Abstract

Resolution and prediction of the structure underground at Mount Isa from the surface geology is now possible using evidence for north to south movement on large-scale thrusts during the first deformation at 1610 Ma. This evidence for the direction of movement during thrusting is preserved (1) macroscopically within three successive thrust imbricates as the north-south alignment of respective truncations of older stratigraphic units by the angular unconformity at the base of the Mount Isa Group; (2) macroscopically and mesoscopically as a zone of vertical north-south-striking foliation with a horizontal stretching lineation that formed on a sinistral transform fault during D 1 along the Wonga-Duchess belt; (3) mesoscopically as a regionally distributed north-south-oriented stretching lineation (L 11 ) that is locally well preserved on S (sub 0,1) surfaces in the hinges of folds with north-south axial planes that formed during the second regional deformation; and (4) microscopically as a horizontal, north-south-aligned mineral elongation lineation defined by inclusion trails preserved in porphyroblasts that grew early during the second deformation adjacent to the Wonga-Duchess belt.The swing in strike of a large-scale 70 degrees -west-dipping lateral ramp on the western margin of the large-scale Kokkalukkanurker duplex from north-south (parallel to the direction of movement during thrusting) to north-northwest-south-southeast caused a portion of the Meerenurker roof thrust lying on this lateral ramp to be overturned for at least 50 km near Mount Isa. This overturning resulted from tilting the 70 degrees -west-dipping lateral ramp through another 70 degrees as it rode obliquely (north-south) over the north-northwest-south-southeast-striking portion of the same lateral ramp on the next imbricate to the south. This generated the gross structural relationships seen in three dimensions at Mount Isa whereby the greenstone basement that truncates Mount Isa Group rocks below the mine overlies them to the west.The second deformation involved intense east-west shortening and formed macroscopic folds that conveniently expose varying levels of the thrust pile generated during the first deformation. In higher grade rocks west of Mount Isa and to either side of the Wonga belt, axial plane schistosity development and/or reactivation of bedding during this deformation was quite intense and tended to obliterate and/or rotate foliations and lineations formed during the first deformation. Only in the hinges of the folds formed during the second deformation, where the deformation was less intense or silicification had occurred, are earlier lineations preserved relatively unaffected by this event.The geology of the Mount Isa Valley and the rocks to the west of the mine is controlled by a series of north-south-striking high-angle reverse faults that slice through the Meerenurker thrust as it crosses the above-mentioned lateral ramp on the western edge of the large-scale duplex. Two of the faults, Mount Gordon and Mount Isa, dip steeply west with similar displacements of 4 to 6 km west side up. The other two faults, Holly and Tastoka, dip steeply east, with the latter fault having a more significant displacement of 2.5 to 3 km east side up that greatly affects the geology on the eastern side of the Mount Isa Valley. The effects of these faults on the geometry produced by thrusting during the first deformation and folding during the second allow accurate prediction of the third dimension anywhere in the Mount Isa Valley and the region immediately to the west. Consequently, they can be used to predict the location of structurally controlled sites of potential copper mineralization similar to those found in the Mount Isa mine where mineralization formed during the third deformation.

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