18.18.

17.17. CASE STUDY 2 SUMMARY

PROJECT SUMMARY

Dilation deformation bands have evolved into highly permeable fluid conduits within the Eyre Formation, increasing the effective permeability between Four Mile uranium deposit and source rock.

Deformation Band Sets 4 and 5 are directly associated with uranium- thorium-rich palaeofluid redox roll front deposits.

Deformation Band Set 6 is increasing permeability between compartmentalized sections of sandstone in the Eyre Formation.

The principle stress regime at the Deadtree Section, proximal to the Four Mile and Beverly uranium deposits, has evolved from extension to compression since deposition of the Eyre Formation in the Eocene.

Deformation bands are excellent indicators of discretechanges in the local stress regimes.

Deformation bands can evolve into fluid conduits and support fluid flow in mineral systems.

The Paralana Fault and the Willunga Fault show the same palaeostress evolution, evolving from extension tocompression with similar σH orientations.

16.16. DEFORMATION BANDS AS FLUID CONDUITS

Deformation bands are observed to have increase fluid flow in the centre of the palaeofluid roll front (Fig. 24). The inverse roll front form is interpreted to be a result of enhanced permeability along the plane of deformation bands. A 3D model of the dilation bands shown in micrographs in Figure 4 was constructed using micro computed tomography (Fig. 25). The results depict the large, extremely well-connected and elongated pore space network, interpreted as highly permeable fluid conduits for uranium-bearing fluid migration (Fig. 26; Fig. 27). The total deformation band porosity is 16.1% with 14.5% open porosity, thus 90% of the total deformation band pore spaces hydraulically conductive, and currently open to fluid flow.

EW low K

high KDeformation band

Deformation band

Deformation band

Roll front deposit

2.5 cmFigure 25. 3-D model of dilation band porosity.

Figure 26. Dilatational deformation band stainedwith iron hydroxide from overlying rock.

Figure 27. Deformation band cross-cutting a palaeochannel.

Figure 24. Photograph of deformation band intersecting a palaeofluid roll front rich in uranium and thorium.

15.15.

2.5 mm

Undeformed host

PPL

RL

Skem

atic

Increasing time, strain, and fluid volumeDilation+ cement

Dilation+cement +fracture

Gradient dilation fracture+cement

Dilation+cement+fracture+host

Eyre Formation deformation bands capture a distinct dilatant evolution (Fig. 23). Grains boundaries are observed to be dilatant with interstitial iron-hydroxide cement. Open elongated vugs are observed forming within cemented dilatant bands, providing a network of large channels for fluid to flow through. This is interpreted to be a result of corrosive fluids passing through the dilation deformation bands. A raft of host rock is observed to be floating within the cement filled fracture, resulting from high strain and high fluid volume. These micrographs support the notion that vast amount of fluids have used these structures in the past and are using these structures at present day as fluid conduits.

EFFECTS ON FLUID FLOW

Figure. 23. Petrographic analysis and evolution of the dilation bands (boundaries marked with white lines)hosted within the Eyre Formation. PPL: plane polarized light, RL: reflected light.

14.14.

Figure 22. Facemap of the Dead Tree section highlighting deformation bands.

We measured 240 deformation bands within the Eyre Formation at the Dead Tree section, which defines five neotectonic generations of conjugate sets through conventional mapping methods (Fig. 22). The Eyre Formation is composed of interbedded coarse sands (iron stain) and fine sands and silts (kaolinized).

STUCTURAL DATA FROM THE EYRE FORMATION

2.2.13.13.

image

Time 4

DBS 1Time 5

Time 3

Time 2

Time 1

Time 6

92 4

C. 10 Ma uplift

Paleo-roll front oxidizing fluid migration

Key

1,2,3,4,5

Maximum horizontal stressYounging indicator

Number of deformation bandsDeformation Band Set

N

22

3

40

6

27

1

34

5

24

2

b

Deformation Band Set 1 - 3 represents conjugate sets of shear-enhanced compaction bands that initiated under a normal fault stress regime and evolved into a strike-slip fault stress regime (Fig. 20). Deformation Band Set 4 - 5 are conjugate shear-enhanced dilation bands that capture the principle stress evolution into a reverse fault stress regime, rotating from NE-SW to NW-SE; this period of deformation is interpreted to be associated with the neotectonic uplift of the Cambrian Flinders Ranges (Fig. 21). Deformation Band Set 6 is a set of pure dilation bands.

STRUCTURAL EVOLUTION OF DEFORMATION BANDS

Figure 21. Stereonet frequency distribution diagrams representing the neotectonic paleostress evolution of the Dead Tree section.

Figure 20. Deformation band from the Eyre Formation.

image

2.2.12.12. INTRODUCTION

354000 356000 358000

354000 356000 358000

6668

000

6666

000

6664

000

6668

000

6666

000

6664

000

Dead Tree

Mesoproterozoicmetasediments

Mesoproterozoicradiogenic Mt. Painter InlierU- Th- source

Mesozoic/Cenozoicsediments

Four Mile West

FourMileEast

Km1.5

Figure 19. Geological map depicting the eastern flank of the northern Flinders Ranges.

Fig. 19

Deformation bands have recently been recognised within Eocene Eyre Formation sandstone adjacent to the Four Mile East uranium deposit in the Frome Basin in the northern Flinders Ranges, arguably Australia’s most p rospec t i ve r eg i on f o r sedimentary-hosted uranium (Fig. 19). The Dead Tree section located along the eastern flank of the northern Flinders Ranges at the base of Four M i le Creek Gu l l y provides a natural laboratory to study the interplay between deformat ion bands and palaeofluid roll front deposits rich in thorium and uranium (Hore & Hill, 2009).

CASE STUDY 2: FLINDERS RANGES, SOUTH AUSTRALIAEFFECTS OF DEFORMATION BANDS ON URANIUM-BEARING

FLUID MIGRATION IN SEDIMENTARY SEQUENCES