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Geology of the volcanic-hosted Brockman rare-metals deposit, Halls Creek Mobile Zone, northwest Australia. II. Geochemistry and petrogenesis of the Brockman volcanics

Geologie der an Vulkanite gebundenen Seltene-Metalle-Lagerstatte Brockman, Halls Creek Mobile Zone, Nordwest-Australien. If. Geochemie und Petrogenese der Brockman Vulkanite

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Summary

Lavas and subvolcanic intrusions of the 1.87 Ga Brockman volcanics comprise a cogenetic suite of alkaline,Qz-normative, metaluminous trachyandesites, trachytes and trachydacites/rhyolites. They are genetically related to the rare-metal-bearing “Niobium Tuff” which contains extreme enrichments in high-field-strength incompatible elements (av. 1660 ppm Y, 9700 ppm Zr, 3200 ppm Nb, 175 ppm Yb). Neodymium isotopic data indicate the Brockman parent magma was mantle-derived with εNd(initial) + 3, analogous to basaltic magmas generated in some modern intraplate “hot-spot” volcanic provinces. The geochemical evolution and incompatible element enrichments in the Brockman suite can be modelled by AFC processes involving extensive degrees of crystallization and progressive contamination of derivative magmas with granitic/ metasedimentary upper crust. The large degrees of crystallization required to derive the more differentiated members of the Brockman suite are best accommodated by a process of “liquid fractionation” resulting in internal compositional stratification of the magma chamber with extreme differentiates such as the Niobium Tuff forming a volatile-enriched “cap” in the magma chamber roof-zone. The high fluorine content of the Brockman magmas played a crucial role in enhancing rare-metal contents by increasing the efficiency of crystal-liquid separation and decreasing mineral-melt Kd's. There appears to be no special role for fluorine-rich fluids in generating the rare-metal enrichments. However, leaching of fluorocarbonate minerals by late hydrothermal solutions, rather than fractionation of a LREE-selective phase, caused marked LREE-depletion in the Niobium Tuff.

Zusammenfassung

Laven and subvulkanische lntrusionen der 1.87 Mrd. J. alten Brockman-Vulkanite umfassen eine kogenetische Abfolge von alkalinenQz-normativen Trachyandesiten, Trachyten and Trachydaziten/Rhyoliten. Diese sind genetisch in Beziehung zu dem Seltene-Metalle-führenden “Niob-Tuff”, der extreme Anreicherungen an inkompatiblen Elementen hoher Feldstdrke führt (im Durchschnitt 1660 ppm Y, 9700 ppm Zr, 3200 ppm Nb, 175 ppm Yb, zu stellen. Nd-Isotopen zeigen, daß das Muttermagma der Brockman-Lagerstatte Mantelursprungs 'St mit εNd (initial) ≈ + 3, analog zu basaltischen Magmen, die an manchen modernen “intraplate hot-spot” Vulkanprovinzen gebildet werden. Die geochemische Evolution and die Anreicherung inkompatibler Elemente in der Brockman-Abfolge kann lurch AFC Prozesse modelliert werden, die extensive Kristallisation and progressive Kontamination der entstehenden Magmen mit granitischer/metasedimentarer Oberkruste beinhalten. Der hohe Grad von Kristallisation der erforderlich ist, um die mehr differenzierten Anteile der Brockman-Abfolge zu erhalten, läßt sich am besten lurch einen Prozeß von “liquid fractionation” erklären, der zu einer inneren Stratifikation der Magmenkammer führt, wobei extreme Differentiate, sowie der Niob-Tuff, eine an Volatilen angereicherte Kappe im Dachbereich der Magmenkammer bildeten. Der hohe Fluor-Gehalt der Brockman-Magmen spielte eine wichtige Rolle bei der Anhebung der Seltene-Metalle-Gehalte, and zwar dadurch, daß er die Effizienz der Kristall-Schmelze-Trennung erhöhte and abnehmende MineralSchmelze Kd's-Werte ermöglichte. Fluor-reiche Fluide scheinen keine spezielle Rolle bei der Bildung der Seltene-Metalle-Anreicherungen zu spielen. Auslaugung von Fluorokarbonaten durch späte hydrothermale Lösungen and nicht die Fraktionierung einer LREE selektiven Phase, bewirkte eine deutliche LREE-Abreicherung im Niob-Tuff.

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Authors and Affiliations

  1. Department of Geological Sciences, University College London, Gower Street, WC1E 6BT, London, United Kingdom

    W. R. Taylor

  2. Faculty of Resource Science and Management, University of New England, Northern Rivers Campus, Lismore, N.S.W., Australia

    G. Esslemont

  3. Australian Geological Survey Organisation, Canberra, Australia

    S. -S. Sun

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  2. G. Esslemont
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  3. S. -S. Sun
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Taylor, W.R., Esslemont, G. & Sun, S.S. Geology of the volcanic-hosted Brockman rare-metals deposit, Halls Creek Mobile Zone, northwest Australia. II. Geochemistry and petrogenesis of the Brockman volcanics. Mineralogy and Petrology 52, 231–255 (1995). https://doi.org/10.1007/BF01163247

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