The Batman and Quigleys gold deposits of the Mt. Todd (Yimuyn Manjerr) Mine, Australia: structural, petrographic and mineralogical investigations of coeval quartz sulphide vein and lode/stockwork systems

Abstract

The Mt. Todd Mine (renamed the Yimuyn Manjerr in August 2000) in the Northern Territory of Australia is located at the southern end of the Pine Creek Inlier (PCI). The mine is host to several discrete ore bodies that strike NNE within a broad NE-trending corridor of gold mineralisation. These deposits exhibit a spatial association with local granite plutons and, in particular, are situated within the intermediate contact metamorphic aureole of the Tennysons Leucogranite. They exhibit a marked difference in ore style, ore system and dominant sulphide mineralogy. The Batman and Quigleys deposits of the Mt. Todd Mine are located 4–5 km apart within the NE-trending corridor of gold mineralisation. The Batman deposit comprises a stack of discontinuous, steeply west-dipping sheetlike veins within an ore envelope that defines a broad zone of normal faulting. It constitutes a quartz–pyrrhotite–pyrite dominant system of >500-m depth continuity. The Quigleys deposit comprises a series of discontinuous, east-dipping lodes that are interconnected and subtended by stockwork veins within a single continuous fault plane. The deposit constitutes an arsenopyrite–pyrite-dominant system of limited depth continuity. Despite their differences, detailed structural, petrographic and mineragraphic studies of the vein/lode systems have established that the deposits are similar in vein/lode morphology, principal orebody-forming process, tectonic history, alteration assemblage and relative geological timing. There is broad agreement in the type of host lithofacies and an association with a NE-trending regional magnetic lineament. The studies suggest that mineralogic differences between the deposits are the result of a gross metal and sulphide zonation about the Cullen Batholith that corresponds with decreasing temperature and increasing distance from the margin of the batholith as noted by other workers. The deposits are therefore interpreted to have formed at the same time in response to similar structural and tectonic controls. Gold mineralisation in the Mt. Todd Mine is thereby constrained to a single mineralising event. The quartz sulphide veins and lodes of the Batman and Quigleys deposits are typified by hydrothermal fracturing and/or normal faulting of the upper crust during sinistral strike–slip displacement on a NE-trending basement strike–slip fault. The deposits are similar to other orogenic gold deposits within the PCI, but with affinity to the thermal aureole gold style. However, in the Batman deposit, the association of gold with bismuth and reduced ore mineralogies makes this deposit unique in the PCI.