Defining the mineral chemistry footprints of the Starra iron oxide-copper gold deposits in Northwest Queensland
Creators: name: Hohl, Max
Publisher: name: University of Tasmania
Year: 2024

description: Located in the Eastern Fold Belt (EFB), part of the Mount Isa Inlier in Northwest Queensland, Starra is a gold-rich member of the IOCG group. Starra comprises five physically separate ore bodies emplaced along a circa 6 km long interval of the north-trending Starra Shear. The ore bodies are intimately associated with hematite-magnetite lenses, which extend beyond the known mineralization along the Starra Shear and are hosted by the Proterozoic Staveley Formation (maximum depositional age: 1,743 ± 4 Ma). <br>At Starra four alteration stages (I-IV) have been recognized, which are associated with IOCG mineralization. Metasedimentary calc-silicate rocks of the Staveley Formation are overprinted by widespread stage I Na-Ca alteration, composed of albite ± scapolite ± calcite. More localized K-Fe(-Ca) alteration overprints Na-Ca alteration assemblages and is most prominent in the footwall to the Starra ore bodies. Predominant mineral phases are magnetite, biotite, pyrite and actinolite. The third alteration stage represents the main mineralization event, which produced abundant chalcopyrite, hematite, quartz, chlorite and minor bornite and Au-tellurides. Post-main mineralization carbonate ± barite veins asso?ciated with minor muscovite alteration occur in the hanging wall representing the fourth alteration stage. Mineral phases associated with those veins are ankerite, calcite, barite, chalcopyrite, bornite, chalcocite and minor native Au and Bi, Au-tellurides, and wittichenite (Cu<sub>3</sub>BiS<sub>3</sub> ). Copper-Au mineralization is subdivided into two stages – a main mineralization stage III comprising chalcopyrite and pyrite with lesser bornite and rare chalcocite, Au-Ag-tellurides, and native Au. This stage was followed by subordinate carbonate - quartz - barite vein formation with minor but locally high-grade Cu-Au mineralization during stage IV.<br>Mafic – intermediate intrusions, most pronounced below the Starra 222 ore body are interpreted to have exerted a primary control on Fe oxide mineralogy along the mineralized trend. Mushketovite (magnetite pseudomorphs after hematite) in mineralized magnetite-dominated iron lodes suggests fluctuating redox conditions during mineralization. The close spatial and mineralogical associations of the mafic-intermediate bodies, K-Fe(-Ca), and mineralization point to a stronger link between magmatic activity and IOCG mineralization than has previously been recognized at Starra. These features imply that the Cu-Au deposits of the Starra trend contain hybrid magmatic-hydrothermal deposits that features mineralogical and geochemical elements common to both Cloncurry IOCG deposits (i.e., abundant magnetite) and IOCG deposits of the Tennant Creek and Olympic Dam districts (i.e., Bi-Au-Cu mineralization, hematite - bornite - chalcocite). <br>Trace element variations in pyrite from Starra are defined by Co, As, Ni and Se. Gold occurs usually as telluride inclusion in pyrite. Starra pyrite has Co concentration of up to 6 wt.% and shows microscale chemical zonation. Most pyrites have a Co/Ni concentration of &gt;50, interpreted to reflect a highly saline, high temperature fluid of likely mafic origin. Nickel, As and Te occur in variable concentrations in pyrite, showing an antithetic relationship with Co concentrations. Highly variable trace element concentrations correlate with a wide spread of δ 34S values. In situ SIMS analysis yielded δ 34S values ranging from -7.1 to +15.8 ‰ from six pyrite samples from across the Starra Line. An intergrain range from -7.0 to +6.9 ‰, which correlates with a range in Co/Ni ratio of five orders of magnitude, reflects pyrite formation during alteration stages I-III. A continues trend between negative δ <sup>34</sup>S values in early-stage pyrite to positive δ <sup>34</sup>S values correlating with Se/S ratio in pyrite suggests mixing between magmatic-derived and seawater-derived fluids. Principal component analysis (PCA) of pyrite trace element chemistry defines distinct chemical domains reflecting changes in fluid source, redox conditions, and temperature. Gold inclusions form preferentially at the borders of those chemical domains, implying that Au deposition was driven by the mixing of hydrothermal fluids. <br>Five types of magnetite (Mag-1 – -5) have been identified at Starra based on their texture and co-precipitating alteration assemblages. Magnetite formed throughout the K-Fe(-Ca) alteration events (stage II) and early main Cu-Au mineralization (stage III). Late Cu-Au veins (stage IV) contain minor magnetite. Magnetite of this stage has Si concentration &gt;1 wt.% and is classified as silician magnetite (Si-magnetite). Partial Least Square (PLS) analysis reveals that variation in magnetite chemistry from across all five types is mostly defined by redox sensitive elements, such as V, Mn, and Cr. This suggests that during K-Fe(-Ca) alteration and early main mineralization, fluid conditions were oxidizing. In addition, Mag-1 has elevated Ni concentrations, which implies a relative primitive fluid source. Titanium and Sn concentrations, elements interpreted to reflect temperature changes, show no significant variation across the Starra magnetite paragenetic stage. Magnetite from the barren Eastern Hematites has consistent V and variable Ga concentration. Both elements discriminate magnetite from barren and mineralized lithologies in the Starra area. This relationship is potentially of significance for IOCG and iron oxide apatite (IOA) deposits as low V in magnetite discriminating Cu mineralized iron oxide systems from barren ones.<br>Oxidation of magnetite to hematite along the Starra line occurred independent of mineralization. Early mineralization was associated with mushketovite formation indicating an initially reduced ore-forming fluid. Near neutral pH and <i>f</i>O<sub>2</sub> conditions around the hematite-magnetite buffer allowed for the presence of Bi-melts, which refined Au concentrations at Starra. Two distinct sources of sulfur have been identified. Most of the sulfur was derived from a magmatic source, that was mixed with seawater-derived sulfur. Cobalt/Ni ratios &gt;50 in hydrothermal pyrite is commonly associated with fluids derived from or leaching of mafic rocks. Magnetite chemistry suggests that mineralization occurred after an increase in <i>f</i>O<sub>2</sub> , which coincided with an increase in <i>f</i>H<sub>2</sub>S. Subsequent chalcopyrite formation decreases <i>f</i>H<sub>2</sub>S of the hydrothermal fluid allowing for Au mineralization.