates the twofold subdivision. If felsic granite melts undergo fractional crystallisation, the major element compositions are not affected to any significant extent, but the concentrations of trace elements can vary widely. For some trace elements, fractional crystallisation causes the trace element abundances to diverge, so the I– and S– type granites are again easily separated. Such fractionated S‐type granites can be distinguished, for example, by high P and low Th and Ce, relative to their I‐type analogues. Our observations in the Lachlan Fold Belt show that there is no genetic basis for subdividing peraluminous granites into more mafic and felsic varieties, as has been attempted elsewhere. The subdivision of felsic peraluminous granites into I– and S‐types is more appropriate, and mafic peraluminous granites are always S–type. In a given area, associated mafic and felsic S‐type granites are likely to be closely related in origin, with the former comprising both restite‐rich magmas and cumulate rocks, and the felsic granites corresponding to melts that may have undergone fractional crystallisation after prior restite separation. We propose a subdivision of I‐type granites into two groups, formed at high and low temperatures. The high‐temperature I–type granites formed from a magma that was completely or largely molten, and in which crystals of zircon were not initially present because the melt was undersaturated in zircon. In comparison with low‐temperature I–type granites, the compositions extend to lower SiO 2 contents and the abundances of Ba, Zr and the rare earth elements initially increase with increasing SiO 2 in the more mafic rocks. While the high‐temperature I–type granite magmas were produced by the partial melting of mafic source rocks, their low‐temperature analogues resulted from the partial melting of quartzofeldspathic rocks such as older tonalites. In that second case, the melt produced was felsic and the more mafic low‐temperature I–type granites have that character because of the presence of entrained and magmatically equilibrated restite. High temperature granites are more prospective for mineralisation, both because of that higher temperature and because they have a greater capacity to undergo extended fractional crystallisation, with consequent concentration of incompatible components, including H 2 O.
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Appendix A—Analytical methods; Appendix B—Supplementary tables B1 (summary of zircon characteristics), B2 (zircon U-Pb age deduction summary and notes), B3 (summary of titanite characteristics), and B4 (titanite U-Pb age deduction summary and notes); and Appendix C—zircon and titanite U-Pb data.
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In monomagmatic chambers density differences between overlying country rock and magma should, following development of ring fractures, result in mafic ring dikes above mafic magma, and in foundering of the roof into felsic magma with the development of felsic stocks. The observed prevalence of felsic ring dikes may be due to shallow floors in felsic magma chambers, or to shallow constructed floors at the top of the mafic fraction in polymagmatic chambers.
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Petrographic studies on the mafic and felsic dykes from the Piratini region, RS, reveal a porphyritic texture with an aphanitic to fine-grained matriz, with glomeroporphyritic and spherulitic textures in the case of felsic, and ophitic to sub-ophitic with myrmekitic intergrowths in the case of mafic dykes. These dykes are intrusive in granitic rocks of the Pelotas Batholith along NW-SE and N-S trends associated with high angle shear zones. They are strongly metaluminous (mafic) and slighty metaluminous to peraluminous (felsic), with an alumina saturation index between 0.60 and 0,65 and 0,75 and 1,2, respectively. The Si02 content in the mafic dykes varies from 44 to 48 wt % and in the felsic varies from 67 to 75 wt %. The mafic dykes present higher Ti, Mg, Ca, Fe, Mn and P contents in comparison with the felsic dykes. The behavior of several major and trace elements (Fe, Mn, Mg, Ti, P, Ca and Sr, and, in minor degree, Ca and Mg show the importance of crystallization of iron-magnesium and feldspars in both dykes. The Nb/Ta e U/Th ratios of trace elements of these rocks show evidence of crustal contamination. The HREE display sub horizontallized patterns, showing Tb/Lu ratios from 1,5 to 2,7 in the samples of mafic dykes and from 1,6 to 2,5 in the felsic dykes. The samples have shown Lu concentration from 0,3 ppm to 0,6 ppm in mafic, and from 18 ppm to 41 ppm in the felsic dykes, respectively. The felsic dykes are more enriched in LREE, with La values from 73 ppm to 155 ppm, whereas mafic dykes present 18 ppm to 41 ppm of La and La/Sm ratios in the range of 3,0 to 5,0.
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Abstract Zircon shape is commonly reported during geochronology and geochemistry analyses of igneous, metamorphic, and sedimentary rocks, but the relationship of zircon shape to primary growth environmental conditions remains poorly constrained. Current models for the control on igneous zircon shape focus on the relative growth of crystal prisms and pyramids, which are not discernible in the imaging techniques used for rapid quantification of zircon shape in geochronology sample mounts. We model the relationship between whole‐rock composition and zircon 2D shape in mineral separates from 45 mafic to felsic igneous samples, representative of Archean and Proterozoic crust in Western Australia. Shape parameters are derived from semi‐automated measurement of photomicrographs of polished zircon crystals in epoxy resin mounts. Whole‐rock composition shows a statistically significant relationship to median magmatic zircon crystal area and mathematically defined “roundness.” Zircon populations show reduced median area and increased median roundness as whole‐rock silica decreases. Phase equilibrium modeling based on whole‐rock composition, and automated electron microscopy mineral maps, indicates that the compositional predisposition of zircon shape is influenced by fundamentally different physical growth environments in mafic versus felsic melts. Specifically, influential factors that differ between mafic and felsic liquids include crystallization sequence and duration—which influence unconstrained growth space—and the potential for absorption/exsolution of zirconium from the accompanying mineral assemblage. We present quantitative, explanatory models for the relationship between zircon 2D shape and whole‐rock silica and demonstrate that the relationships are adhered to across a broad spectrum of whole‐rock compositions.
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