Multiple deformations with successive subvertical and subhorizontal axial planes in the Mount Isa region; their impact on geometric development and significance for mineralization and exploration

Abstract

The first orogeny to affect most rocks exposed in the Mount Isa province caused significant repetition of stratigraphy through north-south shortening. Geological maps are dominated, however, by younger folds with subvertical, north-south-striking axial planes. These folds were produced by east-west shortening during a second orogeny. Both orogenies produced several foliations. However, some foliations have negligible geometric consequences, allowing the geometry of the region to be described generally in terms of just the three deformations, D 1 D 2 , and D 3 , which produced macroscopic folds, and the younger deformations, which produced brittle faults.One of the weak events that appears superficially to have had little or no geometric impact played a highly significant role in the generation of structurally controlled mineralization. That role has been identified following advances in conceptual understanding of fold development and associated foliation. The deformation, which occurred between the D 2 and D 3 events, produced geometric effects that were masked by overprinting of north-south-trending D 2 folds by similarly trending D 3 folds. Significantly, these D 3 folds, which control the localization or upgrading of much mineralization in the Mount Isa province, did not form in rocks unaffected by this intermediate event because the near-parallelism of S 2 and S 3 in such rocks resulted in intensification of S 2 rather than development of a new S 3 .The intermediate deformation event, called D (sub 2.5) , locally produced highly asymmetric deflections in bedding and S 2 , with subhorizontal axial planes that were subsequently overprinted by D 3 folds and an S 3 cleavage. No mesoscopic foliation was produced during D (sub 2.5) , and its rotational effects on bedding were commonly subsumed within younger D 3 structures. Thus D 3 disguises the presence of D (sub 2.5) , except in relatively rare locations where D 2 folds were isoclinal, such as in the Tommy Creek block and on the northern end of the Snake Creek anticline southeast of Cloncurry, and northwest of Mount Isa in these three locations. In the latter locations, both limbs were affected, resulting in large-scale overturning of bedding on one limb prior to overprinting by D 3 .The asymmetric folds developed during D (sub 2.5) created sites where bedding could be pulled apart during D 3 , resulting in implosion because of the high fluid pressure in the surrounding country rock, with brecciation, jostling, fluid access, and mineralization. Large-scale zones of mineralization form where D (sub 2.5) was localized at an earlier formed heterogeneity, such as a thrust surface (e.g., at Mount Isa mine) or a granite-country rock contact. In these locations, early formed angular bedding relationships were locally rotated into orientations which created sites that could implode during D 3 . This role of D (sub 2.5) in the generation of D 3 allows recognition of regional zones for exploration that previously have received little attention.Recognition of the role of D (sub 2.5) reveals a cyclicity in successive subvertical and subhorizontal events, not recognized by most workers in this terrain, that may occur in other orogens around the world but has similarly gone unnoticed. After the first orogeny finished, a succession of three near-vertical and two subhorizontal events occurred during the east-west-shortening orogeny at Mount Isa. These were succeeded by younger kinks with east-west- to northeast-southwest-trending axial planes that appear to be the youngest ductile deformation to have affected the Mount Isa province.