The nature of “quartz eyes” hosted by dykes associated with Au-Bi-As-Cu, Mo-Cu, and Base-metal-Au-Ag mineral occurrences in the Mountain Freegold region (Dawson Range), Yukon, Canada
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Abstract:
Various origins have been assigned to rounded to subrounded and elliptical quartz megacrysts ("quartz eyes") in dyke rocks associated with mineral deposits/occurrences worldwide.An exact interpretation of their nature is likely to tightly constrain the petrogenesis of the host rocks, and by association may be critical in evaluating genetic models for spatially associated ore minerals.ChromaSEM-CL imaging and electron probe microanalysis (EPMA) of "quartz eyes" within porphyry dykes associated with Au-Bi-Cu-As, Mo-Cu, and base-metal-Au-Ag mineral occurrences across the Northern Freegold Resources (NFR) property in the Dawson Range of Yukon Territory, Canada, reveals that the cathodoluminescence (CL) response of quartz is a function of its trace-element abundance(s).Bright blue luminescent growth zones are in most cases richer in Fe (up to 8839 ppm) and Ti (up to 229 ppm) relative to CL dark growth zones, with up to 41 times lower concentration of these elements.Assuming a TiO 2 = 1, the Ti-poor dull cores consistently recorded lower temperatures (mostly < 600 °C) compared to Ti-rich brighter blue rims (up to 860 °C).This suggests either overgrowth on xenocrystic cores or an increase in crystallization temperature.The temperature rise likely reflects magma mixing, and is therefore consistent with the phenocryst/phenoclasts having formed in a magma chamber rather than by secondary processes.Also, the great variability in composition and temperature of crystallization and/or reequilibration of brighter blue growth zones of two quartz crystals (660 °C and 855 °C) from a single sample suggests that multiple episodes of magma mixing and incremental growth of parental magma chambers occurred.Some "quartz eyes" are overprinted by variably oriented, bifurcating, and anastomosing fluid migration trails ("splatter and cobweb textures") of red to reddish-brown CL quartz that is in most cases of low-temperature origin, and trace-elements poor, thus implying interaction of deuteric fluids with quartz phenocrysts/phenoclasts.The presence of "quartz eye" crystals with broken and angular blue cores, overgrown by oscillatory-zoned rims in which the zoning pattern does not correspond with the crystal boundaries, further suggests that some quartz crystals had been explosively fragmented (phenocrysts) and are now hosted in a recrystallized tuffisitic groundmass.The volatile exsolution that likely accompanied both magma mixing and decompression (as suggested by dendritic quartz, fine-grained recrystallized tuffisitic groundmass, and corroded quartz grains) was probably an important process that could have favoured the ore formation.Keywords:
Phenocryst
Igneous differentiation
Petrogenesis
Cathodoluminescence
Trace element
Fractional crystallization (geology)
An extravagant hypothesis 'no phenocrysts, no post-emplacement differentiation' has been put forward by Marsh, in a series of papers, for the development of mafic–ultramafic intrusions. This hypothesis is based on an assertion that the majority of these intrusions are structureless and undifferentiated because they lack residual granitic rocks. To explain this, the hypothesis postulates that phenocryst-free magmas are not able to differentiate in crustal chambers because all evolved interstitial liquid is locked in solidification fronts, producing compositionally uniform magmatic bodies. Layered, well-differentiated intrusions are attributed to the successive emplacement of magma pulses with phenocrysts of different phase and chemical compositions, rather than to slow magma cooling and fractional crystallization, as conventional models imply. Such phenocryst-laden magma pulses are supposed to be derived from an underlying magmatic mush column. However, structureless, undifferentiated, mafic–ultramafic bodies simply do not exist in nature. All well-studied mafic–ultramafic bodies, with or without residual granitic rocks, that crystallized from parental magmas of non-eutectic composition, tend to reveal clear evidence of internal compositional differentiation in terms of crystallization sequences (e.g. Ol, Opx, Opx + Pl, Opx + Pl + Cpx), mineral compositions (e.g. An in plag, En in cpx) and compatible/incompatible major and trace element geochemistry (e.g. Mg-number, Cr, rare earth elements). This is especially evident in layered intrusions that represent the key evidence against the hypothesis. Essentially, by denying the ability of magma to differentiate in intrusive bodies, the hypothesis forbids magmatic differentiation in any sub-chamber related to the entire magmatic mush column. As a result, magma pulses in which phenocrysts progressively change in composition cannot be derived from the column to form layered intrusions. The hypothesis is thus contradictory, baseless and fundamentally flawed. It should be abandoned in favour of a classical fractional crystallization model based on the pioneering experiments of Bowen and amply confirmed over almost a century by subsequent studies of layered intrusions. The classical model was, is, and will most probably remain, the best explanation for the origin of differentiated magmatic bodies.
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Igneous differentiation
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Ultramafic rock
Fractional crystallization (geology)
Layered intrusion
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Phenocryst
Dacite
Igneous differentiation
Hornblende
Basaltic andesite
Alkali basalt
Fractional crystallization (geology)
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Thirty-six basalt samples from near East Pacific Rise 13°N are analyzed for major and trace elements. Different types of zoned plagioclase phenocrysts in basalts are also backscatter imaged, and major element profiles scanned and analyzed for microprobe. Basalts dredged from a restricted area have evolved to different extents (MgO=9.38wt%—6.76wt%). High MgO basalts are modeled for crystalliza-tion to MgO of about 7wt%, and resulted in the Ni contents (≈28 ppm) that are generally lower than that in observed basalts (60 ppm). It suggests that low MgO basalts may have experienced more intensive magma mixing. High MgO (9.38wt%) basalt is modeled for self-mixing-crystallization, and the high Ni contents in low MgO basalts can be generated in small scale and periodical self-mixing of new magma (high MgO). Mixing-crystallization processes that low MgO magmas experienced accord with recent 226Ra/230Th disequilibria studies for magma residence time, in which low MgO magmas have experi-enced more circles of mixing-crystallization in relatively longer residence time. Magma mixing is not homogeneous in magma chamber, however, low MgO magmas are closer to stable composition pro-duced by periodical mixing-crystallization, which is also an important reason for magma diversity in East Pacific Rise. Zoned plagioclase phenocrysts can be divided into two types: with and without high An# cores, both of which have multiple reversed An# zones, suggesting periodical mixing of their host magmas. Cores of zoned plagioclase in low MgO (7.45wt%) basalt differ significantly with their mantle in An#, but are similar in An# with microlite cores (products of equilibrium crystallization) in high MgO (9.38wt%) basalt, which further shows that plagioclase phenocryst cores in low MgO basalts may have formed in their parental magmas before entering into the magma chamber.
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The Batur volcanic field (BVF), in Bali, Indonesia, underwent two successive caldera-forming eruptions that resulted in the deposition of silicic ignimbrites. The magmas erupted during and between these eruptions show a broad range of compositions from low-SiO2 andesite to high-SiO2 dacite. On the basis of their geochemistry and mineralogy these magmas may be assigned to six groups: (1) homogeneous andesites with phenocryst compositions essentially in equilibrium with the whole-rock composition; (2) remobilized crystal-rich low-SiO2 andesites with resorbed phenocrysts in equilibrium with the whole-rock composition; (3) mixed low-SiO2 dacite with a relatively large range of phenocryst compositions, with most phenocrysts slightly too evolved to be in equilibrium with the whole-rock; (4) extensively mixed low-SiO2 dacites with a very large and discontinuous range of phenocryst compositions, with most phenocrysts either more Mg-rich or more evolved than the equilibrium compositions; (5) remobilized crystal-rich low-SiO2 dacites with resorbed and euhedral phenocrysts; (6) homogeneous high-SiO2 dacites lacking evidence for magma mixing and showing narrow ranges of phenocryst compositions in equilibrium with the whole-rock composition. This range of silicic magmas is interpreted to reflect a combination of closed- and open-system fractional crystallization, magma mixing and remobilization of cumulate piles by heating. The variety of magmas erupted simultaneously during the caldera-forming eruptions suggests that the magmatic system consisted of several independent reservoirs of variable composition and degree of crystallization. The magmatic evolution of individual reservoirs varied from closed-system fractional crystallization to fully open-system evolution, thereby resulting in simultaneous production of magmas with contrasted compositions and mineralogy. Extensive emptying of the magmatic system during the caldera-forming eruptions led to successive or simultaneous eruption of several reservoirs.
Phenocryst
Silicic
Fractional crystallization (geology)
Igneous differentiation
Dacite
Caldera
Magma chamber
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Silicic
Igneous differentiation
Felsic
Magma chamber
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Igneous differentiation
Fractional crystallization (geology)
Magma chamber
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