Pre-ore hydrothermal alteration in an unconformity-type uranium deposit |
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| Authors: | A Komninou D A Sverjensky |
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| Institution: | (1) Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, MD 21218, USA, TP |
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| Abstract: | Hydrothermal alteration associated with the formation of unconformity-type uranium deposits is manifested mainly by intense
chloritic replacement of metamorphic mineral assemblages. At the Koongarra deposit (northern Australia), three stages of pre-ore
alteration were identified in chloritized metamorphic muscovite using optical microscopy, the electron microprobe, and transmission
and analytical electron microscopy (TEM and AEM). During the first stage metamorphic muscovite was altered to a mica with
very low interlayer occupancy (K content as low as 0.35 compared to 1 for ideal muscovite), high octahedral cation content
(up to 2.3 whereas ideal muscovite has 2) and high Si content. High resolution TEM images show that these changes in composition
were accompanied by frequent layer terminations in contrast to the metamorphic muscovite, whereas selected area electron diffraction
patterns indicate that the altered muscovite remained a two-layer polytype. The alteration of muscovite appears more extensive
in the outer alteration halo and has been almost completely eradicated in the ore zone where chloritization is more complete.
During the second stage of the pre-ore alteration the altered muscovite was replaced by Mg-rich serpentine (lizardite) and
clinochlore. High resolution TEM images show two 10 A˚ mica layers coalescing to one 14 A˚ chlorite layer. This type of coalescence
appears to involve transformation of one dioctahedral layer of the mica structure to one trioctahedral brucite layer in the
chlorite structure through dissolution of the tetrahedral sheets and cation influx in the octahedral site. The final stage
of the pre-ore alteration resulted in replacement of lizardite and clinochlore by Fe-rich chlorite. Comparison of the alteration
observed in metamorphic muscovite with the alteration observed in metamorphic biotite and garnet shows clearly that chloritization
of these phases was also a pre-ore alteration event. Pre-ore alteration was principally characterized by the evolution of
chlorite from a Mg-rich phase to an Fe-rich one.
Received: 29 July 1994/Accepted: 8 January 1995 |
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