Stratigraphic and structural controls on ironstone mineralization in the Tennant Creek goldfield, Northern Territory, Australia

Abstract

The Warramunga Group has undergone one major period of east‐west sub‐horizontal upright folding with the development of an associated axial plane cleavage. Calculated bedding‐cleavage intersection lines show some fold plunge variation on a hundred metre scale attributed to local non‐cylindrical folding. Reverse faulting and thrusting occurred simultaneously with the major folding event, and the deeper structure of the Warramunga Group may have been dominated by a large thrust‐fault system. Strike‐slip along northwest‐ and northeast‐trending faults occurred in conjunction with small‐scale kink folding, locally deforming the regional cleavage and truncating the east‐west fold structures. Hematite‐magnetite‐quartz‐chlorite ironstone, including the Peko, Argo, Juno and Nobles Nob gold‐copper deposits, are spatially associated with hematite shale horizons occurring at three to four discrete stratigraphic levels above and below a locally stratiform porphyry sill(?). The shapes of the ironstone bodies and associated alteration zones indicate the regional cleavage, and locally reverse faulting, has channelled ironstone mineralizing fluids. Ironstone formation has occurred where cleavage or fault fluid conduits have intersected relatively oxidized shale in the stratigraphy, commonly within fold hinges. The intermittent spaced distribution of the ironstone does not necessarily coincide with regions of rapidly changing fold axes (doubly plunging folds) as suggested by previous studies. Economic gold and copper mineralization is more unevenly distributed than the ironstone suggesting controlling factors other than structure and stratigraphy.