Discussion

3.8.1 TIMING OF CLEAVAGE FORMATION

Having discussed the presence and characteristics of one major cleavage and the possibility of at least two others within the CSA deposit as well as taking into account their presence in the region, it can be concluded that the S2 cleavage is the second, and most pervasive, cleavage formed in the Cobar Mining Field. In a variety of locations throughout the Cobar Mining district, the major regional (S2) cleavage is observed to be slightly oblique to the axial traces of folds (Glen, 1985; Stegman, 2007). This relationship is also observable within the CSA deposit and suggests that the regional cleavage formed after a first stage of folding took place. Figure 3.14 shows the stereographic projection of S2 cleavage-bedding intersections. The pattern observed here is consistent with folds that have been transected by S2. If we consider the major fault orientations observed on the level plans, a strong link can be made between the NNW striking faults and the S2 cleavage as well as the NNE striking faults and one mode in the composite SX cleavage. This correlation contributes to the discussion on the timing of formation of the two cleavages with respect to the formation of the faults. As was discussed in Chapter 3 Section 3.4.6, cross-cutting relationships of NNW-striking faults (sub- parallel to S2) and NNE striking faults (sub-parallel to a common orientation in SX) showed that NNW-striking faults offset NNW-striking faults in 80% of recognisable cases. This suggests that S2 was formed later than SX. A few examples were found of Sx overprinting S2. At this point it is important to note that the cleavage defined as SX is likely an amalgamation of a few different cleavages and that some of these could have formed after S2. However, the orientation of SX cleavage data suggests that most of SX probably formed prior to the S2 cleavage. This leaves three options:

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1) There is a sequential pattern of events with early ESE shortening followed by ENE shortening and fault offsets complicated by later events

2) The two fault generations are synchronous and cleavages near faults form parallel to faults defining an orthorhombic fault pattern

3) Transpressional events, as suggested by Glen (1992), lead to rotation of folds and transection due to rotational strain.

In the Cobar Mining Field, the S2 cleavage has a 12° different strike within the indentor area (Table 3.1). The presence of an indenter, first recorded by Glen (1990) located in the south of the Cobar Mining Field, could affect the orientation of S2 if it was active during D2 as Glen (1992) suggests. The very consistent nature of the S2 orientation throughout the entire Cobar Mining Filed suggests that this indenter must have been formed prior to the formation of S2 otherwise it would have caused a more widespread change in S2 orientation.

3.8.2 TIMING OF MINERALIZATION

The ore is intensely deformed and is concentrated along high strain zones (see Chapter 4, section 4.5 for details). Minerals including chalcopyrite, pyrrhotite, and pyrite show obvious microscopic signs of deformation including twins, folds and brittle cracking (Fig. 3.32).

Macroscopically there are signs of deformation within mineralization as well. Figure 3.16D shows folds within the Pb-Zn-rich ores and is just one example of deformation easily observable within mineralized zones. While no discrete faults are recognisable within ore lenses present in diamond drill core, the occurrence of sulfide-rich replacement of breccias and folds, in accordance with previous mapping of mining levels, suggest that ore lenses are indeed hosted in faults. In all the ore lenses the level of deformation is much higher than in the wall rocks. The textural evidence within the wall rocks suggests increasing cleavage development approaching the ore lenses and then very intense

deformation within all mineralized material. The ore lenses are the highest strain zones in the mine area. The host sequence is intensely deformed near the Cobar Fault but otherwise very few high strain features were found away from the mineralization. No mineralization was found to post-date cleavage development. Thus all structural datacoupled with the deformation visible in the ore suggests that mineralization was pre- or syn D2

3.9 SUMMARY

A structural study of the folds, faults, breccias, lineations, beddings and cleavages at CSA was undertaken as a means of understanding the spatial geometries of the ore bodies present within the deposit. Methods of data generation employed during this study included mapping of diamond drill

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core as well as synthesis of data from the CSA mine site. Data was analysed using a variety of methods including reorientation by CoreSolutions, stereographic projection, and boundary element modeling (i.e.Poly3D). Note, all data collected during this study was reoriented using a defined reference fabric (the S2 regional cleavage) whose orientation was determined to be 85º/264º.

• Bedding was found to have an orientation of 80º/270º. Younging direction and vergence was used to interpret small-scale folds within drill core and to determine larger-scale folds in relation to regional structures.

• Two cleavage sets were identified: 1) A regional cleavage S2 with orientation 85º/264º and 2) SX a group of variable cleavages which have distinctive orientations from S2. S2 is identified by uniform breaks observable best in diamond drill core and additionally in alteration zones by straight or anastomosing dark, Fe-Mg-rich chlorite bands which are regularly spaced and have smooth fractured surfaces. S2 cleavage planes are generally decorated by either elongate chlorite grains with steep stretching lineations or by the alignment of pyrrhotite blebs. When plotted on cross-sections (Fig. 3.17-3.21), S2 changes in orientation from 80°/090° in the upper portions of the mine to 85°/264° in the bottom portions the mine. This change occurs between 9200 mRL and 9000 mRL (Fig. 3.2). The SX cleavages are mainly recognised by their discontinuous nature and their tendency to occur at opposite or widely differing angles to S2. The intensity of SX cleavages are spatially correlated with micro-folds suggesting that a proportion of SX measurements are axial planar to the early folds and thus precede S2.

• Lineations can be grouped into three categories. First, bedding-cleavage intersections (S2∧S0) produced by the oblique intersection of S2 and bedding (S0). Here, the majority of measurements (steeply plunging) suggest that S2 transects earlier asymmetrical folds with long, steeply dipping western limbs while the minor subset of measurements (shallowly plunging) suggest that S2 transects these early folds along fold hinges and short, shallow easterly dipping limb. Second, steeply dipping stretching lineations are characterized by elongate chlorite grains and pyrrhotite blebs. These lineations were formed in compression most likely producing dip-slip movement, and not during transpression (as has previously been suggested). In fact, no evidence for transpression was observed within the confines of this study. Third, late-stage sub-horizontal, chlorite-rich slickenfibers and slickenlines occur in conjunction with intense chlorite alteration. Formed post mineralization during NS compression these lineations are distinctive due to overprinting and shallow dips.

• Folds are common throughout the region. These parasitic, asymmetric folds are open to tight with subhorizontal NNW trending fold axis and wavelengths of 0.5-5. Folds have long, steeply dipping western limbs and short, shallowly dipping eastern limbs. Folds, vergence and younging direction

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were used to produce cross-sections (Fig. 3.17-3.21). The relationship between folds, vergence and younging direction suggest the presence of a large-scale anticline to the east and a large-scale syncline to the west (Myrt syncline). However, the presence of the Cobar Fault to the east precludes the presence of an anticline.

• There are four types of breccias present within the CSA deposit. These brecciations can be classed as hydrothermally produced quartz and sulfide cemented and tectonically produced clay (fault-gouge)-matrix rich breccias and cataclasites.

• Faults play a major role in the localization of ore zones and lenses. These faults are

predominantly sub-vertical, have narrow widths (a few centimetres up to ten metres), have small lateral extents (a few meters to tens of meters), and have large, continuous vertical extents (>100 m). Faults are best identified by the presence of mineralization. Faults occur in four major sets suggestive of orthorhombic fault arrays (Fig. 3.24 and 3.25). These four fault systems have strikes sub-parallel with the dominant subset of SX and with the S2 regional cleavage. Detailed studies of level plans suggest that while the reverse scenario does occur, S2 sub-parallel faults most

commonly cross-cuts SX sub-parallel faults.

• Poly3D (a boundary element method program) was used to model the effects of stress on

structures both locally and regional. Before this model could be produced, a study of calcite twins using he methods of Rowe and Rutter (1990) found that the paleostress at the time CSA was formed to be 200 MPa. This information was inputted into Poly3D and used to model the activation of the Plug Tank and Cobar Faults. While they do not directly cross or interact with each other, these two faults are responsible for producing an area of minimum σ3 directly beneath the location of the CSA deposit in the hanging-wall of the Cobar Fault in response to a

compression of 200 MPa applied directly EW (Fig. 3.30 and 3.31). This zone of minimum σ3 has the potential to draw fluids towards the eastern margin of the basin along the Plug Tank Fault or through deeper aquifers within the basin. The location of the complex orthorhombic fault array within the hanging-wall of the Cobar Fault and the location of the Cobar Fault itself represent an obvious permeability path for the overpressured water to localize.

• Microstructures associated with both sulfide and gangue minerals present in the CSA deposit record a history of deformation occurring both syn and post mineralization. Quartz exhibits recrystallization textures such as irregular grain boundaries, sweeping undulose extinction, and grain size reduction (Fig.3.32). Calcite exhibit intense twining and textures which suggest it is stronger than quartz (Fig. 3.33). Pyrite exhibits two distinct growth periods. The early stage shows fracturing and inclusions within the cores. The second stage is euhedral and overgrows the early pyrite. Pyrrhotite displays twinning that is irregular, variable in thickness, and discontinuous

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throughout individual grains. Pyrrhotite has also been fractured and chalcopyrite injected into the cracks during deformation (Fig. 3.34). Chalcopyrite shows signs of ductile deformation having both been folded and injected into the cracks of other minerals (Fig. 3.34). Sphalerite exhibits intense folds and deformation, however annealing is advanced and microscopic deformation features are not distinguishable. Sphalerite also occurs as skeletal crystals within chalcopyrite (Fig. 3.34).

• Within the ore lenses deformation is much higher than in the wall rocks. The intensely deformed mineralization found within the CSA deposit is located along zones of high strain only and was not found to post-date cleavage development suggesting that mineralization was pre- and/or syn D2.

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