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Early extension in the Yilgarn Craton: Evidence from Leonora

 

Roberto Weinberg, Monash University, Australia

 

 

 

Main Conclusions
  • Normal shear zones abound in the area between Leonora and Diorite King, following the margin of the Raeside Batholith
  • These shear zones predate a NNW-SSE folding phase and pervasive foliation, interpreted as the regional D2
  • Normal shear zones have an extension direction away from the Raeside Batholith: north of the batholith, stretching is to the north, east of the batholith stretching is to the east (see Skwarnecki 1987, and Vearncombe 1992)
  • Extension post-dates an earlier deformation event which produced steep foliation and interpreted to be D1.
  • Normal shear zones developed at amphibolite facies and host gold: e.g. ~2t of gold were produced in the very large pit at Jasper Flat (Williams 1999/9), in Harbour Lights (Vearncombe 1992), and also in the many small historic diggings in the area around Diorite King.
  • Normal shear zones are associated with tight to isoclinal folds with axes plunging towards somewhere between NE to NW and parallel to the mineral stretching lineation
  • This early event is folded around D2 NNW-trending folds at a range of scales, and this is associated with a retrogression to greenschist facies, most obvious at Victor Well pit (331479 x 6811694 WGS84)
  • D2 is also associated with gold in quartz veins in shear zones (e.g. Victor Well)
  • Along the margins of the Raeside batholith, from Leonora to Diorite King, mineralization formed during amphibolite facies metamorphism and early extensional phase De (Diorite King, Jasper Flat, Harbour Lights) and during greenschis facies metamorphism and D2 (Victor Well)
  • Extension was triggered by a regional multidirectional crustal stretching event De and close to the batholith the stretching direction was controlled by its margin

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    Questions and Implications
  • In the literature normal shear zones have been thought to predate a D1 folding phase (Williams and Currie 1993) or postdate an early D1 folding (Witt 1994)
  • We see the extensional event overprinting an early formed foliation, but we know not the age or the nature of that event
  • The extensional event accomplished the uplift of the batholith and the downward movement of the greenstones. We suggest that this movement if prolonged could have led to the formation of unconformities in the Black Flag Beds and the creation of late Basins such as the Pig Well basin
  • Goleby et al. (2004) state (p. 127), but do not further develop, that the "variability witin the upper (crustal) layer suggests a complex deformation style, with suggestion of contractional as well as extensional deformation. Based on our work and the literature, we would expect an early extensional phase overprinted by NNW-ESE trending folds and thrusts
  • The seismic expression of the deformation sequence we determined at the surface would be one of folded listric shear zones. These could easily be misinterpreted as antiformal thrust stacks, as interpreted for the seismic expression of the middle crustal layer beneath Leonora by Goleby et al. (2004, p. 127)

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  • Why is this well-defined event in the Leonora area has not been found elsewhere in the Eastern Goldfields?
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    Introduction

     

    In this page we cover the geological evolution of the area north and northwest of Leonora around the Raeside Batholith and the Boxers Well Pluton. One of the main features of this area is the curved boundary between the Raeside Batholith and the overlying mafic-ultramafic greenstones of the Leonora Terrane (Vanderhor and Witt, 1992), the NNW-trending Mount George Zone (Passchier 1994) which separates the Leonora Terrane from the Keith-Kilkenny Tectonic Zone (Hallberg 1985). The latter is comprised not only of the lower mafic-ultramafic greenstones but also calc-alkaline volcanic rocks (dacites and rhyolites) and the late sedimentary sequence of the Pig Well Basin. Passchier (1994) concluded that the Mt George Zone north of Leonora is not a strike-slip shear zone as previously thought, but an early extensional structure overprinted by D2 folding. He suggests that the Mt George Zone could have accommodated E-directed extension to form a graben into which the upper stratigraphic sequences comprising the calc-alkaline volcanic rocks and the Pig Well basin were deposited.

     

    Williams and Currie (1993) reported a significant drop in metamorphic grade over a very short distance indicating a loss of crustal section in a drill core on the eastern side of the Raeside Batholith. Passchier (1994) argued against Skarnecki's (1987) suggestion that granite diapirism controlled the structures based on the lack of radial stretching lineation around the Raeside Batholith and to the existence of extension structures away from granite margins.

     

    Findings

     

  • At Diorite King, and along the margin of the Boxers Well pluton, NNE-trending shear zones are characterized by NNE-trending lineation and oblique dextral-normal movement on steep W-dipping planes (confirms the Passchier 1994 description of this area)
  • Closer to the Raeside Batholith, at and around Mount Ross, the main foliation strikes N70E/50-60N and down-dip lineation, and S-C fabric indicate normal movement to the NNW. The main trend of the foliation is parallel to the contact between amphibolites and the batholith.
  • These early foliations are folded around NNW (340) steeply dipping foliations and axis trending north and plunging moderately, which we interprete to correspond to regional D2.
  • To the East, in Jasper Flat/Auckland (328411E x 6817300N), an open pit which produced 2t Au (Williams 1998/9), the early foliation strikes 300-320/40-60N, with a broadly similar strike to the contact of the Raeside Batholith, and S-C relationships and shear folds suggest normal movement???
  • Further SE along the margin of the Raeside Batholith at both Victor Well (331480E x 6811700N WGS84) and Riviera (332800E x 6808700N WGS84), normal faults with gentle dips overprint an earlier formed steep WNW-trending foliation. These normal faults have a variety of strikes suggesting widespread stretching on the horizontal place and a vertical maximum shortening axis (Figure 1).
  • Moving to the N-S margin of the Raeside Batholith, structural work by others on the large gold mines of Sons of Gwalia and Harbour Lights have documented E-directed extension Skwarnecki 1987, Vearncombe 1992, Williams and Currie 1993, Passchier 1994, Witt 2001).
  • In all places, extension features developed isoclinal folds with axis parallel to the stretching lineation (see also Skwarnecki 1987), and commonly overprints early formed NW-trending foliation (Riviera pit)

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    Diorite King: Isoclinal folds in dextral-normal shear zones (N30E/80NW)

     

    Figure 1
    Rootless isoclinal folds
    A) Rootless isoclinal quartz folds    		 Rootless isoclinal folds
    B) Line drawing

     

    Mt Ross: Normal Shear Zones

     

    Figure 1
    normal shear zone hosting gold
    A) Line drawing of a 2m normal shear zone hosting gold in Mt Ross area (310875 x 6823056)

     

    Mt Ross: Refolded isoclinal folds

     

    Figure 1
    Refolded isoclinal folds
    A) Refolded F1 folds    		 Refolded isoclinal folds
    B) Line drawing

     

    Riviera Pit: Generalized Horizontal Extension

     

    Figure 1
    Normal fault
    A) Normal fault in weathered granite     		Normal fault
    B) Line drawing of fault and dragging of foliation
    Figure 2
     stereonet, normal faults at Riviera pit
     Equal area projection of all planes measured in the Riviera pit (5km north of Leonora 332833 x 6808700 WGS84) showing normal sense of movement. They define a cone which suggests a vertical axis of shortening and multidirectional horizontal extension.

    Conical Best Fit:
    Maximum shortening axis 313/87 (nearly vertical), Half angle 33o
    Minimum and intermediate extensional axes are indistinguishable and pattern suggests generalized horizontal extension

     

    Jasper Flat/Auckland

     

    Jasper Flat: F1

     

    Figure 1
    Granite F1 fold, Jasper Flat

    Granite F1 fold in northern wall of Jasper Flat

     

    Jasper Flat: Refolded folds

     

    Figure 1
    Normal fault
    A) Normal fault in weathered granite     		Normal fault
    B) Line drawing of fault and dragging of foliation

     

     

    Summary

    From Diorite King in the NW to Sons of Gwalia in the SE, over an arc of 90o and a length of 40km, the extension direction gradually rotates in tandem with and roughly at 90o to the orientation of the batholiths margins: extension to the NNW along ENE-trending margin at Mt Ross, to the NNE along the WNW-trending margin at Jasper Flat/Auckland, multidirectional in Riviera/Victor Well area, and to the E along the N-trending margin close to Leonora.
    Discussion

    Passchier (1994) suggested that the lack of radial stretching lineations around the Raeside Batholith was, together with a number of other evidence, indicative that extension was not related to gneissic doming as had been suggested by Skwarnecki (1987). We showed above that over an arc of 90o extension was radial close to the margins of the batholith, and that it required a vertical axis of maximum shortening.

    The fact that north directed extension also occurs at Mt Malcolm and at Mt Newman, where deformation cannot be related to granites (Passchier 1994) implies a regional north-directed extension event was not induced by granite doming confirming Hammond and Nisbett (1992) and WIlliams and Whitaker (1993).However, the radial pattern around the Raeside Batholith suggests that extension direction was reoriented at granite contacts and doming of the batholith remains plausible. If this was so, the fact that the smaller Boxers Well pluton was descending in relation to the Raeside Batholith is simply a result of the former being on the margins of the much larger batholith.

    Develop: Passchier suggests E directed faulting on Mt George to give rise to a graben. This would have to imply a multidirectional extension

     

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