The alteration and structure of the Maw Zone subeconomic REE deposit Marie Barker Department of Eart

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The alteration and structure of the Maw Zone
sub-economic REE depositMarie BarkerDepartment
of Earth Science, St. Francis Xavier University
Special thanks to James Sykes and Billy Kerr from
Denison Mines and Dr. Brendan Murphy from StFX,
whose advice and assistance, helped improve and
give direction to this project
Introduction
Background

• Structure plays and important role in some
  uranium deposits . Faulted and brecciated areas
  are sites of increased permeability. These areas
  focus fluid flow and provide conduits where
  oxidizing basin and reducing basement fluids can
  mix (Hanly 2001, Fayek Kyser 1997).
• Fluid interaction can cause precipitation of new
  minerals and dissolution of early diagenetic
  cements in the Athabasca Basin (Jefferson et al.
  2006)
• U deposits are the result of prolonged fluid flow
  and the formation of a stationary redox front.
  This fluid activity results in an alteration
  halo which extends far beyond the ore body
  (Jefferson et al. 2006)

Purpose

• Significance of the Athabasca Basin
• 1.7 billion year old sedimentary basin, mostly
  sandstones and conglomerates, deposited
  unconformably over high grade metamorphic and
  igneous rocks of the Canadian Shield (the
  basement)
• known for unconformity-type U deposits, hosts the
  highest grade deposits in the world, leading
  producer of U (Jefferson et al. 2006)

• The Maw Zone is a sub-economic rare earth element
  (REE) deposit located in the south-eastern corner
  of the Athabasca Basin in northern Saskatchewan.
  The area was discovered in the 1980's and is
  enriched in the heavy REE's and Yttrium.
• As well as having REE enrichment, the Maw Zone
  also exhibits alteration and structural patterns
  similar to those found around unconformity-type
  uranium (U) deposits.
• This study examines the surface extent of
  alteration and structure at the Maw Zone for
  trends and patterns and compares them to others
  found around U deposits to acquire a better
  understanding of the processes that led to the
  formation of the Maw Zone.

Alteration associated with unconformity-type
uranium
1. fracturing
/- brecciation
2. silicification

• Unconformity-type U deposits
• named for the association with an unconformity
  surface
• U is soluble in oxidizing aqueous fluids, but
  insoluble in reducing aqueous fluids.
• U is deposited at a redox front, where reducing
  brines from the basement meet oxidizing fluids
  from the basin (Jefferson et al. 2006)

3. outer hematite
4. bleached zone
5. inner clays
- dravite (tourmaline group clay mineral)
- chlorite
Methods
- illite

• Stereonet Analysis
• A structural analysis assesses the potential role
  of deformation in fluid alteration, U
  mineralization and the origin of brecciation and
  fracturing (faulting, hydraulic fracturing or
  primary sedimentary). Shown below are stereonets
  of the 2 most intensely faulted outcrops.

• Mapping
• Mapping of the Maw Zone was completed in August
  2006 to determine the type of alteration present
  and its surface extent, as well as to find
  relationships between alteration and structure.
  Maps of the entire 200m x 300m area and
  individual outcrops were completed.

• A stereonet is a 2D representation of a lower
  hemisphere projection of planar and linear
  features. This method facilitates the analysis
  of the 3-D geometry of these features. The
  points on this stereonet show the poles to the
  main faults at the Maw Zone.

Results and Conclusion
Faults in the basement, formed during the
Trans-Hudson Orogen, are similar in orientation
to the Maw Zone (135-160o, 35-60o, 175-200o and
85-100o). This reflects reactivation of basement
faults throughout the history of the Athabasca
Basin (Fayek Kyser 1997)

• Structure at the Maw Zone is controlled by
  basement faults
• Structure strongly controls alteration
• Most intensely faulted areas are also the most
  intensely brecciated, silicified and bleached
• Outcrops with visible dravite were intensely
  brecciated and silicified.
• Some faults had visible illite clay
• Large U deposits have extensive alteration halos
  where the alteration and clay mineral development
  is more pervasive (Jefferson et al. 2006)
• The Maw Zone has alteration limited to areas
  close to faults and breccias, clays are limited
  to fractures and pore spaces.
• Alteration at the Maw Zone is not nearly as
  intense as that of U deposits, suggesting that
  alteration was relatively short-lived, and did
  not result in the formation of a standing redox
  front that would have allowed precipitation of
  significant U-bearing minerals.

Results
Bleaching reflects upward migration of reducing
basement fluids into the overlying sandstones
Jigsaw Breccia No bleaching
Partial Bleaching Bleaching of Breccia Matrix
Intense bleaching and silicification
Maw Zone Maps Breccia, Alteration
Unaltered Maw Zone sandstone with wave ripples
Legend
Compared to...
M. Fayek, T. Kyzer. Characterization of
multiple fluid flow events and REE mobility
associated with the formation of
unconfromity-type U deposits in the Athabasca
Basin. The Canadian Mineralogist 35 (1997)
627-658 A. J. Hanly (2001). The Mineralogy,
Petrology, and REE Geochemistry of the Maw Zone,
Athabasca Basin. Unpublished Masters Thesis.
University of Saskatchewan, Canada C.W.
Jefferson, D.J. Thomas, S.S. Gandhi, P.
Ramaekers, G. Delaney, D. Brisbin, C. Cutts,
P.Portella, and R.A. Olson. Mineral Deposits of
Canada. (2006). Natural Resources of Canada
http//gsc.nrcan.gc.ca/mindep/synth_dep/uranium/in
dex_e.php

Silicification
Hematite, Bleaching, Dravite
Breccia
Bleached and brecciated sandstone from the Maw
Zone