Rock magnetism Research Papers - Academia.edu (original) (raw)

The Cloncurry district is renowned for Iron-Oxide Copper-Gold (IOCG) mineralization, but includes a wide spectrum of deposit styles, including: IOCG, Iron Sulfide Copper-Gold, Broken Hill Type (BHT), Sedex, Skarn and intrusion related Cu-Mo. Measurements were made of: density, magnetic susceptibility and Natural Remanent Magnetization (NRM), and Koenigsberger ratios calculated, on 1731 specimens from 14 different mineral deposits and prospects from the Cloncurry district. The deposits studied have a wide variety of petrophysical properties, primarily dictated by the relative contents of magnetic minerals (e.g., magnetite, monoclinic pyrrhotite, and hematite) and other non-magnetic minerals (e.g., hexagonal pyrrhotite, pyrite, galena, sphalerite, barite). Any combination of these minerals can be associated with high densities. High magnetic susceptibilities (and hence high amplitude magnetic anomalies) are invariably associated with coarse magnetite. Monoclinic pyrrhotite is associated with moderate magnetic susceptibility and high remanence, whereas hematite is only weakly magnetic. Many deposits and prospects contain mixtures of different Fe-oxide and sulfide phases, which can be related to redox and/or overprinting. Assemblages usually fall into one of three basic categories: 1.Oxidized assemblages, which contain hematite, no pyrrhotite, but typically pyrite and variable magnetite, are associated with retrograde (magnetite-destructive) alteration, often in association with Cu ± Au-and/or Mo-mineralization. 2. Intermediate assemblages are typically magnetite-rich, but can contain pyrrhotite and/ or pyrite. These are often only weakly mineralized, but are associated with large positive magnetic anomalies, caused by substantial quantities of multidomain magnetite. Magnetite-rich lithologies are advantageous for further mineralization, insomuch as they can be further oxidized or reduced. 3. Reduced assemblages are typically pyrrhotite dominant, contain no hematite, but often do contain magnetite. Hexagonal (non-magnetic) pyrrhotite is typically associated with galena and sphalerite, whereas magnetic pyrrhotite is more typically associated with chalcopyrite. Where monoclinic pyrrhotite is present the remanence is oriented sub-vertical up, and as such, is difficult to isolate from the steep, upward north oriented inducing magnetic field.