Economic Geology Vol. 93, 1998, pp. 1369-1389 Multiple Deformations withSuccessive Subvertical and Subhorizontal Axial Planes in theMount IsaRegion: Their Impact on Geometric Development and Significance for Mineralization and Exploration T H. BELL • AND K. A. HICKEY Department ofEarthSciences, James Cook University, Townsville, Queensland 4811, Australia Abstract Thefirst orogeny to affect most rocks exposed in theMount Isaprovince caused significant repetition of stratigraphy through north-south shortening. Geological maps are dominated, however, byyounger folds with subvertical, north-south-striking axial planes. These folds were produced by east-west shortening during a sec- ond orogeny. Both orogenies produced several foliations. However, some foliations have negligible geometric consequences, allowing the geometry oftheregion to bedescribed generally in terms ofjust thethree defor- mations, D1, D2, and Ds, which produced macroscopic folds, and the younger deformations, which produced brittle faults. One of the weak events that appears superficially to have had little ornogeometric impact played a highly significant role in thegeneration of structurally controlled mineralization. That role has been identified fol- lowing advances in conceptual understanding of fold development and associated foliation. Thedeformation, which occurred between the D,z and Dsevents, produced geometric effects that were masked by overprinting ofnorth-south-trending D2 folds by similarly trending D sfolds. Significantly, these Da folds, which control the localization or upgrading of much mineralization in theMount Isa province, did not form in rocks unaffected by this intermediate event because the near-parallelism ofS,z and Ss in such rocks resulted in intensification of S,z rather than development of a new Sa. The intermediate deformation event, called Da.5, locally produced highly asymmetric deflections inbedding and Sa, with subhorizontal axial planes that were subsequently overprinted by Dsfolds and an Ss cleavage. No mesoscopic foliation was produced during Da.s, and its rotational effects on bedding were commonly subsumed within younger Ds structures. Thus Dsdisguises the presence ofD,z.s, except in relatively rare locations where D.z folds were isoclinal, such as in the Tommy Creek block and onthenorthern end of theSnake Creek anti- cline southeast ofCloncurry, and northwest of Mount Isa in these three locations. In the latter locations, both limbs were affected, resulting in large-scale overturning ofbedding on one limb prior to overprinting byDs. Theasymmetric folds developed during Da.s created sites where bedding could bepulled apart during Da, resulting in implosion because of the high fluid pressure in thesurrounding country rock, with brecciation, jostling, fluid access, and mineralization. Large-scale zones of mineralization form where D2.s was localized at anearlier formed heterogeneity, such as a thrust surface (e.g., at Mount Isamine) 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 s. This role ofD2 s inthe generation ofD sallows recognition of regional zones forexploration that previously have received little attention. Recognition ofthe role of D2.5 reveals a eyelieity in successive subvertical and subhorizontal events, not rec- ognized by most workers in this terrain, that may occur in other orogens around the world buthas similarly gone unnoticed. Afterthe firstorogeny finished, a succession of three near-vertical andtwosubhorizontal 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 betheyoungest ductile de- formation tohave affected theMount Isa province. Introduction SOME OROGENIC terrains are dominated byshallowly dipping foliations (e.g., theScandinavian Caledonides, theEuropean Alps,and the Otago Schists: Trouw, 1973;Dieterich and Casey, 1989;and Johnson, 1990;respectively), whereas others, such asthe Mount Isaprovince (Fig. 1), aredomi- nated by steeply dipping foliations. Yet, in terranes dominated by shalloMy dipping foliations, evidence forsteeply dipping foliations is commonly preserved within porphyroblasts and in strain-shadow regions adjacent to deformed quartz veins (Johnson, 1990; Bell et al., 1992; Hayward, 1992; Aerden, 1994, 1995; Jones, 1994). Similar evidence forshallowly dip- ping foliations iscommonly preserved in terranes dominated Corresponding author: email, Tim. Bell@jcu.edu.au by steeply dipping foliations (e.g., Steinhardt, 1989; Bellet al., 1992; Davis, 1993, 1994, 1995; Davis and Forde, 1994). What arethegeometric effects of theoverprinting of suc- cessive orthogonal events at outcrop and map scale in areas dominated byvertical, or alternatively, horizontal foliations? Does such a succession resolve theproblem of fold develop- ment in terrains where successive generations of folds have very similarly oriented axial planes? Resolution ofthese ques- tions is particularly significant for the MountIsa province because thisregion contains a number of structurally con- trolled world-class orebodies of copper (Mount Isa: Perkins, 1984; Swager, 1985; vanDijk, 1991), Ag-Pb-Zn (Mount Isa: Perkins, 1997; Hilton: Myers et al., 1996; Dugald River: Xu, 1996, 1997) and gold (Eloise: Baker and Laing, 1998; Osborne: Adshead, 1995; Ernest Henry: Craske, 1995) that developed 0361-0128/98/2032/1369-21 $6.00 1369