title: Geochronological constraints on the formation, setting and evolution of the Olympic Dam IOCG deposit and the Olympic IOCG Province, South Australia
Creators: name: Cherry, AR
Publisher: name: University of Tasmania
Year: 2019

description: The Olympic Dam Cu-U-Au-Ag deposit in the Olympic Cu-Au Province (Gawler Craton, South Australia) is unique in the iron oxide-copper-gold (IOCG) deposit class for its polymetallic tenor and size; it is one of the largest economic accumulations of metals in the world. The deposit is hosted within the Olympic Dam Breccia Complex (ODBC) within the Roxby Downs Granite. The breccia complex also contains clasts and domains of other lithologies, including surficial lithologies (bedded clastic facies and felsic volcanic rocks). The Olympic Dam deposit, along with the other deposits and prospects in the Olympic Province, was originally suggested to have developed entirely within a geologically brief magmatic-hydrothermal event at ca. 1590 Ma. However, recent work has suggested Olympic Dam has been episodically modified after 1590 Ma. This thesis aims to clarify aspects of the architecture and timing of events at Olympic Dam as well as the nearby Olympic Province primarily through application of geochronology as well as mineral chemistry and textural analyses. This aim is addressed through: (1) determining the timing and relationships of the surficial lithologies in the ODBC with the Roxby Downs Granite, (2) demonstrating that Olympic Dam was originally overlain by the U-prospective Pandurra Formation and (3) exploring the complex history of hydrothermal mineralisation in the nearby Acropolis prospect. New high-precision CA-TIMS geochronology has constrained the Roxby Downs Granite (1593.87 ± 0.21 Ma) to be slightly younger than felsic volcanic clasts (Gawler Range Volcanics -1594.73 ± 0.30 Ma) in the ODBC. The absence of older country rock fragments in the ODBC and shallow emplacement textures in the Roxby Downs Granite suggest the granite intruded already-present Gawler Range Volcanics. The age of tuffaceous mudstone intervals (1590.97 ± 0.58 Ma) in the bedded clastic facies indicates a basin was present at Olympic Dam and the bedded clastic facies were being deposited ca. 3 myr after emplacement of the Roxby Downs Granite. The provenance of the bedded clastic facies is suggested to have changed from initially volcanic-dominated to granitoid-dominated as Hiltaba Suite granitoids within the vicinity of Olympic Dam were unroofed. The onset of formation of the breccia complex and the hydrothermal system is therefore constrained by the age of the Roxby Downs Granite; the breccia complex and the hydrothermal system were active after deposition of the bedded clastic facies. These age constraints and data from other workers are used to propose a model for the deposition and incorporation of the bedded clastic facies involving faults being responsible for their segmentation and entrainment into the breccia complex. A quartz-rich sandstone facies association recently discovered in the ODBC is entirely brecciated and has characteristics distinct from, and is not interbedded with, the other facies associations of the bedded clastic facies. In addition to the ca. 1590 Ma detrital zircon population present in all of the bedded clastic facies, the quartz-rich sandstone contains significant Palaeoproterozoic and Archaean age detrital zircon populations. The detrital and cement mineralogy, sedimentary textures, diagenetic age, and detrital zircon age populations match most closely sandstones of the ca. 1440 Ma Pandurra Formation which was originally deposited in the regionally extensive intracratonic Cariewerloo Basin. This correlation indicates the Cariewerloo Basin originally extended over the ODBC and that it was incorporated by tectonic activity at least 150 myr after the breccia complex first formed at ca. 1590 Ma. The Cariewerloo Basin is also speculated to have been a source of or conduit for oxidised U-bearing fluids that may have interacted with, and possibly added to, the Olympic Dam U resource long after 1590 Ma. Further evidence of post-1590 Ma events affecting the Olympic Province was obtained from a study of apatite in the hydrothermal mineral assemblage (comprising an initial magnetite-apatite assemblage and a later hematite-dominated assemblage) of the nearby Acropolis prospect. The prospect is structurally simpler and less brecciated than Olympic Dam, and the initial magnetite-dominated assemblage is well preserved. The apatite grains contain zones with abundant inclusions of REE-phosphate minerals (xenotime and monazite) as well as inclusion-free zones. The inclusion-rich zones are interpreted to have formed from the fluid-aided recrystallisation of original, inclusion-free apatite, resulting in the remobilisation of REE, U and Th from apatite into REE-phosphate inclusions. U-Th-Pb geochronology of apatite, xenotime and monazite revealed multiple ages; both inclusion-free and inclusion-rich zones of apatite yield ages coincident with the age of the host volcanic units (ca. 1590 Ma). The xenotime and monazite inclusions have ages that indicate alteration events at ca. 1370 Ma and possibly at ca. 500 Ma. Although the ca. 500 Ma age corresponds to the Delamerian Orogeny in the Adelaide Fold Belt adjacent to the Gawler Craton, the ca. 1370 Ma age does not correspond to any known event in or near the Gawler Craton but instead corresponds best with an event in Laurentia. Challenges in the interpretation of the monazite data imply xenotime is a more robust geochronometer in this setting. This thesis establishes a precise geochronological framework for the setting of significant lithologies at Olympic Dam and constrains the maximum age of the ODBC and the Olympic Dam deposit. Furthermore, the presence of the significantly younger Pandurra Formation in the ODBC implies that tectonic activity affected Olympic Dam long after 1590 Ma and has raised the potential for a late contribution of U to the deposit resource. The identification of multiple post-1590 Ma tectonothermal events affecting the Acropolis prospect suggests the wider Olympic Province has also experienced episodic modification. These findings contribute to the theory that the endowment of the Olympic Dam deposit and the Olympic Province did not occur within a single geologically brief event and may be due to episodic modification. The formation of such well-endowed deposits and metallogenic provinces may, in fact, require prolonged or episodic processes and offers the potential to assist in future exploration targeting for large IOCG deposits (i.e. in regions with a complex and long-lived geological history).