Microstructural evidence for an epigenetic origin of a Proterozoic zinc-lead-silver deposit, Dugald River, Mount Isa Inlier, Australia

Abstract

The Zn-Pb-Ag deposit contained in the metasediments of middle Proterozoic age at Dugald River forms one of a number of significant zones of sulphide mineralization within the Eastern Fold Belt of the Mount Isa Inlier. It is characterized by its high Zn + Pb grade, high Zn/(Zn+Pb) ratio and strong structural controls with the present resource standing at 38 million tons averaging 13% Zn, 2.1% Pb and 42 g/t Ag. Microstructural timing relationships and a variety of microscopic ore textures indicate that the deposit resulted from replacement and partial infill of carbonaceous and pyritic host rocks by hydrothermal ore-forming fluids during the D4 deformation event. This genesis is contrary to earlier syngenetic-based interpretations, but accords with the discordant nature and structural controls on emplacement of the mineralization. Key timing criteria include (1) truncation of S2 and/or S3 by the late ore minerals, (2) replacement textures in undeformed mineral paragenesis, (3) slightly preferred dimensional orientation and undulating extinction of quartz and muscovite that are intergrown but which crystallized earlier than sulphide minerals in veins. The presence of these microstructural relationships throughout the deposit and the complete absence of any syngenetic stratiform precursor to zinc-lead-silver mineralization indicates that the zinc, lead and silver at Dugald River were epigenetically introduced rather than just having undergone syngenetic deposition during sedimentation and remobilization during deformation. The regional distribution of the mineralization in a specific stratigraphic zone is most likely due to the partitioning of deformation between different rock types. This caused the weaker lithology to accommodate significantly higher strain than adjacent more competent units, resulting in fracturing and localization of the syntectonic mineralization.