A phosphate and fluorine bearing aluminous assemblage developed in the lower palaeozoic deformed volcanics of the Balcooma - dry river region

Timmins, Anna Louise (1990) A phosphate and fluorine bearing aluminous assemblage developed in the lower palaeozoic deformed volcanics of the Balcooma - dry river region. Honours thesis, James Cook University of North Queensland.

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View at Publisher Website: https://doi.org/10.25903/b460-qr76
 
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Abstract

Quartzite Ridge is an extensive, linear zone of siliceous and high alumina alteration within the Cambrian - Ordovician Balcooma Metavolcanics. These acid to intermediate metavolcanics and lesser metasedimentary host rocks are metamorphosed to greenschist facies and steeply dipping. A regional submylonitic foliation was developed during the early Silurian, when the rocks formed the axial zone of a major orogeny. This foliation is subparellel to stratigraphy and coeval and parallel with the Balcooma mylonite zone lkm to the west.

The alteration occurs along a high strain zone of the main foliation and cuts the stratigraphy at a low angle. It consists of an elongate, fine grained, mylonitic siliceous body in which widespread kyanite and andalusite occur along foliation planes and in crosscutting veins. It also contains elongate pods up to 8m across of structureless kyanite or andalusite, with lesser corundum, diaspore, muscovite, topaz and apatite. Aluminium phosphates scorzalite, lazulite, goyazite and svanbergite occur abundantly with apatite and andalusite in a restricted area.

Topaz occurs also as extremely fine grained irregular aggregates and veins within fine grained quartz and angular or finely milled quartz - topaz breccias. Topaz and abundant foliated muscovite are present in some areas within the foliated quartz. Apatite, pyrite and fluorite are common in topaz rich rocks. Fine grained rutile is present ubiquitously in the alteration zone. Pyrophyllite and kaolinite extensively vein and replace aluminosilicates throughout the zone.

The paragenesis of the above assemblage can be divided into four stages: (1) silicification of host rocks (2) aluminous alteration (3)topaz/muscovite alteration (4) clay alteration. All phases of alteration were part of a continuous evolving hydrothermal event with gradually waning temperature and increasing AHF. The physiochemical conditions during formation of the alteration assemblage can be inferred by a study of the system F₂O₋₁ -K₂O-Al₂O₃-SiO₂-H₂O and mineral reactions infered from textures.

Silicification probably occured in conjunction with hydrolytic cation leaching of the host rocks producing a residual enrichment of silica, aluminium, TiO₂ and possibly phosphate. Aluminium was mobilized and extensively veined and replaced the silicification. Increasing temperature and low aSiO₂ causes nonstochiometric disolution of SiO₂ over Al₂O₃. Low aSiO₂ within aluminous veins formed corundum and diaspore in high fluid flow areas. Muscovite formed as K⁺/H⁺ increased. AHF can increase due to cooling, thus topaz usurped Al₂SiO₅ as the stable aluminosilicate, and fluorite formed. Kaolinite and pyrophyllite alteration occured as the low aSiO₂ fluid cooled futher.

Widespread apatite shows the fluid was high in H₃PO₄. Scorzalite/lazulite (Mg/Fe bearing aluminium phosphates) formed from H₃PO₄ alteration of andalusite. Sulphate bearing aluminium phophates (goyazite and svanbergite) are isostructural with alunite and formed from in response to increased H₂SO₄ and Sr.

P/T considerations of the assemblage andalusite-kyanite-corundum indicate the assemblage was not the product of prograde regional metamorphism and that it probably formed between 2.5 - 3.0 Kb and over a range of 435-220°C. This suggests the event was of Silurian age. This is consistant with foliation developement within the alteration suggesting it developed in the closing stages of the mylonitic event A possible source for an acidic F rich fluid of appropriate age is the Ringwood Park Microgranite.

High alumina alteration zones of extremely analogous mineral assemblages and similar styles occur in other Palaeozoic felsic metavolcanic belts in the world, particularly the Carolina Slate Belt. These high alumina zones are sometimes associated with Au and sulphide mineralization however a drilling program at Quartzite Ridge failed to detect significant mineralization. These zones represent a distinctive alteration style characterised by high F and PO₄ and having genetic affinities with both porphyry Cu-Mo systems and greisens.

Item ID: 67666
Item Type: Thesis (Honours)
Keywords: Quartzite Ridge, Greenvale Subprovince, Minerals, Geochemistry, Volcanic rocks, Topaz
Copyright Information: Copyright © 1990 Anna Louise Timmins.
Date Deposited: 05 Apr 2021 23:57
FoR Codes: 37 EARTH SCIENCES > 3705 Geology > 370505 Mineralogy and crystallography @ 50%
37 EARTH SCIENCES > 3703 Geochemistry > 370399 Geochemistry not elsewhere classified @ 50%
SEO Codes: 28 EXPANDING KNOWLEDGE > 2801 Expanding knowledge > 280107 Expanding knowledge in the earth sciences @ 100%
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