The Lady Loretta formation : sedimentology and stratiform sediment-hosted base metal mineralisation
Creators: name: Dunster, John N
Publisher: name: University Of Tasmania
Year: 2023

description: The Palaeoproterozoic Lady Loretta Formation of the Mount Isa Basin, northwestern Queensland, was deposited during a series of fluctuations in relative sealevel that constitute an overall regression. The interpreted sedimentary environments range from sub-wavebase to supratidal. Carbonaceous lagoonal sediments host economic stratiform base metal mineralisation. Previous tectono-sedimentary models for the Lady Loretta Formation that invoked a rift-setting and interpreted a regional \basal breccia\" as evidence of syn-sedimentary uplift and erosion are disputed. This study interprets the setting as a ramp/shelf with no evidence of large-scale syn-sedimentary tectonic activity. The \"basal breccia\" is interpreted as a duricrust that has no palaeoenvironmental implications for the depositional setting. Sedimentary structures microbialites (stromatolites) evaporite pseudomorphs and the arrangement of interpreted depositional environments indicate fluctuations in water depth tidal influence and storm activity during the deposition of the Lady Loretta Formation. Shale laminated to massive argillaceous dolostone and fine grained sandstone containing hummocky and swaley cross-stratification are interpreted as the deepest-water facies present. Shallow marine carbonates now consist of variably silicified dolostone and include microbialites that range from prone mat to biostromes and bioherms of domal digitate and columnar forms with significant synoptic relief. Ooid shoals developed in areas of shallow agitated water. Widespread mixed carbonate/siliciclastics and carbonates in the northern outcrops of the Lady Loretta Formation appear to be cyclic and developed as a facies mosaic. They are interpreted as peritidal on the basis of bipolar-bimodal palaeocurrent directions and the prevalence of flaser to lenticular bedded units tidal rhythmites interference wave ripples and herringbone cross-stratification with reactivation surfaces. Storm deposits such as imbricated plate breccias and gutter casts are common. Widespread casts and moulds of halite and pseudomorphs of discoidal gypsum enterolithic anhydrite and cauliflower cherts are interpreted to represent an evaporitic overprint produced during regression when a marine sabkha developed locally. Associated sedimentary features include desiccation cracks synaeresis cracks washout rills scour pits and wrinklemarks. Highly carbonaceous pyritic shale variably diagenetically altered Fe- and Mn-rich carbonates and dolomitic siltstone were deposited in isolated areas laterally and stratigraphically associated with tidal and possible subaerial facies. These pyritic facies occur at several localities in the Lady Loretta Formation one of which hosts the Lady Loretta ore body consisting of 8.3 Mt of 12% Zn-Pb-Ag combined. The ore and its host rocks contain a variety of microbialites including silicified or pyritised prone mat and low-relief small diameter elongate and inclined digitate forms. Parts of the host sequence also contain wave ripples crossbeds with bipolar palaeocurrent directions and a possible sulphate evaporite overprint. Sulphur isotope data for pyrite and base metal sulphides are interpreted to indicate a closed system with abundant microbial sulphate reduction. Collectively this is interpreted to indicate that the host rocks were deposited in a restricted lagoon developed within a regional tidal-flat environment. Similar potential host lithologies are recognisable elsewhere in the formation by their sedimentary features the high Mn and Fe content of the carbonate and the abundant bedded pyrite interpreted to have originally been prone microbial mat. Theoretically the distribution of such lagoonal facies can be predicted by sequence stratigraphy. On a rimmed shelf extensive lagoonal facies can develop during a lowstand as the barrier becomes exposed by falling sealevel. Smaller perched lagoons may develop on the tidal flat of an unrimmed shelf. In highstand systems tracts on a rimmed shelf microbialite growth may keep pace with rising sealevel to create large lagoons during minor regressions. Subwavebase shales that correspond to maximum flooding surfaces also have the potential to host base metal mineralisation in the Lady Loretta Formation. Previously proposed genetic mineralisation models for the Lady Loretta ore body have been revised based on the new interpretation of the deposition setting. The classic SEDEX model that relies on exhalation into deep anoxic water confined within a graben is untenable in the shallow lagoonal setting proposed herein. A new model in which mineralisation is interpreted to have formed in unconsolidated sediments in the shallow subsurface is based on geochemical isotopic and textural data. It is also consistent with a lagoonal setting where carbonaceous and pyritic sediments would act as reductants. Alternatively a late epigenetic model involving the long distance migration of brines and mineralisation by replacement of or void-fill in consolidated rock is supported by the difference between the SHRIMP U-Pb zircon age of a tuff in the footwall of the host sediments (1647¬¨¬±4 Ma) and the Pb isotopic model age of the mineralisation (within range of 1600- 1570 Ma). Comparison of the geochemistry of the ore sequence with other carbonaceous pyritic packages elsewhere in the formation suggests that geochemical exploration should be directed towards the direct detection of metals and pathfinder elements such as Ba Cd Hg and TI and the use of alteration indices and carbon and oxygen stable isotopes to map haloes."