Structural controls on Zn-Pb-Ag mineralisation determined by scale integrated analysis at Mount Isa, Queensland, Australia

Davis, Toby Patrick (2004) Structural controls on Zn-Pb-Ag mineralisation determined by scale integrated analysis at Mount Isa, Queensland, Australia. PhD thesis, James Cook University.

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[Part A] The Mount Isa Zn-Pb-Cu-Ag deposit contained almost 30 million tonnes of base metals, prior to mining, in spatially separate copper and Zn-Pb-Ag orebodies. The origin of the Zn-Pb- Ag ores is debated due to apparently conflicting features at intermediate to microscales. Ambiguity is associated with small scale features, which can be interpreted either in terms of syndeformation mineralisation or in terms of remobilisation of a predeformation orebody when considered in isolation of larger scale characteristics of the orebody. Understanding the relationship between metal distribution and the structural framework at the mine-scale helps to determine whether the orebody is deformed and leads to a better understanding of its formation.

The deposit contains ten stratabound Zn-Pb-Ag lenses in an en echelon array. The extremities of the orebodies correlate with F4 folds, and high grade shoots are centred on F4 hinges and short limbs that contain older F2 folds. Contours of Pb/Zn ratios throughout the lenses are parallel to F4 hinges and silica-dolomite alteration fronts. Restoring the large scale effects of folding by rotating bedding and the lenses to horizontal indicates that a sedimentary exhalative style of mineralisation cannot account for the present geometries of the Zn-Pb-Ag lenses. This reconstruction places the depositional basin in a compressional setting, or places the ores on topographic highs. These scenarios are considered to be incompatible with synsedimentary processes.

There are a number of important similarities between the geometries of the Zn-Pb-Ag lenses and the copper orebodies, which are interpreted to have a syntectonic origin. The Zn-Pb- Ag lenses display the same structural controls as the syntectonic copper ores and appear to have been emplaced at the same time in D4. Older F2 folds are preserved on the hinges and short limb areas of F4 folds and are interpreted to have behaved as structural heterogeneities during D4, which caused the dilation that led to metal deposition. F4 folds closest to the copper orebodies contain the highest grade Zn-Pb-Ag ore shoots, possibly indicating decreasing metal deposition away from the copper ores as fluids became progressively depleted in metals and/or concentration of fluid flow near the copper orebodies. In some areas, Zn-Pb-Ag ores wrap around silica-dolomite alteration associated with syntectonic copper mineralisation, suggesting a similarly late timing. The continuity of metal grades and Pb/Zn ratios throughout the Zn-Pb- Ag lenses indicates that the ores are not the result of local remobilisation. Instead, large-scale processes whereby the metals were introduced from an external source in D4 must have been involved in the formation of the Zn-Pb-Ag orebody.

[Part B] Asymmetric folds that are intimately associated with the development of the Zn-Pb-Ag and copper orebodies in the Mount Isa Lead Mine were examined and found to have formed over several deformations (D3 and D4). The mechanism of folding provides insights into dilation mechanisms that led to orebody formation. Folds with vertical NNW-SSE-striking axial planes were initiated from zones affected by localised horizontal shearing during an earlier deformation event. These initially horizontal folds were tightened significantly during D4 by opposite shear senses operating on either limb along the actively developing axial planar foliation, S4. Shearing along S4 in the long limb domains reflects the bulk shear sense for this deformation. The shear sense operating along S4 on the minor limbs of asymmetric folds was caused by the presence of a D3 high strain zone. Reactivation of the D3 high strain zone and bedding surrounding it in D4, prior to establishment of the axial plane cleavage in the short limb domain, imposes a clockwise sense of rotation (looking north), which is opposite to that along the actively forming foliation in the long limb areas. The continuity of folds is related to the scale of competency domains controlling localisation of the fold initiating shear band in D3. A general model for folding is presented in which alternate limbs of folds form in different deformations. Fold limbs should be viewed as shear domains with fold hinges forming from strain gradients near the margins of domains belonging to separate deformations with one determining the location of the fold.

[Part C] New mechanisms for the formation of asymmetric boudins and flanking structures are linked to the deformation history by examining these structures from exposure to microscopicscales. It was found that these structures formed by overprinting deformations, so they can only be used as kinematic indicators when their precise relationship to the deformation history is known. The processes of determining this relationship reveals much information about the deformation of the volume in which the structures are situated. The development of asymmetric boudins commenced with D2 segmentation of competent layers interbedded with weaker rock types. Disjunctive cleavages in massive siltstones and mudstones formed interboudin planes at the same time as distributed crenulation cleavage was produced in adjacent shale beds. Rotation and separation of boudins occurred to some degree in all subsequent deformations, but the bulk of these effects are attributed to D4. The formation of the interboudin planes during cleavage formation demonstrates that the maximum principal stress was perpendicular to them and at a low angle to bedding. Flanking structures at Mount Isa formed by dilation of S2 shear bands in D4 followed by rotation of the external host element by shearing along S4. Dilation of the shear band occurred to accommodate D4 deformation in the surrounding rocks. Relative rotation between the external and internal host element domains is caused by more intense deformation in the former, as shown by the density of S4 seams. In this process the internal host element remains more or less stationary, which is substantially different from present models. Flanking structures and asymmetric boudins preserve details of deformation that may have been destroyed or largely masked by ongoing deformation in the rocks around them and as such can be useful in understanding the deformation history. These structures should therefore be given significant attention when attempting to determine the deformation history of an area, especially in high strain areas where few other heterogeneities are preserved.

[Part D] The Mount Isa Zn-Pb-Ag orebody consists of conformable sulphide bands and breccias whose enveloping surfaces are parallel to layering at hand specimen and exposure-scales. The range of textural variation in the bands and breccias and their distribution has been determined by logging drill core throughout the orebody. The deposit is zoned with respect to ore textures and it contains a core of breccias surrounded by sulphide band textural styles. All ore textural styles have NNW-SSE-striking trends, which is the same orientation as fold-hosted mine-scale high-grade shoots. All sulphide band and breccia textural variants exhibit a similar paragenetic sequence indicating that they formed at similar times. There are no overprinting relationships indicating remobilisation occurred within the deposit. Microstructural examination of each band and breccia style using oriented samples shows that all sulphide deposition was structurally controlled and the kinematics of processes involved in orebody development. Localisation of shearing strain throughout the deformation history played an important role in the formation of the host structures to the sulphides. It produced heterogeneities at a range of scales that were vital for dilation during mineralisation. Dilation occurred to accommodate differential progressive deformation between adjacent volumes. Localisation between competency domains defined by rock type led to mineralisation being limited to some specific beds or zones and ultimately producing stratiform style ores. Higher strains occurred in shaly rock types compared to siltstones and mudstones and produced the bedding parallel sulphide textures that are commonly misinterpreted to represent predeformation mineralisation. The difference in scale of deformation partitioning between the copper and Zn-Pb-Ag orebodies could explain the separation of metals between the two orebodies. Such a difference in the scale of structures hosting ore minerals would produce a sharp contrast in the fluid/rock ratio between the ore bodies, which would ultimately influencing the chemical environment. Consequently, copper was deposited within the silica-dolomite bodies but lead and zinc were permitted to pass beyond it before being deposited.

Item ID: 24945
Item Type: Thesis (PhD)
Keywords: Mount Isa; zinc; lead; silver; mineralisation; mine-scale structural controls; NNW-plunging; asymmetric folds; asymmetric boudins; microstructural analysis; copper ores; North Queensland; Northern Australia
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Davis, Toby P. (2004) Mine-scale structural controls on the Mount Isa Zn-Pb-Ag and Cu orebodies. Economic Geology, 99 (3). pp. 543-559.

Date Deposited: 11 Feb 2013 06:22
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040307 Ore Deposit Petrology @ 30%
04 EARTH SCIENCES > 0403 Geology > 040312 Structural Geology @ 70%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 50%
84 MINERAL RESOURCES (excl. Energy Resources) > 8499 Other Mineral Resources (excl. Energy Resources) > 849999 Mineral Resources (excl. Energy Resources) not elsewhere classified @ 50%
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